Beyond Tumor Accumulation in Oncology Peptide Drug Conjugates: A Critical Narrative Review of Release-Competent Exposure and a Stage-Gated Translational Roadmap

Abstract

Peptide–drug conjugates (PDCs) combine peptide-mediated targeting or transport with a pharmacologically active payload. This critical narrative review organizes oncology PDC development around release-competent exposure (RCE): the connection between conjugate integrity, formation of an identified active compound, delivery to its tumor compartment, pharmacodynamic effect, and therapeutic margin. Tumor uptake or accumulation is distinguished from the concentration–time profile of an identified active species in its relevant compartment. PubMed, ClinicalTrials.gov, and regulatory sources informed the review through 15 September 2026; the recorded retrieval procedures were re-executed and archived on 24 October 2026. We distinguish payload-releasing PDCs from enzyme-potentiated prodrugs and non-cleaving modalities, assemble established, adapted, and conceptual exposure measures, and integrate analytical requirements into a stage-gated development framework. Clinical examples include zelenectide pevedotin, PEN-221, zoptarelin doxorubicin, ANG1005, and melflufen. Five preclinical illustrations and a separate calculation from published tissue AUCs show how different observations inform the framework. ADC-derived principles are qualified by PDC-specific disposition, while sparse tissue sampling, imaging, and modeling are evaluated as translational strategies. Use of these exposure measures within RCE remains hypothesis-generating and requires construct-specific validation; established PK quantities are not thereby validated clinical surrogate endpoints.

1. Introduction

An estimated 20 million new cancer cases and 9.7 million deaths occurred in 2022 [1]. Treatment is constrained by injury to metabolically or proliferatively active normal cells. ADCs demonstrate targeted delivery of highly potent agents, but benefit depends on target expression, conjugate exposure, linker stability, released molecular species, and organ-specific toxicity [2,3]. PDCs occupy a related but kinetically distinct framework; smaller carriers support rapid tissue distribution, chemically defined attachment, and access to receptors or transport pathways difficult to reach with antibodies [4,5,6,7]. Their liabilities include proteolysis, renal clearance, brief intact-conjugate circulation, and premature deconjugation [8,9,10].

PDC reviews describe tumor-homing and cell-penetrating peptides, linkers, payloads, and clinical programs [11,12,13,14]; 2026 reviews extend coverage to development hurdles, hematological applications, imaging, and translation [15,16,17,18]. This critical narrative review focuses on the evidence linking conjugate integrity, active-compound formation, spatial tumor delivery, PD, and therapeutic margin. Its contribution is an author-defined framework integrating established PK, PDC-specific adaptations, and a conceptual kinetic index into testable development hypotheses.

PDCs are not smaller ADCs with transferable PK parameters. Circulating stability can coexist with efficient cleavage after tumor retention or uptake. Rapid plasma clearance can limit delivery while retained conjugate or active payload sustains activity. Linker selection must integrate systemic stability, productive tumor access, local release, and active-compound persistence [19,20,21]. Section 7.4 separates transferable principles from carrier-dependent assumptions and identifies measurement consequences.

2. Review Scope and Methods

2.1. Review Design

This is a critical narrative review, not a systematic review or meta-analysis. Its objective is to develop and critically examine a hypothesis-generating framework linking conjugate integrity, active-compound formation, spatial tumor delivery, PD, and therapeutic margin. This approach is appropriate because the question requires integration of chemical, analytical, preclinical, clinical, and regulatory evidence addressing different steps in a mechanistic chain. The synthesis evaluates what each evidence type establishes, which assumptions transfer between modalities, and where validation is missing; it does not seek exhaustive study identification or a pooled treatment-effect estimate. Article development was informed by the six quality domains of the Scale for the Assessment of Narrative Review Articles (SANRA): importance, aims, literature search, referencing, scientific reasoning, and relevant endpoint data [22]. SANRA is a narrative-review appraisal tool, not a systematic-review protocol or reporting guideline.

2.2. Search Strategy and Evidence Selection

The original narrative search used PubMed, ClinicalTrials.gov, regulatory sources, publisher searches, and citation chaining, with a 15 September 2026 cutoff. Exact historical queries and filter settings are unavailable. A written revision-stage record dated 9 October 2026 was available, but its corresponding raw-response archive was unavailable for proof-stage verification. The recorded PubMed and ClinicalTrials.gov query strings were therefore re-executed and archived on 24 October 2026. Supplementary Materials S1 provides the executed strings, interfaces, fields, date limits, filters, retrieval yields, and official-document lookup records; Table 1 summarizes their scope. Nine PubMed queries cover PDC terminology, carrier classes, platform aliases, and relevant mechanisms across PubMed, including records outside MEDLINE, with a publication-date upper limit of 15 September 2026. ClinicalTrials.gov retrieval used product aliases and the same cutoff for first posting; returned records reflect their versions at the new retrieval date. This proof-stage check documented retrieval procedures and tested a newly assembled benchmark of known citations. It neither reconstructed earlier database states nor comprehensively screened the retrieved records. PubMed search-field syntax and ClinicalTrials.gov API documentation are described in the official resources cited in Supplementary Materials S1 [23,24].

Table 1. Search documentation and proof-stage retrieval verification for the critical narrative review. Complete strategies and dated records are provided in Supplementary Materials S1 and the companion archive.

Source	Date and Scope	Documented Procedure	Role
PubMed (NCBI ESearch)	24 October 2026; publications through 15 September 2026	Nine complete queries (Q01–Q09); title/abstract, MeSH, and publication-date fields. No language, study-design, species, access, or license filters. Full strings and technical settings: Supplementary Materials S1.	Literature retrieval and selected known-item checks
ClinicalTrials.gov API v2	24 October 2026; first posting through 15 September 2026	Product aliases in query.term with a StudyFirstPostDate cutoff; no other eligibility filters. Current record versions. Exact query and parameters: Supplementary Materials S1.	Registry retrieval and identifier verification
FDA and EMA	24 October 2026; specified documents issued by the review cutoff	Direct retrieval of named documents. Titles, versions, URLs, access dates, and lookup outcomes are recorded in Supplementary Materials S1.	Regulatory and product-document verification
ICH	24 October 2026; specified versions issued by the review cutoff	Q8(R2), Q9(R1), Q10, Q11, Q14, and M7(R2). Direct retrieval of versioned official PDFs is documented in Supplementary Materials S1.	Quality-guidance verification
Original publisher searches and citation chaining	Original review cutoff: 15 September 2026	Historical query strings and chaining logs are unavailable and cannot be reconstructed. These activities are distinguished from the new verification.	Context and primary-literature leads

Sources were selected for relevance to target or carrier selection; post-conjugation binding and trafficking; linker stability and release; active-compound identity, permeability, or mechanism; tumor or safety-organ distribution; multi-analyte PK or PD; clinical efficacy or safety; manufacturing and analytical control; or biomarker and regulatory strategy. Product claims prioritized primary clinical reports, trial records, labels, and assessments. Mechanistic claims are distinguished as direct PDC evidence in the reported construct and experimental setting; ADC evidence proposed to transfer only at a stated chemical or analytical level; or author extrapolation where PDC validation is absent. These evidence origins are distinct from the parameter classes in Section 3.2. Section 7.4 separates source observations, transfer limits, and our proposed measurement implications. Radioligand evidence informs target-access and organ-handling questions; radiation dosimetry remains modality-specific, and retained radiolabel does not establish cytotoxic-payload release (Section 8.6). Reviews supplied context and primary-literature leads, not primary outcome data.

The review scope excluded unconjugated peptides, nanoparticles with peptide decoration but no chemically defined payload relationship, and uptake-only reports lacking activation, retention, pharmacology, or safety information. English-language reports were prioritized, but non-English reports were not excluded solely because of language. No language filter was applied to the proof-stage PubMed or registry searches. Pre-2015 studies were retained for foundational carrier mechanisms or linker principles, clinical or quantitative-exposure benchmarks, or regulatory precedents. The evidence base contains 142 full peer-reviewed articles, 141 with DOIs; six DOI-bearing conference abstracts, one DOI-bearing corrigendum, and 24 regulatory, registry, sponsor, or technical-documentation records were retained as additional sources and excluded from the full-article total. Exhaustive study screening, independent dual-reviewer selection, protocol registration, and formal study-level risk-of-bias assessment were not undertaken. The cited-source totals describe the assembled bibliography, not a systematic screening flow; prevalence, comparative effect size, and literature completeness are not estimated.

The five preclinical illustrations in Section 10.1 are a purposive, availability-constrained sample with a convenience component, not a representative or systematic preclinical PDC sample. Selection combined relevance to distinct delivery or activation questions, numerical results extractable without image digitization, sufficient experimental context, and CC BY 4.0 licensing for adaptation. The five figures predate the proof-stage retrieval check and did not result from a systematic CC BY–only search for tabular endpoints. Licensing governed figure reuse, not review eligibility. Numerical extractability was not restricted to tables: some values came from published Results text and supplementary numerical tables.

These considerations describe the illustrative subset, not prospectively registered eligibility criteria. The five complementary studies do not provide balanced or maximum-diversity coverage of carriers, linkers, and payloads. No complete historical candidate inventory or study-by-study exclusion log is available. We therefore cannot identify all eligible unplotted studies, establish that these were the only studies meeting the presentation conditions, or assign retrospective exclusion reasons. Absence from these five illustrations means nonselection for illustration, not scientific ineligibility. Section 10.1 states each example’s contribution and inferential limits.

2.3. Critical Synthesis and Limits

Evidence was synthesized by position in the mechanistic evidence hierarchy defined in Section 3.2. Section 8 presents the authors’ narrative interpretations, informed by study design and comparator, population and evaluable denominators, endpoint and follow-up maturity, reporting completeness, and reported efficacy and safety findings. Randomized comparisons support treatment-effect conclusions that uncontrolled studies cannot establish; regulatory status and publication type provide context rather than independent grades of evidentiary certainty. No numerical score, formal evidence-grading system, or cross-agent ranking was applied. Conference abstracts are identified as abstract-level evidence, with 2026 updates explicitly treated as preliminary; publication in a journal supplement is not equivalent to a full peer-reviewed clinical report. Registry results are labeled separately. Non-peer-reviewed sponsor updates inform program status, not independent efficacy confirmation or agency decisions. Study design and reporting format are considered separately; these sources are not assigned the same evidentiary weight as full peer-reviewed randomized clinical reports.

3. Defining the PDC Modality

3.1. Operational Taxonomy

Here, a PDC comprises a peptide or peptidomimetic carrier covalently linked to a pharmacologically active small-molecule or macromolecular payload. The carrier mediates recognition, transport, or conditional uptake; the complete conjugate or a released compound produces the effect. An intact conjugate denotes the covalently linked carrier–linker–payload entity, not the carrier alone. The payload-releasing cytotoxic subset requires formation of an active payload or catabolite; Table 2 distinguishes this subset from other peptide-guided modalities.

For the authorization statement in Section 8, the subgroup is narrower: a peptide or peptidomimetic ligand must itself provide receptor or cell-surface recognition/transport and be covalently linked to a distinct, nonradioactive small-molecule cytotoxic payload. Classification follows the targeting component and effector, not peptide content alone; macromolecular cargo remains a separate category in Table 2. Authorization means marketing approval in the relevant jurisdiction at the cutoff, not trial registration, orphan designation or a pediatric waiver. These are scientific scope criteria, not regulatory definitions.

Peptide-activated prodrugs use enzyme and transport context rather than receptor-mediated tumor homing. Melflufen’s peptide-like promoiety promotes uptake and intracellular trapping of alkylating metabolites. Its placement in this mechanistic group does not imply an inactive parent; EMA describes it as a peptide-conjugated alkylating drug with intrinsic alkylating capacity [25].

Peptide radioligands used for PRRT deliver radiation without requiring linker cleavage; absorbed dose and microdistribution therefore replace active-catabolite exposure [26,27,28]. Photosensitizers require appropriate light and oxygen. Peptide-decorated nanoparticles, co-administered penetration enhancers, and unconjugated peptides fall outside the strict PDC boundary unless the peptide–payload relationship is chemically and experimentally defined.

Table 2 is an author-defined scientific framework for experimental comparison and regulatory dialogue. It neither creates a legal product class nor assigns an approval pathway or evidence standard; regulatory status concerns the complete product in its jurisdiction.

EMA’s synthetic-peptide guideline addresses conjugation quality, including cytotoxic-payload and imaging-agent delivery [29]. FDA’s ADC guidance concerns antibody- or antibody-fragment conjugates carrying cytotoxic small molecules [30]. Neither adopts this taxonomy. FDA’s ADC-specific drug–biologic combination-product description and preferred biologics license application route cannot automatically be extended to peptide-carrier conjugates [30]. Table 2 maps these scope and classification boundaries.

Table 2. Operational boundaries, exposure variables, and regulatory relationships of peptide-guided payload modalities [10,11,26].

Modality	Defining Architecture	Primary Productive-Exposure Variable	Qualification and Regulatory Relationship
Payload-releasing cytotoxic PDC	Targeting or transport peptide plus linker plus cytotoxic small-molecule payload	Tumor concentration–time profile of active payload or catabolite	Post-conjugation binding, released-compound identity, intracellular access, and free-payload safety exposure. Regulatory: overlap with EMA conjugation-quality guidance; peptide architecture alone does not meet FDA’s antibody-based ADC description [29,30].
Peptide-activated prodrug	Peptide-like substrate or promoiety with an enzyme-potentiated drug moiety	Formation and retention of active metabolites at the intended site	Enzyme activity, tissue distribution, metabolite kinetics, and patient subgroup effects. Regulatory: a mechanistic subclass, compatible with EMA’s description of melflufen as a peptide-conjugated alkylating drug; it does not define a separate approval class [25].
Peptide radioligand	Receptor ligand plus chelator plus radionuclide	Tumor-absorbed dose and spatial microdistribution	Radiochemical stability, image-based selection, dosimetry, kidney and marrow limits. Regulatory: aligns with the EU radiopharmaceutical definition (Directive 2001/83/EC, Article 1(6)) [31]. PRRT is a treatment; EMA peptide guidance covers component synthesis/starting materials only [29].
Peptide-conjugated photosensitizer	Targeting peptide plus photosensitizer	Tumor photosensitizer exposure multiplied by delivered light and available oxygen	Illumination geometry, oxygen dependence, phototoxicity, and procedure feasibility. Regulatory: the light-delivery configuration matters. FDA includes light-activated drugs labeled for a specific light-source device among combination-product types; peptide conjugation alone does not establish that status [32].
Peptide-guided macromolecular cargo	Targeting or cell-penetrating peptide plus protein, toxin, oligonucleotide, or degrader	Functional cargo in the intended compartment; cytosolic or nuclear exposure where required	Cargo integrity and function; endosomal escape and cytosolic/nuclear access where required; immune effects. Regulatory: no single assignment follows from this cargo grouping. Recombinant biological products are outside EMA’s synthetic-peptide guideline; the actual cargo and manufacturing process require separate assessment [29].
Adjacent but excluded systems	Peptide-decorated carrier with undefined conjugate state or co-administered penetration enhancer	Platform-specific	Cannot support a PDC claim without defined composition and causal delivery evidence. Regulatory: exclusion is a boundary of this review, not a finding that the platform is unregulated or lacks a valid development pathway.

