• Individuals assigned male or female at birth, 18 years of age or older, able to understand and give written informed consent.

• Histologically or cytologically locally confirmed TNBC.

• Phase 1: Individuals with unresectable, locally advanced or metastatic TNBC who are refractory to or relapsed after at least one prior standard-of-care chemotherapy regimen or systemic therapy given for locally advanced or metastatic disease.

• Phase 2: Individuals with unresectable, locally advanced or metastatic TNBC who have not received previous systemic therapy for advanced disease.

• Phase 2: Tumors must be PD-L1 negative, defined as tumor PD-L1 combined positive score (CPS) \< 10 using the PD-L1 immunohistochemistry (IHC) 22C3 assay. Alternatively, individuals with tumor CPS ≥ 10 will be eligible if they received an anti-PD-(L)1 agent (ie, checkpoint inhibitor) in the adjuvant or neoadjuvant setting or if they cannot be treated with an anti-PD-(L)1 agent. due to a comorbidity.

• Uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) genotype status.

⁃ During Phase 1 safety run-in, individuals must be UGT1A1 wild-type.

⁃ After Phase 1 safety run-in, individuals with any UGT1A1 genotype may be eligible.

• Measurable disease by computed tomography (CT) or magnetic resonance imaging (MRI) according to RECIST Version 1.1 criteria.

• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.

• Adequate hematologic counts within 2 weeks prior to enrollment.

• Adequate hepatic and renal function.