Any regression of tumor (ART) within 12 weeks versus RECIST 1.1 response category as an intermediate endpoint to assess the activity of immune checkpoint inhibitors (ICIs) for metastatic urothelial carcinoma (mUC).

473 Background: Many ICIs are approved to treat mUC following platinum-based chemo and as 1st-line therapy for selected patients (pts). While RECIST 1.1 responses are generally durable and associated with prolonged survival, the benefit of ICIs extends beyond this group of pts. Stable disease (SD) consists of a heterogeneous population of pts with both increase and decrease in tumor size and is confounded by the impact of …