The phase 2 randomised Young-PEARL study demonstrated that palbociclib plus exemestane with ovarian function suppression significantly prolonged progression-free survival compared with capecitabine in premenopausal women with hormone receptor-positive, HER2-negative metastatic breast cancer. Here, we report results of the protocol-specified secondary endpoint of overall survival.

Young-PEARL was a multicentre, randomised, open-label, phase 2 study conducted at 14 institutions in South Korea. Premenopausal women aged 19 years or older with histologically confirmed hormone receptor-positive, HER2-negative metastatic breast cancer that recurred or progressed during or after previous tamoxifen treatment, who were aromatase inhibitor naive, and had an Eastern Cooperative Oncology Group performance status of 0–2 were eligible. One previous line of chemotherapy was permitted in the metastatic setting. Eligible patients were randomly assigned (1:1), using block randomisation (block size of two) stratified by previous chemotherapy for metastatic breast cancer and presence of visceral metastasis, to receive either palbociclib (orally, 125 mg per day on a 3-weeks-on, 1-week off schedule) plus exemestane (orally 25 mg daily) with leuprorelin (subcutaneously 3·75 mg on day 1 of each 28-day cycle) or capecitabine (orally, 1250 mg/m² twice a day on a 2-weeks-on, 1-week-off schedule) until disease progression or unacceptable toxicity). The primary endpoint was progression-free survival. Overall survival was a secondary endpoint. All analyses were done in the modified intention-to-treat population (ie, included all patients randomly assigned to treatment who had at least one post-baseline CT scan and excluded those who did not receive study medication and who had any major violation of the eligible criteria). Safety was assessed in all patients who received any study treatment. This study is registered with ClinicalTrials.gov, NCT02592746, and is now complete.

Between June 15, 2016, and Dec 10, 2018, 189 patients were enrolled. 184 patients were randomly assigned to the palbociclib plus endocrine therapy group (n=92) or the capecitabine group (n=92), of whom 174 were included in the modified intention-to-treat population (n=90 in the palbociclib plus endocrine therapy group and n=84 in the capecitabine group). All patients were female and ethnicity data were not collected. As of data cutoff (Feb 29, 2024), median follow-up was 54·0 months (IQR 34·1–74·4). Median progression-free survival was 19·5 months (90% CI 14·3–22·2) for palbociclib plus endocrine therapy and 14·0 months (11·7–18·7) for capecitabine (hazard ratio 0·74 [90% CI 0·57–0·98]; one-sided log-rank p=0·036). 52 (58%) of 90 patients in the palbociclib plus endocrine therapy group and 48 (57%) of 84 in the capecitabine group died, with a median overall survival of 54·8 months (95% CI 48·9–77·1) in the palbociclib plus endocrine therapy group versus 57·8 months (46·3–89·2) in the capecitabine group (hazard ratio 1·02 [95% CI 0·69–1·51]; p=0·92). The most common grade 3 or worse adverse event was neutropenia (59 [64%] of 92 in the palbociclib plus endocrine therapy group vs 15 [18%] of 85 in the capecitabine group) . No treatment-related deaths occurred.

With extended follow-up, palbociclib plus exemestane with ovarian function suppression continued to show a significant benefit in progression-free survival compared with capecitabine in premenopausal patients with hormone receptor-positive, HER2-negative metastatic breast cancer who had been previously treated with tamoxifen; however, no improvement in overall survival was seen. Given the progression-free survival benefit, the upfront use of palbociclib plus endocrine therapy is the preferred option for premenopausal women, although a capecitabine-first strategy might be an alternative treatment strategy for maintaining overall survival in resource-limited settings.

Pfizer and Ministry of Health & Welfare, South Korea.