Novel diphenyl ethers: design, docking studies, synthesis and inhibition of enoyl ACP reductase of Plasmodium falciparum and Escherichia coli

X-ray structural analysis of Plasmodium falciparum enoyl acyl carrier protein reductase as a pathway toward the optimization of triclosan antimalarial efficacy

The x-ray crystal structures of five triclosan analogs, in addition to that of the isoniazid-NAD adduct, are described in relation to their integral role in the design of potent inhibitors of the malarial enzyme Plasmodium falciparum enoyl acyl carrier protein reductase (PfENR). Many of the novel 5-substituted analogs exhibit low micromolar potency against in vitro cultures of drug-resistant and drug-sensitive strains of the P. falciparum parasite and inhibit purified PfENR enzyme with IC50 values of <200 nM. This study has significantly expanded the knowledge base with regard to the structure-activity relationship of triclosan while affording gains against cultured parasites and purified PfENR enzyme. In contrast to a recent report in the literature, these results demonstrate the ability to improve the in vitro potency of triclosan significantly by replacing the suboptimal 5-chloro group with larger hydrophobic moieties. The biological and x-ray crystallographic data thus demonstrate the flexibility of the active site and point to future rounds of optimization to improve compound potency against purified enzyme and intracellular Plasmodium parasites.

Biochemical characterization of enoyl reductase involved in Type II fatty acid synthesis in the intestinal coccidium Eimeria tenella (Phylum Apicomplexa)

An enoyl reductase (EtENR) closely related to those of green algae and involved in Type II fatty acid synthesis was characterized and localized to the apicoplast in the coccidium Eimeria tenella. Biochemical analysis using native EtENR protein extracted from parasites confirmed its function as an enoyl reductase using NADH as a cofactor. However, the recombinant form (rEtENR) expressed in bacteria was only able to oxidize NADH, but unable to transfer the electron to enoyl-CoA, possibly due to the inappropriate folding of rEtENR expressed in bacteria. The functions of both native and recombinant EtENR could be inhibited by triclosan (IC(50)=1.45 microM), suggesting that this enzyme may be explored as a drug target against coccidiosis.

Discovery of a rhodanine class of compounds as inhibitors of Plasmodium falciparum enoyl-acyl carrier protein reductase

Enoyl acyl carrier protein (ACP) reductase, one of the enzymes of the type II fatty acid biosynthesis pathway, has been established as a promising target for the development of new drugs for malaria. Here we present the discovery of a rhodanine (2-thioxothiazolidin-4-one) class of compounds as inhibitors of this enzyme using a combined approach of rational selection of compounds for screening, analogue search, docking studies, and lead optimization. The most potent inhibitor exhibits an IC(50) of 35.6 nM against Plasmodium falciparum enoyl ACP reductase (PfENR) and inhibits growth of the parasite in red blood cell cultures at an IC(50) value of 750 nM. Many more compounds of this class were found to inhibit PfENR at low nanomolar to low micromolar concentrations, expanding the scope for developing new antimalarial drugs. The structure-activity relationship of these rhodanine compounds is discussed.