Comparative neuromuscular blocking actions of levamisole and pyrantel-type anthelmintics on rat and gastrointestinal nematode somatic muscle

The basis for the comparative toxicity to parasitic nematodes and their mammalian hosts of the anthelmintics levamisole, pyrantel, and several related analogs on somatic nicotinic cholinergic transmission was examined. Measurements of muscle contractility and membrane potential were made using the isolated hemidiaphragm preparation of the rat and isolated axial muscle segments from the gastrointestinal nematode Haemonchus contortus. Pyrantel caused a dose- and time-dependent reduction of nerve-evoked twitches in the rat diaphragm. These effects were exacerbated by increasing the frequency of phrenic nerve stimulation from 0.5 to 50 Hz. Levamisole was less potent and the onset of its effects slower than pyrantel. Neither drug significantly affected twitches evoked from d-tubocurarine-blocked preparations following direct stimulation of the diaphragm. Twitch depression was reversed by washing, but not by application of physostigmine. In H. contortus, both drugs stimulated a spastic contraction and sustained paralysis in the concentration range of 1-10 microM, mimicking the action of nicotine. Neither nicotinic nor muscarinic antagonists blocked these responses. Moreover, neither nicotinic antagonists nor muscarinic agonists or antagonists had any independent effect on contractility of the parasite muscle segments. The blocking actions of levamisole and pyrantel on H. contortus axial muscle were associated with membrane depolarization at the muscle. In the rat-isolated hemidiaphragm, pyrantel, but not levamisole, depolarized end-plate regions of muscle fibers. d-Tubocurarine blocked the depolarizing action of pyrantel but not levamisole on rat-isolated hemidiaphragm. In axial muscle fibers of H. contortus, d-tubocurarine did not block the depolarizing actions of pyrantel, levamisole, or nicotine. 3-Bromo and 3-amino derivatives of levamisole were equipotent with and mimicked the actions of the parent compound on H. contortus axial muscle contractility. In the rat preparation, the 3-bromo derivative was more potent than levamisole or 3-amino-levamisole. 3-Amino-levamisole, but not 3-bromo-levamisole, depolarized muscle end-plate membrane in the rat diaphragm. Results of the present study are consistent with the following conclusions: (a) both levamisole and pyrantel block contractility of nematode axial muscle by causing sustained depolarization of the muscle membrane; (b) both drugs block neuromuscular transmission at the mammalian neuromuscular junction but their mechanisms appear to differ; (c) levamisole and pyrantel are more potent blockers of neuromuscular transmission in H. contortus than in the rat. These results suggest that potentially important pharmacological differences exist between nematode and mammalian somatic nicotinic receptors.

Effects of intermediary metabolites and electron transport inhibitors on action of chloroquine on Brugia pahangi and Onchocerca volvulus

We examined the possibility that chloroquine is interfering with aerobic energy-generating processes in the adult filarial parasites, Brugia pahangi and Onchocerca volvulus. Using motility of these parasites as an assay of drug effect, we found that micromolar concentrations of chloroquine caused significant paralysis, but only in alkaline medium (pH 8.4). The addition of 12 mM glutamine or 10 mM albizziin to the medium completely antagonized drug-induced paralysis. In addition, in B. pahangi, all of the tricarboxylic acid cycle intermediates (10 mM) except citrate and pyruvate antagonized the effect of chloroquine on motility; in O. volvulus, oxaloacetate as well as glutamine inhibited the effect of the drug. The effect of chloroquine on both parasites was enhanced when it was used in combination with 10 microM acivicin, a glutamine antimetabolite. Here motility of B. pahangi was reduced significantly within 24-48 hr at acidic (6.8) neutral (7.4) and alkaline (8.4) pH. This effect was partially reversible by glutamine (12 mM). Motility of O. volvulus was reduced to near zero within 4 hr with this drug combination. Antimycin A and rotenone, both electron transport inhibitors, also synergized with chloroquine at any pH to produce paralysis in B. pahangi. The effects of the rotenone and chloroquine combination were reversed in the presence of 10 mM succinate. However, glutamine (12 mM) was unable to antagonize the effects of chloroquine plus antimycin A on the motility of B. pahangi. These findings suggest that chloroquine may be inhibiting aerobic energy metabolism in the filariae, possibly at the level of electron transport. Furthermore, since chloroquine is well-tolerated but only weakly filaricidal in vivo, the data indicate that use of this drug in combination with other inhibitors of aerobic energy metabolism may be a chemotherapeutically useful approach to the treatment of filariases.

Glutamine- vs glucose-supported motor activity in Schistosoma mansoni: physiological relevance of aerobic metabolism

The ability of Schistosoma mansoni to generate energy through aerobic metabolic processes was examined in adult parasites in vitro. Parasite catabolism of radiolabeled glucose, glutamine, and other amino acids to CO2 and Krebs cycle intermediates was measured under a variety of incubation conditions. L-Glutamine was metabolized to CO2 via the intermediates glutamate, alpha-ketoglutaramate, and alpha-ketoglutarate in worms incubated in a balanced salts solution containing this amino acid as the only organic constituent. Of the other amino acids tested, CO2 production was detected from L-glutamate and L-asparagine. The catabolism of L-glutamine to CO2 was reduced by the respiratory inhibitor antimycin A. The motility of schistosomes in culture was maintained for at least 24 hr when L-glutamine was the only carbon source available to the worms. Under these conditions, motility was reduced when parasites were exposed to a respiratory inhibitor such as KCN, antimycin A, rotenone, or oligomycin, but it was completely restored by the addition of glucose to the medium. These results suggest that while the schistosome is capable of limited aerobic energy-generating processes under certain conditions, survival is not contingent upon these processes in the presence of glucose.

