A phase II trial of the CDK4/6 inhibitor palbociclib (P) as single agent or in combination with the same endocrine therapy (ET) received prior to disease progression, in patients (pts) with hormone receptor positive (HR+) HER2 negative (HER2−) metastatic breast cancer (mBC) (TREnd trial).

Abstract

1002 Background: P is approved for treatment of HR+/HER2− mBC combined with ET. There is paucity of clinical data of single-agent P in ET resistant pts. Pre-clinical data suggest P may partially reverse endocrine resistance, though this is yet to be tested in pts. Methods: This Phase II, open-label, multicenter study enrolled post-menopausal pts with HR+ HER2− mBC who progressed on 1 or 2 prior ETs. Pts were randomized to P (125 mg/d 3 w on/1 w off) alone or to continue their current ET (aromatase inhibitor or fulvestrant) in combination with P (same schedule as P arm). The primary endpoint was clinical benefit rate (CBR) [complete response (CR), partial response (PR) and stable disease (SD) for > 6 months (mo)]. Secondary endpoints were adverse events (AE) and additional measures of efficacy. A two-stage optimal design assessed treatment activity in each arm assuming activity as CB≥40% (α and β = 10%). Exploratory comparisons were planned for safety and efficacy endpoints. Results: 115 pts were enrolled (ITT population) 58 in the P arm and 57 in the P+ET arm. In both arms, 67% of pts had the study treatment as second line ET, 33% as third line, and about 1/3 of pts also received 1 prior chemotherapy for mBC. CBR was similar in both arms: 54% (95% CI 42 - 67%) with P+ET, and 60% (95% CI 48 -73%) with P alone. Median duration of CB was longer with P+ET (11.5 mo; 95% CI 8.6 – 17.8) than with P (6 mo; 95% CI 3.9 - 9.9) (HR 0.31, 95% CI 0.1 - 0.7, p-value 0.001, exploratory). Objective response rate (ORR; CR, PR) was 11% (95% CI 3 - 19%) and 7% (95% CI 0.4 -13%) with P+ET and P, respectively. PFS was 10.8 mo (95% CI 5.6 - 12.7) with P+ET and 6.5 mo (95% CI 5.4 - 8.5) with P alone (HR 0.69, 95% CI 0.4 - 1.1, p-value 0.13, exploratory). AEs were in line with previous data. Conclusions: Single agent P has clinical activity in ET pre-treated HR+/HER2– mBC pts. The observed increase in PFS and duration of CB with P+ET may suggest that P could reverse resistance to the prior line of ET. Translational studies are ongoing to explore potential biomarkers in this setting. Clinical trial information: NCT02549430.