Ivermectin metabolites reduce Anopheles survival

Abstract

Ivermectin mass drug administration to humans or livestock is a potential vector control tool for malaria elimination. The mosquito-lethal effect of ivermectin in clinical trials exceeds that predicted from in vitro laboratory experiments, suggesting that ivermectin metabolites have mosquito-lethal effect. The three primary ivermectin metabolites in humans (i.e., M1 (3″-O-demethyl ivermectin), M3 (4-hydroxymethyl ivermectin), and M6 (3″-O-demethyl, 4-hydroxymethyl ivermectin) were obtained by chemical synthesis or bacterial modification/metabolism. Ivermectin and its metabolites were mixed in human blood at various concentrations, blood-fed to Anopheles dirus and Anopheles minimus mosquitoes, and mortality was observed daily for fourteen days. Ivermectin and metabolite concentrations were quantified by liquid chromatography linked with tandem mass spectrometry to confirm the concentrations in the blood matrix. Results revealed that neither the LC₅₀ nor LC₉₀ values differed between ivermectin and its major metabolites for An. dirus or An. minimus., Additionally, there was no substantial differences in the time to median mosquito mortality when comparing ivermectin and its metabolites, demonstrating an equal rate of mosquito killing between the compounds evaluated. These results demonstrate that ivermectin metabolites have a mosquito-lethal effect equal to the parent compound, contributing to Anopheles mortality after treatment of humans.

Figure 1

Molecular structure of ivermectin and its primary human metabolites. Molecular structures of (A) ivermectin parent compound, (B) M1 (3″-O-demethyl ivermectin) which has a methyl group removed from the second saccharide ring, (C) M3 (4-hydroxymethyl ivermectin) which has an extra hydroxyl group at C-4, and (D) M6 (3″-O-demethyl, 4-hydroxymethyl ivermectin) which has both the extra hydroxyl group at C-4 and a methyl group removed from the second saccharide ring. The red circles denote changes in the metabolite structures compared to parent compound (B–D).

Figure 2

Anopheles dirus mortality results when fed ivermectin and metabolites in human blood. Open circles represent cumulative mosquito mortality at 14 days after blood meal ingestion. Solid lines represent the mean concentration–response relationship and the shaded area represents the 95% confidence interval associated with the nonlinear fit. Dashed black lines represent the fixed maximum effect of 100% mortality and the estimated minimum effect associated with baseline mortality observed from control mosquitoes.

Figure 3

Anopheles minimus mortality results when fed ivermectin and metabolites in human blood. Open circles represent cumulative mosquito mortality at 14 days after blood meal ingestion. Solid lines represent the mean concentration–response relationship and the shaded area represents the 95% confidence interval associated with the nonlinear fit. Dashed black lines represent the fixed maximum effect of 100% mortality and the estimated minimum effect associated with baseline mortality observed from control mosquitoes.