Inclusion criteria: * Has centrally confirmed unresectable, locally advanced, or metastatic breast cancer with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) status * Has radiographic disease progression on one or more lines of endocrine therapy for unresectable, locally advanced/metastatic HR+/HER2- breast cancer, in combination with a CDK4/6 inhibitor * Is a candidate for chemotherapy * Has an Eastern Cooperative Oncology Group (ECOG) functional status of 0 to 1 assessed within 7 days prior to randomization * Has adequate organ function * Participants infected with human immunodeficiency virus (HIV) must have well-controlled HIV with antiretroviral therapy * Participants who are positive for hepatitis B surface antigen (HBsAg) are eligible if they have received antiviral therapy against HBV for at least 4 weeks and have an undetectable HBV viral load * Participants with a history of infection Patients with hepatitis C virus (HCV) are eligible if their HCV viral load is undetectable. Exclusion Criteria: * Has breast cancer amenable to curative treatment * Has experienced early recurrence (<6 months after completing adjuvant/neoadjuvant chemotherapy) and is therefore eligible for second-line (2L) treatment * Has advanced symptomatic/metastatic visceral dissemination at risk of rapidly progressing to life-threatening complications * Has received prior chemotherapy for locally advanced or unresectable metastatic breast cancer * Active autoimmune disease requiring systemic treatment within the past 2 years * History of (non-infectious) pneumonitis/interstitial lung disease requiring steroids, or has current pneumonitis/interstitial lung disease * Has an active infection requiring systemic therapy