Search Results (124)

Background/Objectives: This study aimed to evaluate the potential utility of plasma 4β-hydroxycholesterol (4β-OHC) as an endogenous biomarker of hepatic CYP3A activity for predicting ribociclib metabolism and systemic pharmacokinetics in patients with primary brain cancer. Methods: Plasma concentration data for ribociclib and its major metabolite, LEQ803, were obtained from 46 patients receiving oral ribociclib at daily doses of 400, 600, or 900 mg for 5 days. Plasma 4β-OHC concentrations and 4β-OHC/cholesterol ratios were determined by LC–MS/MS at baseline and at 4 and 24 h after ribociclib administration on day 5. A parent–metabolite population pharmacokinetic model was developed to simultaneously characterize ribociclib and LEQ803 disposition and identify clinical covariates influencing their pharmacokinetics. Results: The parent–metabolite model adequately described the multiple-dose plasma concentration–time profiles of ribociclib and LEQ803. On-treatment plasma 4β-OHC concentration was identified as a significant predictor for CYP3A-mediated ribociclib metabolism to LEQ803, accounting for ~36% of interindividual variability in the metabolic clearance. Plasma 4β-OHC showed limited ability to predict total apparent oral clearance or systemic exposure of ribociclib. Conclusions: These findings support the clinical utility of plasma 4β-OHC as an endogenous biomarker of hepatic CYP3A activity for predicting CYP3A-mediated metabolism of substrate drugs such as ribociclib, while highlighting its limitation for predicting total apparent oral clearance of drugs undergoing intestinal first-pass metabolism and elimination through multiple pathways. These results underscore the importance of considering the relative contribution of hepatic CYP3A metabolism to overall drug disposition when evaluating translational utility of endogenous CYP3A biomarkers in clinical pharmacology. Full article

Background and Objectives: Palbociclib and ribociclib are widely used with letrozole as first-line treatment for postmenopausal patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer. Pivotal trials have reported different overall survival signals, but they were not designed as direct comparisons. This study evaluated real-world survival outcomes with these regimens and explored the prognostic performance of a composite clinical-inflammatory risk score. Materials and Methods: This retrospective single-center cohort included 140 postmenopausal patients treated with first-line palbociclib/letrozole or ribociclib/letrozole. Overall survival, progression-free survival, best documented response, baseline characteristics, metastatic pattern, and baseline pan-immune-inflammation value were evaluated. The composite risk score assigned one point each for progesterone receptor expression <20%, visceral metastasis, and pan-immune-inflammation value ≥477.8. Pan-immune-inflammation value was additionally evaluated as a continuous variable. The prognostic performance of the score was assessed using Harrell’s c-index, bootstrap internal validation, and comparison with a parsimonious clinical model. Results: Among 140 patients, 67 received palbociclib/letrozole and 73 received ribociclib/letrozole. Median follow-up was 52.3 months. A total of 93 deaths and 118 progression-free survival events occurred. Median overall survival was 36 months with palbociclib/letrozole and 29 months with ribociclib/letrozole, with no statistically significant difference between groups (HR, 1.23; 95% CI, 0.82–1.86; p = 0.322). Median progression-free survival was 15.4 and 16.4 months, respectively (HR, 1.05; 95% CI, 0.73–1.52; p = 0.778). Median overall survival was 34 months for CRS 0–1 and 30 months for CRS 2–3 (univariable HR, 1.68; 95% CI, 1.11–2.54; p = 0.014). The apparent and optimism-corrected c-indices of the score were 0.573 and 0.570, respectively. Addition of the score to the parsimonious clinical model did not significantly improve model fit (likelihood-ratio p = 0.143). Conclusions: No statistically detectable difference in overall survival or progression-free survival between the treatment groups was observed in this cohort. Although grouped CRS was associated with overall survival in univariable analysis, the score showed limited discrimination and no statistically significant incremental prognostic value. These findings remain exploratory and require external validation. Full article

