Triclabendazole biotransformation and comparative diffusion of the parent drug and its oxidized metabolites into Fasciola hepatica

Protein Modelling and Molecular Docking Analysis of Fasciola hepatica β-Tubulin's Interaction Sites, with Triclabendazole, Triclabendazole Sulphoxide and Triclabendazole Sulphone

PURPOSE

Fasciola hepatica is a globally distributed trematode that causes significant economic losses. Triclabendazole is the primary pharmacological treatment for this parasite. However, the increasing resistance to triclabendazole limits its efficacy. Previous pharmacodynamics studies suggested that triclabendazole acts by interacting mainly with the β monomer of tubulin.

METHODS

We used a high-quality method to model the six isotypes of F. hepatica β-tubulin in the absence of three-dimensional structures. Molecular dockings were conducted to evaluate the destabilization regions in the molecule against the ligands triclabendazole, triclabendazole sulphoxide and triclabendazole sulphone.

RESULTS

The nucleotide binding site demonstrates higher affinity than the binding sites of colchicine, albendazole, the T7 loop and pβVII (p < 0.05). We suggest that the binding of the ligands to the polymerization site of β-tubulin can lead a microtubule disruption. Furthermore, we found that triclabendazole sulphone exhibited significantly higher binding affinity than other ligands (p < 0.05) across all isotypes of β-tubulin.

CONCLUSIONS

Our investigation has yielded new insight on the mechanism of action of triclabendazole and its sulphometabolites on F. hepatica β-tubulin through computational tools. These findings have significant implications for ongoing scientific research ongoing towards the discovery of novel therapeutics to treat F. hepatica infections.

Spatial visualization of drug uptake and distribution in Fasciola hepatica using high-resolution AP-SMALDI mass spectrometry imaging

Understanding drug penetration, distribution, and metabolization is fundamental for understanding drug efficacy. This also accounts for parasites during antiparasitic treatment. Recently, we established matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) in blood flukes and liver flukes. This label-free technique is capable of visualizing the molecular distribution of endogenous and exogenous molecules, such as drug compounds. Here, we conducted atmospheric-pressure scanning microprobe MALDI MSI (AP-SMALDI MSI) of tissue sections of adult Fasciola hepatica that have been treated in vitro with 100 µM of triclabendazole (TCBZ), the drug of choice for treatment of fasciolosis, and its main metabolite triclabendazole sulfoxide (TCBZ-SO). Measurements covered an m/z mass range of 250–1,000 and provided a high spatial resolution using a pixel size of 10 µm. To support the interpretation of drug distribution, we first identified endogenous lipids that mark characteristic tissues such as the gastrodermis, the tegument, and the parenchyma. The obtained results suggested an early tegumental route of TCBZ uptake within 20 min, followed by spreading throughout the parasite after 4 h, and an even distribution in most tissues after 12 h. This coincided with a strong reduction of parasite vitality. TCBZ-SO treatment demonstrated the accumulation of this metabolite in the same tissues as the parent drug compound. These data demonstrate the auspicious potential of MALDI MSI to visualize uptake and distribution patterns of drugs or drug-candidate compounds in parasites, which might contribute to preclinical drug discovery in liver fluke research and beyond.

Investigation of Bemethyl Biotransformation Pathways by Combination of LC-MS/HRMS and In Silico Methods

Bemethyl is an actoprotector, an antihypoxant, and a moderate psychostimulant. Even though the therapeutic effectiveness of bemethyl is well documented, there is a gap in knowledge regarding its metabolic products and their quantitative and qualitative characteristics. Since 2018, bemethyl is included to the Monitoring Program of the World Anti-Doping Agency, which highlights the challenge of identifying its urinary metabolites. The objective of the study was to investigate the biotransformation pathways of bemethyl using a combination of liquid chromatography-high-resolution mass spectrometry and in silico studies. Metabolites were analyzed in a 24 h rat urine collected after oral administration of bemethyl at a single dose of 330 mg/kg. The urine samples were prepared for analysis by a procedure developed in the present work and analyzed by high performance liquid chromatography–tandem mass spectrometry. For the first time, nine metabolites of bemethyl with six molecular formulas were identified in rat urine. The most abundant metabolite was a benzimidazole–acetylcysteine conjugate; this biotransformation pathway is associated with the detoxification of xenobiotics. The BioTransformer and GLORY computational tools were used to predict bemethyl metabolites in silico. The molecular docking of bemethyl and its derivatives to the binding site of glutathione S-transferase has revealed the mechanism of bemethyl conjugation with glutathione. The findings will help to understand the pharmacokinetics and pharmacodynamics of actoprotectors and to improve antihypoxant and adaptogenic therapy.

Similarities and differences in the biotransformation and transcriptomic responses of Caenorhabditis elegans and Haemonchus contortus to five different benzimidazole drugs

We have undertaken a detailed analysis of the biotransformation of five of the most therapeutically important benzimidazole anthelmintics - albendazole (ABZ), mebendazole (MBZ), thiabendazole (TBZ), oxfendazole (OxBZ) and fenbendazole (FBZ) - in Caenorhabditis elegans and the ruminant parasite Haemonchus contortus. Drug metabolites were detected by LC-MS/MS analysis in supernatants of C. elegans cultures with a hexose conjugate, most likely glucose, dominating for all five drugs. This work adds to a growing body of evidence that glucose conjugation is a major pathway of xenobiotic metabolism in nematodes and may be a target for enhancement of anthelmintic potency. Consistent with this, we found that biotransformation of albendazole by C. elegans reduced drug potency. Glucose metabolite production by C. elegans was reduced in the presence of the pharmacological inhibitor chrysin suggesting that UDP-glucuronosyl/glucosyl transferase (UGT) enzymes may catalyze benzimidazole glucosidation. Similar glucoside metabolites were detected following ex vivo culture of adult Haemonchus contortus. As a step towards identifying nematode enzymes potentially responsible for benzimidazole biotransformation, we characterised the transcriptomic response to each of the benzimidazole drugs using the C. elegans resistant strain CB3474 ben-1(e1880)III. In the case of albendazole, mebendazole, thiabendazole, and oxfendazole the shared transcriptomic response was dominated by the up-regulation of classical xenobiotic response genes including a shared group of UGT enzymes (ugt-14/25/33/34/37/41/8/9). In the case of fenbendazole, a much greater number of genes were up-regulated, as well as developmental and brood size effects suggesting the presence of secondary drug targets in addition to BEN-1. The transcriptional xenobiotic response of a multiply resistant H. contortus strain UGA/2004 was essentially undetectable in the adult stage but present in the L3 infective stage, albeit more muted than C. elegans. This suggests that xenobiotic responses may be less efficient in stages of parasitic nematodes that reside in the host compared with the free-living stages.

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C. e. & H. c. display hexose conjugation (likely glucose) and excretion of 5 BZs.

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C. elegans (C.e.) biotransformation of ABZ reduces drug potency.

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UGT inhibitor chrysin reduces ABZ biotransformation by C. elegans.

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Transcriptomic response of C. e. (ben-1) to 5 BZs dominated by xenobiotic response and additional targets for FBZ.

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Minimal transcriptomic response of H. contortus to ABZ exposure.

