The Growing Role of CDK4/6 Inhibitors in Treating Hormone Receptor-Positive Advanced Breast Cancer

Single-agent endocrine therapy has been the standard therapeutic choice for the management of hormone receptor (HR)-positive, Her2-negative advanced breast cancer (ABC) for decades. However, the rapidly accumulating data regarding the biological role and safety of CDK4/6 inhibitors and the first-in-class approval of palbociclib have made these novel agents an essential component of treatment for HR-positive ABC. In the frontline setting, palbociclib in combination with endocrine therapy showed an improvement in progression-free survival (PFS) by 10 months to nearly 25 months when compared with endocrine therapy alone and a clinical benefit rate (CBR = stable disease >24 weeks + partial response + complete response) of 85%. Furthermore, clinically meaningful improvements in PFS were seen in combination with fulvestrant for patients with prior endocrine therapy, including premenopausal women. While neutropenia is experienced by most patients, it is typically uncomplicated and palbociclib is otherwise well tolerated. Recent analysis also demonstrated improved quality of life and reassuring evidence of no compromise in benefit from subsequent therapies after progression on palbociclib. Along with palbociclib, the CDK4/6 inhibitors ribociclib and abemaciclib are being evaluated in a variety of settings (metastatic, neoadjuvant, and adjuvant), alone and in combination with endocrine therapy, chemotherapy, and targeted therapies. Future research is needed to address challenges regarding the potential competition of these agents as the preferred partner in endocrine-sensitive disease, their use as single agents or in combination in the endocrine-refractory setting, and the clinical and molecular criteria for use as an alternative to chemotherapy. Unfortunately, despite efforts to determine predictive biomarkers for response, RB1 expression and HR-positive disease have been the only clear predictors of therapeutic benefit. Once more mature data become available, we hope to confirm a significant impact on long-term survival. Meanwhile, given the multiple therapies patients with ABC will receive, prolonged PFS with a well-tolerated oral regimen is a clinically meaningful endpoint. Palbociclib's impact on PFS, high CBR, and tolerability have made its use a preferred option for treating many HR-positive, Her2-negative ABC patients.