Prospective Cohort Study of Palbociclib Treatment in Postmenopausal Patients With Unresectable and Metastatic Hormone Receptor-Positive Breast Cancer: Study Protocol for a CSPOR-BC Palbociclib Cohort Trial.

Background: Combining palbociclib with letrozole or fulvestrant improved progression-free survival in hormone receptor (HR)-positive and human epidermal growth factor receptor-negative metastatic breast cancer. However, combining palbociclib to endocrine treatment increases toxicity and cost compared with the endocrine treatment alone. Moreover, palbociclib treatment may affect the outcome of the subsequent treatment because its benefit in terms of overall survival has not been observed yet. Therefore, it is crucial to examine whether palbociclib can improve clinical outcomes and quality of life (QoL) of patients in the real world. Methods: A prospective observational study with palbociclib is planned in 3 cohorts (A, B, and C) as per the line of endocrine treatment (i.e., first-line, second-line, or third-line or later-line treatment) for postmenopausal metastatic or unresectable breast cancer. The primary endpoint is progression-free survival in each treatment line, the most commonly used endpoint for global phase 3 studies. As per the results of these studies, the planned sample size is 700 cases: cohort A, 340 cases; cohort B, 200 cases; and cohort C, 130 cases. The secondary endpoints are overall survival, clinical benefit rate, time to chemotherapy, adverse events (AEs), patient-reported outcomes (PROs), and health-related QoL (HRQoL). These endpoints are evaluated again during the subsequent treatment. This study will examine whether the efficacy, safety, and QoL effects of palbociclib treatment in daily clinical practice are not inferior compared to those in clinical trials and whether palbociclib treatment affects the efficacy and safety of the subsequent treatment. Moreover, this study would provide information on the most effective time of adding palbociclib to endocrine treatment. Discussion: The reproducibility of randomized clinical trials (RCTs) using real-world data must be confirmed to evaluate whether real-world treatment benefits are similar to those observed in RCTs. Although the efficacy of palbociclib has been confirmed in RCTs, Aes of this drug, including its toxicities and cost, are not comparable to those of mono-hormone therapies. Thus, PROs/HRQoL is an important element of this study because several patients with HR-positive metastatic breast cancer have diseases for which sequential hormone therapy is preferential.