Discovery of TH-302: An achiral hypoxia-activated cytotoxic prodrug

Abstract

3515 Background: Selective targeting of the hypoxic compartment of the tumor microenvironment is both a challenge and an opportunity for discovering and developing new cancer therapeutics. Chronic and transient hypoxia underlie resistance to chemotherapeutics and radiation. We have taken a modular design approach to synthesize hypoxia-activated prodrugs. Different nitro-containing heterocyclic triggers were linked to bis alkylating phosphorous mustard toxins and profiled in in vitro and in vivo assays predictive of compound efficacy and pharmacokinetics (PK). Methods: Drug candidates were synthesized and characterized using standard synthetic methods. In vitro efficacy was assessed with cancer cell line cytotoxicity and clonogenic assays. In vivo efficacy was assessed with xenograft models in monotherapy and combination therapy modes. Results: Screening trigger-toxin combinations in H460 lung cancer cell proliferation assays under anoxic, hypoxic, and aerobic conditions yielded structure-activity relationships (SAR) and identified TH- 302, a 2-nitroimidazole triggered bromo analog of ifosfamide, as a potent and selective molecule. TH-302 exhibits in vitro proliferation IC50 values of 0.2, 5, and 90μM and clonogenic IC90 values of 0.1, 5, and 30μM under conditions of 0%, 0.6%, and 21% O2, respectively. In vitro profiling showed sufficient solubility, plasma and microsomal stability, resistance to drug efflux mechanisms, and PK. In vivo efficacy was assessed with the H460 ectopic xenograft model. TH-302 administered alone (100mpk, ip, Q3Dx5) or in combination with cisplatin (6mpk, iv, Q7Dx2) showed mean tumor volume growth delays to 500mm3 (TGD500) of 10 days for TH-302 alone, 4 days for cisplatin alone, and 20 days for the combination. These results have been confirmed and extended using different tumor models, dosing regimens, and combinations which included Taxol and 5-FU. TH-302 demonstrates dose-proportional PK in multiple species. Conclusions: TH-302, a novel hypoxia-activated cytotoxic prodrug, has promising preclinical in vitro and in vivo efficacy and selectivity and has entered IND-enabling studies. Acute and subchronic toxicology is underway in rats and dogs. We expect to file an IND for clinical studies of TH-302 this year. No significant financial relationships to disclose.