Host pharmacokinetics and drug accumulation of anthelmintics within target helminth parasites of ruminants

Anthelmintic drugs require effective concentrations to be attained at the site of parasite location for a certain period to assure their efficacy. The processes of absorption, distribution, metabolism and excretion (pharmacokinetic phase) directly influence drug concentrations attained at the site of action and the resultant pharmacological effect. The aim of the current review article was to provide an overview of the relationship between the pharmacokinetic features of different anthelmintic drugs, their availability in host tissues, accumulation within target helminths and resulting therapeutic efficacy. It focuses on the anthelmintics used in cattle and sheep for which published information on the overall topic is available; benzimidazoles, macrocyclic lactones and monepantel. Physicochemical properties, such as water solubility and dissolution rate, determine the ability of anthelmintic compounds to accumulate in the target parasites and consequently final clinical efficacy. The transcuticular absorption process is the main route of penetration for different drugs in nematodes and cestodes. However, oral ingestion is a main route of drug entry into adult liver flukes. Among other factors, the route of administration may substantially affect the pharmacokinetic behaviour of anthelmintic molecules and modify their efficacy. Oral administration improves drug efficacy against nematodes located in the gastroinestinal tract especially if parasites have a reduced susceptibility. Partitioning of the drug between gastrointestinal contents, mucosal tissue and the target parasite is important to enhance the drug exposure of the nematodes located in the lumen of the abomasum and/or small intestine. On the other hand, large inter-animal variability in drug exposure and subsequent high variability in efficacy is observed after topical administration of anthelmintic compounds. As it has been extensively demonstrated under experimental and field conditions, understanding pharmacokinetic behaviour and identification of different factors affecting drug activity is important for achieving optimal parasite control and avoiding selection for drug resistance. The search for novel alternatives to deliver enhanced drug concentrations within target helminth parasites may contribute to avoiding misuse, and prolong the lifespan of existing and novel anthelmintic compounds in the veterinary pharmaceutical market.

Enantiomers of triclabendazole sulfoxide: Analytical and semipreparative HPLC separation, absolute configuration assignment, and transformation into sodium salt

Direct HPLC separation of the enantiomers of triclabendazole sulfoxide (TCBZ-SO), which is the main metabolite of the anthelmintic drug triclabendazole, was carried out using the polysaccharide-based Chiralpak AS-H and Chiralpak IF-3 chiral stationary phases (CSPs). The chromatographic behaviour of both CSPs was evaluated and compared using normal-phase and reversed-phase eluents at different column temperatures. The eluent mixture of n-hexane-2-propanol-trifluoroacetic acid 70:30:0.1 (v/v/v) and a column temperature of 40°C were identified as the best operational conditions to carry out semipreparative enantioseparations on a 1-cm I.D. AS-H column. Under these conditions, 12.5mg of racemic sample were resolved in a single chromatographic run within 15min. Comparison of calculated and experimental chiroptical properties provided the absolute configuration assignment at the sulfur atom. The salification of the isolated enantiomers of TCBZ-SO by reaction with sodium hydroxide solution produced water-soluble Na salts which are potentially useful in the development of new anthelmintic enantiomerically pure formulations.

Gaining Insights Into the Pharmacology of Anthelmintics Using Haemonchus contortus as a Model Nematode

Progress made in understanding pharmacokinetic behaviour and pharmacodynamic mechanisms of drug action/resistance has allowed deep insights into the pharmacology of the main chemical classes, including some of the few recently discovered anthelmintics. The integration of pharmaco-parasitological research approaches has contributed considerably to the optimization of drug activity, which is relevant to preserve existing and novel active compounds for parasite control in livestock. A remarkable amount of pharmacology-based knowledge has been generated using the sheep abomasal nematode Haemonchus contortus as a model. Relevant fundamental information on the relationship among drug influx/efflux balance (accumulation), biotransformation/detoxification and pharmacological effects in parasitic nematodes for the most traditional anthelmintic chemical families has been obtained by exploiting the advantages of working with H. contortus under in vitro, ex vivo and in vivo experimental conditions. The scientific contributions to the pharmacology of anthelmintic drugs based on the use of H. contortus as a model nematode are summarized in the present chapter.

A single amino acid substitution in isozyme GST mu in Triclabendazole resistant Fasciola hepatica (Sligo strain) can substantially influence the manifestation of anthelmintic resistance

The helminth parasite Fasciola hepatica causes fascioliasis in human and domestic ruminants. Economic losses due to this infection are estimated in U$S 2000-3000 million yearly. The most common method of control is the use of anthelmintic drugs. However, there is an increased concern about the growing appearance of F. hepatica resistance to Triclabendazole (TCBZ), an anthelmintic with activity over adult and young flukes. F. hepatica has eight Glutathione S-Transferase (GST) isozymes, which are enzymes involved in the detoxification of a wide range of substrates through chemical conjugation with glutathione. In the present work we identified and characterized the GST mu gene isolated from the TCBZ-susceptible and TCBZ-resistant F. hepatica strains. Total RNA was transcribed into cDNA by reverse transcription and a 657 bp amplicon corresponding to the GST mu gene was obtained. The comparative genetic analysis of the GST mu gene of the TCBZ susceptible strain (Cullompton) and TCBZ resistant strain (Sligo) showed three nucleotide changes and one amino acid change at position 143 in the GST mu isozyme of the TCBZ-resistant strain. These results have potential relevance as they contribute better understand the mechanisms that generate resistance to anthelmintics.

Time-dependent tegumental surface changes in juvenile Fasciola gigantica in response to triclabendazole treatment in goat

Triclabendazole (TCBZ), the anthelmintic drug active against both mature and immature liver flukes, was used to investigate the effect of in vivo treatment on the tegumental surface of juvenile Fasciola gigantica. Five goats were infected with 150 F. gigantica metacercariae each by oral gavage. Four of them were treated with single dose of TCBZ at 10mg/kg at four weeks post-infection. They were euthanized at 0 (untreated), 24, 48, 72 and 96h post treatment. Juvenile flukes were manually retrieved from the goat livers and processed for scanning electron microscopy. In control flukes, the anterior region was adorned with sharply pointed spines projecting away from the surface, while in the posterior region, spines become shorter and narrower, loosing serration and with the appearance of distinct furrows and papillae. The dorsal surface retained the same pattern of surface architecture similar to that of ventral surface. Flukes obtained from 24h post-treatment did not show any apparent change and were still very active. However, there were limited movements and some blebbing, swelling, deposition of tegumental secretions and some flattening displayed by the flukes of 48h post-treatment. All the worms were found dead 72h post-treatment and showed advanced level of tegumental disruptions, consisting of severe distortion of spines, sloughing off the tegument to expose the basal lamina, formation of pores and isolated patches of lesions. By 96h post-treatment, the disruption was extremely severe and the tegument was completely sheared off causing deeper lesions that exposed the underlying musculature. The disruption was more severe at posterior than anterior region and on ventral than dorsal surface. The present study further establishes the time-course of TCBZ action in vivo with 100% efficacy against the juvenile tropical liver fluke.

