Mechanisms of action of nitazoxanide and related drugs against helminths
The 5-nitrothiazole, nitazoxanide, is a novel compound with a broad spectrum of activity; effective against anaerobic/microaerophilic intestinal protozoa and bacteria. The mechanism of nitazoxanide's anthelmintic activity is unknown. This study examined the mode of action of nitazoxanide and analogues against nematodes, trematodes and cestodes. Caenorhabditis elegans, a free living nematode, was used as a model. Nitazoxanide was found to exert a transient spastic paralytic effect, particularly at the posterior end of the nematode, with 50% effect at 4.62µM. This effect is similar to that of levamisole (a neuromuscular agent) specific for nicotinic receptors. An effect of nitazoxanide at nicotinic receptors was demonstrated using mutants of C. elegans and the specific neuroblockers, mecamylamine and pempidine. A neuromuscular effect was also seen in Ascaris suum at higher concentrations using electrophysiological technique. Nitazoxanide had no effect on L3 larvae of Nematodirus spathiger and Haemonchus contortus. Cestodes and trematodes in vitro showed high sensitivity to nitazoxanide with <µM causing rapid paralysis and tegumental damage in Hymenolepis diminuta and Schistosoma mansoni. This was accompanied by inhibition of α-bungarotoxin binding to nicotinic receptors in S. mansoni, further suggesting an effect on neurotransmission. Uptake of glucose by the schistosome was decreased, and tegumental damage was found, suggesting that nitazoxanide can impair worm nutrition. Analysis of physicochemical properties of nitazoxanide suggested it is a protonophoric uncoupler. Consistent with this, synthesis of ATP was inhibited significantly by 33µM nitazoxanide in C. elegans and S. mansoni. In mitochondria of H. diminuta an increase in oxygen uptake was observed comparable to that seen with known protonophores suggesting that uncoupling of oxidative phosphorylation was occurring. In conclusion, nitazoxanide is likely to affect helminths by a direct effect on neurotransmission and protonophoric uncoupling. Nematodes are less sensitive than trematodes and cestodes in vitro, possibly due to difficulty of drug adsorption through the cuticle.