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.     

Stability and reproductive fitness of Schistosoma mansoni isolates with decreased sensitivity to praziquantel.

These studies are focused on schistosomes derived from human infections not cured by three successive doses of praziquantel that also produced infections in mice that were significantly more difficult to cure than infections with control worms. Half (three of six) of these isolates retained their decreased response to praziquantel after multiple passages through the life-cycle in the absence of therapeutic pressure. Two of the isolates, including the one initially least sensitive to praziquantel; reverted, to a sensitivity not significantly different from controls. For example, the EE6 isolate initially required 680 mg/kg praziquantel to affect a 50% reduction in worm load in murine infections, but after only six passages through the life cycle over 5 years this was reduced to 113 mg/kg, not different from control infections. The stability of some of the isolates and the reversion of others indicates that the biological or genetic factors conferring decreased praziquantel response varies among the isolates. The three isolates that retained decreased sensitivity to praziquantel all showed compromises in reproductive fitness in the laboratory, expressed most frequently as a decreased cercarial production from snails infected with those isolates compared to controls. For example, the total cercarial production of snails infected with the EE10 isolate was only 57% that of controls. The reversion of some of the isolates to a praziquantel sensitive state and the decreased reproductive fitness of those that did not revert suggest that there is some biological cost associated with the relative praziquantel insensitivity of these worms, which could help limit the impact of such isolates in the field. Infections with the less sensitive isolates also produced significantly less circulating schistosomal antigen in mice, suggesting that a decrease in the host immune response elicited by these worms could be one of the factors contributing to the diminished praziquantel efficacy.     

Observations on associated histopathology with Aggregata octopiana infection (Protista: Apicomplexa) in Octopus vulgaris

The effect of cimetidine on the praziquantel concentration in the blood of the rockfish Sebastes schlegeli and the consequent effect on the treatment efficacy against Microcotyle sebastis were investigated. Fish were divided into 7 groups and orally administered praziquantel alone (200 and 100 mg kg(-1) body weight [BW]) or in combination with cimetidine (in doses of 200, 100 or 50 mg kg(-1) BW cimetidine with a praziquantel dose of 100 mg kg(-1) BW). The fish in the sixth group were coadministered 50 mg praziquantel and 200 mg cimetidine kg(-1) BW. The fish in the control group were administered only saline. At 24 h post-treatment, the plasma was analyzed for praziquantel by reversed-phase high-performance liquid chromatography (RP-HPLC) using diazepam as the internal standard, and the gills were examined to confirm the effectiveness of each treatment. The praziquantel concentration in plasma of fish administered 100 mg praziquantel + 200 mg cimetidine kg(-1) BW was not significantly different from that of fish treated with 200 mg praziquantel kg(-1) BW and was significantly (p < 0.05) higher (about 2 times) than that of fish administered 100 mg praziquantel kg(-1) BW. The group of fish administered 50 mg praziquantel + 200 mg cimetidine kg(-1) BW showed a similar plasma praziquantel concentration to that in the fish treated with 100 mg praziquantel kg(-1) BW. The treatment efficacies of the groups of fish coadministered 100 mg praziquantel kg(-1) BW and various concentrations of cimetidine (200, 100 and 50 mg kg(-1) BW) were not significantly different from that of the group of fish administered 200 mg praziquantel kg(-1) BW, but were significantly higher than those of the groups of fish fed 100 mg praziquantel kg(-1) BW alone or coadministered 50 mg praziquantel + 200 mg cimetidine kg(-1) BW.     

Praziquantel: an urgent and exciting challenge

The anthelmintic drug praziquantel has proved useful in the treatment of schistosomiasis. The precise mechanism by which praziquantel kills the parasites has yet to be elucidated. Here, John Kusel and colleagues review the current theories on praziquantel action and suggest future avenues for research, which becomes urgent in the light of some reports of drug resistance.     

