Exploration of side chain branching and halogenation of mosquito-selective novel carbamates for control of the malaria vector Anopheles gambiae

Recently we reported discovery of novel carbamates that have higher selectivity to Anopheles gambiae than Propoxur, a WHO standard. We report results of evaluation of side-chain branching and halogenation of one of the selective carbamates (PRC 331, 3-tert-butylphenyl methylcarbamate) and discuss effects of chemical moieties to the pharmacokinetics of these analogs. The potency of these analogs to A. gambiae AChE and human AChE (hAChE) and their toxicity to mosquitoes is reported. New carbamates were designed based on mosquito and human AChE protein homology modeling and in silico ligand docking. AChE inhibition was performed by the Ellman method and data was plotted in Prism® (Graphpad). IC50 ratios were computed to determine the potency and selectivity of the compounds to A. gambiae. A standard WHO treated paper assay was used in our toxicity assay, mortality was recorded 24 hr post treatment and LC50 values computed using PoloPlus®. The potency of branched analogs of 331 to AgAChE remained unchanged, whereas there was a marked increased in potency of these compounds to hAChE. Fluorination at different positions of the side chain led to a decrease in potency, most drastically in the mosquito, but some analogs had 2-fold gain in selectivity. Low toxicity of modified analogs may be due to decreased penetration through the cuticle, or increased metabolic degradation. The structure-activity relationship (SAR) observed provide insights that are important in the on-going design of more selective carbamates. Synergism and electrophysiological studies will evaluate the possible effects of detoxifying enzymes and blood brain barrier to toxicity.