To Access the Safety and Effects of Intravenous Administration of VIR-5818 Alone and in Combination With Pembrolizumab in Adult Participants With Locally Advanced or Metastatic HER2-Expressing Cancers

This first-in-human (FIH) Phase 1 open-label multicenter dose-escalation and dose-expansion study is designed to evaluate the safety, pharmacokinetics, and preliminary activity of VIR-5818 (Formerly AMX-818) as a single agent and in combination with pembrolizumab in participants with HER2+ tumors across multiple tumor types. The study will be conducted in four parts: * Part 1 (dose escalation): Single-agent VIR-5818 * Part 2 (dose escalation): VIR-5818 plus pembrolizumab * Part 3 (dose expansion): Single-agent VIR-5818 * Part 4 (dose expansion): VIR-5818 plus pembrolizumab The total length of the study, from screening of the first participant to the end of the study, is expected to be approximately 52 months.