In humans, Wnt signaling plays important roles in aspects of embryonic development. In adult tissues, Wnt signaling is tightly regulated to maintain stem cell niche homeostasis and tissue regeneration. Some cancers hijack the Wnt signaling pathway to promote different aspects of tumourigenesis, including colorectal, ovarian, breast cancer, hepatocellular carcinoma, glioblastoma etc. Despite the efforts made to find inhibitors of Wnt signaling, only a few have reached clinical trials and many exert toxicity. Drug repositioning is an attractive strategy to identify new uses for already approved drugs. Research in Prof Katanaev’s lab was the first to identify an anti-leprotic drug clofazimine (CFZ) as a promising Wnt signaling inhibitor in triple-negative breast cancer (TNBC). TNBC is a particularly aggressive subtype of breast cancer without targeted therapies.
This thesis is part of the CFZ project to explore its potential as a Wnt signaling inhibitor, with the aim of potentially repositioning it as a targeted therapy against the broad range of Wnt-dependent cancers. Our preclinical studies have shown that CFZ specifically inhibits canonical Wnt signaling in vitro and in vivo. In mouse TNBC models, CFZ demonstrated an additive combination effect with the chemotherapeutic agent doxorubicin, without additional toxicity. Further in vitro studies in a panel of cancer cell lines demonstrated that CFZ has the potential to be repositioned against a broad range of Wnt-dependent cancers. We observed that the anti-proliferative efficacy of CFZ in a given cell line correlated with the basal level of Wnt signaling activation and the degree of Wnt inhibition produced by CFZ. By optimizing the structure of CFZ, we identified several highly water-soluble CFZ derivatives with improved Wnt inhibition without worsening aspecific cytotoxicity. The derivative MU17 demonstrated on-target tumour growth suppression in mouse model of TNBC. Importantly, MU17 ameliorated the skin discolouration side effect of CFZ.
Despite its long history of clinical use, the mechanism of action of CFZ is still inconclusive. In addition, the newly discovered anti-Wnt properties of CFZ strongly suggest a novel and unexpected target. In this thesis, I have also attempted to identify the target(s) of CFZ within the Wnt signaling pathway. Using a pull-down assay followed by proteomic analysis, I have narrowed down a list of fifteen putative CFZ interacting proteins as potential target(s). However, further work is needed to verify the target of CFZ.
The results of this thesis have a high potential for subsequent clinical impact. Our preclinical data suggest that CFZ is suitable and effective in a combination treatment with chemotherapy and is ready to be tested in clinical trials in TNBC patients. The in vitro efficacy of CFZ against a broad range of Wnt-dependent cancers opens up a new area of research for repositioning CFZ to other cancers beyond TNBC. With the prospect of identifying CFZ’s target within the Wnt signaling pathway and describing its Wnt inhibitory mechanism of action, we hope to provide valuable insights into the design of new Wnt inhibitors for cancer drug development.
