Abstract B30: KRAS wild type tumors exhibit increased efficacy to the combination of AZ1 (a tankyrase inhibitor) + irinotecan in a patient-derived CRC explant model.

Background: Dysregulation of the canonical Wnt signaling pathway has been implicated in colorectal cancer (CRC) development as well as incipient stages of malignant transformation. An important regulator of the Wnt pathway are tankyrases that function in reducing Axin2 levels which is an essential protein in the β-catenin destruction complex. Targeting this pathway with a potent tankyrase inhibitor presents a plausible approach in drug development. In this study, we evaluated the antitumor effects AZ1 (a novel tankyrase 1/2 inhibitor which also carries PARP1 activity) as a single agent and in combination with irinotecan in our patient derived CRC explant xenograft models. Methods: Twenty patient derived CRC explant xenografts were treated with vehicle, AZ1 (50mg/kg/day), irinotecan (30mg/kg/week) or AZ1 (50mg/kg/day) + irinotecan (30mg/kg/week) for 28 days. Pharmacokinetic effect of AZ1 was determined in plasma and tumor over 24h. Both pharmacodynamic effects (Axin2 levels) and changes in biomarkers relevant to the Wnt pathway were determined by Western blotting. Intra-nuclear β-catenin levels were evaluated by immunohistochemistry. A tumor growth index (TGI) was calculated as the ratio in tumour volumes in the treated animals and the control animals (T/C) on the last day of drug treatment to determine efficacy. TGI≤ 50% was considered sensitive and TGI > 50% was resistant. Results: Three out of 20 CRC explants showed some tumour growth inhibition with AZ1. In animals bearing CRC40 xenografts, AZ1 showed a Tmax of 1 hour in plasma and 30 minutes in tumor. In addition, Axin2 stabilization post AZ1 started 15 minutes after drug administration, with the maximum stabilization observed at 8 hours. Treatment with AZ1 + irinotecan resulted in a greater anti-tumor effect (achieving stasis or better) than either agent alone in 4 out of 20 CRC xenografts. A significant association (fisher exact test: p= 0.007) was identified between combinational sensitivity and KRAS wild type. Evaluation of tumors after treatment showed marked stabilization of Tankyrase 1/2 and Axin2; however, active β-catenin levels were not changed. Also, there were no changes to intra-nuclear β-catenin (IHC) levels after treatment. Interestingly, NuMa, a protein involved in maintenance of the mitotic spindle and a target of Tankyrase 1/2 increased following irinotecan in 3 models, 2 showing a combination effect and 1 which did not. Numa levels decreased with the combination of AZ1 and irinotecan compared to irinotecan alone in 2 models showing a significant combination effect. Conclusion: Combination AZ1 and irinotecan achieved greater anti-tumor effect compared to monotherapy in 4 out of 20 CRC xenografts. Activity was limited to CRC xenografts with KRAS WT status. Further studies are warranted to validate these findings in KRAS WT patients and to understanding the specific mechanisms in tumors that responded to combinational therapy. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B30. Citation Format: WM Tai, Kevin Quackenbush, Alicia Purkey, Stacey Bagby, Wells Messersmith, Eun Kee Song, Todd Pitts, John J. Arcaroli. KRAS wild type tumors exhibit increased efficacy to the combination of AZ1 (a tankyrase inhibitor) + irinotecan in a patient-derived CRC explant model. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B30.