Screening a repurposing library, the Medicines for Malaria Venture Stasis Box, against Schistosoma mansoni

In Vitro Antischistosomal Activity of Bridelia ferruginea, Clausena anisata, Khaya senegalensis, and Vernonia amygdalina

Background

Schistosomiasis is caused by parasitic flatworms and the disease is endemic to most countries in sub-Saharan Africa including Ghana. The current therapeutic agent for managing this disease solely relies on praziquantel. The continual dependence on this single available drug could lead to possible drug resistance. This study seeks to evaluate the antischistosomal activity of the following Ghanaian medicinal plants: Khaya senegalensis, Vernonia amygdalina, Clausena anisata, and Bridelia ferruginea. Methodology. Two concentrations (100 μg/mL and 50 μg/mL) of each extract were tested in a 96-well plate containing 30 newly transformed schistosomula (NTS). Moreover, six worms of both sexes of adult Schistosoma mansoni were exposed to the extracts diluted in the RPMI medium. The assay was performed in a 24-well plate. The parasitic worms were examined using an inverted optical microscope.

Results

At 100 μg/mL and 50 μg/mL, all extracts performed better and showed strong activity (p < 0.001) against NTS; thus, 98.08%, 100%, 80.77%, and 100% for Clausena, Vernonia, Bridelia, and Khaya, respectively, when compared to praziquantel. Strong activity was recorded when the extracts underwent testing against Schistosoma mansoni adults at 100 μg/mL; 96.35%, 100%, and 94.55% for Vernonia, Bridelia, and Khaya, respectively, except for Clausena which exhibited weak activity, i.e., 56.02%. There was no significant difference between Vernonia, Bridelia, and Khaya when compared to praziquantel.

Conclusion

At 100 μg/mL, Khaya senegalensis, Vernonia amygdalina, and Bridelia ferruginea extracts demonstrated strong activity against both schistosomula and adult Schistosoma mansoni. These data can serve as baseline information in the quest to find alternative therapeutic agents to treat schistosomiasis.

Trichuris muris egg-hatching assay for anthelminthic drug discovery and characterization

Trichuriasis is a neglected tropical disease widely distributed among tropical and sub-tropical areas and associated with poverty and lack of access to safe drinking water, sanitation and hygiene. Existing drugs have limited efficacy and face a constant risk of developing resistance, necessitating the search for alternative treatments. However, drug discovery efforts are sparse and little research has been performed on anthelminthic effects on embryonated eggs, the infectious life stage of Trichuris spp. We examined bacterial species dependent egg hatching of the murine model parasite Trichuris muris and identified Escherichia coli, Pseudomonas aeruginosa and Enterobacter hormaechei effective as hatching inducers, resulting in hatching yields of 50-70%. Streptococcus salivarius, reported to be associated with reduced drug efficacy of ivermectin-albendazole coadministration in Trichuris trichiura infected patients, did not promote egg hatching in vitro. We optimized hatching conditions using E. coli grown in luria broth or brain-heart infusion media to reach consistently high hatching yields to provide a sensitive, robust and simple egg-hatching assay. Oxantel pamoate demonstrated the strongest potency in preventing hatching, with an EC50 value of 2-4 μM after 24 h, while pyrantel pamoate, levamisole and tribendimidine exhibited only moderate to weak inhibitory effects. Conversely, all tested benzimidazoles and macrolide anthelminthics as well as emodepside failed to prevent hatching (EC50 > 100 μM). Our study demonstrates that egg-hatching assays complement larval and adult stage drug sensitivity assays, to expand knowledge about effects of current anthelminthics on Trichuris spp. Further, the developed T. muris egg-hatching assay provides a simple and cheap screening tool that could potentially lead to the discovery of novel anthelminthic compounds.

