Phase 1/2 Study of BHB810 in Advanced Gastric and GEJ Adenocarcinoma

Inclusion Criteria: * Participant must be ≥ 18 years or the legal age of consent in the jurisdiction in which the study is taking place at the time of signing the ICF. * Histologically confirmed advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma that has progressed on, was nonresponsive to, or for which no standard or available curative therapy exists. * Participants in Phase 1 Backfill Cohorts \& Phase 2 must be CDH17-positive by central testing. * Other gastrointestinal (GI) tumor types may be enrolled in Backfill Cohorts and Phase 2. * At least 1 measurable target lesion at baseline per RECIST 1.1 (Response Evaluation Criteria in Solid Tumors) * Provision of FFPE archival tumor tissue. Additional fresh biopsies at screening are required in Phase 1 Backfill Cohorts and Phase 2. * Adequate organ and marrow function as defined in the protocol Exclusion Criteria: * Prior cancer treatment as follows, relative to the first planned dose of trial intervention: * Chemotherapy or targeted therapy withing 4 weeks or 5-halflives (whichever is shorter) * Monoclonal antibody-based therapy (including ADCs) within 4 weeks * Immune checkpoint inhibitors within 4 weeks * Wide-field radiation therapy (\>30% marrow-bearing bones) within 4 weeks or \< 2 weeks of focal palliative radiation to nontarget lesions * Prior treatment with a CDH17-directed therapy or an ADC with an auristatin (MMAE or MMAF) * Known hypersensitivity or allergic reaction to BHB810 or it'…