Pooled ctDNA analysis of MONALEESA phase III advanced breast cancer trials

CDK4/6 inhibitors in breast cancer therapy: mechanisms of drug resistance and strategies for treatment

Dysregulated cell cycle progression is a well-established hallmark of cancer, driving the development of targeted antitumor therapies that intervene at specific phases of the cell cycle. Among these therapeutic targets, cyclin-dependent kinases 4 and 6 (CDK4/6) have emerged as critical regulators of cell cycle progression, with their aberrant activation being strongly implicated in tumorigenesis and cancer progression. Currently, multiple CDK4/6 inhibitors have received clinical approval for hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer, demonstrating dual therapeutic mechanisms through both cell cycle arrest and enhancement of antitumor immunity. However, clinical implementation faces two major challenges: the inevitable development of acquired resistance during prolonged treatment, and the need for optimized combination strategies with other anticancer agents to achieve synergistic efficacy. This review systematically examines the molecular mechanisms underlying CDK4/6 inhibitor function and characterizes currently approved therapeutic agents. Importantly, it synthesizes recent discoveries regarding resistance mechanisms, including dysregulated cell cycle checkpoints, compensatory signaling pathway activation, and tumor microenvironment adaptations. Furthermore, we critically evaluate emerging combination therapeutic approaches targeting these resistance mechanisms. By integrating mechanistic insights with clinical evidence, this analysis aims to provide actionable strategies for overcoming therapeutic resistance and maximizing the clinical potential of CDK4/6 inhibitors in breast cancer management.

ESR1 Y537S and D538G Mutations Drive Resistance to CDK4/6 Inhibitors in Estrogen Receptor-Positive Breast Cancer

PURPOSE

Breast cancers with ESR1 mutations are resistant to antiestrogen therapy. In this study, we aimed to investigate the association of ESR1 mutations with resistance to CDK4/6 inhibitors (CDK4/6i) using real-world data analysis and experimental validation.

EXPERIMENTAL DESIGN

A total of 3,958 patients with estrogen receptor-positive metastatic breast cancer with DNA sequencing data were analyzed. Breast tumor DNA and ctDNA were sequenced using the Tempus xT tumor assay and Tempus xF liquid biopsy, respectively. Patients were stratified into either treated with CDK4/6i (tumor tissue: 1,070; ctDNA: 1,885) or CDK4/6i naïve (tumor tissue: 750; ctDNA: 253). Engineered MCF7 cells carrying ESR1Y537S or ESR1D538G knock-in mutations were used to study antitumor efficacy of the CDK4/6i palbociclib in vitro and in vivo.

RESULTS

In both xF and xT assays, ESR1 mutations were the only somatic alterations significantly more frequent in patients who received CDK4/6i compared with those who did not. Knock-in of ESR1Y537S or ESR1D538G in MCF7 cells resulted in upregulation of cell cycle-related gene signatures upon treatment with CDK4/6i ± antiestrogen compared with cells with nonmutant ESR1. MCF7 xenografts harboring ESR1Y537S and ESR1D538G mutations established in nude mice were resistant to palbociclib.

CONCLUSIONS

We report herein real-world and preclinical evidence that ESR1 mutations, particularly Y537S and D538G, can drive resistance to CDK4/6i.

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.

Circulating tumor DNA to monitor treatment response in solid tumors and advance precision oncology

Circulating tumor DNA (ctDNA) has emerged as a dynamic biomarker in cancer, as evidenced by its increasing integration into clinical practice. Carrying tumor specific characteristics, ctDNA can be used to inform treatment selection, monitor response, and identify drug resistance. In this review, we provide a comprehensive, up-to-date summary of ctDNA in monitoring treatment response with a focus on lung, colorectal, and breast cancers, and discuss current challenges and future directions.

