Receptor-Targeted Nanomedicine for Cancer Therapy

Abstract

Receptor-targeted drug delivery has been extensively explored for active targeting of therapeutic moiety in cancer treatment. In this review, we discuss the receptors that are overexpressed on tumor cells and have the potential to be targeted by nanocarrier systems for cancer treatment. We also highlight the different types of nanocarrier systems and targeting ligands that researchers have explored. Our discussion covers various therapeutic modalities, including small molecules, aptamers, peptides, antibodies, and cell-based targeting strategies, and focuses on clinical developments. Additionally, this article highlights the challenges that arise during the clinical translation of nanocarrier-based targeting strategies. It also provides future directions for improving research in the area of clinically translatable cancer-targeted therapy to improve treatment efficacy while minimizing toxicity.

1. Introduction

Since cancer is still one of the most prevalent causes of mortality in people, significant efforts have been made to enhance the effectiveness of cancer management [1,2,3,4,5,6]. Cancer therapies have considerably changed throughout time and now include a variety of techniques, such as surgery, radiation, chemotherapy, and immunotherapy. While chemotherapy and immunotherapy are typically used to treat tumors that are resistant to surgery or radiation therapy or those that appear with metastases, surgery and radiotherapy are largely used to treat early-stage, localized, nonmetastatic malignancies [7,8]. Despite having a high level of effectiveness in killing cancer cells, cancer chemotherapy lacks accuracy, which causes drug-induced damage in noncancerous tissues [9]. Patients undergoing treatment regimens with such nonspecific hazardous substances frequently have severe adverse effects and inadequate drug concentrations reaching the tumor site [10]. Recently, nanomedicine has attracted a lot of attention because it has the ability to solve some of the drawbacks of conventional medicinal approaches, providing opportunities for increased efficacy and reduced side effects. The physical and chemical characteristics of nanomedicine, which typically has diameters between 1 and 100 nanometers, can be tailored to improve interactions with biological systems [11]. Nanomedicine is the use of nanotechnology in medicine for the diagnosis and treatment of illnesses. Over 200 nanomedicines have been licensed or are undergoing clinical trials as a result of the significant impact that progress in the sector has had on healthcare [12]. The concept of nanotechnology was first introduced by Richard Feynman in his visionary speech at Caltech in 1959 [13]. Nanomedicines involve a range of many different inorganic, polymeric, and metallic nanostructures, such as carbon nanotubes, dendrimers, micelles, solid lipid nanoparticles (SLNs), and liposomes, which are widely utilized as targeted and controlled drug delivery vehicles [14,15,16,17,18,19,20,21,22] (Figure 1).

Figure 1. Various types of nanocarriers are used in cancer therapy. Adapted with permission from reference [23].

Figure 1. Various types of nanocarriers are used in cancer therapy. Adapted with permission from reference [23].

This review explores the overexpressed receptors on tumor cells that researchers have investigated as possible targets for nanocarrier systems, emphasizing the broad array of nanocarrier systems and targeting ligands that have been explored. With a special focus on clinical advances, the discussion covers a wide range of therapeutic modalities, including small compounds, aptamers, peptides, antibodies, and cell-based targeting techniques. Although there has been progress, there are still impediments in the clinical translation of targeting techniques based on nanocarriers, which calls for a thorough comprehension of the number of variables involved. This article’s conclusion outlines future prospects for research on therapeutically transferable cancer-targeted therapy with the dual objective of reducing toxicity and maximizing treatment efficacy. Overall, the investigation of nanomedicine in cancer treatment is an exciting prospect for future oncology research, providing an all-encompassing plan.

The study of cancer nanomedicine has grown significantly during the past three decades [24]. Cancer nanomedicine is anticipated to transform chemotherapy by providing a variety of payloads with favorable pharmacokinetics and leverage molecular targeting for improved specificity, efficacy, and consequently safety. Nanomaterials with sizes under 100 nm complement the pore sizes in the comparatively leaky tumor vascular endothelium, and poor lymphatic dysfunction in tumors results in poor nanomaterial clearance, allowing for enhanced permeation and retention (EPR) of nanocarriers inside tumors [25]. The EPR effect provides a means for passive tumor targeting that has been viewed as an effective mechanism for the accumulation of nanocarriers inside the tumors [26].

Passive targeting localizes nanocarriers at the target site but does not induce uptake in the tumor microenvironment [27]. The EPR effect is a characteristic of blood vessels; thus, it might vary based on the pathological features and conditions of the patient or tumor (Figure 2A). In particular, tumors with fewer blood vessels, like pancreatic cancer, consistently have a lower EPR effect. Even within a single tumor, the EPR effect can be varied [28]. Alternative targeting approaches, such as tumor vasculature targeting, cell-mediated tumor targeting, iRGD-facilitated transendothelial extravasation, and tumor penetration, as well as locoregional delivery, have all been proposed to improve the accumulation of nanomedicines in low-EPR tumors [29]. Active cellular targeting by attaching ligands to the surface of nanocarriers to improve retention in the target region and uptake by target cells can also be made possible (Figure 2B). These ligands have been selected to bind to receptors that are overexpressed or grouped together on the cell surfaces of tumor tissues such as HER2 (human epidermal growth factor receptor 2) and folate receptors [30]. Surface receptors that support the growth and survival of cancer cells are often upregulated in cancer. These receptors are reliable targets for treatment [31]. To target tumor cells, nanocarriers are typically functionalized with targeting molecules like antibodies and antibody fragments, nucleic acid aptamers, peptides, carbohydrates, and small molecules. These molecules can bind specifically to tumor-specific antigens or receptors expressed on the plasma membrane, which promotes cellular uptake of the conjugated nanocarriers [32]. Figure 2B represents the receptor-mediated targeting in tumor cells.

2. Receptors in Cancer

Cancer often involves intricate molecular interactions that drive aberrant cellular behaviors. Among the key players in these processes are receptors, and understanding the roles of receptors, such as the Epidermal Growth Factor Receptor (EGFR), Folate Receptor, Transferrin Receptor, Integrins, Mucin-1 (MUC1), CD44, Hormone Receptors, and Programmed Death-1 Receptor (PD-1), is paramount in unraveling the mechanisms underpinning cancer development (Table 1). Dysregulation of these receptors can lead to uncontrolled cell growth, survival, and evasion of the immune system. The following section delves into the structural intricacies and functions of these receptors, emphasizing the importance of comprehending their roles in cancer biology. Such understanding serves as a foundation for the development of targeted therapies that hold promise for more effective and personalized cancer treatments.

2.1. Epidermal Growth Factor Receptor

Epidermal Growth Factor Receptor (EGFR), also known as ErbB-1, is a member of the ErbB family of receptor tyrosine kinases. Structurally, EGFR is a transmembrane protein characterized by an extracellular ligand-binding domain, a transmembrane domain, and an intracellular tyrosine kinase domain [33]. Ligands such as epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-α) interact with the extracellular domain, inducing conformational changes that activate the intracellular kinase domain [34]. EGFR can exist as a monomer on the cell surface, but upon ligand binding, it undergoes a conformational change that facilitates the formation of homo- or heterodimers. This dimerization is critical for activating the intrinsic kinase activity of EGFR, leading to the autophosphorylation of specific tyrosine residues [35]. The subsequent EGFR signaling cascades involve multiple pathways, including the Ras/Raf/MAPK pathway, PI3K/Akt pathway, and JAK/STAT pathway [36]. These pathways collectively govern cellular processes crucial for normal development but can become dysregulated in cancer, resulting in uncontrolled cell growth and survival.

In normal tissues, EGFR is expressed in a regulated manner, contributing to physiological processes such as tissue homeostasis, repair, and cell cycle progression from G1 to S phase. EGFR signaling is also involved in promoting cell survival by activating antiapoptotic pathways. This mechanism helps cells resist programmed cell death, a hallmark of cancer [37]. Furthermore, EGFR signaling influences cell differentiation, which is crucial in developing and maintaining various tissues. Additionally, it contributes to angiogenesis, a process vital for supplying nutrients to rapidly growing cancer cells [38].

Dysregulation of EGFR signaling is commonly associated with various cancers, with multiple mechanisms contributing to its oncogenic role. These mechanisms include mutation, overexpression, amplification, and promoting apoptosis resistance. Gene amplification, a prominent cause, involves an increased copy number of the EGFR gene, often resulting from chromosomal alterations such as duplications or aneuploidy. This leads to an elevated abundance of EGFR receptors on the cell surface [39]. Additionally, epigenetic modifications, such as promoter hypermethylation, contribute to the continuous transcription and subsequent overexpression of EGFR [40]. Mutations in the EGFR gene, particularly within the tyrosine kinase domain, can lead to aberrant activation of the receptor independent of ligand binding, contributing to tumorigenesis [41]. Point mutations, exon deletions (e.g., exon 19), and insertions (e.g., exon 20) are common alterations observed in the EGFR gene [42,43]. Importantly, the presence of EGFR mutations is not uniform within a tumor, contributing to intratumoral heterogeneity [44].

EGFR gene amplification involves the replication and overabundance of EGFR copies within the cancer cell genome. Mechanisms contributing to this genomic amplification include chromosomal duplications, unequal crossing-over events during DNA replication, and genomic instability [45]. The well-established association between EGFR amplification and oncogenic phenotypes includes enhanced tumorigenic potential, increased resistance to apoptosis, and a higher propensity for metastasis [46]. The intricate molecular interplay between EGFR signaling and apoptosis resistance emerges as a hallmark feature in various cancers. EGFR activation triggers the upregulation of anti-apoptotic proteins, such as Bcl-2 and Survivin, inhibiting key elements of the apoptotic machinery and creating an environment conducive to cell survival, even in the face of cellular stress or damage [47,48].

2.2. Folate Receptor

Folate receptors (FR) are integral proteins facilitating the transport of folate (vitamin B9) across cell membranes, a pivotal process in diverse biological functions, including DNA synthesis, repair, and methylation [49,50,51]. As humans cannot synthesize folate, dietary intake is essential. FR, belonging to the SLC19 family, particularly the superfamily of solute carriers, prominently features FR alpha, a well-studied high-affinity variant. This glycosylphosphatidylinositol (GPI)-anchored protein encompasses an extracellular domain for folate binding, a transmembrane domain, and a brief cytoplasmic tail [52].

The chief role of folate receptors is to facilitate cellular folate uptake, achieved through receptor-mediated endocytosis upon folate binding to the extracellular domain. Subsequently, intracellular folate release contributes to various cellular processes, particularly nucleotide synthesis and methylation reactions [53]. There are three isoforms of FR: FR alpha, FR beta, and FR gamma, which display distinct properties and tissue distribution. FR alpha is extensively studied, and prevalent in tissues like kidneys, lungs, and placenta, while FR beta primarily manifests in the placenta, and FR gamma associates with hematopoietic cells, notably in the bone marrow [54]. Folate’s role in one-carbon metabolism, providing methyl groups for methylation reactions across DNA, RNA, proteins, and lipids, is pivotal [55,56]. During pregnancy, folate’s crucial role in neural tube formation and fetal development is emphasized [57]. Proper folate receptor functioning is essential for neurotransmitter syntheses like serotonin, dopamine, and norepinephrine—critical for mood regulation and cognitive function [58].

Cancer often exhibits upregulated folate receptors, a phenomenon linked to heightened folate demand during rapid cell division. Metabolic reprogramming in cancer involves changes in folate metabolism to sustain nucleotide synthesis demand [59]. Folate receptor upregulation adapts to this metabolic shift, ensuring efficient extracellular folate uptake. Nutrient and oxygen deprivation in the tumor microenvironment trigger folate receptor upregulation, enhancing folate capture. This upregulation is associated with metastatic potential, providing a selective advantage in nutrient uptake for distant site survival [60].

The impact of folate receptors in cancer extends beyond folate uptake, involving intricate interactions with molecular pathways. Folate receptors influence Wnt/β-catenin signaling, affecting cell proliferation and differentiation [61]. Studies suggest that folate receptor expression modulates β-catenin stability and activity, influencing downstream signaling [62]. Cross-talk with the PI3K/Akt/mTOR pathway links folate metabolism to cellular processes such as survival, growth, and metabolism. Folate-receptor-mediated activation or suppression of Notch signaling influences cell fate, differentiation, and the maintenance of cancer stem cells [63].

