A Study to Evaluate the Safety and Tolerability of Venetoclax Tablets in Combination With Capecitabine Tablets in Adult Participants With Hormone Receptor-Positive, HER2-Negative Locally Advanced or Metastatic Breast Cancer Who Had Disease Progression During or After CDK4/6 Inhibitor Therapy

Endocrine therapy is the initial treatment for most hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) breast cancers. This study will evaluate the use of venetoclax in combination with capecitabine in adult participants with HR+, HER2-, metastatic breast cancer (MBC) who had disease progression following treatment that included a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor.

Venetoclax is an investigational drug being developed for the treatment of breast cancer. This study is open-label meaning both the participants and study doctors will know what treatment is being given. The study includes two phases: dose escalation and dose expansion. In dose escalation, participants will receive various doses of venetoclax in combination with capecitabine. In dose expansion, participants will receive the recommended dose of venetoclax determined during dose escalation in combination with capecitabine. Adult participants with locally advanced or MBC that is not amenable to curative therapy will be enrolled. Around 42 participants will be enrolled at approximately 20 sites worldwide.

Venetoclax and capecitabine will be administered on a 21-day cycle. During dose escalation, participants will take various doses of venetoclax as a tablet by mouth once a day and capecitabine as a tablet by mouth twice per day on days 1 - 14 of each cycle for approximately 30 weeks. During dose expansion, participants will take venetoclax at the dose identified during dose escalation as a tablet by mouth once a day and capecitabine as a tablet by mouth twice per day on days 1 - 14 of each cycle for approximately 30 weeks.

There may be a higher burden for participants in this trial compared to standard of care. Participants will attend weekly visits during the course of the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, and evaluating for side effects.