Time-course and accumulation of triclabendazole and its metabolites in bile, liver tissues and flukes collected from treated sheep

The flukicidal compound triclabendazole (TCBZ) has a complex metabolic pattern that includes the systemic presence of its sulphoxide (TCBZ.SO) and sulphone (TCBZ.SO2) metabolites, usually recovered from the bile of treated animals. The aim of the current work was to evaluate the time-course and pattern of in vivo accumulation of TCBZ/metabolites into adult Fasciola hepatica specimens recovered from infected sheep. Twelve (12) healthy Corriedale sheep were orally infected with one hundred (100) metacercariae of the TCBZ-susceptible Cullomptom isolate of F. hepatica. Sixteen weeks after infection, animals were intraruminally treated with TCBZ (10mg/kg). At 3, 24, 48 and 60h post-treatment (pt), animals were sacrificed (n=3/time period) and samples of blood, bile, liver tissue and adult F. hepatica specimens were collected. The concentrations of TCBZ/metabolites were measured by HPLC. TCBZ.SO and TCBZ.SO2 were the only molecules recovered in the bloodstream, with peak plasma concentrations of 10.8μg/mL (TCBZ.SO) and 12.6μg/mL (TCBZ.SO2). The same metabolites were also the main analytes accumulated within the adult flukes, reaching peak concentrations between 6.35μg/g (TCBZ.SO) and 13.9μg/g (TCBZ.SO2) at 24h pt, which was coincident with the time when the maximum plasma concentration was attained. Low levels of TCBZ parent drug (0.14μg/g at 24h pt) were measured within collected flukes. TCBZ parent drug and its sulpho- and hydroxy-derivatives were recovered in bile collected from treated sheep between 3 and 60h pt. Although relatively high concentrations of hydroxy-TCBZ (ranging from 0.86 to 10.1μg/mL) were measured in bile, this metabolite was not recovered within the flukes at any time pt. Finally, TCBZ parent drug was the main compound accumulated in liver tissue over the 60h pt period. The time-course and drug concentration patterns within the adult liver fluke after TCBZ treatment followed a similar trend to those observed in plasma. Overall, the data reported here confirm that oral ingestion is a main route of drug entry into the trematode in vivo exposed to TCBZ/metabolites. However, the presence of TCBZ within the adult fluke (despite being absent in the systemic circulation) may be related to some degree of trans-tegumental diffusion from bile or by a direct oral ingestion from portal blood.

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

A study was carried out to investigate whether the action of triclabendazole sulphoxide (TCBZ.SO) against the liver fluke, Fasciola hepatica is altered by inhibition of P-glycoprotein (Pgp)-linked drug efflux pumps. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible fluke isolates were used for this in vitro study and the Pgp inhibitor selected was R(+)-verapamil [R(+)-VPL]. For experiments with the Oberon isolate, flukes were incubated for 24 h with either R(+)-VPL (1×10-4 m) on its own, TCBZ.SO (15 μg mL-1) alone, a combination of R(+)-VPL (1×10-4 m) plus TCBZ.SO (15 μg mL-1), TCBZ.SO (50 μg mL-1) on its own, or a combination of TCBZ.SO (50 μg mL-1) plus R(+)-VPL (1×10-4 m). They were also incubated in TCBZ.SO (50 μg mL-1) alone or in combination with R(+)-VPL (1×10-4 m) until they became inactive; and in TCBZ.SO (50 μg mL-1) alone for a time to match that of the combination inactivity time. Flukes from the Cullompton isolate were treated with either TCBZ.SO (50 μg mL-1) alone or in combination with R(+)-VPL (1×10-4 m) until they became inactive, or with TCBZ.SO (50 μg mL-1) alone time-matched to the combination inactivity time. Morphological changes resulting from drug treatment and following Pgp inhibition were assessed by means of scanning electron microscopy. Incubation in R(+)-VPL alone had a minimal effect on either isolate. TCBZ.SO treatment had a relatively greater impact on the TCBZ-susceptible Cullompton isolate. When R(+)-VPL was combined with TCBZ.SO in the incubation medium, however, the surface disruption to both isolates was more severe than that seen after TCBZ.SO treatment alone; also, the time taken to reach inactivity was shorter. More significantly, though, the potentiation of drug activity was greater in the Oberon isolate; also, it was more distinct at the higher concentration of TCBZ.SO. So, the Oberon isolate appears to be particularly sensitive to efflux pump inhibition. The results of this study suggest that enhanced drug efflux in the Oberon isolate may be involved in the mechanism of resistance to TCBZ.

Biotransformation of albendazole and activities of selected detoxification enzymes in Haemonchus contortus strains susceptible and resistant to anthelmintics

The increased activity of drug-metabolizing enzymes can protect helminths against the toxic effect of anthelmintics. The aim of this study was to compare the metabolism of the anthelmintic drug albendazole (ABZ) and the activities of selected biotransformation and antioxidant enzymes in three different strains of Haemonchus contortus: the ISE strain (susceptible to common anthelmintics), the BR strain (resistant to benzimidazole anthelmintics) and the WR strain (multi-resistant). H. contortus adults were collected from the abomasum of experimentally infected lambs. In vitro (subcellular fractions of H. contortus homogenate) as well as ex vivo (living nematodes cultivated in flasks with medium) experiments were performed. HPLC with spectrofluorimetric and mass-spectrometric detection was used in the analysis of ABZ metabolites. The in vitro activities of oxidation/antioxidation and conjugation enzymes toward model substrates were also assayed. The in vitro data showed significant differences between the susceptible (ISE) and resistant (BR, WR) strains regarding the activities of peroxidases, catalase and UDP-glucosyltransferases. S-oxidation of ABZ was significantly lower in BR than in the ISE strain. Ex vivo, four ABZ metabolites were identified: ABZ sulphoxide and three ABZ glucosides. In the resistant strains BR and WR, the ex vivo formation of all ABZ glucosides was significantly higher than in the susceptible ISE strain. The altered activities of certain detoxifying enzymes might partly protect the parasites against the toxic effect of the drugs as well as contribute to drug-resistance in these parasites.

In vitro efficacy of triclabendazole and clorsulon against the larval stage of Echinococcus multilocularis

Alveolar echinococcosis (AE) caused by the cestode Echinococcus multilocularis (E. multilocularis) is endemic in wide areas of the Northern hemisphere. Untreated AE progresses and leads to death in more than 90% of cases. Until the advent of benzimidazoles, no antihelminthic drugs were available to cure AE. Benzimidazoles have greatly improved the prognosis of patients with AE. However, benzimidazoles have only a parasitostatic effect on E. multilocularis. Albendazole (ABZ) must sometimes be withdrawn because of adverse events. Alternative drugs are urgently needed. The antihelminthic triclabendazole (TCZ) and clorsulon (CLS) are more effective than ABZ to cure infections by the liver flukes Fasciola spp. The efficacy of TCZ and CLS was investigated on an in vitro culture of E. multilocularis larval tissue. E. multilocularis vesicles were evaluated for their morphology before and after adding TCZ, TCZ sulfoxide (TCZSX) and CLS to the larval tissue culture. TCZ at the concentrations of 20 μg/ml culture solution led to maximum vesicle damage within 12 days and of 25 μg/ml within 13 days, and TCZSX at the concentrations of 20 μg/ml within 20 days and of 25 μg/ml within 14 days. Contrary, CLS added at 5, 10 and 15 μg/ml to culture solution did not lead to any vesicle damage. TCZ is a promising further candidate drug for the treatment of AE.