Breast cancer: pathogenesis and treatments

Breast cancer, characterized by unique epidemiological patterns and significant heterogeneity, remains one of the leading causes of malignancy-related deaths in women. The increasingly nuanced molecular subtypes of breast cancer have enhanced the comprehension and precision treatment of this disease. The mechanisms of tumorigenesis and progression of breast cancer have been central to scientific research, with investigations spanning various perspectives such as tumor stemness, intra-tumoral microbiota, and circadian rhythms. Technological advancements, particularly those integrated with artificial intelligence, have significantly improved the accuracy of breast cancer detection and diagnosis. The emergence of novel therapeutic concepts and drugs represents a paradigm shift towards personalized medicine. Evidence suggests that optimal diagnosis and treatment models tailored to individual patient risk and expected subtypes are crucial, supporting the era of precision oncology for breast cancer. Despite the rapid advancements in oncology and the increasing emphasis on the clinical precision treatment of breast cancer, a comprehensive update and summary of the panoramic knowledge related to this disease are needed. In this review, we provide a thorough overview of the global status of breast cancer, including its epidemiology, risk factors, pathophysiology, and molecular subtyping. Additionally, we elaborate on the latest research into mechanisms contributing to breast cancer progression, emerging treatment strategies, and long-term patient management. This review offers valuable insights into the latest advancements in Breast Cancer Research, thereby facilitating future progress in both basic research and clinical application.

Efficacy and Safety of CDK4/6 Inhibitors: A Focus on HR+/HER2- Early Breast Cancer

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) have revolutionized the treatment of hormone-receptor positive (HR+), HER2 negative (HER2-) metastatic breast cancer, and are now also established agents in the treatment of high-risk and intermediate-risk HR+ early breast cancer. Several strategies regarding CDK4/6i combinations or continuation beyond progression have been successfully evaluated in the metastatic setting, and are considered a standard of care. Mechanism of action of and resistance mechanisms against CDK4/6i in addition to endocrine resistance represent an important research topic, important for the treatment of HR+ breast cancer. Clinically, CDK4/6i are efficient substances that are usually well tolerated. However, side effects differing between the substances have been reported, and might lead to treatment discontinuation, including in the early disease setting. In the adjuvant setting, the addition of palbociclib to standard endocrine treatment has not improved outcomes, whereas large randomized phase III trials have demonstrated significant disease-free survival benefit for the addition of ribociclib (NATALEE trial) and abemaciclib (monarchE trial). Patient selection, treatment duration, endocrine backbone therapy, and other study details differ between these pivotal trials. This review focuses on both the scientific background as well as all available clinical data of CDK4/6i, with particular emphasis on their use in early breast cancer.

Low Skeletal Muscle Radiodensity Predicts Response to CDK4/6 Inhibitors Plus Aromatase Inhibitors in Advanced Breast Cancer

BACKGROUND

Recent evidence indicates that a dysregulated host metabolism influences treatment outcomes in patients with breast cancer. We investigated the association of computed tomography (CT)-derived body composition indices with therapeutic responses in patients with hormone receptor-positive, HER2-negative advanced breast cancer (ABC) on endocrine plus CDK4/6 inhibitor (CDK4/6i) treatment.

METHODS

The study involved a retrospective cohort of patients with ABC at the Yonsei Cancer Center who received CDK4/6i and aromatase inhibitors as first-line therapy between January 2017 and October 2020. Body composition parameters were estimated from the non-enhanced CT images of the third lumbar spine by commercialized deep learning software. Patients with low skeletal muscle radiodensity (SMD) were defined as patients with SMD of low tertile (≤ 28.7 Hounsfield Units). The primary outcome was progression-free survival (PFS).

RESULTS

Among the 247 female participants (median age, 53 years; mean body mass index [BMI], 23.7 kg/m2), 45.7% had disease progression or death during a median follow-up of 36.4 months. After adjusting for age and visceral metastasis, SMD was the only independent predictor among body composition parameters for worse PFS (adjusted hazard ratio [HR] = 1.20 per standard deviation decrement, 95% CI: 1.01-1.42, p = 0.041), whereas BMI, muscle area, and fat area were not. Participants with low SMD had a higher risk of progression than those without (PFS, 27.2 vs. 51.1 months, p = 0.009; adjusted HR 1.84, 95% CI: 1.22-2.76, p = 0.003). Strong associations between low SMD and poor PFS were observed in groups with pre-menopause status (HR, 3.04 vs. 1.19 in post-menopause; 95% CI: 1.54-5.99, p for interaction < 0.05) and without visceral metastases (HR, 2.95 vs. 1.19 in with visceral metastases; 95% CI: 1.59-5.49, p for interaction < 0.05).

