BGB-43395 Alone or as Part of Combination Therapies in Participants With Breast Cancer and Other Advanced Solid Tumors

Inclusion Criteria: * Phase 1a (Dose Escalation): Participants with histologically or cytologically confirmed advanced, metastatic, or unresectable solid tumors associated with dependency on CDK4, including HR+ breast cancer, ovarian cancer, endometrial cancer, non-small cell lung cancer, and others. For combination with elacestrant, participants must have received at least 1 prior line of treatment for advanced/metastatic disease including prior endocrine therapy and CDK4/6 inhibitor in either the adjuvant or advanced/metastatic setting. * Phase 1a Safety Expansion: For combination with fulvestrant in regions where approved and available, participants with HR+ breast cancer must have received at least 1 prior line of treatment including endocrine therapy and a CDK4/6 inhibitor. For combination with letrozole, participants must be CDK4/6 inhibitor treatment naïve and have not received any previous systemic treatment for advanced disease. * Phase 1b: Participants with HR+/HER2- breast cancer. * Phase 1b: For combination with fulvestrant, participants with HR+/HER2- breast cancer enrolled in regions where CDK4/6 inhibitors are approved and available must have received 1-2 lines of therapy for advanced/metastatic disease including endocrine therapy and a CDK4/6 inhibitor. Participants can have received up to 2 lines of prior cytotoxic chemotherapy for advanced disease. Prior cytotoxic treatment is prohibited. For combination cohorts with letrozole, participants must be CDK4/6 i…