The Binding Mode of N-Hydroxyamidines to Indoleamine 2,3-Dioxygenase 1 (IDO1).

Indoleamine 2,3-dioxygenase 1 (IDO1) is an important target in cancer immunotherapy. The most advanced clinical compound, epacadostat (INCB024360), binds to the heme cofactor of IDO1 through an N-hydroxyamidine function. Conflicting binding modes have recently been proposed, reporting iron binding either through the hydroxyamidine oxygen or through the hydroxyamidine nitrogen atom. Here, we use quantum chemical calculations, docking, and quantum mechanics/molecular mechanics calculations based on available X-ray data to resolve this issue and to propose a physically meaningful binding mode. Our findings will aid the design of novel IDO1 ligands based on this pharmacophore.