A Phase II Study of Abemaciclib for Patients with Retinoblastoma-Positive, Metastatic Triple-Negative Breast Cancer.

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.