Activity of RX-3117, an oral antimetabolite nucleoside, in subjects with metastatic bladder cancer resistant to gemcitabine: Preliminary results of a phase Ia/IIb study.

Abstract

4544 Background: RX-3117 is an oral small molecule antimetabolite, cyclopentyl pyrimidyl nucleoside that is activated by uridine cytidine kinase 2. RX-3117 has shown efficacy in xenograft models of gemcitabine resistant pancreatic, bladder and colorectal cancer. Preliminary data from an analysis of a phase 1b/2a clinical study of RX3117 in metastatic bladder cancer is described. Methods: This phase 1b/2a study (NCT02030067) was designed to evaluate safety, tolerability and efficacy following treatment with 700 mg administered orally once-daily for 5 consecutive days with 2 days off per week for 3 weeks with 1 week off in each 4 week cycle in a 2-stage design. Eligible subjects (aged ≥ 18 years) were those with relapsed/refractory metastatic bladder cancer with any number of prior therapies. Prior therapy with platinum-based chemotherapy was required. The primary endpoint was to assess the efficacy and safety of RX-3117 in metastatic bladder cancer, with secondary aims of evaluating PFS and CBR. Results: With 9 subjects enrolled, median age was 66 years, ECOG PS was 0-1. All subjects had received gemcitabine/cisplatin in the perioperative or metastatic setting, and 4 subjects had received 3 or more prior therapies. The most frequent related adverse events were anemia, mild-moderate fatigue, vomiting and diarrhea. No dose limiting toxicities were observed. PFS and CBR will be presented at the meeting, as 5 subjects continue to receive therapy at the time of this submission. One subject continues on treatment at 139 days with persistent stable disease. Molecular profiling of his bladder tumor showed alterations in ARID1A, FBXW7, FGFR3, NF1, and TERT. The patient previously responded to an FGFR3 inhibitor but progressed after 9 months, with ctDNA assessments showing incurrence of TP53 alteration. Clinical benefit with RX-3117 was achieved in spite of incurrence of this alteration. Conclusions: RX-3117 demonstrated an excellent safety profile, and prolonged stable disease was seen in 1 subject who failed prior cisplatin/gemcitabine and FGFR3 inhibition. Activity persisted despite development a putative resistance alteration detected by ctDNA. Clinical trial information: NCT02030067.