Co-targeting CDK2 and CDK4/6 overcomes resistance to aromatase and CDK4/6 inhibitors in ER+ breast cancer

Discovery and development of steroidal enzyme inhibitors as anti-cancer drugs: state-of-the-art and future perspectives

Steroidal compounds have emerged as effective therapeutic agents in oncology. Beyond natural-occurring and synthetic steroids that act as cytotoxic anti-tumoral agents, steroidal derivatives can be designed to mime the endogenous substrates of key metabolic enzymes in steroidogenesis, thus reducing the circulating levels of relevant oestrogenic and androgenic hormones responsible for cancer survival and proliferation. Therefore, enzyme inhibition represents an intriguing endocrine approach for the treatment of hormone-dependent tumours, such as breast and prostate cancer, with well-known approved drugs and several pre-clinical and clinical candidates under investigation. This review summarises the key advancements over the past decade (2014-2024) in the development of steroidal enzyme inhibitors endowed with anticancer activity, illustrating their mechanisms of action, therapeutic potential, drug design approaches, and current clinical applications. Furthermore, we discuss challenges related to drug resistance, off-target effects, and future strategies to optimise their efficacy in oncology.

Pharmacological modulation of cellular senescence: Implications for breast cancer progression and therapeutic strategies

Senescence, defined by the cessation of cell proliferation, plays a critical and multifaceted role in breast cancer progression and treatment. Senescent cells produce senescence-associated secretory phenotypes (SASP) comprising inflammatory cytokines, chemokines, and small molecules, which actively shape the tumor microenvironment, influencing cancer development, progression, and metastasis. This review provides a comprehensive analysis of the types and origins of senescent cells in breast cancer, alongside their markers and detection methods. Special focus is placed on pharmacological strategies targeting senescence, including drugs that induce or inhibit senescence, their molecular mechanisms, and their roles in therapeutic outcomes when combined with chemotherapy and radiotherapy. By exploring these pharmacological interventions and their impact on breast cancer treatment, this review underscores the potential of senescence-targeting therapies to revolutionize breast cancer management.

Baseline cell cycle and immune profiles indicate CDK4/6 inhibitor response in metastatic HR + /HER2- breast cancer

While CDK4/6 inhibitors (CDK4/6i) and endocrine therapy are standard-of-care for metastatic HR + /HER2- breast cancer, patient selection for durable efficacy remains undefined. Here, we assessed baseline cell cycle and immune profiles in a CDK4/6i-treated patient cohort with differential progression-free survival (PFS < 6 months vs. >23 months) using transcriptomic and protein-based imaging approaches. Cell cycle, polo-like kinase signaling and transcription gene sets are largely enriched among pre-treatment tissue of patients with short PFS. Pre-treatment tumors express cyclin A or E significantly higher in patients with short PFS and correlate with macrophage accumulation. Patients with long PFS display gene set enrichment for growth factor and immune signaling pre-treatment, while gene set enrichment for immune activation emerges during CDK4/6i therapy. Our data highlight baseline tumor-intrinsic and tumor microenvironments-associated indicators of CDK4/6i response in the "real-world" setting and offer implications for precision-based therapeutic combinations to enhance CDK4/6i efficacy. Clinical trial registration number: NCT04526587.

After a CDK4/6 Inhibitor: State of the Art in Hormone Receptor-Positive Metastatic Breast Cancer

CDK 4/6 inhibitors (CDK4/6i) remain part of the standard first-line treatment for patients with hormone receptor-positive metastatic breast cancer, offering demonstrable improvements in both progression-free survival and overall survival. However, resistance inevitably develops, and the optimal treatment sequencing after CDK4/6i progression remains undefined. Tumor heterogeneity and diverse resistance mechanisms-including alterations in ESR1 and PIK3CA-complicate treatment decisions in the post-CDK4/6i setting. Genomic profiling has helped to characterize these and other clinically relevant alterations, uncovering new avenues for therapeutic intervention. Building on these insights, a growing number of novel endocrine agents, phosphoinositide-3-kinase/AKT pathway-targeted therapies, and antibody-drug conjugates (ADCs) have demonstrated efficacy in biomarker-selected populations and are reshaping the treatment landscape beyond CDK4/6i progression. This chapter reviews current standards of care, emerging therapeutic options, and evolving combination strategies across biomarker-defined subgroups. We also highlight how ongoing clinical trials and advances in molecular profiling are informing personalized approaches to overcome endocrine resistance and improve patient outcomes.

Mechanism and Predictive Role of NUB1 Protein in Oestrogen Receptor Pathway of FEC-Treated Breast Cancer Patients

Introduction: NEDD8 Ultimate Buster 1 (NUB1) is a regulator of the cell cycle and a prognostic marker in cancer patients. However, its role in breast cancer (BC) and its response to 5-fluorouracil, epirubicin, and cyclophosphamide (FEC) treatment remain unclear. This study investigated NUB1's predictive value in FEC treatment and its mechanistic interaction with the oestrogen receptor (ER) in BC. Methods: MDA-MB-231 and MCF-7 cells were treated with FEC and analysed via flow cytometry for cell cycle distribution. Western blotting assessed NUB1 and ERα expression, while immunohistochemistry was conducted on a retrospective cohort (n = 85) from Malaysian hospitals to evaluate the clinical significance of NUB1 expression. Results: FEC treatment induced S and G2 phase cell cycle arrest in MDA-MB-231 cells (p = 0.04 and p = 0.02, respectively), accompanied by NUB1 upregulation. In MCF-7 cells, G2/M arrest was observed (p = 0.01), with reduced ERα expression and increased NUB1 levels in both cell lines. Lower cytoplasmic NUB1 expression was associated with poorer overall survival (OS) (HR = 0.60; 95% CI = 0.32-1.11; p = 0.10). Patients with low NUB1 and low ER expression showed the worst OS outcomes. Discussion: NUB1 upregulation following FEC treatment led to cell cycle arrest in ER-negative cells, whereas ERα suppression failed to induce S-phase arrest in ER-positive cells. Low NUB1 expression predicted poorer OS and increased BC recurrence. Conclusions: By integrating in vitro and clinical data, this study suggests that NUB1 may serve as a predictive biomarker in FEC-treated breast cancer. Larger studies are needed to validate and establish NUB1's predictive role in FEC-treated patients.

