Inclusion Criteria:
- Participants with histologically or cytologically confirmed advanced, metastatic, or unresectable solid tumors not amenable to therapy with curative intent or for whom treatment is not available or not tolerated.
- Participants must be able to provide archival tissue formalin-fixed paraffin-embedded (FFPE) block containing tumor tissue or approximately 10 to 15 freshly cut unstained FFPE slides) or recently obtained fresh tumor biopsy samples at screening.
- Participants must have ≥ 1 measurable lesion as assessed by RECIST v1.1.
- Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1, as assessed ≤ 14 days before the first dose of study drug.
- Adequate bone marrow and organ function as indicated by the following laboratory values ≤ 14 days before the first dose of study drug
- Female participants 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. They must also have a negative serum pregnancy test result ≤ 3 days before the first dose of study drug.
- Nonsterile male participants must be willing to use a highly effective method of birth control and refrain from sperm donation for the duration of the study and for ≥ 4 months after the last dose of study drug.
Exclusion Criteria:
- History of severe allergic reactions or hypersensitivity to BG-T187 or other monoclonal antibodies, or to the active ingredient and excipients of the study drug or camptothecins.
- For Phase 1a Part B Safety Expansion and Phase 1b only: Prior treatment with an EGFR-targeting ADC or mesenchymal-epithelial transition (MET)-targeting antibody-drug conjugate (ADC), or any ADC with topoisomerase I (TOPO1) inhibitor payload.
- Active leptomeningeal disease or uncontrolled, untreated brain metastasis. Participants with a history of treated and, at the time of screening, stable central nervous system (CNS) metastases are eligible, provided they meet all the following:
1. Brain imaging at screening shows no evidence of interim progression, is clinically stable for ≥ 4 weeks, and has no evidence of new brain metastases
2. Have measurable disease and/or evaluable disease outside CNS
3. No ongoing requirement for corticosteroids as therapy for CNS disease; off corticosteroids ≥ 14 days before dosing with study drug; anticonvulsants at a stable dose are allowed
4. No stereotactic radiation or whole-brain radiation ≤ 14 days before the first dose of study drug
- History of interstitial lung disease (ILD), or ≥ Grade 2 noninfectious pneumonitis ≤ 2 years before the first dose of the study drug, or has current ILD/noninfectious pneumonitis, or where suspected active ILD/noninfectious pneumonitis cannot be ruled out by imaging during screening.
- Participants with active or chronic corneal disorder, including but not limited to Sjögren's, Fuch's corneal dystrophy, history of corneal transplantation, corneal keratitis, keratoconjunctivitis, keratopathy, corneal abrasion, inflammation or ulceration, other active ocular conditions and any clinically significant corneal disease that prevents adequate monitoring of drug-induced keratopathy.
Note: Other protocol defined Inclusion/Exclusion criteria may apply.