Structural and enzymatic insights into species-specific resistance to schistosome parasite drug therapy.

The antischistosomal prodrug oxamniquine is activated by a sulfotransferase (SULT) in the parasitic flatworm Schistosoma mansoni. Of the three main human schistosome species, only S. mansoni is sensitive to oxamniquine therapy despite the presence of SULT orthologs in Schistosoma hematobium and Schistosoma japonicum The reason for this species-specific drug action has remained a mystery for decades. Here we present the crystal structures of S. hematobium and S. japonicum SULTs, including S. hematobium SULT in complex with oxamniquine. We also examined the activity of the three enzymes in vitro; surprisingly, all three are active toward oxamniquine, yet we observed differences in catalytic efficiency that implicate kinetics as the determinant for species-specific toxicity. These results provide guidance for designing oxamniquine derivatives to treat infection caused by all species of schistosome to combat emerging resistance to current therapy.

Structural and Functional Characterization of the Enantiomers of the Antischistosomal Drug Oxamniquine.

BACKGROUND: For over two decades, a racemic mixture of oxamniquine (OXA) was administered to patients infected by Schistosoma mansoni, but whether one or both enantiomers exert antischistosomal activity was unknown. Recently, a ~30 kDa S. mansoni sulfotransferase (SmSULT) was identified as the target of OXA action. METHODOLOGY/PRINCIPAL FINDINGS: Here, we separate the OXA enantiomers using chromatographic methods and assign their optical activities as dextrorotary [(+)-OXA] or levorotary [(-)-OXA]. Crystal structures of the parasite enzyme in complex with optically pure (+)-OXA and (-)-OXA) reveal their absolute configurations as S- and R-, respectively. When tested in vitro, S-OXA demonstrated the bulk of schistosomicidal activity, while R-OXA had antischistosomal effects when present at relatively high concentrations. Crystal structures R-OXA•SmSULT and S-OXA•SmSULT complexes reveal similarities in the modes of OXA binding, but only the S-OXA enantiomer is observed in the structure of the enzyme exposed to racemic OXA. CONCLUSIONS/SIGNIFICANCE: Together the data suggest the higher schistosomicidal activity of S-OXA is correlated with its ability to outcompete R-OXA binding the sulfotransferase active site. These findings have important implications for the design, syntheses, and dosing of new OXA-based antischistosomal compounds.

Schistosomiasis control: praziquantel forever?

Since no vaccine exists against schistosomiasis and the molluscs acting as intermediate hosts are not easy to attack, chemotherapy is the main approach for schistosomiasis control. Praziquantel is currently the only available antischistosomal drug and it is distributed mainly through mass administration programs to millions of people every year. A number of positive features make praziquantel an excellent drug, especially with regard to safety, efficacy, cost and ease of distribution. A major flaw is its lack of efficacy against the immature stages of the parasite. In view of its massive and repeated use on large numbers of individuals, the development of drug resistance is a much feared possibility. The mechanism of action of praziquantel is still unclear, a fact that does not favor the development of derivatives or alternatives. A large number of compounds have been tested as potential antischistosomal agents. Some of them are promising, but none so far represents a suitable substitute or adjunct to praziquantel. The research of new antischistosomal compounds is an imperative and urgent matter.

Genetic and molecular basis of drug resistance and species-specific drug action in schistosome parasites.

Oxamniquine resistance evolved in the human blood fluke (Schistosoma mansoni) in Brazil in the 1970s. We crossed parental parasites differing ~500-fold in drug response, determined drug sensitivity and marker segregation in clonally derived second-generation progeny, and identified a single quantitative trait locus (logarithm of odds = 31) on chromosome 6. A sulfotransferase was identified as the causative gene by using RNA interference knockdown and biochemical complementation assays, and we subsequently demonstrated independent origins of loss-of-function mutations in field-derived and laboratory-selected resistant parasites. These results demonstrate the utility of linkage mapping in a human helminth parasite, while crystallographic analyses of protein-drug interactions illuminate the mode of drug action and provide a framework for rational design of oxamniquine derivatives that kill both S. mansoni and S. haematobium, the two species responsible for >99% of schistosomiasis cases worldwide.

Synthesis of new praziquantel analogues: potential candidates for the treatment of schistosomiasis.

An efficient synthesis of antischistosomal drug praziquantel and analogues was achieved and the synthetic route designed was to afford structurally diverse analogues for better structure-activity relationship understanding. Total of nineteen PZQ analogues with structural variations at amide, piperazine and aromatic moieties have been synthesized and fully characterized. Among all the new analogues tested for antischistosomal activity, one dimethoxy tetrahydroisoquinoline analogue and two tetrahydro-ß-carboline analogues exhibited moderate activity against adult Schistosoma mansoni. Tetrahydro-ß-carboline analogues showed moderate activity whereas the presence of p-trifluoromethylbenzoyl and p-toluenesulphonyl moieties resulted in complete suppression of antischistosomal activity.