CONCLUSIONS

CT-defined low SMD predicts poor treatment outcomes in patients with ABC undergoing first-line treatment with aromatase inhibitors and CDK4/6i.

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.

Targeting the mitotic kinase NEK2 enhances CDK4/6 inhibitor efficacy by potentiating genome instability

Selective inhibitors that target cyclin-dependent kinases 4 and 6 (CDK4/6i) are approved by the U.S. Food and Drug Administration (FDA) for treatment of a subset of breast cancers and are being evaluated in numerous clinical trials for other cancers. Despite this advance, a subset of tumors are intrinsically resistant to these drugs and acquired resistance is nearly inevitable. Recent mechanistic evidence suggests that in addition to stalling the cell cycle, the antitumor effects of CDK4/6i involve the induction of chromosomal instability (CIN). Here, we exploit this mechanism by combining CDK4/6i with other instability-promoting agents to induce maladaptive CIN and irreversible cell fates. Specifically, dual targeting of CDK4/6 and the mitotic kinase NEK2 in vitro drives centrosome amplification and the accumulation of CIN that induces catastrophic mitoses, cell cycle exit, and cell death. Dual targeting also induces CIN in vivo and significantly decreases mouse tumor volume to a greater extent than either drug alone, without inducing overt toxicity. Importantly, we provide evidence that breast cancer cells are selectively dependent on NEK2, but nontransformed cells are not, in contrast with other mitotic kinases that are commonly essential in all cell types. These findings implicate NEK2 as a potential therapeutic target for breast cancer that could circumvent the dose-limiting toxicities that are commonly observed when blocking other mitotic kinases. Moreover, these data suggest that NEK2 inhibitors could be used to sensitize tumors to FDA-approved CDK4/6i for the treatment of breast cancers, improving their efficacy and providing a foundation for expanding the patient population that could benefit from CDK4/6i.

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.

Acquired gene alterations in patients treated with ribociclib plus endocrine therapy or endocrine therapy alone using baseline and end-of-treatment circulating tumor DNA samples in the MONALEESA-2, -3, and -7 trials

BACKGROUND

A prior pooled analysis of the MONALEESA-2, -3, and -7 trials identified baseline markers predictive of sensitivity or resistance to ribociclib plus endocrine therapy (ET). We report the results of an analysis of paired baseline and end-of-treatment (EOT) circulating tumor DNA (ctDNA) samples across the MONALEESA trials.

PATIENTS AND METHODS

Paired baseline and EOT ctDNA samples from MONALEESA-2, -3, and -7 were sequenced using a targeted next-generation sequencing panel. Genes with an EOT alteration prevalence of >5% were included. A McNemar test was carried out on paired samples and adjusted for multiple testing to control the false discovery rate. A Bayesian mixed-effects model was used to adjust for ctDNA fraction at both time points and for study differences.

RESULTS

The analysis included 523 paired samples. At EOT, 21 genes had a >5% alteration prevalence. A trend for higher ctDNA fraction at EOT versus baseline (P = 0.08) was observed. Prevalence of alterations was higher at EOT versus baseline in RB1, SPEN, TPR, PCDH15, and FGFR2 in the ribociclib arm; PBRM1 in the placebo arm; and ESR1 in both arms. The mixed-effects model demonstrated that the same trends for increased prevalence of these alterations at EOT were observed after adjusting for ctDNA fraction and that the increased rate of RB1 and SPEN alterations at EOT were specific to ribociclib plus ET. Analysis of ESR1 indicated a similar increase at EOT in both arms. The most common acquired ESR1 mutations at EOT included Y537C/N/S/D, D538G, E380Q, and L536H/R/P/LC. The prevalence of PIK3CA hotspot mutations at baseline and EOT was similar.

CONCLUSIONS

This analysis identified acquired gene alterations in patients with hormone receptor-positive/human epidermal growth factor receptor-2 negative advanced breast cancer treated with ribociclib plus ET or placebo plus ET. These data may support further studies on acquired resistance mechanisms and inform future systemic interventions in the post-cyclin-dependent kinase 4/6 inhibitor setting.