The metabolic fate of ivermectin in host (Ovis aries) and parasite (Haemonchus contortus)

Ivermectin (IVE), one of the most important anthelmintics, is often used in the treatment of haemonchosis in ruminants. The objective of our work was (1) to find and identify phase I and II metabolites of IVE formed by the Barber's pole worm (Haemonchus contortus), and (2) to compare IVE metabolites in helminths with IVE biotransformation in sheep (Ovis aries) as host species. Ultrahigh-performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) was used for this purpose. During in vitro incubations, microsomes (from adult worms or from ovine liver) and a primary culture of ovine hepatocytes were incubated with IVE. In the ex vivo study, living H. contortus adults were incubated in the presence of 1 μM IVE for 24 h. The results showed that the H. contortus enzymatic system is not able to metabolize IVE. On the other hand, 7 different phase I as well as 9 phase II IVE metabolites were detected in ovine samples using UHPLC/MS/MS analyses. Most of these metabolites have not been described before. Haemonchus contortus is not able to deactivate IVE through biotransformation; therefore, biotransformation does not contribute to the development of IVE-resistance in the Barber's pole worm.

The metabolism of flubendazole and the activities of selected biotransformation enzymes in Haemonchus contortus strains susceptible and resistant to anthelmintics

Haemonchus contortus is one of the most pathogenic parasites of small ruminants (e.g. sheep and goat). The treatment of haemonchosis is complicated because of recurrent resistance of H. contortus to common anthelmintics. The aim of this study was to compare the metabolism of the anthelmintic drug flubendazole (FLU) and the activities of selected biotransformation enzymes towards model xenobiotics in 4 different strains of H. contortus: the ISE strain (susceptible to common anthelmintics), ISE-S (resistant to ivermectin), the BR strain (resistant to benzimidazole anthelmintics) and the WR strain (resistant to all common anthelmintics). H. contortus adults were collected from the abomasums from experimentally infected lambs. The in vitro as well as ex vivo experiments were performed and analysed using HPLC with spectrofluorimetric and mass-spectrometric detection. In all H. contortus strains, 4 different FLU metabolites were detected: FLU with a reduced carbonyl group (FLU-R), glucose conjugate of FLU-R and 2 glucose conjugates of FLU. In the resistant strains, the ex vivo formation of all FLU metabolites was significantly higher than in the susceptible ISE strain. The multi-resistant WR strain formed approximately 5 times more conjugates of FLU than the susceptible ISE strain. The in vitro data also showed significant differences in FLU metabolism, in the activities of UDP-glucosyltransferase and several carbonyl-reducing enzymes between the susceptible and resistant H. contortus strains. The altered activities of certain detoxifying enzymes might protect the parasites against the toxic effect of the drugs as well as contribute to drug-resistance in these parasites.

Update on trematode infections in sheep

Trematode parasites live in the liver, fore stomachs or blood vessels of a wide range of animals and humans. Most of them have a special economic and veterinary significance. Liver fluke disease of sheep and other animal species is caused by the common liver fluke Fasciola hepatica. Hepatic fasciolosis occurs throughout the world, where climatic conditions are suitable for the survival of aquatic intermediate host snails. Also of importance for ruminants, in some parts of the world, are Fasciola gigantica and Fascioloides magna. Other trematodes infecting ruminants include Dicrocoelium dendriticum; Eurytrema pancreaticum and Eurytrema coelomaticum. Among the Paramphistomidae, some species can infect sheep and other ruminants. Finally, Schistosoma spp. are found in the blood vessels of ruminants and are of minor importance in temperate regions. The manuscript concentrates on trematode species of veterinary importance for domestic sheep.

Erratum to: inhibition of triclabendazole metabolism in vitro by ketoconazole increases disruption to the tegument of a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The cytochrome P450 (CYP 450) enzyme pathway was inhibited using ketoconazole (KTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 system was inhibited by a 2-h pre-incubation in ketoconazole (40 μM), then incubated for a further 22 h in NCTC medium containing either KTZ, KTZ + nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM), KTZ + NADPH + TCBZ (15 μg/ml), or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO; 15 μg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with KTZ + TCBZ, but more particularly KTZ + TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own: for example, there was severe swelling of the basal infolds and their associated mucopolysaccharide masses, accompanied by an accumulation of secretory bodies just below the apex. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis, production, and transport of secretory bodies were badly disrupted. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes and this process may play a role in the development of drug resistance.

In vitro oxidative metabolism of xenobiotics in the lancet fluke (Dicrocoelium dendriticum) and the effects of albendazole and albendazole sulphoxide ex vivo

Dicrocoeliosis, a parasitic infection caused by Dicrocoelium dendriticum (lancet fluke), is often treated by the anthelmintic drug albendazole (ABZ). In the lancet fluke, ABZ metabolism via enzymatic sulphoxidation was found, but no information about ABZ oxidases has been available. The aim of our project was to find out which enzyme of the lancet fluke is responsible for ABZ sulphoxidation, as well as to assay the activities of oxidation enzymes. We also studied whether ex vivo 24-h exposures of flukes to ABZ or its sulphoxide (ABZ.SO) would alter ABZ sulphoxidation rate and the activities of tested enzymes. In subcellular fractions from flukes, marked activities of peroxidase (Px), glutathione Px (GPx), catalase (CAT), superoxide dismutase, and thioredoxin glutathione reductase were found. Using specific inhibitors, the participation of flavine monooxygenases in ABZ-oxidation was found. The ex vivo exposition of flukes to ABZ or ABZ.SO did not change the rate of ABZ sulphoxidation in vitro, but the ex vivo exposure of flukes to anthelmintics increased Px, CAT, and GPx activity. The modulation of these enzyme activities after ABZ or ABZ.SO exposition may be characteristic of the parasite&#x2019;s protective mechanism against oxidative stress caused by drug treatment.

Inhibition of triclabendazole metabolism in vitro by ketoconazole increases disruption to the tegument of a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The cytochrome P450 (CYP 450) enzyme pathway was inhibited using ketoconazole (KTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 system was inhibited by a 2-h pre-incubation in ketoconazole (40 μM), then incubated for a further 22 h in NCTC medium containing either KTZ, KTZ + nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM), KTZ + NADPH + TCBZ (15 μg/ml), or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO; 15 μg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with KTZ + TCBZ, but more particularly KTZ + TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own: in the syncytium, for example, there was severe swelling of the basal infolds and their associated mucopolysaccharide masses, accompanied by an accumulation of secretory bodies just below the apex. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis, production, and transport of secretory bodies were badly disrupted. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes and this process may play a role in the development of drug resistance.

