A Study of Disitamab Vedotin With Other Anticancer Drugs in Solid Tumors

Inclusion Criteria: General Inclusion Criteria * Measurable disease according to RECIST v1.1 * Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1 Dose Escalation and Optimization Phase Inclusion Criteria * Histologically or cytologically confirmed diagnosis of gastric or gastroesophageal junction adenocarcinoma or breast carcinoma * Locally-advanced, unresectable, or metastatic stage * Must have experienced disease progression on or after standard of care therapies or be intolerant of standard of care therapies. Cohort A (HER2-Low Breast Cancer) Inclusion Criteria * Histologically or cytologically confirmed diagnosis of breast carcinoma * Locally-advanced, unresectable, or metastatic stage * HER2-low status determined by most recent local assessment (IHC 1+ or IHC 2+/ISH-negative) * Prior therapies requirements * No more than 3 prior systemic cytotoxic chemotherapy regimens (including ADCs) for LA/mBC. * Participants with known BRCA mutation must have received a PARP-inhibitor where available and not medically contraindicated * Have progression on or after, or intolerant to, T-DXd, sacituzumab govitecan, or other topoisomerase I inhibitor therapies, if available as local standard of care therapy * Participants with HR+ tumors must have intolerance to endocrine therapy or endocrine therapy refractory disease: * Progressed on ≥2 lines of endocrine therapy for LA/mBC AND had received a CDK4/6 inhibitor in the adjuvant or metastatic setting OR * Progressed on 1 line of endocrine therapy for LA/mBC AND had a relapse while on adjuvant endocrine therapy after definitive surgery for primary tumor AND had received a CDK4/6 inhibitor in the adjuvant or advanced setting * Participants with HR negative, HER2-low and PD-L1-positive (CPS 10 or greater) tumors must have received pembrolizumab with chemotherapy if available as local standard of care therapy. * Participants with HR negative, HER2-low and PD-L1-positive (CPS 10 or greater) tumors must have received pembrolizumab (or other PD-(L)1 inhibitor) with chemotherapy if available as local standard of care therapy and not medically contraindicated. Cohort B (HER2+ Breast Cancer) Inclusion Criteria * Histologically or cytologically confirmed diagnosis breast carcinoma * Locally-advanced, unresectable, or metastatic stage * HER2+ status determined by most recent local assessment (IHC 3+ or IHC 2+/ISH+) * Participants must have: * Received prior trastuzumab, pertuzumab and a taxane if available as local standard of care therapy for advanced disease. * Have progression on or after, or intolerant to, T-DXd or other topoisomerase I inhibitor therapies * No more than 3 prior systemic cytotoxic chemotherapy regimens (including ADCs) for LA/mBC Cohort C (HER2-Low Gastric or Gastroesophageal Junction Adenocarcinoma) Inclusion Criteria * Histologically or cytologically confirmed diagnosis of gastric or gastroesophageal junction adenocarcinoma * Locally-advanced, unresectable, or metastatic stage * HER2-low expression defined as IHC 1+ or IHC 2+/ISH-negative determined by most recent local assessment * Willing and able to provide archival or newly obtained formalin-fixed paraffin-embedded (FFPE) tumor tissue blocks * Participants must have received: * Prior systemic therapy with platinum, fluorouracil, or taxane for locally advanced unresectable or metastatic disease * Progression within 6 months of last dose of (neo)adjuvant cytotoxic chemotherapy is considered as 1 line of systemic therapy for LA/mGC/GEJC * Prior anti-PD-(L)1 therapy is allowed * No more than 2 prior systemic cytotoxic chemotherapy regimens (including ADC) for LA/mGC/GEJC * Must not have received prior treatment with HER2 directed therapy Cohort D (HER2+ LA/mGC/GEJC) Inclusion Criteria * Histologically or cytologically confirmed diagnosis of gastric or gastroesophageal junction adenocarcinoma * Locally-advanced, unresectable, or metastatic stage * HER2+ status determined by most recent local assessment (IHC 3+ or IHC 2+/ISH+) * Participants must have: * Received prior trastuzumab plus fluoropyrimidine and platinum containing chemotherapy if no contraindication. * Prior T-DXd treatment is allowed * Prior PD1 inhibitor therapy is allowed * No more than 2 prior systemic cytotoxic chemotherapy regimens (including ADCs) for LA/mGC/GEJC Exclusion Criteria: * Known hypersensitivity to any excipient contained in the drug formulation of disitamab vedotin or tucatinib * Prior therapy with ADCs with MMAE payload * Prior therapy with tucatinib * Active CNS and/or leptomeningeal metastasis. * Participants who have received prior systemic anticancer treatment including investigational agents within 4 weeks prior to first dose of study treatment * History of other invasive malignancy within 3 years before the first dose of study intervention, or any evidence of residual disease from a previously diagnosed malignancy. * Unable to swallow oral tablets or capsules or any significant GI disease which would preclude the adequate oral absorption of medications