2.3. Transferrin Receptor

Transferrin receptors (TfRs), encoded by TFRC, are integral membrane glycoproteins composed of α and β subunits, forming a homodimer with a molecular weight of 180 kDa. They play a pivotal role in cellular iron homeostasis, an essential element for processes like DNA synthesis, energy production, and cell proliferation [64]. To fulfill iron requirements, cells express TfRs, facilitating the uptake of iron-bound transferrin, the primary carrier of iron in the blood. The extracellular domain binds to transferrin, the carrier protein for iron [65].

In humans, two main isoforms, transferrin receptor 1 (TfR1) and transferrin receptor 2 (TfR2), are expressed at higher levels in rapidly dividing cells due to increased iron demand. These receptors have a transferrin-binding C-terminal domain, a short N-terminal domain, and a single transmembrane domain with transferrin-binding sites, enabling receptor-mediated endocytosis for iron internalization [66]. TfR1, abundant in rapidly dividing cells, is expressed in most tissues, particularly in the bone marrow and developing embryos. TfR2, primarily found in the liver, contributes to systemic iron homeostasis, and mutations are associated with hereditary hemochromatosis [67].

In malignancies, transferrin receptors are frequently overexpressed, promoting uncontrolled growth and survival of cancer cells. Various cancers exhibit distinct TfR expression patterns, reflecting its role in cancer biology heterogeneity [68]. Factors influencing transferrin receptor expression in cancer cells include hypoxia, which upregulates TfR expression via hypoxia-inducible factors (HIFs), and aberrant signaling pathways like PI3K/Akt/mTOR [69,70]. Transferrin receptors play a dual role in cancer progression by meeting increased iron demand for elevated metabolic activity and promoting cell survival and proliferation through modulating processes like DNA synthesis and repair. Upregulation of transferrin receptors is often associated with poor prognosis in cancer patients [71].

Among the isoforms, TfR1 mechanisms are extensively studied. Tyrosine phosphorylation at position 20 by Src enhances antiapoptotic pathways, supporting breast cancer cell survival [72]. TfR1, acting as a mitochondrial regulator, activates the JNK signaling pathway and induces Reactive Oxygen Species (ROS) production, as observed in pancreatic ductal adenocarcinoma (PDAC) [73]. In breast cancer, iron regulatory protein 2 (IRP2) influences TfR1 expression transcriptionally. Sphingosine kinase 1 and mTOR contribute to TfR1 modulation, linking lipid metabolism, iron homeostasis, and inflammation. TfR1-induced oxidant accumulation alters cellular signaling, affecting cell cycle regulators and promoting entry into the S phase [74,75]. Notably, c-Myc, a key player in cancer, directly activates TfR1 expression, establishing TfR1 as a critical downstream target [76].

2.4. Integrins

Integrins, a pivotal family of cell adhesion receptors, are integral in governing essential cellular processes for normal physiological functions. Functioning as transmembrane proteins, they serve as vital links between cells and their extracellular environment, primarily engaging with components of the extracellular matrix (ECM) [77]. Comprising alpha and beta subunits, integrins exhibit heterodimeric structures, and their classification is rooted in these subunits, yielding a diverse array of receptors, each possessing unique ligand specificity. This diversity enables interactions with a broad spectrum of ECM molecules, influencing diverse cellular behaviors and fates [78].

Upon ligand binding, integrins initiate intracellular signaling cascades regulating critical aspects of cell behavior, including adhesion, migration, survival, and proliferation [79]. Key players in this intricate signaling network encompass focal adhesion kinase (FAK), mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K), among others [80,81]. These pathways intricately intersect with other signaling networks, creating a complex web crucial for cellular responses. Fundamental to integrins is their mediation of cell adhesion to the ECM, a process vital for maintaining tissue integrity. Additionally, they link the ECM to the actin cytoskeleton by recruiting proteins like Talin, Paxillin, α-Actinin, Tensin, and Vinculin [82]. Furthermore, a ternary complex involving integrin-linked kinase, PINCH (LIMS1), and Parvin regulates crucial scaffolding and signaling functions influencing integrin-mediated effects on cell migration and survival [83].

In the context of tumor progression, integrins serve as a nexus connecting cancer cells with their microenvironment. Epithelial-derived tumors particularly exhibit retained expression of specific integrins, such as a6β4, a6β1, αvβ5, a2β1, and a3β1, introducing dynamic alterations in migration, proliferation, and survival of malignant cells [84]. Notably, integrin expression displays significant variability between normal and tumor tissues. Integrins like αvβ3, a5β1, and αvβ6, typically expressed at low levels in adult epithelia, can be markedly upregulated in certain tumors. Established correlations between integrin expression and disease progression identify αvβ3, αvβ5, a5β1, a6β4, a4β1, and αvβ6 as pivotal players in cancer advancement across various tumor types [85].

Operating as versatile sensors, integrins respond to environmental cues to enhance cell survival or initiate apoptosis. Ligand-bound integrins transmit signals favoring cell survival through pathways like PI3K-Akt and NF-κB, while unligated integrins may trigger proapoptotic cascades [86]. These survival-enhancing effects extend to cancer stem cells, where integrin signaling is implicated in maintaining this highly tumorigenic subset. Moreover, integrins collaborate with oncogenes and receptor tyrosine kinases, amplifying tumorigenesis and showcasing a nuanced interplay in the molecular orchestra of cancer [87].

Angiogenesis, a hallmark of tumor progression, is profoundly influenced by integrins. Critical integrins, including αvβ3, αvβ5, α1β1, α2β1, α4β1, α5β1, α6β1, α9β1, and α6β4, drive angiogenesis by interacting with growth factor receptors [88]. This integration of cues from the ECM and growth factors orchestrates specific intracellular signaling events. Perivascular cells and desmoplasia underscore integrin-mediated regulation of tumor vasculature and the ECM. Integrins not only contribute to vessel maturation but also play a role in recruiting bone-marrow-derived cells, immune cells, and platelets to tumors, influencing both tumor suppression and angiogenesis [89,90].

2.5. Mucin-1

MUC1, a crucial macromolecular protein in the mucin family, plays a pivotal role in maintaining the integrity of epithelial tissues by forming a protective mesh. In normal physiological conditions, MUC1 acts as a surface cover, creating a tight mesh that serves as a robust barrier, protecting epithelial cells from adverse environmental conditions. This glycosylated transmembrane protein is a frontline defender, establishing a mucosal surface barrier [91]. However, in the context of cancer, MUC1 extends beyond its structural role, assuming intracellular signaling functions and emerging as a significant player in cancer development. Its frequent overexpression in epithelial adenocarcinomas establishes MUC1 as a prominent oncogene, exerting regulatory influence over critical cancer processes such as growth, proliferation, metastasis, apoptosis, and developmental processes [92]. As a member of the transmembrane mucin family, MUC1 undergoes structural alterations in cancer, including overexpression, loss of polarity, and modifications in carbohydrate side chains. The extracellular N-terminus of MUC1 contains highly conserved tandem repeats, extensively modified by O-linked glycans, serving as an effective cell barrier. The C-terminus, crucial in signal transduction, houses tyrosine residues and potential docking sites for kinases, influencing multiple signaling pathways associated with cancer progression [93].

In healthy tissues, MUC1 acts as a versatile guardian, functioning as a physical barrier through its extracellular SEA domain. It protects the apical cell membrane from rupture, immune attack, and harmful environments. MUC1 inhibits immune responses, contributes to lubrication and surface hydration, and collaborates with other mucins, as seen in the normal oral mucosal epithelium [94]. MUC1 acts as a proinflammatory agent in tumors, orchestrating interactions with immune cells and fostering inflammation in the tumor microenvironment. It serves as an immunomodulatory switch, influencing the inflammatory response in infection-induced cancers. However, in its proinflammatory role, MUC1 facilitates the recruitment of inflammatory cells, promoting tumor immune escape. Overexpression of MUC1 hampers the efficacy of chemotherapeutic agents, reducing drug uptake and contributing to resistance [95].

MUC1 actively promotes the migration and invasion of cancer cells through intricate regulatory mechanisms, inducing epithelial–mesenchymal transition, a crucial process for metastasis. It interacts with factors such as TGF-β, KL-6/MUC1 glycosylation, and PDGF-A, thereby enhancing invasive capabilities [96]. MUC1’s expression correlates with VEGF in breast cancer, promoting angiogenesis by activating intracellular signaling pathways such as Ras/MAPK, JAK/STAT, and PI3K/Akt/mTOR [97]. Additionally, MUC1 interacts with apoptotic pathways, inhibiting cisplatin-induced apoptosis in colon cancer cells through JNK1 activation [98]. The intricate and context-dependent functions of MUC1 in cancer tissues underscore its significance as a central player in the complex biology of cancer. A comprehensive understanding of these functions not only sheds light on the underlying mechanisms of cancer development but also opens avenues for innovative therapeutic interventions and personalized treatment strategies [99].

2.6. CD44

CD44, a transmembrane glycoprotein encoded by a single gene on chromosome 11p13, is recognized as a multifunctional cell surface receptor. Also referred to as P-glycoprotein 1, CD44 exhibits a molecular weight spanning 85 to 200 kDa and is pivotal in diverse physiological processes, holding significance in both normal and pathological conditions. The structural diversity of CD44 is a consequence of alternative splicing of its mRNA [100]. The standard isoform, sCD44, with a molecular weight of approximately 85-90 kDa, comprises exons 1–5 and 16–20 [101]. Variant isoforms, collectively termed CD44v, result from inserting alternative exons (exons 6–15) at specific sites within the extracellular domain. The expression of distinct CD44 isoforms is crucial in the progression of various human tumors, with patterns varying across different cancer types. Notably, the transition from standard CD44 to variant isoforms has been associated with heightened survival and adhesion, particularly in cancers like prostate cancer [102].

CD44 is ubiquitously expressed in normal adult and fetal tissues, having been initially identified in hematopoietic cells. While the standard CD44 isoform is widely distributed, variant isoforms exhibit a more restricted expression, often found in specific epithelial cells. CD44 plays a vital role in regulating hyaluronic metabolism, lymphocyte activation, and cytokine release in normal tissues [103]. Loss of CD44 function disrupts critical processes such as wound healing and keratinocyte proliferation [104]. In the context of cancer, CD44 expression is implicated in tumor progression, with specific isoforms, such as CD44v6, serving as markers in colorectal cancer prognosis [105].

The functionality of CD44 is intricately linked to its interaction with various ligands, including hyaluronic acid (HA), osteopontin (OPN), collagens, and matrix metalloproteinases (MMPs). HA binds to CD44v isoforms, activating cytoskeleton and MMP signaling involved in tumor progression [106,107]. The size of the HA ligand is critical for its biological function, with high molecular weight HA involved in tumorigenesis and low molecular weight HA promoting cell motility and angiogenesis [108]. CD44-OPN interaction induces cell migration, promoting cell spreading, motility, and chemotactic behavior [109]. CD44 associates with MMPs, particularly MMP-9, crucial in invading prostate cancer cells derived from bone metastases. CD44’s ability to localize proteolytically active MMP-9 to the tumor cell surface is pivotal in tumor invasion [110].

CD44 plays a central role in the complex landscape of cancer progression, influencing migration, invasion, angiogenesis, and bone metastasis [111]. CD44 receptors integrate adhesive and signaling activities, regulating migration and invasion during cancer progression. Interactions with MMP9 contribute to the secretion of active MMP9, facilitating migration and invasion [112]. CD44 variants, particularly CD44v6 and CD44v3, exhibit distinct roles with implications for cancer stage and disease progression. Disruption of CD44/MMP9 interaction impedes migration and invasion, underscoring its critical role [113].

CD44 expression on endothelial cells is a key regulator of angiogenesis, essential for tumor cell dissemination and migration to distant organs. CD44 variants bind to growth factors, including VEGF, linking CD44 to angiogenic processes [114]. The interaction of CD44 with immobilized HA enhances its role in tumor angiogenesis, contributing to the efficiency of distant metastasis to bone [115]. CD44 is considered a marker for cancer stem cells, influencing the likelihood of bone metastases. CD44 signaling in prostate cancer cells regulates key proteins, such as RANKL and MMP9, involved in osteoclast differentiation and tumor metastasis [116].