Cyclin-dependent kinase 4/6 inhibitors have fundamentally changed the management of hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancer. However, these drugs are not interchangeable and the field is moving away from the notion of a uniform “class effect.” In early breast cancer, adjuvant abemaciclib and ribociclib improve invasive disease-free survival in patients at a high risk of recurrence, whereas palbociclib does not. This difference likely stems from agent-specific pharmacological profiles, differences in trial design, and patient selection, rather than simply dosing nuances. In metastatic breast cancer, all three agents prolong progression-free survival when combined with endocrine therapy, but only ribociclib and potentially abemaciclib have shown an overall survival advantage. In addition, resistance remains a major obstacle in clinical practice. We propose that resistance mechanisms can be meaningfully grouped into two categories: target-driven (e.g., RB1 loss, CDK6 amplification) and bypass-driven (e.g., ESR1 mutations, PI3K/AKT pathway activation, APOBEC3-mediated mutagenesis). Distinguishing between these classes helps in the design of rational sequencing algorithms and combinatorial regimens. Emerging strategies, such as next-generation protein degraders, oral selective estrogen receptor degraders, antibody–drug conjugates, and inhibition of autophagy, are promising methods for overcoming resistance. Moving forward, the greatest need in breast cancer treatment will be not simply developing additional agents but using current therapies more intelligently by refining biomarker-guided patient selection, tailoring treatment duration, and ensuring broad global access. Full article

Background/Objective: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy are the standard first-line treatment for HR+/HER2− advanced/metastatic breast cancer (a/mBC), yet no head-to-head randomized trials have compared palbociclib, ribociclib, and abemaciclib. This systematic literature review (SLR) aimed to synthesize available real-world evidence (RWE) on the comparative effectiveness of these three CDK4/6i in the first-line setting. Methods: An SLR was conducted following PRISMA guidelines. Searches of MEDLINE, Embase, Cochrane, and gray literature (January 2015–September 2025) identified RWE studies reporting real-world progression-free survival (rwPFS) and/or overall survival (OS) for first-line CDK4/6i regimens. Eligible studies included adults with HR+/HER2− a/mBC receiving palbociclib, ribociclib, or abemaciclib, combined with endocrine-based therapy. Comparative outcomes were summarized qualitatively; study quality was assessed using the Newcastle–Ottawa Scale, ISPOR-AMCP-NPC questionnaire, and ESMO-GROW checklist. Results: From 13,345 records, 39 publications (32 unique studies) were identified, of which 21 were full-text studies meeting inclusion criteria. Most included studies used retrospective cohort designs, and approximately 50% evaluated all three CDK4/6is. Given the comparative nature of the included studies, quality issues were found to be related to study design and analytical approaches. Several analyses were unadjusted, and reporting of follow-up duration varied across studies, with some analyses providing incomplete follow-up information. Study sample sizes differed substantially across agents, with smaller populations generally reported for ribociclib and abemaciclib. Among the studies reporting rwPFS hazard ratios (HRs), 8/11 studies showed comparable rwPFS and 5/6 studies reported comparable OS outcomes between CDK4/6i regimens. Three full-text studies reported HRs favoring ribociclib or abemaciclib relative to palbociclib; these analyses primarily included populations receiving palbociclib, with smaller comparator cohorts for ribociclib and abemaciclib, as well as varied follow-up times. Conclusions: Most real-world studies included in the SLR suggested similar effectiveness of the three CDK4/6is in first-line HR+/HER2− a/mBC. Heterogeneity in patient characteristics, definitions of outcomes, follow-up time, sample size, and statistical approach may have important implications on study interpretability. Full article