Perspective on Schistosomiasis Drug Discovery: Highlights from a Schistosomiasis Drug Discovery Workshop at Wellcome Collection, London, September 2022

In September 2022, the Drug Discovery Unit at the University of Dundee, UK, organised an international meeting at the Wellcome Collection in London to explore the current clinical situation and challenges associated with treating schistosomiasis. The aim of this meeting was to discuss the need for new treatments in view of the clinical situation and to ascertain what the key requirements would be for any potential new anti-schistosomals. This information will be essential to inform ongoing drug discovery efforts for schistosomiasis. We also discussed the potential drug discovery pathway and associated criteria for progressing compounds to the clinic. To date, praziquantel (PZQ) is the only drug available to treat all species causing schistosomiasis, but it is often unable to completely clear parasites from an infected patient, partially due to its inactivity against juvenile worms. PZQ-mediated mass drug administration campaigns conducted in endemic areas (e.g., sub-Saharan Africa, where schistosomiasis is primarily prevalent) have contributed to reducing the burden of disease but will not eliminate the disease as a public health problem. The potential for Schistosoma to develop resistance towards PZQ, as the sole treatment available, could become a concern. Consequently, new anthelmintic medications are urgently needed, and this Perspective aims to capture some of the learnings from our discussions on the key criteria for new treatments.

Molecular Docking Studies of Rifampicin - rpoB complex: Repurposing Drug Design Implications for against Plasmodium falciparum Malaria through a Computational Approach

Malaria is one of the world's most devastating diseases, infecting well over 300 million people annually and killing between 2 and 3 million worldwide. Increasing parasite resistance to many existing drugs is exacerbating disease. Resistance to commonly used malarial drugs is increasing the need to develop new drugs urgently. Due to the slow pace and substantial costs of new drug development, repurposing of old drugs which is recently increasingly becoming an attractive proposition of highly efficient and effective way of drug discovery led us to study the drug rifampicin for this purpose. The present paper aims to investigate the route of Plasmodium falciparum apicoplast-targeted proteins that putatively encode β subunits of RNA polymerase with an objective to develop an effective antimalarial drug. Homology searching for conserved binding site to the rifampicin drug and the functional analysis of rpoB gene were done. Multiple Sequence alignment analysis of rpoB was compared with that in E.coli - rpoB and M. tuberculosis - rpoB. Docking studies of Rifampicin - rpoB complex was also done for finding binding affinity. The results of computational studies showed that rifampicin is a potential drug for malaria.

Schistosoma mansoni α-N-acetylgalactosaminidase (SmNAGAL) regulates coordinated parasite movement and egg production

α-galactosidase (α-GAL) and α-N-acetylgalactosaminidase (α-NAGAL) are two glycosyl hydrolases responsible for maintaining cellular homeostasis by regulating glycan substrates on proteins and lipids. Mutations in the human genes encoding either enzyme lead to neurological and neuromuscular impairments seen in both Fabry- and Schindler/Kanzaki- diseases. Here, we investigate whether the parasitic blood fluke Schistosoma mansoni, responsible for the neglected tropical disease schistosomiasis, also contains functionally important α-GAL and α-NAGAL proteins. As infection, parasite maturation and host interactions are all governed by carefully-regulated glycosylation processes, inhibiting S. mansoni's α-GAL and α-NAGAL activities could lead to the development of novel chemotherapeutics. Sequence and phylogenetic analyses of putative α-GAL/α-NAGAL protein types showed Smp_089290 to be the only S. mansoni protein to contain the functional amino acid residues necessary for α-GAL/α-NAGAL substrate cleavage. Both α-GAL and α-NAGAL enzymatic activities were higher in females compared to males (p<0.05; α-NAGAL > α-GAL), which was consistent with smp_089290's female biased expression. Spatial localisation of smp_089290 revealed accumulation in parenchymal cells, neuronal cells, and the vitellaria and mature vitellocytes of the adult schistosome. siRNA-mediated knockdown (>90%) of smp_089290 in adult worms significantly inhibited α-NAGAL activity when compared to control worms (siLuc treated males, p<0.01; siLuc treated females, p<0.05). No significant reductions in α-GAL activities were observed in the same extracts. Despite this, decreases in α-NAGAL activities correlated with a significant inhibition in adult worm motility as well as in egg production. Programmed CRISPR/Cas9 editing of smp_089290 in adult worms confirmed the egg reduction phenotype. Based on these results, Smp_089290 was determined to act predominantly as an α-NAGAL (hereafter termed SmNAGAL) in schistosome parasites where it participates in coordinating movement and oviposition processes. Further characterisation of SmNAGAL and other functionally important glycosyl hydrolases may lead to the development of a novel anthelmintic class of compounds.