2.7. Hormone Receptors

Hormone receptors, specialized proteins integral to cellular signaling, exhibit a distinctive presence either on cell surfaces or within intracellular domains. Serving as molecular switches, these receptors orchestrate the transduction of hormone signals into the cellular interior, thereby modulating gene expression and influencing overarching cellular functions [117]. Key protagonists in hormonal regulation are estrogen, progesterone, androgen, and glucocorticoid receptors, each wielding discrete regulatory roles across various physiological processes. Within the nuclear receptor superfamily, estrogen receptors (ERs) comprise two primary subtypes: ERα and ERβ. Evident in tissues such as the uterus, breast, bone, and brain, ERs mediate estrogenic effects on gene expression, thereby exercising considerable influence over cellular growth and differentiation [118,119]. In the context of ER-positive breast cancer, sustained estrogen stimulation through ERs propels cancer cell proliferation, precipitating tumor initiation and progression. Dysregulated ER signaling not only instigates genomic instability, a hallmark of cancer but also instigates an upregulation of anti-apoptotic proteins, thereby fostering the survival and persistence of malignant cells [120].

Progesterone receptors (PRs), constituents of the nuclear receptor superfamily, encompass two distinct isoforms, PR-A and PR-B. Crucial in orchestrating the menstrual cycle, preparing the uterus for pregnancy, and maintaining gestation, PRs manifest in tissues, including the uterus, mammary glands, and reproductive tissues [121]. Aberrant progesterone signaling can engender uncontrolled cellular proliferation, contributing to hormone-induced tumorigenesis in select reproductive tissues. The intricate interplay between ER and PR signaling pathways synergistically augments cellular proliferation in hormone-responsive cancers [122,123].

Androgen receptors (ARs), members of the steroid hormone receptor family, govern the effects of androgens, prominently testosterone. Prevalent in the testes, prostate, muscle, and various other tissues, AR activation propels the progression of prostate cancer, particularly in its early stages [124]. Notably, resistance to androgen deprivation therapy (ADT) in advanced prostate cancer may ensue, facilitated by mutations in the AR gene, culminating in ligand-independent activation [125]. The intricate crosstalk between AR signaling and other pathways perpetuates cell survival and proliferation, even without androgens [126]. Also belonging to the nuclear receptor superfamily, glucocorticoid receptors (GRs) feature a ligand-binding domain, DNA-binding domain, and activation function domains. They mediate the effects of glucocorticoids, exemplified by cortisol, thereby regulating diverse physiological processes, including metabolism, immune response, and stress response [127]. Widely distributed across virtually all bodily tissues, GR expression is subject to circadian rhythms and stressors. While glucocorticoids induce apoptosis in lymphoid cells, rendering them efficacious in treating hematological malignancies, their immune-suppressive effects in certain cancers contribute to tumor evasion from the host immune system. Depending on the cellular milieu, glucocorticoids may engender prosurvival effects by delicately regulating the equilibrium between apoptosis and cell proliferation [128].

A nuanced comprehension of how estrogen, progesterone, androgen, and glucocorticoid receptors intricately contribute to cancer etiology and progression is paramount for delineating targeted therapeutic interventions. Therapies aimed at perturbing these intricate signaling cascades constitute indispensable components of cancer treatment strategies, particularly in instances of hormone-responsive cancers wherein receptor signaling assumes a central role in dictating tumor growth and viability.

2.8. Programmed Death-1 Receptor

Programmed Death-1 (PD-1) receptors, an essential facet of the immune system, were unveiled in the 1990s. They wield a pivotal influence in orchestrating immune responses via the intricate immune checkpoint pathway, meticulously regulating immune activity to forestall hyperactivation and uphold self-tolerance [129]. Classified within the CD28/CTLA-4 family, PD-1 receptors predominantly grace the surfaces of T cells, B cells, and natural killer (NK) cells, constituting indispensable operatives in both adaptive and innate immune responses [130]. The structural architecture of PD-1 receptors encompasses an extracellular region, a transmembrane domain, and a cytoplasmic tail. The extracellular domain features an immunoglobulin variable (IgV)-like domain, an elemental unit in the ligand-binding process. Meanwhile, the cytoplasmic tail harbors immunoreceptor tyrosine-based inhibitory motifs and immunoreceptor tyrosine-based switch motifs, instrumental in orchestrating the inhibitory signaling cascade instigated by PD-1 [131].

Functionally characterized as immune checkpoints, PD-1 receptors operate as modulators of T-cell activation. The binding of PD-1 to its cognate ligands, PD-L1 or PD-L2, initiates a downstream signaling cascade that effectively dampens T-cell activation and curtails cytokine production [132]. This meticulously orchestrated mechanism serves to mitigate the risk of unrestrained immune responses, ensuring the preservation of self-tolerance and precluding the onset of autoimmune reactions. Of the two known ligands, PD-L1 and PD-L2, the former is ubiquitously expressed in various cell types, including the notorious cohort of cancer cells. In contrast, the latter is selectively expressed on specific immune cells [133]. PD-1 receptors, in turn, become activated on T cells, B cells, and myeloid cells. The intensity and duration of immune responses correlate intricately with the induced expression levels of PD-1 receptors, particularly post-T-cell activation [134].

In the context of malignancy, the role of PD-1 receptors becomes a linchpin in the immune system’s capacity to discern and eliminate aberrant cells, notably cancer cells. The PD-1 pathway, however, becomes a double-edged sword as neoplastic entities adeptly exploit it for immune evasion. Cancer cells, including those residing within the tumor microenvironment, exhibit heightened expression of PD-L1, a pivotal ligand for PD-1 [135]. This orchestrated engagement of PD-1 with its ligands sets off a cascade of inhibitory signals within T cells, leading to the suppression of their activation and effector functions [136]. Prolonged exposure to cancer cells expressing PD-L1 manifests in T cell exhaustion and the recruitment of immunosuppressive cells, ultimately fostering an immunosuppressive microenvironment conducive to unabated tumor growth [137,138]. This intricate interplay culminates in the escape of cancer cells from immune surveillance, thereby promoting metastasis and endowing resistance to immune-mediated destruction [139].

2.9. Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are transmembrane allosteric protein that mediates the transduction of chemoelectric signals [140]. These are homo- or heteropentameric ligand-gated ion channels present in the central nervous system, peripheral nervous system, and neuromuscular junctions. nAChRs are also found in mammalian sperm [141]. nAChRs receptors are also considered targets during the treatment of a variety of medical conditions. Several subunits of these receptors have been identified, such as α, β, γ, δ, and ϵ, which share 35–50% homology [142]. Table 1 summarizes receptor signaling in cancer.

Table 1. Receptor signaling in cancer: Key roles, dysregulation mechanisms, and contributions to tumorigenesis.

Receptor	Key Role in Cancer	Dysregulation Mechanisms	Contribution to Tumorigenesis	Predominant/Prevalent Cancer Types	Reference
Epidermal Growth Factor Receptor	Triggers uncontrolled cell growth via Ras/Raf/MAPK, PI3K/Akt, and JAK/STAT pathways.	Involves mutations (point mutations, exon deletions, insertions), overexpression, and amplification.	Activation, even without ligands, due to mutations is a hallmark in various cancers, driving tumorigenesis.	Non-Small Cell Lung Cancer Colorectal Cancer Head and Neck Cancer	[143,144,145]
Folate Receptor	Upregulated in cancer, modulates Wnt/β-catenin, PI3K/Akt/mTOR, and Notch pathways.	Upregulation adapts to metabolic shifts, ensuring efficient extracellular folate uptake.	Enhances angiogenesis, providing a selective advantage in nutrient uptake for distant site survival and metastasis.	Ovarian Cancer Adenocarcinoma and Squamous Cell Carcinoma of the Lung	[146,147]
Transferrin Receptor	Overexpressed in cancers, meets iron demand, and influences DNA synthesis.	Hypoxia-induced upregulation via hypoxia-inducible factors (HIFs), a dual role in cancer progression.	Upregulation is linked to the tumor microenvironment, influencing processes like DNA synthesis and repair.	Breast Cancer Leukemia (specific types)	[148,149]
Integrins	Pivotal in cell adhesion, impacts migration, survival, and more.	Altered expression influences cancer cell behavior, including changes in migration, proliferation, and survival.	Drive angiogenesis by interacting with growth factor receptors, impacting tumor vasculature and angiogenic processes.	Breast Cancer Prostate Cancer Melanoma	[150,151,152]
Mucin-1	Acts as a guardian in healthy tissues and becomes an oncogene.	Serves as a proinflammatory agent, regulates apoptosis, and inhibits the immune response.	Influences cancer cell survival, migration, and chemoresistance; facilitates recruitment of inflammatory cells in the tumor microenvironment.	Breast Cancer Pancreatic Cancer Ovarian Cancer	[153,154]
CD44	Multifunctional receptor in physiological processes and cancer.	Impacts migration, invasion, and angiogenesis through interactions with ligands (HA, OPN, MMPs).	Considered a marker for cancer stem cells, regulates key proteins involved in osteoclast differentiation and tumor metastasis.	Breast Cancer Prostate Cancer Pancreatic Cancer	[155,156,157]
Hormone Receptors	Distinct roles in hormone-responsive cancers.	The interplay between estrogen and progesterone receptors synergistically augments cellular proliferation.	Androgen receptor activation propels prostate cancer progression; mutations may lead to resistance to androgen deprivation therapy.	Breast Cancer (Estrogen, Progesterone) Prostate Cancer (Androgen)	[158,159]
PD-1 Receptor	Operates as a modulator of T-cell activation, and immune checkpoint.	Engagement by cancer cells initiates inhibitory signals, fostering an immunosuppressive microenvironment.	Prolonged engagement leads to T cell exhaustion, contributing to unabated tumor growth, metastasis, and resistance to immune-mediated destruction.	Melanoma Non-Small-Cell Lung Cancer Non-Hodgkin Lymphoma	[160,161,162]

Table 1. Receptor signaling in cancer: Key roles, dysregulation mechanisms, and contributions to tumorigenesis.

Receptor	Key Role in Cancer	Dysregulation Mechanisms	Contribution to Tumorigenesis	Predominant/Prevalent Cancer Types	Reference
Epidermal Growth Factor Receptor	Triggers uncontrolled cell growth via Ras/Raf/MAPK, PI3K/Akt, and JAK/STAT pathways.	Involves mutations (point mutations, exon deletions, insertions), overexpression, and amplification.	Activation, even without ligands, due to mutations is a hallmark in various cancers, driving tumorigenesis.	Non-Small Cell Lung Cancer Colorectal Cancer Head and Neck Cancer	[143,144,145]
Folate Receptor	Upregulated in cancer, modulates Wnt/β-catenin, PI3K/Akt/mTOR, and Notch pathways.	Upregulation adapts to metabolic shifts, ensuring efficient extracellular folate uptake.	Enhances angiogenesis, providing a selective advantage in nutrient uptake for distant site survival and metastasis.	Ovarian Cancer Adenocarcinoma and Squamous Cell Carcinoma of the Lung	[146,147]
Transferrin Receptor	Overexpressed in cancers, meets iron demand, and influences DNA synthesis.	Hypoxia-induced upregulation via hypoxia-inducible factors (HIFs), a dual role in cancer progression.	Upregulation is linked to the tumor microenvironment, influencing processes like DNA synthesis and repair.	Breast Cancer Leukemia (specific types)	[148,149]
Integrins	Pivotal in cell adhesion, impacts migration, survival, and more.	Altered expression influences cancer cell behavior, including changes in migration, proliferation, and survival.	Drive angiogenesis by interacting with growth factor receptors, impacting tumor vasculature and angiogenic processes.	Breast Cancer Prostate Cancer Melanoma	[150,151,152]
Mucin-1	Acts as a guardian in healthy tissues and becomes an oncogene.	Serves as a proinflammatory agent, regulates apoptosis, and inhibits the immune response.	Influences cancer cell survival, migration, and chemoresistance; facilitates recruitment of inflammatory cells in the tumor microenvironment.	Breast Cancer Pancreatic Cancer Ovarian Cancer	[153,154]
CD44	Multifunctional receptor in physiological processes and cancer.	Impacts migration, invasion, and angiogenesis through interactions with ligands (HA, OPN, MMPs).	Considered a marker for cancer stem cells, regulates key proteins involved in osteoclast differentiation and tumor metastasis.	Breast Cancer Prostate Cancer Pancreatic Cancer	[155,156,157]
Hormone Receptors	Distinct roles in hormone-responsive cancers.	The interplay between estrogen and progesterone receptors synergistically augments cellular proliferation.	Androgen receptor activation propels prostate cancer progression; mutations may lead to resistance to androgen deprivation therapy.	Breast Cancer (Estrogen, Progesterone) Prostate Cancer (Androgen)	[158,159]
PD-1 Receptor	Operates as a modulator of T-cell activation, and immune checkpoint.	Engagement by cancer cells initiates inhibitory signals, fostering an immunosuppressive microenvironment.	Prolonged engagement leads to T cell exhaustion, contributing to unabated tumor growth, metastasis, and resistance to immune-mediated destruction.	Melanoma Non-Small-Cell Lung Cancer Non-Hodgkin Lymphoma	[160,161,162]

