Nationwide real-world practice pattern and clinical data of palbociclib in HR (+), HER2 (−) metastatic breast cancer patients in Korea (KCSG BR21-15)

Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors have remarkably improved the survival outcome in hormone-receptor-positive (HR+)/human epidermal growth factor-2-negative (HER2-) metastatic breast cancer (mBC). Although PALOMA-2 has met its primary outcome, overall survival (OS) was relatively shorter compared to ribociclib and abemaciclib. In Korea, use of palbociclib + aromatase inhibitor (AI) + gonadotropin-releasing hormone agonist (GnRHa) in premenopausal women is limited, and bilateral salpingo-oophorectomy (BSO) is necessary before treatment. We analyzed the real-world clinical outcome and patient characteristics of letrozole + palbociclib in Korea. Methods: Between August 2016 and December 2022, 1017 HR+/HER2-postmenopausal women treated with first-line letrozole + palbociclib were enrolled. Primary endpoints were real-world progression-free survival (rwPFS) in total population and survival differences according to menopausal status (natural or induced menopause via BSO). Results: Patients’ median age was 56 (range 27–92) years. Median rwPFS, real-world OS (rwOS) were 28.0 months (95 % confidence interval [CI] 25.5–32.1) and 61.8 months (95 % CI 57.7–70.5), with a median follow-up of 45.1 (IQR, 31.0–56.6) months. BSO group demonstrated similar median rwPFS compared to natural menopause group. Adjuvant tamoxifen ± GnRHa was most frequently prescribed (73.3 %). Primary endocrine resistant mBC patients showed inferior median rwPFS compared to secondary resistant mBC (14.6 vs. 27.1 months, p = 0.0063). Overall response rate was 47.5 %, with a disease control rate of 89.6 %. Conclusion: This is the largest country-based real-world study on palbociclib + letrozole in Asia. Palbociclib demonstrated median rwOS over 60 months, comparable to other pivotal trials.