Key Inclusion Criteria:  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1 Measurable disease by computed tomography (CT) or magnetic resonance imaging (MRI) as per RECIST Version 1.1 criteria Adequate hematologic counts without transfusional or growth factor support within 2 weeks of study drug initiation Adequate hepatic function (bilirubin ≤ 1.5 upper limit of normal (ULN)), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 ULN Creatinine clearance ≥ 30 mL/min Male and female individuals of childbearing potential who engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception. Phase 1 only: Histologically or cytologically confirmed advanced solid tumor that is refractory to or intolerant of all standard therapy or for which no standard therapy is available. Phase 2 only: Expansion; Histologically- or cytologically-confirmed TNBC per American Society of Clinical Oncologists/College of American Pathologists criteria, based on the most recent analyzed biopsy or other pathology specimen. Refractory to or relapsed after at least 2 prior standard-of-care chemotherapy regimens for unresectable, locally advanced or metastatic breast cancer (mBC).  Key Exclusion Criteria:  Positive serum pregnancy test, or females who may possibly be pregnant Known Gilbert's disease Have previously received antibody drug conjugate containing topoisomerase I inhibitors Presence of bulky disease (defined as any single mass > 7 cm in greatest dimension). Known to be HIV positive, or hepatitis B virus (HBV) surface antigen positive or hepatitis C virus (HCV) antibody positive at screening Known history of significant cardiac disease Known history of clinically significant active chronic obstructive pulmonary disease, or other moderate-to-severe chronic respiratory illness History of interstitial lung disease History of clinically significant gastrointestinal (GI) bleeding, have active chronic inflammatory bowel disease (ulcerative colitis, Crohn's disease) or GI perforation Individuals with a history of anaphylactic reaction to irinotecan.  Note: Other protocol defined Inclusion/Exclusion criteria may apply.