A Study to Evaluate TAlazoparib, Radiotherapy and Atezolizumab in gBRCA 1/2 Negative Patients With PD-L1+ Metastatic Triple Negative Breast Cancer

This is a Phase II study designed to assess efficacy and safety of talazoparib, high dose radiation, and atezolizumab in patients with metastatic TNBC that is PD-L1 positive. A total of 23 gBRCA pathogenic variant negative patients will be enrolled. All patients will be treated with induction talazoparib of 1mg PO daily starting Day 1. Patients will then receive 8 Gy x 3 fractions to 2-4 metastatic lesions beginning Day 12,13, or 14 and given QOD. 840 mg of atezolizumab will be given intravenously (IV) on Day 15 of the 1st cycle and then on Day 1 and Day 15 of the remaining cycles. The sequence of administration is not specified on the days in which talazoparib and atezolizumab are given on the same day. Each cycle equals 28 days. Treatment will continue until progression or severe toxicity. A safety lead in of up to 6 patients will be performed. Immune-related and non-immune related adverse events will be tracked up to 12 weeks post initiation of atezolizumab, as the majority of treatment-related toxicities from talazoparib, radiation, and atezolizumab occur within this time period.