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

Abstract

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.

Conflict of interest statement

Adam Brufsky has received consulting fees from Pfizer Inc, AstraZeneca, Sanofi, Eli Lilly, and Novartis. Xianchen Liu, Benjamin Li, and Lynn McRoy are employees of and own stock in Pfizer Inc. Rachel M. Layman’s institution has received research funding from Pfizer Inc, Novartis, and Eli Lilly; and Dr. Layman has participated in advisory boards for Pfizer Inc, Eli Lilly, and Novartis.

© 2021. The Author(s).

Figures

Fig. 1

Real-world best tumor response. ECOG Eastern Cooperative Oncology Group, LET letrozole, OR odds ratio, PAL palbociclib, PSM propensity-score matching. Indeterminate response includes situations in which the clinician explicitly states that he/she is not able to make a determination of the assessment

Fig. 2

Real-world best overall tumor response rates by subgroup. ECOG Eastern Cooperative Oncology Group, LET letrozole, PAL palbociclib, rwBTR real-world best tumor response aVisceral disease was defined as metastatic disease in the lung and/or liver; patients could have had other sites of metastases. No visceral disease was defined as no lung or liver metastases. bBone-only disease was defined as metastatic disease in only the bone. cMultiple metastases at the same site were counted as one site (e.g., if a patient had three bone metastases in the spine, it was considered only one site)

Fig. 3

Real-world progression-free survival among patients with one or more tumor response assessments. LET letrozole, PAL palbociclib, PSM propensity-score matching, rwPFS real-world progression-free survival

Fig. 4

Overall survival among patients with one or more tumor response assessments. LET letrozole, NE not estimable, NR not reached, OS overall survival, PAL palbociclib, PSM propensity-score matching