Discovery of novel antischistosomal scaffolds from the open access Pandemic Response Box

BACKGROUND

Treatment and control of schistosomiasis rely on a single drug, praziquantel. New orally active antischistosomals featuring novel molecular scaffolds are urgently needed to prevent the emergence of resistance.

METHODS

We screened 400 drug-like compounds contained in the open-access Pandemic Response Box (PRB) against newly transformed schistosomula (NTS) at a concentration of 10 µM scoring death, changes in motility, and morphological alterations. Compounds displaying an activity ≥66% at 72 h underwent testing against adult Schistosoma mansoni in vitro. Fast-acting (≥66% at 24 h), nontoxic drugs focusing on late-stage and approved drugs were investigated in the patent S. mansoni mouse model.

RESULTS

We identified 26 hits active against NTS, of which 17 elicited ≥66% activity against adult S. mansoni following 24 h of drug exposure. The highest activity against adult S. mansoni was observed with MMV1581558 (EC₅₀ value of 0.18 ± 0.01 µM) and nitazoxanide (0.47 ± 0.07 µM). Of the five compounds tested in vivo, MMV1581558 and the approved drug ozanimod reduced average worm burden versus controls by 42 % and 36 %, respectively, after a single oral dose of 200 mg/kg bodyweight in mice harboring a chronic S. mansoni infection.

CONCLUSION

MMV1581558 discovered from screening the PRB represents a novel antischistosomal scaffold with high in vitro antischistosomal activity amenable to chemical modification for drug development.

Identification of hit compounds with anti-schistosomal activity on in vitro generated juvenile worms in cell-free medium

Background

Anthelminthic treatment options against schistosomiasis are limited. The current treatment relies almost exclusively on a single drug, praziquantel (PZQ). As a consequence, the development of resistance to PZQ and limited activity of PZQ against earlier development stages are respectively a risk and a limitation to achieving the goals of the new WHO roadmap towards elimination. For the discovery of new chemical starting points, the in vitro drug screening on Schistosoma mansoni (S. mansoni) against newly transformed schistosomula (NTS) is still the most predominant approach. The use of only NTS in the initial screening limits sensitivity to potential new compounds which are predominantly active in later developmental stages. Using our recently described highly standardized, straightforward and reliable culture method that generates high rates of juvenile worms, we aimed to repurpose a subset of the National Center for Advancing Translational Sciences (NCATS) Pharmaceutical Collection (340 compounds) to identify new hits with an in vitro worm culture assay.

Methodology/Principal findings

Cercariae were mechanically transformed into skin-stage (SkS) schistosomula and continuously cultured for 3–6 weeks to the liver stage (LiS). A commercial source of serum was identified, and decrease of NTS/well along with optimal drug testing conditions was established to test compounds on early and late LiS worms. The library was screened in 96-well format assays using praziquantel (PZQ) as a positive control. Primary screening allowed a 5.9% hit rate and generated two confirmed hits on adult worms; a prophylactic antianginal agent and an antihistaminic drug.

Conclusion

With this standardized and reliable in vitro assay, important S. mansoni developmental stages up to LiS worms can be generated and cultured over an extended period. When exposed to a subset of the NCATS Pharmaceutical Collection, 3 compounds yielded a defined anti-schistosomal phenotype on juvenile worms. Translation of activity on perfused adult S. mansoni worms was achieved only for perhexiline (a prophylactic antianginal agent) and astemizole (an antihistaminic drug).

Schistosomiasis continues to be a major public health problem, mainly in developing countries. Although there have been some advances in finding new drugs, praziquantel is still the drug of choice. Certainly, one of the most important advances in the search for new treatments was the ability to in vitro transform cercariae and to grow schistosomula in culture. To reduce animal use in future drug discovery efforts (3Rs), we optimized a previously established reliable and robust in vitro cell-free culture system for the generation of liver-stage worms that we applied to the screening of a compound library stemming from the National Center for Advancing Translational Sciences (NCATS) Pharmaceutical Collection.

