CDK4/6 inhibition in early and metastatic breast cancer: A review

Breast cancer (BC) is responsible for 14% of cancer-related deaths in women [1]. Cyclin-dependent kinase (CDK) 4/6 inhibitors are a new class of selective drugs, offering an effective and tolerable treatment. CDK4/6 inhibitors induce cell cycle arrest in the G1 phase, and may thereby prevent tumour progression. Three CDK4/6 inhibitors have been tested in clinical BC trials: palbociclib, ribociclib, and abemaciclib. The Food and Drug Administration (FDA) and European Commission (EMA) have approved palbociclib for the treatment of patients HR+ HER2- locally advanced or metastatic BC (aBC) in combination with an aromatase inhibitor as initial therapy in postmenopausal women or in combination with fulvestrant in women who have received prior endocrine therapy. Ribociclib has been approved by the FDA in combination with an aromatase inhibitor as initial therapy for postmenopausal women with HR+ HER2- aBC. Moreover, CDK4/6 inhibitors have shown promising results in the (neo)adjuvant setting. In this review, the principal completed and ongoing clinical trials in aBC are reviewed for both the metastatic as (neo)adjuvant setting. Tables will provide a complete overview of the ongoing clinical trials. At last, the future perspectives of these CDK4/6 inhibitors are discussed.     

The safety and efficacy of palbociclib in the treatment of metastatic breast cancer

Introduction: Palbociclib (Ibrance®) is the first-in-class CDK4/6 inhibitor which has been introduced into clinical practice for the treatment of estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC). It is an orally administered drug, which acts by selectively inhibiting cyclin-dependant kinases CDK4 and CDK6. Given together with anti-estrogens like letrozole and fulvestrant it enhances the antiproliferative effect of these drugs without compromising the favorable toxicity profile of endocrine therapy.

Areas covered: This review gives a concise overview of the current available preclinical and clinical data about pabociclib including its chemistry, mechanism of action, pharmacokinetics and evidence of its substantial anti-tumor efficacy and safety profile when combined with endocrine therapy

Expert commentary: Palbociclib doubles treatment efficacy of letrozole and fulvestrant with a restricted and well manageable toxicity profile. Efficacy as well as safety results from the three registration trials are remarkably consistent. Patients’ quality of life is maintained on palbociclib treatment. The treatment landscape in ER+ HER2? MBC will be highly influenced by this compound and development of two other CDK4/6 inhibitors.     

Ribociclib for the treatment of hormone receptor-positive,human epidermal growth factor receptor 2-negative advanced breast cancer

Introduction: The emergence of cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors marked a significant advancement in the treatment of advanced breast cancer. Ribociclib is an orally bioavailable, highly selective inhibitor of CDK4/6. In combination with various endocrine therapies, ribociclib has demonstrated clinical activity as a first-line therapy for patients with HR+, HER2? advanced breast cancer, without compromising the favorable toxicity profile associated with endocrine therapy. Thus, ribociclib is now considered a new standard of care for HR+, HER2? advanced breast cancer.

Areas covered: This review provides a concise overview of the preclinical and clinical development of ribociclib, including evidence of its clinical activity and safety profile when combined with endocrine therapy in HR+, HER2? advanced breast cancer.

Expert commentary: CDK4/6 inhibition represents a promising treatment option for patients with HR+ metastatic breast cancer. Ribociclib significantly improved progression-free survival in patients receiving first-line endocrine therapy for HR+, HER2? advanced breast cancer. Planned and ongoing trials investigating ribociclib in combination with other endocrine therapies and in various clinical settings will help to determine the optimal treatment sequence for different patient populations.     

CDK4/6 Inhibition as a therapeutic strategy in breast cancer: palbociclib,ribociclib, and abemaciclib

With 40,920 American women expected to die from breast cancer in 2018 and global health estimates that more than 508,000 women died in 2011 from this disease, the identification of novel therapeutic strategies for the treatment of breast cancer cannot be ignored. A breakthrough class of cancer drugs that has emerged in recent years and has had an impact in the treatment of breast cancer are the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, with palbociclib the first in class to have received regulatory approval for breast cancer. In this article we will compare and contrast three CDK4/6 inhibitors - palbociclib, ribociclib and abemaciclib - that have received regulatory approval for the treatment of metastatic breast cancer. Ribociclib and abemaciclib developed after the success of palbociclib represent examples of "me-too" therapies increasingly being deployed in oncology.     

