Background: UC is 2nd most common genitourinary cancer. Current SOC offers platinum-based (PB) first line chemotherapy (chemo) with ddMVAC or gemcitabine-cisplatin (GC) regimens. For cisplatin ineligible patients (pts), SOC includes gemcitabine-carboplatin (GCa), and in select pts pembrolizumab. Erdafitinib is approved for pts with FGFR alterations and Enfortumab vedotin (EV) is approved for previously treated pts. A phase I/II CTEP study of eribulin (E) for metastatic UC (mUC) established the activity of E in UC with objective response rate (ORR) of 37.5% and a median progression free survival (PFS) of 4.1 months (mo) and median overall survival (OS) of 9.5 mo (N=150). A phase II CTEP study of gemcitabine-eribulin (GE) in cisplatin ineligible mUC showed an ORR of 50%, median OS of 11.9 mo and median PFS of 5.3 mo (N=24). The most common Grade 3-4 toxicities included: neutropenia 63%, anemia and fatigue 29%. Pts with liver metastases benefited from therapy with 5 responders in 7 pts for GE vs 12 out 49 E. Methods: This is a phase III, randomized 3 arm study comparing E vs. GE vs. SOC (docetaxel, paclitaxel, or gemcitabine). E is given at 1.4mg/m2 on day (D) 1 and 8 of a 21 D cycle. In the GE arm, gemcitabine is added to E at 1000 mg/m2 dose to D1 and D8. SOC follows standard dosing of the agents. There is no limit to the number/sequence of prior regimens. A simplified summary of eligibility criteria is presented here. All pts must have: received frontline systemic treatment such as PB chemo or a non-platinum regimen; received PD1/PDL1 Ab or be deemed ineligible for PD1/PDL1 Ab; received EV. Assuming a median OS for the SOC arm of 7 mo the study seeks to find at least a 50% increase in median OS to 10.5 mo (Hazard Ratio (HR) = 0.667). One-sided 0.0125 type I error to account for testing of two primary hypotheses (Each arm vs. SOC). 87% power to detect a 3.5 mo improvement in OS. We require 140 eligible (155 total) pts in each arm for a total of 465. The study was activated in Feb 2021 and accrual is ongoing. Clinical trial information: NCT04579224.