Chemotherapy of schistosomiasis: present and future

Schistosomiasis is a chronic parasitic disease in tropical and subtropical regions and is associated with a variety of clinical syndromes that may lead to severe morbidity. Over the past 25 years, therapy and control of schistosomiasis has come to rely heavily on one drug, praziquantel (PZQ). This reliance is of concern should widespread treatment failure arise, particularly as measures are being undertaken to increase PZQ's availability. This review summarizes the use, possible modes of action and limitations of PZQ, and recent attempts to derive synthetic analogs. Alternative artemisinin-based chemotherapies that have shown applicability in certain disease settings are also similarly examined. Looking forward, the review highlights some of the more experimental anti-schistosomals being evaluated (e.g. the trioxolanes), including those where knowledge of the parasite target (e.g. cysteine proteases and hemozoin formation) is more defined.     

In vitro effect of schistosomicidal drugs on hepatic arylsulfatase B from the schistosoma-infected mouse

Arylsulfatase B (ASB) hydrolyzes the desulfation of N-acetylgalactosamine-4-sulfate at the non-reducing terminal of glycosaminoglycans. This enzyme activity was found to be elevated in mice schistosomiasis. In the present study, the catalytic and immunological properties of purified ASB from the liver of Schistosoma-infected mouse was investigated in the presence and absence of the schistosomicidal drugs praziquantel and Commiphora extract. The in vitro effect of praziquantel was found to be inhibitory with a Ki value of 5.5 x 10(-4) M while that of commiphora extract was as an activator. Furthermore, these drugs did not have an observed effect on the immunological properties of ASB with regard to its binding to its polyclonal rabbit antibody. These results indicate that some schistosomicidal drugs may reverse the alteration of the catalytic properties of the enzyme in schistosomiasis.     

A direct competitive enzyme-linked immunosorbent assay (ELISA) for determination of praziquantel concentration in serum

A simple and reproducible enzyme-linked immunosorbent assay (ELISA) for the determination of the concentration of praziquantel in the serum was developed. Since praziquantel has no functional group to conjugate with carrier protein, praziquantel was first converted to a compound with an amino group similar to praziquantel. This compound was then conjugated to bovine serum albumin for use as an immunogen, and to horseradish peroxidase, as enzyme-labeled praziquantel, respectively. The conjugate of praziquantel-bovine serum albumin conjugate was used to raise anti-praziquantel antiserum in mice. The direct competitive ELISA was conducted by simultaneously incubating praziquantel and horseradish peroxidase-labeled praziquantel conjugate with anti-praziquantel antiserum over a second antibody and the enzyme activity of the remaining horseradish peroxidase-labeled praziquantel conjugate was measured. The intra- and inter-assay coefficient of variation was < 10% in the range of 1.0 to 30 ng ml(-1), and the limit of the detection was 0.3 ng ml(-1). The cross reactivities of anti-praziquantel antibody with compounds related to praziquantel were negligible. Using this ELISA, serum levels of praziquantel were easily determined in male Wistar rats up to 8 h following a single intraperitoneal injection at 2 mg kg(-1) of body weight.     

Are Ca2+ channels targets of praziquantel action?

Praziquantel is the current drug of choice for the control of schistosomiasis. It is highly effective against all species of schistosomes and shows minimal adverse effects. Though introduced for the treatment of schistosomiasis more than 20 years ago, the mode of action of praziquantel remains to be elucidated. This review will focus on advances in defining the molecular target of praziquantel action, with particular emphasis on recent work indicating an important role for voltage-gated calcium channels.     