CDK2 inhibition produces a persistent population of polyploid cancer cells

Aneuploidy, a cancer hallmark, drives chromosomal instability, drug resistance, and clinically aggressive tumors. Cyclin-dependent kinase 2 (CDK2) antagonism with independent inhibitors or CDK2 knockdown triggered anaphase catastrophe. This disrupts supernumerary centrosome clustering, causing multipolar division and apoptosis. Time-lapse fluorescence microscopy of fluorescent ubiquitination-based cell cycle indicator (FUCCI) cell cycle probes transduced into aneuploid lung cancer cells revealed distinct fates of bipolar and polyploid cells after CDK2 inhibition. Apoptosis occurred in multipolar progeny but was repressed in persistent polyploid cancer cells. RNA-Seq analyses after CDK2 inhibition of 4N versus 2N lung cancer cells were enriched for CDK1 pathway and KIF family members. The Cancer Genome Atlas (TCGA) analysis of lung cancers indicated that CDK1 and KIF family member overexpression was associated with an unfavorable survival. Intravital microscopy of transplanted lung cancer cells in mice extended findings from the in vitro to in vivo settings. CDK2 inhibition of tumor-bearing mice produced polyploid cancer cells in vivo. These cancer cells were resistant to apoptosis and proliferated despite CDK2 inhibition. In contrast, polyploid populations were rarely detected in CDK2-inhibited human alveolar epithelial cells. These findings are translationally relevant. Combined targeting of CDK2 with CDK1 or kinesin family member antagonists should eliminate polyploid cancer cells, promote apoptosis, and augment antineoplastic effects.

CDK4/6 inhibition in early and advanced hormone receptor-positive, HER2-negative breast cancer

INTRODUCTION

Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibition in combination with endocrine therapy is the mainstay of treatment for hormone receptor-positive, HER2-negative (HR  +  /HER2-) advanced breast cancer; the CDK4/6 inhibitors abemaciclib and ribociclib are also approved for high-risk, early-stage, HR  +  /HER2- breast cancer. Numerous studies exploring CDK4/6 inhibitors in the early-stage setting are ongoing, as well as many more exploring novel combinations in the metastatic setting.

AREAS COVERED

Here, we review the basis of CDK4/6 inhibition for HR  +/HER2- breast cancer, the pivotal clinical trials which led to regulatory approval, and ongoing trials evaluating novel combinations to further improve outcomes for those with both early and advanced HR+/HER2- breast cancer. Current literature was reviewed by a comprehensive search of PubMed MEDLINE (1/1/2000-12/31/2024).

EXPERT OPINION

CDK4/6 inhibitors are integral in the management of both advanced and high-risk, early-stage HR  +  /HER2- breast cancer. Biomarkers predictive of CDK 4/6 inhibitor (CDK4/6i) benefit remain elusive, and clinical and pathological features remain key to identifying those who are candidates for CDK4/6 inhibition in the early-stage setting. Numerous trials evaluating the role of a CDK4/6i with novel endocrine therapy partners and other targeted agents are ongoing, with the goal of improving outcomes for those with HR  +  /HER2- disease.

Molecular Mechanisms and Therapeutic Strategies to Overcome Resistance to Endocrine Therapy and CDK4/6 Inhibitors in Advanced ER+/HER2- Breast Cancer

Approximately 70-80% of breast cancers are estrogen receptor-positive (ER+), with 65% of these cases also being progesterone receptor-positive (ER+PR+). In most cases of ER+ advanced breast cancer, endocrine therapy (ET) serves as the first-line treatment, utilizing various drugs that inhibit ER signaling. These include tamoxifen, a selective estrogen receptor modulator (SERM); fulvestrant, a selective estrogen receptor degrader (SERD); and aromatase inhibitors (AIs), which block estrogen synthesis. However, intrinsic or acquired hormone resistance eventually develops, leading to disease progression. The combination of ET with cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6is) has been shown to significantly increase progression-free survival (PFS) and, in some cases, overall survival (OS). CDK4/6is works by arresting the cell cycle in the G1 phase, preventing DNA synthesis, and enhancing the efficacy of ET. This review highlights the key mechanisms of resistance to ET, whether used alone or in combination with biological agents, as well as emerging therapeutic strategies aimed at overcoming resistance. Addressing ET resistance remains a work in progress, and in the near future, better patient selection for different therapeutic approaches is expected through the identification of more precise biological and genetic markers. In particular, liquid biopsy may provide a real-time portrait of the disease, offering insights into mechanisms driving ET resistance and cancer progression.

Discovery of 2,4-quinazolinedione derivatives as LC3B recruiters in the facilitation of protein complex degradations

Targeted protein degradation through autophagosome-tethering compounds (ATTECs) that bypasses the ubiquitination process has garnered increasing attention. LC3B, a key protein in autophagosome formation, recruits substrates into the autophagy-lysosome system for degradation. In this study, we systematically optimized 2,4-quinazolinedione derivatives as LC3B-recruiting fragments, utilizing the CDK9 indicator. By attaching the designed LC3B-recruiting fragment to CDK9 inhibitor SNS-032 through a linker, the resulting bifunctional ATTEC molecule simultaneously degraded CDK9 and its associated Cyclin T1. Two-dimensional NMR experiments confirmed the direct interaction between the novel LC3B-recruiting fragments and LC3B. Mechanistic studies elucidated that degradation occurred via an LC3B-dependent autophagy-lysosomal pathway. Additionally, the general applicability of leveraging LC3B-recruiting fragments linked to inhibitors for the targeted degradation of protein complexes was validated with PRC2 and CDK2/4/6 along with their respective Cyclins. This work provides a series of novel LC3B-recruiting fragments that enrich the ATTEC toolbox and can be applied to the degradation of diverse intracellular disease-causing proteins.

Profiling the Activity of the Potent and Highly Selective CDK2 Inhibitor BLU-222 Reveals Determinants of Response in CCNE1-Aberrant Ovarian and Endometrial Tumors

BLU-222 is an investigational, potent, highly selective, orally bioavailable cyclin-dependent kinase 2 (CDK2) inhibitor in clinical development. BLU-222 demonstrated robust antitumor activity in select CCNE1-high ovarian and endometrial cancer models. We used a combination of CRISPR whole-genome screens coupled with targeted genetic and pharmacologic approaches in ovarian and endometrial cell lines to identify biological determinants to predict BLU-222 monotherapy activity. Rb and p16 expression were biomarkers that enriched for CDK2-dependency/BLU-222 sensitivity in CCNE1-overexpressed, nonamplified cells. Furthermore, intact Rb and low p16 expression predicted a BLU-222 and CDK4/6 inhibitor combination response. BLU-222 demonstrated robust activity in combination with carboplatin or paclitaxel in CCNE1-aberrant models, rendering chemotherapy-resistant tumors strongly sensitive to the combination. These findings demonstrate that response to CDK2 inhibition by BLU-222 can be further predicted using a combinatorial biomarker signature that could refine patient selection criteria in CCNE1-high patients and support clinical development. Significance: The identification of biomarkers of response to the CDK2-selective inhibitor BLU-222 and effective combinations with CDK4/6 inhibitors or chemotherapy could enable precision medicine strategies for CDK2 inhibition in ovarian and endometrial cancer. See related article by Dommer and colleagues, p. 1310.

