Clinical pharmacokinetics of ivermectin metabolites and their activity against Anopheles stephensi mosquitoes

Abstract Background Ivermectin (22,23-dihydroavermectin B1a: H2B1a) is an endectocide used to treat worm infections and ectoparasites including lice and scabies mites. Furthermore, survival of malaria transmitting Anopheles mosquitoes is strongly decreased after feeding on humans recently treated with ivermectin. Currently, mass drug administration of ivermectin is under investigation as a potential novel malaria vector control tool to reduce Plasmodium transmission by mosquitoes. A “post-ivermectin effect” has also been reported, in which the survival of mosquitoes remains reduced even after ivermectin is no longer detectable in blood meals. In the present study, we analyzed existing material from human clinical trials to understand the pharmacokinetics of ivermectin metabolites and performed feeding experiments in Anopheles stephensi mosquitoes to assess whether ivermectin metabolites contribute to the mosquitocidal action of ivermectin and whether they may be responsible for the post-ivermectin effect. Methods Ivermectin was incubated in the presence of recombinant human cytochrome P450 3A4/5 (CYP 3A4/5) to produce ivermectin metabolites. In total, nine metabolites were purified by semi-preparative high-pressure liquid chromatography. The pharmacokinetics of the metabolites were assessed over three days in twelve healthy volunteers who received a single oral dose of 12 mg ivermectin. Blank whole blood was spiked with the isolated metabolites at levels matching the maximal blood concentration (Cmax) observed in pharmacokinetics study samples. These samples were fed to An. stephensi mosquitoes, and their survival and vitality was recorded daily over three days. Results Human CYP3A4 metabolized ivermectin more rapidly than CYP3A5. However, 24-hydroxy- H2B1a (M9) was solely produced by CYP3A5, whereas ivermectin metabolites M1–M8 were predominantly formed by CYP3A4. Both 3’’-O-desmethyl-H2B1a (M1) and 4-hydroxy-H2B1a (M2) killed all mosquitoes within three days post-feeding, while administration of 3’’-O-desmethyl, 4-hydroxy-H2B1a (M4) reduced survival to 35% over an observation period of three days. Ivermectin metabolites that underwent deglycosylation or hydroxylation at spiroketal moiety were not active against An. stephensi. Half-lives of M1 (54.2 ± 4.7 h) and M4 (57.5 ± 13.2 h) were considerably longer than that of the parent compound IVM (38.9 ± 20.8 h). Conclusion In conclusion, both metabolites contribute to the activity of ivermectin against mosquitoes and could be responsible for the “post-ivermectin effect”.