Abstract IA14: Targeting the Wnt signaling pathway in cancer
Authors: McLaughlin Margaret 
Published: May 2016

Abstract
Mutations in the adenomatous polyposis coli (APC) gene, resulting in aberrant Wnt pathway activation, were first identified in hereditary and sporadic colorectal cancers in 1991. Today, 24 years later, there are still no approved Wnt pathway inhibitors on the market. In contrast, oncogenic BRAF mutations were discovered in various solid tumors in 2002. Within 9 years the first RAF inhibitor, vemurafenib, received FDA approval for the treatment of BRAF-mutant metastatic melanoma. Why has it been so difficult to develop Wnt pathway inhibitors?
The first and most significant challenge has been to identify protein targets within the Wnt pathway that are amenable to pharmacologic inhibition. Other prominent signaling pathways dysregulated in cancer (for example, the MAPK pathway) consist of a series of activating phosphorylation events carried out by kinases, many of which can be inhibited by small molecules. However, the critical regulatory event in the Wnt pathway is the degradation of the transcriptional co-activator β-catenin by the β-catenin destruction complex, composed of APC, Axin, casein kinase 1 (CK1), and glycogen synthase kinase 3 (GSK3). Instead of activating the pathway, the kinases CK1 and GSK3 serve an inhibitory role, sequentially phosphorylating the amino terminus of β-catenin, triggering its ubiquitination and degradation. Confident that the Wnt pathway was important for tumorigenesis, albeit difficult to drug, Novartis initiated a high-throughput screen to identify small molecule inhibitors of Wnt secretion without a priori knowledge of their protein targets. This effort yielded WNT974, a potent and selective inhibitor of Porcupine, the O-acyltransferase that palmitoylates Wnt ligands, enabling them to be secreted.
With the first synthesis of WNT974 in 2009, and efforts by Novartis and others to target additional upstream Wnt pathway components (including Frizzled, LRP6 and R-spondin), a second challenge had to be faced. Upstream Wnt pathway inhibitors are unlikely to be efficacious in tumors harboring downstream mutations in APC or β-catenin. Which, if any, human cancers are Wnt ligand-dependent, and therefore, likely to be sensitive to upstream Wnt pathway inhibitors? Recent insights into how the R-spondin/Lgr5/Rnf43 module regulates surface levels of Wnt receptors provided a critical first step towards a patient selection strategy. We now know that inactivating mutations in the Wnt antagonist RNF43 occur frequently in microsatellite-unstable colorectal, gastric, and endometrial cancers (80%, 55%, and 51%, respectively), pancreatic ductal adenocarcinoma (10%), and liver fluke-associated cholangiocarcinoma (9%). Its homolog ZNRF3 is commonly mutated in adrenocortical carcinoma (21%), and fusions that increase the expression of the Wnt agonists RSPO2 or RSPO3 are reported in 10% of colorectal cancers. Our preclinical studies in cell line and patient-derived xenograft models have confirmed that inactivating mutations of RNF43 or R-spondin fusions confer Wnt ligand dependency.
Currently, a Phase 1 clinical trial with WNT974 is underway in patients with advanced cancer. Preliminary clinical data suggest WNT974 has a manageable safety profile, with dysguesia (49%) as the most common adverse event suspected to be drug related. Based on pharmacokinetic, pharmacodynamic and tolerability data, a recommended dose for expansion of 10 mg QD has been selected. In response to the emerging scientific evidence implicating the R-spondin/Lgr5/Rnf43 module in a wide variety of cancers, we adapted the clinical trial design, restricting the expansion part to patients harboring upstream Wnt pathway mutations. While there was no evidence of WNT974 anti-tumor activity in the dose escalation, one patient in the dose expansion with an RNF43-mutant appendiceal cancer had stable disease with tumor shrinkage of -27%, raising the possibility that WNT974 may have anti-tumor activity in a molecularly selected population. The intense efforts to understand the basic biology of the Wnt pathway and Wnt-driven cancers have made progress toward development of Wnt pathway inhibitors possible.
Citation Format: Margaret McLaughlin. Targeting the Wnt signaling pathway in cancer. [abstract]. In: Proceedings of the AACR Special Conference: Developmental Biology and Cancer; Nov 30-Dec 3, 2015; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(4_Suppl):Abstract nr IA14.