A Study of Runimotamab in Participants With Locally Advanced or Metastatic HER2-Expressing Cancers

This study will evaluate the safety, tolerability, and pharmacokinetics of Runimotamab administered intravenously as a single agent and in combination with Trastuzumab in participants with locally advanced or metastatic Human Epidermal Growth Factor Receptor 2 (HER2)-expressing cancers.

Inclusion Criteria * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Life expectancy of at least 12 weeks * Adequate hematologic and end-organ function * Acute, clinically significant treatment-related toxicity from prior therapy must have resolved to Grade \</=1 prior to study entry * Left Ventricular Ejection Fraction (LVEF) \>/=50% HER2-Expressing Breast Cancer-Specific Inclusion Criteria * Locally tested, Human Epidermal Growth Factor Receptor 2 (HER2)-expressing BC * Locally advanced or metastatic BC that has relapsed or is refractory to established therapies HER2-Expressing Gastric/Gastroesophageal (GEJ) Cancer-Specific Inclusion Criteria * Adenocarcinoma of the stomach or GEJ with inoperable locally advanced or recurrent and/or metastatic disease, not amenable to curative therapy * HER2-expressing tumor (primary tumor or metastasis) as assessed by local lab testing * HER2-positive gastric/GEJ cancer must have received prior trastuzumab, cisplatin (or carboplatin or oxaliplatin or investigational platinum agent) and 5-fluorouracil (5-FU)/capecitabine HER2-Positive Solid Tumor Specific Inclusion Criteria * HER2-positive tumor (primary tumor or metastasis) as assessed by local (non-central) laboratory testing * Locally advanced, recurrent, or metastatic incurable malignancy that has progressed after at least one available standard therapy; or for whom standard therapy has proven to be ineffective or intolerable, or is considered inappropriate; or for whom a clinical trial of an investigational agent is a recognized standard of care; or for whom a clinical trial of an investigational agent is considered an acceptable treatment option Exclusion Criteria * Pregnant or breastfeeding, or intending to become pregnant during the study or within 140 days after the last dose of runimotamab * Significant cardiopulmonary dysfunction * Known clinically significant liver disease * Positive for acute or chronic Hepatitis B virus (HBV) infection * Acute or chronic Hepatitis C virus (HCV) infection * Human Immunodeficiency Virus (HIV) seropositivity * Poorly controlled Type 2 diabetes mellitus * History of ventricular dysrhythmias or risk factors for ventricular dysrhythmias * Current treatment with medications that are well known to prolong the Q-wave/T-wave (QT) interval * Known clinically significant liver disease * Primary central nervous system (CNS) malignancy, untreated CNS metastases, or active CNS metastases (progressing or requiring corticosteroids for symptomatic control) * Leptomeningeal disease * Spinal cord compression that has not definitively treated with surgery and/or radiation * History of autoimmune disease * Prior allogeneic stem cell or solid organ transplantation