Abstract
BACKGROUND AND PURPOSE Mass drug administration of ivermectin has been
proposed as a possible malaria elimination tool. Ivermectin exhibits a
mosquito-lethal effect well beyond its biological half-life, suggesting
the presence of active slowly eliminated metabolites. EXPERIMENTAL
APPROACH Human liver microsomes, primary human hepatocytes, and whole
blood from healthy volunteers given oral ivermectin were used to
identify ivermectin metabolites by ultra-high performance liquid
chromatography coupled with high resolution mass spectrometry. The
molecular structures of metabolites were determined by mass spectrometry
and verified by nuclear magnetic resonance. Pure cytochrome P450 enzyme
isoforms were used to elucidate the metabolic pathways. KEY RESULTS
Thirteen different metabolites (M1-M13) were identified after incubation
of ivermectin with human liver microsomes. Three (M1, M3, and M6) were
the dominant metabolites found in microsomes, hepatocytes, and blood
from volunteers after oral ivermectin administration. The chemical
structure defined by LC-MS/MS and NMR indicated that M1 is 3″-O-demethyl
ivermectin, M3 is 4-hydroxymethyl ivermectin, and M6 is 3″-O-demethyl,
4-hydroxymethyl ivermectin. Metabolic pathway evaluations with
characterized cytochrome P450 enzymes showed that M1 was produced by
CYP3A4 and CYP3A5, and that M3 and M6 were produced by CYP3A4.
CONCLUSIONS AND IMPLICATIONS Demethylated and hydroxylated ivermectin
are the main human metabolites in vivo. Further study to characterize
their pharmacokinetic properties and mosquito-lethal activity is now
needed.