“The combination of palbociclib plus exemestane plus leuprolide showed a consistent significant improvement in PFS [progression-free survival] compared to the capecitabine arm,” Yeon Hee Park, MD, PhD, from Samsung Medical Center, Sungkyunkwan University, Seoul, South Korea, reported at the annual meeting of the American Society of Clinical Oncology.
Study Methods and Results
Young-PEARL, a prospective, multicenter, open-label, randomized phase 2 study, included 184 patients, median age 44 years, who had relapsed or progressed during previous tamoxifen therapy, with one line of previous chemotherapy for mBC allowed. Patients were randomized to palbociclib plus endocrine therapy (oral palbociclib 125 mg per day for 21 days every 4 weeks, oral exemestane 25 mg per day for 28 days, plus leuprolide 3.75 mg subcutaneously every 4 weeks) or chemotherapy (oral capecitabine 1250 mg/m2, twice daily for 2 weeks every 3 weeks).
Previously published initial results (Lancet Oncol. 2019 Dec;20[12]:1750-1759) for the primary endpoint showed a median PFS of 20.1 months in the palbociclib group versus 14.4 months in the capecitabine group, (hazard ratio [HR] 0.659, P = .0235) after median follow-up of 17 months.
Updated results showed this benefit was maintained after a median of 54 months, with a PFS of 19.5 months in the palbociclib arm, versus 14 months in capecitabine arm (HR 0.744, P = .0357), Dr. Park reported. However, this PFS benefit did not lead to an overall survival (OS) benefit, with median OS being similar: 54.8 versus 57.8 months in the palbociclib and capecitabine groups, respectively (HR = 1.02, P = .92).
To explore why PFS — but not OS — was better in the palbociclib arm, the researchers conducted a multivariate analysis which showed that going on to an additional CDK4/6 inhibitor treatment after the end of the study was as an independent variable favoring OS. Because more patients in the capecitabine arm received a post-study CDK4/6 inhibitor (49.3%) compared with in the palbociclib group (15%), this weighted the OS to the capecitabine arm, Dr. Park explained in an interview.
“In the capecitabine arm, excluding post-study CDK4/6 inhibitor use, the median OS was 38.8 months.” This was inferior to the 49 months OS seen in the palbociclib arm (P = .065), she said.
“As expected, hematologic toxicity was more common in the palbociclib arm compared with in the capecitabine arm,” Dr. Park said (92% vs 86%), with neutropenia topping the list [of all adverse events] (65.2% vs 27.9%, all grades). However, “most [adverse events] were not that serious,” Dr. Park said. Arthralgia was more common in the palbociclib arm (25% vs 7%), and diarrhea and hand-foot syndrome were more common in the capecitabine arm (15.2% vs 39.5% and 79.1% vs 2.2%).
Study Validates Endocrine Therapy + CDK4/6 Inhibitor as First Line
Commenting on Young-PEARL in an interview, Harold Burstein, MD, PhD, said, “The point of this study was to compare whether upfront chemotherapy would be better than upfront hormonal therapy for patients who had metastatic ER positive breast cancer.”
“This is the first study in probably 20 years that has compared these two approaches, and it validated that for the vast majority of patients with ER positive metastatic breast cancer, the appropriate first treatment is endocrine therapy with a CDK4/6 inhibitor,” continued Dr. Burstein, a breast cancer expert at Dana-Farber Cancer Institute, and professor at Harvard Medical School in Boston.
Dr. Park disclosed honoraria from AstraZeneca, Daiichi Sankyo, Eisai, Lilly, MSD, Novartis, Pfizer, and Roche; consulting or advisory roles for AstraZeneca, Boryung, Daiichi Sankyo, Eisai, Gilead Sciences, Lilly, Menarini, MSD, Novartis, Pfizer, and Roche; research funding from AstraZeneca, Gencurix, Genome Insight, NGeneBio, Pfizer; and Roche; and travel/accommodations/expenses from Gilead. Dr. Burstein disclosed a research grant from the National Cancer Institute.