A single-arm, phase ii study assessing the efficacy of pembrolizumab (pembro) plus radiotherapy (RT) in metastatic triple negative breast cancer (mTNBC)

1017Background: Overall response rates of 5-7% have been reported with checkpoint inhibitor monotherapy in PD-L1-unselected mTNBC as second line or subsequent therapy. RT is frequently used to enhance local control in mTNBC and has been reported to induce distant (abscopal) tumor responses when combined with immunotherapy. In this study, we evaluate the safety and efficacy of RT combined with pembro, a programmed death 1 (PD-1) inhibitor, in a phase II, single-arm, Simon two-stage, study in mTNBC. Methods: Eligible women had biopsy-proven mTNBC and ≥2 measurable sites of metastatic disease with at least one site requiring RT. PD-L1 expression was not required for study entry. A total RT dose of 3000 cGy was delivered in 5 daily fractions. Pembro was given intravenously at 200 mg within 3 days of the first RT fraction, then every 3 weeks +/-3 days until disease progression. The primary endpoint was overall response rate at week 13 in the non-irradiated lesions by RECIST v1.1. Secondary endpoints included s...