Litomosoides carinii: Mode of action in vitro of benzothiazole and amoscanate derivatives with antifilarial activity

It is suggested that the recently developed benzothiazole and amoscanate derivatives with antifilarial activity exert their action in vitro by an inhibition of mitochondrial-derived respiration. It was confirmed that the drugs CGP 20376, 21835, 20308, 21306, and 6140 cause a rapid immobilization in vitro of the adult filarial worm, Litomosoides carinii, the time required being similar to rotenone at the same concentration. The other drugs investigated, CGPs 20309, 21833, 24589, 23518, and 13231, were also effective; however, they required much longer incubation times. Submitochondrial particles (SMP) were prepared from Ascaris muscle and rat liver. The concentration of drug causing 50% inhibition of respiration (IC₅₀) was calculated. It was found that the drugs most rapidly inhibiting respiration have IC₅₀s for NADH oxidase of less than 25 μM in both Ascaris and rat liver SMP. This effect on SMP respiration could be overcome by using succinate as a substrate, indicating the site of inhibition to be within complex I of the mitochondrial respiratory chain. Further experiments showed that whereas the respiratory chain's NADH:ferricyanide reductase was unaffected by these drugs, there were pronounced effects on both Ascaris and rat liver NADH:quinone reductase activity. This suggests that the inhibition within complex I occurs after the flavoprotein dehydrogenase, but before the site of the quinone reduction. The other compounds examined, which had a slower effect on motility, also showed inhibition of the NADH oxidase, but not to as great an extent as the aforementioned compounds. The compounds most active against motility were also most effective at inhibiting respiration in intact adult L. carinii. Analysis of the aerobic end products produced by L. carinii showed that acetate production was greatly reduced even in the presence of low concentrations of the drugs. There was also a slight decrease in lactate production. However, a direct effect on the glycolytic pathway was ruled out by two observations. One, that the production of lactate from cell-free extracts of L. carinii is unaffected by the presence of the drugs, and secondly, that a protozoan, Giardia lamblia, reliant on glycolysis for energy production, can survive for long periods of time in the presence of high concentrations of the drugs. A correlation can be observed between the time for immobilization of the filarial worm and the strength of inhibition of mitochondrial respiration. Therefore, it is suggested that, at least in vitro, the mechanism of toxicity of these antifilarials in L. carinii is due to the blocking of the respiratory chain at a site similar to that of rotenone.