Optimal Methods to Characterize ADC Resistance in Solid Tumors and Identify Clinically Useful Biomarkers

This study is collecting tumor samples from patients receiving antibody-drug conjugate (ADC) therapy — a type of targeted cancer treatment — to understand why some cancers stop responding to these drugs over time and to identify markers that predict effectiveness. **You may be eligible if...** - You are 18 or older - You have confirmed advanced or metastatic breast, urothelial (bladder/urinary tract), non-small cell lung, or gastric cancer - You are about to receive an approved or early-access ADC treatment - You have at least 2 measurable tumor lesions and are willing to provide biopsy samples **You may NOT be eligible if...** - You are unable or unwilling to provide biopsy samples before and after treatment - Your general health or blood tests do not meet safety thresholds Talk to your doctor to see if this trial is right for you.