Abstract 1358: The selective HDAC6 inhibitor ITF3756 stimulates an antitumor immune response and leads to tumor regression in combination with anti CTLA-4 antibody in a colon carcinoma murine model

Stimulation of tumor immune response with antibodies blocking the repressive activity of immune checkpoints gives impressive results with long-lasting tumor regression that is achieved regrettably only in a fraction of patients. To overcome these suboptimal results, different targets and combination therapies are the aim of active non-clinical and clinical research. Importantly, clinical trials of combination therapies have demonstrated better efficacy but also an increase of adverse events. HDAC6 is a member of the Zn-dependent histone deacetylase family of enzymes with peculiar structure and functions. In contrast to what is observed with many other HDAC family members, HDAC6 KO mice are viable and have no peculiar phenotype indicating that selective HDAC6 inhibitors should be well tolerated. Since HDAC6 is an obligate regulator of cytokine-induced PD-L1 expression, HDAC6 inhibitors might have a role in modulating tumor immune responses. We here report the in vivo efficacy of ITF3756, a potent and selective HDAC6 inhibitor that reduces in vitro the expression of PD-L1 on human monocytes and on CD8 T cells, counters immune exhaustion in CD8 T cells but shows no direct cytotoxic effects on a panel of murine tumor cell lines. In syngeneic tumor models ITF3756 showed anti-tumor activity that was comparable to the efficacy of an anti PD1 antibody and that went along with increased immune cell infiltration and the generation of tumor-reactive CD8 and CD4 cells. ITF3756 was inactive in immune-deficient animals and selective depletion of either CD8 or CD4 cells severely blunted the antitumor activity of ITF3756. In combination with an anti CTLA-4 antibody, ITF3756 lead to a complete tumor eradication in 50% of the animals. Re-challenge of these animals did not result in tumor growth indicating that the combination therapy had elicited tumor immunity. Blocking the PD-1/PD-L1 axis either alone or in combination with inhibition of CTLA4 in NOD mice strongly accelerated the development of auto-immune diabetes that was lethal in some treatment groups. In contrast neither ITF3756 alone nor the combination with anti CTLA4 lead to any significant induction of diabetes despite the activity of ITF3756 on the PD-1/PD-L1 axis. We conclude that ITF3756 induces an in vivo antitumor immune response and a durable antitumor activity when combined with an anti CTLA-4 antibody. Neither ITF3756 monotherapy nor the combination with anti CTLA4 accelerated the induction of autoimmune