Background/Objectives: First-generation CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) target the conserved ATP-binding pocket of CDK4 and, despite clinical success, are limited by acquired resistance and insufficient exploration of alternative regulatory sites. This study aimed to identify a putative allosteric small-molecule candidate at the CDK4 αE-helix–Cyclin D1 α1-helix protein–protein interaction (PPI) interface within the CDK4/Cyclin D1/p21 ternary complex using RapidFunnel-AI, a decision-interpretable virtual-screening pipeline. Methods: Starting from 50,000 ChEMBL 33 molecules, the pipeline sequentially applied a Q-Fold/RapidFunnel topological Tanimoto scan based on clinical CDK4/6 inhibitor motifs, fragment-level electronic-property enrichment, ADMET/PAINS filtering, dry Vina-GPU docking, hydration-mediated AutoDock-GPU (Version 1.6) docking, explicit-solvent molecular dynamics, contact-retention analysis, and MM-GBSA energy decomposition. The Q-Fold Thermo-Core surrogate model provided fragment-level enrichment, predicting the HOMO–LUMO gap (R² = 0.93) and isotropic polarizability (R² = 0.98) on QM9. Candidate selection did not rely on the lowest docking or MM-GBSA score alone, but on pose persistence, contact continuity, and energy-component consistency. Results: The workflow reduced the initial library to 43 topologically prioritized candidates, 25 ADMET/PAINS-filtered ligands, and 9 docking-derived complexes for MD validation. Ligand_020 emerged as the only candidate that preserved a persistent binding mode at Site 2 during a 500 ns simulation—an interface engagement reproduced across three independent 500 ns replicates with no full dissociation in any replicate—with a protein Cα RMSD of 2.88 ± 0.32 Å, a ligand heavy-atom RMSD of 3.56 ± 0.28 Å, and a van der Waals-dominated MM-GBSA profile (ΔGbind = −28.23 ± 3.57 kcal/mol). In contrast, palbociclib and ribociclib, forcibly placed at Site 2 as negative controls, lost most initial contacts within 5 ns and tended to detach despite more favorable MM-GBSA values. Conclusions: These results suggest that single-score docking or MM-GBSA ranking can generate false positives at shallow PPI interfaces. By integrating AI-assisted prioritization, multipocket docking, explicit-solvent MD, contact-retention analysis, and energy-component consistency, RapidFunnel-AI nominated Ligand_020 as an experimentally testable putative allosteric hit targeting the CDK4/Cyclin D1 interface, offering a reusable platform for PPI-focused oncological drug discovery. Full article

Germline BRCA1/2-mutated (gBRCAm) hormone receptor-positive/HER2-negative (HR+/HER2-) metastatic breast cancer (MBC) is a biologically distinct subset in which the efficacy of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors remains incompletely characterized. We evaluated real-world outcomes and prognostic factors in a multicenter retrospective Turkish cohort treated with a CDK4/6 inhibitor plus endocrine therapy (June 2020–September 2025). Progression-free survival (PFS) and overall survival (OS) were estimated using Kaplan–Meier and Cox methods. Among 121 patients, 30 (24.8%) had BRCA1, 88 (72.7%) had BRCA2, and three (2.5%) had dual mutations; 66.9% received first-line therapy, with ribociclib in 69.4% and palbociclib in 29.8%. Objective response rate was 69.4% and the clinical benefit rate was 82.6%. Median PFS was 17.0 months and OS 47.0 months. PFS was numerically longer in BRCA1 than in BRCA2 carriers (25.0 vs. 14.0 months), although the difference was not statistically significant in the pairwise comparison (HR 1.50, 95% CI 0.88–2.56; log-rank p = 0.135); the dual BRCA1/2 subgroup (n = 3) had the poorest outcomes and was assessed descriptively. OS did not differ significantly between BRCA1 and BRCA2 carriers (57.0 vs. 49.0 months; log-rank p = 0.520). PFS did not differ between ribociclib and palbociclib (p = 0.192); OS favored ribociclib at borderline significance (p = 0.050), but this was not confirmed in Cox regression. In multivariable analysis, ECOG ≥ 1 (HR 1.85; p = 0.010) and fulvestrant-based therapy (HR 1.74; p = 0.041) predicted shorter PFS; fulvestrant also predicted worse OS (HR 2.39; p = 0.008). CDK4/6 inhibitor-based therapy shows meaningful activity in gBRCAm HR+/HER2- MBC; the numerically poorer outcomes observed in BRCA2 carriers are hypothesis-generating and warrant validation in larger cohorts. Full article