This author-derived mapping distinguishes guidance scope, product terminology, and legal classification; it is not an agency decision for unapproved constructs. RCE measures are scientific tools, not regulatory acceptance criteria. The authorization statement concerns the peptide–small-molecule subgroup specified in Section 3.1; Section 8 considers boundary cases under broader definitions.

Table 2. Operational boundaries, exposure variables, and regulatory relationships of peptide-guided payload modalities [10,11,26].

Modality	Defining Architecture	Primary Productive-Exposure Variable	Qualification and Regulatory Relationship
Payload-releasing cytotoxic PDC	Targeting or transport peptide plus linker plus cytotoxic small-molecule payload	Tumor concentration–time profile of active payload or catabolite	Post-conjugation binding, released-compound identity, intracellular access, and free-payload safety exposure. Regulatory: overlap with EMA conjugation-quality guidance; peptide architecture alone does not meet FDA’s antibody-based ADC description [29,30].
Peptide-activated prodrug	Peptide-like substrate or promoiety with an enzyme-potentiated drug moiety	Formation and retention of active metabolites at the intended site	Enzyme activity, tissue distribution, metabolite kinetics, and patient subgroup effects. Regulatory: a mechanistic subclass, compatible with EMA’s description of melflufen as a peptide-conjugated alkylating drug; it does not define a separate approval class [25].
Peptide radioligand	Receptor ligand plus chelator plus radionuclide	Tumor-absorbed dose and spatial microdistribution	Radiochemical stability, image-based selection, dosimetry, kidney and marrow limits. Regulatory: aligns with the EU radiopharmaceutical definition (Directive 2001/83/EC, Article 1(6)) [31]. PRRT is a treatment; EMA peptide guidance covers component synthesis/starting materials only [29].
Peptide-conjugated photosensitizer	Targeting peptide plus photosensitizer	Tumor photosensitizer exposure multiplied by delivered light and available oxygen	Illumination geometry, oxygen dependence, phototoxicity, and procedure feasibility. Regulatory: the light-delivery configuration matters. FDA includes light-activated drugs labeled for a specific light-source device among combination-product types; peptide conjugation alone does not establish that status [32].
Peptide-guided macromolecular cargo	Targeting or cell-penetrating peptide plus protein, toxin, oligonucleotide, or degrader	Functional cargo in the intended compartment; cytosolic or nuclear exposure where required	Cargo integrity and function; endosomal escape and cytosolic/nuclear access where required; immune effects. Regulatory: no single assignment follows from this cargo grouping. Recombinant biological products are outside EMA’s synthetic-peptide guideline; the actual cargo and manufacturing process require separate assessment [29].
Adjacent but excluded systems	Peptide-decorated carrier with undefined conjugate state or co-administered penetration enhancer	Platform-specific	Cannot support a PDC claim without defined composition and causal delivery evidence. Regulatory: exclusion is a boundary of this review, not a finding that the platform is unregulated or lacks a valid development pathway.

This author-derived mapping distinguishes guidance scope, product terminology, and legal classification; it is not an agency decision for unapproved constructs. RCE measures are scientific tools, not regulatory acceptance criteria. The authorization statement concerns the peptide–small-molecule subgroup specified in Section 3.1; Section 8 considers boundary cases under broader definitions.

Figure 1 adapts Hoppenz et al.’s peptide–linker–drug architecture and uptake schemes [11] to distinguish payload-releasing conjugates, enzyme-potentiated prodrugs, and non-cleaving modalities, each with different activation and exposure requirements.

Figure 1. Operational taxonomy of peptide-guided payload systems and the modality-specific meaning of release competence. (A) Payload-releasing PDCs, shown in teal, are evaluated through active-payload or active-catabolite exposure in the tumor. (B) Peptide-activated prodrugs, shown in orange, depend on intracellular active-metabolite formation. (C) Peptide radioligands, shown in purple, are evaluated through tumor absorbed dose and microdistribution. (D) Adjacent peptide-guided systems, shown in gray, require platform-specific evidence. Downward arrows link each modality to its architecture and productive-exposure variable; colors distinguish categories and do not encode quantitative values. Adapted from Figures 1 and 2 of Hoppenz et al. [11] under the Creative Commons Attribution 4.0 International License. Changes: completely redrawn, combined, simplified, recolored, and expanded with modality-specific decision variables.

3.2. Release Competent Exposure

Release-competent exposure links conjugate integrity, active-compound formation, spatial tumor delivery, pharmacodynamic effect, and therapeutic margin. For payload-releasing PDCs, its central quantity is the time-integrated exposure of an identified active payload or catabolite in the intended anatomical and subcellular compartment, with payload-target engagement or linked downstream PD and unbound systemic and dose-limiting-organ exposure compatible with the intended therapeutic window. Evaluation requires a pharmacologically relevant analyte, the correct compartment, appropriate timing, and organ-specific safety assessment. RCE is an evidence profile, not a universal score.

For internalizing, payload-releasing PDCs, the proposed causal sequence is as follows: administration → intact-PDC exposure → tumor access → cellular internalization/trafficking → active-species formation → active-site exposure → payload-target engagement → downstream PD → efficacy.

These are distinct evidence levels. Arrows denote mechanistic hypotheses to test, not proof supplied by the preceding measurement. Bulk tumor accumulation does not identify active species or their compartment; formation does not establish sufficient active-site exposure; exposure does not establish target engagement, and downstream PD does not guarantee efficacy. Carrier–receptor binding supports access/uptake and is distinct from engagement of the payload’s pharmacological target. Extracellular activation may precede active-product entry into cells, bypassing carrier internalization [33]. Active species may also reach the tumor from circulation; chemical identity alone does not prove local formation. Toxicity follows a parallel systemic/normal-organ exposure → tissue engagement or other injury → toxicity branch, modified by tissue susceptibility and possible carrier, linker or additional-product effects. Table 3 locates each measure within this hierarchy.

Implementation differs by development stage. Preclinical discovery and qualification can use time-resolved sampling across animal cohorts with chemically selective assays to estimate tumor and organ exposure, alongside spatial analysis and validated compartment-specific methods to examine localization. Cohort-derived tissue AUC, spatial distribution and subcellular access are distinct observations; homogenate measurements alone do not establish intracellular active-site exposure.

In clinical development, active-site AUC is a mechanistic reference, not a routine measurement requirement: repeated spatially resolved or subcellular tumor sampling sufficient for individual AUC is generally impractical. Realistically obtainable evidence includes serial plasma PK of quantifiable PDC-related species [34,35,36] and clinical efficacy/safety data, supplemented in selected substudies by ethically justified timed biopsies or planned resections [37,38], mechanism-linked PD and imaging. A biopsy provides a local concentration at one time; plasma, imaging and PD observations do not directly replace active-compound concentration–time data in the intended tumor compartment. Model-derived tumor exposure should be labeled as predicted, with assumptions and uncertainty reported; where data cannot identify exposure, they are reported as unmeasured or not estimable. Section 12 details these clinical strategies and limitations.

Throughout this review, tumor uptake denotes delivery into tumor tissue, whereas accumulation denotes material or signal present or retained there; neither alone establishes cellular internalization or active-payload identity. Tumor exposure specifies an analyte, compartment and temporal measure, such as a concentration–time profile, AUC or peak concentration. Active-site exposure additionally requires localization of the active species in its relevant compartment. Radioactivity, total drug after cleavage, and fluorescence can obscure molecular identity. A single tumor concentration establishes presence at the sampling time, not AUC, exposure duration, or spatial coverage.

ADC experiments show that linker-dependent catabolites differ in permeability and potency, so greater total tumor-metabolite AUC need not imply greater activity [39,40]. In separate ADC models, released MMAE concentrations correlated with cytotoxicity and tumor-growth inhibition [41]. These findings support chemical identification and exposure–PD testing; PDC-specific uptake, release kinetics, sampling, and thresholds require separate justification (Section 7.4).

Table 3 separates six core quantitative measures for payload-releasing PDCs (Part A, numbered 1–6) from supporting and modality-specific approaches (Part B), which are excluded from that count. The core measures are independently classified by methodological provenance: Class 1, an established PK/PD parameter; Class 2, an adaptation of an established parameter to PDCs; or Class 3, a new conceptual metric proposed by the authors. Active-compound AUC is established; dose-normalized tumor gain, apparent unbound tumor/plasma exposure, tumor-to-organ exposure ratios, and temporal coverage are adaptations; the kinetic-separation index is conceptual. Classification concerns methodological provenance, not measurement validity or clinical prediction. The selection and interpretation of these quantities within RCE remain hypothesis-generating. Interpretable estimates require analyte-selective assays, defined compartments, explicit assumptions and, for exposure comparisons, appropriate dose normalization and matched AUC intervals. These conditions do not establish a validated endpoint or universal cutoff.

Active-site exposure uses the established AUC parameter, ${\mathit{AUC}}_{P^{*},T,k}\left(0,\tau \right)$, where P^* is the chemically identified active compound and k is the intended anatomical or subcellular tumor compartment. The adapted dose-normalized tumor gain compares ${\mathit{AUC}}_{P^{*},T,k}$ after the PDC with that after an equimolar free-payload comparator, following established relative-exposure comparisons [42]. For the adapted unbound exposure ratio [43], where unbound concentrations are measurable, $K_{p,\mathit{uu}}^{P^{*}}={\mathit{AUC}}_{u,\mathit{tumor}}^{P^{*}}/{\mathit{AUC}}_{u,\mathit{plasma}}^{P^{*}}$ describes the apparent unbound tumor-to-plasma exposure ratio. Local formation, trapping, and elimination can influence this ratio, so it is not necessarily an equilibrium partition coefficient.

The adapted tumor-to-organ exposure ratio for organ j is $M_{\mathit{tumor}/\mathit{organ},j}={\mathit{AUC}}_{P^{*},\mathit{tumor}}/{\mathit{AUC}}_{P^{*},\mathit{organ}\hspace{0.25em}j}$. An exposure ratio does not encode tissue susceptibility or establish therapeutic index [44]. Kidney, liver, marrow, peripheral nerve, skin, and gastrointestinal tissue are not interchangeable; each plausible dose-limiting organ should be reported separately where estimable, with unavailable human organ AUC recorded as not determined (Section 12). Temporal coverage adapts time-above-threshold PK/PD [45] as the fraction of the dosing interval during which tumor P^* exceeds a mechanism-matched concentration such as an experimentally justified EC_x. The author-proposed kinetic-separation index, K_sep = t_{1/2,plasma cleavage}/t_{1/2,target matrix release}, is an exploratory in vitro comparison, not a validated ranking rule or proof of in vivo selectivity. Its assumptions and restricted use are specified in Section 5.1.

These AUC ratios do not measure the fraction of payload released; distribution, binding, metabolism, and clearance prevent their interpretation as a molar mass balance. Spatial productive coverage and concordance between tumor P^*, payload-target engagement, and downstream PD remain necessary to determine whether bulk exposure reaches viable payload-susceptible cells, including target-negative neighboring cells when bystander killing is intended.

Table 3. Six core quantitative RCE measures (Part A) and supporting or modality-specific approaches (Part B): classification, definitions, and assumptions [26,27,28,41,42,43,44,45,46,47,48].

Measure	Definition or Comparison	Classification and Interpretation	Essential Controls and Limitations
Part A. Six core quantitative measures for payload-releasing PDCs
1. Active-site exposure ${\mathit{AUC}}_{P^{*},T,k}$	Chemically identified active compound in the intended tumor or subcellular compartment over a prespecified interval	Class 1—Established PK parameter. AUC quantifies exposure; “active-site” interpretation requires evidence for the sampled compartment. Chain position: active-site exposure.	Preclinical: specify active species, compartment, total/unbound basis and interval; capture early distribution and the active-compound tail. Clinical: routine spatial/subcellular tumor AUC is not proposed. A single biopsy cannot determine individual AUC; homogenates do not establish cytosolic/nuclear exposure. Plasma PK, imaging, and PD are supportive; model predictions are labelled (Section 12).
2. Dose-normalized tumor gain	Tumor P* AUC per molar dose for PDC divided by the corresponding value for free payload	Class 2—Adaptation to PDCs. Relative active-tumor exposure per molar payload dose; the name does not imply a new AUC operation. Chain position: comparative tumor active-species exposure.	Primarily nonclinical: match molecular species, molar payload dose, compartment, route, schedule, and interval. Dose normalization does not correct nonlinear uptake or release. Requires a suitable free-payload comparator and quantified denominator; an increased ratio need not improve response.
3. Apparent unbound tumor/plasma exposure $K_{p,\mathit{uu}}^{P^{*}}$	Unbound tumor P* AUC divided by unbound plasma P* AUC	Class 2—Adaptation to PDCs. Apparent unbound tumor/plasma exposure, not necessarily an equilibrium partition coefficient. Chain position: active-species exposure in tumor vs. plasma.	Match unbound species, compartment and AUC interval; report denominator uncertainty. Local formation, trapping and elimination confound a simple partition interpretation. Microdialysis requires recovery validation and samples extracellular, not intracellular, drug. Total tissue/plasma ratios are not Kp,uu.
4. Tumor-to-organ exposure ratio Mtumor/organ,j	Active-compound tumor AUC divided by active-compound AUC in a prespecified organ at risk	Class 2—Adaptation to PDCs. Tissue-distribution comparison; not a therapeutic index or safety margin. Chain position: tumor/normal-organ exposure comparison preceding toxicity interpretation.	Report each relevant organ and both absolute AUCs. Organ sensitivity, cellular localization and other toxic molecular species are not captured. Low or uncertain denominators can inflate ratios. Human organ AUC is often unavailable; toxicity monitoring cannot replace it. Unity is not a safety threshold.
5. Temporal coverage	Fraction of the dosing interval during which tumor P* exceeds a mechanism-matched ECx	Class 2—Adaptation to PDCs. Time-above-threshold analysis with a construct-specific biological hypothesis. Chain position: exposure duration linked to an assumed PD threshold.	Requires time-resolved P* and an ECx matched to species, compartment, total/unbound basis, pulse duration and schedule. A plasma paclitaxel threshold [45] does not validate a tumor PDC threshold. Persistent injury or delayed PD can invalidate simple threshold interpretation; a single sample cannot establish coverage.
6. Kinetic-separation index Ksep	Plasma chemical-cleavage half-life divided by the half-life of authentic active-product formation in a specified target-relevant matrix	Class 3—New conceptual metric proposed by the authors. An in vitro screening hypothesis built from familiar kinetic quantities, without a claim of mathematical novelty. Chain position: in vitro hypotheses about conjugate stability and active-product formation.	Exploratory comparison within a mechanistically comparable series under standardized, matrix-specific assays (Section 5.1). Exclude systemic elimination and parent loss alone. Report both profiles, half-lives, release yields, kinetic fits and uncertainty; use bounds or non-estimability when needed. Interpret absolute release time against productive residence. A larger ratio does not establish tumor access, normal-organ sparing or in vivo selectivity.
Part B. Supporting and modality-specific approaches (not included in the six core measures)
Spatial PD concordance	Colocalization of active compound with payload-target engagement or downstream damage/cell-death markers over time	Supportive evidence outside the six measures. Tests spatial exposure–PD concordance; no universal scalar is defined. Chain position: spatial association of exposure, payload-target engagement and downstream PD.	Spatial analyte and mechanism-linked PD observations provide supportive evidence; sparse biopsies and indirect markers do not establish active-site AUC
Radionuclide dosimetry	Tumor absorbed dose and dose rate relative to kidney, marrow, or other organ dose	Established absorbed-dose quantity outside the six measures; a modality-specific dosimetry approach. Chain position: absorbed energy preceding radiation response; chemical release is not required.	Requires patient-level dosimetry, radiochemical stability, and microdistribution assumptions

Report each quantity as observation-supported, model-predicted, unmeasured, or not estimable; below-quantification results are a separate category. Part B does not add to the six-measure count: spatial PD concordance supplies supporting evidence without a universal scalar definition, whereas radionuclide dosimetry is an established modality-specific quantitative approach. Section 12 discusses clinical alternatives; Section 10.1 provides a worked calculation. Section 7.4 specifies PDC-dependent sampling and interpretation. The six core measures do not cover every step; intact-PDC PK, uptake/trafficking, direct payload-target engagement and outcomes require complementary evidence.