Expanding the Activity Profile of Pyrido[1,2-a]benzimidazoles: Synthesis and Evaluation of Novel N1-1-Phenylethanamine Derivatives against Schistosoma mansoni

Praziquantel is the only widely available drug to treat schistosomiasis. With very few candidates currently in the drug development pipeline, there is an urgent need to discover and develop novel antischistosomal drugs. In this regard, the pyrido[1,2-a]benzimidazole (PBI) scaffold has emerged as a promising chemotype in hit-to-lead efforts. Here, we report a novel series of antischistosomal PBIs with potent in vitro activity (IC₅₀ values of 0.08-1.43 μM) against Schistosoma mansoni newly transformed schistosomula and adult worms. Moreover, the current PBIs demonstrated good hepatic microsomal stability (>70% of drug remaining after 30 min) and were nontoxic to the Chinese hamster ovarian and human liver HepG2 cells, though toxicity (selectivity index, SI < 10) against the rat L6 myoblast cell line was observed. The compounds showed a small therapeutic window but were efficacious in vivo, exhibiting moderate to high worm burden reductions of 35.8-89.6% in S. mansoni-infected mice.

Validation of a human-serum-based in vitro growth method for drug screening on juvenile development stages of Schistosoma mansoni

Background

Schistosomiasis affects over 200 million people worldwide but only praziquantel is available for treatment and control. Drug discovery is often based on phenotypic drug screening, involving different parasite stages retrieved from infected mice. Aiming to reduce animal use, we validated an in vitro growth method for juvenile Schistosoma mansoni for the purpose of drug sensitivity assays.

Methodology/Principal findings

We compared inter–batch variability of serum, worm size and organ development, gender distribution, and drug sensitivity between in vitro and in vivo grown worms over different life stages. In vitro developed S. mansoni in Hybridoma medium supplemented with 20% human serum were similar in size as in vivo worms until 28 days of incubation (males 1.4 ± 0.2 mm, females 1.1 ± 0.5 mm long). qPCR analysis revealed similar gender distribution both on newly transformed schistosomula and worms grown for 21 days. Worms developed in vitro and in vivo were similarly sensitive to praziquantel from 7 to 35 days of development with the exception of 21 days of development, where a slightly lower activity was observed for the in vitro grown worms (IC₅₀: 0.54 μM in vitro, 0.14 μM in vivo 72 hours post-incubation). The evaluation of five additional drugs revealed a similar sensitivity on worms developed for 21 days, with the exception of mefloquine, where we observed a 10-fold lower sensitivity on in vitro developed schistosomes when compared to in vivo grown (IC₅₀: 4.43 μM in vitro, 0.48 μM in vivo).

Conclusion

A large number of juvenile S. mansoni worms can be grown in vitro, which show similar drug sensitivity, gender distribution, size and morphology as the worms recovered from rodents, supporting the use of this method in drug screening efforts.

Schistosomiasis is a water-borne disease affecting over 200 million people worldwide and praziquantel is the only drug available for treatment. Although this drug is effective on all Schistosoma species on the adult stage of development, its activity is only partial against early infection stages. In addition, this has been the only drug in use for over 40 years, a situation that poses selective pressure towards resistant worms and highlights the urgent need of new drugs. The discovery and development of new drugs is often based on drug screenings, performed on worms collected from infected mammals. In line with animal welfare recommendations and aiming to increase throughput and reduce the assay costs, we validated a recently established in vitro method to grow juvenile S. mansoni as a drug screening tool. The in vitro established worms have similar gender distribution, size and drug sensitivity as worms collected from mice.

Chemotherapy for human schistosomiasis: how far have we come? What's new? Where do we go from here?

Globally, schistosomiasis threatens more than 700 million lives, mostly children, in poor localities of tropical and sub-tropical areas with morbidity due to acute and chronic pathological manifestations of the disease. After a century since the first antimonial-based drugs were introduced to treat the disease, anti-schistosomiasis drug development is again at a bottleneck with only one drug, praziquantel, available for treatment purposes. This review focuses on promising chemotypes as potential starting points in a drug discovery effort to meet the urgent need for new schistosomicides.