In vitro responses of praziquantel-resistant and -susceptible Schistosoma mansoni to praziquantel

The resistance status of five praziquantel-susceptible and five praziquantel-resistant isolates was confirmed by chemotherapy in CD(1) mice with 3 x 200mg/kg micronised praziquantel. Micronised praziquantel had higher efficacy than two other praziquantel formulations (prepared without milling). The five resistant isolates were less responsive to praziquantel than the five susceptible isolates (59-74% reduction in worm burden in resistant isolates compared with 92-100% in susceptible isolates). Observations were made on the in vitro responses of different stages of 10 isolates to praziquantel. There were different in vitro responses to praziquantel at the egg, miracidial, cercarial and adult stages of Schistosoma mansoni between praziquantel-resistant and praziquantel-susceptible isolates. There were differences in the response of resistant and susceptible isolates following exposure of freshly hatched miracidia to 10(-6)M praziquantel for 1 min and observing the percent change in shape. Using this test it should be possible to determine whether failed therapy in patients infected with S. mansoni is due to the presence of praziquantel-resistant worms. Similarly, by exposing freshly shed cercariae to 4 x 10(-7)M praziquantel and observing the percent of tail shedding over 80 min it should be possible to monitor for the presence of praziquantel-resistant worms in snails collected in the field.     

A paragonimiasis patient with allergic reaction to praziquantel and resistance to triclabendazole: successful treatment after desensitization to praziquantel

Paragonimiasis is an infectious disease caused by trematodes of the genus Paragonimus. This trematode can be treated successfully with praziquantel in more than 90% of the cases. Although praziquantel is generally well tolerated, anaphylactic reactions to this drug have been reported in a few cases. We report here a 46-year-old Korean female with paragonimiasis, presumed to be due to Paragonimus westermani, who displayed an allergic reaction to praziquantel and resistance to triclabendazole treatment. The patient was successfully treated with praziquantel following a rapid desensitization procedure. Desensitization to praziquantel could be considered when no alternative drugs are available.     

Interpreting low praziquantel cure rates of Schistosoma mansoni infections in Senegal

Praziquantel is currently the drug of choice for the treatment of schistosomiasis. Selective treatment of Schistosoma mansoni infections in various endemic countries usually present cure rates of >70% when using the standard dose of 40 mg kg(-1) body weight of praziquantel. However, unusually low cure rates (18-38%) have been reported from Senegal, raising fears for emergence of resistance (or tolerance) to praziquantel. One major problem is the precise quantitative interpretation of cure rates, which allows an unequivocal distinction between drug failure and normal drug performance. This article reviews studies on praziquantel treatment of population by standardizing the data through an innovative meta-analysis and provides empirical evidence concerning the extent to which the reported low cure rates from Senegal are atypical.     

Treatment of Microcotyle sebastis infestation in cultured rockfish Sebastes schlegeli by oral administration of praziquantel in combination with cimetidine

The effect of cimetidine on the treatment efficacy of praziquantel against Microcotyle sebastis infestation in cultured rockfish Sebastes schlegeli was investigated. Juvenile rockfish were divided into 7 groups, and orally administered praziquantel alone (50, 100 and 200 mg kg(-1) body wt, BW) or in combination with cimetidine at a dose of 200 mg kg(-1) BW for each praziquantel dose. The fish in the control group were administered only saline. The results clearly showed that coadministration of cimetidine with praziquantel led to a significantly increased treatment efficacy of the latter drug, and consequently would lead to a lowering of the total dose of praziquantel, and a reduction in the administration times and costs for the treatment of M. sebastis infestation in cultured rockfish.     

The effect of praziquantel on Schistosoma mansoni.

3 x 10(-6)M praziquantel fails to completely paralyse miracidia and cercariae in a short time but they are not infective when maintained in the solution. 3 x 10(-5)M praziquantel prevents infected snails shedding cercariae but does not kill daughter sporocyts or developing cercariae. As the action of praziquantel on adult worms is not blocked by 10(-2)M mecamylamine, pempidine or carbachol, but is reduced by calcium depletion, it is suggested that praziquantel may act by permitting calcium influx to muscle cells causing them to contract.     

Potential Cost-Effectiveness of Schistosomiasis Treatment for Reducing HIV Transmission in Africa – The Case of Zimbabwean Women

Background

Epidemiological data from Zimbabwe suggests that genital infection with Schistosoma haematobium may increase the risk of HIV infection in young women. Therefore, the treatment of Schistosoma haematobium with praziquantel could be a potential strategy for reducing HIV infection. Here we assess the potential cost-effectiveness of praziquantel as a novel intervention strategy against HIV infection.

