Increased action of triclabendazole (TCBZ) in vitro against a TCBZ-resistant isolate of Fasciola hepatica following its co-incubation with the P-glycoprotein inhibitor, R(+)-verapamil

Pivotal role of permeability-glycoprotein in absorption of praziquantel into Dicrocoelium dendriticum parasite

Dicrocoelium dendriticum affects the livers of ruminants and causes several deleterious effects on animal health status. The aim of this study was to investigate the role of permeability-glycoprotein (P-gp) in absorption of praziquantel (PZQ) into D. dendriticum flukes by co-incubation with verapamil (VPL), an inhibitor of P-gp, under in vitro conditions. Mature flukes of D. dendriticum were collected from naturally infected sheep livers. The flukes were incubated with different concentrations of PZQ and VPL (50 and 100 μg/ml) in culture media and after several times of exposure (2, 6, 12, and 24 h), the concentration of PZQ absorbed in the parasites was measured by high-performance liquid chromatography. At 2-h post-incubation, the highest concentration of PZQ was noted as 0.92 µg/ml in the flukes treated with 100 μg/ml of each PZQ and VPL. After 24-h of exposure, VPL at all tested concentrations resulted in significant increase in absorption of PZQ into the parasite. Co-incubation of lancet flukes with VPL and PZQ roughly doubled the absorption of PZQ into them. Results of tegumental structures analysis by light microscopy confirmed higher efficacy of combination of VPL and PZQ. In conclusion, co-administration of VPL, especially at the concentration of 100 μg/ml, was able to increase PZQ uptake in Dicrocoelium flukes at all time points of the study.

Understanding anthelmintic resistance in livestock using "omics" approaches

Widespread and improper use of various anthelmintics, genetic, and epidemiological factors has resulted in anthelmintic-resistant (AR) helminth populations in livestock. This is currently quite common globally in different livestock animals including sheep, goats, and cattle to gastrointestinal nematode (GIN) infections. Therefore, the mechanisms underlying AR in parasitic worm species have been the subject of ample research to tackle this challenge. Current and emerging technologies in the disciplines of genomics, transcriptomics, metabolomics, and proteomics in livestock species have advanced the understanding of the intricate molecular AR mechanisms in many major parasites. The technologies have improved the identification of possible biomarkers of resistant parasites, the ability to find actual causative genes, regulatory networks, and pathways of parasites governing the AR development including the dynamics of helminth infection and host-parasite infections. In this review, various "omics"-driven technologies including genome scan, candidate gene, quantitative trait loci, transcriptomic, proteomic, and metabolomic approaches have been described to understand AR of parasites of veterinary importance. Also, challenges and future prospects of these "omics" approaches are also discussed.

A major locus confers triclabendazole resistance in Fasciola hepatica and shows dominant inheritance

Fasciola hepatica infection is responsible for substantial economic losses in livestock worldwide and poses a threat to human health in endemic areas. The mainstay of control in livestock and the only drug licenced for use in humans is triclabendazole (TCBZ). TCBZ resistance has been reported on every continent and threatens effective control of fasciolosis in many parts of the world. To date, understanding the genetic mechanisms underlying TCBZ resistance has been limited to studies of candidate genes, based on assumptions of their role in drug action. Taking an alternative approach, we combined a genetic cross with whole-genome sequencing to localise a ~3.2Mbp locus within the 1.2Gbp F. hepatica genome that confers TCBZ resistance. We validated this locus independently using bulk segregant analysis of F. hepatica populations and showed that it is the target of drug selection in the field. We genotyped individual parasites and tracked segregation and reassortment of SNPs to show that TCBZ resistance exhibits Mendelian inheritance and is conferred by a dominant allele. We defined gene content within this locus to pinpoint genes involved in membrane transport, (e.g. ATP-binding cassette family B, ABCB1), transmembrane signalling and signal transduction (e.g. GTP-Ras-adenylyl cyclase and EGF-like protein), DNA/RNA binding and transcriptional regulation (e.g. SANT/Myb-like DNA-binding domain protein) and drug storage and sequestration (e.g. fatty acid binding protein, FABP) as prime candidates for conferring TCBZ resistance. This study constitutes the first experimental cross and genome-wide approach for any heritable trait in F. hepatica and is key to understanding the evolution of drug resistance in Fasciola spp. to inform deployment of efficacious anthelmintic treatments in the field.

