Paraherquamide is a novel natural anthelmintic product with a mode of action that is incompletely characterized. Nicotine and cholinergic-anthelmintic agonists of different chemical classes were used to produce contraction in Ascaris muscle strips. Paraherquamide and a semisynthetic derivative, 2-deoxy-paraherquamide, antagonized these responses. Analysis of the actions of the antagonists was made using the simple competitive model and nonlinear regression to estimate the pK B values of the antagonists. The analysis was tested using Clark plots. The pK B values for paraherquamide were: nicotine, 5.86 Ϯ 0.14; levamisole, 6.61 Ϯ 0.19; pyrantel, 6.50 Ϯ 0.11; and bephenium, 6.75 Ϯ 0.15. The pK B of nicotine was significantly different from the pK B values for levamisole, pyrantel, and bephenium, showing that paraherquamide can distinguish a subtype of cholinergic receptors sensitive to nicotine and a subtype of cholinergic receptors sensitive to levamisole, pyrantel, and bephenium. The pK B values for 2-deoxy-paraherquamide were: levamisole, 5.31 Ϯ 0.13; pyrantel, 5.63 Ϯ 0.10; and bephenium, 6.07 Ϯ 0.13. The Clark plots of the antagonism illustrated the degree of fit to the competitive model for 2-deoxy-paraherquamide. 2-Deoxy-paraherquamide selectively antagonized the effects of bephenium; the pK B values of levamisole and pyrantel were significantly different from the pK B of bephenium. Paraherquamide and 2-deoxy-paraherquamide are selective competitive cholinergic antagonists that distinguish subtypes of cholinergic receptor in Ascaris muscle corresponding to nicotine-, levamisole-, and bephenium-sensitive receptors.Nematode parasite infections of humans and animals cause disease with loss of productivity, debility, and occasionally death. Ascariasis and hookworm infections are carried by 1.6 billion people throughout the world and in 2% of cases cause loss of life. The use of therapeutic compounds forms a major component of control, and the development of novel therapeutic agents is required to deal with the increasing levels of resistance to existing drugs.Paraherquamide (Fig. 1) is a novel anthelmintic (Yamazaki et al., 1981) that is an alkaloid fermentation product originally isolated from Penicillium paraherquii. The anthelmintic property of paraherquamide was first identified using jirds infected with Trichostrongylus colubriformis (Ostlind et al., 1990). Paraherquamide produces paralysis of parasitic nematodes in culture, without an effect on ATP, suggesting that it does not act as a metabolic poison (Thompson et al., 1996). Interestingly, one of the toxic effects of paraherquamide in the dog (Shoop et al., 1990) is a prolapsed nictitating membrane, an effect that suggests antagonism of neuronal nicotinic receptors (nAChRs). Recently, it has been reported (E.

No abstract

Here we review recent studies on the mode of action of the cholinergic anthelmintics (levamisole, pyrantel etc.). We also include material from studies on the free living nematode Caenorhabditis elegans. The initial notion that these drugs act on a single receptor population, while attractive, has proven to be an oversimplification. In both free living and parasitic nematodes there are multiple types of nicotinic acetylcholine receptor (nAChR) on the somatic musculature. Each type has different (sometimes subtly so) pharmacological properties. The implications of these findings are: (1) combinations of anthelmintic that preferentially activate a broad range of nAChR types would be predicted to be more effective; (2) in resistant isolates of parasite where a subtype has been lost, other cholinergic anthelmintics may remain effective. Not only are there multiple types of nAChR, but relatively recent research has shown these receptors can be modulated; it is possible to increase the response of a parasite to a fixed concentration of drug by altering the receptor properties (e.g. phosphorylation state). These findings offer a potential means of increasing efficacy of existing compounds as an alternative to the costly and time consuming development of new anthelmintic agents.

No abstract

1 The development of resistance to all chemotherapeutic agents increases and needs to be addressed. We are interested in resistance in parasitic nematodes to the anthelmintic levamisole. During studies on methyridine, we found that it gave us a new insight into pharmacological changes associated with levamisole resistance. Initially, electrophysiological investigation using a two-micropipette currentclamp recording technique revealed that methyridine acts as a cholinergic agonist on nematode muscle receptors (Ascaris suum). Methyridine (430 mM) produced reversible concentration-dependent depolarizations and increases in input conductance. Mecamylamine (30 mM) and paraherquamide (0.3 mM) produced reversible antagonism of the depolarization and conductance responses to methyridine. These observations suggest that methyridine, like acetylcholine and levamisole, gates ion channels on the muscle of parasitic nematodes. 2 The antagonistic effects of dihydro-b-erythroidine and paraherquamide on methyridine-induced contractions of A. suum muscle flaps were then examined to determine if methyridine showed subtype selectivity for N-subtype (nicotine-sensitive) or L-subtype (levamisole-sensitive) acetylcholine receptors. Dihydro-b-erythroidine weakly antagonized the effects of methyridine (but had no effect on levamisole responses). The antagonism of methyridine (pA 2 , 5.9) and nicotine (pA 2 , 6.1) by paraherquamide was similar, but was less than the antagonism of levamisole (pA 2 , 7.0). The antagonist profiles suggested that methyridine has a selective action on the N-subtype rather than on the L-subtype. 3 A novel use for a larval inhibition migration assay was made using L 3 larvae of Oesophagostomum dentatum. Inhibitory effects of nicotine, levamisole, pyrantel and methyridine on the migration of larvae of levamisole-sensitive (SENS) and levamisole-resistant (LEV-R) isolates were tested at different concentrations. Levamisole and pyrantel (putative L-subtype-selective agonists) concentration -response plots were displaced to the right in LEV-R isolates. Nicotine (an N-subtype-selective agonist) and methyridine produced little shift in concentration -response plots in the LEV-R isolates. Resistance dose ratios were used to calculate the relative selectivity, r L , for the L-type receptor (levamisole r L ¼ 1.0; pyrantel r L ¼ 0.93; methyridine r L ¼ 0.17; nicotine r L ¼ 0.06). These observations reveal an N-subtype-selective action of methyridine and suggest that levamisole resistance may be associated with a loss of the L-subtype, but not the N-subtype receptors. The pharmacology of methyridine suggests an approach for the treatment of levamisole-resistant parasites. British Journal of Pharmacology (2003) 140, 1068-1076. doi:10.1038/sj.bjp.0705528 Keywords: Methyridine; levamisole; nicotine; pyrantel; levamisole-resistance; Ascaris suum; Oesophagostomum dentatum; ACh receptor subtypes; N-subtype; L-subtype.Abbreviations: ANOVA, analysis of variance; AChR, acetylcholine-gated ion-channel receptor; DMSO, dimethylsulfox...