Methods

We developed a mathematical model of female genital schistosomiasis (FGS) and HIV infections in Zimbabwe that we fitted to cross-sectional data of FGS and HIV prevalence of 1999. We validated our epidemic projections using antenatal clinic data on HIV prevalence. We simulated annual praziquantel administration to school-age children. We then used these model predictions to perform a cost-effectiveness analysis of annual administration of praziquantel as a potential measure to reduce the burden of HIV in sub-Saharan Africa.

Findings

We showed that for a variation of efficacy between 30–70% of mass praziquantel administration for reducing the enhanced risk of HIV transmission per sexual act due to FGS, annual administration of praziquantel to school-age children in Zimbabwe could result in net savings of US$16–101 million compared with no mass treatment of schistosomiasis over a ten-year period. For a variation in efficacy between 30–70% of mass praziquantel administration for reducing the acquisition of FGS, annual administration of praziquantel to school-age children could result in net savings of US$36−92 million over a ten-year period.

Conclusions

In addition to reducing schistosomiasis burden, mass praziquantel administration may be a highly cost-effective way of reducing HIV infections in sub-Saharan Africa. Program costs per case of HIV averted are similar to, and under some conditions much better than, other interventions that are currently implemented in Africa to reduce HIV transmission. As a cost-saving strategy, mass praziquantel administration should be prioritized over other less cost-effective public health interventions.     

Therapeutic failure of praziquantel in the treatment of Schistosoma haematobium infection in Brazilians returning from Africa

Several cases of therapeutic failure of praziquantel used for the treatment of urinary schistosomiasis have been reported. Alternative drugs, like niridazol and metrifonate, have shown a lower therapeutic effect and more side effects than praziquantel. Twenty-six Brazilian military men (median age of 29 years) with a positive urine parasitological exam who were part of a United Nation peace mission in Mozambique in 1994 were treated with 40 mg/kg body weight praziquantel, single dose. They swam in Licungo river (Mocuba city, Mozambique) during the weekends. After this, they presented haematuria, dysuria, polakiuria, and lumbar pain. Control cystoscopy examinations carried out between 6 and 24 months after each treatment (including two additional treatments at a minimum interval of 6 months) revealed the presence of viable eggs. Granulomas in the vesical submucosa were observed in 46.2% (12/26) of the individuals. A vesical biopsy confirmed the presence of granulomas in all of these patients and the presence of viable eggs in 34.3% (9/26) of individuals who no longer excreted eggs in urine. The eggs filled with miracidia showed characteristics of viability. Histopathological examination using different strains demonstrated therapeutic failure and the need for repeated treatment. In this study, we demonstrated a low efficacy of praziquantel in the treatment of schistosomiasis haematobia, and the necessity of the urinary bladder biopsy as criterion of cure.     

Use of benzimidazole chemotherapy in human helminthiases: indications and efficacy

The past two decades have seen some remarkable developments in anthelmintic chemotherapy in clinical medicine. Whereas praziquantel has revolutionized the management of many cestode and trematode infections, Gordon Cook explains how the introduction of the benzimidazoles - most importantly thiabendazole, mebendazole and recently albendazole (flubendazole, cambendazole, ciclobendazole, and triclabendazole have also been used on a very limited scale) - has had a major impact upon the safe and effective management of several important intestinal and systemic nematode and, to a leser extent, cestode infections.     

Physicochemical Properties and Anti-Opisthorchosis Effect of Mechanochemically Synthesized Solid Compositions of Praziquantel with Glycyrrhizic Acid Disodium Salt

The mechanochemical preparation of solid compositions of praziquantel with plant saponin (glycyrrhizic acid disodium salt) is described. The study of a number of physicochemical parameters showed that dissolving solid compositions in water is accompanied by the inclusion of praziquantel molecules into micelles, which are formed in the solution of the glycyrrhizic acid disodium salt. Using the opisthorchiasis model caused by Opisthorchis felineus, we found a 4- to 11-fold increase in the anthelmintic activity of praziquantel in the composition as compared to the official praziquantel. According to the pharmacokinetic data, the use of the composition increased the bioavailability of praziquantel 3 times.     

