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

2021 Year in Review - HER2-Negative Breast Cancer —January 21, 2022


Breast Cancer

The objective of this study was to compare the real-world tumor response of palbociclib plus the aromatase inhibitor letrozole versus letrozole alone in patients with hormone receptor (HR)-positive, HER2-negative metastatic breast cancer in the United States.1

Using the Flatiron Health Analytics database, procedures and patients with HR-positive, HER2-negative metastatic breast cancer who received palbociclib plus letrozole or letrozole monotherapy as first-line treatment between February 2015 and September 2018 were included in this retrospective research. Patients were followed up through December 2018. The best tumor response in the real world was calculated using physicians’ assessments of radiologic evidence for changes in disease burden.1

Approximately 70% of the 1383 eligible patients who received the palbociclib plus letrozole combination (N = 662) or letrozole monotherapy (N = 306) as first-line therapy had ≥1 tumor response evaluations. In the first-line scenario, the real-world best tumor response rate (full response plus partial response) was 59.8% in the palbociclib plus letrozole group and 39.2% in the letrozole monotherapy group (P <.0001). The real-world best tumor response rate in the palbociclib plus letrozole group was 58.6% versus 39.1% in the letrozole group after 1:1 propensity score matching.1

In this large cohort of general oncology practice patients in the United States, patients with HR-positive, HER2-negative metastatic breast cancer treated with first-line palbociclib plus letrozole had a considerably higher likelihood of tumor response than those treated with letrozole alone. Furthermore, when compared with letrozole alone, treatment with first-line palbociclib plus letrozole resulted in significant improvements in real-world progression-free survival and overall survival.1

These findings support palbociclib plus letrozole as a standard-of-care treatment for patients with HR-positive, HER2-negative metastatic breast cancer, and they add to the therapeutic benefit shown with palbociclib plus endocrine therapy in randomized clinical studies.1


  1. Brufsky A, Liu X, Li B, et al. Real-world tumor response of palbociclib plus letrozole versus letrozole for metastatic breast cancer in US clinical practice. Target Oncol. 2021;16:601-611. Erratum in: Target Oncol. 2021;16:865.
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Last modified: January 21, 2022

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