Ribociclib,a CDK 4/6 inhibitor,plus endocrine therapy in Asian women with advanced breast cancer

The ongoing, Phase Ib MONALEESASIA study is evaluating the efficacy and safety of ribociclib plus endocrine therapy in Asian patients with hormone receptor‐positive, human epidermal growth factor receptor 2‐negative advanced breast cancer. Eligible patients from Japan, Hong Kong, and Singapore were enrolled in this 2‐phase study consisting of a dose‐escalation phase to determine the maximum‐tolerated dose and the recommended Phase II dose of ribociclib plus letrozole, and a dose‐expansion phase to evaluate safety and tolerability of ribociclib plus letrozole, fulvestrant, or tamoxifen. An exploratory biomarker analysis evaluating expression of target genes was also conducted. In the dose‐escalation phase, the maximum‐tolerated/recommended Phase II doses of ribociclib were lower in Japanese patients (300 mg) than in Asian non‐Japanese patients (600 mg). Ribociclib plus endocrine therapy at the recommended Phase II dose had a manageable safety profile, with neutropenia and elevated liver transaminases being the most common adverse events leading to dose modifications or discontinuations, and it demonstrated evidence of clinical activity in both Japanese and Asian non‐Japanese patients. Preliminary efficacy in Asian populations is similar to that observed in White populations studied in previous ribociclib (MONALEESA) trials. Biomarker analysis demonstrated suppression of pharmacodynamic biomarker gene expression, indicating inhibition of target genes by ribociclib combined with endocrine therapy. Results from the ongoing study support the use of ribociclib in combination with letrozole in Asian non‐Japanese patients at the same dose (600 mg) as White patients. In Japanese patients, a lower dose of ribociclib (300 mg) should be considered. Clinicaltrials.gov: NCT02333370.     

Progression-Free Survival for Real-World Use of Palbociclib in Hormone Receptor-Positive Metastatic Breast Cancer

BackgroundAdditional use of cyclin-dependent kinase 4/6 inhibitors with endocrine therapy improves progression-free survival (PFS) in advanced hormone receptor (HR)-positive HER2-negative breast cancer. However, neutropenia is a common reason for dose reductions, leading to concerns about palbociclib efficacy at lower doses. A safety analysis confirmed no PFS differences between palbociclib doses in the second-line setting, but to our knowledge, this has not been evaluated for first-line treatment.Patients and MethodsIn this retrospective, single-center cohort study we evaluated real-world use of first-line palbociclib with aromatase inhibitor (AI) treatment in HR-positive, HER2-negative metastatic breast cancer patients who received treatment between February 2015 and July 2017. The primary objective was to determine PFS of treatment with palbociclib and an AI in a real-world first-line setting. Secondary objectives included determining the PFS for patients treated with palbociclib on the basis of final doses, time to first dose reduction, time to treatment failure (TTF), and safety.ResultsSeventy patients were included in the final analysis. Median PFS was 26.4 months. No significant differences in PFS were observed on the basis of final doses of palbociclib (P = .77). Time to first dose reduction was 2.3 months. Median TTF was 26.1 months. Dose delays, reductions, and Grade 3/4 neutropenia were common (63%, n = 44; 57%, n = 40; and 62%, n = 43, respectively).ConclusionReal-world first-line palbociclib treatment produced outcomes similar to those in PALOMA-2 (Palbociclib and Letrozole in Advanced Breast Cancer) (median PFS 26.4 months vs. 24.8 months) despite more dose reductions (57%, n = 40 vs. 36%, n = 160) and shorter time to first dose reduction (2.3 vs. 3.0 months). No significant differences in PFS were observed for the varying palbociclib doses. Palbociclib dose reductions might not significantly affect PFS in the first-line setting.     

