‘Surprising’ lack of benefit to adding palbociclib to endocrine therapy in early HR+/HER2– breast cancer

Not the final word?

An interim analysis of the phase 3 monarchE trial did not align with the PALLAS trial.

The monarchE trial found that adding the CDK4/6 inhibitor abemaciclib (Verzenio) to endocrine therapy for 2 years significantly reduced the risk of early recurrence, compared with endocrine therapy alone in the same patient populations – those with early HR-positive, HER2-negative breast cancer. The researchers reported the combination was associated with a 25% relative risk reduction of invasive disease-free survival (HR, 0.75; P =.0096).

The research was presented at the ESMO Virtual Congress 2020 and simultaneously published in the Journal of Clinical Oncology.

Dr. Miller speculated how about how these two drugs that look so similar in the metastatic setting have given such different results in the adjuvant setting. One potential reason is pure chance.

“Any study, no matter how many zeros in the P value, could be simply the play of chance,” she said in an interview. “And that is true for negative and positive studies.”

The fault could also lie in the study design. “Remember, these are agents that we think of as reversing endocrine resistance and extending the benefit of hormone therapy,” she pointed out. “And yet we looked at very early results. Perhaps the study design was just wrong for palbociclib.”

Yet another possibility: The relative potency of those two CDK4/6 inhibitors could differ. “In a metastatic setting, that did not seem to affect effectiveness, but it clearly affected the toxicity profile. Perhaps in the adjuvant setting, those differences really do drive differences in efficacy,” she said.

Dr. Gnant also speculated that differences in the treatment schedules for the two drugs, as abemaciclib is taken continuously without a break, could potentially explain the different efficacies in the early breast cancer setting.

He called for long-term follow up, saying it’s essential for comprehensively examining outcomes in HR-positive luminal breast cancers.

“Ongoing analyses in the Trans-PALLAS translational and clinical science program, with almost 6,000 tumor blocks and tens of thousands of blood samples, will improve understanding of CD4/6 inhibition as well as contemporary management of HR-positive, HER2-negative breast cancer,” Dr. Gnant said.

The trial was funded by Pfizer, who provided study drug and financial support. In addition, the academic organizations ABCSG and AFT supported the trial by providing human resources. Dr. Gnant reported employment at Sandoz; receiving honoraria from Amgen, Novartis, AstraZeneca, Lilly; and consulting or advisory roles at Daiichi Sankyo, Veracyte, Tolmar¸ LifeBrain, and Lilly.

A version of this article first appeared on Medscape.com.