Background/Objectives: The prognostic significance of the uric acid-to-albumin ratio (UAR) in patients with hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2−) metastatic breast cancer treated with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors has not been adequately investigated. This study aimed to investigate the association between baseline UAR and survival outcomes in this patient population. Methods: This retrospective study included HR-positive/HER2-negative metastatic breast cancer patients treated with ribociclib or palbociclib at Necmettin Erbakan University between May 2020 and April 2025. UAR was calculated by dividing the serum uric acid level (mg/dL) by the serum albumin level (g/dL). Based on receiver operating characteristic (ROC) analysis, the optimal cut-off value for UAR was identified as 1.0 (AUC = 0.67; sensitivity 68%; specificity 58%). Patients were subsequently classified into two groups as UAR < 1 and UAR ≥ 1. Progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan–Meier method and compared with the log-rank test. Independent prognostic factors were evaluated using Cox regression analyses. Results: A total of 118 eligible patients were included in the analysis, including 34 (28.8%) in the UAR < 1 group and 84 (71.2%) in the UAR ≥ 1 group. The proportion of postmenopausal patients was significantly higher in the UAR ≥ 1 group (p = 0.01). Kaplan–Meier analysis showed that median PFS was not reached in the UAR < 1 group, whereas it was 33.05 months in the UAR ≥ 1 group (log-rank p = 0.06). Median OS was not reached in the UAR < 1 group and was 50.7 months in the UAR ≥ 1 group (p = 0.017). Multivariate Cox regression analysis demonstrated that UAR < 1 was associated with improved PFS (HR = 0.65; 95% CI: 0.34–0.89; p = 0.04). Postmenopausal status emerged as an independent adverse prognostic factor for PFS (HR = 1.92; 95% CI: 1.10–4.05; p = 0.04). In addition, UAR < 1 was associated with a reduced risk of mortality in the OS analysis (HR = 0.61; 95% CI: 0.26–0.87; p = 0.01). Conclusions: Lower baseline UAR was associated with more favorable survival outcomes in HR-positive/HER2-negative metastatic breast cancer patients treated with CDK4/6 inhibitors. As an inexpensive and easily accessible biomarker derived from routine laboratory parameters, UAR may provide additional prognostic information for clinical risk stratification. Full article

Introduction: In the treatment of hormone receptor-positive (HR+), HER2-negative (HER2−) metastatic breast cancer (MBC), the current guidelines recommend endocrine therapy combined with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors as the preferred first-line treatment to preserve quality of life. Ribociclib is a CDK4/6 inhibitor that has been used in combination with aromatase inhibitors or fulvestrant in patients with HR+, HER2− metastatic breast cancer. Various adverse drug reactions associated with ribociclib have been reported, including cutaneous reactions, hepatotoxicity, and hematologic toxicity. In this study, we aimed to evaluate the clinical manifestations and risk factors of dermatologic toxicities in patients with metastatic breast cancer treated with ribociclib. Methods: This retrospective study included patients with metastatic/recurrent breast cancer who were prescribed ribociclib from April 2021 to December 2024 at a single institution. We retrospectively reviewed the medical records of these patients to identify the frequency of cutaneous adverse events, the time of onset, and the clinical characteristics of skin reactions. Logistic regression analysis was performed on several clinical factors, including body surface area (BSA) and concomitant medications, to identify risk factors associated with the occurrence of cutaneous adverse events. Results: A total of 110 patients with MBC were enrolled during the study period. The median age was 53 years (range, 28–82); all 110 patients (100.0%) were female; the median BSA was 1.56 m² (range, 1.29–2.07); and 32 patients (29.1%) were premenopausal. Ribociclib plus letrozole was administered in 48 patients (43.6%) and ribociclib plus fulvestrant in 29 patients (26.4%). An additional 33 patients (30.0%) received ribociclib plus letrozole with a gonadotropin-releasing hormone (GnRH) agonist. Cutaneous adverse events occurred in 29 patients (26.4%), and the median time to onset was 84 days (range, 3–498). The cutaneous adverse event patterns included pruritus, erythematous macular rash, eczematous rash/contact dermatitis, vitiligo, urticarial reactions, polymorphous light eruption, toxic epidermal necrolysis (TEN), and desquamation. Grade 1 or 2 cutaneous adverse events occurred in 93.1% of patients; Grade 3 toxicity occurred in one patient; and Grade 4 toxicity, namely toxic epidermal necrolysis (TEN), was reported in one patient. Dose reduction was required in three patients (10.3%), and permanent discontinuation of ribociclib occurred in one patient. Clinical improvement was achieved in the majority of patients (86.2%) with cutaneous adverse events following supportive care. Logistic regression analysis revealed that age, Eastern Cooperative Oncology Group (ECOG) performance status, body surface area (BSA), treatment regimen, and use of cholesterol-lowering medications were not independently associated with the development of cutaneous adverse events. Conclusion: CDK4/6 inhibitors represent one of the most important treatment options for HR+/HER2− metastatic breast cancer. Regardless of their clinical efficacy, cutaneous adverse events remain a common source of patient discomfort. Therefore, careful clinical attention and appropriate supportive care are essential to improve patients' quality of life. Full article

