The metabolism of avermectins B1a, H2B1a, and H2B1b by liver microsomes.

The avermectins area a new class of structurally related antiparasitic agents isolated from Streptomyces avermitilis. The major polar metabolites isolated from in vitro incubations of [3H]avermectins B1a, H2B1a, and H2B1b with either rat or steer liver microsomes have been isolated and identified as the C24-methyl alcohols of the parent compounds. A smaller quantity of a more polar metabolite has also been identified as the monosaccharide of the C24-methyl alcohols of avermectin H2H1b from rat liver microsomal incubation and avermectin H2B1a from steer liver microsomal incubation. The mass spectra and 300-MHz 1H-NMR spectra permitted assignment of structures to these metabolites. Together these two metabolites represent 50-80% of the total radioactivity more polar than the parent compounds. The metabolite profiles on reverse-phase HPLC demonstrate that the rat and steer are qualitatively similar in the production of these two polar metabolites.