An oral dual inhibitor of IDO and TDO enhances anti-cancer immunity and synergizes with immune checkpoint blockade

Abstract

Background: Indolamine 2,3-dioxygenase (IDO) blockade is a new therapeutic strategy to enhance cancer immunity. However, IDO blockade alone cannot completely block the immunosuppressive tryptophan-kynurenine (Trp-Kyn) pathway in the tumor microenvironment. Recent studies have demonstrated that Trp 2,3-dioxygenase (TDO) is an alternative enzyme employed by various tumors that can be used as a target for the Trp-Kyn pathway; therefore, here we developed an orally available dual inhibitor that targets IDO and TDO. Methods: Small-molecule inhibitors for IDO and TDO were synthesized and screened by in vitro IDO/TDO enzyme and cell-based assays. CT26 colon or 4T1 breast tumorbearing mice were treated with CB548 either alone or in combination with an anti-PD1 antibody. We monitored tumor growth and analyzed the tumor microenvironment using flow cytometry, qPCR, and confocal microscopy. Results: A lead compound, CB548, showed potent inhibition of IDO and TDO in the enzyme and cell-based assays with various human and murine cancer cell lines. Oral administration of CB548 revealed a good pharmacokinetic profile, and the conversion of Trp to Kyn in tumors was effectively suppressed. Moreover, the CB548 monotherapy revealed a dose-dependent inhibition of CT26 colon or 4T1 breast cancer growth as well as markedly increased CD8+ T cell infiltration in the tumor microenvironment. Additionally, the combination immunotherapy of CB548 and anti-PD1 antibody suppressed tumor growth to a greater extent than did the monotherapy, and led to durable tumor regression. There was no significant systemic toxicity with the CB548 treatment. Conclusions: Overall, our study demonstrates that CB548, a novel IDO/TDO dual inhibitor, can elicit a robust anti-cancer immunity and synergistically inhibit cancer progression in combination with an immune checkpoint inhibitor.