The induction and inhibition of UDP-glycosyltransferases in Haemonchus contortus and their role in the metabolism of albendazole

Albendazole (ABZ) is an anthelmintic frequently used to treat haemonchosis, a common parasitosis of ruminants caused by the gastrointestinal nematode Haemonchus contortus. This parasite is able to protect itself against ABZ via the formation of inactive ABZ-glycosides. The present study was designed to deepen the knowledge about the role of UDP-glycosyltransferases (UGTs) in ABZ glycosylation in H. contortus. The induction effect of phenobarbital, a classical inducer of UGTs, as well as ABZ and ABZ-sulphoxide (ABZSO, the main active metabolite of ABZ) on UGTs expression and UGT activity toward ABZ was studied ex vivo in isolated adult nematodes. The effect of three potential UGT inhibitors (5-nitrouracil, 4,6-dihydroxy-5-nitropyrimidine and sulfinpyrazone) on ABZ glycosylation was tested. Pre-incubation of nematodes with ABZ and ABZSO led to increased expression of several UGTs as well as ABZ-glycosides formation in subsequent treatment. Phenobarbital also induced UGTs expression, but did not affect ABZ biotransformation. In the nematode's subcellular fraction, sulfinpyrazone inhibited UGT activity toward ABZ, although no effect of other inhibitors was observed. The inhibitory potential of sulfinpyrazone on the formation of ABZ-glycosides was also proved ex vivo in living nematodes. The obtained results confirmed the role of UGTs in ABZ biotransformation in H. contortus adults and revealed sulfinpyrazone as a potent inhibitor of ABZ glycosylation in this parasite. The possible use of sulfinpyrazone with ABZ in combination therapy merits further research.

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UDP-glycosyl transferases catalyse biotransformation of benzimidazole anthelmintics.

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Gene expression of several UGTs in H. contortus is affected by albendazole.

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The inhibition of UGTs leads to a decrease in the amounts of glycosylated metabolites.

Drug resistance in liver flukes

Liver flukes include Fasciola hepatica, Fasciola gigantica, Clonorchis sinensis, Opisthorchis spp., Fascioloides magna, Gigantocotyle explanatum and Dicrocoelium spp. The two main species, F. hepatica and F. gigantica, are major parasites of livestock and infections result in huge economic losses. As with C. sinensis, Opisthorchis spp. and Dicrocoelium spp., they affect millions of people worldwide, causing severe health problems. Collectively, the group is referred to as the Food-Borne Trematodes and their true significance is now being more widely recognised. However, reports of resistance to triclabendazole (TCBZ), the most widely used anti-Fasciola drug, and to other current drugs are increasing. This is a worrying scenario. In this review, progress in understanding the mechanism(s) of resistance to TCBZ is discussed, focusing on tubulin mutations, altered drug uptake and changes in drug metabolism. There is much interest in the development of new drugs and drug combinations, the re-purposing of non-flukicidal drugs, and the development of new drug formulations and delivery systems; all this work will be reviewed. Sound farm management practices also need to be put in place, with effective treatment programmes, so that drugs can be used wisely and their efficacy conserved as much as is possible. This depends on reliable advice being given by veterinarians and other advisors. Accurate diagnosis and identification of drug-resistant fluke populations is central to effective control: to determine the actual extent of the problem and to determine how well or otherwise a treatment has worked; for research on establishing the mechanism of resistance (and identifying molecular markers of resistance); for informing treatment options; and for testing the efficacy of new drug candidates. Several diagnostic methods are available, but there are no recommended guidelines or standardised protocols in place and this is an issue that needs to be addressed.

