Pharmacological effects of ivermectin, an antiparasitic agent for intestinal strongyloidiasis: its mode of action and clinical efficacy. Ivermectin is an oral semi-synthetic lactone anthelmintic agent derived from avermectins isolated from fermentation products of Streptomyces avermitilis. Ivermectin showed a concentration-dependent inhibitory effect on motility of a free-living nematode, Caenorhabditis elegans (C. elegans). There exist specific binding sites having a high affinity for ivermectin in the membrane fraction of C. elegans, and a strong positive correlation was detected between the affinity for these binding sites and the suppressive effect on motility of C. elegans in several ivermectin-related substances. These results suggested that the binding to these binding sites is important for the nematocidal activity of ivermectin. In oocytes of Xenopus laevis injected with the Poly (A)(+) RNA of C. elegans, expression of a chloride channel, which is irreversibly activated by ivermectin, was recognized. The pharmacological properties of this channel suggest that the ivermectin-sensitive channel is a glutamate-activated chloride channel. As to the glutamate-activated chloride channel, two subtypes (GluCl-alpha and GluCl-beta) were cloned, suggesting these subtypes constitute the glutamate-activated chloride channel. These findings suggest that ivermectin binds to glutamate-activated chloride channels existing in nerve or muscle cells of nematode with a specific and high affinity, causing hyperpolarization of nerve or muscle cells by increasing permeability of chloride ion through the cell membrane, and as a result, the parasites are paralyzed to death. In experimental infections in sheep and cattle, ivermectin exhibited potent dose-dependent anthelmintic effects on Haemonchus, Ostertagia, Trichostrongylus, Cooperia, Oesphagostomum, and Dictyocaulus. Anthelmintic effects were reported also in dogs, horses, and humans infected with Strongyloides. In the clinical Phase III trial in Japan, 50 patients infected with Strongyloides stercoralis were administered approx. 200 microg/kg of ivermectin to be given orally twice at an interval of 2 weeks. As a result, the Strongyloides stercoralis-eradicating rate was 98.0% (49/50).