Characterisation of a beta-tubulin gene from the liver fluke, Fasciola hepatica

Spectrum subtraction as a complementary method for six resolution techniques resolving overlapping spectra; application to multicomponent veterinary formulation with greenness and whiteness assessment

Mathematical filtration is an efficient tool to resolve the overlapping spectra of binary mixtures in zero or first order form. Herein, a comparative study was conducted between six economic, accurate and precise spectrophotometric methods for determination of Triclabendazole (TCB) and Levamisole HCl (LVM). Each component was resolved with minimum mathematical steps in its zero-order absorption spectrum by ratio subtraction, constant multiplication, and the recent factorized response method; coupled with spectrum subtraction. In addition, the mixture was resolved in its first derivative form by derivative subtraction, D1 constant multiplication, and the recent D1 factorized response method; coupled with spectrum subtraction. Results obtained were also compared to those obtained from constant value, concentration value, and derivative ratio methods. The linearity range was found to be either 1.0-10.0 µg/mL or 2.0-20.0 µg/mL for TCB, and 2.0-14.0 µg/mL for LVM with LOD of 0.08 µg/mL and 0.19 µg/mL, respectively. Validation of the proposed methods was performed according to VICH guidelines. Results obtained from the statistical data showed no significant difference regarding accuracy and precision compared to the reported methods. The developed spectrophotometric methods followed the principles of green analytical chemistry, in which the green assessment was done through four tools, called, National Environmental Methods Index (NEMI), Analytical Eco-Scale (AES), Green Analytical Procedure Index (GAPI) and Analytical greenness metric (AGREE). Also, a white assessment was performed using RGB model. The proposed methods could offer an economic alternative for the routine analysis of bulk materials and combined veterinary dosage form.

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.

Exploration of interaction zones of β-tubulin colchicine binding domain of helminths and binding mechanism of anthelmintics

Numerous studies postulated the possible modes of anthelmintic activity by targeting alternate or extended regions of colchicine binding domain of helminth β-tubulin. We present three interaction zones (zones vide -1 to -3) in the colchicine binding domain of Haemonchus contortus (a helminth) β-tubulin homology model and developed zone-wise structure-based pharmacophore models coupled with molecular docking technique to unveil the binding hypotheses. The resulted ten structure-based hypotheses were then refined to essential three point pharmacophore features that captured recurring and crucial non-covalent receptor contacts and proposed three characteristics necessary for optimal zone-2 binding: a conserved pair of H bond acceptor (HBA to form H bond with Asn226 residue) and an aliphatic moiety of molecule separated by 3.75±0.44Å. Further, an aliphatic or a heterocyclic group distant (11.75±1.14Å) to the conserved aliphatic site formed the third feature component in the zone-2 specific anthelmintic pharmacophore model. Alternatively, an additional HBA can be substituted as a third component to establish H bonding with Asn204. We discern that selective zone-2 anthelmintics can be designed effectively by closely adapting the pharmacophore feature patterns and its geometrical constraints.

Liver fluke β-tubulin isotype 2 binds albendazole and is thus a probable target of this drug

Albendazole is a benzimidazole drug which can be used to treat liver fluke (Fasciola hepatica) infections. Its mode of action is believed to be the inhibition of microtubule formation through binding to β-tubulin. However, F. hepatica expresses at least six different isotypes of β-tubulin, and this has confused, rather than clarified, understanding of the molecular mechanisms of benzimidazole drugs in this organism. Recombinant F. hepatica β-tubulin proteins were expressed in, and purified from, Escherichia coli. These proteins were then used in pull-down assays in which albendazole was covalently linked to Sepharose. β-Tubulin isotype 2 was pulled down in this assay, and this interaction could be reduced by adding competing albendazole. Molecular modelling of β-tubulin isotypes suggests that changes in the side change conformations of residue 200 in the putative albendazole binding site may be important in determining whether, or not, a particular isotype will bind to the drug. These results, together with previous work demonstrating that albendazole causes disruption of microtubules in the liver fluke, strongly suggest that β-tubulin isotype 2 is one of the targets of this drug.

Echinococcus metacestodes as laboratory models for the screening of drugs against cestodes and trematodes

Among the cestodes, Echinococcus granulosus, Echinococcus multilocularis and Taenia solium represent the most dangerous parasites. Their larval stages cause the diseases cystic echinococcosis (CE), alveolar echinococcosis (AE) and cysticercosis, respectively, which exhibit considerable medical and veterinary health concerns with a profound economic impact. Others caused by other cestodes, such as species of the genera Mesocestoides and Hymenolepis, are relatively rare in humans. In this review, we will focus on E. granulosus and E. multilocularis metacestode laboratory models and will review the use of these models in the search for novel drugs that could be employed for chemotherapeutic treatment of echinococcosis. Clearly, improved therapeutic drugs are needed for the treatment of AE and CE, and this can only be achieved through the development of medium-to-high throughput screening approaches. The most recent achievements in the in vitro culture and genetic manipulation of E. multilocularis cells and metacestodes, and the accessability of the E. multilocularis genome and EST sequence information, have rendered the E. multilocularis model uniquely suited for studies on drug-efficacy and drug target identification. This could lead to the development of novel compounds for the use in chemotherapy against echinococcosis, and possibly against diseases caused by other cestodes, and potentially also trematodes.

