James L. Bennett

A benzodiazepine derivative and praziquantel: effects on musculature of Schistosoma mansoni and Schistosoma japonicum.

SummaryThe benzodiazepine derivative (Ro 11-3128) which has central nervous effects similar to other benzodiazepines, and praziquantel (PZ), are two new antischistosomal drugs. At low concentrations these drugs will produce a marked spastic paralysis of male Schistosoma mansoni musculature. An analysis of the action of these drugs on the parasites musculature shows that Ro 11-3128 and PZ produced a rapid rise in the tension of the musculature of male schistomomes. Various compounds known to interact with the schistosomes neuroreceptive sites did not block or potentiate the action of these drugs. Removal of Ca2+ or addition of Mg2+ to the incubation medium blocked the action of these drugs on the schistosomes musculature. Uptake studies of inorganic cations by male schistosomes indicate that Ro 11-3128 and PZ decrease the influx of K+ but stimulate the influx of Ca2+ and Na+ into the male schistosome. It is suggested that this interference with inorganic ion transport mechanisms causes the contraction of the schistosome musculature.

Praziquantel: The Enigmatic Antiparasitic

Praziquantel (PZQ), a pyrazinoisoquinoline, was introduced as a novel anthelmintic in 1975. PZQ is currently the drug of choice for the treatment of a wide range of both veterinary and human trematode and cestode infections, including human schistosomiasis. Current estimates suggest that 150 million humans are infected with schistosomes, and it is expected that PZQ will play the lead role in chemotherapeutic control of those infections. Despite the time that has passed since its introduction and its obvious importance in global health care, it is not yet understood why PZQ is so selective and effective. The target molecules for PZQ have not been defined, nor are the sites of its effects within the parasites known. Here, Tim Day, James Bennett and Ralph Pax summarize some of the progress that has been made toward reaching these objectives in recent years.

Praziquantel-induced tegumental damage in vitro is diminished in schistosomes derived from praziquantel-resistant infections.

The aggressive use of praziquantel to combat schistosomiasis in Egpyt raises concern about the possible emergence of resistance. Eggs from Egyptian patients with praziquantel-resistant infections (not cured by 3 doses of praziquantel) have been used to establish infection-specific schistosome isolates in mice. The response of these worms to the drug was observed in vitro, in order to determine if the isolates obtained from these resistant infections were, in fact, less responsive to praziquantel. One of the hallmark effects of praziquantel on schistosomes in vitro is a disruption of the worms outer surface, the tegument. Here, praziquantel-induced tegumental damage is observed in 3 distinct isolates, 2 derived from resistant infections and 1 from an infection cured by a single dose. The isolates from the resistant infections were less susceptible to praziquantel-induced tegumental damage in vitro, suggesting that the worms are in some way less responsive to the drug.

Schistosoma mansoni: Differences in acetylcholine, dopamine, and serotonin control of circular and longitudinal parasite muscles

The physiological and pharmacological properties of circular and longitudinal somatic musculature in adult male Schistosoma mansoni were compared using cut muscle sections. Carbachol reduced tone in both circular and longitudinal muscle, but was without effect on circular muscle bathed in high Mg2+, indicating that cholinergic receptors were not associated with circular muscle membrane. 5-Hydroxytryptamine (5-HT) induced rhythmic contractile activity in both sets of muscle. It decreased muscle tone in circular muscle but increased the tone of longitudinal muscle. Metergoline blocked 5-HT effects on both sets of muscle. 5-HT continued to be effective on both sets of muscle bathed in high-Mg2+ medium, indicating that serotonergic receptors were present on both circular and longitudinal muscle membranes. Dopamine decreased both circular and longitudinal muscle tone. Its effects on circular muscle were still present after exposure to high Mg2+, but its effects on the longitudinal muscle were significantly reduced, leading to the conclusion that dopaminergic sites were probably associated with circular muscle membrane but not that of longitudinal muscle. Also, spiroperidol blocked stimulus responsiveness of the circular muscle but not that of the longitudinal muscle. From these studies it appears that there are significant physiological and pharmacological differences between circular and longitudinal muscles in the adult male schistosome.

Praziquantel, potassium and 2,4-dinitrophenol: Analysis of their action on the musculature of Schistosoma mansoni

Praziquantel (PZ) (10(-6) M), an antischistosomal drug, 60 mM K+ and 2,4-dinitrophenol (DNP) induce a rapid contracture of the musculature in adult male S. mansoni. This event is accompanied, except for DNP, by an increased accumulation of 45Ca2+ in the parasite. K+-induced contractures had phasic and tonic components while DNP and PZ produced only tonic contracture of the parasites musculature. The Ca2+ antagonist, D-600 (10(-4) M) as well as La3+ (10 mM) and Co2+ (10 mM) prevented the 45Ca2+ accumulation produced by high K+. La3+ and Co2+ blocked the tension increasing effects of high K+; D-600 blocked only the tonic contractures. D-600 and Co2+ did not prevent the 45Ca2+ increase produced by PZ but La3+ did. D-600 did not block the PZ response but Co2+ and La3+ partially attenuated it. La3+ also partially attenuated the DNP response. Co2+ and D-600 prevented sustained contractures in response to DNP. Thus, although K+, PZ and DNP have the same final effect on the musculature of S. mansoni, it appears that their effects are not mediated by the same mechanisms.

