A Study of Cobimetinib Plus Paclitaxel, Cobimetinib Plus Atezolizumab Plus Paclitaxel, or Cobimetinib Plus Atezolizumab Plus Nab-Paclitaxel as Initial Treatment for Participants With Triple-Negative Breast Cancer That Has Spread

This three-cohort, multi-stage, randomized, Phase II, multicenter trial will evaluate the safety and tolerability and estimate the efficacy of cobimetinib plus paclitaxel versus placebo plus paclitaxel in Cohort I, of cobimetinib plus atezolizumab plus paclitaxel in Cohort II, and of cobimetinib plus atezolizumab plus nab-paclitaxel in Cohort III in participants with metastatic or locally advanced, triple-negative adenocarcinoma of the breast who have not received prior systemic therapy for metastatic breast cancer (MBC). Participants may continue on study treatment until the development of progressive disease (PD) or the loss of clinical benefit, unacceptable toxicity, and/or consent withdrawal. The Cohort I target sample size is 12 participants for the safety run-in stage and approximately 90 participants in the expansion stage. Each of Cohorts II and III will consist of a safety run-in stage of approximately 15 participants followed by an expansion stage of approximately 15 participants.