Piperonyl butoxide enhances triclabendazole action against triclabendazole-resistant Fasciola hepatica

A study has been carried out to determine whether the action of triclabendazole (TCBZ) against the liver fluke, Fasciola hepatica is altered by inhibition of the cytochrome P450 (CYP 450)-mediated drug metabolism pathway. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible fluke isolates were used for these experiments, the basic design of which is given in the paper by Devine et al. (2010a). Piperonyl butoxide (PB) was the CYP P450 inhibitor used. Morphological changes resulting from drug treatment and following metabolic inhibition were assessed by means of transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than TCBZ-resistant isolate. However, co-incubation with PB+TCBZ, but more particularly PB+TCBZ.SO, led to greater changes to the TCBZ-resistant isolate than with each drug on its own, with blebbing of the apical plasma membrane, severe swelling of the basal infolds and their associated mucopolysaccharide masses in the syncytium and flooding in the internal tissues. Golgi complexes were greatly reduced or absent in the tegumental cells and the synthesis and production of secretory bodies were badly disrupted. The mitochondria were swollen throughout the tegumental system and the somatic muscle blocks were disrupted. With the TCBZ-susceptible Cullompton isolate, there was a limited increase in drug action following co-incubation with PB. The results provide evidence that the condition of a TCBZ-resistant fluke can be altered by inhibition of drug metabolism. Moreover, they support the concept that altered drug metabolism contributes to the mechanism of resistance to TCBZ.

Enhancement of the drug susceptibility of a triclabendazole-resistant isolate of Fasciola hepatica using the metabolic inhibitor ketoconazole

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) is altered by using the metabolic inhibitor, ketoconazole (KTZ) to inhibit the cytochrome P450 (CYP 450) system within Fasciola hepatica. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP 450 enzyme system was inhibited by a 2 h pre-incubation in KTZ (40 microM). Flukes were then incubated for a further 22 h in NCTC medium containing either KTZ; KTZ + nicotinamide adenine dinucleotide phosphate (NADPH; 1 nM); KTZ + NADPH + TCBZ (15 microg/ml); or KTZ + NADPH + triclabendazole sulphoxide (TCBZ.SO;15 microg/ml). Morphological changes resulting from drug treatment and following metabolic inhibition were assessed using scanning electron microscopy. After treatment with either TCBZ or TCBZ.SO alone, there was greater disruption to the TCBZ-susceptible isolate than the TCBZ-resistant isolate. However, co-incubation with KTZ and TCBZ/TCBZ.SO led to more severe surface changes to the TCBZ-resistant isolate than with each drug on its own, with greater swelling and blebbing of the tegument and even the loss of the apical plasma membrane in places. With the Cullompton isolate, there was limited potentiation of drug action in combination with KTZ, and only with TCBZ.SO. The results support the concept of altered drug metabolism within TCBZ-resistant isolates and indicate that this process may play a role in the development of drug resistance.

Potentiation of triclabendazole sulphoxide-induced tegumental disruption by methimazole in a triclabendazole-resistant isolate of Fasciola hepatica

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The flavin monooxygenase system (FMO) was inhibited using methimazole (MTZ) to see whether a TCBZ-resistant isolate could be made more sensitive to TCBZ action. The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The FMO system was inhibited by a 2-h pre-incubation in methimazole (100 microM), then incubated for a further 22 h in NCTC medium containing either MTZ; MTZ+nicotinamide adenine dinucleotide phosphate (NADPH) (1 nM); MTZ+NADPH+TCBZ (15 microg/ml); or MTZ+NADPH+triclabendazole sulphoxide (TCBZ.SO) (15 microg/ml). Changes to fluke ultrastructure following drug treatment and metabolic inhibition were assessed using transmission electron microscopy. After treatment with either TCBZ or TCBZ.SO on their own, there was greater disruption to the TCBZ-susceptible than triclabedazole-resistant isolate. However, co-incubation with MTZ+TCBZ, but more particularly MTZ+TCBZ.SO, led to more severe changes to the TCBZ-resistant isolate than with each drug on its own, with severe swelling of the basal infolds and mucopolysaccharide masses in the syncytium, accompanied by a reduction in numbers of secretory bodies. The synthesis and production of secretory bodies in the tegumental cells was severely affected as well. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action. The results support the concept of altered drug metabolism in TCBZ-resistant flukes, and this process may play a role in the development of drug resistance.

A transmission electron microscope study on the route of entry of triclabendazole into the liver fluke, Fasciola hepatica

Uptake of triclabendazole by the liver fluke, Fasciola hepatica has been studied by experiments designed to block either oral uptake of drug, by use of ligatures, or trans-tegumental diffusion, by allowing the drug to bind to an excess of bovine serum albumin (BSA) in the medium. Changes to the tegumental system, musculature and gut were assessed using transmission electron microscopy. Flukes were incubated in vitro for 24 h in TCBZ.SO (15 microg/ml). Disruption to the tegument and muscle was similar in ligatured and non-ligatured flukes, suggesting that closing the oral route did not affect drug uptake. The ultrastructure of the gastrodermal cells remained unchanged. Non-ligatured flukes were also incubated for 24 h in vitro in TCBZ.SO (15 microg/ml) in the presence of red blood cells (RBCs). Oral uptake of blood was demonstrated, but gut ultrastructure remained normal, whereas the tegument was severely disrupted. In separate experiments, ligatured and non-ligatured flukes were incubated in TCBZ.SO (15 microg/ml) in the presence of BSA (30 mg/ml) for 24 h in vitro. There was a marked decrease in the degree of tegumental disruption observed compared with TCBZ.SO action alone; again, the gut remained normal. The findings support previous morphological and pharmacological studies indicating that trans-tegumental uptake of triclabendazole predominates in the liver fluke.

Inhibition of cytochrome P450-mediated metabolism enhances ex vivo susceptibility of Fasciola hepatica to triclabendazole

A study has been carried out to investigate whether the action of triclabendazole (TCBZ) against Fasciola hepatica is altered by inhibition of drug metabolism. The cytochrome P450 (CYP P450) system was inhibited using piperonyl butoxide (PB). The Oberon TCBZ-resistant and Cullompton TCBZ-susceptible isolates were used for these experiments. The CYP P450 system was inhibited by a 2 h pre-incubation in PB (100 mum). Flukes were then incubated for a further 22 h in NCTC medium containing either PB; PB+nicotinamide adenine dinucleotide phosphate (NADPH) (1 nm); PB+NADPH+TCBZ (15 microg/ml); or PB+NADPH+TCBZ.SO (15 microg/ml). Morphological changes resulting from drug treatment and following metabolic inhibition were assessed using scanning electron microscopy. After treatment with either TCBZ or TCBZ.SO alone, there was greater disruption to the TCBZ-susceptible than the resistant isolate. However, co-incubation with PB and TCBZ/TCBZ.SO lead to more severe surface changes to the TCBZ-resistant Oberon isolate than with each drug on its own. With the TCBZ-susceptible Cullompton isolate, there was limited potentiation of drug action, and only with TCBZ.SO. The results support the concept of altered drug metabolism in TCBZ-resistant flukes and this process may play a role in the development of drug resistance.

