High-throughput screening for potent and selective inhibitors of Plasmodium falciparum dihydroorotate dehydrogenase

Plasmodium falciparum is the causative agent of the most serious and fatal malarial infections, and it has developed resistance to commonly employed chemotherapeutics. The de novo pyrimidine biosynthesis enzymes offer potential as targets for drug design, because, unlike the host, the parasite does not have pyrimidine salvage pathways. Dihydroorotate dehydrogenase (DHODH) is a flavin-dependent mitochondrial enzyme that catalyzes the fourth reaction in this essential pathway. Coenzyme Q (CoQ) is utilized as the oxidant. Potent and species-selective inhibitors of malarial DHODH were identified by high-throughput screening of a chemical library, which contained 220,000 drug-like molecules. These novel inhibitors represent a diverse range of chemical scaffolds, including a series of halogenated phenyl benzamide/naphthamides and urea-based compounds containing napthyl or quinolinyl substituents. Inhibitors in these classes with IC₅₀ values below 600 nM were purified by high pressure liquid chromatography, characterized by mass spectroscopy, and subjected to kinetic analysis against the parasite and human enzymes. The most active compound is a competitive inhibitor of CoQ with an IC₅₀ against malarial DHODH of 16 nM, and it is 12,500-fold less active against the human enzyme. Site-directed mutagenesis of residues in the CoQ-binding site significantly reduced inhibitor potency. The structural basis for the species selective enzyme inhibition is explained by the variable amino acid sequence in this binding site, making DHODH a particularly strong candidate for the development of new anti-malarial compounds.