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

This study is testing a new cancer drug called MT-304 in adults with advanced solid tumors (cancers that have spread) that produce high levels of a protein called HER2. The study is evaluating the drug's safety, how the body processes it, and whether it shows early signs of effectiveness. **You may be eligible if...** - You are 18 or older - You have been diagnosed with a metastatic or advanced cancer that overexpresses HER2 (confirmed by testing) - Your cancer can be measured on imaging scans - Your overall health and organ function are adequate (ECOG 0–1) **You may NOT be eligible if...** - You have active cancer that has spread to the brain - You currently have a fever or acute illness - You have had significant heart failure or serious lung inflammation in the past - You have uncontrolled fluid buildup around the lungs, heart, or in the abdomen - You have an active autoimmune disease requiring systemic treatment Talk to your doctor to see if this trial is right for you.