Background: RX-3117 is an oral small molecule nucleoside analogue (cyclopentyl pyrimidyl nucleoside) that is activated by uridine cytidine kinase 2. RX-3117 shows efficacy in various xenograft models, including those of gemcitabine resistant bladder cancer. Preliminary data from Stage 2 of a Phase 2a clinical study of RX‐3117 as a single agent in subjects with advanced urothelial cancer (aUC) is described below. Methods: The efficacy of oral RX‐3117 was evaluated in eligible patients (aged ≥ 18 years) with refractory aUC in a Phase 2a study. Primary objectives include safety and efficacy of 700 mg of administered orally RX‐3117 for 5 consecutive days followed by 2 days off per week in each 4-week cycle for 4 continuous weeks. The primary endpoint is a ≥ 20% rate of progression free survival (PFS) benefit (i.e., proportion of subjects with stable disease for at least 4 months) and/or 10% of evaluable patients with a partial or better response by RECIST criteria. Results: As of February 2018, 27 patients (19 males, 8 females) with aUC were treated with RX‐3117. The median age was 67 years and ECOG performance status was ≤ 1. Prior treatment with gemcitabine or immunotherapy was 85% or 67% of patients, respectively. Five patients achieved stable disease for 4 cycles of RX‐3117 treatment; one patient received treatment for 315 days and another patient continues treatment beyond 4 months. One patient achieved a partial response after 2 cycles. Four patients have shown a tumor reduction ranging from 13.9 to 20% as measured by RECIST. All reductions occurred after 1 cycle of RX-3117 treatment, except for one patient’s reduction, which occurred after 4 cycles. The most frequent related adverse events were G1/2 diarrhea (14%), fatigue (9%), nausea (9%), vomiting (9%), G1/G2 anemia (7%), and G3 thrombocytopenia (7%). Conclusions: RX‐3117 is safe and well tolerated and shows preliminary evidence of anti-tumor activity in heavily pretreated patients. The study continues to enroll subjects with aUC in Stage 2. Clinical trial information: NCT02030067