Limited clinical activity of palbociclib and ribociclib monotherapy in advanced cancers with cyclin D-CDK4/6 pathway alterations in the Dutch DRUP and Australian MoST trials

The Dutch Drug Rediscovery Protocol (DRUP) and the Australian Cancer Molecular Screening and Therapeutic (MoST) Program are similar nonrandomized, multidrug, pan-cancer trial platforms that aim to identify signals of clinical activity of molecularly matched targeted therapies or immunotherapies outside their approved indications. Here, we report results for advanced or metastatic cancer patients with tumors harboring cyclin D-CDK4/6 pathway alterations treated with CDK4/6 inhibitors palbociclib or ribociclib. We included adult patients that had therapy-refractory solid malignancies with the following alterations: amplifications of CDK4, CDK6, CCND1, CCND2 or CCND3, or complete loss of CDKN2A or SMARCA4. Within MoST, all patients were treated with palbociclib, whereas in DRUP, palbociclib and ribociclib were assigned to different cohorts (defined by tumor type and alteration). The primary endpoint for this combined analysis was clinical benefit, defined as confirmed objective response or stable disease ≥16 weeks. We treated 139 patients with a broad variety of tumor types; 116 with palbociclib and 23 with ribociclib. In 112 evaluable patients, the objective response rate was 0% and clinical benefit rate at 16 weeks was 15%. Median progression-free survival was 4 months (95% CI: 3-5 months), and median overall survival 5 months (95% CI: 4-6 months). In conclusion, only limited clinical activity of palbociclib and ribociclib monotherapy in patients with pretreated cancers harboring cyclin D-CDK4/6 pathway alterations was observed. Our findings indicate that monotherapy use of palbociclib or ribociclib is not recommended and that merging data of two similar precision oncology trials is feasible.     

Molecular mechanisms of resistance to CDK4/6 inhibitors in breast cancer: A review

Deregulation of the cyclin D-CDK4/6-INK4-RB pathway leading to uncontrolled cell proliferation, is frequently observed in breast cancer. Currently, three selective CDK4/6 inhibitors have been FDA approved: palbociclib, ribociclib and abemaciclib. Despite promising clinical outcomes, intrinsic or acquired resistance to CDK4/6 inhibitors has limited the success of these treatments; therefore, the development of various strategies to overcome this resistance is of great importance. We highlight the various mechanisms that are directly or indirectly responsible for resistance to CDK4/6 inhibitors, categorizing them into two broad groups; cell cycle-specific mechanisms and cell cycle-nonspecific mechanisms. Elucidation of the diverse mechanisms through which resistance to CDK4/6 inhibitors occurs, may aid in the design of novel therapeutic strategies to improve patient outcomes. This review summarizes the currently available knowledge regarding mechanisms of resistance to CDK4/6 inhibitors, and possible therapeutic strategies that may overcome this resistance as well.     

Palbociclib or Ribociclib in First-Line Treatment in Patients With Hormone Receptor–Positive/Human Epidermal Receptor 2–Negative Advanced or Metastatic Breast Cancer? A Perspective Based on Pharmacologic Costs

PurposeTo assess the pharmacologic costs of CDK4/6 inhibitors (palbociclib and ribociclib) in hormone receptor–positive (HR⁺)/human epidermal receptor 2–negative (HER2^?) advanced or metastatic breast cancer (BC). Pivotal phase 3 randomized controlled trials (RCTs) were considered.DiscussionTwo phase 3 RCTs including 1334 patients were considered. European Society for Medical Oncology Magnitude of Clinical Benefit Scale reached grade 3 for the PALOMA-2 and MONALEESA-2 trials. Pharmacologic costs of palbociclib and ribociclib at full dose were similar, at €3864 and €4002 per month of progression-free survival (PFS) gained, respectively. The reduction of dose of ribociclib (36.1% in the pivotal RCT vs. 36.0% of palbociclib in pivotal RCT) resulted in €2718 and €1348 per month of PFS gained at 400 and 200 mg daily, respectively.ConclusionWhen pharmacologic costs of drugs are combined with the measure of efficacy represented by PFS, both palbociclib and ribociclib are cost-effective first-line treatments in postmenopausal women with HR⁺/HER2^? advanced or metastatic BC, with a lower cost in favor of ribociclib in patients with dose reduction.     