Liquid chromatography/mass spectrometric identification of benzimidazole anthelminthics metabolites formed ex vivo by Dicrocoelium dendriticum

With further use of chemical agents in the control of parasitic infections, an increased number of drug resistance occurrences to antiparasitic drugs has been reported. Induction of enzymes responsible for detoxification of given drugs can contribute to drug resistance development in a parasitic organism. The identification of formed metabolites allows the characterization of the enzymes participating in biotransformation and possibly in drug resistance development. The objective of our work was to find and identify phase I and phase II metabolites of the anthelminthic drugs albendazole, flubendazole and mebendazole formed in ex vivo incubations by the parasitic helminth Dicrocoelium dendriticum, a parasite of ruminants and other grazing animals, using liquid chromatography/mass spectrometric (LC/MS) techniques. In the ex vivo study, approximately 50 living D. dendriticum adults were incubated in 5 mL RPMI-1640 medium in the presence of 10.0 micromol L(-1) benzimidazole drug (5% CO(2), 38 degrees C) for 24 h. The bodies of the parasite were then removed from the medium. After homogenization of parasites, both parasite homogenates and medium from the incubation were separately extracted using solid-phase extraction. The extracts were analyzed using LC/MS with electrospray ionization. The results showed that D. dendriticum enzymatic systems are capable of phase I oxidation and reduction as well as phase II conjugation reactions. Detected phase I metabolites comprised albendazole sulfoxide, reduced flubendazole and reduced mebendazole. As for phase II metabolites, methyl derivatives of both reduced flubendazole and reduced mebendazole were observed.

Xenobiotic metabolizing enzymes and metabolism of anthelminthics in helminths

Anthelminthics remain the only accessible means in the struggle against helminth parasites, which cause significant morbidity and mortality in man and farm animals. The treatment of helminthic infections has become problematic because of frequent drug resistance of helminth parasites. The development of drug resistance can be facilitated by the action of xenobiotic metabolizing enzymes (XMEs). In all organisms, XMEs serve as an efficient defense against the potential negative action of xenobiotics. The activities of XMEs determine both desired and undesired effects of drugs, and the knowledge of drug metabolism is necessary for safe, effective pharmacotherapy. While human and mammalian XMEs have been intensively studied for many years, XMEs of helminth parasites have undergone relatively little investigation, so far. However, many types of XMEs, including oxidases, reductases, hydrolases, transferases, and transporters, have been described in several helminth species. XMEs of helminth parasites may protect these organisms from the toxic effects of anthelminthics. In case of certain anthelminthics, metabolic deactivation was reported in helminth larvae and/or adults. Moreover, if a helminth is in the repeated contact with an anthelminthic, it defends itself against the chemical stress by the induction of biotransformation enzymes or transporters. This induction can represent an advantageous defense strategy of the parasites and may facilitate the drug-resistance development.

Triclabendazole progress report, 2005–2009: an advancement of learning?

Triclabendazole (TCBZ) remains the drug of choice for treating infections of the liver fluke,Fasciola hepaticain livestock and has become the main drug used to treat human cases of the disease as well. Cases of resistance in livestock continue to be reported, suggesting that the problem is increasing. In order to address the problem, there is a need for better understanding of drug action. A ‘state-of-play’ review on different aspects of TCBZ activity was published by the present author in 2005. The main purpose of the current review is to assess what progress has been made in the past four years towards understanding the main aspects of drug activity, including drug pharmacokinetics and pharmacodynamics and an understanding of the mechanism(s) of resistance. Also, what advances have been made in identifying alternative compounds and using drug combinations to enhance TCBZ activity. Stemming from a number ofin vivostudies, it has become evident that fluke isolates of differing sensitivity to TCBZ differ in some of their biological parameters, and information on this interesting phenomenon will be presented. An update on the use of TCBZ for human fascioliasis is also given. The review will indicate what progress has been made, but will also highlight areas that remain inadequately understood and require greater research focus.

A scanning electron microscope study on the route of entry of triclabendazole into the liver fluke, Fasciola hepatica

SUMMARY

Studies have been carried out to establish the relative importance of oral and trans-tegumental uptake of triclabendazole by the liver fluke, Fasciola hepatica. Experiments were designed to block either oral uptake of drug, by use of ligatures, or trans-tegumental diffusion, by allowing the drug to bind to bovine serum albumin (BSA) in the medium. Changes to the surface morphology of the tegument and gut were assessed by scanning electron microscopy. Flukes were incubated in vitro for 24 h in TCBZ.SO at a concentration of 15 microg/ml. Tegumental disruption in ligatured and non-ligatured flukes was similar, suggesting that closing the oral route did not affect drug uptake. The gut remained unaffected by drug treatment. When BSA (30 mg/ml) was present in the medium, there was a marked decline in the level of tegumental disruption. Again, the gut retained a normal morphology. Non-ligatured flukes were also incubated for 24 h in vitro in TCBZ.SO (15 microg/ml) in the presence of red blood cells. Oral ingestion of blood was demonstrated, although the gut surface retained a normal morphology. In contrast, the tegumental surface was severely affected by the drug. The findings support previous pharmacological studies which suggest that trans-tegumental uptake of triclabendazole predominates in the liver fluke.

Relative activity of triclabendazole metabolites against the liver fluke, Fasciola hepatica

A study has been carried out to determine the relative activity of triclabendazole (TCBZ) and its sulphoxide (TCBZSO) and sulphone (TCBZSO(2)) metabolites against the adult stage of the liver fluke, Fasciola hepatica. Flukes were incubated for 24h in vitro in 15mug/ml of each of the compounds and prepared for scanning and transmission electron microscopy. All three compounds induced changes to the surface morphology of the fluke, the changes comprising swelling and blebbing to a greater or lesser extent in different regions of the fluke. TCBZSO(2) was more disruptive anteriorly and TCBZSO posteriorly. Internal ultrastructural changes were evident following incubation with each of the compounds, with an order of severity TCBZSO(2)>TCBZSO>TCBZ. Swelling of the basal infolds and mitochondria were observed in the tegumental syncytium. In the tegumental cell bodies, there was a reduction in the number of secretory bodies, disruption of the Golgi complexes and swelling of the mitochondria. Severe flooding of the internal tissues was observed with TCBZSO(2) and, to a lesser extent, with TCBZSO and TCBZ. The results demonstrate that both TCBZ and TCBZSO(2) are capable of disrupting the fluke in vitro and are not the inactive compounds they were assumed to be previously. They may well contribute to drug action in vivo as well, indicating that drug action is due to the additive effects of several metabolites, rather than being due to a single active metabolite, namely, TCBZSO.

Phase I biotransformation of albendazole in lancet fluke (Dicrocoelium dendriticum)

Dicroceliosis, a lancet fluke infection, is a frequent parasitosis of small ruminants and the anthelmintic drug albendazole (ABZ) is effective in control of this parasitosis. The aim of our project was to study the metabolism of ABZ and ABZ sulphoxide (ABZ.SO) in lancet fluke. Both invitro (subcellular fractions of fluke homogenates) and exvivo experiments (adult flukes cultivated in medium) were performed for this purpose. ABZ was metabolised invitro by lancet fluke NADPH-dependent enzymes by two oxidative steps (sulphoxidation and sulphonation). The apparent kinetic parameters of these reactions have been determined. In the exvivo experiments, only ABZ sulphoxidation was observed. The stereospecificity in ABZ sulphoxidation invitro was slight, with preferential formation of (+)-ABZ.SO enantiomer. In contrast (-)-ABZ.SO formation predominated in exvivo experiments. Sulphoreduction of ABZ.SO occurred neither invivo nor exvivo. The detection of ABZ oxidative metabolites indicates the presence of drug metabolising oxidases in lancet fluke.

