Prognostic role of tumor subtype and germline BRCA mutation in advanced breast cancer patients treated with palbociclib plus endocrine therapy.

Purpose: Palbociclib is a cyclin-dependent kinase 4 and 6 inhibitor which shows promising effect in hormone receptor-positive breast cancer. The purpose of this study is to evaluate the real-world efficacy and toxicity of palbociclib plus endocrine therapy.

Methods: This is a retrospective study performed in two tertiary referral hospitals in Korea. Advanced breast cancer patients who were treated with 1st-line palbociclib plus endocrine therapy were enrolled.

Results: A total of 216 patients were included between August 2016 and May 2019. Median age was 56 (29-89) years old and 75 patients (34.7%) were premenopausal. Median progression-free survival (PFS) was 33.0 months [95% confidence interval (CI) 24.7 to 41.3] and objective response rate was 59.3%. Luminal B patients had shorter PFS (33.0 months vs. Not reached, p = 0.019) and tendency of lower ORR (58.3 vs. 62.0%, p = 0.19) compared to luminal A patients. Multivariate analysis revealed luminal B (adjusted hazard ratio 1.90, p = 0.038) and germline BRCA mutation (adjusted hazard ratio 5.57, p = 0.002) as an independent poor prognostic factor for PFS. The most common grade 3 or 4 adverse event was neutropenia (86.7%).

Conclusion: The efficacy and toxicity of palbociclib in the real world were comparable to those of clinical trials. In addition, palbociclib with endocrine therapy was an effective treatment option for young patients. Luminal B and germline BRCA mutation were associated with inferior outcome.