Dihydroquinazolines as a Novel Class of Trypanosoma brucei Trypanothione Reductase Inhibitors: Discovery, Synthesis, and Characterization of their Binding Mode by Protein Crystallography

Abstract

Trypanothione reductase (TryR) is a genetically validated drug target in the parasite Trypanosoma brucei, the causative agent of human African trypanosomiasis. Here we report the discovery, synthesis, and development of a novel series of TryR inhibitors based on a 3,4-dihydroquinazoline scaffold. In addition, a high resolution crystal structure of TryR, alone and in complex with substrates and inhibitors from this series, is presented. This represents the first report of a high resolution complex between a noncovalent ligand and this enzyme. Structural studies revealed that upon ligand binding the enzyme undergoes a conformational change to create a new subpocket which is occupied by an aryl group on the ligand. Therefore, the inhibitor, in effect, creates its own small binding pocket within the otherwise large, solvent exposed active site. The TryR–ligand structure was subsequently used to guide the synthesis of inhibitors, including analogues that challenged the induced subpocket. This resulted in the development of inhibitors with improved potency against both TryR and T. brucei parasites in a whole cell assay.

Induction of Oxidative Stress in Trypanosoma brucei by the Antitrypanosomal Dihydroquinoline OSU-40

“…Trypanosomatids also lack glutathione reductase, thioredoxin reductase, catalase, and selenium-containing glutathione peroxidases, possibly making them deficient in defense against oxidative stress. Fairlamb et al proposed that the essential parasite enzymes trypanothione synthetase (32,41) and trypanothione reductase (24,25) are promising drug targets and reported different classes of small molecule inhibitors (24,25,32) that interfere with the ROS-scavenging system. Taken together, these studies support the idea that the induction of oxidative stress can be an effective strategy for developing antitrypanosomal agents.…”

Ebsulfur Is a Benzisothiazolone Cytocidal Inhibitor Targeting the Trypanothione Reductase of Trypanosoma brucei

“…DL-␣-Difluoromethylornithine inhibits the biosynthesis of spermidine, a constituent of trypanothione (15), and antimonials form complexes with T(SH) 2 and TryR (16). Several classes of compounds have been shown to inhibit TryR (17)(18)(19)(20)(21)(22).…”

Dibenzosuberyl substituted polyamines and analogs of clomipramine as effective inhibitors of trypanothione reductase; molecular docking, and assessment of trypanocidal activities

“…Our docking studies (section 2.4) indicate that the chlorine is involved in binding interactions with residues in the active site, also there are several examples of TR inhibitors containing a halogenated aryl group being more effective than analogs without the halogen. 34,56,58,67 Another possible factor impacting the inhibiting activities is that (2) and (3) contain an exocyclic nitrogen (versus the endocyclic nitrogen of the DBA group of (1)). However, since the shapes of the DBS and DBA rings are similar and docking (section 2.4) showed that both tricycles bind to the same locations in the active site, any effects on binding resulting from differences in ring nitrogen location between compounds (1) to (3) are likely to be small compared to binding differences due to the presence of chlorine in the DBA ring of (1).…”

“…7 Also, several genetic studies on Leishmania and Trypanosoma have shown the importance of TR, including studies in which trypanosomatids modified to produce decreased levels of TR were found to be acutely sensitive to oxidative stress with severely compromised abilities to infect cells. [28][29][30][31] Additionally, the structure and properties of TR from different genera of trypanosomatids are very similar, 16,[32][33][34] indicating that an inhibitor of TR may be active against a range of trypanosomatids.…”