First-Line Treatment for Endocrine-Sensitive Bone-Only Metastatic Breast Cancer: Systematic Review and Meta-analysis

In the last decade, several clinical trials have investigated novel endocrine combinations for the first-line treatment of hormone receptor–positive metastatic breast cancer. Nevertheless, the use of combinations for the first-line treatment of bone-only disease is widely discussed as a result of its indolent natural history. We performed a comprehensive search of phase 3 randomized clinical trials published in the literature through September 2018. Our aim was to explore the role of the new endocrine approaches in bone-only metastatic breast cancer, suggesting a possible strategy for their selection. In particular, we evaluated the comparative risk of adverse event occurrence during these treatments. A total of 6 studies were deemed suitable for meta-analysis: the Monaleesa-2, Monaleesa-7, Monarch-3, Paloma-2, SWOG, and Alliance trials. Overall, the novel strategies were shown to improve progression-free survival in bone-only disease (hazard ratio = 0.65; 95% confidence interval, 0.49-0.86; P = .003). Combinations with cyclin-dependent kinase inhibitors improved progression-free survival (hazard ratio = 0.54; 95% confidence interval, 0.39-0.75; P < .001) with an acceptable toxicity profile. Abemaciclib was associated with increased anemia and gastrointestinal toxicity (especially diarrhea), whereas palbociclib was associated with increased leukopenia (but not neutropenia) compared to the other compounds. Increased aspartate aminotransferase levels were reported for both ribociclib and abemaciclib. The combination of cyclin-dependent kinase 4/6 inhibitors and endocrine therapy represents an effective and well-tolerated approach for first-line treatment in bone-only disease settings. Because no direct comparison between the 3 cyclin-dependent kinase 4/6 inhibitors is available, the selection of the most appropriate treatment should be based on toxicity profile as well as patient preference and copathologies.     

Mechanisms of therapeutic CDK4/6 inhibition in breast cancer

Cyclin dependent kinase (CDK) 4/6 inhibitors have advanced the treatment of metastatic breast cancer by targeting the cell cycle machinery, interrupting intracellular and mitogenic hormone signals that stimulate proliferation of malignant cells. Preclinical evidence demonstrated that derangements of cyclin D1, CDK4/6, and retinoblastoma expression are common in breast cancer, and suggested a therapeutic benefit from interrupting this axis required for cell cycle progression. Studies of cell lines and animal models of breast cancer have demonstrated the complex interplay between the cell cycle and estrogen receptor and human epidermal growth receptor 2 signaling, which informs our understanding of synergistic use of CDK4/6 inhibitors with endocrine therapy, as well as mechanisms of resistance to endocrine therapy. Interestingly, estrogen receptor activity leads to upregulation of cyclin D1 expression, but the estrogen receptor is also in turn activated by cyclin D1, independent of estrogen binding. Early CDK inhibitors were nonspecific and limited by systemic toxicities, while the current generation of CDK4/6 inhibitors have shown promise in the treatment of hormone receptor-positive breast cancer. Preclinical investigations of the three CDK4/6 inhibitors approved by the US Food and Drug Administration (palbociclib, ribociclib, and abemaciclib) lend further insight into their mechanism of action, which will hopefully inform the future use and refinement of these therapies. Finally, we summarize evidence for additional novel CDK4/6 inhibitors currently in development.     

Clinical considerations of the role of palbociclib in the management of advanced breast cancer patients with and without visceral metastases

