RecruitingPhase 1Drug trialSafety, Pharmacokinetics, Pharmacodynamics and Preliminary Efficacy of MT-304 in Adults With Advanced HER2-Expressing Solid TumorsThis clinical trial is designed to evaluate the safety, pharmacokinetics, pharmacodynamics, and preliminary efficacy of MT-304 in adults with advanced HER2-expressing solid tumors. The main questions it aims to answer are: * What is the safety profile of MT-304 when administered alone or with nivolumab? * What is the recommended Phase 2 dose (RP2D) of MT-304? Participants will: * Receive MT-304 alone (every 14 days) or with nivolumab (every 28 days). * Attend regular clinic visits for assessments and monitoring. * Continue treatment until disease progression, unacceptable toxicity, or study discontinuation.