3. Common Nanocarrier Types

3.1. Lipidic

3.1.1. Liposomes

Liposomes, which are artificially prepared vesicles, can be used as drug delivery systems in various disease conditions [163,164,165,166,167,168,169]. One of the advantages of using liposomes is that they can improve the pharmacokinetic and pharmacodynamic properties of drugs, thereby enhancing their efficacy and reducing their toxicity. Additionally, the functionalization of liposomes with antibodies for overexpressed receptors on tumor surfaces can selectively target cancer cells and deliver drugs directly to tumor sites, which can reduce the side effects of chemotherapy and improve the quality of life for cancer patients. Researchers have presented a promising approach to improving the efficacy of anticancer drugs, paclitaxel (PTX) and rapamycin, using immunoliposomes, the surface decorated with an anti-HER2 monoclonal antibody, Trastuzumab. They utilized liposomes as a delivery vehicle to target HER2-positive breast cancer cells and coloaded them with PTX and RAP to inhibit cancer cell growth and suppress mTOR activity. The study employed various techniques to characterize the immunoliposomes, including drug loading efficiency, cytotoxicity, and in vivo antitumor efficacy. The results show the potential of this approach for synergistic anticancer therapy and imaging of HER2-positive breast cancer [170].

Another approach to improving the delivery and efficacy of chemotherapy drugs is the use of immunoliposomes coated with an anti-MUC-1 antibody. Researchers encapsulated the drug doxorubicin (DOX) inside liposomes and targeted them to MUC-1-positive breast cancer cells. The immunoliposomes had high specificity for cancer cells and demonstrated enhanced anticancer effects due to the delivery of DOX [171] The researchers suggest that their immunoliposomes could be further optimized for improved anticancer effects and could be combined with other therapeutic modalities. Finally, cationic liposomes (CTL) have been used as carriers for two drugs, PTX and crizotinib (CRI), and a small interfering RNA (siRNA) targeting Bcl-xL, a protein that promotes cancer cell survival. The CTL had enhanced anticancer effects due to the co-delivery of PTX, CRI, and siRNA, and could be further optimized for improved therapeutic outcomes [172].

3.1.2. Solid Lipid Nanoparticle

Solid lipid nanoparticles (SLNs) are nanospheres made of solid lipids and stabilized by surfactants that prevent particle aggregation and coalescence [173]. SLNs have a photon correlation spectroscopy (PCS) size ranging from 50 to 1000 nm, depending on the formulation parameters and production method [174]. The solid lipid matrix of SLNs can solubilize hydrophilic or lipophilic drugs, and the particles can be modified to achieve specific properties such as stealth, targeting, or mucoadhesion [175]. Various techniques can be used to prepare SLNs, including high-pressure homogenization, microemulsion, high-speed stirring and ultrasonication methods, microemulsion method, solvent emulsification–diffusion method, solvent emulsification–evaporation, solvent injection, double emulsion method, phase inversion temperature method, coacervation method, and supercritical fluid-based methods [176]. SLNs offer several advantages over other colloidal carriers, such as biodegradability, biocompatibility, physical stability, protection of labile drugs, and the ability for large-scale production. SLNs can be administered through various routes, including oral, parenteral, topical, ocular, nasal, and pulmonary [177]. Researchers have recently developed new drug delivery systems for cancer therapy using SLNs. One of these systems uses hyaluronic acid (HA)-coated SLNs to deliver vorinostat (VRS) to CD44 overexpressing cancer cells. The HA coating enhances the cellular uptake of the nanoparticles, and SLNs protect VRS from degradation. The HA-VRS-SLNs exhibit pH-sensitive drug release behavior, enhanced cellular uptake, cytotoxicity, and apoptosis induction in CD44 overexpressing cancer cells, as well as improved pharmacokinetics, biodistribution, and antitumor efficacy in tumor-bearing mice. This drug delivery platform offers the advantages of prodrugs and nanoparticles for cancer therapy [178]. Another drug delivery system for cancer therapy involves magnetic solid lipid nanoparticles (MSLNs) that release PTX in response to magnetic hyperthermia. MSLNs are composed of solid lipids that can encapsulate hydrophobic drugs and magnetic nanoparticles. The PTX-MSLNs were prepared using glyceryl monostearate, poloxamer 188, and manganese ferrite nanoparticles. These particles exhibit enhanced cytotoxicity and apoptosis induction in MCF-7 breast cancer cells compared with free PTX in vitro. The PTX-MSLNs also show improved pharmacokinetics, biodistribution, and antitumor efficacy compared with free PTX in vivo. This research represents a promising drug delivery platform for cancer therapy [179].

3.1.3. Phytosomes

Phytosomes are specialized delivery systems that consist of natural phospholipids and phytoconstituents. These complexes are designed to improve the bioavailability and efficacy of plant extracts, making them a promising option for cancer treatment. Phytosomes are nanosized complexes that can enhance the delivery and efficacy of the extract, which has several biological activities, including anti-inflammatory, antioxidant, immunomodulatory, and anticancer effects. Scientists have developed a new drug delivery system for cancer treatment using phytosomes made from aloe vera extract. This innovative system has demonstrated improved solubility, stability, and bioavailability of the extract, as well as enhanced cytotoxicity and apoptosis induction in human breast and cervical cancer cells. Furthermore, in vivo testing has shown improved pharmacokinetics, biodistribution, and antitumor efficacy. This research offers a promising drug delivery platform for cancer therapy by combining the advantages of natural products and phytosomes [180]. Researchers have also developed a drug delivery system for breast cancer therapy using a self-assembled fisetin–phospholipid complex. Fisetin is a natural flavonoid that has shown anticancer, anti-inflammatory, and antioxidant effects. However, it suffers from poor solubility, stability, and bioavailability. The Fisetin–phospholipid complex (FPC) improved the solubility, stability, and bioavailability of Fisetin compared with conventional methods. In vitro and in vivo studies have demonstrated enhanced cytotoxicity, apoptosis induction, and antitumor efficacy of the FPC. This research provides a sound scientific basis for the development of a promising drug delivery platform for breast cancer therapy [181].

3.1.4. Nanostructured Lipid Carrier

Nanostructured lipid carriers (NLCs) are a promising drug delivery system that can encapsulate hydrophobic drugs and improve their solubility, stability, and bioavailability. Scientists have developed various drug delivery platforms using NLCs for cancer therapy. One such platform is a chitosan (CS)-coated NLC system that delivers tetrahydrocurcumin (THC) to the skin for breast cancer therapy. THC is a natural metabolite of curcumin and has potent anti-inflammatory and antioxidant properties. The THC-CS-NLCs showed improved skin permeation, cytotoxicity, and apoptosis induction in MCF-7 breast cancer cells compared with uncoated THC-NLCs [182]. Another promising drug delivery system is an NLC system that co-delivers erlotinib (ELT) and resveratrol (RES) for non-small-cell lung cancer therapy. The ELT-RES-NLCs showed improved solubility, stability, and bioavailability of ELT and RES compared with free drugs or single-drug-loaded NLCs. They also demonstrated enhanced cytotoxicity, apoptosis induction, and inhibition of ROS generation and autophagy in NSCLC cells compared with free drugs or single-drug-loaded NLCs in vitro [183]. NLCs have also been used to deliver piperine (PIP) for hepatocellular carcinoma (HCC) therapy. PIP-P-NLCs (pectin-coated NLCs with piperine) showed enhanced cytotoxicity, cellular uptake, and apoptosis induction in HepG2 HCC cells compared with free PIP or PIP-NLCs in vitro, as well as improved pharmacokinetics, biodistribution, and antitumor efficacy compared with free PIP or PIP-NLCs in vivo [184]. Another exciting drug delivery system is an NLC system that contains two drugs, docetaxel (Dtxl) and tariquidar (TRQ), and is functionalized with polyethylene glycol (PEG) and a peptide called RIPL (PRN) to improve their biocompatibility and targeting ability for breast cancer therapy. This system showed sustained drug release behavior, enhanced cellular uptake, and improved pharmacokinetics and antitumor efficacy compared with free drugs. The combination of synergistic drug combination and nanoparticles in this promising platform shows great potential for cancer therapy [185]

3.2. Polymeric

3.2.1. Dendrimer

Dendrimers, nanoparticles with functionalizable terminal groups branching out from a central core, have unique properties that make them a promising tool in various scientific fields. They can incorporate multiple therapeutic agents through encapsulation within the central cavity or conjugation to functional end groups [186]. Researchers have conducted studies to explore the potential applications of dendrimers in biomedical research. One study evaluated the effectiveness of Starburst polyamidoamine (PAMAM) dendrimers as a delivery system for antisense oligonucleotides and “antisense expression plasmids” to regulate gene expression. The researchers assessed the potential of DNA-dendrimer complexes to transfer oligonucleotides and plasmid DNA for antisense inhibition in an in vitro cell culture system [187].

Another research group developed a drug delivery system using a dendrimer called PAMAM that can effectively deliver the chemotherapeutic drug; DOX to target cells. The system uses a cell-penetrating peptide from a transactivating transcriptional activator (TAT) along with EGFR-binding peptide 1 (EBP-1) to encapsulate DOX [188]. Yet another study demonstrated that PEG-citrate dendrimer conjugated with folic acid serves as a SPECT (single-photon emission computed tomography) contrast agent for detecting breast cancer. The conjugation of folic acid with dendrimers provides targeted delivery and improved imaging capabilities, making them an essential tool in oncological applications [189]. Moreover, metal preligation techniques in dendrimer-based contrast agents have shown promising results in enhancing their relaxivities, making them suitable for MRI applications. This approach has great potential for advancing scientific research in the field of medical imaging [190]. Studies have shown promising results in regulating gene expression, delivering DOX to targeted cells, detecting breast cancer, and enhancing relaxivities for MRI applications. Dendrimers can be a valuable tool for biomedical research in the field of drug delivery and medical imaging. In conclusion, dendrimers have the potential to serve as drug delivery systems and imaging agents in various medical applications.

3.2.2. Polymeric Micelle

Polymer micelles, formed via the self-assembly of amphiphilic block copolymers in water, are a type of nanostructure that ranges in size from 10 to 100 nm. They can be functionalized with custom core-forming polymers or targeting ligands for controlled degradation [191]. These micelles have been highlighted in recent literature as a promising drug delivery system for cancer chemotherapy. Supramolecular polymeric prodrug (SPP) micelles, composed of an amphiphilic poly(ethylene glycol) derivative, an adamantane-modified DOX, and a β-cyclodextrin derivative, offer a precise drug molecular structure and loading content that can be easily controlled by the stoichiometric ratio of the components. The micelles are stable in blood circulation but can release DOX rapidly in tumor cells due to the acidic pH and the presence of cathepsin B. They have shown enhanced antitumor efficacy and safety compared with free DOX in vitro and in vivo [192]. Enzyme-responsive polymeric micelles of cabazitaxel (CTX) have been developed as a drug delivery system for prostate cancer chemotherapy. The micelles have two different types of amphiphilic block copolymers that self-assemble in water and encapsulate CTX in their hydrophobic cores. They can release CTX in a controlled manner in response to the acidic and enzymatic conditions in the tumor cells [193]. Polymeric micelles targeted against the CD44v6 receptor have been developed as a drug delivery system for colorectal cancer therapy. They are loaded with niclosamide (NCS), which inhibits cancer stem cell proliferation and survival. The micelles are composed of CD44v6-specific peptides, poly(ethylene glycol), and cholesterol. They can self-assemble in water, encapsulate NCS, and release it into the cells in response to the acidic pH and esterases. These micelles show enhanced cytotoxicity, apoptosis induction, and CSC inhibition compared with free NCS in vitro, and improved pharmacokinetics, biodistribution, and antitumor efficacy in vivo [194]. Researchers have developed a new pH-redox responsive polymer-DOX prodrug micelle system for targeted drug delivery and controlled release of DOX. The system comprises a copolymer conjugated with DOX via a pH-sensitive bond and a redox-sensitive bond. The micelles can respond to the acidic and reducing environment in tumor cells, and release DOX in a two-stage manner. This new drug delivery platform holds promise for cancer chemotherapy [195].

