Palbociclib Does Not Improve OS in ER+, HER2- Advanced Breast Cancer

Adding palbociclib to letrozole did not improve overall survival (OS) in postmenopausal patients with estrogen receptor (ER)-positive, HER2-negative advanced breast cancer in the phase 3 PALOMA-2 trial.¹

At a median follow-up of 7.5 years, there was no improvement in OS with palbociclib, even when the researchers recovered data from patients who had been lost to follow-up or withdrawn from the trial. These results were published in the Journal of Clinical Oncology.

The PALOMA-2 trial (ClinicalTrials.gov identifier: NCT01740427) included 666 postmenopausal patients with ER-positive, HER2-negative advanced breast cancer. The patients were randomly assigned to receive palbociclib plus letrozole (n=444) or placebo plus letrozole (n=222). Baseline characteristics were well balanced between the arms.

Prior results from this trial showed that palbociclib improved progression-free survival, meeting the study’s primary endpoint.²

In the current analysis, the median follow-up was 90.1 months.¹ The median OS was 53.9 months in the palbociclib-letrozole arm and 51.2 months in the placebo-letrozole arm (hazard ratio [HR], 0.96; 95% CI, 0.78-1.18; P =.34).

The researchers also conducted an analysis with recovered data because, at the data cutoff, 15.9% of the intent-to-treat population had been lost to follow-up or withdrew consent.

With the recovered data, the median OS was 53.8 months in the palbociclib-letrozole arm and 49.8 months in the placebo-letrozole arm (HR, 0.92; 95% CI, 0.76-1.12; P =.21).

The researchers speculated that several factors could have affected the OS results. “The long follow-up duration of the study, potential cross-over affecting the longer survival for the control arm with multiple subsequent treatments post-study, and an imbalance in the CDK4/6 inhibitor(s) use post-study could represent potential confounders to OS results,” the researchers wrote.

In the palbociclib arm, 322 patients received at least 1 post-study systemic therapy, including endocrine therapy (n=262), chemotherapy (n=221), and a CDK4/6 inhibitor (n=47). In the placebo arm, 190 patients received at least 1 post-study systemic therapy, including endocrine therapy (n=161), chemotherapy (n=134), and a CDK4/6 inhibitor (n=58).

Disclosures: This study was supported by Pfizer Inc. Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.

This article originally appeared on Cancer Therapy Advisor