The Schistosoma mansoni tegumental allergen protein, SmTAL1: Binding to an IQ-motif from a voltage-gated ion channel and effects of praziquantel

SmTAL1 is a calcium binding protein from the parasitic worm, Schistosoma mansoni. Structurally it is comprised of two domains - an N-terminal EF-hand domain and a C-terminal dynein light chain (DLC)-like domain. The protein has previously been shown to interact with the anti-schistosomal drug, praziquantel (PZQ). Here, we demonstrated that both EF-hands in the N-terminal domain are functional calcium ion binding sites. The second EF-hand appears to be more important in dictating affinity and mediating the conformational changes which occur on calcium ion binding. There is positive cooperativity between the four calcium ion binding sites in the dimeric form of SmTAL1. Both the EF-hand domain and the DLC-domain dimerise independently suggesting that both play a role in forming the SmTAL1 dimer. SmTAL1 binds non-cooperatively to PZQ and cooperatively to an IQ-motif from SmCa_v1B, a voltage-gated calcium channel. PZQ tends to strengthen this interaction, although the relationship is complex. These data suggest the hypothesis that SmTAL1 regulates at least one voltage-gated calcium channel and PZQ interferes with this process. This may be important in the molecular mechanism of this drug. It also suggests that compounds which bind SmTAL1, such as six from the Medicines for Malaria Box identified in this work, may represent possible leads for the discovery of novel antagonists.

Multi-center screening of the Pathogen Box collection for schistosomiasis drug discovery

BACKGROUND

Over the past five years, as a public service to encourage and accelerate drug discovery for diseases of poverty, the Medicines for Malaria Venture (MMV) has released box sets of 400 compounds named the Malaria, Pathogen and Stasis Boxes. Here, we screened the Pathogen Box against the post-infective larvae (schistosomula) of Schistosoma mansoni using assays particular to the three contributing institutions, namely, the University of California San Diego (UCSD) in the USA, the Swiss Tropical and Public Health Institute (Swiss TPH) in Switzerland, and the Fundação Oswaldo Cruz (FIOCRUZ) in Brazil. With the same set of compounds, the goal was to determine the degree of inter-assay variability and identify a core set of active compounds common to all three assays. New drugs for schistosomiasis would be welcome given that current treatment and control strategies rely on chemotherapy with just one drug, praziquantel.

METHODS

Both the UCSD and Swiss TPH assays utilize daily observational scoring methodologies over 72 h, whereas the FIOCRUZ assay employs XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide) at 72 h to measure viability as a function of NAD⁺/NADH redox state. Raw and transformed data arising from each assay were assembled for comparative analysis.

RESULTS

For the UCSD and Swiss TPH assays, there was strong concordance of at least 87% in identifying active and inactive compounds on one or more of the three days. When all three assays were compared at 72 h, concordance remained a robust 74%. Further, robust Pearson's correlations (0.48-0.68) were measured between the assays. Of those actives at 72 h, the UCSD, Swiss TPH and FIOCRUZ assays identified 86, 103 and 66 compounds, respectively, of which 35 were common. Assay idiosyncrasies included the identification of unique compounds, the differential ability to identify known antischistosomal compounds and the concept that compounds of interest might include those that increase metabolic activity above baseline.

CONCLUSIONS

The inter-assay data generated were in good agreement, including with previously reported data. A common set of antischistosomal molecules for further exploration has been identified .

Life cycle maintenance and drug-sensitivity assays for early drug discovery in Schistosoma mansoni

Drug discovery for schistosomiasis is still limited to a handful of academic laboratories worldwide, with only a few novel antischistosomal lead compounds being actively researched. Despite recent international mobilization against the disease to stimulate and promote antischistosomal drug discovery, setting up a drug-screening flow with schistosome parasites remains challenging. Whereas numerous different protocols to obtain and cultivate schistosomes have been published, those describing the drug-screening process are scarce, and none gather together parasite cultivation and early drug discovery procedures. To help overcome this hurdle, we provide here a set of integrated methods either adapted from already-published protocols or based on our long-term experience in schistosomiasis research. Specifically, we detail the establishment and maintenance of the complex and several-week-long Schistosoma mansoni life cycle in a laboratory setting, as well as the means of retrieving and culturing the parasites at their relevant life stages. The in vitro and in vivo assays that are performed along the drug-screening cascade are also described. In these assays, which can be performed within 5 d, the effect of a drug is determined by phenotypic assessment of the parasites' viability and morphology, for which stage-specific scoring scales are proposed. Finally, the modalities for testing and evaluating a compound in vivo, constituting a procedure lasting up to 10 weeks, are presented in order to go from in vitro hit identification to the selection of early lead candidates.

