Background: Tumor cells residing in tumor hypoxic zones are a major cause of drug resistance and tumor relapse. In this study we investigated the efficacy of evofosfamide,(formerly TH302)) a hypoxia-activated prodrug, and its combination with topotecan in neuroblastoma (NBL) preclinical models.
Methods: A panel of five NBL cells lines (SKNEB2 , CHLA15, CHLA20 CHLA90, LAN5) were tested in vitro to assess the effect of evofosfamide on cell proliferation, both as a single agent and in combination with low dose metronomic administration of topotecan daily x 2 weeks , both under normoxic and various hypoxic conditions. In vivo antitumor activity was evaluated in different xenograft models. Animal survival was studied with the NBL metastatic model.
Results: Under normoxic condition, all the tested lines responded to evofosfamide in a dose-dependent manner, with IC50 values ranging from 4.6 to 66.1µM. When tumor cells were exposed to hypoxia overnight, there was a 22 to 65-fold decrease of evofosfamide IC50 with the IC50 values ranging from 0.07 to 2.4µM. By adding 20nM topotecan, evofosfamide induced cytotoxicity was significantly enhanced under overnight hypoxia as indicated by decreased IC50s in most tested tumor cell lines (p<00.1). In SK-NBE(2) and CHLA-20 xenograft models, after 2 weeks of treatment, tumor growth delay was observed with evofosfamide or topotecan as monotherapies. Complete tumor regression was observed in the combined topotecan / evofosfamide treatment group. From SK-N-BE(2) metastatic model, evofosfamide or topotecan treatment showed a substantial increase in animal survival compared to the control mice with a median survival of 22.5 days for control group, 25 days for evofosfamide group and 36 days for topotecan group. However, treatment with a combination of evofosfamide and topotecan had the greatest impact on animal survival (p < 0.01) with a median survival of 46 days.
Conclusions: Evofosfamide shows antitumor effects in NBL xenografts. Compared to single-agent, evofosfamide / topotecan, combined therapy using a metronomic topotecan regimen improves tumor response and enhances animal survival in preclinical tumor models.