BackgroundThis report assesses the efficacy and safety of palbociclib plus endocrine therapy (ET) in women with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer (ABC) with or without visceral metastases.Patients and methodsPre- and postmenopausal women with disease progression following prior ET (PALOMA-3; N = 521) and postmenopausal women untreated for ABC (PALOMA-2; N = 666) were randomized 2 : 1 to ET (fulvestrant or letrozole, respectively) plus palbociclib or placebo. Progression-free survival (PFS), safety, and patient-reported quality of life (QoL) were evaluated by prior treatment and visceral involvement.ResultsVisceral metastases incidence was higher in patients with prior resistance to ET (58.3%, PALOMA-3) than in patients naive to ET in the ABC setting (48.6%, PALOMA-2). In patients with prior resistance to ET and visceral metastases, median PFS (mPFS) was 9.2 months with palbociclib plus fulvestrant versus 3.4 months with placebo plus fulvestrant [hazard ratio (HR), 0.47; 95% confidence interval (CI), 0.35–0.61], and objective response rate (ORR) was 28.0% versus 6.7%, respectively. In patients with nonvisceral metastases, mPFS was 16.6 versus 7.3 months, HR 0.53; 95% CI 0.36–0.77. In patients with visceral disease and naive to ET in the advanced disease setting, mPFS was 19.3 months with palbociclib plus letrozole versus 12.9 months with placebo plus letrozole (HR 0.63; 95% CI 0.47–0.85); ORR was 55.1% versus 40.0%; in patients with nonvisceral disease, mPFS was not reached with palbociclib plus letrozole versus 16.8 months with placebo plus letrozole (HR 0.50; 95% CI 0.36–0.70). In patients with prior resistance to ET with visceral metastases, palbociclib plus fulvestrant significantly delayed deterioration of QoL versus placebo plus fulvestrant, whereas patient-reported QoL was maintained with palbociclib plus letrozole in patients naive to endocrine-based therapy for ABC.ConclusionsPalbociclib plus ET prolonged mPFS in patients with visceral metastases, increased ORRs, and in patients previously treated for ABC, delayed QoL deterioration, presenting a standard treatment option among patients with visceral metastases amenable to endocrine-based therapy.Clinical trial registrationNCT01942135, NCT01740427     

Abemaciclib induces atypical cell death in cancer cells characterized by formation of cytoplasmic vacuoles derived from lysosomes

In the cell cycle, the G₁/S transition is controlled by the cyclin‐dependent kinase (CDK) 4/6‐cyclin D complex. Constitutive activation of CDK4/6 dysregulates G₁/S transition, leading to oncogenic transformation. We found that 3 CDK4/6 inhibitors, abemaciclib, ribociclib, and palbociclib, exerted a cytocidal effect as well as a cytostatic effect at the G₁ phase in cancer cell lines, including A549 human non–small cell lung cancer cells. Among these inhibitors, abemaciclib exhibited the most potent cytotoxic effect. The cell‐death phenotype induced by abemaciclib, which entailed formation of multiple cytoplasmic vacuoles, was not consistent with apoptosis or necroptosis. Abemaciclib blocked autophagic flux, resulting in accumulation of autophagosomes, however vacuole formation and cell death induced by abemaciclib were independent of autophagy. In addition, methuosis, a cell‐death phenotype characterized by vacuole formation induced by excessive macropinocytosis, was excluded because the vacuoles did not incorporate fluorescent dextran. Of note, both formation of vacuoles and induction of cell death in response to abemaciclib were inhibited by vacuolar‐type ATPase (V‐ATPase) inhibitors such as bafilomycin A1 and concanamycin A. Live‐cell imaging revealed that the abemaciclib‐induced vacuoles were derived from lysosomes that expanded following acidification. Transmission electron microscopy revealed that these vacuoles contained undigested debris and remnants of organelles. Cycloheximide chase assay revealed that lysosomal turnover was blocked by abemaciclib. Furthermore, mTORC1 inhibition along with partial lysosomal membrane permeabilization occurred after abemaciclib treatment. Together, these results indicate that, in cancer cells, abemaciclib induces a unique form of cell death accompanied by swollen and dysfunctional lysosomes.     

CDK4/6-Inhibitoren beim Mammakarzinom

The cyclin-dependent kinase (CDK)4/6 inhibitors palbociclib, ribociclib, and abemaciclib have demonstrated significant improvements in progression-free survival in numerous clinical trials in metastasized hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer. Since palbociclib and ribociclib were approved in Germany, CDK4/6 inhibitors in combination with antihormonal therapy have been a standard treatment in first- and also in second-line therapy. Abemaciclib, the third CDK4/6, inhibitor will be approved in Germany this year. Side effects, particularly neutropenia, are well managed in daily routine by dose reductions and dose interruptions. The necessary intensive treatment monitoring has to be adhered to precisely, in order to administer treatment in the form of a well-tolerated maintenance therapy. Several clinical trials in the neo-/adjuvant setting are currently in progress, in order to transfer the survival advantage from the metastasized situation into early breast cancer. The aim of this article is to elucidate the mode of action, efficacy, and side effects of the CDK4/6 inhibitors, and to put them in a current clinical context.     