CDK4/6 inhibitors such as palbociclib, ribociclib and abemaciclib are commonly used in the clinical treatment of HR-positive, HER2-negative metastatic or locally advanced breast cancer. Patients with metastatic disease often receive palliative radiotherapy for symptom control of bone metastases and/or local lesions, typically administered in close temporal proximity to CDK4/6 inhibitor therapy, although treatment with the inhibitors may be temporarily paused during the radiotherapy period in some cases. In this study, we investigated the extent to which senescence is induced by CDK4/6 inhibitors, ionizing radiation, and the combination of the two, compared to other types of cell fate. Eight breast cancer cell lines with different molecular subtypes and two healthy cell lines (fibroblasts and keratinocytes) were treated with CDK inhibition using palbociclib, ribociclib or abemaciclib and with or without a single dose of 2 Gy ionizing radiation. Cellular senescence, cell death in form of apoptosis and necrosis, and the cell cycle were analyzed using flow cytometry. We focused mainly on understanding how CDK inhibition can trigger cellular senescence. Our data showed that in many cell lines —but not all—the use of CDK inhibitors induced senescence much more strongly than cell death. Except for one cell line, significantly more cell lines died necrotically than apoptotically. Neither apoptosis nor necrosis was responsible for a major cell fate after CDK inhibition. Combination therapy with irradiation did not show a clear additive effect. In cell lines, senescence is clearly triggered by CDK4/6 inhibitors and even more so when in combination with ionizing radiation, which, when transferred to patients, could lead to less damage caused by cell loss, such as necrotic areas. However, it could also lead to more senescence-specific side effects, such as inflammation-induced tumors and fibrosis. Full article

Background: Hormone receptor-positive/HER2-negative (HR+/HER2−) early breast cancer (EBC) presents a persistent risk of relapse, even beyond 5 years, driving the need for adjuvant intensification strategies. This review analyzes the clinical evidence for CDK4/6 inhibitors (CDK4/6i) in the adjuvant setting. This evidence is then integrated with molecular findings to support the concept of the “carry-over” effect, which is understood as a lasting benefit that persists after the end of active treatment, reflected by a sustained separation of invasive disease-free survival (iDFS) curves during follow-up. Relevant Sections: The main adjuvant trials in EBC are reviewed, with consideration of the “carry-over” effect. Emerging biomarkers and the impact of financial toxicity are also described. Results: PALLAS did not demonstrate a clear on-treatment or post-treatment benefit, whereas PENELOPE-B suggested, at most, a transient early advantage that was not maintained with longer follow-up; therefore, neither trial provides convincing evidence of a durable “carry-over” effect. In contrast, monarchE (abemaciclib) and NATALEE (ribociclib) showed significant improvements in iDFS and, in the case of abemaciclib, a signal of benefit in overall survival, supporting the existence of a clinically relevant post-treatment effect. Conclusions: From a biological perspective, the review proposes that the “carry-over” effect should not be considered a uniform class effect, but rather the result of a sequence of events modulated by pharmacological selectivity (CDK4 vs. CDK6 and additional targets), the induction of cellular senescence, and immunomodulatory effects that could favor the control of micrometastases. In addition, elements that influence interpretation and the need to optimize adherence and toxicity management to “materialize” the benefit in a potentially curable context are discussed. Full article

