The Evolving Pathways of the Efficacy of and Resistance to CDK4/6 Inhibitors in Breast Cancer

The approval of cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) has remarkably improved the survival outcomes of patients with advanced hormone receptor-positive (HR+) breast cancer (BC), becoming the new standard of care treatment in these patients. Despite the efficacy of this therapeutic combination, intrinsic and acquired resistance inevitably occurs and represents a major clinical challenge. Several mechanisms associated with resistance to CDK4/6i have been identified, including both cell cycle-related and cell cycle-nonspecific mechanisms. This review discusses new insights underlying the mechanisms of action of CDK4/6i, which are more far-reaching than initially thought, and the currently available evidence of the mechanisms of resistance to CDK4/6i in BC. Finally, it highlights possible treatment strategies to improve CDK4/6i efficacy, summarizing the most relevant clinical data on novel combination therapies involving CDK4/6i.

Systematic review of time to subsequent therapy as a candidate surrogate endpoint in advanced solid tumors

Aim: Time to subsequent therapy (TST) is an end point that may complement progression-free survival (PFS) and overall survival (OS) in determining the treatment effect of anticancer drugs and may be a potential surrogate for PFS and OS. We systematically reviewed the correlation between TST and both PFS and OS in published phase 2/3 studies in advanced solid tumors. Materials & methods: Trial-level correlational analyses were performed for TST versus PFS (by investigator and/or central review) and TST versus OS. Results: Of 21 included studies, nine (43%) used 'time to first subsequent therapy or death' (TFST) as the TST end point; 11 (57%) used different definitions ('other TST end points'). There was a strong correlation between TFST and PFS by investigator (medians: R2 = 0.88; hazard ratio [HR]: R2 = 0.91) and TFST versus PFS by central review (medians: R2 = 0.86; HRs: R2 = 0.84). For TFST versus OS there was medium/poor correlation for medians (R2 = 0.64) and HRs (R2 = 0.02). Conclusion: TFST strongly correlates with PFS, but not with OS.

Cost Effectiveness of Ribociclib in Combination with Fulvestrant for the Treatment of Postmenopausal Women with HR+/HER2- Advanced Breast Cancer Who Have Received No or Only One Prior Line of Endocrine Therapy: A Canadian Healthcare Perspective

BACKGROUND

The MONALEESA-3 trial demonstrated the efficacy and safety of ribociclib plus fulvestrant versus placebo plus fulvestrant for patients with hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+/HER2-) advanced breast cancer (ABC). This analysis evaluated the cost effectiveness of ribociclib plus fulvestrant versus fulvestrant in patients with HR+/HER2- ABC from a Canadian healthcare payer perspective.

METHODS

The incremental cost-effectiveness ratio (ICER), expressed as incremental costs per quality-adjusted life-year (QALY) gained for ribociclib plus fulvestrant versus fulvestrant, was estimated using a semi-Markov cohort model developed in Microsoft Excel, with states for progression-free, post-progression, and dead. A 15-year time horizon was used. Survival distributions for progression-free survival (PFS), post-progression survival (PPS), and time to discontinuation (TTD) were based on parametric survival distributions fit to data from MONALEESA-3. Health-state utilities were estimated using EQ-5D index values collected in MONALEESA-3. Direct costs of ABC treatment (medication and administration costs, follow-up and monitoring, adverse events, subsequent treatments) were based on Canadian-specific values from published sources. Costs (2019 CAN$) and QALYs were discounted at 1.5% annually.

RESULTS

In the base case, ribociclib plus fulvestrant was estimated to result in gains of 1.19 life-years and 0.96 QALYs versus fulvestrant, at an incremental cost of $151,371. The ICER of ribociclib plus fulvestrant versus fulvestrant was $157,343 per QALY gained based on the mean of probabilistic analyses. Results were sensitive to parametric distributions used for projecting long-term TTD, PFS, and PPS.

CONCLUSIONS

For patients with HR+/HER2- ABC, ribociclib plus fulvestrant is projected to result in substantial gains in QALYs compared with fulvestrant. At its current list price, ribociclib used in combination with fulvestrant is likely to be cost effective in these patients at a threshold ICER of $157,343. These results may be useful in deliberations regarding reimbursement and access to this treatment.

Endocrine Therapy With or Without CDK4/6 Inhibitors in Women With Hormone-receptor Positive Breast Cancer: What do we Know About the Effects on Cognition?