Table 3. Six core quantitative RCE measures (Part A) and supporting or modality-specific approaches (Part B): classification, definitions, and assumptions [26,27,28,41,42,43,44,45,46,47,48].

Measure	Definition or Comparison	Classification and Interpretation	Essential Controls and Limitations
Part A. Six core quantitative measures for payload-releasing PDCs
1. Active-site exposure ${\mathit{AUC}}_{P^{*},T,k}$	Chemically identified active compound in the intended tumor or subcellular compartment over a prespecified interval	Class 1—Established PK parameter. AUC quantifies exposure; “active-site” interpretation requires evidence for the sampled compartment. Chain position: active-site exposure.	Preclinical: specify active species, compartment, total/unbound basis and interval; capture early distribution and the active-compound tail. Clinical: routine spatial/subcellular tumor AUC is not proposed. A single biopsy cannot determine individual AUC; homogenates do not establish cytosolic/nuclear exposure. Plasma PK, imaging, and PD are supportive; model predictions are labelled (Section 12).
2. Dose-normalized tumor gain	Tumor P* AUC per molar dose for PDC divided by the corresponding value for free payload	Class 2—Adaptation to PDCs. Relative active-tumor exposure per molar payload dose; the name does not imply a new AUC operation. Chain position: comparative tumor active-species exposure.	Primarily nonclinical: match molecular species, molar payload dose, compartment, route, schedule, and interval. Dose normalization does not correct nonlinear uptake or release. Requires a suitable free-payload comparator and quantified denominator; an increased ratio need not improve response.
3. Apparent unbound tumor/plasma exposure $K_{p,\mathit{uu}}^{P^{*}}$	Unbound tumor P* AUC divided by unbound plasma P* AUC	Class 2—Adaptation to PDCs. Apparent unbound tumor/plasma exposure, not necessarily an equilibrium partition coefficient. Chain position: active-species exposure in tumor vs. plasma.	Match unbound species, compartment and AUC interval; report denominator uncertainty. Local formation, trapping and elimination confound a simple partition interpretation. Microdialysis requires recovery validation and samples extracellular, not intracellular, drug. Total tissue/plasma ratios are not Kp,uu.
4. Tumor-to-organ exposure ratio Mtumor/organ,j	Active-compound tumor AUC divided by active-compound AUC in a prespecified organ at risk	Class 2—Adaptation to PDCs. Tissue-distribution comparison; not a therapeutic index or safety margin. Chain position: tumor/normal-organ exposure comparison preceding toxicity interpretation.	Report each relevant organ and both absolute AUCs. Organ sensitivity, cellular localization and other toxic molecular species are not captured. Low or uncertain denominators can inflate ratios. Human organ AUC is often unavailable; toxicity monitoring cannot replace it. Unity is not a safety threshold.
5. Temporal coverage	Fraction of the dosing interval during which tumor P* exceeds a mechanism-matched ECx	Class 2—Adaptation to PDCs. Time-above-threshold analysis with a construct-specific biological hypothesis. Chain position: exposure duration linked to an assumed PD threshold.	Requires time-resolved P* and an ECx matched to species, compartment, total/unbound basis, pulse duration and schedule. A plasma paclitaxel threshold [45] does not validate a tumor PDC threshold. Persistent injury or delayed PD can invalidate simple threshold interpretation; a single sample cannot establish coverage.
6. Kinetic-separation index Ksep	Plasma chemical-cleavage half-life divided by the half-life of authentic active-product formation in a specified target-relevant matrix	Class 3—New conceptual metric proposed by the authors. An in vitro screening hypothesis built from familiar kinetic quantities, without a claim of mathematical novelty. Chain position: in vitro hypotheses about conjugate stability and active-product formation.	Exploratory comparison within a mechanistically comparable series under standardized, matrix-specific assays (Section 5.1). Exclude systemic elimination and parent loss alone. Report both profiles, half-lives, release yields, kinetic fits and uncertainty; use bounds or non-estimability when needed. Interpret absolute release time against productive residence. A larger ratio does not establish tumor access, normal-organ sparing or in vivo selectivity.
Part B. Supporting and modality-specific approaches (not included in the six core measures)
Spatial PD concordance	Colocalization of active compound with payload-target engagement or downstream damage/cell-death markers over time	Supportive evidence outside the six measures. Tests spatial exposure–PD concordance; no universal scalar is defined. Chain position: spatial association of exposure, payload-target engagement and downstream PD.	Spatial analyte and mechanism-linked PD observations provide supportive evidence; sparse biopsies and indirect markers do not establish active-site AUC
Radionuclide dosimetry	Tumor absorbed dose and dose rate relative to kidney, marrow, or other organ dose	Established absorbed-dose quantity outside the six measures; a modality-specific dosimetry approach. Chain position: absorbed energy preceding radiation response; chemical release is not required.	Requires patient-level dosimetry, radiochemical stability, and microdistribution assumptions

Report each quantity as observation-supported, model-predicted, unmeasured, or not estimable; below-quantification results are a separate category. Part B does not add to the six-measure count: spatial PD concordance supplies supporting evidence without a universal scalar definition, whereas radionuclide dosimetry is an established modality-specific quantitative approach. Section 12 discusses clinical alternatives; Section 10.1 provides a worked calculation. Section 7.4 specifies PDC-dependent sampling and interpretation. The six core measures do not cover every step; intact-PDC PK, uptake/trafficking, direct payload-target engagement and outcomes require complementary evidence.

4. Peptide Carrier Selection

4.1. Addressability and Productive Trafficking

A useful carrier requires addressability and productive trafficking. Addressability is an accessible tumor-to-normal differential under the intended route and schedule; productive trafficking delivers or retains the final conjugate, including its linker and payload, in the pharmacological compartment. Peptide-receptor mapping established receptor-rich tumors as targets while revealing substantial intertumoral and normal-tissue variation [49]. High affinity can therefore fail through perivascular trapping, unproductive recycling, kidney accumulation, or loss of binding after conjugation.

In vivo phage display established accessible vascular and tumor molecular addresses and enabled tumor-homing motif discovery [50,51]. Examples include RGD, NGR-like vascular ligands, somatostatin analogues, gonadotropin-releasing hormone and melanocortin ligands, and library-derived peptides. RGD uptake can involve endothelium, tumor cells, or both [52]; LyP-1 linked selective tumor and lymphatic homing to internalization and growth inhibition [53]. Functional target selection should assess accessible density, internalization or retention, tumor distribution, shedding, normal-organ access, and deployable biomarkers.

Proteolytic exposure of a C-terminal CendR motif can engage neuropilin-1 and increase tissue penetration [54,55,56], potentially countering perivascular capture. Activity depends on the initiating receptor, proteases, perfusion, and neuropilin distribution; penetration must therefore be tested spatially for the final conjugate rather than inferred from the unconjugated peptide.

Cell-penetrating peptides can improve internalization of polar or macromolecular cargo but often lack tumor specificity and remain endosomally trapped [57,58,59]. TAT cargo was shown to enter through macropinocytosis, with a fusogenic element needed to enhance escape; cyclic and topologically constrained cationic peptides demonstrated that early-endosomal escape and cytosolic delivery can diverge from bulk uptake [60,61,62,63], as did an endosomolytic delivery study [64]. These uptake or escape modules need separate targeting or conditional activation. Cationic charge can increase nonspecific tissue and proximal-tubule interactions. Total fluorescence cannot resolve membrane binding, endosomal sequestration, degradation, and cytosolic delivery.

Dimeric RGD–paclitaxel achieved tumor uptake, while αvβ6-directed doxorubicin or paclitaxel constructs showed receptor-dependent internalization and cytotoxicity [65,66,67]. Nevertheless, αvβ6–paclitaxel produced efficacy similar to that of free paclitaxel in its xenograft experiment, showing that receptor selectivity need not improve therapeutic index.

4.2. Constrained Carriers and Kinetic Optimization

Cyclization, stapling, backbone modification, D-amino acids, N-methylation, and bicyclic architectures can improve protease resistance and binding while providing defined attachment sites [68,69,70], but may alter receptor conformation, internalization, solubility, or retention. Hydrophobic modification can prolong circulating peptide residence through albumin association while increasing aggregation or hepatic uptake. Protease resistance, permeability, filtration, and exposure extension remain distinct ADME problems [71]. Reversible transthyretin binding and a high-affinity albumin-binding peptide tag substantially extended half-life in experimental systems but also changed the distribution problem [72,73]. Each modification requires evaluation in the final conjugate.

Affinity must be balanced with transport: slow dissociation improves retention but can strengthen perivascular binding barriers; multivalency increases avidity and target-mediated disposition while reducing diffusion. Carrier panels should assess equilibrium affinity, binding kinetics, receptor competition, internalization and recycling, post-conjugation stability, penetration from vessels, tumor-to-organ ratios, renal handling, solubility, and synthetic scalability.

Figure 2 adapts Hoppenz et al.’s sequence of receptor binding, internalization, endolysosomal processing, and payload release [11], adding renal loss, perivascular retention, recycling, and lysosomal trapping as competing branches with decisive readouts. This compartmental itinerary distinguishes productive delivery from uptake followed by loss or sequestration. Table 4 summarizes carrier classes, principal failure modes, and candidate evidence.

Figure 2. Carrier-dependent itinerary from vascular access to the pharmacological compartment. Adapted from Figure 2 of Hoppenz et al. [11] under the Creative Commons Attribution 4.0 International License, redrawn, simplified, recolored, and relabeled; competing loss pathways and decision readouts have been added. The depicted route represents receptor-mediated internalization; extracellular activation may bypass carrier internalization. Receptor binding denotes carrier–receptor interaction, while active-species identification, payload-target engagement and downstream PD require distinct evidence (Section 3.2). Horizontal arrows indicate the proposed sequence of delivery steps; downward arrows identify competing loss or retention pathways and productive intracellular delivery. Colored upper boxes distinguish stages, while red, orange, purple, and green branch arrows denote proteolysis or renal filtration, perivascular retention, recycling or lysosomal trapping, and productive cytosolic or nuclear delivery, respectively. The numbered dark-blue boxes identify the corresponding measurement readouts; color and arrow size are not quantitative.

Table 4. Carrier classes and candidate translational evidence [8,56,71].

Carrier Class	Best-Fit Use	Principal Failure Mode	Evidence to Prioritize by Stage
Receptor-targeting tumor-homing peptide	Accessible tumor-cell, stromal, or vascular target with a meaningful normal-tissue differential	Off-tumor binding, shedding, recycling, or perivascular capture	Human tumor and normal-tissue distribution; post-conjugation binding kinetics; competition; internalization or retention; analyte-resolved tumor-to-organ exposure
CendR or other tumor-penetrating peptide	Perfusion-accessible tumors in which distribution beyond vessels is limiting	Variable protease activation, neuropilin dependence, and off-context penetration	Protease dependence; neuropilin perturbation; distance-from-vessel analysis; 3D model penetration; biodistribution of final conjugate
Cell-penetrating peptide	Conditional uptake or escape enhancement for polar and macromolecular cargo	Nonspecific uptake, endosomal trapping, and cationic renal retention	Surface vs. internalized fraction; cytosolic-delivery assay; endosomal escape; target-negative controls; analyte-resolved kidney and liver exposure
Cyclic, stapled, or bicyclic peptide	High-affinity recognition requiring chemical definition and rapid distribution	Conjugation-site perturbation, target-mediated clearance, or renal uptake and retention	Post-conjugation affinity and off-rate; plasma stability; penetration–retention balance; solubility; aggregation; scalable synthesis
Half-life-extended peptide	Carrier whose useful intact exposure is shorter than the tumor-access interval	Increased systemic release, lower penetration, or albumin-mediated organ redistribution	Intact-PDC half-life; free-payload exposure; albumin affinity; analyte-matched tumor-to-plasma and tumor-to-organ ratios (state concentration or AUC basis); schedule simulation

5. Linker and Payload Co Design

5.1. Linker Kinetics as a Therapeutic Index Switch

Systemic deconjugation must be slow enough for intact-PDC delivery, while local release or degradation must match carrier residence and payload action. Rapidly cleared constructs require delivery within the available vascular exposure window; subsequent activation depends on productive tumor retention and trafficking rather than plasma half-life alone. A single percentage-intact measurement can conceal biphasic degradation, labile subpopulations, and biological-species differences in cleavage.

For the kinetic-separation index, potentially informative comparisons are limited to a mechanistically comparable series with the same carrier–payload scaffold and matrix pair. Plasma bond cleavage and target-matrix formation of the active product may involve different enzymes or sequential cleavage, self-immolation, and catabolism; they do not measure one common reaction. ADC assays demonstrate enzyme-dependent differences in relative linker cleavage [74]; this informs assay design without validating PDC kinetics. Standardize each assay across constructs, documenting species, matrix source/fraction, substrate concentration, temperature, pH and trigger activity; differences between the paired matrices must be biologically justified. Verify product identity, recovery, stability and release yield. A single half-life requires sufficient sampling and an adequate kinetic model; incomplete, multiphasic or censored profiles may require reporting full curves and justified bounds or non-estimability, not an infinite ratio.

A value above unity means only that assayed plasma cleavage is slower than target-matrix active-product formation. It may help prioritize related constructs for further testing, with both absolute half-lives and uncertainty reported; it is not a universal ranking rule. Even a large ratio may accompany release too slow for productive tumor residence. The ratio does not establish delivery, cellular access, or normal-organ sparing. In vivo selectivity therefore requires construct-specific validation of intact-conjugate and active-product time courses in plasma, tumor and relevant normal tissues, with controls or validated modeling that distinguish local formation from redistribution of circulating payload. Until then, the index remains a Class 3 hypothesis, not an in vivo selectivity measure.