哌柏西利治疗HR+/HER2-乳腺癌新进展

哌柏西利作为第一个上市的细胞周期蛋白依赖性激酶4和6抑制剂，显著改善了激素受体阳性、人表皮生长因子受体-2阴性乳腺癌的生存。哌柏西利的开发是抗肿瘤药物发展的一个重要里程碑。本文回顾了哌柏西利作用机制，总结了药物相关临床试验、相关不良反应以及药物应用等问题。     

A phase II feasibility study of palbociclib in combination with adjuvant endocrine therapy for hormone receptor-positive invasive breast carcinoma

BackgroundThe CDK4/6 inhibitor palbociclib prolongs progression-free survival in hormone receptor-positive/HER2-negative (HR+/HER2−) metastatic breast cancer when combined with endocrine therapy. This phase II trial was designed to determine the feasibility of adjuvant palbociclib and endocrine therapy for early breast cancer.Patients and methodsEligible patients with HR+/HER2− stage II–III breast cancer received 2 years of palbociclib at 125 mg daily, 3 weeks on/1 week off, with endocrine therapy. The primary end point was discontinuation from palbociclib due to toxicity, non-adherence, or events related to tolerability. A discontinuation rate of 48% or higher would indicate the treatment duration of 2 years was not feasible, and was evaluated under a binomial test using a one-sided α = 0.025.ResultsOverall, 162 patients initiated palbociclib; over half had stage III disease (52%) and most received prior chemotherapy (80%). A total of 102 patients (63%) completed 2 years of palbociclib; 50 patients discontinued early for protocol-related reasons (31%, 95% CI 24% to 39%, P = 0.001), and 10 discontinued due to protocol-unrelated reasons. The cumulative incidence of protocol-related discontinuation was 21% (95% CI 14% to 27%) at 12 months from start of treatment. Rates of palbociclib-related toxicity were congruent with the metastatic experience, and there were no cases of febrile neutropenia. Ninety-one patients (56%) required at least one dose reduction.ConclusionAdjuvant palbociclib is feasible in early breast cancer, with a high proportion of patients able to complete 2 years of therapy. The safety profile in the adjuvant setting mirrors that observed in metastatic disease, with approximately half of the patients requiring dose-modification. As extended duration adjuvant palbociclib appears feasible and tolerable for most patients, randomized phase III trials are evaluating clinical benefit in this population.Clinicaltrials.gov registrationNCT02040857.     

Preclinical and clinical development of palbociclib and future perspectives

Cyclin-dependent kinases (CDKs) play a key role in cell cycle regulation, which makes them a clear therapeutic target to interfere with cell division and proliferation in cancer patients. Palbociclib, a specific inhibitor of CDK4/6 with outstanding clinical efficacy data and limited toxicity, has been recently approved for the treatment of hormone receptor (HR)-positive human epidermal growth factor receptor 2 (HER2)-negative locally advanced or metastatic breast cancer, either in combination with an aromatase inhibitor or in combination with fulvestrant in women who have received prior endocrine therapy. This review describes the mechanism of action, preclinical experiences and clinical data of palbociclib, with a special focus on integrating this data with the positioning of palbociclib in the current clinical guidelines for advanced HR-positive/HER2-negative breast cancer. Aspects of the ongoing major studies are also presented, as well as future prospects in the development of palbociclib.     

Cyclin-dependent kinase 4/6 inhibitors as first-line treatment for post-menopausal metastatic hormone receptor-positive breast cancer patients: a systematic review and meta-analysis of phase III randomized clinical trials

Background

To compare the efficacy and toxicity of the combination of cyclin-dependent kinase 4/6 (CDK 4/6) inhibitors and nonsteroidal aromatase inhibitors (AI) versus AI alone as first-line therapy for patients with advanced hormone receptor-positive breast cancer.

Materials and methods

Phase III randomized clinical trials (RCT) were identified after a systematic review of electronic databases. A random-effect model was used to determine the pooled hazard ratio (HR) for progression-free survival (PFS) using the inverse-variance method. The Mantel–Haenszel method was used to calculate the pooled odds ratio (OR) for overall response, clinical benefit rate and treatment-related side effects. Heterogeneity was measured using the tau-squared and I² statistics.

