Abstract C211: Therapeutic utility of GSK‐3 inhibitors for the treatment of cancer

Glycogen synthase kinase‐3 (GSK‐3) is a constitutively active protein kinase that plays a role in diverse signaling pathways. Initially described as a regulator of glycogen synthesis and metabolism, GSK‐3 is now known to play a central role in a variety of cellular functions ranging from maintenance of stem cell function to regulation of cell cycle and apoptosis. In mammals, there are two highly homologous isoforms of GSK‐3 (GSK‐3α and GSK‐3β) which share substrate specificity and functional properties. GSK3β is one of the key components of the β‐catenin destruction complex and phosphorylation of β‐catenin by GSK‐3 targets the substrate protein for ubiquitylation and proteosomal destruction ‐ conversely, inhibition of GSK‐3β activity results in loss of phosphorylation sites and stabilization of β‐catenin in the cellular cytoplasm and nucleus. GSK‐3 expression and/or activity have been reported to be elevated in multiple tumor types, including pancreatic, gastric and colorectal carcinomas. Equally important, inhibition of GSK‐3 expression in colorectal tumor cell lines via specific siRNAs induced apoptosis and attenuated the proliferation of tumor cells. Together, these findings suggest that GSK‐3 inhibitors may represent a new class of therapeutic agents against colorectal cancer. LY‐GSK‐3i is a novel, potent and selective ATP competitive inhibitor of both isoforms of GSK‐3. In vitro, LY potently inhibits the enzymatic activity of GSK‐3α and GSK‐3β with IC50s of 1.5 nM and 0.9 nM, respectively. Pharmacologic activity of other GSK‐3 inhibitors in the literature against cancer is not clear. LY‐GSK‐3i did not evidence useful antitumor activity in its own right against colorectal, ovarian or non‐small cell lung (NSCL) tumor lines. However, when combined with different chemotherapeutic agents, LY was found to potentiate their pro‐apoptotic activity. LY‐GSK‐3i was shown to significantly potentiate caspase‐3 activation when combined cisplatin in HCT‐116 and Colo‐205 colorectal carcinoma lines. Enhancement of caspase‐3 activity was also observed when LY‐GSK‐3i was combined with 5‐fluorouracyl (5‐FU) in HCT‐116 cells. In order to explore the potential of LY‐GSK‐3i in other tumor types, we assessed the pro‐apoptotic activity of the LY/cisplatin combination against ovarian (A2780) and lung (A549 and NCI‐H460) cell lines. In all cases, LY‐GSK‐3i was shown to significantly elevate caspase‐3 activity when compared to cisplatin alone. In vivo, administration of a single dose of LY‐GSK‐3i to xenograft‐bearing mice lead to a time‐and dose‐dependent accumulation of the substrate protein ‐catenin in tumor tissues. In vivo xenograft growth studies revealed that administration of 5 mg/kg LY‐GSK‐3i in combination with cisplatin, given every 7 or 14 days, was sufficient to significantly improve the efficacy as compared to cisplatin alone given at the same intervals. Together, our findings provide evidence for the potential therapeutic utility of GSK‐3 inhibitors for the treatment of cancer. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C211.