Cyclin-dependent kinases 4 and 6 (CDK4/6) play a central role in the regulation of cell cycle progression and represent important therapeutic targets in hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2−) breast cancer. The introduction of selective CDK4/6 inhibitors, including palbociclib, ribociclib, and abemaciclib, in combination with endocrine therapy, has significantly improved clinical outcomes and has become a standard treatment strategy in both metastatic and high-risk early-stage disease. Nevertheless, treatment resistance and disease progression remain major clinical challenges. A deeper understanding of the structural characteristics of CDK4/6 and the molecular basis of inhibitor binding is therefore essential for improving therapeutic strategies and guiding the development of new targeted agents. This review provides an integrated overview of the structural features of CDK4/6 and their role in cell cycle regulation, summarizes the clinical development and major clinical trials of currently approved CDK4/6 inhibitors, and discusses recent computational studies investigating inhibitor binding and conformational dynamics. Particular attention is given to the application of in silico approaches, including molecular docking, molecular dynamics simulations, and binding free-energy calculations, which provide insights into mechanisms of therapy resistance and potential strategies to overcome them and support the identification and optimization of novel CDK4/6-targeted therapeutic candidates. By integrating structural, clinical, and computational perspectives, this review highlights current knowledge and emerging directions in CDK4/6 research that may advance the development of more personalized therapies for HR+/HER2− breast cancer, while accounting for both intrinsic and de novo resistance mechanisms. Full article

Background/Objectives: Epirubicin, Olaparib, and Ribociclib are widely used anticancer agents whose serum concentrations exhibit significant inter-individual variability, supporting the need for reliable and robust analytical methods suitable for pharmacokinetic evaluation and therapeutic exposure assessment. Variations in metabolism, drug–drug interactions, organ function, and treatment regimens may substantially influence systemic exposure, highlighting the importance of accurate quantification in clinical practice. This study describes the development and validation of a solid-phase extraction–liquid chromatography–mass spectrometry (SPE–LC–MS) method for the simultaneous quantification of these drugs in human serum. Methods: Sample preparation was performed using Oasis PRiME HLB^® cartridges to ensure efficient clean-up, optimal recovery, and reduced matrix effects. Chromatographic separation was achieved using gradient elution with 0.1% formic acid and acetonitrile on a reversed-phase column, followed by single-quadrupole mass spectrometric (QDa) detection in the selected ion recording mode. The total run time was 13 min, enabling high-throughput analysis. Results: The method demonstrated good linearity (r > 0.997) over the tested concentration ranges, along with adequate selectivity, precision, accuracy, recovery, and stability, fulfilling the ICH M10 guideline validation criteria. No significant carry-over or interference from endogenous compounds was observed. Conclusions: Application to patient samples confirmed reliable performance in real clinical matrices and consistent quantification across different concentration levels. The proposed approach provides a potentially more accessible alternative in laboratories already equipped with LC-MS systems compared to LC-MS/MS platforms and can be applied in pharmacokinetic studies, representing a proof-of-concept for exposure assessment in oncology. Full article

Background: Ribociclib, initially approved for HR+/HER2− advanced breast cancer (ABC) at a 600 mg dose, was recently approved for HR+/HER2− early breast cancer (EBC) at a 400 mg dose based on the NATALEE trial. Differences in dose and patient population warrant reassessment of ribociclib drug–drug interactions (DDIs) and the impact of hepatic or renal impairment (HI/RI) in EBC patients to guide co-medication management and subpopulation dose recommendations. Methods: Physiologically-based pharmacokinetic (PBPK) modeling based on a healthy volunteer population was conducted to assess ribociclib DDIs with CYP3A4 substrates/modulators in patients with EBC. Subgroup analysis from NATALEE assessed HI/RI impact on ribociclib PK in EBC patients. Existing data from ABC/advanced cancer patients and non-cancer subjects were also integrated to inform dose recommendations for EBC subpopulations. Results: PBPK modeling predicted that ritonavir or erythromycin (strong and moderate CYP3A4 inhibitors) would increase ribociclib steady-state area under the concentration–time curve (AUC) by 1.84-fold or show no meaningful impact, respectively. Steady-state ribociclib AUC was estimated to decrease by 83% and 74% with rifampicin and efavirenz, strong and moderate CYP3A4 inducers, respectively. Ribociclib was estimated to increase CYP3A4 substrate midazolam exposure by 280%. Mild HI or mild/moderate RI did not show an apparent impact on ribociclib PK. Conclusions: Using relevant data and methodology for EBC patients, this analysis informed the approved ribociclib label of no dose adjustment for EBC patients with concomitant use of a moderate CYP3A inhibitor, any degree of HI, or mild/moderate RI, and a reduced 200 mg dose for patients with concomitant use of a strong CYP3A inhibitor or severe RI. Full article

