Ivermectin monosaccharide

Ivermectin is a member of the avermectin family of compounds that are used to treat helminth and arthropod diseases in humans, domestic animals, and plants. A membrane-bound high affinity ivermectin binding site was extracted from Caenorhabditis elegans with the nonionic detergent 1-O-n-octyl-beta-D-glucopyranoside. The free-living nematode C. elegans is highly sensitive to the avermectins and was used as a model of parasitic nematodes. The membrane-bound and detergent-solubilized ivermectin binding sites are stable and exhibit high affinity binding, with dissociation constants of 0.11 nM and 0.20 nM, respectively. The maximum binding of [3H]ivermectin is 0.54 pmol/mg of membrane protein and 0.66 pmol/mg of detergent-soluble protein. Kinetic analysis of ivermectin binding shows that the ivermectin binding sites form a slowly reversible complex with ivermectin. The rates of dissociation of [3H]ivermectin with the solubilized and membrane-bound binding sites are 0.005 min-1 and 0.006 min-1, respectively. The association rate of the soluble binding site is 0.053 nM-1 min-1, slightly slower than that observed for the membrane-bound site, 0.074 nM-1 min-1. To characterize the ivermectin binding site, competition experiments were performed by inhibiting [3H]ivermectin binding with several avermectin derivatives and the neurotransmitter gamma-aminobutyric acid (GABA). The order of potency was 22,23-dihydroavermectin B1a monosaccharide greater than 22,23-dihydroavermectin B1a aglycone greater than 3,4,8,9,10,11,22,23-octahydro B1 avermectin for both the membrane-bound and NOG-soluble binding sites. GABA did not compete with ivermectin binding, although it has been suggested that ivermectin acts at the GABA-gated chloride channel in some invertebrate systems. Optimum ivermectin binding and assay conditions have been determined. The detergent-soluble ivermectin binding site appears to be negatively charged and has a pl of 4.0 and an apparent Mr in Triton X-100 micelles of 340,000. Detergent solubilization of a high affinity ivermectin binding site will enable the subsequent purification and characterization of a putative site of ivermectin action.

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.

Avermectin (AVM) refers to eight macrolides containing a common l-oleandrosyl disaccharide chain indispensable to their antiparasitic bioactivities. We delineated the biosynthetic pathway of TDP-β-l-oleandrose (1), the sugar donor of AVM, by characterizing AveBVIII, AveBV, and AveBVII as TDP-sugar 3-ketoreductase, 5-epimerase, and 3-O-methyltransferase, respectively. On the basis of this pathway, we successfully reconstituted the biosynthesis of1inEscherichia coli. Our work completes the biosynthetic pathway of AVM and lays a solid foundation for further studies.