Genetic analysis of decreased praziquantel sensitivity in a laboratory strain of Schistosoma mansoni.

A laboratory strain of Schistosoma mansoni subjected to repeated in vivo praziquantel (PZQ) treatments for several generations has been previously found to have lesser sensitivity to the drug than the original unselected strain. In this study we have collected evidence on the mode of inheritance of the partial insensitivity exhibited by the PZQ-selected schistosomes. A single male and a single female worm of the two strains, assorted in the four possible combinations, were introduced into the mesenteric veins of mice and the eggs produced by each pair were used as the source of the F(1) progeny. PZQ sensitivity was assessed using both in vivo and in vitro methods. In the first approach, the PZQ ED(50) was determined by infecting mice with cercariae of the strains to be tested, treating at seven weeks with different drug doses and counting the number of surviving worms three weeks later. For the in vitro approach, adult schistosomes kept in culture were exposed overnight to different PZQ concentrations and their survival was monitored during the subsequent 7 days. Results from both approaches lead to the conclusion that hybrid schistosomes of the F(1) generation have a drug sensitivity intermediate between those of the two parental strains and are thus suggestive of a pattern of partial dominance for the trait under study.

Schistosoma mansoni: lack of correlation between praziquantel-induced intra-worm calcium influx and parasite death.

The schistosomicidal activity of praziquantel (PZQ) is accompanied by a large influx of calcium into the worms, suggesting that this phenomenon could be the source of the observed muscular contraction, surface disruption and eventual death of the parasite. We have incubated live adult schistosomes in a medium containing radioactive calcium and we were able to confirm that PZQ does indeed stimulate calcium entry into the parasite. An even higher calcium uptake, however, occurred in schistosomes exposed to PZQ after pre-incubation with cytochalasin D, a condition that suppresses PZQ schistosomicidal effects and allows the complete survival of the parasites. The calcium blockers nicardipine and nifedipine also failed to prevent the calcium influx induced by PZQ. Similarly, a large calcium influx occurred in 28-day-old worms exposed to PZQ, in spite of the fact that these immature worms are largely insensitive to the schistosomicidal effects of the drug. Schistosomes incubated overnight with radioactive calcium and PZQ and then returned to normal medium, retained a calcium content higher than worms pre-incubated with cytochalasin D, but the difference could be a consequence--rather than a cause--of schistosomicidal effects. These results suggest that calcium accumulation by itself, at least as measured in whole parasites maintained in vitro, may not represent an exhaustive explanation for the schistosomicidal effects of PZQ.

Praziquantel derivatives I: Modification of the aromatic ring.

Several analogues of the potent anthelmintic praziquantel were prepared with variation in the aromatic ring. The biological activity of these analogues was evaluated and compared against known analogues. Amination of the ring was tolerated while other variations were not. These results have important implications for drug development for schistosomiasis.

Cytochalasin D abolishes the schistosomicidal activity of praziquantel.

To test the hypothesis that calcium channels of schistosomes are the targets for the action of praziquantel, we subjected schistosomes in vitro to pharmacological agents capable of interfering with the functioning of calcium channels. After 1-h exposure to these agents, praziquantel was added and incubation continued overnight. Worms were then washed, resuspended in drug-free medium and observed during the following 7-10 days. About 50% of schistosomes pre-exposed to the calcium channel blockers nicardipine and nifedipine were able to survive a praziquantel concentration (3 microM) that normally killed the majority of adult male worms. Since the organization of the actin cytoskeleton controls the activity of calcium channels in a number of different systems, we also pre-exposed schistosomes to the actin depolymerizing agent cytochalasin D. This treatment rendered the parasites completely refractory to the effects of very high praziquantel levels (up to 36 microM). These results are consistent with the hypothesis that schistosome calcium channels are involved in the mechanism of action of praziquantel.

The schistosome enzyme that activates oxamniquine has the characteristics of a sulfotransferase

Available evidence suggests that the antischistosomal drug oxamniquine is converted to a reactive ester by a schistosome enzyme that is missing in drug-resistant parasites. This study presents data supporting the idea that the active ester is a sulfate and the activating enzyme is a sulfotransferase. Evidence comes from the fact that the parasite extract loses its activating capability upon dialysis, implying the requirement of some dialyzable cofactor. The addition of the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) restored activity of the dialyzate, a strong indication that a sulfotransferase is probably involved. Classical sulfotransferase substrates like beta-estradiol and quercetin competitively inhibited the activation of oxamniquine. Furthermore, these substrates could be sulfonated in vitro using an extract of sensitive (but not resistant) schistosomes. Gel filtration analysis showed that the activating factor eluted in a fraction corresponding to a molecular mass of about 32 kDa, which is the average size of typical sulfotransferase subunits. Ion exchange and affinity chromatography confirmed the sulfotransferase nature of the enzyme. Putative sulfotransferases present in schistosome databases are being examined for their possible role as oxamniquine activators.