Fasciola hepatica expresses multiple alpha- and beta-tubulin isotypes

We have identified five alpha-tubulin and six beta-tubulin isotypes that are expressed in adult Fasciola hepatica. Amino acid sequence identities ranged between 72 and 95% for fluke alpha-tubulin and between 65 and 97% for beta-tubulin isotypes. Nucleotide sequence identity ranged between 68-77% and 62-80%, respectively, for their coding sequences. Phylogenetic analysis indicated that two of the alpha-tubulins and two of the beta-tubulins were distinctly divergent from the other trematode and nematode tubulin sequences described in this study, whereas the other isotypes segregated within the trematode clades. With regard to the proposed benzimidazole binding site on beta-tubulin, three of the fluke isotypes had tyrosine at position 200 of beta-tubulin, two had phenylalanine and one had leucine. All had phenylalanine at position 167 and glutamic acid at position 198. When isotype RT-PCR fragment sequences were compared between six individual flukes from the susceptible Cullompton isolate and from seven individual flukes from the two resistant isolates, Sligo and Oberon, these residues were conserved.

Genetic heterogeneity of Fasciola hepatica isolates in the northwest of Spain

The liver fluke Fasciola hepatica is a worldwide distributed trematode parasite of great veterinary importance. In Spain, the prevalence of the infection is up to 56% in sheep. Its control is mainly based on the use of anthelmitics, although some resistant populations have been described. Genetic studies of the genus Fasciola have been focused on the 28S ribosomal deoxyribonucleic acid (rDNA) gene, with no intraspecific variations and a few interspecific nucleotide differences. We have designed a specific polymerase chain reaction to amplify and sequence a 510-bp-long fragment of the gene to characterize several F. hepatica ovine isolates from the northwest of Spain. All the isolates rendered the expected results on the agarose gel, while sequencing revealed some differences in the 34 site. The anthelmintic-resistant specimens showed the same nucleotide differing of the sensitive ones. Comparing to other studies, our results conclude that there are some genetic variations in F. hepatica (at the level of the 28S rDNA), including heterozygous specimens. In conclusion, a genetic heterogeneity has been found in the F. hepatica populations of ovine origin in the northwest of Spain, using an assay that could be useful for further epidemiology and characterization studies.

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.

A possible model of benzimidazole binding to beta-tubulin disclosed by invoking an inter-domain movement

Although it is well established that benzimidazole (BZMs) compounds exert their therapeutic effects through binding to helminth beta-tubulin and thus disrupting microtubule-based processes in the parasites, the precise location of the benzimidazole-binding site on the beta-tubulin molecule has yet to be determined. In the present study, we have used previous experimental data as cues to help identify this site. Firstly, benzimidazole resistance has been correlated with a phenylalanine-to-tyrosine substitution at position 200 of Haemonchus contortus beta-tubulin isotype-I. Secondly, site-directed mutagenesis studies, using fungi, have shown that other residues in this region of the protein can influence the interaction of benzimidazoles with beta-tubulin. However, the atomic structure of the alphabeta-tubulin dimer shows that residue 200 and the other implicated residues are buried within the protein. This poses the question: how might benzimidazoles interact with these apparently inaccessible residues? In the present study, we present a mechanism by which those residues generally believed to interact with benzimidazoles may become accessible to the drugs. Furthermore, by docking albendazole-sulphoxide into a modelled H. contortus beta-tubulin molecule we offer a structural explanation for how the mutation conferring benzimidazole resistance in nematodes may act, as well as a possible explanation for the species-specificity of benzimidazole anthelmintics.

Separation and characterization of synthetic impurities of triclabendazole by reversed-phase high-performance liquid chromatography/electrospray ionization mass spectrometry

A simple high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) method for the separation and characterization of impurities in the synthesis of triclabendazole has been developed. The analytical separation was achieved on a reversed-phase C18 column using acetonitrile and water (60:40, v/v) as mobile solvent at a flow-rate of 1.0 ml/min at 25 degrees C, and an UV detection at 230 nm. The on-line HPLC/ESI-MSn examinations were performed using ion trap analyzer with extraction ion chromatography (EIC) technique in positive or negative ion modes. The semi-preparative separation was performed with a reversed-phase column using methanol and water (75:25, v/v) as mobile solvent at a flow-rate of 4 ml/min at 25 degrees C, and an UV detection at 230 nm. Thus, two impurities were detected and identified as 5-chloro-6-(2,3,4-trichlorophenoxy)-2-methylsulfanyl-1H-benzoimidazole and 5-chloro-6-(2,3-dichlorophenoxy)-1-methyl-2-methylsulfanyl-1H-benzoimidazole. Meanwhile, some intermediates of impurity-1 in multi-step synthetic reactions, were tracked. Structural elucidation by 1D and 2D NMR and ESI-MSn was discussed.

Characterisation of a beta-tubulin gene from the monogenean parasite, Gyrodactylus salaris Malmberg, 1957

The cDNA and two partial genomic sequences of beta-tubulin genes have been isolated from the monogenean parasite Gyrodactylus salaris. The cDNA sequence is not represented by either of the genomic sequences, implying that at least three isotypes of the gene exist in G. salaris. The sequences show regions of high homology with other helminth beta-tubulin genes. This represents the first isolation of a beta-tubulin gene from a monogenean and contributes to the overall characterisation of these genes within the helminths. This is an important area, as anthelmintic resistance is increasing against benzimidazole drugs that target the beta-tubulin gene. Benzimidazole drugs have been tested successfully against Gyrodactylus parasites, but their use is not widespread. Should it increase, analysis of the beta-tubulin gene may provide a tool for monitoring resistance development and improving management practises. Use of the beta-tubulin gene in the identification of Gyrodactylus species may prove complex due to the presence of different isotypes.