Palbociclib‐letrozole as first‐line treatment for advanced breast cancer: Updated results from a Japanese phase 2 study

Palbociclib is a highly selective, reversible, oral inhibitor of cyclin‐dependent kinases 4 and 6 that is approved to treat hormone receptor‐positive/human epidermal growth factor receptor 2‐negative advanced breast cancer. An open‐label, single‐arm, Japanese phase 2 study was conducted to investigate the efficacy and safety of palbociclib plus letrozole as first‐line treatment in 42 postmenopausal patients with estrogen receptor‐positive/human epidermal growth factor receptor 2‐negative advanced breast cancer. The probability of progression‐free survival originally reported at 1 year was 75.0% (90% confidence interval, 61.3‐84.4), but median progression‐free survival was not attained at the primary analysis. In this report, updated efficacy and safety results with a longer follow‐up period are presented. The median duration of treatment in the updated analysis was 33.0 months (range, 1.8‐49.2). The probability of progression‐free survival at 1 year was 75.6% (90% confidence interval, 62.4‐84.7). Median progression‐free survival was 35.7 months (95% confidence interval, 21.7‐46.7). Objective response rate and disease control rate were 47.6% (95% confidence interval, 32.0‐63.6) and 85.7% (95% confidence interval, 71.5‐94.6), respectively. Common treatment‐related adverse events (all grades; grade 3/4) were neutropenia (100%; 93%), leukopenia (83%; 60%), and stomatitis (76%; 0%). Treatment‐related febrile neutropenia was reported in one patient. In general, no clinically meaningful deterioration in health‐related quality of life was observed. Palbociclib plus letrozole remained effective and tolerable in Japanese postmenopausal patients with estrogen receptor‐positive, human epidermal growth factor receptor 2‐negative advanced breast cancer in this updated analysis.