A Phase 1a/b, Open-label, Multicenter Study to Investigate the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Preliminary Antitumor Activity of BG-C137, an Antibody-Drug Conjugate Targeting FGFR2b, in Patients With Advanced Solid Tumors

Inclusion Criteria:

1. Histologically or cytologically confirmed advanced or metastatic solid tumors.
2. Life expectancy of ≥ 3 months.
3. Prior standard systemic therapy in the advanced or metastatic setting. Dose Escalation: Participants for whom further standard treatment is not available, not tolerated or determined not appropriate based on the investigator's judgment. Safety Expansion and Dose Expansion: Participants who have received 1 or 2 prior lines of systemic therapy in the advanced or metastatic setting.
4. Participants must provide agreement for collection of archival tissue or recently obtained fresh tumor biopsy for central evaluation of FGFR2b expression levels and other biomarker assessments.
5. ≥ 1 measurable lesion per RECIST v1.1.
6. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1.
7. Adequate organ function as determined per protocol.

Exclusion Criteria:

1. Prior exposure to topoisomerase I inhibitor (TOP1i)-based antibody-drug conjugate (ADC) therapies or FGFR2b-targeted ADC therapies.
2. Active or chronic corneal disorder, history of corneal transplantation, corneal keratitis, keratoconjuntivitis, keratopathy, corneal abrasion, inflammation or ulceration, other active ocular conditions and any clinically significant corneal disease that prevents adequate monitoring of drug-induced keratopathy.
3. Spinal cord compression, or active leptomeningeal disease or uncontrolled, untreated brain metastasis.
4. Systemic antitumor therapy (including targeted therapy and immunotherapy ≤ 14 days, ≤ 28 days for immuno- oncological antibody, ≤ 14 days or 5 half-lives [whichever is shorter] for chemotherapy, ADCs, or investigational therapy) before first dose of study drug(s).
5. Toxicities due to prior therapy that have not recovered.
6. Any malignancy ≤ 2 years before first dose of study drug(s) except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated curatively.
7. History of interstitial lung disease (ILD), noninfectious pneumonitis, oxygen saturation at rest < 92%, or requirement for supplemental oxygen at baseline.