LC-MS-MS identification of albendazole and flubendazole metabolites formed ex vivo by Haemonchus contortus

Resistance of helminth parasites to common anthelminthics is a problem of increasing importance. The full mechanism of resistance development is still not thoroughly elucidated. There is also limited information about helminth enzymes involved in metabolism of anthelminthics. Identification of the metabolites formed by parasitic helminths can serve to specify which enzymes take part in biotransformation of anthelminthics and may participate in resistance development. The aim of our work was to identify the metabolic pathways of the anthelminthic drugs albendazole (ABZ) and flubendazole (FLU) in Haemonchus contortus, a world-wide distributed helminth parasite of ruminants. ABZ and FLU are benzimidazole anthelminthics commonly used in parasitoses treatment. In our ex vivo study one hundred living adults of H. contortus, obtained from the abomasum of an experimentally infected lamb, were incubated in 5 mL RPMI-1640 medium with 10 micromol L(-1) benzimidazole drug (10% CO(2), 38 degrees C) for 24 h. The parasite bodies were then removed from the medium. After homogenization of the parasites, both parasite homogenates and medium from the incubation were separately extracted using solid-phase extraction. The extracts were analyzed by liquid chromatography-mass spectrometry (LC-MS) with electrospray ionization (ESI) in positive-ion mode. The acquired data showed that H. contortus can metabolize ABZ via sulfoxidation and FLU via reduction of a carbonyl group. Albendazole sulfoxide (ABZSO) and reduced flubendazole (FLUR) were the only phase I metabolites detected. Concerning phase II of biotransformation, the formation of glucose conjugates of ABZ, FLU, and FLUR was observed. All metabolites mentioned were found in both parasite homogenates and medium from the incubation.

Biotransformation of flubendazole and selected model xenobiotics in Haemonchus contortus

Haemonchus contortus is one of the most pathogenic parasites of small ruminants (e.g., sheep and goat). The treatment of haemonchosis is complicated because of frequent resistance of H. contortus to common anthelmintics. The development of resistance can be facilitated by the action of drug metabolizing enzymes of parasites that can deactivate anthelmintics and thus protect parasites against the toxic effect of the drug. The aim of this project was to investigate the Phase I biotransformation of benzimidazole anthelmintic flubendazole in H. contortus and to determine the biotransformation of other model xenobiotics. For this purpose, in vitro (subcellular fractions of H. contortus homogenate) as well as ex vivo (live nematodes cultivated in flasks with medium) experiments were used. The results showed that cytosolic NADPH-dependent enzymes of H. contortus metabolize flubendazole via reduction of its carbonyl group. The apparent kinetic parameters of this reaction were determined (V'max=39.8+/-2.1 nM min(-1), K'm=1.5+/-0.3 microM). The reduction of flubendazole in H. contortus is stereospecific, the ratio of (-):(+) enantiomers of reduced flubendazole formed was 90:10. Reduced flubendazole was the only Phase I metabolite found. Effective reduction of other xenobiotics with carbonyl group (metyrapon, daunorubicin, and oracin) was also found. Significant activity of carbonyl-reducing enzymes may be important for H. contortus to survive the attacks of anthelmintics or other xenobiotics with carbonyl group.

Drug transfer into target helminth parasites

The pharmacokinetics of an anthelmintic drug includes the time course of drug absorption, distribution, metabolism and elimination from the host and determines the concentration of the active drug that reaches the location of the parasite. However, the action of the anthelmintic also depends on the ability of the active drug to reach its specific receptor within the target parasite. Thus, drug entry and accumulation in target helminths are important issues when considering how best to achieve optimal efficacy. Passive drug transfer through the external helminth surface is the predominant entry mechanism for most widely used anthelmintics and is discussed in this article. Despite the structural differences between the external surface of nematodes (the cuticle) and the external surface of cestodes and trematodes (the tegument), the mechanism of drug entrance into both types of helminth depends on the lipophilicity of the anthelmintic and this is the major physicochemical determinant for the drug to reach a therapeutic concentration in the target parasite. Understanding the processes that regulate drug transfer into helminth parasites is an important aspect in improving the control of parasites in human and veterinary medicine.

Resistance-induced changes in triclabendazole transport in Fasciola hepatica: ivermectin reversal effect

Triclabendazole (TCBZ) and albendazole (ABZ) are flukicidal benzimidazole compounds extensively used in veterinary medicine. Although TCBZ has excellent activity against mature and immature stages of the liver fluke, Fasciola hepatica, ABZ action is restricted to flukes older than 12 wk. The intensive use of TCBZ has resulted in the development of resistance. To gain insight into the mechanisms of resistance to TCBZ, the ex vivo diffusion of TCBZ, TCBZ sulfoxide (TCBZSO, the active metabolite of TCBZ), and ABZ into TCBZ-susceptible and -resistant adult flukes was compared. TCBZ-susceptible (Cullompton) and -resistant (Sligo) flukes were incubated in Krebs-Ringer Tris buffer with either TCBZ, TCBZSO, or ABZ (5 nmol/ ml) for 90 min. Drug/metabolite concentrations were quantified by high-performance liquid chromatography. All the assayed molecules penetrated through the tegument of both susceptible and resistant flukes. However, significantly lower concentrations of TCBZ and TCBZSO were recovered within the TCBZ-resistant flukes. In contrast, ABZ entrance into the susceptible and resistant flukes was equivalent. The influx/efflux balance for TCBZ, TCBZSO, and ABZ in susceptible and resistant flukes in the presence or absence of a substrate (ivermectin) of the drug transporter P-glycoprotein was assessed. The ivermectin-induced modulation of P-glycoprotein activity decreased TCBZ efflux from the resistant flukes. Higher concentrations of TCBZ and TCBZSO were recovered from the resistant liver flukes in the presence of ivermectin. Thus, an altered influx/efflux mechanism may account for the development of resistance to TCBZ in F. hepatica.

