Abstract 4465: In vivo antitumor characterization of the hypoxia-activated prodrug TH-302 in a preclinical SCLC xenograft model

Abstract

Abstract The hypoxia activated prodrug TH-302, has showed anti-tumor activity in a variety of preclinical in vivo models. In hypoxic conditions, TH-302 acts as a DNA crosslinking agent by the reductase- and oxygen concentration-dependent release of bromo-IPM. In the present study, we investigated the anti-cancer mechanisms of TH-302 in a small cell lung cancer (SCLC) xenograft model with a focus on selective effects on the hypoxic compartment and tumor vasculature. The ectopic xenograft model was set up by s.c. implantation of 5×106 H82 human SCLC cells into the flank of nude mice. When tumor size reached 500 to 700mm3, animals were randomized (5-8 mice/group) and were injected i.p. with a single dose of 150mg/kg TH-302 or vehicle. Tumors were harvested for frozen sections 24, 48 and 72 hours after TH-302 treatment. Pimonidazole, for detection of hypoxia, was administered at 60mg/kg i.p. 1 hour before sacrifice, and to assess the effect of TH-302 on tumor vascular perfusion, Hoechst 33342 was injected at 10mg/kg i.v. into the tail vein 1 min before sacrifice. Anti-CD31 antibody was utilized as a marker for the microvasculature. Morphometric analysis was conducted by point counting and with Image-Pro Plus 6.0 software. Previous studies showed that TH-302 as a single agent, exhibited efficacy in the H82 xenograft model with tumor growth inhibition at about 55%. In the current study, anti-tumor activity was evaluated by assessment of tumor necrosis after H&E staining. The necrotic fraction in the vehicle treated tumor was 7.3% and reached 17.2%, 37.9% and 34.8% after 24, 48 and 72hrs of TH-302 treatment, respectively. A significant increase of necrosis occurred from 48hrs after the treatment (p<0.05). The pimonidazole-positive hypoxic fraction decreased after TH-302 treatment and reached a significant difference at 48hrs (6.3% ± 1.2% in vehicle vs. 1.8% ± 1.1% in the 48hr treatment group, p<0.05). Tumor vessel density and vascular perfusion in the non-necrotic area, as characterized by CD31-positive microvasculature cells and Hoechst 33342-positive cells, showed no significant difference between vehicle and any of the treated groups. There was significant correlation between tumor vessel density (CD31) and tumor vessel flow (Hoechst 33342) (R2=0.28, p<0.05). The results demonstrate an antitumor effect of TH-302 through cytotoxic activity on the hypoxic compartment in the tumor. In addition, a bystander effect is likely, given the observation that the necrosis fraction was 38% 48hrs after TH-302 treatment while the hypoxic fraction was less than 8%. Unlike tirapazamine, another hypoxia-activated prodrug which can cause vascular dysfunction, TH-302 appears to exhibit minor effects on the vasculature. The results support an in vivo tumor hypoxia targeting mechanism of TH-302. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4465.