Abstract 1359: Selective HDAC6 inhibition by GB-1101 revokes tumor immune privilege and synergizes with immune checkpoint therapies to induce tumor regressions

HDAC6 is a cytoplasmic class IIB HDAC isoenzyme that is involved in the deacetylation of cytoplasmic proteins. HDAC6 also binds ubiquitinated proteins, facilitating the formation of the aggresome, to remove misfolded proteins. Previously, HDAC6-selective inhibitors were shown to decrease immunosuppression and enhance immune function of melanoma patient T-cells demonstrating an important role for HDAC6 inhibition for cancer immunotherapy. GB-1101 is a novel, orally bioavailable, nanomolar potent inhibitor of HDAC6. GB-1101 tested in vitro did not induce direct cytotoxicity to CD4+/CD8+ or NK cells at concentrations up to 300 nM. RNA-seq on GB-1101 exposed MHC-null human MSS colon cancer cell lines reveal potent induction of MHC Class I/II genes and expression numerous cancer neoantigens. To test the activity of GB-1101 in vivo, we established large (~450mm3) 4T1-luc syngeneic tumors and initiated treatment of GB-1101 given orally once daily alone and combined with anti-PD1, anti-CTLA4, and anti-PD1/anti-CTLA4 combination. The activity of anti-PD1 in this model was minimal (TGI: 3%); GB-1101 as a single agent showed more potent anti-tumor activity (TGI: 50%) compared to anti-PD1. We observed synergistic benefit when GB-1101 was combined with anti-PD1 (Combination TGI: 78%). GB-1101 showed comparable anti-tumor activity to single agent anti-CTLA4 (TGI: 43%) but revealed significant tumor regressions of large and well-established tumors when combined with anti-CTLA4 (Combination TGI: >100%). As a single agent, GB-1101 was superior to anti-PD1+anti-CTLA4 (TGI: 7%) and induced significant tumor regressions when combined with anti-PD1+anti-CTLA4 (Triple Combination TGI: >100%). In the TC-1, HPV-positive syngeneic model, GB-1101 reprogrammed the tumor immune microenvironment reducing the number of M-MDSC and TReg cells and increase the numbers of CD4+/CD8+ and NK cells. Taken together, these data support GB-1101 as an important new targeted epigenetic immunomodulator able to revoke immune privilege to enhance the clinical activity of immune checkpoint therapies. Citation Format: Kari Kotlarczyk, Mario Sepulveda, Tong Wang, Maggie Murray, Paul Gonzales, Stephen Thomas Gately. Selective HDAC6 inhibition by GB-1101 revokes tumor immune privilege and synergizes with immune checkpoint therapies to induce tumor regressions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1359.