Beyond failure of endocrine-based therapies in HR+/HER2 negative advanced breast cancer: What before chemotherapy? A glimpse into the future

Despite the impressive improvements achieved by endocrine therapy and CDK4/6 inhibitors (CDK4/6i) and the forthcoming availability of alternative endocrine manipulations and targeted therapies, hormone-receptor positive/HER2 negative (HR+/HER2-) advanced breast cancer (ABC) is almost inevitably destined to become endocrine- refractory. At this time chemotherapy has been recently challenged and partly replaced by new targeted options as antibody-drug conjugated (ADCs). Trastuzumab-deruxtecan has been proven meaningfully superior to chemotherapy either in 1st and later lines after progression to CDK4/6i in HER2-low ABC and results with other ADCs as Sacituzumab Govitecan and Datopotamab-deruxtecan are promising, but the definition of cross-resistance between these drugs sharing either antibody or payload is crucial before implementing them in a useful sequence. While PARP inhibitors are the standard 2nd line in patients with gBRCA mutation, it is not still known whether patients with mutations of PALB2 or of other homologous recombinant defect (HRD)-related genes will benefit of the same treatment. On the other hand, the results obtained with immune checkpoint inhibitors (ICIs) in HR+ /HER2-ABC contrarily to the early setting are disappointing up to now, but investigations of ICIs in combination with other targeted drugs which may increase immune response and the search for better markers of activity are under way. Moreover the anticipation in upfront treatment of ADCs or PARPi in patients with features of putative endocrine resistance and/or of less sensitiviy to CDK4/6i and the choice of therapy in patients recurring during or soon after adjuvant CDK4/6i and olaparib represent further challenges for the future.

Recent advances in regulating the cell cycle through inhibiting CDKs for cancer treatment

The inhibition of cyclin-dependent kinases (CDKs) is considered a promising strategy for cancer treatment due to their role in cell cycle regulation. However, CDK inhibitors with no selectivity among CDK families have not been approved. A CDK inhibitor with high selectivity for CDK4/6 exhibited significant treatment effects on breast cancer and has become a heavy bomb on the market. Subsequently, resistance gradually decreased the efficacy of selective CDK4/6 inhibitors in breast cancer treatment. In this review, we first introduce the development of selective CDK4/6 inhibitors and then explain the role of CDK2 activation in inducing resistance to CDK4/6 inhibitors. Moreover, we focused on the development of CDK2/4/6 inhibitors and selective CDK2 inhibitors, which will aid in the discovery of novel CDK inhibitors targeting the cell cycle in the future.

Targeting cyclin-dependent kinase 2 (CDK2) interactions with cyclins and Speedy 1 (Spy1) for cancer and male contraception

The review discusses progress in discovering cyclin-dependent kinase 2 (CDK2) inhibitors for cancer treatment and their potential for male contraception. It summarizes first-, second-, and third-generation CDK inhibitors and selective CDK2 inhibitors currently in clinical trials for cancer. Novel strategies to discover allosteric inhibitors, covalent inhibitors, and degraders are also discussed.

Radiogenomic Landscape of Metastatic Endocrine-Positive Breast Cancer Resistant to Aromatase Inhibitors

Breast cancer poses a significant global health challenge and includes various subtypes, such as endocrine-positive, HER2-positive, and triple-negative. Endocrine-positive breast cancer, characterized by estrogen and progesterone receptors, is commonly treated with aromatase inhibitors. However, resistance to these inhibitors can hinder patient outcomes due to genetic and epigenetic alterations, mutations in the estrogen receptor 1 gene, and changes in signaling pathways. Radiogenomics combines imaging techniques like MRI and CT scans with genomic profiling methods to identify radiographic biomarkers associated with resistance. This approach enhances our understanding of resistance mechanisms and metastasis patterns, linking them to specific genomic profiles and common metastasis sites like the bone and brain. By integrating radiogenomic data, personalized treatment strategies can be developed, improving predictive and prognostic capabilities. Advancements in imaging and genomic technologies offer promising avenues for enhancing radiogenomic research. A thorough understanding of resistance mechanisms is crucial for developing effective treatment strategies, making radiogenomics a valuable integrative approach in personalized medicine that aims to improve clinical outcomes for patients with metastatic endocrine-positive breast cancer.

Targeting CDK4/6 in breast cancer

Dysregulation of the cell cycle machinery, particularly the overactivation of cyclin-dependent kinases 4 and 6 (CDK4/6), is a hallmark of breast cancer pathogenesis. The introduction of CDK4/6 inhibitors has transformed the treatment landscape for hormone receptor-positive breast cancer by effectively targeting abnormal cell cycle progression. However, despite their initial clinical success, drug resistance remains a significant challenge, with no reliable biomarkers available to predict treatment response or guide strategies for managing resistant populations. Consequently, numerous studies have sought to investigate the mechanisms driving resistance to optimize the therapeutic use of CDK4/6 inhibitors and improve patient outcomes. Here we examine the molecular mechanisms regulating the cell cycle, current clinical applications of CDK4/6 inhibitors in breast cancer, and key mechanisms contributing to drug resistance. Furthermore, we discuss emerging predictive biomarkers and highlight potential directions for overcoming resistance and enhancing therapeutic efficacy.