3.2.3. Polymer Drug Conjugate

Polymer drug conjugates have emerged as a promising option for cancer treatment due to their many advantages over conventional chemotherapy. These conjugates consist of a carrier polymer linked to an anticancer drug. The carrier polymer can be designed to selectively target cancer cells, allowing for more precise drug delivery and increased efficacy. Researchers have developed a promising drug delivery system for colon cancer treatment. The system uses a gemcitabine-based polymer–drug conjugate (GPD) that self-assembles into micelles in an aqueous solution. The GPD micelles release gemcitabine in response to the acidic pH in the tumor microenvironment, improving gemcitabine’s cellular uptake and cytotoxicity in colon cancer cells. The GPD micelles also show improved pharmacokinetics, biodistribution, and antitumor efficacy compared with free gemcitabine in vivo [196]. Similarly, another study has investigated the use of polymer-drug nanoparticles (PNPs) for treating colorectal cancer (CRC) that has metastasized to the lung. PNPs are composed of biocompatible and biodegradable block copolymers that can self-assemble into nanosized particles and encapsulate various chemotherapeutic agents. The study examined the efficacy and safety of PNPs loaded with different drugs, such as wortmannin, PX866, and SN-38, which can inhibit the phosphoinositide 3-kinase (PI3K) pathway, a key signaling pathway involved in CRC progression and metastasis. The results demonstrate that PNPs can accumulate preferentially in the lung tissue and deliver the drugs to the lung metastases. Moreover, PNPs can suppress or eliminate lung metastases in mouse models of CRC, depending on the type and dose of the drug. This study shows that PNPs offer a new and effective therapeutic option for treating CRC that has metastasized to the lung, with reduced toxicity and improved outcomes [197].

3.3. Inorganic

3.3.1. Silica Nanoparticle

Silica nanoparticles have emerged as a promising tool for cancer treatment and gene delivery applications [16]. They are more effective and less toxic than traditional methods and have shown great potential in delivering specific agents such as RNase A, pepstatin A, protein antigen, and toll-like receptor 9 agonists. Researchers have developed organosilica nanoparticles that are responsive to oxidative and redox stimuli, which can be used to deliver RNase A for cancer treatment. These nanoparticles have large pores and electrostatic interactions that allow RNase A to be loaded, resulting in improved anticancer activity with low systemic toxicity [198]. In addition, scientists have synthesized and characterized hollow mesoporous silica nanoparticles (HMSNs) that are functionalized with polyethyleneimine (PEI) and polyethylene glycol (PEG) for gene delivery. They found that the functionalized HMSNs have higher gene loading capacity, enhanced cellular uptake, and increased transfection efficiency compared with bare HMSNs and PEI alone [199]. Furthermore, researchers have studied the use of mesoporous silica and organosilica nanoparticles to deliver pepstatin A, a peptide that suppresses an enzyme in breast cancer cells. They found that hollow organosilica nanoparticles performed best, and showed potential for using nanoparticles as carriers for cancer therapy [200]. Finally, extra-large-pore mesoporous silica nanoparticles (XL-MSNs) have been found to effectively deliver protein antigen and toll-like receptor 9 (TLR9) agonists to activate dendritic cells and induce antitumor immune responses. Compared with small-pore MSNs, XL-MSNs have superior loading capacity, reduced cytotoxicity, enhanced cellular uptake, and increased transfection efficiency. Studies on mice with melanoma showed that XL-MSNs efficiently activated DCs, primed cytotoxic T lymphocytes, and suppressed tumor growth, making it a promising platform for cancer vaccines [201].

3.3.2. Silver Nanoparticles

Silver nanoparticles, which are typically less than 100 nanometers in size, possess unique properties that make them ideal for targeted delivery of drugs to cancer cells and their subsequent destruction [20]. Studies have shown that silver nanoparticles can attach themselves to cancer cells, disrupting cellular processes and leading to cell death. They can also enhance the efficacy of existing cancer treatments, such as chemotherapy and radiation therapy. A recent study developed a novel nanoconstruct that combined 5-fluorouracil and nisin on chitosan-coated silver nanoparticles, demonstrating anticancer potential against murine skin cancer. The nanoconstruct significantly reduced the tumor volume and burden while improving the oxidant/antioxidant status and restoring skin histoarchitecture. This promising approach could serve as a platform for creating synergistic single platforms against various cancers [202]. Another research article reported the synthesis of silver nanoparticles from the aqueous extract of a marine brown alga, Dictyota ciliolata. These nanoparticles exhibited anticancer activity against non-small-cell lung cancer cells and could potentially be used as nanomedicine for NSCLC therapy [203]. Furthermore, a study proposed an innovative approach to enhance X-ray radiotherapy on HER2-overexpressing cancer cells using silver nanoparticles conjugated with anti-HER2 antibody. The AgNPs-trastuzumab conjugates were found to be effective in improving the radiosensitization effect of X-ray irradiation on HER2-positive cells, resulting in increased DNA damage, apoptosis, and cell cycle arrest. This approach could significantly improve the specificity of X-ray radiotherapy on HER2-overexpressing cancerous cell lines [204].

3.3.3. Gold Nanoparticles

Gold nanoparticles possess unique physical, optical, and electronic properties that make them valuable tools for cancer therapy. They can be customized with tumor-specific antibodies or ligands to target cancer cells with high precision and sensitivity. In addition, they can function as drug-delivery vectors that specifically transport therapeutic agents to cancer cells. Their photothermal properties can be harnessed to destroy cancer cells by transforming light energy into heat. Studies indicate that gold nanoparticles can enhance the efficacy of chemotherapy drugs like DOX in treating cancer. For instance, in one study, researchers encapsulated DOX within tumor-specific gold nanoparticle clusters (AuNPs) to achieve high cytotoxicity on cancer cells and low toxicity on normal cells. The AuNPs were tested in mice bearing MCF-7 tumors to assess their efficacy, and the results demonstrate their potential in cancer treatment [205]. Gold nanoparticle clusters (AuNPs) that can disassemble in a tumor-specific manner have also been used to deliver DOX. The AuNPs are sensitive to acidic pH, which is characteristic of tumor tissues, and can release DOX into the tumor cells. The study demonstrated the successful preparation and characterization of AuNPs that can disassemble in a tumor-specific manner and release DOX for enhanced anticancer therapy and imaging [206]. Further research has explored the use of gold nanoparticles as carriers for enhancing the anticancer effects of nisin and theaflavin. The results indicate that gold nanoparticle conjugates with these agents demonstrated higher cytotoxicity on cancer cells and lower toxicity on normal cells [207]. Additionally, scientists have developed biotin-conjugated gold photoactive nanoparticles (B-AuPNs) to enhance photothermal therapy (PTT) for brain cancer cells. The B-AuPNs were engineered to target brain cancer cells, penetrate the blood–brain barrier, and generate reactive oxygen species under light irradiation. The in vitro and in vivo studies demonstrated that B-AuPNs can significantly improve the efficacy of PTT in treating brain cancer cells, providing a novel approach to treating brain cancer [208]. In conclusion, gold nanoparticles have enormous potential for cancer treatment, and ongoing research is exploring their safety and efficacy in clinical settings.

4. Targeting Strategies

4.1. Small Molecules

Along with antibodies, proteins, and peptides, large-molecular-weight polymers are also being extensively explored as ligands for targeted activity. Despite recent developments in biological technology, they are still expensive compounds that require time-consuming synthesis procedures. They are challenging to work with due to their higher variability, instability, storage, and additionally complicated characterization techniques [209]. Another significant problem that restricts the investigation of macromolecular targets for active targeting is the potential of peptide molecules to trigger an immunological response. As a lucrative option, small-molecule ligands offer several benefits over biological and macromolecule ligands inexpensive and stable than other biological ligands, easy characterization and manufacturing, and less complexity [210]. The most used small molecule targeted agent is folic acid (FA) which is a water-soluble form of vitamin B. The folate cycle supports important metabolic processes and is necessary for rapidly expanding cells. The production of purine and thymidine nucleotides, which are necessary for the synthesis, the methylation process, and the repair of DNA, utilizes folates [211]. The folate molecule’s pterin ring is positioned at the end of its active site cavity. The 4-aminobenozyl moiety interacts with the cavity’s core area through hydrophobic interactions. However, the glutamyl tail’s g-carboxylate is only partially exposed to the solvent. This group is more solvent-accessible compared with the less reactive pterin amine, making it an appealing site for modification and conjugation while retaining FA-FR affinity [212]. The folic acid receptor (FR), which is only minimally expressed in noncancerous tissues, is overexpressed in a variety of tumor types, including ovarian, triple-negative breast, and lung cancers. Due to this, researchers have extensively investigated FR for cancer targeting, making it a prospective candidate for the development of anticancer drugs [213]. For the dual administration of cisplatin (cis-diaminodichloroplatinum, CDDP) and PTX, He et al. constructed FA conjugated nanoparticles (NPs) from the amphiphilic copolymer composed of PEG-PLGA poly(ethylene glycol)–poly (lactic-coglycolic acid). Both CDDP and PTX were chelated to the center shell and enclosed inside the hydrophobic interior core, respectively. For extended circulation, PEG formed the outer layer. The CDDP+PTX-encapsulated nanoparticles demonstrated a significantly synergistic effect on the suppression of A549 (FR-negative) and M109 (FR-positive) lung cancer cell line growth after a 24 h incubation period. With an 89.96% tumor suppression rate (TSR), the CDDP + PTX-encapsulated nanoparticles inhibited tumor growth when treated with A549 cell xenograft lung tumors in vivo. Compared with nanoparticles containing a single medication or a free chemotherapeutic treatment combination, this TSR was noticeably higher. The TSR reported 95.03% for the M109 cell xenograft tumor. The results in vivo and in vitro suggest that the developed FA-modified copolymer-based NP system demonstrated greater antitumor and targeting activity for M109 cells than A549 cells [214].

In a study performed for the simultaneous delivery of the hydrophobic drug letrozole (L) and the hydrophilic antioxidant ascorbic acid (A), the researcher developed a folate-PEGylated niosome as an effective nanocarrier. The developed folate-PEGylated-LA- niosomes demonstrated notable anticancer and antimetastatic actions on breast cancer cells MDA-MB-231 and SKBR3. The enhanced rate of cell death in the MDA-MB-231 and SKBR3 tumor cells was observed by flow cytometry results, demonstrating the synergistic impact between the two complementary anticancer medications in the course of treatment [215]. Another category of top-notch targeting ligands includes monosaccharides like glucose, mannose, and galactose. For the rapid multiplication and survival of cancerous cells, they require a higher amount of glucose to produce energy [216]. For instance, glucose targets the blood–brain barrier’s overexpressed D-Glucose transporter (GLUT) receptor. Treatment options, including NPs, made up of PEG-PTMC [poly (ethylene glycol)-b-poly (tri-methylene carbonate)] containing PTX modified with deoxy-D-glucose, have been explored for glioblastoma (Figure 3). Rhodamine B isothiocyanate was used as a fluorescent probe to investigate the cellular uptake of the NPs in RG-2 glioma cells, and the fluorescence intensity of DGlu-NP was higher than that of nonglucosylated NP. In addition to this, data on transportation across the in vivo blood–brain barrier (BBB) model and cytotoxicity to RG-2 cells after crossing the BBB indicated that DGlu-NP/PTX considerably outperformed NP/PTX. DGlu-NP demonstrated good selectivity and efficiency in cerebral tumor accumulation, as demonstrated by an in vivo fluorescent picture [217].