Uptake and effect of praziquantel and the major human oxidative metabolite, 4-hydroxypraziquantel, by Schistosoma japonicum

After exposure to praziquantel in vitro at a concentration of 1 microgram/ml for 0.5-2 hr, amounts of praziquantel in Schistosoma japonicum varied from 2.1 +/- 1.2 to 3.7 +/- 1.6 ng/male worm and 1.3 +/- 1.2 to 2.2 +/- 1.5 ng/female worm during the time studied. At 30 micrograms/ml, praziquantel amounts were 11-33-fold higher. However, within 2 hr after removal from a medium containing 30 micrograms/ml praziquantel, 95% of the drug was released from the parasites. When S. japonicum worm pairs were incubated in vitro with 1, 10, and 30 micrograms/ml of 4-hydroxypraziquantel, the major human oxidative metabolite of praziquantel, 0.2 +/- 0.2, 3.8 +/- 1.3, and 7.4 +/- 1.3 ng/worm pair, respectively, were found after a 2-hr incubation. 15-30-fold lower than corresponding worm pair amounts of praziquantel. In vivo, when 4- or 5-wk S. japonicum-infected mice were treated orally with praziquantel (300 mg/kg), peak concentrations of praziquantel in plasma determined by high pressure liquid chromatography were 14.7 +/- 1.5 micrograms/ml (4-wk infection) and 16.7 +/- 2.8 micrograms/ml (5-wk infection) 15 min after treatment. Corresponding in vivo worm praziquantel amounts were 1.8 +/- 0.4 ng/male worm and 2.4 +/- 1.1 ng/female worm, respectively, in the 4-wk infection and 4.6 +/- 1.6 ng/male worm and 5.6 +/- 1.2 ng/female worm in the 5-wk infection. Peak plasma concentrations of 4-hydroxypraziquantel were similar but corresponding in vivo worm amounts were 1-20-fold lower, depending on the time after drug administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Schistosoma mansoni: activity responses in vitro to praziquantel.

The effects of praziquantel, a novel antischistosomal compound, on the activity of adult Schistosoma mansoni (Liverpool strain) in vitro were investigated. Worm activity was modified at all concentrations of praziquantel. High concentrations (above 0.5 microgram/ml) produced rapid paralysis, whilst low concentrations of praziquantel stimulated worm activity. It is concluded that such activity modification could lead to worm displacement in vivo.     

Depletion of praziquantel in plasma and muscle tissue of cultured rockfish Sebastes schlegeli after oral and bath treatment

Depletion of praziquantel in plasma and muscle tissue after oral and bath treatments was studied in cultured rockfish Sebastes schlegeli. In the oral treatment, a single dose of 400 mg praziquantel kg(-1) body weight was administered by intubation of the stomach. A bath treatment at 100 ppm of praziquantel for 4 min was also carried out. Plasma and muscle tissue samples were collected at 3, 6, 12, 24, 48, 72, 96, 120, 144 and 168 h post-treatment, and analyzed for praziquantel by reversed-phase HPLC using diazepam as the internal standard. Following oral treatment, praziquantel was detected in plasma and muscle tissue until 96 h after treatment. In plasma the praziquantel concentration was highest at the 9 h sampling time and declined sharply at the 48 h sampling point. The concentrations of praziquantel in the muscle tissue were lower than those in the plasma, and the highest value was found at the 9 h sampling time. Following bath treatment, praziquantel was found in plasma and muscle tissue until 72 and 24 h after treatment, respectively. In plasma the praziquantel concentration was highest at the 12 h sampling time and declined sharply thereafter. The concentrations of praziquantel in the muscle tissue were significantly lower than those in the plasma, and the concentrations declined consistently with time.