TFAP2C-DDR1 axis regulates resistance to CDK4/6 inhibitor in breast cancer

Breast cancer is the predominant malignancy with the majority of cases are characterized as HR+/HER2-subtype. Although cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) have shown remarkable efficacy in treating this subtype when combined with endocrine therapy, the development of resistance to these inhibitors remains a significant clinical obstacle. Hence, there is an urgent need to explore innovative therapies and decipher the underlying mechanisms of resistance to CDK4/6i. In this study, we employed quantitative high-throughput combination screening (qHTCS) and genomics/proteomics approaches to uncover the molecular mechanisms driving resistance to CDK4/6i (palbociclib) in breast cancer. The comprehensive analyses revealed DDR1 as a potential factor implicated in mediating resistance to CDK4/6i. Specifically, DDR1 inhibition in combination with palbociclib exhibited remarkable synergistic effects, reducing cell survival signaling and promoting apoptosis in resistant cells. In-vivo xenograft model further validated the synergistic effects, showing a significant reduction in the resistant tumor growth. Exploration into DDR1 activation uncovered TFAP2C as a key transcription factor regulating DDR1 expression in palbociclib resistant cells and inhibition of TFAP2C re-sensitized resistant cells to palbociclib. Gene set enrichment analysis (GSEA) in the NeoPalAna trial demonstrated a significant enrichment of the TFAP2C-DDR1 gene set from patitens after palbociclib treatment, suggesting the possible activation of the TFAP2C-DDR1 axis following palbociclib exposure. Overall, this study provides crucial insights into the novel molecular landscape of palbociclib resistance in breast cancer, suggesting TFAP2C-DDR1 axis inhibition as a promising strategy to overcome resistance.

RET inhibition overcomes resistance to combined CDK4/6 inhibitor and endocrine therapy in ER+ breast cancer

Background

Combined CDK4/6 inhibitor (CDK4/6i) and endocrine therapy significantly improve the outcome of patients with advanced estrogen receptor-positive (ER+) breast cancer. However, resistance to this treatment and disease progression remains a major clinical challenge. High expression of the receptor tyrosine kinase REarranged during Transfection (RET) has been associated with resistance to endocrine therapy in breast cancer, but the role of RET in CDK4/6i treatment response/resistance remains unexplored.

Methods

To identify gene expression alterations associated with resistance to combined endocrine therapy and CDK4/6i, we performed RNA sequencing of two ER+ breast cancer cell models resistant to this combined therapy. The functional role of RET was assessed by siRNA-mediated RET silencing and targeted inhibition with the FDA/EMA-approved RET-selective inhibitor selpercatinib in resistant breast cancer cells and patient-derived organoids (PDOs). RET silencing was evaluated mechanistically using global gene expression and pathway analysis. The clinical relevance of RET expression in ER+ breast cancer was investigated by gene array analysis of primary tumors treated with endocrine therapy and by immunohistochemical scoring of metastatic lesions from patients who received combined CDK4/6i and endocrine therapy.

Results

We show that RET is upregulated in ER+ breast cancer cell lines resistant to combined CDK4/6i and fulvestrant compared to isogenic cells resistant to fulvestrant alone. siRNA-mediated silence of RET in high RET-expressing, combined CDK4/6i- and fulvestrant-resistant cells reduced their growth partially by affecting cell cycle regulators of the G2-M phase and E2F targets. Notably, targeting RET with selpercatinib in combination with CDK4/6i inhibited the growth of CDK4/6i-resistant cell lines and resensitized ER+ breast cancer patient-derived organoids resistant to CDK4/6i. Finally, analysis of RET expression in ER+ breast cancer patients treated with endocrine therapy showed that high RET expression correlated with poor clinical outcomes. We further observed a shorter median survival to combined CDK4/6i and endocrine therapy in patients with RET-positive compared to RET-negative tumors, but this difference did not reach statistical significance.

Conclusions

Our findings show that RET is overexpressed in ER+ metastatic breast cancer resistant to combined CDK4/6i and endocrine therapy, rendering RET inhibition a promising therapeutic approach for patients who experience disease progression on combined CDK4/6i and endocrine therapy.

Optimizing therapeutic approaches for HR+/HER2- advanced breast cancer: clinical perspectives on biomarkers and treatment strategies post-CDK4/6 inhibitor progression

This review offers an expert perspective on biomarkers, CDK4/6 inhibitor efficacy, and therapeutic approaches for managing hormone receptor-positive (HR+), human epidermal growth factor receptor-negative (HER2-) advanced breast cancer (ABC), particularly after CDK4/6 inhibitor progression. Key trials have demonstrated that combining CDK4/6 inhibitors with endocrine therapy (ET) significantly improves progression-free survival (PFS), with median durations ranging from 14.8 to 26.7 months, and overall survival (OS), with median durations reaching up to 53.7 months. Actionable biomarkers, such as PIK3CA and ESR1 mutations, have emerged as pivotal tools to guide second-line treatment decisions, enabling the use of targeted therapies like alpelisib and elacestrant and emphasizing the important role of biomarkers in guiding the selection of therapy. This overview aims to provide clinicians with a practical and up-to-date framework to inform treatment decisions and improve patient care in the context of this challenging disease. Additionally, we review emerging biomarkers and novel treatment strategies to address this difficult clinical landscape.

Resistance mechanisms and therapeutic strategies of CDK4 and CDK6 kinase targeting in cancer

Cyclin-dependent kinases (CDKs) 4 and 6 (CDK4/6) are important regulators of the cell cycle. Selective CDK4/6 small-molecule inhibitors have shown clinical activity in hormonal receptor-positive (HR+) metastatic breast cancer, but their effectiveness remains limited in other cancer types. CDK4/6 degradation and improved selectivity across CDK paralogs are approaches that could expand the effectiveness of CDK4/6 targeting. Recent studies also suggest the use of CDK4/6-targeting agents in cancer immunotherapy. In this Review, we highlight recent advancements in the mechanistic understanding and development of pharmacological approaches targeting CDK4/6. Collectively, these developments pose new challenges and opportunities for rationally designing more effective treatments.

CDK4/6 Inhibitor Resistance in ER+ Breast Cancer

The cyclin-dependent kinases 4 and 6 inhibitors are the mainstay of treatment for patients with hormone receptor-positive and HER2-negative breast cancer. The ability of these drugs to improve the outcome of patients both in the metastatic and the early setting has been largely demonstrated. However, resistance, either de novo or acquired, represents a major clinical challenge. In the past years, efforts have been made to identify biomarkers that might help in a better selection of patients or to unravel the mechanisms leading to resistance in order to develop new therapeutic strategies to overcome it. Alterations of cell cycle-related genes and proteins are among the best characterized markers of resistance, and pathways impacting the cell cycle, including nuclear and growth factor receptors signaling, have been thoroughly investigated. Despite this, to date, cyclin-dependent kinases 4 and 6 inhibitors are administered based only on the hormone receptor and HER2 status of the tumor, and patients progressing on therapy are managed with currently available treatments. Here we summarize present knowledge on the cyclin-dependent kinases 4 and 6 inhibitors' mechanisms of action, efficacy data, and mechanisms of resistance.

