Findings in context
PENELOPE-B is the second phase 3 trial to show no benefit for palbociclib in the treatment of early high-risk breast cancer. The first was the PALLAS trial, which was reported only a few months ago at the European Society for Medical Oncology Virtual Congress 2020.
In PALLAS, palbociclib plus endocrine therapy was compared with endocrine therapy alone in the treatment of women with HR+, HER2-negative early breast cancer, but there was no additional benefit seen.
The results of both PENELOPE-B and PALLAS contrast those recently seen with another CDK4/6 inhibitor, abemaciclib, in HR+/HER2-negative early breast cancer.
Results of the phase 3 monarchE trial showed that, when abemaciclib was added to endocrine treatment, there was a significant (P = .0096) 25% relative risk reduction (HR, 0.75; 95% confidence interval, 0.60-0.93) in iDFS, compared with endocrine therapy alone. These results were also presented at the ESMO Virtual Congress 2020 and published in the Journal of Clinical Oncology.
While it’s not clear why one CDK4/6 inhibitor may be of benefit in HR+/HER2-negative early breast cancer while the other is not, “PENELOPE-B is clearly quite a different trial to the other two adjuvant CDK inhibitor trials,” Dr. O’Regan observed.
For one thing, PENELOPE-B participants were only eligible for the trial if they did not have a pathological complete response after NACT. In addition, PENELOPE-B is a smaller trial, enrolling well under a third of the number of patients who participated in the PALLAS and monarchE trials. Furthermore, the CPS-EG score, rather than anatomic staging, was used to determine eligibility.
“Abemaciclib could be a more effective CDK inhibitor; that’s most certainly a possibility,” Dr. O’Regan suggested. “However, it’s not supported by the metastatic first-line trials, which have remarkably similar hazard ratios and favor CDK inhibitors.”
Perhaps the shorter duration of palbociclib treatment in the PENELOPE-B trial (12 months vs. 24 months) had an effect.
“Clearly, we need to await the results of NATALEE that uses 3 years of ribociclib,” Dr. O’Regan said. It also remains to be seen if the results of the monarchE trial hold up with longer follow-up.
The PENELOPE-B trial was sponsored by the German Breast Group in collaboration with Pfizer, the AGO Study Group, NSABP Foundation, and the Breast International Group. Dr. Loibl disclosed grant and other support from Pfizer during the conduct of the study and relationships with other companies outside the submitted work. She also disclosed intellectual property rights and being a pending patent holder (EP14153692.0) for a method to predict response to anti-HER2–containing therapy and/or chemotherapy in patients with breast cancer, and she disclosed a relationship with Medscape, which is owned by the same company as MDedge. Dr. O’Regan disclosed relationships with Novartis, Eli Lilly, Genentech, PUMA, Macrogenics, Immunomedics, Biotheranostics, and Eisai.
SOURCE: Loibl S et al. SABCS 2020, Abstract GS1-02.