Human colon cancer, except for the small subset of microsatellite instable (MSI) colon cancer, does not respond to anti-PD-L1/PD-1 immune checkpoint inhibitor (ICI) immunotherapy. A well-established notion is that MSI colon cancer harbors various mutations that serve as neoantigens to generate tumor-specific cytotoxic T lymphocytes (CTLs). Because CTLs suppress tumor through inducing tumor cell apoptosis, we hypothesized that colon cancer cell intrinsic apoptosis resistance is potentially another mechanism underlying microsatellite stable (MSS) colon cancer nonresponse to ICI immunotherapy. If tumor cells are not sensitive to apoptosis induction, then tumor cells cannot be killed by CTLs regardless of how potent the CTLs are. To test this hypothesis, we analyzed MSI and MSS human colon carcinoma specimens and developed a small molecule that targets H3K9me3-mediated cell death pathways as a sensitizer for colon cancer ICI immunotherapy. As expected, we observed high level of CTL infiltration in all eight MSI colon carcinoma specimens examined. However, five of the nine MSS colon carcinoma specimens also exhibited high level of CTL infiltration, suggesting that CTL level is not the sole mechanism underlying MSS colon cancer non-response to ICI immunotherapy. Analysis of the TCGA database revealed that SUV39H1, a histone methyltransferase that catalyzes H3K9me3, is significantly elevated in human colon carcinoma specimens compared to normal human colon tissues. A SUV39H1 structure-based virtual chemical library screening in combination with functional assays identified a small-molecule SUV39H1 inhibitor. Further analysis of the top forty-three hits identified a potent SUV39H1 inhibitor. Structure modifications were then performed and a chemical synthesis procedure was developed to synthesize a novel SUV39H1 inhibitor, termed F5446. F5446 has an EC50 of 0.496 µM against SUV39H1 enzymatic activity and suppresses human colon carcinoma cell growth in a dose-dependent manner in vitro. F5446 suppresses colon carcinoma growth through upregulating key cell cycle regulators to induce cell cycle arrest at the S phase. Furthermore, F5446 also decreased H3K9me3 at the FAS promoter to upregulate Fas expression. Consequently, a sublethal dose of F5446 effectively overcame human colon carcinoma cell resistance to apoptosis induced by FasL, a ligand on activated CTLs in vitro. Treatment of colon carcinoma-bearing mice with F5446 and anti-PD-1 both significantly suppress tumor growth in vivo. Our data determined that F5446 is a novel SUV39H1 inhibitor that has the potential to be further developed as a sensitizer to overcome colon carcinoma resistance to ICI immunotherapy through activating the Fas-FasL pathway.

Citation Format: Chunwan Lu, Dafeng Yang, John D. Klement, Aaron H. Colby, Mark W. Grinstaff, Cedric Pearce, Nicholas H. Oberlies, Thomas Albers, Iryna Lebedyeva, Kebin Liu. The SUV39H1-H3K9me3 axis mediates colon carcinoma cell intrinsic apoptosis and immune evasion [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 4966.