Targeting CDK2 to circumvent treatment resistance in HR+ breast cancer

Genetic and epigenetic defects of the p53 system have previously been associated with resistance to CDK4/6 inhibitors in women with HR+ breast cancer. Recent data from Kudo et al. demonstrate that CDK2-targeting agents may offer an effective strategy to circumvent such resistance by enforcing cellular senescence downstream of RBL2 dephosphorylation.

Targeting CDK2 to combat drug resistance in cancer therapy

Drug resistance remains a major obstacle in cancer treatment, leading to treatment failures and high mortality rates. Despite advancements in therapies, overcoming resistance requires a deeper understanding of its mechanisms. This review highlights CDK2's pivotal role in both intrinsic and acquired resistance, and its potential as a therapeutic target. Cyclin E upregulation, which partners with CDK2, is linked to poor prognosis and resistance across various cancers. Specifically, amplifications of CCNE1/CCNE2 are associated with resistance to targeted therapies, immunotherapy, endocrine therapies and chemo/radiotherapy. Given CDK2's involvement in resistance mechanisms, investigating its role presents promising opportunities for developing novel strategies to combat resistance and improve treatment outcomes.

Unveiling the Potential of Cyclin-Dependent Kinases 4 and 6 Inhibitors Beyond Progression in Hormone Receptor Positive/Human Epidermal Growth Factor Negative Advanced Breast Cancer - A Clinical Review

OPINION STATEMENT

Cyclin-dependent kinases 4 and 6 inhibitors (CDK4/6i) have revolutionized the management of hormone receptor-positive (HR +) breast cancer. However, resistance to CDK4/6i remains an unavoidable challenge, with limited evidence to guide the choice of subsequent treatments. Continuation of CDK4/6 inhibition raises as a compelling treatment option and is currently an active area of research. This approach encompasses multifaceted strategies regarding CDK4/6i sequence (same or switched agent), endocrine therapy (ET) partner and potential combination with a third drug. Continuing CDK4/6 inhibition while targeting ET resistance in tumours still dependent on ER activity (i.e., ESR1 mutation) through a ctDNA-guided approach has the potential of becoming practice-changing, pending the results of ongoing phase III studies. Conversely, the efficacy of this strategy in cases of radiological progression in a biomarker-unselected population appears to be rather unsatisfactory. While some benefit, albeit modest, has been observed from switching to a different CDK4/6i after progression (e.g. ribociclib after palbociclib in the MAINTAIN trial and abemaciclib after both palbociclib and ribociclib in the postMONARCH trial), the current evidence (mainly with palbociclib) clearly argues against maintaining the same CDK4/6i. Biomarker analyses to optimally identify patients suitable for this approach yielded inconsistent findings that do not apply to daily clinical decision making. Attractive preliminary efficacy has recently emerged from combining a third agent (immunotherapy, AKT/ PIK3CA/mTOR inhibitor, new ET agents, CDK2 inhibitors) to CDK4/6i and ET, but further validation in larger ongoing trials is required to also determine the optimal timing for incorporating these agents into the therapeutic timeline. To date, CDK4/6i after CDK4/6i progression is far from being a standard of care. However, selected patients with indolent disease, prolonged exposure to previous CDK4/6i treatment (especially palbociclib) and without actionable molecular alterations, may be suitable for suchmaintenance strategy beyond progression. In this challenging and rapidly evolving treatment landscape, ongoing studies can refine the optimal approach and identify clinical and molecular factors to select the best treatment for the right patient.

Therapeutic benefits of maintaining CDK4/6 inhibitors and incorporating CDK2 inhibitors beyond progression in breast cancer

The combination of CDK4/6 inhibitors (CDK4/6i) and endocrine therapy has revolutionized treatment for hormone receptor-positive (HR+) metastatic breast cancer. However, the emergence of resistance in most patients often leads to treatment discontinuation with no consensus on effective second-line therapies. The therapeutic benefits of maintaining CDK4/6i or incorporating CDK2 inhibitors (CDK2i) after disease progression remain unclear. Here, we demonstrate that sustained CDK4/6i therapy, either alone or combined with CDK2i, significantly suppresses the growth of drug-resistant HR+ breast cancer. Continued CDK4/6i treatment induces a non-canonical pathway for retinoblastoma protein (Rb) inactivation via post-translational degradation, resulting in diminished E2F activity and delayed G1 progression. Importantly, our data highlight that CDK2i should be combined with CDK4/6i to effectively suppress CDK2 activity and overcome resistance. We also identify cyclin E overexpression as a key driver of resistance to CDK4/6 and CDK2 inhibition. These findings provide crucial insights into overcoming resistance in HR+ breast cancer, supporting the continued use of CDK4/6i and the strategic incorporation of CDK2i to improve therapeutic outcomes.

Inhibition of GPX4 enhances CDK4/6 inhibitor and endocrine therapy activity in breast cancer

CDK4/6 inhibition in combination with endocrine therapy is the standard of care for estrogen receptor (ER+) breast cancer, and although cytostasis is frequently observed, new treatment strategies that enhance efficacy are required. Here, we perform two independent genome-wide CRISPR screens to identify genetic determinants of CDK4/6 and endocrine therapy sensitivity. Genes involved in oxidative stress and ferroptosis modulate sensitivity, with GPX4 as the top sensitiser in both screens. Depletion or inhibition of GPX4 increases sensitivity to palbociclib and giredestrant, and their combination, in ER+ breast cancer models, with GPX4 null xenografts being highly sensitive to palbociclib. GPX4 perturbation additionally sensitises triple negative breast cancer (TNBC) models to palbociclib. Palbociclib and giredestrant induced oxidative stress and disordered lipid metabolism, leading to a ferroptosis-sensitive state. Lipid peroxidation is promoted by a peroxisome AGPAT3-dependent pathway in ER+ breast cancer models, rather than the classical ACSL4 pathway. Our data demonstrate that CDK4/6 and ER inhibition creates vulnerability to ferroptosis induction, that could be exploited through combination with GPX4 inhibitors, to enhance sensitivity to the current therapies in breast cancer.

