Abstract 535: Hypoxia-dependent in vivo activity of the hypoxia-activated prodrug (HAP) TH-302

Abstract

Abstract Hypoxia is a defining feature of solid tumors, associated with treatment failure, poor prognosis, cancer relapse and increased metastatic potential. Tumor-specific hypoxia-targeted therapies have been pursued, including hypoxia-activated prodrugs (HAPs). TH-302, a HAP of the cytotoxic warhead bromo-isophosphoramide mustard (Br-IPM), was designed with the goal of high hypoxia selectivity, in vivo efficacy, and an acceptable safety profile. TH-302 has been previously shown to be hypoxia-selective in human tumor cell line in vitro cytotoxicity assays. Here, using a panel of human tumor xenograft models, we demonstrate selective hypoxic-compartment targeting of TH-302 in vivo. Eleven different xenograft models were employed to investigate the antitumor activity of TH-302. The hypoxic fraction (HF) of the tumors was assessed with the hypoxia-specific stain pimonidazole and semi-quantitative morphometrics. Tumor growth inhibition (TGI) was the read-out of antitumor activity. Localization of TH-302-induced DNA damage was by γH2AX immunohistochemistry. TH-302 demonstrated antitumor activity in nine of eleven xenograft models, including NSCLC, SCLC, melanoma, pancreatic, colon, fibrosarcoma and prostate cancer. The two xenografts in which TH-302 showed lower efficacy (SU.86.86 pancreatic and 786-O RCC) were highly vascular, well-perfused and exhibited very small hypoxic fractions. The relative antitumor activity between the xenograft models significantly correlated with the magnitude of the tumor hypoxia. In an efficacy study employing H460 NSCLC xenografts, the tumor-bearing animals were exposed to different oxygen concentrations (10%, 21% or 95%) using controlled atmospheric chambers. Oxygen concentration control began ½ hour before dosing and continued for 2 hr after dosing. A regimen of daily dosing for 2 weeks (Qdx5 x2wk) was employed. TGI was 77% in the 10%, 68% in the 21%, and 56% in the 95% oxygen groups. To assess TH-302 effects on hypoxic compartments, we measured the change of HF in the tumors after a single dose of TH-302. 48 hours after TH-302 treatment, tumor hypoxia in the Hs766t pancreatic cancer xenograft was significantly reduced (11.5% ± 1.4% in vehicle vs. 4.5% ± 0.9% in the TH-302 treatment group, p<0.05). Similar hypoxic volume reductions after TH-302 treatment were also observed in the other models (H82 SCLC, H460 NSCLC and A375 melanoma xenograft models). After TH-302 treatment, DNA damage was assessed by γH2AX and detected first in the pimonidazole-positive hypoxic regions of the tumors and then radiated outward in a time-dependent manner. Taken together, these results demonstrate TH-302 targets the hypoxia compartment selectively in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 535. doi:10.1158/1538-7445.AM2011-535