The PENELOPE-B phase III trial did not show a benefit to the addition of one year of the CDK4/6 inhibitor palbociclib to adjuvant endocrine therapy in patients with hormone receptor-positive/HER2-negative breast cancer and residual disease after neoadjuvant chemotherapy.

TRANSCRIPT

This JCO podcast provides observations and commentary on the JCO article “Palbociclib for Residual High-Risk Invasive Hormone Receptor-positive, HER2 negative Early Breast Cancer – The PENELOPE-B Trial,” by Sybille Loibl and colleagues. My name is Erica Mayer, and I am a clinical investigator at the Dana-Farber Cancer Institute in Boston, MA who specializes in breast cancer.

Since the initial approval of the CDK4/6 inhibitor palbociclib in 2015, this class of agents has become a standard part of management for most patients with hormone receptor positive HER2 negative advanced breast cancer. Use of one of the available CDK4/6 inhibitors - palbociclib, abemaciclib, or ribociclib - in combination with endocrine therapy, improves progression-free, and even overall survival, in the advanced disease setting, as shown in the respective trial series PALOMA, MONARCH, or MONALEESA. Although differentiated by their toxicity profiles, with palbociclib and ribociclib contributing more neutropenia and abemaciclib more diarrhea, across-the-board the benefit gained from use of these agents is remarkably preserved between trials, without apparent differential drug activity.

Following the observation of benefit from use of CDK 4/6 inhibitors in the advanced setting, clinical trials evaluating the three available agents were subsequently launched to evaluate these agents in the adjuvant setting, after prior treatment with surgery, chemotherapy, and radiation if necessary. The first two trials to report in 2020, PALLAS and MONARCH-E, enrolled patients with early hormone receptor positive HER2- breast cancer, randomizing them to receive a CDK 4/6 inhibitor, palbociclib or abemaciclib respectively, for a planned 2-year duration, in addition to ongoing adjuvant endocrine therapy, versus ongoing adjuvant endocrine therapy alone. Notably, PALLAS enrolled patients with stage II and III breast cancer, whereas MONARCH-E targeted a high-risk group identified by tumor size, nodes, and Ki67 status. Initial reports from these trials were presented at an interim follow-up of about 20-24 months, a time when many were still receiving the combination therapy. The MONARCH-E analysis demonstrated a benefit from the addition of abemaciclib to endocrine therapy in prolonging invasive disease-free survival. This benefit was not observed with the addition of palbociclib in the PALLAS trial. The ongoing NATALEE trial, offering 3-years of reduced dose ribociclib in a similar setting, continues accrual.

It is within this context that we place the PENELOPE-B trial. This trial enrolled the highest risk patients with early HR+/HER2- breast cancer: those with residual disease after preoperative chemotherapy and a high CPS-EG score, which is a calculation of anatomic stage and tumor biology meant to identify patients at highest risk of cancer recurrence. A total of 1250 patients were randomized to receive either one year of adjuvant palbociclib with ongoing adjuvant endocrine therapy, or placebo with endocrine therapy, and followed for a primary endpoint of invasive disease-free survival. As expected, the study enrolled a very high-risk population, with many patients having at least T2 disease and 4 involved lymph nodes at time of surgery. At a median follow-up of 43 months, no difference in invasive disease-free survival was observed, with a 4-year iDFS of 73% in the intervention arm, versus 72.4% in the placebo arm. No clinicopathologic subgroup appeared to derive benefit from the research intervention. The toxicity experience was similar to that seen in the metastatic setting, with notable and expected hematologic toxicity, but no significant difference in overall non-hematologic adverse events. Overall, drug exposure was satisfactory, with about 17% of patients unable to complete the planned 1-year duration of therapy.

The PENELOPE-B study makes its mark in the adjuvant CDK4/6 landscape in several important ways. First, PENELOPE-B has the greatest duration of follow-up, allowing analysis of mature data and outcomes during and following adjuvant CDK4/6 inhibition. Although it is tempting to interpret a slight visual divergence of the Kaplan Meyer curves during the palbociclib exposure, where there is an absolute difference in iDFS of 4.3% at 2 years, there is no statistical evidence that the 2-year iDFS estimates are different between the two arms, and therefore no evidence of time-dependent efficacy. Importantly, in regard to experiences within the other CDK4/6 inhibitor trials, which have reported at an earlier time point, longer-term follow-up of those trials will be necessary to determine stability and maturity of the observed results. Additionally, PENELOPE-B is the only placebo-controlled trial presented to date. For that reason, the toxicity data presented represents the purest description of the experience of receiving a CDK4/6 inhibitor in the adjuvant setting in a population with prior chemotherapy exposure. Finally, when discriminating among the CDK4/6 inhibitors study populations, it is important to remember that PENELOPE-B enrolled a population of greatest established risk, suggesting any apparent differential outcome among the trials is unlikely related to differing baseline risk of accrued patients.

Overall, PENELOPE-B represents a significant contribution to the breast cancer field, and substantially adds depth and dimension to the exploration of CDK4/6 inhibitors in the adjuvant setting. At the time of publication, it is unclear if an approved role will emerge for a CDK4/6 inhibitor in adjuvant therapy for our early breast cancer patients. Many questions remain regarding patient/tumor selection, agent differentiation, duration of therapy, and potential biomarkers. Importantly, the correlative analyses planned for all of the CDK4/6 inhibitor trials and molecular exploration of patient samples will substantially improve our understanding of the impact of these drugs in the early setting, and our overall ability to treat this subset of breast cancer. While waiting for these data, we should continue our focus on providing the best possible care for our patients with hormone receptor-positive, HER2-negative breast cancer who are being treated with endocrine therapy.

This concludes this JCO podcast. Thank you for listening.

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PENELOPE-B: Exploration of CDK4/6 Inhibition in the Adjuvant Setting