Pleiotropic alterations in gene expression in Latin American Fasciola hepatica isolates with different susceptibility to drugs

Background

Fasciola hepatica is the main agent of fasciolosis, a zoonotic disease affecting livestock worldwide, and an emerging food-borne disease in humans. Even when effective treatments are available, drugs are costly and can result in tolerance, liver damage and normally they do not prevent reinfection. Drug-resistant strains in livestock have been reported in various countries and, more worryingly, drug resistance in human cases has emerged in South America. The present study aims to characterize the transcriptome of two South American resistant isolates, the Cajamarca isolate from Peru, resistant to both triclabendazole and albendazole (TCBZR/ABZR) and the Rubino isolate from Uruguay, resistant to ABZ (TCBZS/ABZR), and compare them to a sensitive strain (Cenapa, Mexico, TCBZS/ABZS) to reveal putative molecular mechanisms leading to drug resistance.

Results

We observed a major reduction in transcription in the Cajamarca TCBZR/ABZR isolate in comparison to the other isolates. While most of the differentially expressed genes are still unannotated, several trends could be detected. Specific reduction in the expression levels of cytoskeleton proteins was consistent with a role of tubulins as putative targets of triclabendazole (TCBZ). A marked reduction of adenylate cyclase might be underlying pleiotropic effects on diverse metabolic pathways of the parasite. Upregulation of GST mu isoforms suggests this detoxifying mechanism as one of the strategies associated with resistance.

Conclusions

Our results stress the value of transcriptomic approaches as a means of providing novel insights to advance the understanding of drug mode of action and drug resistance. The results provide evidence for pleiotropic variations in drug-resistant isolates consistent with early observations of TCBZ and ABZ effects and recent proteomic findings.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2553-2) contains supplementary material, which is available to authorized users.

RNA interference in Fasciola gigantica: Establishing and optimization of experimental RNAi in the newly excysted juveniles of the fluke

Fasciolosis caused by Fasciola gigantica is a neglected tropical disease but a constraint on the growth and productivity of cattle, buffaloes and sheep in the tropical countries of Asia and Africa. Resistance to commonly used anthelmintics in Fasciola has increased the need to search for alternative therapeutic targets. RNA interference is the current tool of choice in the search for such targets in Fasciola. The susceptibility of juvenile Fasciola hepatica to double stranded (ds) RNA induced RNAi has been established but in F. gigantica a single preliminary report on RNAi induced mRNA transcript knockdown is available. Here we optimized conditions for RNAi in the liver fluke F.gigantica targeting six genes including superoxide dismutase (SOD), σ class of glutathione-s-transferase (GST), cathepsin (Cat) L1-D, Cat B1, Cat B2 and Cat B3 that showed robust transcriptional silencing of the targets following exposure of the newly excysted juveniles (NEJs) to long (170–223 nt) dsRNA. Knockdown was shown to be concentration dependent with significant mRNA transcript suppression occurring at 5 ng / μl that showed further suppression with the increase in the dsRNA concentration. The dsRNA induced persistent silencing of the mRNA transcript of SOD and σGST up to 15 days of observation. Delivery of the long dsRNA and siRNA to the newly excysted juveniles by soaking method was found to be efficient by tracking the uptake and diffusion of Cy³ labelled siRNA and long dsRNA in the flukes. Off-target effects of dsRNA trigger on some of the non-target genes were detected in the present investigation on RNAi in F. gigantica. The dsRNA induced superoxide dismutase protein suppression while impact of RNAi on other target proteins was not studied. There is no in vitro culture system for prolonged survival of the F. gigantica and in the present study in vitro maintenance of the NEJs is reported for a period of 3 weeks. The present study is the first attempt on optimization of RNAi protocols in F. gigantica where long dsRNA allowed for an efficient and persistent gene silencing, opening prospects for functional validation of putative vaccine and therapeutic targets in this neglected parasite.

