Discovery and Characterisation of Dual Inhibitors of Tryptophan 2,3-Dioxygenase (TDO2) and Indoleamine 2,3-Dioxygenase 1 (IDO1) Using Virtual Screening.

Euphemia Leung,Suat Sari,Jóhannes Reynisson,Petr Tomek

doi:10.3390/molecules24234346

Abstract

Cancers express tryptophan catabolising enzymes indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO2) to produce immunosuppressive tryptophan metabolites that undermine patients’ immune systems, leading to poor disease outcomes. Both enzymes are validated targets for cancer immunotherapy but there is a paucity of potent TDO2 and dual IDO1/TDO2 inhibitors. To identify novel dual IDO1/TDO2 scaffolds, 3D shape similarity and pharmacophore in silico screening was conducted using TDO2 as a model for both systems. The obtained hits were tested in cancer cell lines expressing mainly IDO1 (SKOV3—ovarian), predominantly TDO2 (A172—brain), and both IDO1 and TDO2 (BT549—breast). Three virtual screening hits were confirmed as inhibitors (TD12, TD18 and TD34). Dose response experiments showed that TD34 is the most potent inhibitor capable of blocking both IDO1 and TDO2 activity, with the IC50 value for BT549 at 3.42 µM. This work identified new scaffolds able to inhibit both IDO1 and TDO2, thus enriching the collection of dual IDO1/TDO2 inhibitors and providing chemical matter for potential development into future anticancer drugs.

Highlights

The kynurenine pathway transforms ~95% of the essential amino acid tryptophan into many bioactive metabolites including nicotinamide adenine dinucleotide (NAD+ ), crucial for cellular redox metabolism [1]
Eighty (80) known TDO2 inhibitors were obtained from ChEMBL collection [32], and from the literature [33], thirty-four (34) were defined as active (IC50 ≤ 10 μM) and 46 as inactive ligands (IC50 > 100 μM)
TDO2 was used as a model for both systems, the rational being that the same substrates are metabolised into the same products by both enzymes

Summary

Introduction

The kynurenine pathway transforms ~95% of the essential amino acid tryptophan into many bioactive metabolites including nicotinamide adenine dinucleotide (NAD+ ), crucial for cellular redox metabolism [1]. Haem-containing enzymes indoleamine 2,3-dioxygenase (IDO1) and tryptophan. 2,3-dioxygenase (TDO2) catalyse oxidation of tryptophan into N-formyl-kynurenine, the first and rate-limiting step in the kynurenine pathway [2]. N-formyl-kynurenine is readily converted into kynurenine inside mammalian cells by kynurenine formamidase [3]. Whilst both IDO1 and TDO2 catalyse the identical biochemical reaction, their physiological roles and protein structures differ substantially. TDO2 is a tetramer (167 kDa) expressed predominantly in the liver to maintain tryptophan homeostasis. IDO1 is a monomeric enzyme (37 kDa) essentially absent in most normal tissues but induced during inflammation and mammalian gestation to suppress the immune system [1]

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Conclusion