Assessment of the main metabolism pathways for the flukicidal compound triclabendazole in sheep

Triclabendazole (TCBZ) is an halogenated benzimidazole (BZD) compound worldwide used to control immature and adult stages of the liver fluke Fasciola hepatica. The purpose of this investigation was to characterize in vitro the patterns of hepatic and ruminal biotransformation of TCBZ and its metabolites in sheep. TCBZ parent drug was metabolized into its sulphoxide (TCBZSO), sulphone (TCBZSO2) and hydroxy derivatives by sheep liver microsomes. The same microsomal fraction was also able to oxidize TCBZSO into TCBZSO2 and hydroxy-TCBZSO (HO-TCBZSO). TCBZ sulphoxidation was significantly (P < 0.001) inhibited after inactivation of the flavin-monooxygenase (FMO) system (77% inhibition) as well as in the presence of the FMO substrate methimazole (MTZ) (71% inhibition). TCBZ sulphoxidative metabolism was also reduced (24% inhibition, P < 0.05) by the cytochrome P450 inhibitor piperonyl butoxide (PB). The rate of TCBZSO conversion into TCBZSO2 was also significantly inhibited by PB (55% inhibition), MTZ (52% inhibition) and also following FMO inactivation (58% inhibition). The data reported here indicate that the FMO is the main enzymatic pathway involved in TCBZ sulphoxidation (ratio FMO/P450 = 3.83 +/- 1.63), although both enzymatic systems participate in a similar proportion in the sulphonation of TCBZSO to form the sulphone metabolite (ratio FMO/P450 = 1.31 +/- 0.23). Additionally, ketoconazole (KTZ) did not affect TCBZ sulphoxidation but decreased (66% inhibition, P < 0.05) the formation of TCBZSO2. Similarly, inhibition of TCBZSO2 production was observed after incubation of TCBZSO in the presence of KTZ and erythromycin (ETM). Conversely, thiabendazole (TBZ) and fenbendazole (FBZ) did not affect the oxidative metabolism of both incubated substrates. The sheep ruminal microflora was able to reduce the sulphoxide (TCBZSO) into the parent thioether (TCBZ). The ruminal sulphoreduction of the HO-TCBZSO derivative into HO-TCBZ was also demonstrated. The rate of sulphoreduction of HO-TCBZSO was significantly (P < 0.05) higher than that observed for TCBZSO. The metabolic approach tested here contributes to the identification of the different pathways involved in drug biotransformation in ruminant species. These findings on the pattern of hepatic and ruminal biotransformation of TCBZ and its main metabolites are a further contribution to the understanding of the pharmacological properties of widely used anthelmintics in ruminants. Comprehension of TCBZ metabolism is critical to optimize its flukicidal activity.

Drug transport mechanisms in helminth parasites: Passive diffusion of benzimidazole anthelmintics

Anthelmintic molecules must reach their receptors inside target parasites to exert the pharmacological effect. Available data suggest that the main route of entry of antiparasitic drugs into helminth parasites would be through their external surface. However, it is unclear if trans-tegumental/cuticular penetration is the most important way of entry of benzimidazole (BZD) anthelmintics into their target parasites compared to oral ingestion. The relative involvement of active and passive transport mechanisms has not been defined. The goal of the work reported here was to determine the main processes involved in the entry of BZD anthelmintic molecules into the three main classes of helminth parasites. Adult specimens of Moniezia benedeni (cestode), Fasciola hepatica (trematode) and Ascaris suum (nematode) were incubated in Kreb's Ringer Tris buffer (pH 7.4, 37 degrees C) (1g parasite/10 ml incubation medium) for 15, 45, and 90 min, respectively, in the presence of a concentration gradient of either fenbendazole (FBZ), oxfendazole or triclabendazole sulphoxide (TCBZSO) (1-30 mol/ml, n=4). Dead helminth specimens were also incubated with the same drug concentration gradient. Specimens of F. hepatica with the oral route closed off by ligation were incubated with TCBZSO in the presence or absence of bovine serum albumin. After the incubation time elapsed, samples of parasite material were chemically extracted and prepared for high performance liquid chromatography analysis to measure drug/metabolite concentrations. Equivalent drug concentrations were measured within ligated and non-ligated liver flukes, demonstrating that BZD do mainly penetrate by trans-tegumental diffusion. The higher the concentration of BZD molecules in the incubation medium, the greater their concentration recovered within the helminth parasites. High correlation coefficients (>0.98) were obtained between initial drug concentration in the incubation medium and those measured inside the nematode, cestode, and trematode parasites. FBZ concentrations recovered from tissues of dead cestodes/nematodes over time were significantly greater compared to those measured in living parasites. These differences in drug diffusion may be related to the morphological/functional properties of the parasite's external surfaces. The outcome of the work reported here indicates that passive drug transfer through the external helminth surface is the main transport mechanism accounting for BZD accumulation into target parasites.

Altered drug influx/efflux and enhanced metabolic activity in triclabendazole-resistant liver flukes

Triclabendazole (TCBZ) is a halogenated benzimidazole compound that possesses high activity against immature and adult stages of the liver fluke, Fasciola hepatica. The intensive use of TCBZ in endemic areas of fascioliasis has resulted in the development of liver flukes resistant to this compound. TCBZ sulphoxide (TCBZSO) and TCBZ sulphone (TCBZSO2) are the main molecules recovered in the bloodstream of TCBZ-treated animals. In order to gain some insight into the possible mechanisms of resistance to TCBZ, the goals of the work described here were: to compare the ex vivo transtegumental diffusion of TCBZ parent drug and its sulpho-metabolites (TCBZSO and TCBZSO2) into TCBZ-susceptible and -resistant liver flukes; and to assess the comparative pattern of TCBZ biotransformation by TCBZ-susceptible and -resistant F. hepatica. For the tegumental diffusion studies, TCBZ-susceptible (Cullompton) and -resistant (Sligo) adult flukes collected from untreated infected sheep were incubated (15-180 min) in KRT buffer containing either TCBZ, TCBZSO or TCBZSO2 (5 nmol.ml-1). For the metabolism studies, microsomal fractions obtained from TCBZ-susceptible and -resistant flukes were incubated for 60 min with TCBZ (40 microM), and the amount of the formed metabolic product (TCBZSO) was measured. Drug/metabolite concentrations were quantified by HPLC. All the assayed TCBZ-related molecules penetrated through the tegument of both TCBZ-susceptible and -resistant flukes. However, significantly lower (approximately 50%) concentrations of TCBZ and TCBZSO were recovered within the TCBZ-resistant flukes compared to the TCBZ-susceptible ones over the 180 min incubation period. The rate of TCBZ sulphoxidative metabolism into TCBZSO was significantly higher (39%) in TCBZ-resistant flukes. The flavin-monooxigenase (FMO) enzyme system appears to be the main metabolic pathway involved in the formation of TCBZSO in both TCBZ-susceptible and -resistant flukes. The altered drug influx/efflux and enhanced metabolic capacity identified in TCBZ-resistant liver flukes may account for the development of resistance to TCBZ.

Triclabendazole: new skills to unravel an old(ish) enigma

Triclabendazole was introduced in the early 1980s for the treatment of Fasciola hepatica infections in livestock. Due to its high activity against immature flukes, it has become established as the principal anti-fluke drug on the market. More recently, triclabendazole has been used to treat human cases of fascioliasis and is now the drug of choice for this infection, too. Resistance to triclabendazole was first reported in 1995 in a field population in Australia and, since that time, resistant populations have been identified in several countries in Europe. Parallel to the spread of resistance has been a sharp increase in the prevalence of fascioliasis, which has been attributed largely to climate changes. Consequently, farmers are faced with an alarming scenario, as none of the other fasciolicides on the market possess such high activity against the damaging immature stages of fluke. The main aim of this review is to assess current understanding of the mechanism of action of triclabendazole against the fluke and the mechanism by which the fluke has become resistant to it. The use of triclabendazole against animal and human infections is summarized and suggestions are given on ways to deal with resistance. Gaps in the knowledge of various aspects of its use are highlighted and this may serve to open up future research areas.