RNA interference (RNAi) is a powerful method for selectively silencing genes for the validation of potential targets for drug and vaccine development. The susceptibility of juvenile Fasciola hepatica to double stranded (ds) RNA induced RNAi has been established but in F. gigantica a single report of a preliminary study on knockdown of a single gene transcript exists. In the absence of other tools of reverse genetics, RNAi occupies a centre stage in the validation of gene functions in Fasciola species. This study focuses on F.gigantica, an economically important veterinary parasite with a zoonotic potential. Here in this study, we optimized a set of simple methods for triggering RNAi in the F. gigantica juvenile liver fluke, which shows that a robust transcriptional suppression can be readily achieved across all targets tested and with protein suppression confirmed in one of the targets. These studies also highlight the need for developing an in vitro maintenance system for the fluke for validation of the RNAi protocols. These findings are important for researchers aiming to employ RNAi in investigations of liver fluke biology and target validation.

Co-delivery of hydrophilic gemcitabine and hydrophobic paclitaxel into novel polymeric micelles for cancer treatment

Schematic illustration of the preparation and intracellular performance of GEM–VE and PTX–VE loaded FA–PEG–VE micelle.

Current Threat of Triclabendazole Resistance in Fasciola hepatica

Triclabendazole (TCBZ) is the only chemical that kills early immature and adult Fasciola hepatica (liver fluke) but widespread resistance to the drug greatly compromises fluke control in livestock and humans. The mode of action of TCBZ and mechanism(s) underlying parasite resistance to the drug are not known. Due to the high prevalence of TCBZ resistance (TCBZ-R), effective management of drug resistance is now critical for sustainable livestock production. Here, we discuss the current status of TCBZ-R in F. hepatica, the global distribution of resistance observed in livestock, the possible mechanism(s) of drug action, the proposed mechanisms and genetic basis of resistance, and the prospects for future control of liver fluke infections using an integrated parasite management (IPM) approach.

Disruption of spermatogenesis in the liver fluke, Fasciola hepatica by two artemisinin derivatives, artemether and artesunate

An in vivo study in the laboratory rat model has been carried out to monitor changes to the spermatogenic cells in the testis tubules of adult Fasciola hepatica following treatment with the artemisinins, artemether and artesunate. Rats infected with the triclabendazole (TCBZ)-resistant Sligo isolate were dosed orally with artemether at a concentration of 200 mg/kg and flukes recovered at 24, 48 and 72 h post treatment (pt). Rats infected with the TCBZ-resistant Oberon isolate were dosed orally with artesunate at a concentration of 200 mg/kg and flukes recovered 24, 48, 72 and 96 h pt. The flukes were processed for histological and transmission electron microscope (TEM) examination. Changes to the spermatogenic cells were evident at 24 h pt with artemether. The spermatogonial and spermatocyte cells contained abnormal mitochondria, there were fewer spermatids and spermatozoa in the tubules than normal, and a number of cells showed signs of apoptosis. There was a further decline in cell numbers at 48 h pt and the organization of the spermatocyte and spermatid rosettes was atypical. Sperm formation had become abnormal and those spermatozoa present possessed only a single axoneme. By 72 h pt, the testis tubules were vacuolated and filled with abnormal cells and cell debris. Only spermatogonial cells could be identified and there was widespread evidence of apoptosis in the cells. Distinct cellular changes following artesunate treatment did not become apparent until 48 h pt. The changes seen were similar to those described for artemether, but were generally less severe at matching time-periods. The fine structural changes occurring in the spermatogenic cells were compared to those observed in other cell types and fluke tissues and the overall information was collated to identify the cellular targets for artemisinin action and to establish the time-line for drug action.

