Fasciola hepatica: Action in vitro of triclabendazole on immature and adult stages

Abstract

Under in vitro conditions in a balanced salt solution, triclabendazole was found to accumulate in significant amounts in both immature (3 week old) and adult Fasciola hepatica. A viable parasite was needed to concentrate the drug, but a high percentage of the compound was also bound by the dead worm. The drug could penetrate into liver flukes even when the oral route had been closed off by ligation, indicating that the drug can be taken up by transtegumentary absorption. A 24 hr exposure to triclabendazole, at 10–25 μ M concentrations, was found to result in a strong inhibition of the parasite's motility. This effect was paralleled by dramatic changes in the worm's resting tegumental membrane potential. The onset of these actions was found to develop very slowly, and high drug levels had to accumulate within the parasite to initiate its immobilization. In addition to drug concentration and incubation time, physiological alterations observed were also dependent on other culture conditions, such as the presence or absence of serum albumin and the drug tissue/medium ratio. Biochemical examinations showed that triclabendazole significantly stimulated glucose derived acetate and propionate formation by adult liver flukes. Adenosine triphosphate levels were not changed even in the presence of high triclabendazole concentrations (25 μ M). Likewise, the activities of various membrane associated adenosine triphosphatases were not altered by the drug. However, the ability of the drug to inhibit colchicine binding to microtubular protein purified from adult liver flukes suggested an interference of the drug with microtubular structure and function. This interpretation of the drug's action was supported by the finding that triclabendazole strongly inhibited the release of proteolytic enzymes from immature and adult parasites, a process which is likely to be dependent on microtubular function. Similar physiological and biochemical effects as those described for triclabendazole were also found with its major metabolite, triclabendazole sulfoxide. Although triclabendazole was found to initiate a variety of potent physiological and metabolic effects on immature and adult liver flukes, the precise molecular mode of action of the fasciolicidal drug remains to be elucidated.