Synthesis and Molecular Modeling Studies of N'-Hydroxyindazolecarboximidamides as Novel Indoleamine 2,3-Dioxygenase 1 (IDO1) Inhibitors

Abstract

Indoleamine 2,3-dioxygenase 1 (IDO1) is an immunosuppressive enzyme that is highly overexpressed in various cancer cells and antigen-presenting cells. It has emerged as an attractive therapeutic target for cancer immunotherapy, which has prompted high interest in the development of small-molecule inhibitors. To discover novel IDO1 inhibitors, we designed and synthesized a series of N'-hydroxyindazolecarboximidamides. Among the compounds synthesized, compound 8a inhibited both tryptophan depletion and kynurenine production through the IDO1 enzyme. Molecular docking studies revealed that 8a binds to IDO1 with the same binding mode as the analog, epacadostat (INCB24360). Here, we report the synthesis and biological evaluation of these hydroxyindazolecarboximidamides and present the molecular docking study of 8a with IDO1.

Keywords: N’-hydroxyindazolecarboximidamide; cancer immunotherapy; indoleamine 2,3-dioxygenase 1; kynurenine production; tryptophan depletion.

Figure 1

Structure of IDO1 inhibitors in clinical trials.

Figure 2

Rational design of novel IDO1 inhibitor (1) from the known molecules.

Scheme 1

Synthesis of compounds 8, 12 and 16. Reagents and conditions: (a) 10% Pd/C, H₂, EtOH, r.t.; (b) NaNO₂, H₂O, HCl, H₂SO₄, KI; (c) Zn(CN)₂, Pd₂(dba)₃, dppf, Zn, DMA, reflux; (d) hydroxylamine hydrogen chloride, NaHCO₃, H₂O, EtOH, 85 ^°C; (e) (i) 6N HCl, H₂O, AcOH, 45 °C, (ii) HCl, NaNO₂, H₂O, 0 °C, 3 h; (f) amines, NaHCO₃, H₂O, THF, 60 °C; (g) NCS, DMF, r.t., 6 h.

Figure 3

Docking model 8a against IDO1. (a) Binding mode of 8a (pink, ball and stick style) coordinated to heme; (b) The surface model of the active site bound to 8a is colored because of hydrophobicity. Hydrogen bonds of inter- and intra-molecular interactions are shown as green dashed lines and interaction residues are represented by the stick model.