Cleavable linkers support near-parent payload release, permeability, or bystander killing—the death of neighboring target-negative cells exposed to released payload. Protease-cleavable linkers often use self-immolative spacers; PABC enabled rapid unmodified-doxorubicin release after dipeptide cleavage in model-linker and immunoconjugate studies [75]. Protease sensitivity is not tumor specificity; lysosomal enzymes occur in normal tissues, and circulating or extracellular enzymes vary across biological species and disease states [19,21]. In ADC studies, cathepsin-B deletion did not abolish nominally cathepsin-B-cleavable conjugate processing, indicating compensatory routes; added glutamate improved mouse-plasma Val–Cit stability while preserving activation [74,76]. Selectivity may depend on receptor-mediated uptake. We propose testing these candidate mechanisms in PDCs using human plasma and whole blood, relevant organ and lysosomal preparations, and efficacy and toxicology species.

Acid-labile and redox-sensitive linkers exploit pH or thiol gradients but face contextual limits; endolysosomal pH varies across cell states, extracellular tumor acidity is heterogeneous, disulfides can exchange with circulating thiols, and glutathione abundance does not establish cytosolic access. Bioorthogonal release reduces dependence on endogenous triggers but requires coordinated distribution, reaction kinetics, timing, and safety of two agents.

Non-cleavable linkers require carrier processing when activity depends on liberated products. ADC maytansinoid studies demonstrate linker-dependent catabolites with different permeability and extracellular potency [39,40]. We extrapolate the need to identify and characterize PDC-derived products, not antibody lysosomal processing or specific catabolite structures. Reduced bystander killing, improved systemic safety, and poorer coverage of heterogeneous tumors remain construct-specific PDC hypotheses. Processing site, product identity, potency, permeability, target access, and toxicity therefore require direct evaluation.

Conjugation chemistry and payload loading can change disposition before release. In ADC studies, hydrophobic loading accelerated clearance, hydrophilic design improved exposure, and higher loading did not proportionally improve in vivo efficacy or tolerability [77,78]. We propose transferring final-conjugate physicochemical characterization, not ADC clearance effects, loading optima, or antibody catabolism. Direct PDC evidence shows lower solubility of BT8009 than its precursor peptide [79]; this does not establish a PDC loading–clearance rule. PDC binding, filtration, proteolysis, and tissue distribution require direct measurement. ADC reviews identify conjugation site, payload and release chemistry as coupled variables [80,81]; their quantitative effects require PDC confirmation.

Figure 3 adapts Hoppenz et al.’s linker mechanisms and Vrettos et al.’s protease-triggered PABC self-immolation cascade [11,82], to compare five release mechanisms and their required analytical observations. The released molecular species, rather than the nominal trigger, is central to linker evaluation.

5.2. Identity of Released Payloads and Active Catabolites

Our proposed PDC release-characterization strategy applies established structural-analysis methods; it does not assert that ADC catabolites occur in PDCs. Released products and relevant intermediates are identified using LC–MS/MS or high-resolution MS/MS, authentic standards where available, and orthogonal characterization. Matrices are selected and sampled by activation route and development stage, distinguishing formation during sample processing from formation in vivo (Section 3.2 and Section 12).

We further propose characterizing each active PDC-derived species for potency, permeability, target access, protein binding, metabolic stability, and transporter interactions. The ADC literature describes extracellular-release and bystander mechanisms [83]; these are candidate mechanisms for PDC testing. Direct BT8009 experiments have measured intact conjugate and released MMAE [33], but these did not quantify the relative contributions of extracellular and intracellular release in vivo.

Figure 3. Author-proposed linker activation routes and analytical tests. Lysosome dependence requires construct-specific confirmation and is not assumed for all non-cleavable PDCs. Adapted from Figure 5 of Hoppenz et al. [11] and Figure 5 of Vrettos et al. [82] under the Creative Commons Attribution 4.0 International License, redrawn, combined, simplified, recolored, and expanded to include contextual kinetics and active-compound verification. Rows are color-coded by linker logic: teal, non-cleavable; green, protease-sensitive; orange, acid-labile; purple, redox-sensitive; and red, bioorthogonal. Arrows connect each linker class with its proposed activation event, the molecular species to identify, and the required evidence; they do not indicate measured rates.

Figure 3. Author-proposed linker activation routes and analytical tests. Lysosome dependence requires construct-specific confirmation and is not assumed for all non-cleavable PDCs. Adapted from Figure 5 of Hoppenz et al. [11] and Figure 5 of Vrettos et al. [82] under the Creative Commons Attribution 4.0 International License, redrawn, combined, simplified, recolored, and expanded to include contextual kinetics and active-compound verification. Rows are color-coded by linker logic: teal, non-cleavable; green, protease-sensitive; orange, acid-labile; purple, redox-sensitive; and red, bioorthogonal. Arrows connect each linker class with its proposed activation event, the molecular species to identify, and the required evidence; they do not indicate measured rates.

5.3. Payload Selection

Payload selection must match potency to the amount delivered while ensuring target access, sustained activity, activity against resistant tumors, and compatibility with conjugation and manufacturing processes. The desired bystander-killing profile should be specified before selection.

ADC experiments link payload permeability to bystander killing [41,84,85]; this mechanism is also discussed in the review by Giugliano et al. [86]. The resulting spatial coverage and normal-tissue risk require direct PDC evaluation. Maytansinoid analogue chemistry illustrates how side chains and disulfides connect conjugation with activation [87]. Microtubule-targeting payloads require control of toxicity, including neurotoxicity, whereas camptothecins act through TOP1–DNA cleavage complexes [88,89]. A HER2-targeted ADC carrying a topoisomerase I inhibitor provides a preclinical example of this payload class [90]. PBD dimers are potent, but their therapeutic application is particularly sensitive to toxicity [91]. Toxicity can originate from the carrier, linker, payload, or other released products [92].

Radionuclide selection must match half-life, emission, chelator stability, receptor retention, tumor size, heterogeneity, and organ dose. Beta emitters provide longer-range cross-fire; alpha emitters deliver high linear-energy-transfer radiation over shorter ranges. Photosensitizers require light delivery and management of oxygen limitation. Protein toxins, oligonucleotides, and degraders require intact cargo delivery and escape into the cytosol or nucleus. Table 5 summarizes the proposed linker and payload co-design considerations.

Table 5. Author-proposed linker and payload co-design matrix. ADC precedents inform chemical-payload rows; validation is construct- and modality-specific [33,39,75,76,83].

Release or Payload Class	Intended Effect	Required Active-Compound Property	Main Translational Risk	Decisive Assay
Protease-cleavable linker with permeable tubulin inhibitor	Protease-dependent release with intended bystander killing	Productive extracellular or intracellular release; membrane permeability; preserved tubulin activity	Plasma or normal-tissue cleavage, neuropathy, neutropenia, and unintended killing of neighboring normal cells	Analyte-selective plasma and target-matrix PK plus mixed target-positive and target-negative assay
Non-cleavable linker with tubulin inhibitor	Potentially target-cell-confined cytotoxicity	Active catabolite after carrier processing; identity, charge and compartment require PDC confirmation	Limited bystander coverage if the active compound is poorly permeable	Authentic-catabolite potency and permeability with processing-pathway test
Cleavable topoisomerase or DNA-active payload	Potent killing with nuclear target access	Chemically defined, sufficiently permeable active compound with nuclear exposure	Narrow systemic safety margin and marrow or gastrointestinal toxicity	Active-compound plasma and tumor PK linked to DNA-damage PD and organ exposure
Acid- or redox-responsive linker	Conditional release in acidic or reducing compartment	Trigger rate faster at target site than in circulation	Heterogeneous trigger abundance and nonspecific hydrolysis or thiol exchange	Full pH or thiol kinetic panel in human matrices and intact cells
Peptide radioligand	Receptor-mediated radiation delivery	Radiochemical stability and target residence matched to isotope	Kidney, marrow, salivary, or other organ dose	Patient-level quantitative imaging and dosimetry
Photosensitizer or macromolecular payload	Local physical activation or functional cytosolic delivery	Light and oxygen access, or intact cargo escape to its target compartment	Procedure limits, hypoxia, endosomal trapping, and immune effects	Spatial activation or cytosolic functional assay rather than total uptake

6. Tumor and Intracellular Delivery Barriers

Small size does not overcome abnormal perfusion, interstitial pressure, dense matrix, stroma, hypoxia, necrosis, or heterogeneous receptors [93,94]. Short-circulating peptides cannot reach regions unperfused during exposure; strong perivascular binding can increase uptake without deeper delivery. Imaging mass cytometry, autoradiography, fluorescence with validated label stability, mass spectrometry imaging, ex vivo microscopy, and spatial PD should establish whether intact conjugate and active molecular species reach the same tumor regions. Three-dimensional spheroids and organoids support ranking but lack blood flow, filtration, and organ-specific processes; their results should be linked to in vivo exposure rather than substitute for it.

Trafficking must match activation and payload action: lysosomal-protease linkers require degradative-compartment access; extracellular activation does not. Non-cleavable conjugates may require carrier proteolysis into active payload–linker–amino-acid or peptide-bearing catabolites, whose identity and activity must be demonstrated. Disulfides require reducing-thiol access, potentially extracellularly; DNA-directed payloads require nuclear access. Uptake studies should resolve surface binding, internalization, recycling, degradation, and active-compound delivery to the pharmacological compartment [59,95].

Patient selection must account for TME differences in perfusion, stroma, macrophage uptake, extracellular proteases, hypoxia, and vascular permeability despite similar receptor expression. Receptor assessment should be matched to appropriate IHC methods; receptor imaging can help quantify the accessible target burden. Biomarkers should address the relevant uncertainty, using spatial pathology for heterogeneity and receptor imaging for accessibility.

SORT1-targeted TH1902 showed receptor-mediated uptake, apoptosis, xenograft activity, and tolerability in SORT1-positive breast cancer models, with subsequent studies extending investigation to ovarian and triple-negative breast cancer stem-like models [96,97]. Later work associated TH1902 treatment with cGAS–STING signaling and enhanced anti-PD-L1 activity; interpretation must use the corrected figures [98]. These findings support mechanisms and combination hypotheses, but they cannot replace analyte-resolved clinical evidence.

Figure 4 adapts Sheng et al.’s vascular, matrix, interstitial, and cellular barriers [99] to brief intact-PDC circulation, adding competing kidney and liver uptake and premature systemic release. These nanomedicine findings provide mechanistic analogies for evaluating spatial delivery in viable tumor regions.

Figure 4. Sequential barriers and competing organ uptake that separate administered PDC dose from active-compound exposure in viable tumor cells. Adapted from Figure 1 of Sheng et al. [99] under the Creative Commons Attribution 4.0 International License, redrawn, simplified, recolored, and relabeled for peptide-conjugate kinetics, spatial pharmacology, and safety-organ exposure. This illustrates an intracellular-delivery route: “target binding and uptake” denotes carrier-mediated entry, not payload-target engagement. Extracellular-release mechanisms need not follow this route (Section 3.2). Horizontal arrows indicate sequential delivery barriers; colored downward arrows identify competing loss pathways: red, plasma deconjugation; orange, renal filtration and reabsorption; purple, hepatic uptake or metabolism; and gray, recycling, trapping, or inactive-catabolite formation. Colors of the upper boxes distinguish stages, with the dark-blue final box denoting intracellular active species; colors and arrow dimensions are schematic.

7. Multi Analyte Pharmacokinetics and Pharmacodynamics

7.1. Analyte Definition and Assay Architecture

For payload-releasing cytotoxic PDCs and peptide-activated prodrugs, we propose a mechanism-dependent analyte plan from the first in vivo study: intact conjugate or parent, total conjugated payload where meaningful, released payload, relevant active metabolites or catabolites, and dominant inactive or toxic degradants. ADC methodological literature motivates analyte separation and warns that biotransformation may complicate quantification [46,47,48]; the expanded PDC panel proposed here requires construct-specific justification. Intact-acting constructs and other modalities require a plan matched to their active entity (Section 10.2). Nonclinical tumor and safety-organ sampling should support AUC estimation or a justified model. Human tissue sampling belongs in justified substudies; serial plasma PK, imaging, and PD can support translation (Section 12). Total drug or radioactivity supports mass balance, not release competence. Sampling must capture early intact-PDC distribution and persistent active compounds, using common AUC intervals, stated integration and extrapolation methods, and adequate analyte-specific sensitivity.

Bioanalysis must control ex vivo deconjugation, adsorption, degradation, and poor tissue recovery through chemistry-appropriate stabilization, temperature control, matrix-specific recovery, incurred-sample reanalysis, and identity confirmation. Direct PDC clinical bioanalysis separates intact BT5528 from unconjugated MMAE [34] and quantifies intact zelenectide pevedotin [36]. The BT1718 assay instead measures total DM1-related material after reduction and derivatization; it does not selectively quantify DM1 released in vivo [35]. FDA bioanalytical and ADC clinical-pharmacology guidance supplies validation and analyte-selection principles requiring PDC-specific justification [30,100].

Joint intact-PDC, payload, and active-catabolite curves do not identify release location; tissue-generated payload can return to plasma. Estimating uptake, local formation, and elimination requires identifiable models supported by independent tissue, metabolism, or clearance observations. Unsupported processes should remain unestimated or sensitivity scenarios. ADC structures and Fc-dependent parameters cannot be imported unchanged (Section 7.4).

7.2. Renal and Hepatic Distribution and Clearance

Renal filtration can limit intact-PDC exposure, whereas proximal-tubule reabsorption and retention may increase kidney toxicity. Charge, hydrophobicity, protein binding, reabsorption, metabolic stability, and linker chemistry influence both processes. Figure 5 is our conceptual PDC adaptation of analyte separation in Glassman’s ADC modeling review [101], extending it to plasma, tumor, kidney, liver and other safety organs. Its validity and analytes remain construct-specific.

Figure 5. Multi-analyte disposition of a peptide–drug conjugate with tumor active-compound formation and renal or hepatic uptake and clearance. Conceptually adapted from Figure 1 of Glassman [101] under the Creative Commons Attribution 4.0 International License. The ADC framework has been completely redrawn, simplified, recolored, relabeled in PDC terms, and expanded to include tumor and safety-organ matrices. The transferred element is analyte separation; antibody FcRn recycling, Fc-receptor uptake, clearance constants, and compartmental exchange rates are not assigned to PDCs (Section 7.4). Blue, green, orange, and purple boxes distinguish plasma, tumor, kidney, and liver or other organs, respectively. Colored arrows from plasma indicate delivery to the corresponding tissues; gray double-headed arrows denote comparisons of tissue exposure, not direct organ-to-organ transport. Arrows within boxes indicate local processing. The lower boxes identify the exposure measures; no arrow specifies a measured transfer rate.

