Abstract 5196: The discovery of small molecule inhibitors of Tankyrases 1 and 2, which modulate Axin homeostasis and inhibit Wnt signaling in vivo

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Deregulated Wnt signaling has been implicated in a wide range of cancer types, both through truncating mutations in the tumor suppressor protein Adenomatous Polyposis Coli (APC), most prevalent in colorectal cancer, as well as overexpression of Wnt ligands and receptors. Canonical Wnt signaling involves the regulated degradation of the beta catenin protein. In the absence of Wnt signaling, cytosolic beta catenin levels are maintained at low levels, as a result of its phosphorylation-dependent ubiquitination and subsequent proteasomal degradation. This degradation is mediated by the ‘destruction complex’, whose constituents include Glycogen Synthase Kinase 3 (GSK3β), Casein Kinase 1 (CK1) and the scaffolding proteins Axin and APC. The efficient assembly of this complex depends upon the steady state levels of its components, such as Axin, which when overexpressed has been shown to decrease beta catenin levels and inhibit Wnt signaling. Recent reports suggest that the poly-ADP-ribosylating Tankyrase enzymes can bind to and regulate Axin levels, hence providing promising targets for treating Wnt-dependent tumors. We have identified potent, low nM small molecule inhibitors of the Wnt pathway, which inhibit Tankyrases 1 and 2, stabilize Axin, deplete beta catenin protein levels and modulate a set of Wnt-regulated genes in colorectal cell lines in a similar manner as seen when using siRNA to beta catenin. Furthermore, using siRNAs to Tankyrase 1 and 2 we have shown that co-depletion of both enzymes is required for Axin stabilization and Wnt pathway inhibition. We have also successfully demonstrated that our lead molecules cause prolonged Axin stabilization and Wnt pathway inhibition in vivo following oral BID dosing in colon xenografts carrying APC mutations. However, in contrast to previous reports, using molecular tools and our lead compounds (e.g. inducible Axin cell lines, compound rescue experiments using siRNAs to Axin 1/2), we have shown that Axin stabilization has no effect on the viability/proliferation of APC mutant colorectal cell lines, under conditions where siRNA to beta catenin was able to cause significant growth inhibition. Some potential mechanistic rationales for this lack of phenotypic effect will be presented. Understanding the consequences of Tankyrase inhibition in vivo in relevant disease models will be important to establish the clinical utility of these molecules in colorectal cancer and beyond. 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 5196. doi:10.1158/1538-7445.AM2011-5196