Members of the FAD/NAD-linked reductase family are recognized as crucial targets in drug development for cancers, inflammatory disorders, and infectious diseases. However, individual FAD/NAD reductases are difficult to inhibit in a selective manner with off-target inhibition reducing usefulness of identified compounds. Thioredoxin glutathione reductase (TGR), a high molecular weight thioredoxin reductase-like enzyme, has emerged as a promising drug target for the treatment of schistosomiasis, a parasitosis afflicting more than 200 million people. Taking advantage of small molecules selected from a high-throughput screen and using X-ray crystallography, functional assays, and docking studies, we identify a critical secondary site of the enzyme. Compounds binding at this site interfere with well-known and conserved conformational changes associated with NADPH reduction, acting as a doorstop for cofactor entry. They selectively inhibit TGR from Schistosoma mansoni and are active against parasites in culture. Since many members of the FAD/NAD-linked reductase family have similar catalytic mechanisms, the unique mechanism of inhibition identified in this study for TGR broadly opens new routes to selectively inhibit homologous enzymes of central importance in numerous diseases.
Schistosomiasis
is a widespread human parasitic disease currently affecting over 200
million people. Chemotherapy for schistosomiasis relies exclusively
on praziquantel. Although significant advances have been made in recent
years to reduce the incidence and intensity of schistosome infections,
these gains will be at risk should drug-resistant parasites evolve.
Thioredoxin glutathione reductase (TGR) is a selenoprotein of the
parasite essential for the survival of schistosomes in the mammalian
host. Several high-throughput screening campaigns have identified
inhibitors of Schistosoma mansoni TGR. Follow up
analyses of select active compounds form the basis of the present
study. We identified eight compounds effective against ex
vivo worms. Compounds 1–5 are active against all major species and development stages. The
ability of these compounds to target immature worms is especially
critical because praziquantel is poorly active against this stage.
Compounds 1–5, 7, and 8 displayed schistosomicidal activity even after only 1 h
incubation with the worms. Compounds 1–4 meet or exceed standards set by the World Health Organization for
leads for schistosomiasis therapy activity. The mechanism of TGR inhibition
was studied further with wild-type and mutant TGR proteins. Compounds 4–6 were found to induce an nicotinamide
adenine dinucleotide phosphate (NADPH) oxidase activity in TGR, leading
to the production of superoxide and hydrogen peroxide. Collectively,
this effort has identified several active compound series that may
serve as the basis for the development of new schistosomicidal compounds.
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