Methotrexate (MTX) was investigated as a potential FR ligand because of the structural resemblance between FA and MTX, and the similarity in structure increased the binding affinity to FR [218]. Wong et al. and Thomas et al. published a proof-of-concept study using systematic investigation to demonstrate the dual characteristic of MTX employing PAMAM dendrimer [219,220]. The results of the investigation that MTX decorated dendrimers have a strong affinity for FR are encouraging. Additionally, they demonstrated that MTX’s anticancer activity was unaffected by conjugated MTX as opposed to free drug. Besides these aforementioned small-molecule-based targeting ligands, several other vitamins like riboflavin [221], vitamin H (biotin) [222], carbohydrate-based ligands like mannose [223,224], fructose [225], and N-acetylglucosamine [226,227], nucleosides like adenosine [228] and some amino acids [229,230,231] were also explored for targeting cancer therapy. Li and colleagues developed mixed micelles for the treatment of non-small-cell lung cancer by combining biotin with PEG, PCL, and acrylate polymer. According to a sixfold reduction in IC₅₀ values, the self-assembled micelles could transport the encapsulating quercetin to cancer cells at a substantially higher concentration than other delivery systems [232]. Another mannose ligand, P-aminophenyl-d-mannopyranoside (PAM), has been utilized by Zhang et al. to actively target cancer. GLUT1 which is expressed in endothelium and brain cancer cells, was targeted by conjugating PAM with the liposomal surface. When compared with nonconjugated NPs, live cell imaging revealed that the targeted NPs induced dye to migrate in the brain [233].

Small molecule drug conjugates (SMDC) prepared by conjugation of small-molecule targeting ligands with anticancer drugs are also explored for active targeting of therapeutics to the cancer tissue. They consist of three components: small-molecule targeting ligands, cytotoxic compounds, and linkers. Compared with antibody–drug conjugates, SMDCs can be disseminated into tumor tissues faster and evenly, with minimal cost and no immunogenicity [234]. SMDC developed by linking acetazolamide and monomethyl auristatin E (MMAE) via dipeptide linkers exhibits substantial antitumoral efficacy in mice carrying xenografted SKRC-52 renal cell carcinomas. Here, acetazolamide (a small heteroaromatic sulfonamide) was used to target carbonic anhydrase (a membrane-bound enzyme) that is overexpressed mostly in renal carcinoma cells [235]. Even though the increasing domain of small molecule–drug conjugates, only Lutathera (¹⁷⁷Lu-DOTATATE), which targets peptide receptors, is licensed for the treatment of gastroenteropancreatic neuroendocrine tumors and several folates receptor-targeted SMDCs, including vintafolide (folatedesacetylvinblastine hydrazide), OTL-38 (Pte-Tyr-NIR-dye), EC17 (folate-fluorescein isothiocyanate), and etarfolatide (folate-^99mTc), are in clinical trials. ¹⁷⁷Lu-DOTATATE is a somatostatin analog labeled with ¹⁷⁷Lu, a radioactive isotope that emits beta and Gamma rays. Targeted radioisotope therapy enhances somatostatin’s anticancer properties, inhibiting tumor growth while reducing radiation toxicity. ¹⁷⁷Lu is a b-emitter with minimal tissue penetration (less than 2 mm) and a respectable half-life of 6.7 days. This makes it a promising therapeutic and safety option, as 99% of the active ingredient is removed from the body within two weeks [235,236]. The PTX-degarelix conjugate was developed for targeted chemotherapy. PTX was conjugated to a degarelix analog using disulfide bonds, that are stable within normal physiological conditions but break to free thiols in the presence of reducing agents such as glutathione as it is abundant in the cell cytoplasm (~10 mM) but present at a low concentration in blood plasma (~0.002 mM). It was found to be more cytostatic to cancer cells than to normal cells as per the in vitro cell line study conducted using MCF-7 human breast cancer cells and HT-29 human colon cancer cells [237].

4.2. Peptides

Peptide ligands are intermediate in size between small molecules and antibody ligands. Peptide ligands have substantially better binding affinities and specificities than small-molecule ligands since they can mimic protein–protein interactions and have broad binding surfaces with receptors [166,238,239,240]. Peptides provide a powerful tool for the targeted delivery of drugs. Peptide ligands for targeted drug delivery are typically discovered through bio-inspired methods (biomimetic peptides) or by thorough screening of peptide pools (like phage display peptide pools and chemical peptide pools) [241]. Targeting peptides can therefore be coupled directly to drugs to produce peptide–drug conjugates (PDC); biomaterials to create peptide-modified nanocarriers; and other peptides to generate supramolecular self-assembly [242,243]. Peptides and peptidomimetics that contain the RGD sequence (arginine–glycine–aspartic acid), a crucial recognition mechanism for cell adhesion, have the potential to bind to a variety of integrins that are excessively present on endothelial cells, which are found in tumor neovasculature [244]. Numerous receptors, including SSTR, integrin, transferrin, HER2, APN (Aminopeptidase N), LHRH (Luteinizing hormone-releasing hormone), EGFR, EpCAM (Epithelial cell adhesion molecule, and CD133, have been associated in the malignant progression of cancer. These receptors have been used to develop peptide-based functionalized nanodelivery systems for targeted cancer therapy, as well as diagnosis [245].

The specific targeting peptide (cyclic RGD) was covalently linked to the NPs by PEG-400. NP core was composed of inulin multimethacrylate encapsulating DOX. After intravenous administration, this drug’s anticancer effectiveness has been improved by having its surface tailored using a cyclic peptide bearing the RGD sequence, cyclo-(Arg-Gly-Asp-D-PheCys) [246]. Colon cancer cells that express the integrin α₅β₁ can be targeted using PEGylated liposomes encapsulating 5-Fluorouracil (5-FU) utilizing PR_b as a targeting moiety (a peptide sequence that resembles the cell adhesion region of fibronectin). The cytotoxicity of these surface-functionalized PEGylated liposomes was greater than that of non-surface-decorated 5-FU-loaded PEGylated liposomes on CT-26 wt cells after they were internalized through α₅β₁ mediated endocytosis [247]. The efficiency of systemic administration of siRNA is constrained by the small interfering RNA’s (siRNA) poor ability to cross cellular plasma membranes and its poor blood stability. To get through these challenges, poly(propyleneimine) (PPI) dendrimers were used to create the siRNA NPs, which were then stabilized using PEG coating and cross-linkers that included dithiol (Figure 4). To target the siRNA NPs, particularly the cancer cells, a synthetic analog of the luteinizing hormone-releasing hormone (LHRH) peptide was attached to the distal part of PEG. The layer-by-layer modification and targeting strategy provided stability in plasma and intracellular bioavailability to the siRNA NPs, and it allowed for their specific uptake by tumor cells and ensured the accumulation of siRNA within the cytoplasm of malignant cells and effective gene silencing. These findings were supported by in vivo experiments. Additionally, biodistribution results support this targeting delivery system’s great specificity [248].

nAChRs are present throughout the central nervous system, including across the blood–brain barrier, allowing for intracranial drug delivery. A study revealed that the peptide KC2S obtained from the loop 2 fragment of the toxin of Ophiophagus Hannah has a high binding affinity towards nAChRs. The results of in vivo quantitative and qualitative distribution of fluorescent dyes (DiR or coumarin-6) suggest that KC2S-functionalized poly (ethylene glycol)-poly (lactic acid) micelles (KC2S-PEG-PLA micelles) lead to increased drug delivery into the brain. Additionally, intravenous administration of KC2S-PEG-PLA-PTX micelles (PTX-encapsulated) provided strong suppression of cerebral glioma. Mice treated with mPEG-PLA-PTX micelles (41.5 days), PTX (38.5 days), or saline (34 days) had median survival times that were all considerably shorter than those of mice treated with KC2S-PEGPLA-PTX-micelles (47.5 days) [249].

In the treatment of glioma, enzymatic barriers in the bloodstream and brain, the blood–brain tumor barrier (BBTB), and BBB severely restrict the ability of drugs and drug carriers to reach the tumor site. Wei et al. designed a liposomal system that passed through different barriers by altering the liposome exterior with, two peptides that are proteolytically stable. One is DCDX which is the D-peptide ligand of nAChRs on the BBB, and another is c(RGDyK) targets integrin highly expressed in the BBTB and glioma cells. Due to the extraordinary stability of both peptide ligands in lysosomal homogenate, intact ligands were able to target gliomas and exocytose brain capillary endothelial cells. In the cellular uptake assays, dual-labeled liposomes successfully penetrated the BBB and BBTB monolayers, overcame the enzymatic barrier, and targeted three-dimensional tumor spheroids in addition to brain capillary endothelial cells and tumor cells. Histological examination and ex vivo imaging were used to confirm in vivo targeting capabilities in cerebral gliomas. In contrast to liposomes without modification and liposomes modified with individual peptide ligands, DOX liposomes modified with both DCDX and c(RGDyK) demonstrated improved antiglioma activity with prolonged median survival in nude mice with glioma [250].

4.3. Aptamers

Aptamers are artificial single-stranded (ss) DNA or RNA molecules that bind to target molecules with high affinity, utilizing three-dimensional structures. For diagnostic or therapeutic purposes, aptamers are a class of substances that function as molecular recognition elements or affinity probes. The name “aptamer” is a combination of the Latin term “aptus”, which means “to fit”, and the Greek word “meros”, which means “part” [251]. Aptamers are known as chemical antibodies and are usually composed of 20–80 nucleotide bases [252]. Systematic Evolution of Ligands by Exponential Enrichment (SELEX) is the method utilized to select aptamers from the library of aptamers, it was initially developed for the first time in 1990 by Tuerk and Gold [253], Ellington and Szostak [254].

An aptamer folds into a special tertiary structure for interacting with particular targets; the complementary forms of aptamers and targets enable them to bind. Aptamers have several advantages over other ligands, including small size, ease of synthesis, great chemical stability, fully synthesized, and minimal immunogenicity [255]. Aptamers are appealing for targeted therapy because of these features. Several aptamers have been investigated thus far and have demonstrated significant promise in a variety of applications, including the diagnosis, prognosis, and treatment of human viral and cancer diseases [256]. Aptamer-conjugated nanomaterials may offer a more effective and safe way to fulfill the growing demand for novel cancer-targeting strategies by combining the intrinsic properties of nanomaterials with the accurate recognition capabilities of aptamers [257,258,259]. Langer and coworkers developed Dtxl NPs from a copolymer made up of poly (d, l-lactic-co-glycolic acid)-block-poly (ethylene glycol) (PLGA-b-PEG) and RNA aptamers, A10 2′-fluoropyrimidine is conjugated to the surface of the NPs, which can recognize the extracellular domain of the prostate-specific membrane antigen (PSMA) present on the surface of prostate cancerous cells. The results of the study show reduced toxicity of the nanoparticle aptamer conjugate system (Dtxl-NP-Apt) in contrast to Dtxl-NP and exhibited remarkable efficacy in animal studies [260].

Dhar et al. also developed NPs incorporating a platinum IV compound with the same PLGA-b-PEG polymeric material and aptamer, which targets PSMA in prostate cancer cells. The aptamer-conjugated copolymeric NPs showed greater efficacy and were rapidly uptaken by LNCaP epithelial cells but not by PC3 cancer cells additionally as compared with nontargeted NPs or free cisplatin, aptamer-Pt(IV) contained NPs were found to be effective in delivering drugs to the LNCaP cells [261]. Another study involving redox-responsive mesoporous silica nanoparticles (MSNs) for triplex tumor-targeted therapy released the drug DOX by cleavage of disulfide bonds in the cancer cells due to the higher concentration of reductant glutathione in the tumor microenvironment (Figure 5). The surface of this carrier was functionalized by attaching immobilized cytochrome c (CytC) and the DNA aptamer AS1411, which can bind with nucleolin, is involved in tumorigenesis and angiogenesis, and is overexpressed in cancerous cells. Additionally, CytC, after detaching from the MSNs, binds to the apoptotic protease activating factor (Apaf-1) and promotes cell death. More significantly, the nanocarrier demonstrated excellent tumor-targeting potential and produced triplex treatment effects on tumor inhibition in vivo with lesser side effects of body weight reduction [262].