Schistosome resistance to praziquantel: Fact or artifact?

Praziquantel is the current drug of choice for human schistosomiasis. Recent reports from laboratory and field studies concerning reduced praziquantel efficacy against Schistosoma mansoni have generated some controversy. The prevailing question is whether the emergence of strains of schistosome resistant to praziquantel is a fact, or an artifact resulting from erroneous field or laboratory experimentation. In this article, Padraic Fallon, Liang-feng Tao, Magdi Ismail and James Bennett examine the available evidence for schistosome resistance to praziquantel. Contributory factors to the schistosomicidal activity of praziquantel, which may interfere with evaluation of drug efficacy or resistance, are also considered.

Platyhelminth FMRFamide-related peptides (FaRPs) contract Schistosoma mansoni (Trematoda: Digenea) muscle fibres in vitro

Molluscan FMRFamide and two recently discovered platyhelminth FMRFamide-related peptides (FaRPs), GNFFRFamide from the cestode Moniezia expansa and RYIRFamide from the terrestrial turbellarian Artioposthia triangulata, cause dose-dependent contractions of individual muscle fibres from Schistosoma mansoni in vitro. The most potent FaRP tested was the turbellarian peptide RYIRFamide, which produced a concentration-dependent effect between 10(-9) and 10(-7) M. FMRFamide and GNFFRFamide were less potent, inducing contractions between 10(-8)-10(-6) M and 10(-7)-10(-5) M respectively. The contractile effect of each of these peptides was blocked by the presence of 1 microM FMR-D-Famide. FMRF free acid did not elicit contraction of the muscle fibres. The FaRP-induced contractions did not occur if the Ca2+ was omitted and 0.5 microM EGTA was added to the extracellular medium. The FaRP-induced contractions were not blocked by the Ca2+ channel blockers nicardipine, verapamil or diltiazem, although high K+-induced contractions of these fibres were blocked by nicardipine. These data indicate the presence of FaRP receptors on schistosome muscle fibres and demonstrate their ability to mediate muscle contraction. The action of these endogenous flatworm peptides on schistosome muscle is the first demonstration of a direct excitatory effect of any putative neurotransmitter on the muscle of a flatworm, and establishes a role for FaRPs in neuromuscular transmission in trematodes. In addition, it provides the first evidence that the peptidergic nervous system is a rational target for chemotherapeutic attack in parasitic platyhelmiths.

Schistosoma mansoni: Direct method for simultaneous recording of electrical and motor activity

Abstract A method utilizing suction electrodes for simultaneous recording of electrical activity and motility of adult male Schistosoma mansoni is described. Spontaneous body contractions, similar to peristaltic waves, occur in an approximately rhythmic manner. Surface electrical potentials are associated with these contractions. Stretch increases the rate of spontaneous contractions. 5-Hydroxytryptamine at a concentration of 10 −6 M increases the contraction rate and also the complexity of the contractions. Carbachol at 10 −5 M decreases the rate at which contractions occur and also decreases the complexity of each contraction burst and at 10 −4 M completely suppresses activity.

Micromotility meter: an instrument designed to evaluate the action of drugs on motility of larval and adult nematodes

An instrument for measuring the motility of larval and adult nematodes is described along with an analysis of its use as a tool to analyse drug action on these parasites. Motility was detected from larval parasites of Nippostrongylus brasiliensis and Ascaris suum in the absence and presence of various anti-nematodal drugs. These agents produced, within 48 h, a significant decrease in larval parasite motility. The instrument was also capable of detecting the motility of Caenorhabditis elegans, adult female Brugia pahangi and their response to anti-nematodal drugs. The design of the instrument allows us to accurately measure motility in a single sample within 60 sec.

Pharmacokinetics of praziquantel in healthy volunteers and patients with schistosomiasis.

The pharmacokinetics of a novel praziquantel preparation (Distocide) were investigated in Sudanese patients with hepatosplenic schistosomiasis and in healthy volunteers, and compared with those of Biltricide. The results of the first study indicated greater (P less than 0.05) plasma concentrations of Biltricide at 1.5, 2, 3 and 5 h after administration than with Distocide; plasma elimination half-lives (t 1/2) were not significantly different. In patients with hepatosplenic schistosomiasis, higher plasma levels of Distocide were noted (P less than 0.05 at 8 h) compared to healthy controls; however, due to wide inter-individual variations, there were no significant differences in maximum plasma concentration, time to maximum plasma concentration, area under the plasma concentration curve (AUC), volume of distribution, or clearance; t 1/2 was greater (P less than 0.05) in patients (11.9 +/- 5.4 h) than controls (2.3 +/- 0.4 h). In the presence of food, higher plasma concentrations of Distocide occurred compared to the fasting state; AUCs were greater (P less than 0.01) in both food groups, although the values of t 1/2 were shorter. The lower plasma levels and longer duration of action of Distocide may be advantageous in reducing side effects and prolonging exposure of the schistosomes to the drug.