Real-world comparative effectiveness of palbociclib plus letrozole versus letrozole in older patients with metastatic breast cancer

Background: Palbociclib, the first available cyclin-dependent kinase 4/6 inhibitor, plus endocrine therapy is approved for hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2−) metastatic breast cancer (MBC). This study compared real-world effectiveness of palbociclib plus letrozole versus letrozole in older patients with MBC in US clinical practice. Methods: This retrospective analysis included patients from the Flatiron Health longitudinal database. Overall, 796 women with HR+/HER2− MBC aged ≥65 years starting palbociclib plus letrozole or letrozole as first-line therapy between February 2015 and September 2018 were included. Patients were evaluated from treatment start until December 2018, death, or last visit, whichever came first. Real-world progression-free survival (rwPFS), overall survival (OS), and real-world best tumor responses (rwBTR) were endpoints. Stabilized inverse probability treatment weighting (sIPTW) balanced patient characteristics. Results: After sIPTW, 450 patients treated with palbociclib plus letrozole and 335 treated with letrozole were included; median age was 74.0 years. Median rwPFS was 22.2 (95% CI, 20.0–30.4) months for palbociclib plus letrozole versus 15.8 (12.9–18.9) months for letrozole (hazard ratio, 0.59 [0.47–0.74]; P<0.001). Median OS was not reached for palbociclib plus letrozole versus 43.4 months (30.0–not estimable) with letrozole (hazard ratio, 0.55 [0.42–0.72]; P<0.001). No interactions between age groups (65–74 and ≥75 years) and treatment groups were observed for rwPFS or OS. Rate of rwBTR was significantly higher for palbociclib plus letrozole (52.4%) versus letrozole (22.1%; odds ratio, 2.0 [1.4–2.7]; P<0.001). Conclusion: This analysis demonstrates the effectiveness of palbociclib combination therapy as standard-of-care for older patients with HR+/HER2− MBC in the first-line setting.