Safety, Pharmacokinetics, Pharmacodynamics and Preliminary Efficacy of MT-304 in Adults With Advanced HER2-Expressing Solid Tumors

This multicenter, open-label, Phase 1 trial is designed to assess the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and preliminary efficacy of MT-304 in adults aged 18 and older with advanced HER2-expressing solid tumors. The study consists of two treatment modules: * Module 1 (Monotherapy): Participants receive MT-304 every 14 days for 28-day cycles, with dosing adjustments based on clinical benefit and safety evaluations. * Module 2 (Combination Therapy): Participants receive MT-304 in combination with nivolumab, administered every 14 days and 28 days, respectively, also allowing for dosing adjustments. The Bayesian Optimal Interval (BOIN) design will guide dose escalation, overseen by a Safety Review Committee to establish the recommended Phase 2 dose (RP2D). Regular assessments, including vital signs and laboratory tests, will monitor safety and efficacy throughout the trial, with follow-up visits for up to 2 years post-treatment.