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.

Long-term breast cancer response to CDK4/6 inhibition defined by TP53-mediated geroconversion

Inhibition of CDK4/6 kinases has led to improved outcomes in breast cancer. Nevertheless, only a minority of patients experience long-term disease control. Using a large, clinically annotated cohort of patients with metastatic hormone receptor-positive (HR+) breast cancer, we identify TP53 loss (27.6%) and MDM2 amplification (6.4%) to be associated with lack of long-term disease control. Human breast cancer models reveal that p53 loss does not alter CDK4/6 activity or G1 blockade but instead promotes drug-insensitive p130 phosphorylation by CDK2. The persistence of phospho-p130 prevents DREAM complex assembly, enabling cell-cycle re-entry and tumor progression. Inhibitors of CDK2 can overcome p53 loss, leading to geroconversion and manifestation of senescence phenotypes. Complete inhibition of both CDK4/6 and CDK2 kinases appears to be necessary to facilitate long-term response across genomically diverse HR+ breast cancers.

Current Clinical Utility of Circulating Tumor DNA Testing in Breast Cancer: A Practical Approach

Circulating tumor DNA (ctDNA) refers to DNA fragments released from cancer cells into the bloodstream. Clinical utility of ctDNA in breast cancer has been explored in both metastatic breast cancer (MBC) and early-stage breast cancer (EBC) settings. In MBC, ctDNA can detect therapeutically targetable genomic alterations and has shown great potential in predicting treatment response or resistance. Accumulating data suggest that ctDNA might also have prognostic value in MBC. In EBC, emerging data have shown ctDNA's predictive and/or prognostic value in both neoadjuvant and adjuvant settings. Minimal residual disease (MRD) detection via ctDNA to detect clinical recurrence after curative therapy is a rapidly advancing field. In this review, we discuss the existing and emerging data regarding ctDNA utility in both MBC and EBC settings.

CDK4/6 inhibition initiates cell cycle arrest by nuclear translocation of RB and induces a multistep molecular response

CDK4/6 inhibitors are standard of care in the treatment of metastatic breast cancer. Treatment regimen consists of a combination with endocrine therapy, since their therapeutic efficacy as monotherapy in most clinical trials was rather limited. Thus, understanding the molecular mechanisms that underlie response to therapy might allow for the development of an improved therapy design. We analyzed the response to the CDK4/6 inhibitor palbociclib in bladder cancer cells over a 48-hour time course using RNA sequencing and identified a multi-step mechanism of response. We next translated these results to the molecular mechanism in bladder cancer cells upon PD treatment. The initial step is characterized by translocation of the RB protein into the nucleus by activation of importin α/β, a mechanism that requires the NLS sequence. In parallel, RB is proteolyzed in the cytoplasm, a process regulated by gankyrin and the SCF complex. Only hypophosphorylated RB accumulates in the nucleus, which is an essential step for an efficient therapy response by initiating G1 arrest. This might explain the poor response in RB negative or mutated patients. At later stages during therapy, increased expression of the MiT/TFE protein family leads to lysosomal biogenesis which is essential to maintain this response. Lastly, cancer cells either undergo senescence and apoptosis or develop mechanisms of resistance following CDK4/6 inhibition.

Current treatment paradigms for triple-negative breast cancer

Triple negative breast cancer (TNBC) accounts for 15% of all breast cancers and is characterized by more aggressive biology, significant disease heterogeneity, and worse clinical outcomes. In recent years, improved understanding of TNBC tumor biology and its heterogeneity have led to the identification of new molecular targets, opening avenues for novel treatment strategies. Clinical trials evaluating immunotherapy, poly-ADP ribose polymerase (PARP) inhibitors, and antibody drug conjugates have shown improvement in clinical outcomes, leading to their incorporation to the treatment options available for patients with TNBC. This review aimed to provide the internal medicine specialist and primary care provider with a comprehensive overview of the current systemic therapy approaches for TNBC and introduce clinicians to novel therapies that have recently been added to the treatment armamentarium against this disease.

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.