The Role of Xenobiotic-Metabolizing Enzymes in Anthelmintic Deactivation and Resistance in Helminths

Xenobiotic-metabolizing enzymes (XMEs) modulate the biological activity and behavior of many drugs, including anthelmintics. The effects of anthelmintics can often be abolished by XMEs when the drugs are metabolized to an inefficient compound. XMEs therefore play a significant role in anthelmintic efficacy. Moreover, differences in XMEs between helminths are reflected by differences in anthelmintic metabolism between target species. Taking advantage of the newly sequenced genomes of many helminth species, progress in this field has been remarkable. The present review collects up to date information regarding the most important XMEs (phase I and phase II biotransformation enzymes; efflux transporters) in helminths. The participation of these XMEs in anthelmintic metabolism and their possible roles in drug resistance are evaluated.

New tricks for old dogs: countering antibiotic resistance in tuberculosis with host-directed therapeutics

Despite the availability of Mycobacterium tuberculosis (Mtb) drugs for over 50 years, tuberculosis (TB) remains at pandemic levels. New drugs are urgently needed for resistant strains, shortening duration of treatment, and targeting different stages of the disease, especially for treatment during human immunodeficiency virus co-infection. One solution to the conundrum that antibiotics kill the bacillus yet select for resistance is to target the host rather than the pathogen. Here, we discuss recent progress in so-called 'host-directed therapeutics' (HDTs), focusing on two general mechanistic strategies: (i) HDTs that disrupt Mtb pathogenesis in macrophages and (ii) immunomodulatory HDTs that facilitate protective immune responses that kill Mtb or reduce deleterious responses that exacerbate disease. HDTs hold significant promise as adjunctive therapies in that they are less likely to engender resistance, will likely have efficacy against antibiotic-resistant strains, and may have activity against non-replicating Mtb. However, TB is a complex and variegated disease, and human populations exhibit significant diversity in their immune responses to it, which presents a complicated landscape for HDTs to navigate. Nevertheless, we suggest that a detailed mechanistic understanding of drug action, together with careful selection of disease stage targets and dosing strategies may overcome such limitations and allow the development of HDTs as effective adjunctive treatment options for TB.

A comparative study on the impact of two artemisinin derivatives, artemether and artesunate, on the female reproductive system of Fasciola hepatica

An in vivo study in the laboratory rat model has been carried out to monitor changes to the female reproductive system in adult Fasciola hepatica following treatment with the artemisinins, artemether and artesunate. Rats infected with the triclabendazole (TCBZ)-resistant Sligo isolate were dosed orally with artemether at a concentration of 200mg/kg and flukes recovered at 24, 48 and 72 h post-treatment (pt). Rats infected with the TCBZ-resistant Oberon isolate were dosed orally with artesunate at a concentration of 200mg/kg and flukes recovered 24, 48, 72 and 96 h pt. The flukes were processed for histological and transmission electron microscope (TEM) examination of the uterus, Mehlis' gland, ovary and vitellaria. After treatment with artemether, egg production had become abnormal by 72 h pt, with free vitelline cells and masses of shell protein material within the uterus; spermatozoa were absent. The Mehlis' gland and ovary retained a normal morphology over the 3-day period. A change in the cell population in the vitelline follicles was seen at 48 h pt, with a decline in the number of immature cells. This became more marked by 72 h and the follicles became progressively vacuolated over the 3-day period. At the TEM level, there were changes in the immature vitelline cells at 24h pt, as evidenced by a decrease in shell protein production and the presence of lipid droplets and abnormal mitochondria. Spaces in the follicles separated the cells from each other. The changes became progressively more severe with time, so that, by 72 h pt, the follicles were very disrupted, containing cells in the advanced stages of apoptotic breakdown. In extreme cases, the follicles were scarcely recognisable and had become filled with cellular debris. Fine structural changes to the vitelline cells induced by artesunate treatment were similar to those described for artemether, but generally occurred more quickly and were greater; this was particularly true of the swelling of the ger cisternae. Overall, the results have shown that artemisinin treatment has a severe impact on egg production by TCBZ-resistant flukes, an effect that is mediated by disruption of the vitelline cells.