For PDCs, extending intact-conjugate circulation may increase delivery to tumors but also prolong opportunities for systemic release. Albumin binding, lipidation, PEG-like polymers, and multivalency alter filtration, target access, binding, and distribution [102,103]. Reversible protein binding extended peptide half-life up to more than 20-fold in selected peptide-only systems, but unbound fraction and distribution require measurement [72,73]. Conjugation also changed clearance and hepatic distribution of structurally distinct anti-STEAP1 ADCs [104]. Decisions therefore require intact-conjugate and tumor active-compound exposure, absolute safety-organ exposure, schedule, and exposure–response; half-life or an unchanged distribution ratio alone is insufficient.

Direct PDC evidence from BT5528 and BT8009 preclinical studies shows brief intact-conjugate plasma persistence alongside sustained intratumoral MMAE concentrations [33,79,105]. First-in-human BT5528 showed intact-conjugate and released-MMAE plasma half-lives of approximately 0.4–0.7 h and 35–47 h, respectively, an objective response rate of 6.7%, and grade ≥ 3 neutropenia in 22.2% [106]. This kinetic divergence requires separate analyte measurement.

7.3. Exposure Response and Candidate Evidence

Exposure–response analysis should test mechanism-relevant AUC, peak concentration, or duration above a justified threshold; pulses and continuous exposure may differ, and PD damage may outlast exposure. Relate tumor active-compound exposure to payload-target engagement, downstream PD and efficacy, and systemic and organ exposure to toxicity. Radioligands require absorbed dose and dose rate. Construct-specific population PK can guide dosing without importing ADC thresholds. Section 10.2 proposes development questions and candidate evidence; Table 3 classifications do not supply mandatory thresholds or a requirement to obtain every measure.

7.4. Transferability of ADC Principles to PDC Metric Design

For two Nectin-4-directed MMAE conjugates, zelenectide pevedotin plasma half-life was 0.42–0.91 h across dose groups, vs. a population-PK estimate of 3.6 days for the enfortumab vedotin ADC analyte [107,108]. This approximately two-order difference is neither a class constant nor a controlled comparison; assays, doses, populations, and methods differ. Unconjugated-MMAE half-lives were approximately 37–50 h and 2.6 days, respectively. Short intact-conjugate circulation therefore does not justify truncating systemic payload AUC.

Disrupting FcRn binding with H435A altered ADC clearance and efficacy and tolerability despite similar in vitro cytotoxicity [109]. Fc-free peptides lack IgG salvage, although protein binding or size extension can modify disposition [72,73]. ADC elimination includes target-mediated and nontarget cellular uptake and catabolism opposed by FcRn salvage, not exclusively reticuloendothelial clearance. For proposed rapid renal elimination of small Bicycle conjugates [33,105], the cited imaging used a radiolabeled peptide analogue, not intact-PDC mass balance. Elimination of each final construct and released molecular species therefore requires evidence; neither universal renal PDC nor exclusively hepatic ADC clearance is justified [104].

Reduced kidney uptake in megalin-deficient mice demonstrated tubular reabsorption of radiolabeled peptides [110], supporting pathway testing in susceptible PDCs. It establishes neither universal megalin dependence nor active-payload AUC from retained radioactivity. Table 6 links these distinctions to Table 3; its sampling and modeling recommendations are author-proposed, not validated cross-platform thresholds.

Table 6. Evidence provenance and transferability of ADC pharmacology to PDCs: observations, limits and author-proposed RCE implications.

Domain	ADC Evidence or Precedent	PDC Evidence and Transfer Limits	Authors’ Proposed RCE Implications
Circulation and FcRn	FcRn-binding perturbation changes ADC disposition [109]. Clinical enfortumab conjugate and MMAE half-lives differ [108].	Zelenectide and MMAE had distinct clinical half-lives [107]. Fc-free peptides lack IgG salvage; peptide-only extension studies [72,73] do not establish final-PDC kinetics.	Resolve early intact PDC and later active compound separately. Retain an adequate active-compound AUC interval; do not substitute carrier half-life for temporal coverage.
Elimination and organ accumulation	Cellular uptake/catabolism, target binding, Fc-related processes, and conjugation-dependent hepatic uptake contribute [104,109].	Final-PDC elimination routes require verification. Radiolabeled-peptide megalin data [110] support a renal hypothesis; label retention is not active-payload AUC, and universal renal clearance is not established.	Measure kidney and other relevant organ analytes where feasible. Select organs from disposition and toxicity; report absolute exposures alongside each tumor-to-organ ratio.
Linker activation and active compound	Linker chemistry changes maytansinoid catabolites; lysosomal processing was demonstrated in the tested ADCs [39,40,76].	BT8009 studies measured intact conjugate and MMAE in plasma/tumor [33]; the release compartment was not isolated. ADC processing routes, catabolites and release rates require PDC confirmation.	Confirm authentic active compound and the productive compartment. Interpret chemical cleavage/release rates against measured target residence; plasma elimination is not the numerator of Ksep.
Loading and spatial delivery	Loading and hydrophobicity changed ADC disposition and efficacy [77,78]; payload permeability affected bystander killing [41,85].	BT8009 conjugation reduced solubility vs. its precursor peptide [79]. This supports characterization, not ADC loading optima. BT8009/EV-analogue comparisons remain confounded by architecture and dosing [33].	Normalize by molar payload and retain dose/schedule context. Do not extrapolate through saturation. Test spatial active-compound/PD coverage and intended bystander activity directly.
Bioanalysis and exposure–response	ADC methodological literature supports analyte separation [46,47,48]; ADC experiments and clinical analyses examine relationships between released-payload concentrations or exposure and effects [41,108].	PDC plasma assays resolve BT5528/MMAE and intact zelenectide [34,36]; clinical BT5528 confirms kinetic divergence [106]. These do not establish tumor active-site AUC, equilibrium partition, or transferable effect thresholds.	For unbound ratios, report both AUCs and denominator uncertainty; local formation is not passive partitioning. Select AUC, peak, or duration measures by payload mechanism and distinguish PD persistence from drug persistence.

ADC observations and PDC observations apply to the cited constructs and settings. The final column gives our proposed measurement implications, not cross-platform validation; transferred principles should be confirmed in the final PDC. Human AUC remains classified as observed, model-predicted, or not-estimable (Section 12), without added tissue-sampling requirements. FcRn, neonatal Fc receptor; EV, enfortumab vedotin.

8. Clinical Evidence and Mechanistic Reanalysis

Within the receptor-/cell-surface-targeted peptide–small-molecule cytotoxic subgroup defined in Section 3.1, the targeted narrative search identified no product with FDA approval or EU marketing authorization in force on 15 September 2026. This conclusion depends on the stated definition and search scope; it is not a claim that all products described as PDCs lack authorization.

Broader definitions include melflufen (Pepaxti), an EU-authorized peptide-conjugated alkylator with passive cellular entry and enzyme-potentiated activity rather than a receptor-binding carrier; its prior US approval (Pepaxto) was withdrawn [25,111]. Lutathera (lutetium-177 oxodotreotide) and the PSMA-directed small-molecule radioligand Pluvicto (lutetium-177 vipivotide tetraxetan) are EU-authorized, but their cytotoxic effect is mediated by radiation [112,113].

Broad polypeptide–toxin definitions would also encompass denileukin diftitox-cxdl (Lymphir; FDA-approved), an IL-2–diphtheria-toxin fusion, and tagraxofusp (Elzonris; EU-authorized), an IL-3–diphtheria-toxin fusion [114,115]. These receptor-directed cytotoxins are recombinant protein fusions, not peptide–small-molecule conjugates. ADCs remain antibody-carrier systems irrespective of peptide structures in their linkers or payloads [30]. These boundary cases are considered explicitly without treating their authorization as validation of the narrower cytotoxic PDC subgroup.

8.1. Zelenectide Pevedotin

Zelenectide pevedotin (BT8009, not PEN-221) couples a Nectin-4-targeting bicyclic peptide to MMAE. Duravelo-1 monotherapy dose escalation treated 49 patients with advanced solid tumors; among 42 efficacy-evaluable patients across tumor types, ORR was 24% (10/42; 95% CI 12–40), with median DOR of 11.1 months among 10 confirmed responders. In urothelial carcinoma patients, ORR was 38% (8/21; 95% CI 18–62), CBR 57% (12/21), median DOR 13.1 months among eight confirmed responders, and median PFS 7.4 months (95% CI 1.8–12.4). Grade ≥ 3 TRAEs occurred in 39%, including neutropenia in 16%; higher-schedule DLTs included grade 3 asthenia, fatigue, appetite loss, febrile neutropenia, and grade 4 sepsis [107].

Preliminary 2026 conference abstracts, rather than full peer-reviewed clinical reports, described confirmed ORRs of 32% (17/53) with monotherapy in pretreated enfortumab-vedotin-naive metastatic urothelial cancer patients and 50% (11/22) with pembrolizumab in first-line cisplatin-ineligible disease [116,117]. For zelenectide plus pembrolizumab in Duravelo-2, a further preliminary conference abstract reported blinded-independent-review ORRs of 55% (16/29; 95% CI 36–74) at 5 mg/m² weekly and 58% (15/26; 95% CI 37–77) at 6 mg/m² on days 1 and 8 every 21 days; the latter schedule was selected [118]. Survival remained immature without a standard-of-care efficacy comparison. In a non-peer-reviewed March 2026 corporate update, the sponsor reported that the prior phase II/III design was not an acceptable approval pathway and deprioritized internal development [119]. This was a sponsor account of development status, not a published agency decision or peer-reviewed efficacy assessment.

8.2. PEN 221

PEN-221 (nendratareotide uzatansine) is an SSTR2-targeted somatostatin-analogue–DM1 conjugate. A phase IIa conference abstract in GI midgut NET reported CBR of 88.5% (23/26; 95% CI 70–98), entirely stable disease, with ORR 0% and median PFS 9.0 months (95% CI 5.0–16.5) [120]. Posted registry results for pancreatic-NET and small-cell lung-cancer cohorts also showed ORR 0%, with median PFS 3.2 months (95% CI 2.0–5.9) and 1.4 months (95% CI 1.2–1.8), respectively [121]. Table 7 details phase I dose-limiting hepatotoxicity at 25 mg reported in a conference abstract [122]. Disease stabilization must remain distinct from tumor response.

SSTR2-binding maytansinoid optimization achieved selective uptake, tumor DM1 delivery, and regressions; subsequent small-cell lung-cancer experiments showed receptor-dependent internalization and complete regressions in selected xenografts [123,124]. The clinical ORR of 0% therefore exposes a translation gap despite preclinical delivery and activity.

8.3. Zoptarelin Doxorubicin

Zoptarelin doxorubicin (AEZS-108) tests whether receptor targeting improves therapeutic margin. A single-arm endometrial-cancer study reported ORR 23% and median OS 15 months [125]. In 511 patients, the randomized ZoptEC trial, reported in a conference abstract and registry results, did not demonstrate superiority over doxorubicin in OS (HR 1.06; 95% CI 0.87–1.30; p = 0.544) or PFS (HR 0.89; 95% CI 0.71–1.11; p = 0.309) [126,127]. Table 7 reports median survival, response, neutropenia, and serious-adverse-event results. Receptor expression and early activity did not establish improved therapeutic index.

In platinum-refractory/resistant ovarian cancer, phase II AEZS-108 yielded partial response in 14.3% and stable disease in 38% despite frequent LHRH-receptor positivity [128]. In castration- and taxane-resistant prostate cancer, phase I demonstrated circulating-tumor-cell internalization and a 210 mg/m² maximum tolerated dose; phase II reported one confirmed partial response among 25 treated patients [129,130]. These findings did not predict randomized superiority.

8.4. Paclitaxel Trevatide

Paclitaxel trevatide (ANG1005) links three paclitaxel molecules to Angiopep-2 for LRP1-mediated blood–brain barrier transport. In recurrent breast-cancer brain metastases, phase II response and safety results, including investigator vs. independent response assessment, are detailed in Table 7; the independent-response criterion was not met. In leptomeningeal disease, independent intracranial disease control was 79% (19/24 response-evaluable patients), and median OS in the full 28-patient subgroup was 8.0 months (95% CI 5.4–9.4), without a control arm [131]. A subsequent high-grade-glioma study failed arm-specific response or PFS endpoints; overall median PFS was 1.4 months (95% CI 1.4–2.1) [132]. CNS delivery has not established comparative benefit.

Preclinical ANG1005 studies showed approximately 4.5-fold greater brain uptake than free paclitaxel, LRP1-sensitive uptake into brain metastases, and microenvironment-dependent LRP1 expression and transport [133,134,135]. Early trials established 650 mg/m² every three weeks, reported responses in advanced solid tumors and recurrent glioma, and measured conjugate and paclitaxel concentrations in resected tumor [37,136]. Exploratory FLT-PET showed a median 20.9% decline in maximum standardized uptake value after one cycle, correlated with MRI response [137]. These observations support tissue access, without proving therapeutic-index improvement.

8.5. Melflufen as a Boundary Case

Melflufen is an enzyme-potentiated lipophilic peptide-conjugated alkylator, classified here as a peptide-activated prodrug (Section 3.1). Single-arm HORIZON reported ORR 29%, median PFS 4.2 months, and median OS 11.6 months; grade ≥ 3 neutropenia, thrombocytopenia, and anemia occurred in 79%, 76%, and 43% [138]. OCEAN reported median PFS 6.8 vs. 4.9 months (HR 0.79; 95% CI 0.64–0.98), although FDA’s prespecified analysis did not establish superiority; median OS was 19.8 vs. 25.0 months (HR 1.10; 95% CI 0.85–1.44) [139]. FDA withdrew Pepaxto on 23 February 2024, because confirmatory benefit was unverified and safety/effectiveness for its US indication unestablished; EU Pepaxti authorization remained under a narrower indication at cutoff [25,111]. Activation therefore requires interpretation alongside survival, subgroup risk, and regulatory context.

Melflufen studies demonstrated rapid intracellular melphalan accumulation, aminopeptidase-dependent activity, irreversible DNA damage, and activity in bortezomib-resistant and tumor-initiating myeloma cells [140,141,142], alongside endothelial and angiogenic effects [143]. Aminopeptidase associations with progression and ex vivo sensitivity remain an exploratory biomarker hypothesis [144].

After first-in-human solid-tumor testing, phase I–II myeloma development selected 40 mg, with concurrent responses and hematologic toxicity [145,146]. Longer follow-up retained a 31% response rate and median OS 20.7 months [147]. ANCHOR combination-cohort response rates exceeded 70%, but cohorts were small and nonrandomized; LIGHTHOUSE randomized only 54 of 240 planned patients, precluding definitive comparison [148,149].

In vitro whole-blood experiments showed blood-cell peaks of melflufen and melphalan at approximately 1 and 6 min, respectively; clinical PK yielded melflufen distribution and terminal half-lives of 1.24 and 26.7 min [150]. These mechanistic and PK findings could not override survival and safety findings.