Results

After a systematic search, three phase III RCT (n = 1827) were included. The use of CDK 4/6 inhibitors (abemaciclib, palbociclib, and ribociclib) in combination with an AI was significantly associated with longer PFS compared to the use of letrozole or anastrozole alone (HR: 0.57; 95% CI 0.50–0.65; p < 0.00001), with no significant heterogeneity among trials. Similarly, overall response rate and clinical benefit rate were higher for patients who received the combination therapy than for patients allocated to AI alone. Grade 3 or higher treatment-related side effects were more frequently reported for patients who received CDK 4/6 inhibitors (OR: 7.51; 95% CI 6.01–9.38; p < 0.00001), these included mainly neutropenia, leukopenia and anemia.

Conclusion

The addition of CDK 4/6 inhibitors (either abemaciclib, palbociclib, or ribociclib) to an AI (anastrozole or letrozole) significantly improved PFS, overall response rate, and clinical benefit rate in comparison with a nonsteroidal AI alone.

     

Cyclin-dependent kinase 4/6 inhibitors in hormone receptor-positive early breast cancer: preliminary results and ongoing studies

The cyclin D-cyclin-dependent kinase (CDK) 4/6-inhibitors (CDK4/6i) induce cell cycle arrest in the G1 phase what eventually can prevent the proliferation of cancer cells. The CDK4/6i have changed the landscape of treatment options for ER-positive, HER2-negative metastatic breast cancer. Currently, palbociclib, ribociclib, and abemaciclib are approved by the US Food and Drug Administration in this setting. This success encouraged the researchers to examine CDK4/6i activity in (neo)adjuvant setting. In this review, clinical data to date and ongoing clinical trials with palbociclib, ribociclib, and abemaciclib in the early breast cancer are discussed. A literature search of these topics was carried out using PubMed and data reported at international oncology meetings and clinicaltrials.gov were included. Currently, we have the early promising data from Phase II clinical trials of CDK4/6i efficacy in the neoadjuvant setting in women with HR-positive breast cancer. Moreover, there are numerous studies that are in progress today in (neo)adjuvant setting.     

Real-World Palbociclib Use in HR+/HER2− Advanced Breast Cancer in Canada: The IRIS Study

Background: Palbociclib is a selective cyclin-dependent kinase (CDK) 4/6 inhibitor used in combination with aromatase inhibitors or fulvestrant for patients with hormone receptor-positive (HR+) human epidermal growth factor receptor 2 (HER2)-negative advanced/metastatic breast cancer (ABC/MBC). Palbociclib was the first CDK 4/6 inhibitor approved for HR+/HER2− ABC/MBC treatment in Canada in combination with letrozole (P+L) as an initial endocrine-based therapy (approved March 2016), or with fulvestrant (P+F) following disease progression after prior endocrine therapy (approved May 2017). The Ibrance Real World Insights (IRIS) study ({"type":"clinical-trial","attrs":{"text":"NCT03159195","term_id":"NCT03159195"}}NCT03159195) collected real-world outcomes data for palbociclib-treated patients in several countries, including Canada. Methods: This retrospective chart review included women with HR+/HER2− ABC/MBC receiving P+L or P+F in Canada. Physicians reviewed medical records for up to 14 patients, abstracting demographic and clinical characteristics, treatment patterns, and clinical outcomes. Progression-free rates (PFRs) and survival rates (SRs) at 6, 12, 18, and 24 months were estimated via Kaplan–Meier analysis. Results: Thirty-three physicians examined medical records for 247 patients (P+L, n = 214; P+F, n = 33). Median follow-up was 8.8 months for P+L and 7.0 months for P+F. Most patients were initiated on palbociclib 125 mg/d (P+L, 90.2%; P+F, 84.8%). Doses were reduced in 16.6% of P+L and 14.3% of P+F patients initiating palbociclib at 125 mg/d. The PFR for P+L was 90.3% at 12 months and 78.2% at 18 months; corresponding SRs were 95.6% and 93.0%. For P+F, 6-month PFR was 91.0%; 12-month SR was 100.0%. Conclusions: Dose reduction rates were low and PFR and SR were high in this Canadian real-world assessment of P+L and P+F treatments, suggesting that palbociclib combinations are well tolerated and effective.