The development of aptamer-conjugated DOX-MSNs (Ap-MSN-DOX) allowed for the targeted delivery of DOX to colon cancer cells that expressed the epithelial cell adhesion molecule (EpCAM), which was showing increased uptake and enhanced cytotoxic effect of DOX in EpCAM-expressing colon cancer cells in contrast to nonconjugated MSNs (MSN-DOX). These findings indicated that Ap-MSN-DOX could improve the therapeutic index while lowering side effects through the targeted delivery of drugs to colon cancer cells expressing EpCAM [263]. Aptamer AS1411-functionalized nanocapsules (NCs) (target for nucleolin) made up of a copolymer of chitosan and poly (N-vinylpyrrolidone-alt-itaconic anhydride) containing 5-FU was also developed, and the results of the study demonstrate that the prepared aptamer-conjugated NCs produced a greater cytotoxic effect on human breast cancer cells (MCF-7) and produced rapid cell death as compared with free 5-FU [264]. Using three distinct aptamers, namely MUC1, AS1411, and ATP, Taghdisi et al. developed a DNA dendrimer. Targeted delivery of epirubicin (Epi) from the developed dendrimer was studied in breast carcinoma cell lines (MCF-7) and murine colon cancer cells (C26). The results of flow cytometry analysis suggest that, with multiple aptamer components assembled on the surface of a dendrimer, the Apts–Dendrimer–Epi complex significantly enhanced the fluorescence intensity of Epi in MCF-7 and C26 cells (target cells) when compared with free Epi, showing more internalization of Epi through Apts–Dendrimer–Epi complex-treated target cells. Using allograft animal models implanted with C26 cells, the therapeutic activity of the Apts–Dendrimer–Epi complex was investigated, and the results of the in vivo study suggest that, following a 16-day course of treatment, the tumor volumes for the MUC1 aptamer Dendrimer–epirubicin conjugate and AS1411 aptamer Dendrimer–epirubicin were 1802 mm³ and 850 mm³, respectively. However, the dendrimer that was marked with both aptamers exhibited a decreased tumor volume of 585 mm³. As a result, the research conducted by this group offers compelling proof of the improved effectiveness and specificity of drug-loaded dendrimers after modification with single or double aptamers [265].

To deliver targeted therapy to patients with non-small-cell lung cancer (NSCLC), Engelberg et al. synthesized aptamer-functionalized NPs made up of block copolymer PEG-PCL packed with PTX. They used the ssDNA-based S15 aptamer as a targeting ligand on NPs. S15-APTs were shown to be an aptamer specific to A549 cells when it was found that cellular uptake of S15-APT-attached NPs was significantly higher in human NSCLC (A549) cells compared with human bronchial epithelial (BEAS2B), human embryonic kidney (HEK-293) cells, and neonatal foreskin fibroblast (FSE), alongside cervical carcinoma (HeLa) and colon adenocarcinoma (CaCo-2) cells. Concerning A549, BEAS2B, HeLa, CaCo2, FSE, and HEK-293 cells, the aptamer-conjugated PTX NPs’ IC50 values were 0.03, 1.7, 4.2, 43, 87, and 980 M of PTX, respectively. According to their research, these S15 aptamer-functionalized NPs have a significant preclinical potential for precisely targeting and eliminating human NSCLC cells while posing minimal risk to healthy tissues [266]. In another study, Sgc8 aptamer with DOX-coupled MSNPs (Sgc8-MSNDOX) for targeted drug delivery was synthesized by Yang et al. using a nanoprecipitation technique. The in vitro cellular uptake of the produced NPs was examined using CEM T lymphocyte leukemia cells and Ramos B lymphoma cells. The result demonstrated that cellular uptake of Sgc8-DOX-MSNPs was much higher than that of DOX-MSNPs in CEM T cells, although uptake of both types of NPs was identical in Ramos B cell lines. This was due to the Sgc8 aptamer’s selectivity for PTK-7 (protein tyrosine kinase-7) receptors present on CEM T cells, which led to a rise in uptake. In addition to this, Sgc8-DOX-MSNPs were significantly more cytotoxic to CEM T cells in vitro than DOX-MSNPs, which suggests that Sgc8 Apt might be used as a targeting molecule for effective anticancer therapy since it was more selective for cells that overexpressed the PTK-7 receptor [267].

Aptamer-conjugated liposomes were developed by Naznin et al. for tumor targeting by endothelial cells. With the help of the Ara HH001 aptamer, the tumor’s endothelium cells were targeted. Apt-PEG-LPs were developed by using a lipid hydration technique and decorating the PEG-spacer’s outer surface with a DNA aptamer. Aptamer-coupled liposomes resulted in a 3.8-fold increase in fluorescence in mouse tumor endothelial cells (mTECs) compared with control liposomes (PEG-LPs). According to confocal laser scanning microscopy (CLSM) images, aptamer-associated liposomes have been found in the lysosomal region of mTECs cells and have shown higher cellular absorption. After intravenous administration of aptamer-conjugated liposomes, CLSM demonstrated in vivo aptamer targeting ability to epithelial cells in mice possessing human renal cell cancer. Regular liposomes bind to TECs at a rate of 3%, but aptamer-linked liposomes attach at a rate of 25%. The higher binding in this instance was caused by the aptamer’s affinity for TECs. Darabi et al. synthesized anti-EGFR/CD44 dual-RNA aptamers attached to DOX-loaded SLNs, which are overexpressed in triple-negative breast cancer (TNBC) [268]. Dexamethasone (Dexa) was chemically bonded to the surface of nanoparticles to deliver the DOX to the nucleus. When compared with SLNs/DOX/Dexa/CD44 and SLNs/DOX/Dexa/EGFR, SLNs/DOX/Dexa/CD44/EGFR reduced cell viability, suggesting that TNBC therapy using dual targeting aptamers that target multiple proliferation pathways was found beneficial [269].

4.4. Proteins

In challenging disease situations, solvent-based, hydrophobic chemotherapeutic drugs have given oncologists much-needed treatment choices. But in addition to the toxicities associated with the chemotherapy drugs themselves, the delivery vehicles of water-insoluble chemotherapy agents have also been linked to certain very serious toxicities. Taxanes are a highly active class of cytotoxic medicines, and solvent-based formulations of taxanes are linked to hypersensitivity responses, neutropenia, and neuropathy. Utilizing natural pathways, human protein albumin-based nanoparticle technology targets tumors with greater medication concentrations while avoiding some of the toxicities associated with solvent-based formulations. Many of the issues with solvent-based formulations can potentially be resolved by nanoparticle protein engineering treatment. Protein-based drug delivery has the potential to provide greater intratumor concentrations of the medicine by avoiding solvent-based toxicities and exploiting endogenous albumin pathways. The FDA approved 130-nanometer albumin-bound paclitaxel (nabTM-paclitaxel; Abraxane^®) in January 2005 as the first commercial product based on protein nanoparticles in oncology for the treatment of breast cancer in patients who fail combination chemotherapy for metastatic disease or relapse within 6 months of adjuvant chemotherapy. The only ingredients in nab-Paclitaxel are unmodified paclitaxel and human albumin. It is CrEL-free (polyethoxylated castor oil). Nab-paclitaxel has a lower risk of hypersensitivity due to the removal of CrEL from its formulation. It also does not need to be premedicated, may be administered in less time (30 min), and does not require specific intravenous tubing. Albumin is a particularly suitable vehicle for targeted medication delivery in oncology due to several special characteristics.

Hydrophobic molecules (including vitamins, hormones, and other components of plasma) are naturally transported by albumin, which has good noncovalent binding properties. According to preliminary findings, nab-paclitaxel may be used alone or in combination therapy to treat metastatic breast cancer as first-line treatment as well as other solid tumors such as non-small-cell lung cancer, ovarian cancer, and malignant melanoma. Clinical trials utilizing nab formulations of additional water-insoluble anticancer medications, such as Dtxl and rapamycin, are currently being conducted. The nab technology holds the potential for broad application in cancer therapy. Dtxl in the nanometer-sized form known as nab-Dtxl (ABI-008) will be investigated in a range of solid tumors. Nab-Dtxl may avoid some of the toxicities of Dtxl, such as hypersensitivity responses, high rates of severe myelosuppression, and fluid retention, by eliminating polysorbate 80 in its formulation. By preferentially accumulating in tumours of albumin-bound complexes nab-Dtxl may also be more effective than Dtxl which is formulated traditionally. Up until recently, rapamycin’s potential as an anticancer treatment was constrained by both its poor chemical stability and poor water solubility. Rapamycin can be administered intravenously using albumin-bound technology of nab-Rapamycin (ABI-009) [270].

4.5. Antibodies

Lipid nanoparticles have been functionalized with antibodies that bind to receptors overexpressed in cancer cells or angiogenic endothelial cells to increase selectivity for target cells and, consequently, therapeutic efficacy. The development of antibody-functionalized lipid nanoparticles for anticancer drug administration has been widely adopted owing to improvements in bioconjugation and antibody engineering techniques, and it has produced encouraging results in vitro and small animal models [271]. Numerous scientific studies have attested to the enhancement of cancer nanomedicines by Ab-NMs. As potential future candidates for the treatment of solid tumors, several promising Ab-NMs, including SGT-53, SGT-94, MM-302, Anti-EGFR ILs-DOX, Lipovaxin-MM, MCC-465, and Erbitux-EDVspac, are now undergoing various stages of clinical studies. SGT-53 is an antitransferrin-receptor-targeted lipoplex with p53 plasmid DNA that is now undergoing a Phase Ib clinical trial to determine the optimal Phase II doses and assess the formulation’s safety when combined with Dtxl. SGT-94, an antitransferrin-receptor-targeted cationic liposomesis loaded with RB94 plasmid DNA. The Phase I clinical trial for SGT-94 is now taking place to determine its safety and maximum tolerated dose (MTD). A Phase I clinical trial for MM-302, a DOX-loaded immunoliposomes (ILs-DOX) targeting HER2 receptor, is now being conducted to obtain MTD. Similar to this, a Phase II clinical trial is currently being conducted on Anti-EGFR targeted ILs-DOX to ascertain its efficacy as first-line therapy for treating advanced triple-negative, EGFR-positive breast cancer. Phase I clinical studies for Lipovaxin-MM, an anti-CD209 targeted lipid-based vaccination for metastatic melanoma immunotherapy, are being conducted to assess its safety and efficacy. An antihuman GAH antibody-conjugated liposomal DOXMCC-465 is in Phase I clinical trial. Selective antibody binding to tumor tissue was seen, even though its precise target was not well characterized. An anti-EGF-receptor-targeted immunomicelle Erbitux-EDVspachas completed Phase I clinical trials the MTD is recommended for Phase II study 7, 172, 173. The major concern for translating laboratory research to therapeutic applications is the safety and efficacy of Ab-NMs. There are several Phase I and Phase II clinical trials going on Ab-NMs, which promise the scientific possibility of Ab-NMs on the selective administration of medications with narrow therapeutic indices and/or limited accessibility to intracellular targets [272]. As a steroid saponin found in Anemarrhenaasphodeloides, timosaponin AIII (TAIII) offers promising potential as an anticancer agent. However, its in vivo anticancer effectiveness is severely constrained by its hydrophobicity and limited bioavailability. To address this issue, TAIII-loaded liposomes (LP) were developed to increase TAIII solubility and increase its duration in circulation. Also, to enhance the therapeutic index of TAIII, anti-CD44 antibody-modified LP (CD44-LP) was prepared. Target-specific binding and uptake, in vivo pharmacokinetics, and biological activity were also used to characterize the LP and CD44-LP. The findings showed that CD44-LP and LP can both significantly lengthen TAIII circulation time, boost tumor-targeted accumulation, and improve anticancer activity. With significant anticancer benefits, the anti-CD44 antibody-modified liposome is a viable choice for treating CD44-positive carcinoma [273].

