Background: RX-3117 is an oral small molecule antimetabolite that is activated by uridine cytidine kinase 2 (UCK2) which is predominantly expressed in cancer cells. RX-3117 has shown efficacy in xenograft models of gemcitabine resistant pancreatic, bladder and colorectal cancer. Preliminary data from an analysis of a Phase 2a clinical study of RX3117 in advanced urothelial cancer is described. Methods: In the Phase 2a study designed to evaluate safety, tolerability and efficacy, subjects were treated with oral RX-3117 (700 mg) once-daily for 5 consecutive days on and 2 days off for 3 of 4 weeks or all 4 weeks in a 28-day cycle. Eligible subjects (aged ≥ 18 years) had relapsed/refractory metastatic urothelial cancer, ECOG PS of 0 to 1, normal organ function (hepatic, renal and hematology) with no limit on the number of prior therapies. The primary Phase 2a endpoints are progression free survival (PFS) and/or objective clinical response with secondary endpoints of safety, TTP, DOR and ORR. Results: As of October 5, 2018; 33 subjects were treated (23 males and 10 females, median age 67.5 years); 29 subjects were evaluable having completed more than 1 cycle of therapy or discontinued due to a related adverse event. Twenty subjects had received 3 or more prior therapies; 30 received gemcitabine/cisplatin and 25 received a checkpoint inhibitor. The most common related adverse events were anemia (G1-2%, G2-3%, G3-3%), fatigue (G1-6%, G2-3%), neutropenia (G2-2%, G3-5%, G4-2%), diarrhea (G1-4%, G2-2%), and thrombocytopenia (G2-2%, G3-3%, G4-1%). One subject had a complete response after 4 cycles of therapy and continues therapy beyond 10 cycles; 5 subjects had PFS ranging from 133 to 315 days. Conclusions: RX-3117 appears to be safe and well-tolerated in chemotherapy and immunotherapy refractory advanced urothelial cancer with acceptable toxicities. Preliminary results show anti-tumor activity in heavily pre-treated patients. Clinical trial information: NCT02030067