Adjuvant endocrine therapy (ET) is the cornerstone of treatment for hormone-receptor positive breast cancer. Recently, ET is increasingly combined with "cyclin-dependent kinases 4 and 6'' (CDK4/6) inhibitors. Given the importance of estrogens in neural processes and the role of cyclin D in hippocampal cell proliferation, it is plausible that these therapies affect cognition, but studies on these potential cognitive effects are sparse. In this review, we summarize existing knowledge on the cognitive effects of ET and CDK4/6 inhibitors in pre-, peri- and postmenopausal patients with breast cancer. We show that several clinical studies support adverse cognitive effects, especially on verbal memory, after ET-induced decrease of estrogen-levels or inactivation of estrogen-receptors. Clinical studies on the cognitive effects of CDK4/6 inhibitors are virtually non-existent and no conclusions can yet be drawn. Longitudinal studies on the cognitive effects of the combined ET-CDK4/6 inhibitors are highly needed to properly inform patients about potential short-term and long-term cognitive side effects. These studies should preferably include cognitive assessments (including a measurement prior to ET), and be designed in such a way that they can account for variables such as type and duration of ET, CDK4/6 inhibition, menopausal status, and other disease- and treatment-related symptoms that can impact cognition, such as fatigue and distress.

Cost Effectiveness of Ribociclib Plus a Nonsteroidal Aromatase Inhibitor in Pre-/Perimenopausal, HR+ and HER2- Advanced Breast Cancer: A Canadian Healthcare Perspective

BACKGROUND AND OBJECTIVES

The MONALEESA-7 trial demonstrated the efficacy and safety of ribociclib plus a nonsteroidal aromatase inhibitor (NSAI) [with goserelin] for pre-/perimenopausal women with hormone receptor-positive and human epidermal growth factor receptor 2-negative advanced breast cancer. This analysis evaluated the cost effectiveness of ribociclib plus NSAI vs NSAI monotherapy and tamoxifen monotherapy from the perspective of the Canadian healthcare system.

METHODS

The incremental cost-effectiveness ratio expressed as incremental costs per quality-adjusted life-year (QALY) gained for ribociclib plus an NSAI vs an NSAI and vs tamoxifen was estimated using a semi-Markov cohort model developed in Microsoft Excel with a 15-year time horizon and states for progression-free survival, post-progression survival, and dead. Survival distributions for progression-free survival, post-progression survival, and time to discontinuation as well as health-state utilities were estimated using data from MONALEESA-7. Direct costs of advanced breast cancer treatment were based on Canadian-specific values from published sources. Costs ($CAN 2019) and QALYs were discounted at 1.5% annually.

RESULTS

Ribociclib plus an NSAI was estimated to yield gains of 1.42 life-years and 1.17 QALYs vs an NSAI, and 2.61 life-years and 2.12 QALYs vs tamoxifen, at incremental costs of $209,701 and $220,836, respectively. In probabilistic analyses, the incremental cost-effectiveness ratio for ribociclib plus an NSAI was estimated to be $178,872 per QALY gained vs an NSAI and $104,400 per QALY gained vs tamoxifen. Results of deterministic analyses were similar (incremental cost-effectiveness ratios of $177,245 and $103,316 vs NSAI and tamoxifen, respectively). Results were sensitive to parametric distributions used for projecting progression-free survival and the time horizon.

CONCLUSIONS

At its current list price, ribociclib used in combination with NSAI is likely to be co-effective relative to an NSAI alone or tamoxifen alone if the willingness-to-pay threshold is less than approximately $178,000 per QALY. These results have informed deliberations regarding reimbursement and access to this treatment in Canada and may be useful for decision makers in other settings.

Ribociclib-Induced Pneumonitis: A Case Report

BACKGROUND

Cyclin-dependent kinase 4/6 (CDK 4/6) inhibitors have been a significant breakthrough in the management of hormone receptor-positive, HER2-negative metastatic breast cancer based on the results of several large phase III randomized trials. The most common reported toxicity is myelosuppression due to disease such as leukopenia, neutropenia, and thrombocytopenia. Other toxicities associated with CDK 4/6 inhibitors include mucositis, fatigue, gastrointestinal side effects, hepatic toxicities, and QTc prolongation. Despite a good toxicity profile in pivotal studies, the increased rates of use in clinical practice may show less prevalent but lethal toxicity such as lung injury.

CASE PRESENTATION

Here, we describe a female patient with metastatic hormone receptor-positive/human epidermal growth factor 2-negative breast cancer who developed lung toxicity while on ribociclib.

DISCUSSION

Lung injury is a possible side effect of CDK 4/6 inhibitors and there is an increasing need to understand the management of this side effect.

