Rational Design of 4-Aryl-1,2,3-Triazoles for Indoleamine 2,3- Dioxygenase 1 Inhibition Ute F. Rö hrig, †,‡,# Somi Reddy Majjigapu, †,§,# Aure ́ lien Grosdidier, ‡ Sylvian Bron, ‡,∥ Vincent Stroobant, ⊥ Luc Pilotte, ⊥ Didier Colau, ⊥ Pierre Vogel, §,† Benoît J. Van den Eynde, ⊥ Vincent Zoete,* ,‡ and Olivier Michielin ∥, * ,†,‡ † Ludwig Center for Cancer Research of the University of Lausanne, CH-1015 Lausanne, Switzerland ‡ Swiss Institute of Bioinformatics, Molecular Modeling Group, CH-1015 Lausanne, Switzerland § Laboratory of Glycochemistry and Asymmetric Synthesis, Ecole Polytechnique Fe ́ de ́ rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ∥ Pluridisciplinary Centre for Clinical Oncology (CePO), Centre Hospitalier Universitaire Vaudois (CHUV), Lausanne, Switzerland ⊥ Ludwig Institute for Cancer Research, Brussels Branch, and de Duve Institute, Universite ́ Catholique de Louvain, B-1200 Brussels, Belgium * S Supporting Information ABSTRACT: Indoleamine 2,3-dioxygenase 1 (IDO1) is an important therapeutic target for the treatment of diseases such as cancer that involve pathological immune escape. Starting from the scaffold of our previously discovered IDO1 inhibitor 4-phenyl-1,2,3- triazole, we used computational structure-based methods to design more potent ligands. This approach yielded highly efficient low molecular weight inhibitors, the most active being of nanomolar potency both in an enzymatic and in a cellular assay, while showing no cellular toxicity and a high selectivity for IDO1 over tryptophan 2,3-dioxygenase (TDO). A quantitative structure−activity relationship based on the electrostatic ligand−protein interactions in the docked binding modes and on the quantum chemically derived charges of the triazole ring demonstrated a good explanatory power for the observed activities. ■ INTRODUCTION Many tumors develop the capacity to actively suppress a potentially effective immune response. 1 The enzyme indole- amine 2,3-dioxygenase 1 (IDO1, EC 1.13.11.52) is one of the key players in this pathological immune escape and has therefore been selected as a therapeutic target for pharmaco- logical interventions. 2−4 IDO1 catalyzes the initial and rate- limiting step in the catabolism of tryptophan (Trp) along the kynurenine pathway. 5,6 By depleting Trp and accumulating Trp catabolites, IDO1 exerts a local immunosuppressive effect on T- lymphocytes. 7−10 The observations that many human tumors constitutively express IDO1 11 and that increased IDO1 expression in tumor cells is correlated with poor prognosis for survival in several cancer types 12 led to the hypothesis that its inhibition might enhance the efficacy of cancer treatments. Indeed, results from in vitro and in vivo studies have suggested that the efficacy of therapeutic vaccination or chemotherapy may be improved by concomitant administration of an IDO1 inhibitor. 11,13−16 Very recently, it has been shown that the functionally related enzyme tryptophan 2,3-dioxygenase (TDO) may be a complementary anticancer target. 17,18 The IDO1-like protein Indoleamine 2,3-dioxygenase 2 (IDO2) 19,20 shares 44% of sequence homology with IDO1. However, its physiological role remains unclear due to (i) very low Trp degradation activity, (ii) the presence of poly- morphisms abolishing its enzymatic activity in about 50% of Caucasians, and (iii) the presence of multiple splice variants. 21−23 IDO1 is an extrahepatic heme-containing enzyme that displays less substrate specificity than TDO. 6 In the first step of the catalytic cycle, IDO1 binds both the substrate and molecular oxygen in the distal heme site. The enzyme catalyzes the cleavage of the pyrrole ring of the substrate and incorporates both oxygen atoms before releasing N-formyl kynurenine, which is subsequently hydrolyzed to kynurenine by a cytosolic formamidase. 24 The two available crystal structures of IDO1 include the heme-bound ligands cyanide and 4- phenylimidazole (PIM), respectively. 25 Mutant analyses showed that none of the polar amino acid residues in the distal heme site are essential for the activity of the enzyme, suggesting a reaction mechanism involving only the substrate and the dioxygen molecule. 25−27 In the active form of IDO1, the heme iron is in its ferrous state (Fe 2+ ), while in its inactive form, the heme iron is in the ferric (Fe 3+ ) state. Formally, the catalytic cycle of IDO1 does not alter the oxidation state of the Received: February 24, 2012 Published: May 22, 2012 Article pubs.acs.org/jmc © 2012 American Chemical Society 5270 dx.doi.org/10.1021/jm300260v | J. Med. Chem. 2012, 55, 5270−5290