8.6. Radioligand Success and Limits of Transfer to Cytotoxic PDCs

PRRT provides evidence of clinical benefit in selected receptor-positive tumors. NETTER-1 improved PFS in progressive midgut NET (HR 0.21; 95% CI 0.13–0.33; p < 0.001) [26]; final OS was nonsignificant with crossover [151]. In first-line higher-grade well-differentiated gastroenteropancreatic NET, NETTER-2 median PFS was 22.8 vs. 8.5 months (HR 0.276; 95% CI 0.182–0.418), with ORRs 43.0% vs. 9.3% [152]. Both compared ¹⁷⁷Lu-DOTATATE plus octreotide with high-dose octreotide (Table 7). These results establish regimen- and population-specific benefit, not a general advantage of peptide-guided modalities.

PRRT combines imaging-based SSTR selection with a retained radiation source, accompanied by amino-acid renal protection and hematologic/renal monitoring [26,112,152]. A plausible mechanistic advantage is that beta irradiation does not require linker cleavage, formation of an active small-molecule catabolite, or cytosolic access. Finite-range cross-fire can irradiate neighboring cells without each internalizing conjugate, but it does not eliminate dependence on retention, spatial distribution, or radiosensitivity [112]. These features may reduce particular delivery barriers; their individual contributions to the contrast with cytotoxic programs were not established by these trials.

The transferable principles are functional patient/lesion selection, assessment of tumor residence and spatial coverage, normal-organ protection, and comparator-controlled confirmation of benefit. For cytotoxic PDCs, these require qualification of the final conjugate, chemically identified active-species exposure, linked PD and exposure–toxicity relationships. Radiation absorbed dose and cross-fire cannot substitute for chemical active-site exposure or bystander killing. PRRT uptake thresholds, dosing schedules, and amino-acid protocols require separate PDC validation. The NETTER trials tested fixed administered-activity regimens; their success does not validate personalized dosimetry or RCE metrics [26,112,152].

Zoptarelin and ANG1005 outcomes do not identify which additional requirement—active-species formation, access to the payload target, payload susceptibility, or tolerated exposure—limited benefit [126,127,131,132]. These remain mechanistic hypotheses. Differences in disease, selection, prior treatment, comparators, and endpoints prevent cross-trial attribution of success or failure to modality alone.

Figure 6 synthesizes five contrasting delivery hypotheses from the studies by Baldini, Halperin, Miller, Kumthekar, and Schjesvold and colleagues [107,120,126,131,139], identifying the clinically challenged step rather than treating development phase as evidence quality.

Figure 6. Clinical programs interpreted as tests of distinct peptide-guided delivery hypotheses. This figure is an original synthesis by the authors from the studies cited in the preceding text; no source artwork was reproduced. PEN-221 and ZoptEC inputs include conference-abstract evidence [120,126]; source formats and study designs are distinguished in Table 7. Colors distinguish programs: orange, PEN-221; teal, ANG1005; green, zelenectide pevedotin; red, zoptarelin doxorubicin; and purple, melflufen. Each upward arrow links a program to the type or phase of evidence discussed; the labels do not describe its current development status or rank evidence quality. Regulatory statements reflect the review cutoff of 15 September 2026. The melflufen summary distinguishes withdrawal of US approval from retained EU authorization under its narrower indication.

Table 7. Clinical maturity, efficacy, safety, and authors’ qualitative interpretations of representative peptide-targeted therapeutics.

Class and Agent	Target or Construct	Most Informative Evidence	Principal Efficacy Result	Principal Safety Finding	Authors’ Qualitative Interpretation, 15 September 2026
Cytotoxic PDC: zelenectide pevedotin	Nectin-4 bicyclic peptide–MMAE	Full peer-reviewed report: Duravelo-1 phase I/II monotherapy dose escalation; 49 treated and 42 efficacy-evaluable patients [107]	All tumor types: ORR 24% (10/42), median DOR 11.1 months among confirmed responders. Urothelial carcinoma: ORR 38% (8/21), median DOR 13.1 months among confirmed responders, and median PFS 7.4 months	Grade ≥ 3 TRAEs: 39% (19/49), including neutropenia in 16% (8/49); DLTs in 3 of 26 DLT-evaluable patients	Encouraging but noncomparative; no authorization; 2026 activity and dose-selection updates are preliminary conference abstracts [116,117,118]
Cytotoxic PDC: PEN-221	SSTR2 somatostatin analogue–DM1	Phase I/IIa; 89 patients overall; GI midgut-NET cohort: 32 treated and 26 response-evaluable patients (conference abstract [120]; posted registry results [121])	GI midgut-NET CBR 88.5% (23/26), entirely stable disease; ORR 0% (0/26). ORR was also 0% in the pancreatic-NET and SCLC response-evaluable cohorts (0/12 each)	Phase I conference abstract: reversible grade 3 ALT/AST elevations were dose-limiting in 2 of 3 patients receiving 25 mg; one also developed grade 3 hyperbilirubinemia and mucositis [122]	Completed without randomized follow-up; disease stabilization without objective responses; comparative benefit unestablished
Cytotoxic PDC: zoptarelin doxorubicin	LHRH agonist–doxorubicin	ZoptEC randomized open-label phase III; 511 randomized patients, 256 vs. 255 (conference abstract [126]; posted registry results [127])	Zoptarelin doxorubicin vs. doxorubicin: median OS 10.9 vs. 10.8 months; median PFS 4.7 vs. 4.7 months; ORR 12.5% (32/256) vs. 14.1% (36/255)	Grade ≥ 3 neutropenia: 47% vs. 45%; all-cause serious adverse events: 36.5% (92/252) vs. 30.1% (75/249)	Randomized trial did not demonstrate superiority over doxorubicin; no clear safety advantage in the studied population
Transport PDC: paclitaxel trevatide	LRP1-binding Angiopep-2 with three paclitaxel moieties	Breast-CNS phase II; 72 treated and 60 evaluable for intracranial response [131]	Intracranial response: 15% (9/60) by investigator assessment, including unconfirmed responses, and 8% (5/60) by independent review, all confirmed; prespecified independent-response criterion not met	Grade ≥3 febrile neutropenia: 17% (12/72); grade ≥ 3 decreased neutrophil count: 64% (46/72)	Intracranial activity without established comparative benefit; subsequent glioma study did not meet its prespecified endpoints
Peptide-activated prodrug: melflufen	Lipophilic peptide-conjugated alkylator with enzyme-enhanced activity	OCEAN randomized phase III; 495 randomized patients, 246 vs. 249 [139]	Melflufen–dexamethasone vs. pomalidomide–dexamethasone: median OS 19.8 vs. 25.0 months; PFS conclusions differed between the publication and the prespecified regulatory analysis [111,139]	Grade 3–4 treatment-emergent adverse events in the melflufen safety population (n = 228): thrombocytopenia 63% (143/228), neutropenia 54% (123/228), and anemia 43% (97/228)	US authorization withdrawn; EU authorization retained under a restricted indication; not a receptor-targeted PDC
Peptide radioligand comparator: 177Lu-DOTATATE	SSTR-binding octreotate–DOTA–177Lu	NETTER-1 randomized phase III; 229 patients in the primary report and 231 in the final OS analysis [26,151]	PFS HR 0.21 in the primary report; final OS HR 0.84, without a statistically significant OS difference, in the setting of crossover to peptide receptor radionuclide therapy	Initial grade 3–4 neutropenia, thrombocytopenia, and lymphopenia: 1%, 2%, and 9%, respectively, in the 177Lu-DOTATATE safety population (n = 111); MDS reported during long-term follow-up	Randomized PFS benefit in the studied population; authorized radioligand, mechanistically distinct from cytotoxic PDCs
Peptide radioligand comparator: 177Lu-DOTATATE	Same construct in first-line higher-grade NET	NETTER-2 randomized phase III; 226 randomized patients, 151 vs. 75 [152]	Median PFS 22.8 vs. 8.5 months; HR 0.276; ORR 43.0% (65/151) vs. 9.3% (7/75)	All-cause grade ≥ 3 adverse events: 35% (52/147) vs. 27% (20/73); no study drug-related deaths during the treatment period	Positive randomized evidence strengthens the comparator, not chemical-release claims

“Vs.” denotes experimental/control order; NETTER trials compare ¹⁷⁷Lu-DOTATATE plus octreotide with high-dose octreotide. Response-evaluable and safety denominators are distinguished; DOR applies to responders. Adverse-event categories retain source definitions and are not directly comparable across studies. The final column contains contextual author interpretations, not formal evidence grades or comparative rankings across agents; their basis is described in Section 2.3. Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; CBR, clinical-benefit rate; CI, confidence interval; CNS, central nervous system; DLT, dose-limiting toxicity; DOR, duration of response; GI, gastrointestinal; HR, hazard ratio; LHRH, luteinizing-hormone-releasing hormone; MDS, myelodysplastic syndrome; MMAE, monomethyl auristatin E; NET, neuroendocrine tumor; ORR, objective response rate; OS, overall survival; PFS, progression-free survival; SCLC, small-cell lung cancer; SSTR, somatostatin receptor; TRAE, treatment-related adverse event.

Table 7. Clinical maturity, efficacy, safety, and authors’ qualitative interpretations of representative peptide-targeted therapeutics.

Class and Agent	Target or Construct	Most Informative Evidence	Principal Efficacy Result	Principal Safety Finding	Authors’ Qualitative Interpretation, 15 September 2026
Cytotoxic PDC: zelenectide pevedotin	Nectin-4 bicyclic peptide–MMAE	Full peer-reviewed report: Duravelo-1 phase I/II monotherapy dose escalation; 49 treated and 42 efficacy-evaluable patients [107]	All tumor types: ORR 24% (10/42), median DOR 11.1 months among confirmed responders. Urothelial carcinoma: ORR 38% (8/21), median DOR 13.1 months among confirmed responders, and median PFS 7.4 months	Grade ≥ 3 TRAEs: 39% (19/49), including neutropenia in 16% (8/49); DLTs in 3 of 26 DLT-evaluable patients	Encouraging but noncomparative; no authorization; 2026 activity and dose-selection updates are preliminary conference abstracts [116,117,118]
Cytotoxic PDC: PEN-221	SSTR2 somatostatin analogue–DM1	Phase I/IIa; 89 patients overall; GI midgut-NET cohort: 32 treated and 26 response-evaluable patients (conference abstract [120]; posted registry results [121])	GI midgut-NET CBR 88.5% (23/26), entirely stable disease; ORR 0% (0/26). ORR was also 0% in the pancreatic-NET and SCLC response-evaluable cohorts (0/12 each)	Phase I conference abstract: reversible grade 3 ALT/AST elevations were dose-limiting in 2 of 3 patients receiving 25 mg; one also developed grade 3 hyperbilirubinemia and mucositis [122]	Completed without randomized follow-up; disease stabilization without objective responses; comparative benefit unestablished
Cytotoxic PDC: zoptarelin doxorubicin	LHRH agonist–doxorubicin	ZoptEC randomized open-label phase III; 511 randomized patients, 256 vs. 255 (conference abstract [126]; posted registry results [127])	Zoptarelin doxorubicin vs. doxorubicin: median OS 10.9 vs. 10.8 months; median PFS 4.7 vs. 4.7 months; ORR 12.5% (32/256) vs. 14.1% (36/255)	Grade ≥ 3 neutropenia: 47% vs. 45%; all-cause serious adverse events: 36.5% (92/252) vs. 30.1% (75/249)	Randomized trial did not demonstrate superiority over doxorubicin; no clear safety advantage in the studied population
Transport PDC: paclitaxel trevatide	LRP1-binding Angiopep-2 with three paclitaxel moieties	Breast-CNS phase II; 72 treated and 60 evaluable for intracranial response [131]	Intracranial response: 15% (9/60) by investigator assessment, including unconfirmed responses, and 8% (5/60) by independent review, all confirmed; prespecified independent-response criterion not met	Grade ≥3 febrile neutropenia: 17% (12/72); grade ≥ 3 decreased neutrophil count: 64% (46/72)	Intracranial activity without established comparative benefit; subsequent glioma study did not meet its prespecified endpoints
Peptide-activated prodrug: melflufen	Lipophilic peptide-conjugated alkylator with enzyme-enhanced activity	OCEAN randomized phase III; 495 randomized patients, 246 vs. 249 [139]	Melflufen–dexamethasone vs. pomalidomide–dexamethasone: median OS 19.8 vs. 25.0 months; PFS conclusions differed between the publication and the prespecified regulatory analysis [111,139]	Grade 3–4 treatment-emergent adverse events in the melflufen safety population (n = 228): thrombocytopenia 63% (143/228), neutropenia 54% (123/228), and anemia 43% (97/228)	US authorization withdrawn; EU authorization retained under a restricted indication; not a receptor-targeted PDC
Peptide radioligand comparator: 177Lu-DOTATATE	SSTR-binding octreotate–DOTA–177Lu	NETTER-1 randomized phase III; 229 patients in the primary report and 231 in the final OS analysis [26,151]	PFS HR 0.21 in the primary report; final OS HR 0.84, without a statistically significant OS difference, in the setting of crossover to peptide receptor radionuclide therapy	Initial grade 3–4 neutropenia, thrombocytopenia, and lymphopenia: 1%, 2%, and 9%, respectively, in the 177Lu-DOTATATE safety population (n = 111); MDS reported during long-term follow-up	Randomized PFS benefit in the studied population; authorized radioligand, mechanistically distinct from cytotoxic PDCs
Peptide radioligand comparator: 177Lu-DOTATATE	Same construct in first-line higher-grade NET	NETTER-2 randomized phase III; 226 randomized patients, 151 vs. 75 [152]	Median PFS 22.8 vs. 8.5 months; HR 0.276; ORR 43.0% (65/151) vs. 9.3% (7/75)	All-cause grade ≥ 3 adverse events: 35% (52/147) vs. 27% (20/73); no study drug-related deaths during the treatment period	Positive randomized evidence strengthens the comparator, not chemical-release claims

“Vs.” denotes experimental/control order; NETTER trials compare ¹⁷⁷Lu-DOTATATE plus octreotide with high-dose octreotide. Response-evaluable and safety denominators are distinguished; DOR applies to responders. Adverse-event categories retain source definitions and are not directly comparable across studies. The final column contains contextual author interpretations, not formal evidence grades or comparative rankings across agents; their basis is described in Section 2.3. Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; CBR, clinical-benefit rate; CI, confidence interval; CNS, central nervous system; DLT, dose-limiting toxicity; DOR, duration of response; GI, gastrointestinal; HR, hazard ratio; LHRH, luteinizing-hormone-releasing hormone; MDS, myelodysplastic syndrome; MMAE, monomethyl auristatin E; NET, neuroendocrine tumor; ORR, objective response rate; OS, overall survival; PFS, progression-free survival; SCLC, small-cell lung cancer; SSTR, somatostatin receptor; TRAE, treatment-related adverse event.

9. Chemistry Manufacturing and Regulatory Strategy

9.1. Product Definition and Control Strategy

PDC quality control must connect molecular attributes to binding, stability, release, biodistribution, and potency. Relevant attributes include peptide sequence and stereochemistry, conjugation site, linker integrity, payload-related impurities, unconjugated peptide, free payload, residual reagents, counterions, aggregation, and degradation products.

