Colorectal cancer (CRC) is one of deadliest malignancies and approximately 25% of patients present with metastatic disease at the time of diagnosis, necessitating efficacious therapeutic options. Treatment of CRC has been greatly improved by targeted therapies such as the FDA-approved multi-kinase inhibitor, regorafenib, which elicits its anti-cancer effects by inducing apoptosis. Ours and other studies identified the anti-apoptotic Mcl-1 protein as a significant resistance factor to regorafenib treatment in CRC, associated with inactivating mutations in the tumor suppressor F-box and WD repeat domain-containing 7 (FBW7). These mutations result in abrogated Mcl-1 degradation, which contributes to survival of cancer cells and ineffective regorafenib induced apoptosis. Therefore, there is sound rationale for Mcl-1 inhibition to circumvent FBW7 inactivity and restore the efficacy of regorafenib. Our group has discovered and developed several classes of selective small molecule Mcl-1 inhibitors that bind in the conserved BH3 groove. In HCT116 colon cancer cells, the lead compound, 483LM, engages endogenous Mcl-1 as demonstrated by a CETSA assay and leads to a rapid accumulation of Mcl-1 protein levels, indicative of protein stabilization. 483LM induces BAX/BAK dependent apoptosis in cancer cells, upregulates the pro-apoptotic protein, Noxa, and displays favorable in vivo DMPK properties. Using a genetic Mcl-1 knock-in (KI) CRC cell line to mimic the effect of FBW7 loss, we confirmed the in vitro resistance of these KI cells to single agent regorafenib treatment and restored apoptosis when treated in combination with 483LM. Importantly, the in vivo combination treatment of 483LM and regorafenib completely overcome Mcl-1-mediated regorafenib resistance in HCT116 KI CRC xenograft tumors. Pharmacodynamic evaluation of tumor samples revealed significant caspase-3 activation in 483LM treated tumors. The proximity ligation assay revealed the ability of 483LM to induce significant disruption of PUMA:Mcl-1 complexes in treated tumor samples. These data confirm the previous reports of the involvement of Mcl-1 in regorafenib resistance in CRC and provides rationale for further investigation into the clinical application of pharmacological Mcl-1 inhibition in combination therapies to combat resistance. In addition, these results provide evaluation and characterization of a novel Mcl-1 selective small molecule inhibitor with potent in vivo activity and potential to treat various malignancies.

Citation Format: Karson J. Kump, Jingshan Tong, Ahmed S. Mady, Lei Miao, Siwei Li, Duxin Sun, Jian Yu, Lin Zhang, Zaneta Nikolovska-Coleska. Novel Mcl-1 inhibitor effectively restores in vivo regorafenib sensitivity in resistant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1970.

©2018 American Association for Cancer Research.
2018
American Association for Cancer Research.