Real-World Tumor Response of Palbociclib Plus Letrozole Versus Letrozole for Metastatic Breast Cancer in US Clinical Practice

Background: Limited information exists regarding tumor response to palbociclib plus an aromatase inhibitor (AI) versus AI alone in real-world practice. Objective: To evaluate the real-world tumor response of palbociclib plus letrozole (PAL+LET) versus LET alone as first-line treatment for patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer (HR+/HER2‒ MBC) in routine US clinical practice. Patients and Methods: This retrospective analysis included HR+/HER2‒ MBC patients who initiated PAL+LET or LET as first-line treatment between February 2015 and September 2018 in the Flatiron Health Analytics database. Patients were followed until December 2018. Real-world best tumor response (rwBTR) was determined based on physicians’ assessment of radiologic evidence for change in burden of disease. Results: Of the 1383 eligible patients who initiated PAL+LET or LET as first-line therapy in the Flatiron database, 968 patients had ≥ 1 tumor response assessment (662 received PAL+LET and 306 received LET). The rwBTR rate (complete response+partial response) in the first-line setting was 59.8% in the PAL+LET group and 39.2% in the LET group (odds ratio 2.31 (95% CI 1.75‒3.04), P < 0.0001). After 1:1 propensity-score matching, the rwBTR rate was 58.6% in the PAL+LET group versus 39.1% in the LET group (odds ratio 2.21 (95% CI 1.50‒3.25), P < 0.0001). Conclusions: This real-world analysis demonstrated that HR+/HER2‒ MBC patients were more likely to respond to PAL+LET compared to LET. These findings further showed the effectiveness of PAL+LET therapy in the real-world setting and support the combination as a standard of care for MBC. Study Registration: Pfizer; NCT04176354; registered November 25, 2019.