Advances in the mechanism of CDK4/6 inhibitor resistance in HR+/HER2- breast cancer

Among women, breast cancer is the most prevalent form of a malignant tumour. Among the subtypes of breast cancer, hormone receptor (HR) positive and human epidermal growth factor receptor (HER2) negative kinds make up the biggest proportion. The advent of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, which are dependent on cell cycle proteins, has greatly enhanced the prognosis of patients with advanced HR+/HER2- breast cancer. This is a specific treatment that stops the growth of cancer cells by preventing them from dividing. Nevertheless, the drug resistance of the disease unavoidably impacts the effectiveness of treatment and the prognosis of patients. This report provides a thorough analysis of the current research advancements about the resistance mechanism of CDK4/6 inhibitors in HR+/HER2- breast cancer. It presents an in-depth discussion from numerous viewpoints, such as aberrant cell cycle regulation and changes in signalling pathways. In response to the drug resistance problem, subsequent treatment strategies are also being explored, including switching to other CDK4/6 inhibitor drugs, a combination of novel endocrine therapeutic agents, an optimal combination of targeted therapies and switching to chemotherapy. An in-depth study of the resistance mechanism can assist in identifying creative tactics that can overcome or postpone drug resistance, alleviate the problem of restricted treatment strategies following drug resistance and enhance the prognosis of patients.

Targeting cyclin-dependent kinases: From pocket specificity to drug selectivity

The development of selective modulators of cyclin-dependent kinases (CDKs), a kinase family with numerous members and functional variations, is a significant preclinical challenge. Recent advancements in crystallography have revealed subtle differences in the highly conserved CDK pockets. Exploiting these differences has proven to be an effective strategy for achieving excellent drug selectivity. While previous reports briefly discussed the structural features that lead to selectivity in individual CDK members, attaining inhibitor selectivity requires consideration of not only the specific structures of the target CDK but also the features of off-target members. In this review, we summarize the structure-activity relationships (SARs) that influence selectivity in CDK drug development and analyze the pocket features that lead to selectivity using molecular-protein binding models. In addition, in recent years, novel CDK modulators have been developed, providing more avenues for achieving selectivity. These cases were also included. We hope that these efforts will assist in the development of novel CDK drugs.

Mechanisms of sensitivity and resistance to CDK4/CDK6 inhibitors in hormone receptor-positive breast cancer treatment

Cell cycle dysregulation is a hallmark of cancer that promotes eccessive cell division. Cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase 6 (CDK6) are key molecules in the G1-to-S phase cell cycle transition and are crucial for the onset, survival, and progression of breast cancer (BC). Small-molecule CDK4/CDK6 inhibitors (CDK4/6i) block phosphorylation of tumor suppressor Rb and thus restrain susceptible BC cells in G1 phase. Three CDK4/6i are approved for the first-line treatment of patients with advanced/metastatic hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) BC in combination with endocrine therapy (ET). Though this has improved the clinical outcomes for survival of BC patients, there is no established standard next-line treatment to tackle drug resistance. Recent studies suggest that CDK4/6i can modulate other distinct effects in both BC and breast stromal compartments, which may provide new insights into aspects of their clinical activity. This review describes the biochemistry of the CDK4/6-Rb-E2F pathway in HR+ BC, then discusses how CDK4/6i can trigger other effects in BC/breast stromal compartments, and finally outlines the mechanisms of CDK4/6i resistance that have emerged in recent preclinical studies and clinical cohorts, emphasizing the impact of these findings on novel therapeutic opportunities in BC.

Molecular Engineering of Functional SiRNA Agents

Synthetic biology constitutes a scientific domain focused on intentional redesign of organisms to confer novel functionalities or create new products through strategic engineering of their genetic makeup. Leveraging the inherent capabilities of nature, one may address challenges across diverse sectors including medicine. Inspired by this concept, we have developed an innovative bioengineering platform, enabling high-yield and large-scale production of biological small interfering RNA (BioRNA/siRNA) agents via bacterial fermentation. Herein, we show that with the use of a new tRNA fused pre-miRNA carrier, we can produce various forms of BioRNA/siRNA agents within living host cells. We report a high-level overexpression of nine target BioRNA/siRNA molecules at 100% success rate, yielding 3-10 mg of BioRNA/siRNA per 0.25 L of bacterial culture with high purity (>98%) and low endotoxin (<5 EU/μg RNA). Furthermore, we demonstrate that three representative BioRNA/siRNAs against GFP, BCL2, and PD-L1 are biologically active and can specifically and efficiently silence their respective targets with the potential to effectively produce downstream antiproliferation effects by PD-L1-siRNA. With these promising results, we aim to advance the field of synthetic biology by offering a novel platform to bioengineer functional siRNA agents for research and drug development.

Derived from fangchinoline, LYY-35 exhibits an inhibiting effect on human NSCLC cancer A549 cells

Although fangchinoline has been widely used as an adjunct therapy for a variety of inflammatory and cancerous diseases, its mechanism of action on tumor cells remains unclear. Fangchinoline derivative LYY-35 reduced the number of A549 cells, deformed cell morphology and increased cell debris. Cell viability was significantly reduced, while the same concentration of LYY-35 had little effect on BEAS-2B viability of normal lung epithelial cells. In addition, LYY-35 can also reduce the migration, proliferation and invasion ability of A549 cells. Levels of β-catenin, ZO-1 and ZEB-1 proteins, biomarkers of cell adhesion and epithelial mesenchymal transformation, were significantly reduced. The levels of superoxide dismutase and lactate dehydrogenase decreased gradually, while the levels of glutathione, malondialdehyde and intracellular and extracellular ROS increased significantly. At the same time, LYY-35 induced increased apoptosis, increased expression of Bax, cleaved caspase3, cleaved PARP1, and decreased expression of Bcl-xl, which blocked the cell cycle to G0/G1 phase. The expressions of cell cycle checkpoint proteins Cyclin B1, Cyclin E1, CDK6, PCNA and PICH were significantly decreased. With the increase of LYY-35 concentration, the trailing phenomenon was more obvious in single cell gel electrophoresis. DNA damage repair proteins: BLM, BRCA-1 and PARP-1 expression decreased gradually.LYY-35 can inhibit the proliferation of non-small cell lung cancer A549 cells, block cell cycle, promote apoptosis, increase ROS production, cause DNA damage and interfere with DNA replication. LYY-35 is promising for the treatment of non-small cell lung cancer in the future.