Peptide synthesis introduces sequence errors, epimerization, oxidation, and aggregation; conjugation adds regioisomers, incomplete products, hydrolysis products, payload dimers, and potent residual reagents. Co-eluting molecular species can differ pharmacologically, requiring orthogonal chromatography and mass spectrometry, appropriate peptide mapping and stereochemical controls, residual-solvent and elemental-impurity assessment, and stability-indicating assays linked to biological function.

The EMA synthetic-peptide guideline, effective 1 June 2026, covers starting materials, synthesis, conjugation, characterization, specifications, and drug-product considerations [29]. Applying the scope distinctions in Section 3.1/Table 2, recombinant biological products and complete radiopharmaceuticals are excluded, while radiopharmaceutical synthetic-peptide components remain covered for synthesis and starting materials. ICH Q8(R2), Q9(R1), Q10, Q11, Q14, and M7(R2) complement development, risk management, lifecycle control, analytical development, and mutagenic-impurity assessment [153,154,155,156,157,158], subject to their own scope exclusions.

Payload-releasing PDC release testing requires free-payload limits, intact-conjugate quantification, related-substance profiles, binding or function, and degradation or release assays sensitive to clinically meaningful changes. Changes in the process affecting size, hydrophobicity, counterion, impurities, or conformation require comparability of binding, release kinetics, cell potency, and relevant PK.

Collins et al. reported 95% less waste with wash-free solid-phase peptide synthesis in their studied platform [159]. Figure 7 adapts this process scheme to PDC manufacturing while retaining controls for molecular identity, conjugation, free payload, related substances, potency, stability, and comparability; the waste reduction is not a universal PDC result.

9.2. Bioanalysis and Clinical Pharmacology

We propose a construct-specific clinical assay plan for intact conjugate, released payload, and relevant catabolites identified nonclinically. Sampling should characterize their distinct concentration–time profiles; distinguishing release, binding, and elimination mechanisms requires independent observations and identifiable models.

FDA ADC guidance provides clinical-pharmacology precedent, not a PDC definition or default submission pathway (Section 3.1). Its analyte, exposure–response, intrinsic-factor, QT, interaction, and immunogenicity recommendations require construct-specific justification [30]. PDC adaptation must establish kinetics, elimination, relevant peptide immunogenicity risk, and active-catabolite identity. Expected low free-payload concentrations require testing, not assay omission. The proposed RCE measures and gates do not replace agency requirements or product-specific advice.

Figure 7. Manufacturing and CMC readiness workflow for a chemically defined PDC. Adapted from Figure 1 of Collins et al. [159] under the Creative Commons Attribution 4.0 International License, redrawn, simplified, recolored, and expanded from peptide synthesis to PDC conjugation, purification, characterization, release testing, and comparability. Blue, green, orange, purple, and red distinguish peptide synthesis, conjugation, purification and isolation, characterization, and release testing and comparability, respectively. Horizontal arrows indicate the process sequence; downward arrows connect each stage to its associated quality attributes. Colors distinguish stages and do not indicate numerical values.

Figure 7. Manufacturing and CMC readiness workflow for a chemically defined PDC. Adapted from Figure 1 of Collins et al. [159] under the Creative Commons Attribution 4.0 International License, redrawn, simplified, recolored, and expanded from peptide synthesis to PDC conjugation, purification, characterization, release testing, and comparability. Blue, green, orange, purple, and red distinguish peptide synthesis, conjugation, purification and isolation, characterization, and release testing and comparability, respectively. Horizontal arrows indicate the process sequence; downward arrows connect each stage to its associated quality attributes. Colors distinguish stages and do not indicate numerical values.

9.3. Biomarkers and Companion Diagnostics

Biomarkers should test the causative delivery bottleneck. Receptor IHC alone may miss trafficking, amplification, spatial heterogeneity, protease activity, perfusion, and resistance. Imaging can assess whole-body accessibility and organ accumulation; tissue assays resolve cellular localization and heterogeneity. Each readout needs a defined decision. FDA recommends contemporaneous approval or clearance of companion diagnostics and their corresponding therapeutics [160] and provides draft recommendations for their codevelopment [161]; early analytical validation, scoring, specimen requirements, prevalence, and turnaround assessment help separate drug failure from assay failure. Table 8 summarizes the CMC, bioanalytical, and regulatory readiness considerations.

Table 8. CMC, bioanalytical, and regulatory readiness considerations [29,153,154,155,156,157,158].

Domain	Required Evidence	Development Decision	Material Risk If Deferred
Molecular definition	Sequence, stereochemistry, conjugation site, payload identity, counterion, and molecular mass	Confirms a reproducible active substance	Regioisomers or stereochemical impurities may alter binding, release, or PK
Impurity control	Truncations, epimers, oxidized compounds, free payload, linker degradants, residual reagents, and mutagenic-risk assessment	Sets specifications linked to safety and process capability	Low-level potent impurities can dominate systemic toxicity
Functional control	Post-conjugation binding, cell-based potency, and mechanism-relevant release or catabolite assay	Links analytical quality to biological performance	Analytical comparability may conceal altered pharmacology
Stability	Intact conjugate, free payload, aggregation, chemical degradation, and active-compound formation under storage and in-use conditions	Defines formulation, shelf life, and administration controls	Product can release payload before dosing or change during infusion
Clinical bioanalysis	Validated intact-PDC, free-payload, and active-catabolite assays with ex vivo stabilization	Enables exposure–response and dose selection	Mechanism of efficacy or toxicity remains unidentifiable
Biomarker or diagnostic	Assay validity, cutoff or imaging rule, prevalence, heterogeneity, and operational feasibility	Defines the evaluable and intended-use population	A broad target-positive label can dilute benefit or expose target-negative patients
GMP scale-up	Process capability, containment for highly potent payloads, cleaning validation, and comparability	Supports clinical supply and lifecycle changes	Scale-up can alter impurity profile or release behavior after candidate selection

10. Stage Gated Translational Roadmap

10.1. Quantitative Primary Research Examples

Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12 illustrate the purposive subset described in Section 2.2. Published numerical summaries were replotted without digitization or individual-animal reanalysis; captions identify source locations, units, sample sizes, uncertainty, and normalized quantities. Each example addresses a distinct experimental question. A separate historical prodrug dataset supplies the tissue AUCs for the worked calculation and is outside the five CC BY 4.0 adaptations.

Currie et al. compared free docetaxel and SORT1-targeted TH1902 at matched docetaxel-content doses in ovarian and endometrial xenografts [162]. Figure 8 reproduces tabulated endpoint fold changes across three models, including high-dose TH1902 regression in SKOV3 and AN3-CA. This payload-equivalent comparison supports the carrier–payload hypothesis but identifies neither the active molecular species nor productive intratumoral exposure.

Lepland et al. constrained the CD206-binding mUNO motif into MACTIDE and coupled it to verteporfin [163]. Figure 9 uses results-text summaries to trace affinity, proteolytic persistence, delivery to tumor-associated macrophages, and phenotype-dependent payload action in a light-activated setting. It illustrates sequential carrier qualification beyond binding; peptide half-life and per-cell signal are normalized ratios, not absolute concentrations.

Figure 8. Endpoint tumor-size progression after vehicle, free docetaxel, or SORT1-targeted TH1902 in ES-2, SKOV3, and AN3-CA xenografts. (A) ES-2 ovarian xenografts; (B) SKOV3 ovarian xenografts; (C) AN3-CA endometrial xenografts. Gray bars denote vehicle; light-blue and dark-blue bars denote docetaxel at 3.75 and 15 mg/kg; orange and green bars denote TH1902 at 8.75 and 35 mg/kg, respectively. Values are fold change from treatment Day 0 at the respective vehicle-group endpoint; the dashed line at 1 indicates no net change, and values below 1 indicate regression. Adapted from Figures 6 and 7 and Table 1 of Currie et al. [162] under the Creative Commons Attribution 4.0 International License. Changes have been made; exact tabulated values were reorganized into aligned quantitative panels and completely redrawn with standardized ordering, labels, colors, and scales; no error bars were added because the source table did not report them.

Figure 8. Endpoint tumor-size progression after vehicle, free docetaxel, or SORT1-targeted TH1902 in ES-2, SKOV3, and AN3-CA xenografts. (A) ES-2 ovarian xenografts; (B) SKOV3 ovarian xenografts; (C) AN3-CA endometrial xenografts. Gray bars denote vehicle; light-blue and dark-blue bars denote docetaxel at 3.75 and 15 mg/kg; orange and green bars denote TH1902 at 8.75 and 35 mg/kg, respectively. Values are fold change from treatment Day 0 at the respective vehicle-group endpoint; the dashed line at 1 indicates no net change, and values below 1 indicate regression. Adapted from Figures 6 and 7 and Table 1 of Currie et al. [162] under the Creative Commons Attribution 4.0 International License. Changes have been made; exact tabulated values were reorganized into aligned quantitative panels and completely redrawn with standardized ordering, labels, colors, and scales; no error bars were added because the source table did not report them.

Figure 9. Experimental qualification chain for the CD206-targeted MACTIDE platform. (A) Equilibrium dissociation constant; (B) dissociation rate constant; (C) peptide half-life in 4T1 tumor lysate relative to mUNO; (D) FAM signal per CD206-positive tumor-associated macrophage after intraperitoneal administration relative to mUNO; (E) depletion of IL-4-stimulated or LPS + IFNγ-stimulated macrophages after irradiated MACTIDE-V. Panels (A,B) use logarithmic axes. In panels (A–D), gray denotes mUNO and green denotes MACTIDE; in panel (E), green and orange denote IL-4- and LPS + IFNγ-stimulated macrophages, respectively. The arrows in the interpretation note link evidence domains rather than numerical quantities. Adapted from Figure 3A,B, 4B, and 5B of Lepland et al. [163] under the Creative Commons Attribution 4.0 International License. Changes have been made; reported numerical values and fold differences were extracted, normalized only where explicitly indicated, reorganized, and the figure has been completely redrawn in a unified multi-panel style.

Figure 9. Experimental qualification chain for the CD206-targeted MACTIDE platform. (A) Equilibrium dissociation constant; (B) dissociation rate constant; (C) peptide half-life in 4T1 tumor lysate relative to mUNO; (D) FAM signal per CD206-positive tumor-associated macrophage after intraperitoneal administration relative to mUNO; (E) depletion of IL-4-stimulated or LPS + IFNγ-stimulated macrophages after irradiated MACTIDE-V. Panels (A,B) use logarithmic axes. In panels (A–D), gray denotes mUNO and green denotes MACTIDE; in panel (E), green and orange denote IL-4- and LPS + IFNγ-stimulated macrophages, respectively. The arrows in the interpretation note link evidence domains rather than numerical quantities. Adapted from Figure 3A,B, 4B, and 5B of Lepland et al. [163] under the Creative Commons Attribution 4.0 International License. Changes have been made; reported numerical values and fold differences were extracted, normalized only where explicitly indicated, reorganized, and the figure has been completely redrawn in a unified multi-panel style.

Jiang et al. illustrate discordance between intrinsic potency and delivery: DTX-P7 was approximately tenfold less potent than docetaxel in A549 monolayers yet achieved greater tumor-growth inhibition at payload-equivalent doses in A549 xenografts [164]. Figure 10 uses Results and supplementary values to show why in vitro potency cannot establish RCE; neither the mechanism nor magnitude of increased active-compound exposure was established.

Figure 10. DTX-P7 activity in vitro and in vivo. Panel (A) shows reported IC50 values for free docetaxel and DTX-P7 in A549 and H1975 cells. Panel (B) shows reported tumor-growth inhibition after four weekly intraperitoneal treatments in A549 xenografts with n equals 5 mice per group. Adapted from Figures 1A,B and S1, and the accompanying Results of Jiang et al. [164] under the Creative Commons Attribution 4.0 International License. Changes have been made; reported values were extracted and replotted using unified axes, typography, ordering, and colors; no source artwork is reproduced.

Figure 10. DTX-P7 activity in vitro and in vivo. Panel (A) shows reported IC50 values for free docetaxel and DTX-P7 in A549 and H1975 cells. Panel (B) shows reported tumor-growth inhibition after four weekly intraperitoneal treatments in A549 xenografts with n equals 5 mice per group. Adapted from Figures 1A,B and S1, and the accompanying Results of Jiang et al. [164] under the Creative Commons Attribution 4.0 International License. Changes have been made; reported values were extracted and replotted using unified axes, typography, ordering, and colors; no source artwork is reproduced.

Wang et al. tested VEGFR-targeting lytic conjugate QR-KLU by transarterial delivery in VX2 rabbit liver tumors [165]. Figure 11 uses results-text summaries to show restrained enlargement and the highest measured necrosis fraction seven days after embolization. This local-delivery example demonstrates activity under those conditions, without quantifying intratumoral active-compound exposure or establishing systemic targeting.

Figure 11. Tumor-volume change and necrosis after transarterial embolization with normal saline, doxorubicin, KLU, or QR-KLU in the VX2 rabbit liver-tumor model. (A) Tumor volumes at baseline and Day 7; (B) tumor necrosis at Day 7. Gray, blue, orange, and green denote normal saline (NS), doxorubicin (DOX), KLU, and QR-KLU, respectively. Overlapping baseline markers and error bars reflect similar group means and their reported variability; they have not been displaced. Values are mean plus or minus SD with n equals 6 per group. Adapted from Figures 4B and 5B and the accompanying Results of Wang et al. [165] under the Creative Commons Attribution 4.0 International License. Changes have been made: numerical results were extracted and replotted in a unified two-panel style; source magnetic-resonance and histology images are not reproduced.

Figure 11. Tumor-volume change and necrosis after transarterial embolization with normal saline, doxorubicin, KLU, or QR-KLU in the VX2 rabbit liver-tumor model. (A) Tumor volumes at baseline and Day 7; (B) tumor necrosis at Day 7. Gray, blue, orange, and green denote normal saline (NS), doxorubicin (DOX), KLU, and QR-KLU, respectively. Overlapping baseline markers and error bars reflect similar group means and their reported variability; they have not been displaced. Values are mean plus or minus SD with n equals 6 per group. Adapted from Figures 4B and 5B and the accompanying Results of Wang et al. [165] under the Creative Commons Attribution 4.0 International License. Changes have been made: numerical results were extracted and replotted in a unified two-panel style; source magnetic-resonance and histology images are not reproduced.

Wang et al. tested FGFR2-targeting LLC2B-DM1 with disulfide and maleimide linkers in separate MCF-7 xenograft experiments [166]. Figure 12 reproduces complete tumor-growth and uncertainty series from Supplementary Tables S2 and S3. Contemporaneous controls illustrate architecture- and dose-dependent performance; the separate experiments do not support a controlled head-to-head linker comparison.