The T687G SNP in a P-glycoprotein gene of Fasciola hepatica is not associated with resistance to triclabendazole in two resistant Australian populations

Triclabendazole (TCBZ) is widely used for control of Fasciola hepatica (liver fluke) in animals and humans and resistance to this drug is now widespread. However, the mechanism of resistance to TCBZ is not known. A T687G single nucleotide polymorphism (SNP) in a P-glycoprotein gene was proposed as a molecular marker for TCBZ resistance in F. hepatica (Wilkinson et al., 2012). We analyzed this Pgp gene from TCBZ-susceptible and TCBZ-resistant populations from Australia to determine if the SNP was a marker for TCBZ resistance. From the 21 parasites studied we observed 27 individual haplotypes in the Pgp sequences which comprised seven haplotypic groups (A-G), with haplotypes A and B representing 81% of the total observed. The T687G SNP was not observed in either of the resistant or susceptible populations. We conclude that the T687G SNP in this Pgp gene is not associated with TCBZ resistance in these Australian F. hepatica populations and therefore unlikely to be a universal molecular marker for TCBZ resistance.

Theranostic nanoemulsions: codelivery of hydrophobic drug and hydrophilic imaging probe for cancer therapy and imaging

Aim: To develop a theranostic nanoemulsion (TNE) that can codeliver the conjugates of a hydrophobic drug paclitaxel (PTX) and a hydrophilic imaging probe sulforhodamine B (SRB). Materials & methods: The TNE was established using core-matched technology, and can achieve high encapsulation efficiency and synchronized release of the loaded cargo. It has been examined for a correlation between the dynamic uptake of PTX and the intensity of SRB imaging signal in different organs. Results & discussion: Our data demonstrate that the TNE, with improved circulation time, increases therapeutic efficacy and imaging efficiency in both drug-sensitive and drug-resistant cancer. The TNE could not satisfy the demand of visual diagnosis in the living animal because of interference. We therefore formulated a long-circulating theranostic nanoemulsion (LCTNE). Results showed that the LCTNE can meet imaging requirements in vivo. Conclusion: The LCTNE plays a good therapeutic and diagnostic role for subcutaneous tumors in the living animal.

Fasciola hepatica tegumental antigens indirectly induce an M2 macrophage-like phenotype in vivo

The M2 subset of macrophages has a critical role to play in host tissue repair, tissue fibrosis and modulation of adaptive immunity during helminth infection. Infection with the helminth, Fasciola hepatica, is associated with M2 macrophages in its mammalian host, and this response is mimicked by its excretory-secretory products (FhES). The tegumental coat of F. hepatica (FhTeg) is another major source of immune-modulatory molecules; we have previously shown that FhTeg can modulate the activity of both dendritic cells and mast cells inhibiting their ability to prime a Th1 immune response. Here, we report that FhTeg does not induce Th2 immune responses but can induce M2-like phenotype in vivo that modulates cytokine production from CD4(+) cells in response to anti-CD3 stimulation. FhTeg induces a RELMα expressing macrophage population in vitro, while in vivo, the expression of Arg1 and Ym-1/2 but not RELMα in FhTeg-stimulated macrophages was STAT6 dependent. To support this finding, FhTeg induces RELMα expression in vivo prior to the induction of IL-13. FhTeg can induce IL-13-producing peritoneal macrophages following intraperitoneal injection This study highlights the important role of FhTeg as an immune-modulatory source during F. hepatica infection and sheds further light on helminth-macrophage interactions.

RNA interference in Fasciola hepatica newly excysted juveniles: long dsRNA induces more persistent silencing than siRNA

