Overall Survival of Ibrance With Letrozole Numerically Longer Over Placebo for ER+, HER2- Metastatic Breast Cancer


With a median follow-up of 90 months, individuals receiving palbociclib in combination with letrozole had numerically longer OS compared to placebo, but it was not deemed statistically significant.

Overall survival (OS) results from the phase 3 PALOMA-2 trial were numerically longer with palbociclib (Ibrance; Pfizer) in combination with letrozole compared to placebo and letrozole for the first-line treatment of postmenopausal women with estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (mBC).

At a median follow-up of 90 months, patients administered the palbociclib combination had numerically longer OS (median [95% CI] 53.9 months (49.8–60.8) versus placebo plus letrozole (median 51.2 months [43.7–58.9]); however, the results were not deemed statistically significant (HR=0.956 [95% CI, 0.777–1.177]).

The PALOMA-2 trial was designed for a primary endpoint of progression-free survival (PFS) with OS as one of the secondary endpoints. The results are being presented today as an oral presentation at the 2022 American Society of Clinical Oncology Annual Meeting.

“[Palbociclib] continues to provide substantial benefit as a first-line treatment for adults with HR+, HER2- mBC based on strong progression-free survival data, which formed the basis of its worldwide approvals,” Chris Boshoff, MD, PhD, chief development officer of Oncology at the Pfizer Global Product Development, said in a statement. “Interpretation of OS in PALOMA-2 is limited by the large and disproportionate censoring of patients with missing survival data between treatment arms. We remain confident in the compelling benefits that [palbociclib] plus endocrine therapy offers to this patient population, which is underscored by data from PALOMA-2 showing delayed time to chemotherapy, maintenance of quality of life and a consistent safety profile.”

PALOMA-2 met its primary endpoint of PFS in 2016, which was published in The New England Journal of Medicine. The results showed that palbociclib plus letrozole resulted in an improved median PFS of 24.8 months compared to placebo and letrozole at 14.5 months.

The trial showed that in addition to substantially delaying disease progression, palbociclib as a first-line treatment with letrozole delayed time to chemotherapy at 38.1 months compared to placebo and letrozole at 29.8 months, while maintaining quality of life with no new identified safety issues. The OS analysis included a large proportion of individuals with missing survival data—such as those who withdrew consent or were lost to follow-up—and those who were assumed to be alive at the time of analysis.

Further, 10% of individuals taking palbociclib and letrozole and 2% of those on the placebo and letrozole were still on study treatment at the time of the final analysis.

The most common adverse events in the PALOMA-2 study included neutropenia, leukopenia, infection, fatigue, and nausea.

“[Palbociclib] transformed the treatment landscape for patients with HR+, HER2- mBC when it was approved in 2015, representing the first new treatment in this patient population in over a decade,” Richard Finn, MD, professor of Medicine at the University of California Los Angeles David Geffen School of Medicine and Jonsson Comprehensive Cancer Center, said in the statement. “PALOMA-2 enrolled a diverse patient population including patients whose disease was first diagnosed in the metastatic stage as well as those with Disease Free Interval less than 12 months from adjuvant treatment and those with greater than 12 months following adjuvant treatment. The median survival of over 50 months in this population represents a significant improvement in the natural history of HR+ breast cancer.”


Pfizer announces overall survival results from phase 3 PALOMA-2 trial of Ibrance (palbociclib) for the first-line treatment of ER+, HER2- metastatic breast cancer. Pfizer. News release. June 4, 2022. Accessed June 4, 2022. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-announces-overall-survival-results-phase-3-paloma-2

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