Combining targeting molecules with nanoengineered drug carriers, such as polymer capsules, micelles, and polymersomes, has significant potential to improve the therapeutic delivery and index of a range of drugs. Even when the target cells make up less than 0.1% of the overall cell population, antibody-functionalized capsules specifically bind to colorectal cancer cells. This precise targeting offers promise for drug delivery applications. Even when the capsule/target cell ratio was low, these functionalized capsules demonstrated highly selective binding to cancer cells expressing the target antigen and minimal nonspecific binding [274]. Delivery of anti-cancer drugs specifically to tumor cells by using advanced nanosystems strongly relies on the expression of cancer-associated targets. For such a delivery method, glycans that cancer cells abnormally express are appealing targets. A potential target for a delivery nanosystem is sialylated glycans, such as Sialyl-Tn (STn), which are aberrantly expressed in several epithelial tumors, including GC. In a study, the NPs surface-functionalized with a specific antibody targeting the STn glycan were developed and further evaluated for nanosystem effectiveness regarding its specificity and recognition capacity. The results show that under static and live cell monitoring flow settings, cells showing the cancer-associated STn antigen readily recognize the NPs surface-functionalized with anti-STn antibody. This reveals the possible use of such NPs for cancer drug delivery. Results indicate that STn glycan, which is highly expressed by cancer cells, may be targeted by polymeric NPs functionalized with B72.3 antibody [275]. In this study, biodegradable antibody-conjugated polymeric NPs were created using tamoxifen (Tam) to accomplish targeted drug administration and sustained release against breast cancer cells. To promote the site-specific intracellular delivery of Tam against HER2 receptor overexpressed breast cancer (MCF-7) cells, copolymer polyvinyl-pyrrolidone (PVP) was used to conjugate Herceptin (antibody) with PLGA NPs. Polyvinyl alcohol (PVA) was used for coating to stabilize PLGA NPs. Compared with Tam-PVP-PLGA NPs, Herceptin-Tam-PVP-PLGA NPs showed greater cellular uptake and cytotoxicity. This is due to HER2 receptor-mediated endocytosis of Herceptin-conjugated NPs by breast cancer cells. Tam is then released from the NPs at an acidic pH, which causes tumor cell proliferation to be suppressed by inducing apoptosis [276].

Preclinical and clinical trials of several tumor-specific antibodies (Pertuzumab, Herceptin, Avastin, Rituxan, Cetuximab, Mylotarg, Erbitux, and MDX-010), angiogenesis inhibitors (vascular growth factor—trap, IMC-1C11, SU5416, SU668, angiostatins, endostatins, and ZD6126), and drugs targeting specific proteins and small molecules (TP38, EM164, NVPADW742, PX-748, Gossypol, Oblimerson, Bortezomib, Celecoxib, Refecoxib, BAY43-9006, Rapamycin, UCN-01, and Bryostatin) are undergoing. The distinctions between malignant cells and normal cells can be exploited if these tumor-specific inhibitors can be conjugated to fit the model of a biodegradable nanoparticle [277]. Ovarian tumors commonly express the protein mesothelin. Studies were conducted to examine the drug targeting capability of immunodendrimers, i.e., antibody-conjugated modified half-generation poly(propylene imine) dendrimers against ovarian cancer using PTX as a model anticancer agent. Immunodendrimers with PTX significantly decreased the tumor volume. The targeting effectiveness and greater biodistribution of immunodendrimers into the mesothelin protein-expressing ovarian cancer cells were further confirmed by the biodistribution tests. According to the findings, the immunodendrimers developed can provide a much higher amount of bioactive and have better therapeutic benefits [278]. For a variety of cancers, the EGFR is essential. Various nations use nimotuzumab (NmAb), an anti-EGFR monoclonal antibody (mAb), to treat EGFR-overexpressed cancers. It is internalized via receptor-mediated endocytosis and targets malignant cells. In a study, mAb-nanoparticle(NmAb) was conjugated with 27 nm spherical gold nanoparticles (AuNPs) to form AuNP-NmAb nanoconjugates. The AuNP-NmAb treatment significantly reduced cancer cell survival when compared with NmAb monotherapy. This study emphasizes the distinct therapeutic potential of AuNP-NmAb in EGFR+ cancers and demonstrates the potential for the development of other mAb nanoparticle complexes for enhanced therapeutic efficacy [279]. Targeting antibodies with nanocarriers holds promise for both therapeutic and diagnostic oncology. Strong Raman signal for cancer cell identification and near-infrared (NIR) absorbance for selective photothermal ablation of tumors are two distinctive optical features of single-walled carbon nanotubes (SWNTs) that can be used for these purposes. In a study, an in vitro model composed of HER2-expressing SK-BR-3 cells and HER2-negative MCF-7 cells was used to demonstrate the dual functionality of the HER2 IgY-SWNT complex for both detection and selective elimination of cancer cells. The HER2 IgY-SWNT complex specifically targeted HER2-expressing SK-BR-3 cells but not receptor-negative MCF-7 cells. Without the need for internalization by the cells, the complex has the potential to be employed for both detection and selective photothermal ablation of receptor-positive breast cancer cells. Thus, novel approaches for cancer diagnosis and therapy may be developed using the distinctive intrinsic features of SWNTs in conjunction with the high specificity and sensitivity of IgY antibodies [280]. In a study, two PAMAM dendrimer–trastuzumab conjugates that carried PTX or Dtxl specifically targeted to cells that overexpressed HER2 were synthesized. The conjugates are promising carriers for HER2 expressing tumor-selective delivery, based on analysis of their cytotoxicity, cellular uptake, and internalization. Trastuzumab, which binds to and inhibits HER2, is used to produce the observed selectivity. In addition, a pH-sensitive linker that breaks in the presence of a tumor environment to permit PAMAM-drug conjugate release was used. Both conjugates have the potential to be used as drug delivery systems, improving therapeutic index and lowering dosage requirements for anticancer medications. These conjugates might be superior for in vivo application due to their increased toxicity for HER2 positive breast cancer due to specific targeting to tumor cells [281] In another study antibody–drug conjugate (Trastuzumab emtansine—T-DM1) was developed by using trastuzumab and microtubule inhibitory agent DM1 (a derivative of maytansine) by employing stable linker moiety. T-DM1 enables targeted drug delivery to HER2-overexpressing cells, improving therapeutic efficacy while reducing exposure to normal tissue. TDM1 has presented therapeutic efficacy in patients with HER2-positive advanced breast cancer in Phase 2 studies [282].

5. Conclusions and Future Remarks

Over the past decade, receptor-targeted nanomedicine has emerged as a focal point in cancer therapy, leveraging precise targeting strategies to deliver therapeutic payloads directly to cancer cells while sparing healthy tissues. The widespread acceptance and effectiveness of COVID-19 vaccines, which utilize nanomedicine principles, have positively shifted perceptions towards the field. With an increasing volume of scientific research and literature on receptor-targeted nanomedicine, there is growing anticipation for their commercial translation, indicating a promising future for personalized and targeted cancer treatments. Nevertheless, certain areas necessitate focused attention to fully harness the potential of receptor-targeted nanomedicine.

Nanocarrier optimization represents a critical avenue for advancing receptor-targeted nanomedicine in cancer therapy. As the research progresses, achieving the delicate balance of targeting strategies is paramount for maximizing clinical benefits while optimizing resource utilization. The scientific community as a whole should make an effort to delve into studies that scrutinize the extent of nanoparticle modification with targeting ligands and their effect on clinical performance. By meticulously examining the interplay between nanoparticle surface modifications and targeting ligand densities, researchers can fine-tune the targeting specificity and efficiency of nanomedicines. To delve into this aspect, researchers can undertake a series of comprehensive studies. For example, employing analytical techniques such as surface plasmon resonance spectroscopy or fluorescence spectroscopy to quantify the density of targeting ligands immobilized on nanoparticle surfaces can allow researchers to systematically vary the ligand density and assess its correlation with receptor binding affinity and specificity. Comparative cell-based assays can be conducted to evaluate the binding kinetics and specificity of ligand-functionalized nanoparticles to cancer cell receptors. Researchers can assess the impact of varying ligand densities on nanoparticle targeting efficiency by employing a panel of cancer cell lines expressing different levels of target receptors. In addition, animal models of cancer can be combined with techniques such as positron emission tomography imaging or near-infrared fluorescence imaging to achieve real-time visualization of nanoparticle accumulation in tumors and nontarget tissues, allowing for quantitative assessment of targeting specificity. By integrating findings from these multidisciplinary approaches, a knowledge base can be generated for the rational design of receptor-targeted nanomedicines with optimized targeting strategies, thereby maximizing clinical benefits while conserving resources.

The advent of personalized or precision medicine approaches in cancer therapy using receptor-targeted nanomedicine holds significant promise for improving patient outcomes. Central to this approach is the integration of advanced molecular profiling techniques, such as genomics, proteomics, and transcriptomics, to elucidate the unique molecular signatures driving tumor growth and progression. Genomic analyses enable the identification of somatic mutations, copy number alterations, and chromosomal rearrangements that drive oncogenesis and influence treatment response. By sequencing the tumor genome and comparing it to the patient’s germline DNA, targetable mutations and aberrant signaling pathways can be identified that can be further exploited for therapeutic intervention. Additionally, proteomic and transcriptomic analyses provide insights into the dynamic changes in protein expression and cellular pathways underlying cancer progression. Once potential molecular targets are identified through molecular profiling, rigorous validation studies should be conducted to assess their functional significance and therapeutic potential. This involves characterizing the biological role of candidate targets in cancer pathogenesis and evaluating their druggability using preclinical models and in vitro assays. Targets that demonstrate oncogenic dependency or association with poor prognosis should be prioritized for further development as therapeutic targets for receptor-targeted nanomedicine. In parallel, efforts must be made to design targeting ligands that selectively bind to the identified molecular targets with high affinity and specificity. This may involve screening combinatorial libraries of peptides, antibodies, or small molecules to identify ligands that exhibit preferential binding to the target receptor expressed on cancer cells. Once lead ligands are identified, they are conjugated or incorporated into nanocarrier formulations to facilitate targeted delivery of therapeutic payloads to tumor cells. Such a personalized approach has the potential to revolutionize cancer treatment by providing patients with targeted therapies optimized for their individual tumor biology.

Patient outcomes and quality of life are paramount considerations in cancer therapy, and receptor-targeted nanomedicine holds tremendous promise for improving both aspects. By exploiting the overexpression of specific receptors on cancer cells, receptor-targeted nanomedicine enhances treatment efficacy by delivering higher concentrations of therapeutic agents to the tumor site, leading to more effective eradication of cancer cells. Moreover, the reduced systemic toxicity minimizes side effects commonly observed with traditional chemotherapy, such as nausea, fatigue, and hair loss, thereby improving patient tolerance and quality of life during treatment. By delivering therapeutic payloads directly to cancer cells, targeted nanomedicine can effectively inhibit tumor growth and metastasis, leading to prolonged progression-free survival and overall survival. Additionally, targeted therapies may offer advantages in managing recurrent or metastatic disease by overcoming resistance mechanisms that commonly limit the efficacy of conventional treatments. Receptor-targeted nanomedicine has the potential to positively impact various domains of quality of life, including physical functioning, symptom management, emotional well-being, and social functioning. By minimizing treatment-related side effects and preserving organ function, targeted therapies enable patients to maintain a higher quality of life during and after treatment. Additionally, targeted nanomedicine may offer advantages in preserving cognitive function and neurocognitive outcomes, particularly in patients receiving treatment for brain tumors or central nervous system metastases. Integrating patient-reported outcomes (PROs) into clinical trials is essential for capturing the full spectrum of treatment effects and patient experiences. Receptor-targeted nanomedicine studies should include PRO assessments to evaluate treatment-related symptoms, functional status, and health-related quality of life from the patient’s perspective.

Addressing regulatory hurdles in the development and commercialization of receptor-targeted nanomedicine presents the last hurdle for commercial translation. It involves navigating complex pathways, ensuring compliance with stringent safety and efficacy standards, and fostering collaboration between industry, academia, and regulatory agencies. Key challenges include scaling up production while maintaining consistency and quality standards, conducting comprehensive preclinical safety assessments, designing well-controlled clinical trials, and leveraging expedited approval pathways when appropriate. Collaboration between industry and academia is essential, with industry partners bringing expertise in drug development and regulatory affairs while academic researchers contribute scientific innovation and clinical expertise. Global regulatory harmonization efforts further streamline the regulatory review process and ensure consistent standards for drug development and approval worldwide. By addressing these challenges and fostering collaborative partnerships, researchers can accelerate the translation of innovative nanomedicine therapies from the laboratory to the clinic, ultimately improving patient outcomes and advancing cancer treatment.

In conclusion, this comprehensive review has provided a deep understanding of the rapidly evolving field of receptor-targeted nanomedicine for cancer therapy. The review has showcased the multifaceted approaches toward precision cancer treatment by delving into the intricate roles of various receptors implicated in cancer progression and elucidating the diverse nanocarrier types and innovative targeting strategies available. By fostering collaborative partnerships and embracing advancements in personalized medicine and precision targeting, the field of receptor-targeted nanomedicine holds immense potential to redefine the landscape of cancer therapy. With continued advancements, receptor-targeted nanomedicine stands poised to significantly enhance patient outcomes and pave the way for a brighter future in the fight against cancer.