Tumor suppressor heterozygosity and homologous recombination deficiency mediate resistance to front-line therapy in breast cancer

The co-occurrence of germline and somatic oncogenic alterations is frequently observed in breast cancer, but their combined biologic and clinical significance has not been evaluated. To assess the role of germline-somatic interactions on outcomes in routine practice, we developed an integrated clinicogenomic pipeline to analyze the genomes of over 4,500 patients with breast cancer. We find that germline (g) BRCA2 -associated tumors are enriched for RB1 loss-of-function mutations and manifest poor outcomes on standard-of-care, front-line CDK4/6 inhibitor (CDK4/6i) combinations. Amongst these tumors, g BRCA2 -related homologous recombination deficiency (HRD) as well as baseline RB1 LOH status promote acquisition of RB1 loss-of- function mutations under the selective pressure of CDK4/6i, causing therapy resistance. These findings suggest an alternative therapeutic strategy using sequential targeting of HRD in g BRCA- associated breast cancers through PARP inhibitors prior to CDK4/6i therapy to intercept deleterious RB1 -loss trajectories and thus suppress the emergence of CDK4/6 inhibitor resistance. More broadly, our findings demonstrate how germline-somatic driven genomic configurations shape response to systemic therapy and can be exploited therapeutically as part of biomarker-directed clinical strategies.

Cracking the Genomic Code of CDK4/6 Inhibitor Resistance

The therapeutic approach to metastatic hormone receptor-positive, human epidermal growth factor-2-negative metastatic breast cancer (HR+/HER2- MBC) has evolved rapidly over recent years. The cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) have become first-line targeted agents of choice, in combination with an antiestrogen. Simultaneously, the clinical landscape of therapeutic options has been rapidly shifting, with novel antiestrogens, signal transduction inhibitors, and next-generation CDK inhibitors in various stages of development. Given these dynamic changes, understanding the genomic and molecular landscape of resistance to currently available antiestrogen therapy and CDK4/6 inhibitors represents a major focus of translational breast cancer research globally. See related article by Goetz et al., p. 2233.

Real-World Experience among Elderly Metastatic Breast Cancer Patients Treated with CDK4/6 Inhibitor-Based Therapy

The largest portion of breast cancer patients diagnosed after 70 years of age present with hormone receptor-positive (HR+) breast cancer subtypes. Cyclin-dependent kinase (CDK) 4/6 inhibitor treatment, in conjunction with endocrine therapy, has become standard-of-care for metastatic HR+ breast cancer. In total, 320 patients with metastatic breast cancer receiving CDK4/6 inhibitor combined with fulvestrant or an aromatase inhibitor were enrolled in an ongoing observational study or were included in an IRB-approved retrospective study. All patients receiving CDK4/6 inhibitor-based therapy that were ≥70 years of age (n = 111) displayed prolonged progression-free survival (27.6 months) as compared to patients <70 years of age (n = 209, 21.1 months, HR = 1.38, p < 0.05). Specifically, patients receiving a CDK4/6 inhibitor with an aromatase inhibitor who were ≥70 years of age (n = 79) displayed exceptionally prolonged progression-free survival (46.0 months) as compared to patients receiving the same treatment who were <70 years of age (n = 161, 21.8 months, HR = 1.71, p < 0.01). However, patients ≥70 years of age also experienced more frequent adverse responses to CDK4/6 inhibitor-based treatment leading to dose reduction, hold, or discontinuation than the younger cohort (69% and 53%, respectively). Treatment strategies that may decrease toxicity without affecting efficacy (such as dose titration) are worth further exploration.

Circulating tumor DNA validity and potential uses in metastatic breast cancer

Following the first characterization of circulating tumor DNA (ctDNA) in the 1990s, recent advances led to its introduction in the clinics. At present, the European Society Of Medical Oncology (ESMO) recommendations endorse ctDNA testing in routine clinical practice for tumor genotyping to direct molecularly targeted therapies in patients with metastatic cancer. In studies on metastatic breast cancer, ctDNA has been utilized for treatment tailoring, tracking mechanisms of drug resistance, and for predicting disease response before imaging. We review the available evidence regarding ctDNA applications in metastatic breast cancer.