Praziquantel for the treatment of schistosomiasis: its use for control in areas with endemic disease and prospects for drug resistance.

Praziquantel became available for the treatment of schistosomiasis and other trematode-inflicted diseases in the 1970s. It was revolutionary because it could be administered orally and had very few unwanted side effects. As a result of marked reductions in the price of praziquantel, the rate at which it is used has accelerated greatly in recent years. For the foreseeable future it will be the mainstay of programs designed to control schistosome-induced morbidity, particularly in sub-Saharan Africa where schistosomiasis is heavily endemic. There is currently no evidence to suggest that any schistosomes have developed resistance to praziquantel as a result of its widespread use. Nevertheless, while resistance may not pose an obvious or immediate threat to the usefulness of praziquantel, complacency and a failure to monitor developments may have serious consequences in the longer term since it will be the only drug that is readily available for large-scale treatment of schistosomiasis.

The schistosome enzyme that activates oxamniquine has the characteristics of a sulfotransferase.

Available evidence suggests that the antischistosomal drug oxamniquine is converted to a reactive ester by a schistosome enzyme that is missing in drug-resistant parasites. This study presents data supporting the idea that the active ester is a sulfate and the activating enzyme is a sulfotransferase. Evidence comes from the fact that the parasite extract loses its activating capability upon dialysis, implying the requirement of some dialyzable cofactor. The addition of the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) restored activity of the dialyzate, a strong indication that a sulfotransferase is probably involved. Classical sulfotransferase substrates like beta-estradiol and quercetin competitively inhibited the activation of oxamniquine. Furthermore, these substrates could be sulfonated in vitro using an extract of sensitive (but not resistant) schistosomes. Gel filtration analysis showed that the activating factor eluted in a fraction corresponding to a molecular mass of about 32 kDa, which is the average size of typical sulfotransferase subunits. Ion exchange and affinity chromatography confirmed the sulfotransferase nature of the enzyme. Putative sulfotransferases present in schistosome databases are being examined for their possible role as oxamniquine activators.

Effect of praziquantel on the immature stages of Schistosoma haematobium.

Schistosoma mansoni is known to be refractory to praziquantel treatment in the pre-patent period of infection. Since Schistosoma haematobium has a much longer pre-patent period (10-12 weeks vs. 5-6 for the former species), we asked the question whether a correspondingly longer period of insusceptibility exists in urinary schistosomiasis. In hamsters treated at different times after infection, S. haematobium was partially refractory to praziquantel when treatment was given at week 5, but showed practically full sensitivity at 7-8 weeks and later times. Schistosoma haematobium worms obtained at different times after infection and exposed in vitro to praziquantel were refractory to low drug concentrations between 4 and 6 weeks, but were clearly affected at higher concentrations and at later time points. We conclude that S. haematobium does not have a praziquantel-insensitive window longer than in S. manson, in spite of its much longer maturation period. In addition, refractoriness of immature stages can be overcome at higher drug concentrations.

Determination of ED50 values for praziquantel in praziquantel-resistant and -susceptible Schistosoma mansoni isolates.

The dose of praziquantel required to kill 50% of adult worms in vivo (i.e. the ED50) was estimated for nine different isolates of Schistosoma mansoni in infected mice. Four of the isolates were selected because they had not knowingly been in contact with the drug (i.e. they were putatively praziquantel-susceptible). Five putatively praziquantel-resistant isolates were chosen because they had been selectively bred for drug-resistance in the laboratory and/or had previously been shown to be relatively resistant to praziquantel in the field. The work was performed in three laboratories in different countries using pre-agreed and comparable experimental protocols. All four praziquantel-susceptible isolates had ED50s estimated to be <100 mg/kg (mean=70+/-7 SD; median=68), while all five putatively praziquantel-resistant isolates had estimated ED50s >100 mg/kg (mean=209+/-48 SD; median=192). Thus, the five praziquantel-resistant isolates, including two that had been subjected to drug pressure during more than 20 passages in mice, had drug ED50s that were approximately three times as great as those of the praziquantel-susceptible isolates. Two of the five isolates in the putatively resistant group had previously been passaged 15 or more times in mice without administration of drug-pressure, but had ED50s consistent with the other three isolates in the group, indicating that the trait of praziquantel-resistance did not necessarily impair biological fitness during laboratory passage. The protocols used here to estimate the praziquantel ED50s of S. mansoni isolates should be useful for establishing and monitoring the drug susceptibility/resistance profiles of parasite isolates freshly obtained from endemic areas, particularly those in which increased usage of the drug is likely to occur.