Screening the Pathogen Box for Identification of New Chemical Agents with Anti-Fasciola hepatica Activity

Fascioliasis is an infectious parasitic disease distributed globally and caused by the liver fluke Fasciola hepatica or F. gigantica This neglected tropical disease affects both animals and humans, and it represents a latent public health problem due to the significant economic losses related to its effects on animal husbandry. For decades, triclabendazole has been the unique anti-Fasciola drug that can effectively treat this disease. However, triclabendazole resistance in fascioliasis has more recently been reported around the world, and thus, the discovery of novel drugs is an urgent need. The aim of this study was to investigate the fasciocidal properties of 400 compounds contained in the Pathogen Box. The first stage of the screening was carried out by measuring the fasciocidal activity on metacercariae at a concentration of 33 μM each compound (the standard dose). Subsequently, the activities of the most active compounds (n = 33) at their 50% inhibitory concentration (IC₅₀) values against metacercariae were assayed, and the results showed that 13 compounds had IC₅₀s of ≤10 μM. The second stage queried the activities of these compounds at 33 μM against adult flukes, with seven of the compounds producing high mortality rates of >50%. Four hit compounds were selected on the basis of their predicted nontoxic properties, and the IC₅₀ values obtained for adult worms were <10 μM; thus, these compounds represented the best fasciocidal compounds tested here. A cytotoxicity assay on four types of cell lines demonstrated that three compounds were nontoxic at their most active concentration. In conclusion, three hit compounds identified in this proof-of-concept study are potential candidates in the discovery of new fasciocidal drugs. Further studies are warranted.

Early Antischistosomal Leads Identified from in Vitro and in Vivo Screening of the Medicines for Malaria Venture Pathogen Box

As part of the control and elimination strategy of human schistosomiasis, preventive chemotherapy relies on a single drug, praziquantel. Facing an almost dry drug development pipeline, screening the Pathogen Box from the Medicines for Malaria Venture (MMV), provides a unique opportunity to possibly expand the pool of potent molecules against schistosomiasis. The activity of 400 compounds from this open-access library was first screened in vitro on the larval stage of Schistosoma mansoni. The hits were then tested on adult worms. Eleven leads were identified and tested for albumin-binding and activity on adult S. haematobium. In parallel, a rudimental structure-activity relationship analysis was performed on the 112 available analogues of three leads, yielding another 30 molecules active against both larval and adult stages of S. mansoni. Seven leads, selected on druglikeness, pharmacokinetic properties, and availability, plus auranofin were tested in mice harboring a chronic S. mansoni infection. MMV022029 and MMV022478 revealed the highest worm burden reductions of 67.8 and 70.7%, respectively. This study provided a series of new potent scaffolds and pharmacophores that could be used to design and develop suitable alternative(s) to praziquantel.

Interactive online application for the prediction, ranking and prioritisation of drug targets in Schistosoma haematobium

BACKGROUND

Human schistosomiasis is a neglected tropical disease caused by parasitic worms of the genus Schistosoma that still affects some 200 million people. The mainstay of schistosomiasis control is a single drug, praziquantel. The reliance on this drug carries a risk of resistance emerging to this anthelmintic, such that research towards alternative anti-schistosomal drugs is warranted. In this context, a number of studies have employed computational approaches to prioritise proteins for investigation as drug targets, based on extensive genomic, transcriptomic and small-molecule data now available.

METHODS

Here, we established a customisable, online application for the prioritisation of drug targets and applied it, for the first time, to the entire inferred proteome of S. haematobium. This application enables selection of weighted and ranked proteins representing potential drug targets, and integrates transcriptional data, orthology and gene essentiality information as well as drug-drug target associations and chemical properties of predicted ligands.

RESULTS

Using this application, we defined 25 potential drug targets in S. haematobium that associated with approved drugs, and 3402 targets that (although they could not be linked to any compounds) are conserved among a range of socioeconomically important flatworm species and might represent targets for new trematocides.

CONCLUSIONS

The online application developed here represents an interactive, customisable, expandable and reproducible drug target ranking and prioritisation approach that should be useful for the prediction of drug targets in schistosomes and other species of parasitic worms in the future. We have demonstrated the utility of this online application by predicting potential drug targets in S. haematobium that can now be evaluated using functional genomics tools and/or small molecules, to establish whether they are indeed essential for parasite survival, and to assist in the discovery of novel anti-schistosomal compounds.