O-alkylhydroxylamines as rationally-designed mechanism-based inhibitors of indoleamine 2,3-dioxygenase-1

Abstract

Indoleamine 2,3-dioxygenase-1 (IDO1) is a promising therapeutic target for the treatment of cancer, chronic viral infections, and other diseases characterized by pathological immune suppression. Recently important advances have been made in understanding IDO1’s catalytic mechanism. Although much remains to be discovered, there is strong evidence that the mechanism proceeds through a heme-iron bound alkylperoxy transition or intermediate state. Accordingly, we explored stable structural mimics of the alkylperoxy species and provide evidence that such structures do mimic the alkylperoxy transition or intermediate state. We discovered that O-benzylhydroxylamine, a commercially available compound, is a potent sub-micromolar inhibitor of IDO1. Structure-activity studies of over forty derivatives of O-benzylhydroxylamine led to further improvement in inhibitor potency, particularly with the addition of halogen atoms to the meta position of the aromatic ring. The most potent derivatives and the lead, O-benzylhydroxylamine, have high ligand efficiency values, which are considered an important criterion for successful drug development. Notably, two of the most potent compounds demonstrated nanomolar-level cell-based potency and limited toxicity. The combination of the simplicity of the structures of these compounds and their excellent cellular activity makes them quite attractive for biological exploration of IDO1 function and antitumor therapeutic applications.

Fused Heterocyclic Compounds as Potent Indoleamine-2,3-dioxygenase 1 Inhibitors

“…We presume that the accuracy of the methylene blue-ascorbate regeneration system to keep IDO1 in its active state (Fe 2+ ) could be the primary reason for the differences in the inhibitory activity values between the pDMAB-and HPLC-based methods. 8,14 The HPLC-based assay also showed that monoamino substituted dipyrazolopyran derivative 4l displayed strongest inhibitory activity (IC 50 = 78 nM) among all the investigated compounds.…”

“…Figure 1B showed that in the absence of compound the deoxy-ferrous-IDO1 enzyme displayed Soret and Q-band at 421 and 556 nm. 14 In the presence of compounds, 4j and 4l the Soret band (421 nm) got blue-shifted, and new Q bands appeared around 520/550 nm ( Figure 1B). This indicates their probable binding to the Fe 2+ -IDO1 enzyme.…”

“…In this regard, we recorded the absorption spectra of ferric-IDO1 and deoxy-ferrous-IDO1 in the absence and presence of the selected compounds ( Figure 1A,B). 14 Figure 1A showed that in the presence of compounds the Soret peak (404 nm) showed no substantial shift, indicating its trivial binding to the ferric-IDO1 enzyme. Figure 1B showed that in the absence of compound the deoxy-ferrous-IDO1 enzyme displayed Soret and Q-band at 421 and 556 nm.…”

“…8,14,15 Successful use of ipilimumab and nivolumab and current clinical development of the inhibitors of IDO1 enzyme inspire researchers to develop IDO1 inhibitors for cancer immunotherapy. 14 To identify a new structural class of IDO1 inhibitors that could be optimized to afford potent and selective inhibitors of IDO1, we synthesized a series of fused di-and triheterocyclic compounds. Activity studies of these compounds revealed their strong and selective IDO1 inhibitory potency under the in vitro conditions with no/negligible cytotoxicity.…”

“…Several reports also described such benefits of halogen substituted aryl ring on the IDO1 inhibitory efficacies. 8,14,17,19 For these fused heterocyclic compounds halogen substituents to meta-and/or para-positions of the aryl ring proved to be quite beneficial (Tables 1 and 2). Such stronger activity of the halogen substituted compounds could be due to the hydrophobic interactions, pi-stacking interaction with aromatic amino acids, or halogen bonding with the Lewis bases present within the active site.…”