CDK4/6 inhibitors: The Devil is in the Detail

PURPOSE OF REVIEW

Update on the most recent clinical evidence on CDK4/6 inhibitors (CDK4/6i) in the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor (HER)2-negative breast cancer.

RECENT FINDINGS

Over the past decade, CDK4/6i have become part of the standard of care treatment of patients with both metastatic and high-risk early HR + /HER2- breast cancers. The three available CDK4/6i (palbociclib, ribociclib and abemaciclib) have been extensively studied in combination with endocrine therapy (ET) in metastatic breast cancer (mBC) with consistent prolongation of progression free survival; however, ribociclib has emerged as the preferred first line agent in mBC given overall survival benefit over endocrine monotherapy. In early BC, abemaciclib is the only currently approved agent while ribociclib has early positive clinical trial data. Toxicities and financial burden limit the use of CDK4/6i in all patients and resource-poor settings, and optimal timing of their use in mBC remains unclear. There is considerable evidence for the use of CDK4/6i in metastatic and early HR + /HER2- breast cancer, but knowledge gaps remain, and further research is necessary to better define their optimal use.

Therapy for Hormone Receptor-Positive, Human Epidermal Growth Receptor 2-Negative Metastatic Breast Cancer Following Treatment Progression via CDK4/6 Inhibitors: A Literature Review

Endocrine therapy (ET) with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) is currently the first-line standard treatment for most patients with hormone receptor-positive (HR+) and human epidermal growth receptor 2-negative (HER2-) metastatic or advanced breast cancer. However, the majority of tumors response to and eventually develop resistance to CDK4/6is. The mechanisms of resistance are poorly understood, and the optimal postprogression treatment regimens and their sequences continue to evolve in the rapidly changing treatment landscape. In this review, we generally summarize the mechanisms of resistance to CDK4/6is and ET, and describe the findings from clinical trials using small molecule inhibitors, antibody-drug conjugates and immunotherapy, providing insights into how these novel strategies may reverse treatment resistance, and discussing how some have not translated into clinical benefit. Finally, we provide rational treatment strategies based on the current emerging evidence.

Cyclin-dependent kinase 2 (CDK2) inhibitors and others novel CDK inhibitors (CDKi) in breast cancer: clinical trials, current impact, and future directions

Aberrant cyclin-dependent kinase 2 (CDK2) activation has been identified as a main resistance mechanism to CDK4/6 inhibition in hormone-receptor positive (HR+) breast cancer. Additionally, consistent preclinical evidence states its crucial role in MYC and CCNE1 overexpressed cancer survival, such as triple-negative breast cancers (TNBC), thus representing an appealing and relatively unexplored target treatment opportunity. Despite emerging initial results of novel CDK2 inhibitors (CDK2i) activity, a comprehensive outcomes collection is currently absent from the scientific literature. We aim to provide an overview of ongoing clinical trials involving CDK2i in the context of metastatic breast cancer (mBC), either as monotherapy or in combination with other agents. The review extends beyond CDK2i to encompass novel emerging CDK4 inhibitors, combined CDK2/4/6 inhibitors, and the well-known pan-CDK inhibitors including those specifically directed at CDK2. Delving into the results, we critically appraise the observed clinical efficacy and offer valuable insights into their potential impact and future applications.

Autophagy and senescence facilitate the development of antiestrogen resistance in ER positive breast cancer

Estrogen receptor positive (ER+) breast cancer is the most common breast cancer diagnosed annually in the US with endocrine-based therapy as standard-of-care for this breast cancer subtype. Endocrine therapy includes treatment with antiestrogens, such as selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), and aromatase inhibitors (AIs). Despite the appreciable remission achievable with these treatments, a substantial cohort of women will experience primary tumor recurrence, subsequent metastasis, and eventual death due to their disease. In these cases, the breast cancer cells have become resistant to endocrine therapy, with endocrine resistance identified as the major obstacle to the medical oncologist and patient. To combat the development of endocrine resistance, the treatment options for ER+, HER2 negative breast cancer now include CDK4/6 inhibitors used as adjuvants to antiestrogen treatment. In addition to the dysregulated activity of CDK4/6, a plethora of genetic and biochemical mechanisms have been identified that contribute to endocrine resistance. These mechanisms, which have been identified by lab-based studies utilizing appropriate cell and animal models of breast cancer, and by clinical studies in which gene expression profiles identify candidate endocrine resistance genes, are the subject of this review. In addition, we will discuss molecular targeting strategies now utilized in conjunction with endocrine therapy to combat the development of resistance or target resistant breast cancer cells. Of approaches currently being explored to improve endocrine treatment efficacy and patient outcome, two adaptive cell survival mechanisms, autophagy, and "reversible" senescence, are considered molecular targets. Autophagy and/or senescence induction have been identified in response to most antiestrogen treatments currently being used for the treatment of ER+ breast cancer and are often induced in response to CDK4/6 inhibitors. Unfortunately, effective strategies to target these cell survival pathways have not yet been successfully developed. Thus, there is an urgent need for the continued interrogation of autophagy and "reversible" senescence in clinically relevant breast cancer models with the long-term goal of identifying new molecular targets for improved treatment of ER+ breast cancer.

The CDK4/6 inhibitors biomarker landscape: The most relevant biomarkers of response or resistance for further research and potential clinical utility

Cyclin-Dependent Kinase 4/6 inhibitors (CDK4/6is) in combination with Endocrine Therapy (ET) represent the standard frontline therapy for patients with Hormone Receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic Breast Cancer (mBC). Clinical activity and efficacy of CDK4/6is-based therapies have been proven both in the endocrine sensitive and resistant settings. Therapy resistance eventually underpins clinical progression to any CDK4/6is-based therapies, yet there is a lack of validated molecular biomarkers predictive of either intrinsic or acquired resistance to CDK4/6is in clinical practice. As the "post-CDK4/6is" landscape for the management of HR-positive/HER2-negative mBC is rapidly evolving with the introduction of novel therapies, there is an urgent need for the definition of clinically relevant molecular biomarkers of intrinsic/acquired resistance mechanisms to CDK4/6is. This narrative review outlines the role of currently approved CDK4/6is-based therapies, describes the most relevant molecular biomarkers of CDK4/6is-resistance, and ultimately provides a perspective on the clinical and research scenario.

