Malarial Dihydrofolate Reductase as Drug Target

Mutant, Drug Resistant PfDHFR

The of PfDHFR has four : N51I, C59R, S108N, and I164L.^[2] These mutations cause decreased binding affinities of inhibitors, a huge factor being steric clashing. For pyrimethamine and cycloguanil specifically, the N51I mutation greatly effects their ability to bind the enzyme due to the drugs' rigid chlorophenyl substituents. In an effort to mitigate the effect of the mutations, the commonly occurring quadruple mutant must be addressed in the research of new PfDHFR targeting drugs.

A first step in this direction was to experiment with the drug that, with its (2,4,5-trichlorophenoxy)propoxy side chain, addressed the steric clash that was subjected to with a S108N mutation. However further research with this drug was stopped because of its gastrointestinal toxicity in humans and low bioavailbility.^[3] Wr99210 was significantly more basic than pyrimethamine, a pyrimidine and slightly acidic substance to match that of the gastrointestinal track, and thus was not readily absorbed in the intestines. Though it was effective at getting deep into the mutant PfDHFR active site, its ineffectiveness as an oral drug would have made it unsuccessful.

It was resolved that a 2,4-diaminopyrimidine anchor on a new drug would allow the steric hindrance found with pyrimethamine to be avoided by replacing its rigid chlorophenyl group. In addition to this anchor allowing deep binding into the active site of PfDHFR, the other end of the molecule, an alpha-carboxylate, would form strong hydrogen bonds with the conserved Arg122.

Species Specificity

There are three regions of the DHFR active site that differ between human and Plasmodium falciparum that allows a drug to target the PfDHFR specifically while not harming that of humans.