Abstract 2424: Association between Chk1 inhibitor AZD7762-mediated modulation of pharmacodynamic biomarkers and potentiation of hypoxia-activated prodrug TH-302 antitumor efficacy in a human tumor xenograft model
Authors: Sun Jessica D. , Meng Fanying , Liu Qian , Ahluwalia Dharmendra , Li Wenwu , Wang Yan , Bhupathi Deepthi , Hart Charles P. 
Published: August 2015

Abstract
Background: TH-302 is a 2-nitroimidazole triggered hypoxia-activated prodrug of the DNA cross-linker bromo-isophosphoramide mustard (Br-IPM). Checkpoint kinase 1 (Chk1) plays a key role in the activation of the DNA damage checkpoint in the cell cycle. We investigated the association between Chk1 inhibitor AZD7762′s-mediated modulation of select downstream pharmacodynamic (PD) biomarkers and potentiation of TH-302 efficacy in the HT29 xenograft model.
Material and methods: HT29 ectopic human colorectal xenograft model was employed. Six animals per group were treated with Vehicle (V), TH-302 (T) at 150 mg/kg on Day 1, AZD7762 (A) at 25 mg/kg on Day 1 and 2, or T + A. Two dosing schedules were used in the combination: 1), A was given first, and 4 h and 24 h later, T and A was dosed, respectively (ATA); and 2), T was given first, and 4 h and 24 h later, A was dosed (TAA). The PD biomarkers were evaluated by immunohistochemistry (IHC) on the samples collected on Day 3. In a separate efficacy study, ten animals per group were treated with V, T at 100 mg/kg on Day 1, 4/wk x 2wks, A at 20 mg/kg on Day 1, 2, 4 and 5/wk x 2wks, ATA or TAA combination. Antitumor activity was assessed by tumor growth inhibition (TGI).
Results: The biomarker study showed phosphorylated Chk1 (Ser345), γH2AX-positive cells (DNA damage), and Caspase-3 positive cells (apoptosis), were significantly increased in ATA, compared with V, A, T or TAA (p&lt;0.05). ATA-induced DNA damage occurred in 38% cells, which were mainly cancer cells (vs. 3%, 8%, 14% and 24% in V, A, T, and TAA, respectively). 12% of the cells underwent apoptosis after ATA, which was observed mainly in endothelial cells and activated fibroblasts of the tumor stroma. Tumor hypoxia was characterized with pimonidazole IHC and the endogenous marker CA-IX, which showed a strong co-localization of staining. The tumoral hypoxic fraction was reduced after ATA or TAA. In the efficacy study, T or A monotherapy did not show any effect. However, the efficacy was significant in the combination groups, in which ATA yielded TGI of 67%, compared to the TAA of 48%. The dosing schedule dependent antitumor activity was consistent with the PD biomarker observation. Maximal body weight loss in ATA was 1% vs. 6% in TAA.
Conclusion: The results suggest that the Chk1 inhibitor A potentiates the efficacy of T in a dose schedule-dependent manner, consistent with dose schedule dependent modulations of PD biomarkers. The detailed mechanistic analysis indicates that the sensitization is due to both disruption of the Chk1 mediated DNA damage checkpoint of the cell cycle and the induction of apoptosis. These studies provide additional support to the preclinical translational rationale for combining TH-302 with Chk1 inhibitors in the clinical trial setting.
Citation Format: Jessica D. Sun, Fanying Meng, Qian Liu, Dharmendra Ahluwalia, Wenwu Li, Yan Wang, Deepthi Bhupathi, Charles P. Hart. Association between Chk1 inhibitor AZD7762-mediated modulation of pharmacodynamic biomarkers and potentiation of hypoxia-activated prodrug TH-302 antitumor efficacy in a human tumor xenograft model. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2424. doi:10.1158/1538-7445.AM2015-2424