Phase 1a/1b Study of BG-C9074, an Antibody Drug Conjugate Targeting B7H4, as Monotherapy and in Combination With Other Anticancer Therapies in Patients With Advanced Solid Tumors

Inclusion Criteria: 1. Able to provide a signed and dated written informed consent prior to any study-specific procedures, sampling, or data collection. 2. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1. 3. Participants with selected histologically or cytologically confirmed advanced, metastatic, and unresectable solid tumors who have previously received standard systemic therapy and whose cancer is not amenable to therapy with curative intent, and for whom further treatment is not available or not tolerated. Enrollment will be limited to participants with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer, cholangiocarcinoma (CCA), endometrial cancer, squamous non-small cell lung cancer (NSCLC), triple-negative breast cancer (TNBC), or ovarian cancer. Enrollment in the Japan cohort will be limited to participants with HR+/HER2- breast cancer, TNBC, endometrial cancer, or ovarian cancer. 4. ≥ 1 measurable lesion per Response Evaluation Criteria in Solid Tumors, version 1.1 (RECIST v1.1) 5. Able to provide an archived tumor tissue sample. 6. Adequate bone marrow and organ function. 7. Females of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study, and for ≥ 7 months after the last dose of study drug(s). 8. Nonsterile males must be willing to use a highly effective method of birth control for the duration of the study treatment period and for ≥ 4 months after the last dose of study drug(s). Exclusion Criteria: 1. Prior treatment with a B7 homolog 4 (B7H4)-targeting antibody-drug conjugate (ADC) or an ADC with a topoisomerase 1 inhibitor (TOP1i) payload. 2. Active leptomeningeal disease or uncontrolled, untreated brain metastasis 3. Any malignancy ≤ 2 years before the first dose of study treatment(s) except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated with curative intent (eg, resected basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix or breast). 4. History of interstitial lung disease, ≥ Grade 2 noninfectious pneumonitis, oxygen saturation at rest \< 92%, or requirement for supplemental oxygen (including intermittent use) at baseline. 5. Uncontrolled diabetes. 6. Infection (including tuberculosis infection) requiring systemic (oral or intravenous) antibacterial, antifungal, or antiviral therapy ≤ 14 days before the first dose of study treatment(s). Note: Other protocol defined Inclusion/Exclusion criteria may apply.