Nifuroxazide boosts the anticancer efficacy of palbociclib-induced senescence by dual inhibition of STAT3 and CDK2 in triple-negative breast cancer

Though palbociclib, a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor has been approved for treating breast cancer, two major clinical challenges remain: (i) Triple-negative breast cancer (TNBC) appears to be more resistant to palbociclib, and (ii) Palbociclib-induced senescence-associated secretory phenotype (SASP) has a pro-tumorigenic function. Here we report that combining palbociclib with the STAT3 inhibitor nifuroxazide uncouples SASP production from senescence-associated cell cycle exit. Moreover, we identified nifuroxazide as a CDK2 inhibitor that synergistically promotes palbociclib-induced growth arrest and senescence in TNBC cells. In vitro, the combination of nifuroxazide with palbociclib further inhibited the TNBC cell proliferation and enhanced palbociclib-induced cell cycle arrest and senescence. The modulation of palbociclib-induced SASP by nifuroxazide was associated with the reduction of phosphorylated-STAT3. Nifuroxazide also blocks SASP-dependent cancer cell migration. Furthermore, thermal shift assay and molecular docking of nifuroxazide with STAT3 and CDK2 revealed that it binds to their active sites and acts as a potent dual inhibitor. In vivo, the combination of nifuroxazide with palbociclib suppressed 4T1 tumor growth and lung metastasis. Our data suggest that nifuroxazide enhances the anticancer effects of palbociclib in TNBC by uncoupling SASP production from senescence-associated cell cycle exit and inhibiting CDK2 to promote tumor senescence.

Regulation of the Cell Cycle by ncRNAs Affects the Efficiency of CDK4/6 Inhibition

Cyclin-dependent kinases (CDKs) regulate cell division at multiple levels. Aberrant proliferation induced by abnormal cell cycle is a hallmark of cancer. Over the past few decades, several drugs that inhibit CDK activity have been created to stop the development of cancer cells. The third generation of selective CDK4/6 inhibition has proceeded into clinical trials for a range of cancers and is quickly becoming the backbone of contemporary cancer therapy. Non-coding RNAs, or ncRNAs, do not encode proteins. Many studies have demonstrated the involvement of ncRNAs in the regulation of the cell cycle and their abnormal expression in cancer. By interacting with important cell cycle regulators, preclinical studies have demonstrated that ncRNAs may decrease or increase the treatment outcome of CDK4/6 inhibition. As a result, cell cycle-associated ncRNAs may act as predictors of CDK4/6 inhibition efficacy and perhaps present novel candidates for tumor therapy and diagnosis.

Novel Therapeutic Combination Targets the Growth of Letrozole-Resistant Breast Cancer through Decreased Cyclin B1

As breast cancer cells transition from letrozole-sensitive to letrozole-resistant, they over-express epidermal growth factor receptor (EGFR), mitogen-activated protein kinase (MAPK), and human epidermal growth factor receptor 2 (HER2) while acquiring enhanced motility and epithelial-to-mesenchymal transition (EMT)-like characteristics that are attenuated and reversed by glyceollin treatment, respectively. Interestingly, glyceollin inhibits the proliferation and tumor progression of triple-negative breast cancer (TNBC) and estrogen-independent breast cancer cells; however, it is unlikely that a single phytochemical would effectively target aromatase-inhibitor (AI)-resistant metastatic breast cancer in the clinical setting. Since our previous report indicated that the combination of lapatinib and glyceollin induced apoptosis in hormone-dependent AI-resistant breast cancer cells, we hypothesized that combination therapy would also be beneficial for hormone independent letrozole-resistant breast cancer cells (LTLT-Ca) compared to AI-sensitive breast cancer cells (AC-1) by decreasing the expression of proteins associated with proliferation and cell cycle progression. While glyceollin + lapatinib treatment caused comparable inhibitory effects on the proliferation and migration in both cell lines, combination treatment selectively induced S and G2/M phase cell cycle arrest of the LTLT-Ca cells, which was mediated by decreased cyclin B1. This phenomenon may represent a unique opportunity to design novel combinatorial therapeutic approaches to target hormone-refractory breast tumors.

CDK4/6 inhibitor resistance in estrogen receptor positive breast cancer, a 2023 perspective

CDK4/6 inhibitors have become game-changers in the treatment of estrogen receptor-positive (ER+) breast cancer, and in combination with endocrine therapy are the standard of care first-line treatment for ER+/HER2-negative advanced breast cancer. Although CDK4/6 inhibitors prolong survival for these patients, resistance is inevitable and there is currently no clear standard next-line treatment. There is an urgent unmet need to dissect the mechanisms which drive intrinsic and acquired resistance to CDK4/6 inhibitors and endocrine therapy to guide the subsequent therapeutic decisions. We will review the insights gained from preclinical studies and clinical cohorts into the diverse mechanisms of CDK4/6 inhibitor action and resistance, and highlight potential therapeutic strategies in the context of CDK4/6 inhibitor resistance.

Position paper on CDK4/6 inhibitors in early breast cancer

Inhibitors of the cyclin-dependent kinases 4/6 (CDK4/6i) have been practice-changing and are now considered the standard of care in combination with endocrine therapy for the first- or second-line treatment in advanced hormone-receptor-positive, human epidermal growth factor receptor 2‑negative breast cancer. Recently, CDK4/6i have also emerged as an appealing targeted cancer therapy in early breast cancer, however results of large clinical trials are controversial. This position paper summarizes the evidence, and provides guidance for clinical practice.

AKT/mTOR signaling modulates resistance to endocrine therapy and CDK4/6 inhibition in metastatic breast cancers

Endocrine therapy (ET) in combination with CDK4/6 inhibition is routinely used as first-line treatment for HR+/HER2- metastatic breast cancer (MBC) patients. However, 30-40% of patients quickly develop disease progression. In this open-label multicenter clinical trial, we utilized a hypothesis-driven protein/phosphoprotein-based approach to identify predictive markers of response to ET plus CDK4/6 inhibition in pre-treatment tissue biopsies. Pathway-centered signaling profiles were generated from microdissected tumor epithelia and surrounding stroma/immune cells using the reverse phase protein microarray. Phosphorylation levels of the CDK4/6 downstream substrates Rb (S780) and FoxM1 (T600) were higher in patients with progressive disease (PD) compared to responders (p = 0.02). Systemic PI3K/AKT/mTOR activation in tumor epithelia and stroma/immune cells was detected in patients with PD. This activation was not explained by underpinning genomic alterations alone. As the number of FDA-approved targeted compounds increases, functional protein-based signaling analyses may become a critical component of response prediction and treatment selection for MBC patients.