Recent studies have suggested that pan inhibitors of dipeptidyl peptidase-4 activity and/or structure homologs (DASH), including
ARI-4175, can mediate
tumor regression by immune-mediated mechanisms. This study assessed the potential of combining
ARI-4175 with
cancer vaccines. We evaluated ARI-4175's effect on immunogenic modulation, ability to sensitize
tumor cells to
antigen-specific CTL killing, effect on immune-cell subsets and function, and antitumor activity in 2
tumor models, both as a monotherapy and in combination with a recombinant viral or dendritic cell (DC)-based
tumor-cell
vaccine. ARI-4175's effects on the growth, surface phenotype, and
antigen-specific CTL-mediated lysis of murine and human
carcinoma cell lines were assessed in vitro. In vivo, C57BL-6 mice were treated orally with
ARI-4175, after which splenocytes were assessed by flow cytometry and functional assays. Antitumor studies were performed in murine models of colon
carcinoma (MC38-CEA(+) in CEA-transgenic C57BL-6 mice) and
rhabdomyosarcoma (M3-9-M in C57BL-6 mice). Mice received oral
ARI-4175 alone or in combination with a
vaccine consisting of recombinant
vaccinia/
fowlpox CEA-TRICOM (colon model) or a DC-based
tumor-cell
vaccine (
rhabdomyosarcoma model). Exposure to
ARI-4175 had no effect on the proliferation or viability of
carcinoma cells in vitro; however, it did alter
tumor phenotype, making murine and human
tumor cells more sensitive to
antigen-specific CTL killing. Assessment of immune-cell subsets and function indicated that
ARI-4175 increased levels of natural killer cells and DCs. Detrimental immune effects, including reduced T effector cells and increased immunosuppressive cells (Tregs, MDSCs), were normalized when treatment stopped, suggesting that scheduling is critical when combining this agent with
vaccine. As a monotherapy,
ARI-4175 had potent antitumor activity in both
tumor models, and had even greater effects when combined with a
vaccine (either DC-based or poxviral vector based). These findings provide the rationale for the combined use of
cancer immunotherapy with DASH
enzyme inhibitors such as
ARI-4175.