Bojana Jovanović

Representativeness of Study Participants

Purpose:Cyclin-dependent kinase 4/6 inhibitors can significantly extend survival when given in combination with endocrine therapy in patients with hormone receptor–positive metastatic breast cancer. However, their activity has been relatively underexplored in patients with metastatic triple-negative breast cancer (mTNBC).Patients and Methods:We conducted a single-arm phase II study of abemaciclib monotherapy in patients with Rb-positive mTNBC. Patients were treated with abemaciclib 200 mg orally twice daily until disease progression, unacceptable toxicity, or withdrawal of consent. The primary endpoint was the objective response rate; secondary endpoints included progression-free survival (PFS), overall survival (OS), clinical benefit rate, disease control rate, and safety and tolerability.Results:A total of 27 patients were enrolled before the trial was closed early because of slow accrual. Patients had received a median of two lines of systemic therapy in the metastatic setting prior to enrollment. After a median follow-up of 28.5 months, the objective response rate was 0%, the clinical benefit rate was 14.8%, and the disease control rate was 22.2%. The median PFS was 1.94 months (95% confidence interval, 1.84–11.47), and the median OS was 8.44 months (95% confidence interval, 4.57–15.57). Median PFS and OS did not differ significantly based on androgen receptor and PD-L1 status. Pretreatment gene expression profiling of tumor tissue provided some hypothesis-generating insights into biological features associated with clinical benefit in this study. The most common treatment-related adverse events of grade 2 or higher were diarrhea (40.7%), neutropenia (40.7%), anemia (29.6%), and nausea (29.6%).Conclusions:Abemaciclib monotherapy did not show clinical activity in patients with pretreated Rb-positive mTNBC.