Strategic Combinations to Prevent and Overcome Resistance to Targeted Therapies in Oncology

Recent advances in the understanding of underlying molecular signaling mechanisms of cancer susceptibility and progression have led to an increase in the use of targeted therapies for cancer treatment. Despite improvements in survival with new treatment options in oncology, resistance to therapy is a major obstacle to the long-term effectiveness of targeted agents in metastatic cancer treatment, culminating in insensitivity to treatment and tumor outgrowth. Adaptive resistance can play an important role in primary and upfront resistance to therapy as well as in secondary or acquired resistance. By focusing on colorectal and breast tumors, we discuss how therapeutic combinations based on specific drivers of tumor biology can be used to overcome resistance. We present how monitoring tumor dynamics over time may allow early adaptation of treatment. Breast cancer is the most common malignancy in women worldwide, and the majority of these cancers are sensitive to endocrine therapy (ET) blocking the production of or response to estrogen. However, primary and acquired resistance limits efficacy. Recent combinations of agents targeted to pathways that drive tumor growth resistance with ET have resulted in remarkable improvements in disease response and control, improving survival in some settings. In this review, we summarize adaptive resistance mechanisms, approaches to combination strategies, and dynamic tumor monitoring to improve efficacy and overcome resistance. We provide examples of combination therapy to enhance the efficacy of targeted therapies in breast and colorectal tumors.

Therapy after cyclin-dependent kinase inhibition in metastatic hormone receptor-positive breast cancer: Resistance mechanisms and novel treatment strategies

Endocrine therapy has been the standard of care for patients with metastatic hormone receptor (HR)-positive, HER2-negative breast cancer since the 1970s, improving survival while avoiding the toxicities associated with cytotoxic chemotherapy. However, all HR-positive tumors ultimately develop resistance to endocrine therapy. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors have more recently become an important component of the management of this breast cancer subtype, significantly delaying time to the disease progression and improving survival when combined with endocrine therapy. However, as with endocrine therapy alone, treatment resistance remains a universal phenomenon. As more women receive CDK4/6 inhibitors as part of their treatment, the management of de novo and acquired resistance to combined CDK4/CDK6 inhibitor plus endocrine therapy regimens has emerged as an important clinical challenge. Several resistance mechanisms have been described, including alterations in the CDK4/6/cyclin D complex or its major effector retinoblastoma protein (pRb), bypass signaling through other cyclin/CDK complexes and activation of upstream signaling pathways, in particular the PI3K/mTOR pathway, but robust biomarkers to predict resistance remain elusive, and the role for continuing CDK4/6 inhibitors after progression remains under investigation. Novel strategies being evaluated in clinical trials include the continuation of CDK4/6 inhibitors through progression, as well as triplet therapy combinations with PI3K inhibitors or immune checkpoint inhibitors.

Biologic therapy for advanced breast cancer: recent advances and future directions

INTRODUCTION

Advanced breast cancer (ABC) is a leading cause of mortality, morbidity, and disability in women worldwide. For decades, treatment of ABC has relied on chemotherapy and endocrine treatments (ET), until HER2 was recognized as a 'druggable' target in the 1990s. Thereafter, various anti-HER2 drugs have been approved for the HER2-positive subtype, but only in the last few years, biologic agents targeting different pathways have entered the therapeutic arsenal of luminal and triple-negative cancers.

AREAS COVERED

The purpose of the present review is to recapitulate the most promising novel biologic agents being developed for the treatment of ABC. New drugs for all breast cancer subtypes are discussed, as well as some potential future directions in ABC treatment.

EXPERT OPINION

Several biologic drugs have been recently approved, revolutionizing ABC treatment algorithms: key examples are CDK4/6-inhibitors and the PI3K-inhibitor alpelisib for endocrine-positive ABC; atezolizumab for triple-negative cancers; two PARP-inhibitors for HER2-negative germinal BRCA-mutated cancers. Additionally, multiple drugs are demonstrating activity in late-phase clinical trials for all subtypes. While some of these represent pharmacological evolutions of previously approved drugs, some others might pave the way for new paradigms in ABC, challenging both its classification and current treatment algorithms.

CDK4/6 Inhibitor Biomarker Research: Are We Barking Up the Wrong Tree?

CDK4/6 inhibitors have emerged as a significant advance for the treatment of patients with advanced estrogen receptor-positive breast cancer. However, the identification of predictive markers that optimize their use is proving harder than expected. In this commentary we advocate for unbiased discovery and a collaborative approach across trials.See related article by Finn et al., p. 110.