Application of two-dimensional high-performance liquid chromatography in metabolism studies of ivermectin.

Two-dimensional high-performance liquid chromatography (HPLC) of alternate reversed-phase and normal-phase columns was used in the purification, quantification and identification of submicrogram quantities of drug residue and metabolites in tissues from animals dosed with ivermectin. In reverse isotope dilution assay of the parent drug, two-dimensional HPLC assured constant specific activity of the drug isolates. For identification of metabolites from liver tissue, HPLC in two dimensions not only facilitated the purification, but also provided information (capacity factor k') in both systems for the elimination of possible metabolite structures in the course of compound identification. These approaches exemplified by the ivermectin studies should be generally applicable in analyses of complex biological samples in which quantity is frequently a limiting factor.

Comparative metabolic disposition of ivermectin in fat tissues of cattle, sheep, and rats.

In the study of the metabolic disposition of ivermectin in cattle, sheep, and rats, a group of nonpolar metabolites was detected in the fat tissues of these animals. Upon saponification or esterase treatment, these nonpolar metabolites gave rise to polar products that were similar to the ivermectin metabolites present in the liver. A hypothesis was thus proposed that the polar ivermectin metabolites produced in the liver were esterified and stored in the fat as nonpolar entities, which can be reconverted back to the polar metabolites chemically or enzymatically. To substantiate this hypothesis, the regeneration of polar metabolites from the nonpolar metabolites was studied and hydrolysis products were characterized to establish the basic structure of the alcohol portion of the metabolites. Furthermore, chromatographic comparisons were made between radiolabeled in vivo metabolites and synthetic fatty acid ester samples. These results established the general structural class of the ivermectin nonpolar metabolites and also confirmed the unusual metabolic pathway of ivermectin disposition in fat tissue.

The metabolism of avermectin-H2B1a and -H2B1b by pig liver microsomes.

The avermectins are a new class of macrocyclic lactone disaccharide antiparasitic agents derived from Streptomyces avermitilis. On incubation of avermectin-H2B1a and avermectin-H2B1b with pig liver microsomes, a group of metabolites slightly more polar than the parent compounds were generated. Two major metabolites have been isolated and purified by repetitive reversed phase and normal phase HPLC. Their structures were established to be the O-demethylation products of the parent compounds, i.e. 3''-O-desmethyl-H2B1a and 3''-O-desmethyl-H2B1b. The structure assignments were based on spectral results of UV, NMR, fast atom bombardment-mass spectrometry, and chemical derivatization studies including acid hydrolysis as well as fluorogenic reaction. Among the in vitro metabolites, there was only a trace amount of the polar metabolites 24-hydroxymethyl-H2B1a and 24-hydroxymethyl-H2B1b which were the major in vitro metabolites of avermectin-H2B1a and -H2B1b by steer or rat liver microsomes.

Metabolic disposition of ivermectin in tissues of cattle, sheep, and rats.

The metabolic disposition of ivermectin, a new antiparasitic drug, has been studied in cattle, sheep, and also in rats dosed with the drug labeled with tritium in the C-22,23 positions. In the edible tissues of these animals, the unaltered drug was the major tissue residue component and was quantitated by HPLC-reverse isotope dilution assay. The depletion half-lives of the drug ranged between 1 and 6 days, similar to those of the total tissue residue in these species. Most metabolites present in the liver tissues were more polar than the parent drug. Based on spectral (NMR, mass spectrometric) analysis and chromatographic comparison with authentic compounds prepared by in vitro rat or steer microsomal incubations, three of these metabolites have been isolated and identified as the hydroxylation derivatives of ivermectin, i.e. 24-hydroxymethyl-H2B1a, its monosaccharide, and 24-hydroxymethyl-H2B1b.

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.