Development of a novel, targeted small molecule inhibitor of the nucleoside salvage pathway through combinatorial murine studies to enable further clinical advancement in prostate tumors

PROJECT SUMMARY Trethera, a clinical stage biopharmaceutical company, has developed a first-in-class small molecule drug, TRE- 515, to target the nucleoside salvage pathway via a key rate-limiting enzyme, deoxycytidine kinase (dCK). dCK activity in tumor cells is a critical, but previously unaddressable, resistance mechanism to widely administered anticancer therapeutics. This proposal will test a TRE-515 anchored combination therapy to prevent the dCK- mediated escape route found in radiation-treated preclinical castration-resistant prostate cancer (CRPC) models and generate data to support a Phase 1b clinical trial. Notably, TRE-515 is the only salvage pathway inhibitor to ever enter clinical development, with no salvage pathway inhibitors approved as cancer therapeutics. Currently, Trethera is evaluating TRE-515 in a Phase 1a open-label, dose escalation study in patients with solid tumors (NCT05055609), which has enrolled 22 patients across 6 separate cohorts over a twelve-fold dose increase, showing the drug to be well tolerated (no dose-limiting clinical or laboratory toxicities were observed in any treated patient) with mild side effects, compelling target engagement, and early signs of clinical benefit. These encouraging signs highlight the immediate potential for TRE-515 to improve clinical outcomes when adminstered as a component of a rationally-designed combination therapy. dCK upregulation is a mechanism of resistance to anticancer therapies that promote DNA damage. The therapy combination and patient population where TRE- 515 has the most compelling preliminary evidence for improved clinical outcomes is with radiation therapy (RT) in CRPC. Building on strong preliminary data showing >95% tumor regression following RT with TRE-515 treatment in an aggressive human prostate cancer in vivo model, we propose to complete key preclinical experiments to demonstrate enhanced antitumor activity of TRE-515 when combined with RT. In Aim 1, we will determine the effects of TRE-515 treatment alone and in combination with computed tomography (CT)-guided focal beam RT on tumor growth inhibition and overall survival in four aggressive human xenograft prostate cancer models. The studies will incorporate two clinically relevant biomarkers that will inform target inhibition by TRE- 515: (i) a dCK-specific positron emission tomography (PET) probe that provides a non-invasive pharmacodynamic biomarker for TRE-515; and (ii) mass spectrometry analysis of plasma dC substrate levels to confirm on-target engagement. The enhanced efficacy of RT with TRE-515 will be confirmed in a tumor model engineered to lack dCK. The in vivo studies will be complemented by focused in vitro experiments to investigate the mechanism of antitumor activity. In Aim 2, we will determine tumor growth inhibition and tolerability in three murine syngeneic prostate cancer models, in immunocompetent hosts, treated with TRE-515 alone and in combination with RT. Successful completion of the proposed Aims will provide necessary efficacy validation for TRE-515 as a combination therapy and inform the Phase 1b/2 trial design that may improve CRPC patient outcomes, as well as attract outisde funding for the proposed trial.