Sex- and stage-related sensitivity of Schistosoma mansoni to in vivo and in vitro praziquantel treatment.

The efficacy of praziquantel against a Puerto Rican strain of Schistosoma mansoni was assessed using both in vivo and in vitro approach. The drug effective dose (50%) in the infected mouse model was about 30 times higher when determined against 28-day-old infections than against 7-week-old parasites. Single-sex female infections were also largely refractory to treatment and single-sex male infections moderately refractory, in comparison with bisexual infections. The in vitro approach consisted of overnight exposure of parasite cultures to various drug concentrations, followed by several days of culture in drug-free medium. In vitro results confirmed in vivo data and allowed for the observation of schistosome morphological phenomena after praziquantel exposure. Early worm contraction was observed in all cases, even after exposure to sub-lethal concentrations of praziquantel or upon exposure of the largely refractory 28-day-old schistosomes. In these instances, however, worms resumed movements and normal shape upon drug removal and were able to survive. The inference of these observations on the clinical use of praziquantel and on its mechanism of action is discussed.

Sequence and level of endogenous expression of calcium channel beta subunits in Schistosoma mansoni displaying different susceptibilities to praziquantel.

Kohn et al. [J. Biol. Chem. 276 (2001) 36873] demonstrated that cells expressing the structurally unusual schistosome beta subunit SmCavbeta1 in their voltage-operated calcium channels, exhibit an increased current amplitude in the presence of praziquantel (PZQ). This suggests that the beta subunit is involved in PZQ activity and is consistent with the known pharmacological effects of the drug. If this is so, the low susceptibility to PZQ noted in some Schistosoma mansoni strains could be due to some mutation(s) in the gene coding for this protein. We have sequenced the cDNAs coding for the SmCavbeta1 and SmCavbeta2 subunits of different sensitive and resistant strains and we have not been able to detect any meaningful differences. As an alternative hypothesis, the different sensitivity of schistosomes to PZQ action could be due to the expression of different beta subunits in the parasite. This interpretation could also explain the low PZQ susceptibility of immature worms (28 days). We analyzed Northern blots of various strains and various developmental stages, but we were unable to demonstrate major quantitative differences in the expression of the beta subunits.

Praziquantel.

Praziquantel is the drug of choice for the treatment of all forms of schistosomiasis. This review summarizes the main features of the drug, with special attention being given to those aspects that may be of interest to the practicing physician. After a brief mention of the history, the chemistry, the major available brands and their costs, doses and administration schedules are reviewed. Pharmacokinetics and drug interactions are analyzed and the low toxicity and mild side effects are stressed. A major weakness of praziquantel is its relative inefficacy against recent infections, a factor that may occasionally result in low cure rates in hyperendemic areas. Recent findings of schistosome isolates with a decreased sensitivity to praziquantel are discussed in the broader context of a possible emergence of drug resistance.

Rapamycin insensitivity in Schistosoma mansoni is not due to FKBP12 functionality.

Rapamycin (RAPA) is a well-known immunosuppressant, the action of which is mediated by the immunophilin FKBP12. Upon RAPA binding, FKBP12 forms ternary complexes with phosphatidyl inositol related kinases known as the target of RAPA (TOR), which can lead to a mitotic block at the G1-S phase transition. Such an antiproliferative effect makes RAPA an attractive anticancer, antifungal or antiparasitic compound. In this study, we found the helminth parasite Schistosoma mansoni to be insensitive to the drug. In order to elucidate the mechanism underlying RAPA resistance, the S. mansoni drug receptor FKBP12 (SmFKBP12) was cloned for functional analysis. Western blot experiments showed that the protein is constitutively expressed in all life cycle stages and in both male and female parasites. The Escherichia coli-synthesised recombinant protein possessed enzymatic activity, which was inhibitable by RAPA. Moreover, SmFKBP12 was able to complement mutant Saccharomyces cerevisiae cells lacking FKBP12 in their RAPA sensitivity phenotype, leading us to conclude that SmFKBP12 is expressed in yeast in a functional form and capable of interacting with the drug and yeast TOR kinase. Even though the wild type SmFKBP12 appeared to restore a large part of RAPA sensitivity, a mutation of Asp(89)-Lys(90) to Pro(89)-Gly(90) in the schistosome protein was found to be more effective and restored drug sensitivity to the same level as the endogenous yeast protein. Despite ternary complex formation, our results suggest that additional unknown factors other than a functional drug receptor are implicated in drug resistance mechanisms.