Background and objectives: Early biomarkers that can reliably predict treatment outcomes during CDK4/6 inhibitor therapy remain an unmet clinical need in patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2−) metastatic breast cancer (MBC). Metabolic changes on ^18F-FDG PET/CT may precede radiologic response and provide insight into tumor biology and early treatment resistance. Methods: This two-center retrospective study included 203 patients with HR+/HER2− MBC who received first-line CDK4/6 inhibitors (ribociclib or palbociclib) plus endocrine therapy between 2018 and 2024. Baseline and interim ^18F-FDG PET/CT scans performed after 2–4 cycles were evaluated. Early metabolic response was defined as a ≥30% reduction in SUVmax on the most metabolically active lesion, consistent with PERCIST 1.0. Progression-free survival (PFS) and overall survival (OS) were analyzed using Kaplan–Meier and multivariable Cox models. ROC analysis assessed the discriminative performance of ΔSUVmax for predicting disease progression. Results: Among 203 patients, 153 (75.4%) achieved a ≥30% SUVmax reduction. Responders had significantly longer PFS (median 44.4 vs. 4.8 months; p < 0.001) and OS (median not reached vs. 32.0 months; p < 0.001). Metabolic response remained independently associated with improved PFS (HR 0.24; 95% CI 0.15–0.37; p < 0.001) and OS (HR 0.37; 95% CI 0.20–0.67; p = 0.001) after adjustment for tumor grade, endocrine resistance, and visceral disease involvement. Non-responders demonstrated more aggressive baseline features, including higher rates of liver (34.0% vs. 15.0%) and brain metastasis (10.0% vs. 1.3%), as well as lower progesterone receptor expression (median 30% vs. 60%). Conclusions: Early metabolic response assessed by SUV-max on interim ^18F-FDG PET/CT is independently associated with substantially improved PFS and OS in HR+/HER2− MBC receiving treatment with CDK4/6 inhibitors. Although the predictive accuracy of ΔSUVmax alone was modest, the strong survival gradient suggests meaningful prognostic value. Prospective studies with standardized imaging time points and comprehensive metabolic metrics are warranted to define the role of PET-guided treatment adaptation: Full article

Background: The prognostic significance of low-level human epidermal growth factor receptor 2 (HER2) expression in hormone receptor-positive/HER2-negative (HR+/HER2−) metastatic breast cancer remains unclear, particularly in patients treated with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i). This study aimed to evaluate the impact of HER2-low status on treatment response and survival outcomes in this setting. Methods: This multicenter retrospective cohort study included patients with HR+/HER2− metastatic breast cancer who received first-line endocrine therapy combined with palbociclib or ribociclib between January 2018 and May 2025. HER2-low tumors were defined as immunohistochemistry (IHC) 1+ or 2+ with negative in situ hybridization, while HER2-zero tumors were classified as IHC 0. Treatment response, progression-free survival (PFS), and overall survival (OS) were compared between groups using Kaplan–Meier analysis and Cox regression models. Results: A total of 309 patients were analyzed, including 122 (39.5%) with HER2-low disease and 187 (60.5%) with HER2-zero disease. Baseline clinicopathological characteristics were well balanced between groups. The overall response rate was 75.4% in the HER2-low group and 72.7% in the HER2-zero group (p > 0.05). Median PFS was 23.9 months for HER2-low patients and 25.2 months for HER2-zero patients (log-rank p = 0.785). Median OS was 49.5 and 53.1 months, respectively, with no statistically significant difference (log-rank p = 0.649). HER2 status was not an independent predictor of PFS or OS in multivariable analyses. Conclusions: In patients with HR+/HER2− metastatic breast cancer treated with first-line endocrine therapy plus CDK4/6 inhibitors, HER2-low expression was not associated with differences in treatment response or survival outcomes. These findings suggest that HER2-low status does not have prognostic or predictive relevance in this endocrine-sensitive population. Full article