In trematodes RNA interference is the current tool of choice for functional analysis of genes since classical reverse genetic approaches remain unavailable. Whereas this approach has been optimized in schistosomes, few reports are available for other trematodes, likely reflecting the difficulties in the establishment of the technology. Here we standardized conditions for RNAi in the liver fluke Fasciola hepatica, the causative agent of fasciolosis, one of the most problematic infections affecting livestock worldwide. Targeting a single copy gene, encoding leucine aminopeptidase (LAP) as a model, we refined delivery conditions which identified electro-soaking, i.e. electroporation and subsequent incubation as efficient for introduction of small RNAs into the fluke. Knock down of LAP was achieved with as little as 2.5 μg/ml dsRNA concentrations, which may reduce or obviate off-target effects. However, at these concentrations, tracking incorporation by fluorescent labeling was difficult. While both long dsRNA and short interfering RNA (siRNA) are equally effective at inducing a short-term knock down, dsRNA induced persistent silencing up to 21 days after treatment, suggesting that mechanisms of amplification of the interfering signal can be present in this pathogen. Persistent silencing of the invasive stage for up to 3 weeks (close to what it takes for the fluke to reach the liver) opens the possibility of using RNAi for the validation of putative therapeutic targets.

Schistosome ABC multidrug transporters: From pharmacology to physiology

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The genuine and hypothesized roles of schistosome ABC transporters are reviewed.

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Evidence suggesting a role for transporters in schistosome drug susceptibility is discussed.

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Potential roles of ABC transporters in normal schistosome biology are outlined.

Praziquantel (PZQ) is essentially the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions worldwide. Though highly effective overall, PZQ has limitations, most notably its significant lack of activity against immature schistosomes. Furthermore, the availability of only a single drug for a disease of this magnitude makes reports of PZQ-resistant isolates particularly troubling. ATP-binding cassette (ABC) multidrug transporters such as P-glycoprotein (Pgp; ABCB1) are efflux transporters that underlie multidrug resistance (MDR); changes in their expression or structure are also associated with drug resistance in parasites, including helminths. This review will discuss the role these transporters might play in modulating schistosome susceptibility to PZQ, and the implications for developing new or repurposed treatments that enhance the efficacy of PZQ. However, in addition to influencing drug susceptibility, ABC transporters play important roles in several critical physiological functions such as excretion and maintenance of permeability barriers. They also transport signaling molecules with high affinity, and several lines of evidence implicate mammalian transporters in a diverse array of physiological functions, including regulation of immune responses. Like their mammalian counterparts, schistosome ABC transporters appear to be involved in functions critical to the parasite, including excretory activity and reproduction, and we hypothesize that they underlie at least some aspects of parasite–host interactions. Thus, in addition to their potential as targets for enhancers of PZQ susceptibility, these transporters might also serve as candidate targets for agents that disrupt the parasite life cycle and act as antischistosomals on their own.

Disruption of vitellogenesis and spermatogenesis by triclabendazole (TCBZ) in a TCBZ-resistant isolate of Fasciola hepatica following incubation in vitro with a P-glycoprotein inhibitor

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of P-glycoprotein (Pgp)-linked drug efflux pumps. The Sligo TCBZ-resistant fluke isolate was used for these experiments and the Pgp inhibitor selected was R(+)-verapamil [R(+)-VPL]. In the first experiment, flukes were initially incubated for 2 h in R(+)-VPL (100 μm), then incubated in R(+)-VPL+triclabendazole sulphoxide (TCBZ.SO) (50 μg mL−1, or 133·1 μm) until flukes ceased movement (at 9 h post-treatment). In a second experiment, flukes were incubated in TCBZ.SO alone and removed from the incubation medium following cessation of motility (after 15 h). In the third experiment, flukes were incubated for 24 h in R(+)-VPL on its own. Changes to the testis tubules and vitelline follicles following drug treatment and following Pgp inhibition were assessed by means of light microscope histology and transmission electron microscopy. Incubation of the Sligo isolate in either R(+)-VPL or TCBZ.SO on their own had a limited impact on the morphology of the two tissues. Greater disruption was observed when the drugs were combined, in terms of the block in development of the spermatogenic and vitelline cells and the apoptotic breakdown of the remaining cells. Sperm formation was severely affected and abnormal. Large spaces appeared in the vitelline follicles and synthesis of shell protein was disrupted. The results of this study support the concept of altered drug efflux in TCBZ-resistant flukes and indicate that drug transporters may play a role in the development of drug resistance.