Abstract A22: Hypoxia-dependent antitumor activity of TH-302, a hypoxia-activated prodrug, in preclinical pancreatic xenograft models
Authors: Jessica D. Sun, Qian Liu, Dharmendra Ahluwalia, Yan Wang, Fanying Meng, Deepthi Bhupathi, John G. Curd, Mark D. Matteucci, Charles P. Hart
Journal: Clinical Cancer Research (2010)

Abstract TH-302 is a 2-nitroimidazole triggered hypoxia-activated prodrug (HAP). In hypoxic cells the DNA cross-linker bromo-isophosphoramide mustard (Br-IPM) is released. TH-302 demonstrates hypoxia-dependent cytotoxicty in cell-based in vitro assays with human tumor cell lines, efficacy both as monotherapy and combination therapy in human tumor xenografts in nude mice, and is currently in multiple clinical trials for the treatment of cancer, including pancreatic cancer. In the present study, we investigated the antitumor activity and mechanism of action of TH-302 in two pancreatic xenograft models. The Hs766t and SU.86.86 human pancreatic cancer lines reflect two different pancreatic adenocarcinoma cell phenotypes. The Hs766t cells are more mesenchymal, having passed through the epithelial- mesenchymal transition (EMT), while the SU.86.86 cell line retains a more epithelial differentiative state. By employing a histological and immunohistochemical approach, we show that the two cell types form xenograft tumors with distinct microenvironmental features. As characterized by CD31 and Hoechst 33342 staining, SU.86.86 is more highly vascularized and well-perfused than Hs766t. With the exogenous hypoxia marker pimonidazole, a much larger hypoxic volume is present in Hs766t tumors compared with that in SU.86.86 (11.5% ± 1.4% vs. 4.5% ± 0.9%, p&amp;lt;0.001, tumors ~250 mm3). We tested TH-302 and gemcitabine, both as single agents and in combination, in these two pancreatic cancer models with in vitro cell-based models and in vivo tumor xenograft models. In the ectopic xenograft models, TH-302 (75mg/kg, Q3Dx5, ip), gemcitabine (60mg/kg, Q3Dx5, ip) or the combination of TH-302 and gemcitabine was administered to the animals. The TH-302 was given 4 hours before the gemcitabine. The schedule of TH-302 and gemcitabine mirrors the clinical trial schedule where TH-302 and gemcitabine are given on the same day weekly (3 weeks on, 1 week off per cycle). In the Hs766t model, tumor growth inhibition (TGI) of TH-302 alone and gemcitabine alone was 75%, and 45%, respectively. When TH-302 was combined with gemcitabine, the antitumor activity was significantly enhanced, and TGI was 89% (p&amp;lt;0.001 vs. Vehicle and p&amp;lt;0.05 vs. gemcitabine alone). In contrast, gemcitabine alone was highly efficacious in the SU.86.86 xenograft model and showed TGI of 104%, while the antitumor activity of TH-302 monotherapy was modest and TGI was only 28% (the combination TGI was 105%). To characterize specific pharmacodynamic effects of TH-302 treatment on the tumors, we examined Hs766t tumors immunohistochemically 48 hours after a single dose of TH-302 treatment (150mg/kg, ip). The hypoxic volume was significantly reduced from 11.5% (n=9) to 4.6% (n=5) (p&amp;lt;0.01) after TH-302 treatment. The gemcitabine and TH-302 showed no synergy when tested together in vitro. The results support the hypothesis that the two agents (TH-302 and gemcitabine) work in a complementary manner by targeting two distinct tumor compartments and add support for the hypothesis of hypoxic compartment selectivity of TH-302's mechanism of action. Recently published studies have implicated the mesenchymal phenotype of the Hs766t model being associated with gemcitabine resistance. Thus the enhanced anti-tumor efficacy observed with the addition of TH-302 to the gemcitabine regimen in the xenografts may model inter-patient response differences to therapy and lead to biomarker strategies for patient stratification. Citation Information: Clin Cancer Res 2010;16(14 Suppl):A22.