Anti-filarial effects of nine quinoline-containing drugs on adult filariae in vitro

The potencies and efficacies of 9 quinoline-containing anti-malarials including chloroquine, (bis)desethylchloroquine, SN6911, SN12108, amodiaquine, CN-2999-2K, primaquine, quinacrine, and quinine were examined in vitro against adult female Brugia pahangi. Parasite motility and lactate excretion were measured as indicators of drug effects. All of the agents tested showed time-dependent increases in potency over a 24-72-hr incubation period. SN12108 was the most potent at 72 hr, reducing motility by greater than or equal to 50% (IC50) at 1.0 x 10(-7) M. Chloroquine (IC50 2.3 x 10(-6) M), desethylchloroquine (IC50 7.0 x 10(-6) M), quinacrine (IC50 1.9 x 10(-6) M), and quinine (IC50 1.5 x 10(-5) M) were the least potent. All of the drugs caused time-dependent decreases in lactate excretion, except quinine; decreases were found to be dose dependent. A high correlation (r greater than 0.85) was seen between time-dependent effects on motility and lactate excretion. The effects of chloroquine (10 microM) on motility were also examined in female Acanthocheilonema viteae, Dirofilaria immitis, Onchocerca volvulus, and male Onchocerca gutturosa. Dirofilaria immitis was less sensitive to chloroquine than B. pahangi; A. viteae was equally sensitive. Species of Onchocerca were the most sensitive parasites studied. Adult O. gutturosa and O. volvulus were affected by 10 microM chloroquine within 4-6 hr; motility was reduced by 80% within 24 hr. Although the mechanism of anti-filarial activity of the quinoline-containing drugs is not known, their in vitro activity against a variety of adult filariae at clinically relevant concentrations, as well as differential sensitivity seen between the different filariae examined, warrants further study of these compounds.

pH-dependent uptake and macrofilaricidal effects of chloroquine on adult filarial parasites in vitro

Uptake and macrofilaricidal effects of chloroquine (CQ) and other aminoquinolines were found to be highly pH dependent in Brugia pahangi, Acanthocheilonema viteae, Onchocerca volvulus, and Dirofilaria immitis. Using [3H]CQ, it was found that all of the parasites took up more drug under alkaline conditions (RPMI 1640 at pH 8.4) than in neutral (pH 7.4) or acidic (pH 6.8) media. Differences were seen in the amount of drug taken up among the filariae studied. B. pahangi and A. viteae took up 7 times more chloroquine per milligram of tissue than did O. volvulus, and 30 times more than D. immitis during a 60-min incubation period at pH 8.4. Sensitivity to the aminoquinolines also increased with increasing media pH, and was measured using parasite motility as an indicator of drug efficacy. Potency of chloroquine against B. pahangi increased 100-fold at pH 8.4 compared to pH 7.4. A. viteae and O. volvulus showed similar sensitivity to chloroquine compared to B. pahangi; D. immitis was less sensitive. While uptake of chloroquine was linear from pH 6.8 to 8.4, B. pahangi was unaffected by 32 microM of the drug below pH 7.6; at any pH above this, motility of this parasite was completely inhibited. Calculations of the internal pH of this parasite indicated that it shifted upwards significantly with changes in media pH. It was concluded that these shifts in internal pH may render parasites more sensitive to the effects of chloroquine.

Comparative effects of anthelmintics on motility in vitro of Onchocerca gutturosa, Brugia pahangi and Acanthocheilonema viteae

The effects of standard anthelmintics on the motor activity in vitro of adult Onchocerca gutturosa, Brugia pahangi and Acanthocheilonema viteae were determined using a micromotility meter. Fresh adult males dissected from bovine tissues were the best source for observations on O. gutturosa. Parasites liber-ated by collagenase digestion showed poor viability and motility. Only segments of O. gutturosa females were obtainable by dissection and these were not able to sustain motility in vitro. Adult males and females of O. volvulus were active after collagenase digestion of human nodular tissue, but behaved so irregularly that satisfactory monitoring of their movements with the meter was not possible on a regular enough basis to permit quantitation of drug-induced changes. Inhibitory effects on motility of O. gutturosa, B. pahangi and A. viteae were produced by anthelmintics which showed macrofilaricidal effects in vivo in a laboratory rodent model, with the exception of the benzimidazoles. O. gutturosa was, however, much more sensitive than B. pahangi or A. viteae to the temporary paralyzing effects of levamisole and pyrantel. The utility of in vitro screening against O. gutturosa and B. pahangi was evaluated by determining the discriminatory capacity of the tests in detecting novel compounds with reproducible in vivo activity in the jird-B. pahangi/A. viteae model. The results suggested that this would be a valuable selective screening procedure. Although false positives were detected at the rate of 15-17% of the novel anthelmintic chemical series tested, no false negatives were allowed through the screen provided both O. gutturosa and B. pahangi were included.2=