Figure 12. Tumor-growth trajectories for FGFR2-targeted LLC2B-DM1 conjugates in separate disulfide-linker and maleimide-linker MCF-7 xenograft experiments. (A) Disulfide-linker series (SS-PDC); (B) maleimide-linker series (Mal-PDC). Lines and markers show group means, and translucent bands show ±SEM. Colors identify the treatments and doses specified in each panel legend. Overlapping trajectories and uncertainty bands represent the reported measurements and are retained without displacement. Values are mean plus or minus SEM with n equals 5 per group; doses are expressed as DM1-equivalent mg per kg for conjugates. Adapted from Figure 6A,C and Supplementary Tables S2 and S3 of Wang et al. [166] under the Creative Commons Attribution 4.0 International License. Changes have been made; the full published mean and SEM series were replotted with standardized axes, labels, typography, and colors; no source artwork is reproduced.

Figure 12. Tumor-growth trajectories for FGFR2-targeted LLC2B-DM1 conjugates in separate disulfide-linker and maleimide-linker MCF-7 xenograft experiments. (A) Disulfide-linker series (SS-PDC); (B) maleimide-linker series (Mal-PDC). Lines and markers show group means, and translucent bands show ±SEM. Colors identify the treatments and doses specified in each panel legend. Overlapping trajectories and uncertainty bands represent the reported measurements and are retained without displacement. Values are mean plus or minus SEM with n equals 5 per group; doses are expressed as DM1-equivalent mg per kg for conjugates. Adapted from Figure 6A,C and Supplementary Tables S2 and S3 of Wang et al. [166] under the Creative Commons Attribution 4.0 International License. Changes have been made; the full published mean and SEM series were replotted with standardized axes, labels, typography, and colors; no source artwork is reproduced.

Worked example using chemically resolved tissue exposure.

The five illustrations lack matched active-compound AUCs for the proposed ratios. For example, Figure 9 measures MACTIDE persistence, not verteporfin release, and cannot supply the kinetic-separation ratio’s target-release denominator [163]. Calculation therefore requires an additional dataset with the relevant analyte and comparators.

Dubois et al. reported such data for the peptide-activated doxorubicin prodrug CPI-0004Na (Section 3.1) [167]. MCF-7/6 xenograft-bearing BALB/c nu/nu mice received CPI-0004Na or doxorubicin hydrochloride by 2 h intravenous infusion at 86.2 μmol/kg; three animals per time point were sampled through 96 h. HPLC distinguished doxorubicin (Dox), intact prodrug, and peptide-bearing metabolites, correcting concentrations for extraction recovery and blood contamination. Table 9 uses source-reported trapezoidal Dox AUCs from zero to the last sample, without digitization or pooling distinct molecular species.

Table 9. Published Dox tissue AUCs and calculated tumor-to-organ ratios for the CPI-0004Na worked example. AUC0–96 h units are pmol·h/mg tissue; inputs are from Table 3 of Dubois et al. [167].

Tissue	Dox AUC After Free Dox·HCl	Dox AUC After CPI-0004Na	Tumor/Organ Ratio After CPI-0004Na
Tumor	1138	2186	—
Heart	1933	209.3	10.44
Spleen	47,184	1641	1.33
Liver	4315	756.7	2.89
Kidney	6433	4696	0.466
Lung	4680	882.1	2.48

Each dimensionless tumor/organ ratio divides the CPI-0004Na tumor Dox AUC (2186) by the corresponding organ AUC, rounding after division. These use published summary AUCs, not new measurements or reconstructed concentration–time curves.

As a Class 2 adaptation, dose-normalized tumor gain compares tumor Dox AUC per molar payload dose after prodrug vs. free drug. Each molecule contains one Dox unit; equal payload-equivalent doses therefore cancel, as follows:

Tumor gain = (2186/86.2)/(1138/86.2) = 1.92.

The tumor/heart ratio uses the same molecular species and tissue-mass units:

Tumor/heart ratio after CPI-0004Na = 2186/209.3 = 10.44.

The free-drug tumor/heart ratio is 1138/1933 = 0.589. However, the prodrug tumor/kidney ratio is 2186/4696 = 0.466; kidney Dox exposure exceeds tumor exposure. Renal exposure and toxicity therefore require separate evaluation despite favorable cardiac distribution. Neither ratio alone establishes overall safety, and unity is not a safety threshold.

This calculation implements two Class 2 adaptations in Part A of Table 3 in bulk tissue. Extractable homogenate Dox does not establish unbound intracellular or nuclear exposure, and thus complete RCE. The ratios exclude peptide-bearing metabolites and organ-specific pharmacological sensitivity; summary AUCs cannot support ratio confidence intervals. Unbound partitioning, mechanism-matched temporal coverage, paired cleavage/release half-lives, and spatial exposure–PD concordance remain unquantified. The example demonstrates calculability, not clinical-endpoint validation or a therapeutic-index claim.

10.2. Decision Gates

Table 10 proposes a sequence of development questions linking target access and conjugate integrity to the modality-appropriate active state, spatial delivery, PD, and therapeutic margin. The analyte, intended compartment, and mechanism determine which Table 3 quantities are informative and feasible. The suggested evidence supports hypothesis testing alongside manufacturing, biomarker, and comparator considerations; it does not constitute a validated scoring system, mandatory gate, or universal numerical rule for advancement.

Table 10. Proposed development questions and modality-specific candidate evidence for PDCs [8,26,27,28,29,30,32,34,35,46,47,48].

Gate	Decision Question	Modality-Specific Candidate Evidence	Potential Reasons to Reassess
Target and indication	Is the intended target, transport, or activation mechanism accessible and relevant in a definable population?	Tumor/normal-tissue context; receptor access and trafficking where required; enzyme/activation context for prodrugs; feasible selection strategy	Inaccessible target or activation site; adverse normal-organ distribution; missing mechanism-required trafficking; unsupported selection strategy
Carrier	Does the final conjugate preserve mechanism-relevant binding or transport, stability, distribution, and manufacturability?	Binding/transport where relevant; proteolytic stability; tissue penetration appropriate to the mechanism; solubility; synthetic feasibility	Loss of required binding/transport, inadequate distribution, dominant kidney or liver accumulation, or unstable sequence
Integrity and activation	Is the intended active form delivered intact or generated at the required site?	Releasing PDCs/prodrugs: relevant-matrix stability; active-product identity, formation, potency and permeability where relevant. Intact-acting constructs: integrity and function. Radioligands: radiochemical stability. Photosensitizers and macromolecular cargo: functional effector and release only when required.	Unintended release/degradation; unidentified consequential products; inadequate intended activation or integrity; unsafe off-target activity
Payload fit	Does the effector mechanism match tumor distribution, target-compartment access, and susceptibility?	Mechanism-matched models and controls; receptor competition where relevant; bystander killing when intended; functional access to the required compartment	Nonspecific cytotoxicity, fluorescence-only uptake evidence, heterogeneous tumors left uncovered, or implausible safety margin
PK and safety	Does modality-relevant tumor exposure or absorbed dose support activity with acceptable normal-organ effects?	Releasing PDCs/prodrugs: relevant parent, payload and metabolite PK. Intact-acting constructs: intact-species PK and activity. Radioligands: time–activity profiles and dosimetry. Photosensitizers: distribution, light delivery and mechanism-relevant oxygen. Macromolecular cargo: functional delivery. Spatial analyte/PD mapping when informative and feasible; human tissue sampling only in justified substudies. Modality-specific exposure–toxicity assessment.	Critical mechanism or exposure claims unsupported by fit-for-purpose evidence; insufficient tumor activity or unacceptable normal-organ toxicity
CMC and assay readiness	Can a clinically representative product be manufactured and measured reproducibly?	Defined conjugation site; scalable process; purity and stability specifications; free-payload control where applicable; validated modality-appropriate analytical plan	Scale-up changes performance, potent impurity is uncontrolled, or a critical quality or exposure claim cannot be supported
Clinical strategy	Can the trial test the delivery hypothesis against a relevant comparator?	Biomarker rule where justified; dose and schedule rationale; endpoint hierarchy; comparator and combination rationale	Trial cannot identify a responsive population or distinguish delivery advantage from payload activity alone

Applicability: select evidence by mechanism and development stage. Non-cleavable linkers can still yield active catabolites through carrier degradation. Spatial tissue PD and tumor/organ AUC are not universal clinical requirements; plasma PK, imaging and PD provide complementary evidence (Section 12). Document non-applicability separately from unavailable measurements; missing data do not establish safety.

Each claimed causal step needs a potentially disconfirming observation chosen for the mechanism; Table 10 makes modality-specific substitutions explicit. Clinical sampling must balance procedural risk and scientific value; advancement need not require all six core quantitative measures in every trial (Section 12).

11. Emerging Directions

Constrained and multicyclic peptides expand the range of binding targets while retaining chemically defined structures. Their value depends on active-compound tumor delivery, beyond plasma persistence or renal excretion. Screens should therefore assess post-conjugation binding, proteolysis, trafficking, and systemic distribution alongside affinity.

Tandem or AND-gate release can increase specificity through two biochemical conditions, but added chemistry may reduce yield and complicate product identification. Bioorthogonal activation adds external control while requiring compatible trigger distribution and reaction kinetics. Added complexity and controls should be justified by active-compound tumor exposure gains over simpler linkers.

Theranostic analogues can assess target access, whole-body distribution, and patient heterogeneity before cytotoxic treatment. Because linker, payload, chelator, charge, and albumin-binding changes can alter disposition, imaging must be validated against the therapeutic construct.

AI and machine learning support peptide generation, structure prediction, stability estimation, and multi-objective ranking [168]. PDC models should include negative results and binding, degradation, conjugation compatibility, release, trafficking, PK, and organ-uptake endpoints; affinity-only optimization repeats the single-variable selection problem addressed by RCE.

Solvent use, purification, low conjugation yields, and potent-payload handling can offset synthetic simplicity. Greener solid-phase synthesis, solvent reduction, convergent routes, and improved purification should be evaluated by product quality and scalability, beyond solvent substitution [159,169].

12. Limitations of the Framework

RCE is an author-proposed research framework, not a validated clinical surrogate or regulatory criterion. Established PK calculations retain their conventional meaning; their proposed predictive use within RCE requires validation. AUC-based estimates require concentration–time data for an identified active compound in a specified compartment. Lesion access, procedural risk, tissue yield, and patient burden often preclude dense biopsies. Research sampling requires a prespecified question, informed consent, independent ethical review, and proportionate risk [38]; routine biopsy-based individual tumor AUC reconstruction is not proposed. A single timed biopsy establishes one concentration, not individual AUC or exposure across lesions.

Microdialysis can serially measure diffusible, unbound extracellular analytes in accessible lesions, but it requires catheter placement and analyte-specific probe-recovery, stability, and adsorption assessment. Clinical carboplatin studies demonstrate feasibility alongside technical failures and variable recovery [170]. Extracellular profiles do not directly measure cytosolic or nuclear active-catabolite exposure; homogenates mix vascular, stromal, extracellular, and cellular material. Chemical identity, tissue localization, and subcellular access therefore require distinct evidence.

A feasible translational strategy could combine complementary evidence. Time-resolved tissue PK, identity of molecular species, and exposure–PD relationships can be established nonclinically; then, serial human plasma assays can be used to assess intact conjugate, released payload, and relevant catabolites. Separate intact-BT5528 and unconjugated-MMAE assays provide a direct PDC precedent [34], without establishing comprehensive catabolite coverage. Selected timed biopsies or planned resections can anchor tissue observations, with concurrent plasma and controls for ex vivo cleavage and residual blood. The GRN1005 phase I surgical cohort separately measured conjugate and paclitaxel in resected tumor with concurrent plasma [37], demonstrating tissue bioanalysis rather than individual tumor AUC, unbound exposure, or subcellular localization.

Molecular imaging can assess lesion accessibility, distribution, and heterogeneity less invasively. ZEPHIR combined radiolabeled trastuzumab PET and early metabolic imaging to examine heterogeneity and T-DM1 response [171], providing an ADC methodological precedent rather than validating PDC release. Carrier or diagnostic-analogue signals cannot locate or quantify released cytotoxic catabolites without validation. Label stability, labeled metabolites, and distribution effects of tracer mass, affinity, charge, and conjugation should be assessed. Uptake and metabolic response support interpretation but cannot directly supply active-catabolite AUC.

Population or physiologically based PK models could integrate nonclinical tissue kinetics, human plasma, and sparse tissue or imaging data to predict analyte-specific tumor concentration–time profiles and derived exposure measures. Antibody plasma/PET modeling provides a precedent and illustrates dependence on model assumptions [172]; PDC use requires construct-specific evaluation. Ethically justified sampling staggered across participants may constrain population profiles, not directly observed individual curves. Identifiability, sensitivity to release and partition assumptions, uncertainty intervals, and validation against independent tissue data should be reported where available. Plasma fit alone may leave tumor release unidentified; that quantity should be reported as not estimable. Transferred antibody models must replace carrier-specific recycling, tissue-exchange, and elimination assumptions with supported PDC processes (Section 7.4).

Payload-target engagement and downstream tissue PD, including damage responses, can support activity without full tumor PK but cannot uniquely determine active-compound concentration or duration above an effect threshold. Human safety-organ AUC is usually inaccessible; nonclinical distribution, clinical exposure–toxicity analysis, and organ monitoring provide safety evidence, not measured tumor/organ AUC ratios. Reporting should distinguish between unmeasured, below-quantification, and non-estimable quantities; missing denominators cannot imply favorable exposure ratios or safety. Plasma, imaging, or PD surrogates require prospective construct-specific calibration against selective tissue measurements of molecular species, then evaluation against benefit and toxicity. Explicit uncertainty should be retained to avoid generalizing thresholds across constructs, payloads, or biological species.

Many PDC programs remain preclinical or early-phase, clinical reporting is inconsistent, and negative programs are underreported. Target, payload, population, dose, and trial-design differences limit cross-study comparison. As a critical narrative review, this synthesis is susceptible to literature-selection and interpretive bias. Language prioritization may add selection bias, and unavailable historical search logs limit retrospective reproducibility. The documented retrieval check and qualitative appraisal do not establish completeness or eliminate bias; the proposed framework requires empirical validation.

The five adaptations also carry author-selection and availability bias. Permissive licensing and numerical accessibility constrain inclusion, and positive, null, and negative findings are unbalanced. Less accessible studies and unsuccessful constructs may be underrepresented. Mechanistic diversity does not establish representativeness or permit estimates of delivery success, platform rankings, or validation of RCE predictions and thresholds. The separate AUC example establishes calculability only.

13. Conclusions

RCE connects conjugate integrity, active-compound formation, active-species exposure in the relevant tumor compartment, PD, and therapeutic margin in a testable development framework. Its measures require defined analytes, compartments, intervals, and comparators, with uncertainty retained when direct measurements are infeasible. Programs should progressively strengthen the links between target access, active-species exposure, PD, and therapeutic margin before major development decisions. Early-stage advancement may proceed with documented uncertainties and a plan to address decision-relevant gaps. The predictive use of these established, adapted, and conceptual measures within RCE awaits construct-specific validation.