Indole-pyridinyl-ethanones as novel inhibitors of indoleamine-2,3-dioxygenase (IDO), a promising target for anti-cancer immunotherapy

Immunotherapy is a promising novel and validated strategy for cancer therapy. It consists of the therapeutic vaccination of patients to stimulate their (natural) immune system against cancer cells. However, this approach showed a limited efficacy in vivo because cancer cells develop enzymatic mechanisms allowing tumours to resist or escape immune rejection. Among the enzymes involved, indoleamine 2,3-dioxygenase (IDO) was identified as a potential actor. IDO catalyses the rapid degradation of tryptophan (Trp) into N-formylkynurenine. This results in a local Trp depletion that severely affects T-cells proliferation and is thereby deeply immunosuppressive. Recently, the team of Prof. Van den Eynde demonstrated that many human tumours express IDO in a constitutive manner and that this expression allows cancer cells to escape immune rejection. IDO was thus clearly identified as an attractive target for the development of inhibitors. The recent elucidation of the three-dimensional structures of IDO, in complex with phenylimidazole and the cyanide ion (CN¯), provide important results for the structure-based drug discovery and design of novel IDO inhibitors. In the present work, we applied virtual screening for the discovery of new IDO inhibitors. As a result, five novel scaffolds with inhibitory potencies in the micromolar range were identified. Among these, the most promising candidate (1: IC50 = 65 µM) was selected and its inhibitory potency improved by chemical modifications. This led to a 7-fold improvement of the inhibitory potency of the hit selected. In this communication, the identification of 1, the synthesis and biological evaluation of a series of analogues as well as a modeling study explaining the SAR's will be presented.