• Phase 1a (Dose Escalation) and 1b (Dose Expansion): 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.

• Phase 1a: Received prior therapy for their condition (if available) and should be refractory to, or intolerant of standard-of-care therapies. In regions where approved and available, participants with HR+ breast cancer must have received at least 2 prior lines of treatment including endocrine therapy and a CDK4/6 inhibitor. 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 1b: Selected tumor cohorts will include HR+/HER2- breast cancer and additional tumor types.

• Phase 1b: Participants with HR+/HER2- breast cancer enrolled in regions where CDK4/6 inhibitors are approved and available must have received at least one line of therapy for advanced disease including endocrine therapy and a CDK4/6 inhibitor. Participants can have received up to 2 lines of prior cytotoxic chemotherapy for advanced disease.

• Stable Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1.

• Female participants with metastatic HR+/HER2- breast cancer must be postmenopausal or receiving ovarian function suppression treatment.

• Adequate organ function without symptomatic visceral disease.