The pharmacodynamics of the flukicidal salicylanilides, rafoxanide, closantel and oxyclosanide

Anthelmintics in the environment: Their occurrence, fate, and toxicity to non-target organisms

Anthelmintics are drugs used for the treatment and prevention of diseases caused by parasitic worms (helminths). While the importance of anthelmintics in human as well as in veterinary medicine is evident, they represent emerging contaminants of the environment. Human anthelmintics are mainly used in tropical and sub-tropical regions, while veterinary anthelmintics have become frequently-occurring environmental pollutants worldwide due to intensive agri- and aquaculture production. In the environment, anthelmintics are distributed in water and soil in relation to their structure and physicochemical properties. Consequently, they enter various organisms directly (e.g. plants, soil invertebrates, water animals) or indirectly through food-chain. Several anthelmintics elicit toxic effects in non-target species. Although new information has been made available, anthelmintics in ecosystems should be more thoroughly investigated to obtain complex knowledge on their impact in various environments. This review summarizes available information about the occurrence, behavior, and toxic effect of anthelmintics in environment. Several reasons why anthelmintics are dangerous contaminants are highlighted along with options to reduce contamination. Negative effects are also outlined.

Therapeutic advantages of the combined use of closantel and moxidectin in lambs parasitized with resistant gastrointestinal nematodes

The serious widespread development of nematode resistance has motivated the use of combined anthelmintic formulations. However, the advantages/disadvantages of the combined use of anthelmintics require further scientific characterization. The goals of the current trial were a) to characterize the pharmacokinetics of closantel (CLO) and moxidectin (MXD) administered both subcutaneously (sc) and orally either separately or co-administered (CLO + MXD) to lambs; b) to compare the nematodicidal activity of both molecules given individually or co-administered to lambs infected with resistant nematodes. Seventy (70) Corriedale lambs naturally infected with multiple resistant gastrointestinal nematodes were involved in the pharmacokinetic and efficacy trials. The animals were allocated into six groups (n = 10) and treated with either CLO, MXD, or with the CLO + MXD combined formulation by both the oral and sc routes. Additionally, an untreated control group (n = 10) was included for the efficacy trial. The efficacy was estimated by the faecal egg count reduction test (FECRT). Higher systemic exposure of both CLO and MXD was observed after the sc compared to the oral administration in lambs. The combined administration of CLO + MXD did not markedly alter their disposition kinetics. At 13 days post-treatment, the administration of both molecules as a single active principle reached efficacy levels ranging between 80% (MXDoral), 84% (CLOoral), 85% (CLOsc), and 92% (MXDsc). The combined oral and sc treatments reached 99% efficacy. No adverse effects were observed after the combined treatment of CLO + MXD, and their co-administration did not show any adverse pharmacokinetic interaction. The combined effect of CLO + MXD successfully restored the maximum efficacy levels, which were not reached by the individual active ingredients.

Rafoxanide, a salicylanilide anthelmintic, interacts with human plasma protein transthyretin

Transthyretin (TTR) is a carrier protein for thyroid hormone thyroxine (T4 ) in plasma, placental cytosol, and cerebrospinal fluid. While the potential toxicity of small molecules that compete with T4 for binding to TTR should be carefully studied, these small molecules can also serve as anti-ATTR amyloidosis drugs by stabilizing the TTR structure. Here, we demonstrated that rafoxanide, an EU-approved anthelmintic drug for domesticated animals, binds to the T4 -binding site of TTR. An intrinsic fluorescence quenching assay showed that rafoxanide also binds to the thyroid hormone-related proteins, including serum albumin and thyroid hormone receptor β. Rafoxanide strongly inhibited TTR amyloidogenesis in fibrillization assay, but the binding of rafoxanide to TTR was interfered with in human plasma, probably due to interactions with thyroid hormone-related proteins. Protein crystallography provided clues for the optimization of binding affinity and selectivity. Our findings emphasize the importance of considering rafoxanide as both a possible thyroid-disrupting chemical and a lead compound for the development of new ATTR amyloidosis inhibitors.

The contribution of temporal analysis of pupillometry measurements to cognitive research

Reaction time (RT) is one of the most frequently used measures to detect cognitive processes. When tasks require more cognitive processes/resources, reaction is slower. However, RTs may provide only restricted information regarding the temporal characteristics of cognitive processes. Pupils respond reflexively to light but also to cognitive activation. The more cognitive resources a task requires, the more the pupil dilates. However, despite being able to use temporal changes in pupil size (advanced devices measure changes in pupil diameter with sampling rates of above 1000 samples per second), most past studies using pupil dilation have not investigated temporal changes in pupil response. In the current paper, we discuss the advantage of the temporal approach to analyze pupil changes compared to a more traditional perspective, specifically, singular value methods such as mean value and peak amplitude value. Using data from two recent studies conducted in our laboratory, we demonstrate the differences in findings arising from the various analyses. In particular, we focus on the advantage of temporal analysis in detecting hidden effects, investigating temporal characterizations of the effects, and validating the experimental manipulation.

Pharmacokinetics and pharmacokinetic interactions of orally administered oxfendazole and oxyclozanide tablet formulation to sheep

The combination of oxfendazole and oxyclozanide is used to provide activity against fluke and gastrointestinal nematodes. This study aimed to determine both the pharmacokinetics of oxfendazole (7.5 mg/kg) and oxyclozanide (15 mg/kg) tablet formulation administered orally to sheep and whether there is a pharmacokinetic interaction between these two drugs. The study was conducted in a three-period, crossover pharmacokinetic design and on six healthy Awassi sheep 1-3 years of age. The plasma concentrations of oxfendazole and its metabolites (fenbendazole and fenbendazole sulphone) and oxyclozanide were determined by high-performance liquid chromatography using an ultraviolet detector. Compounds recovered in plasma when oxfendazole was administered alone or combined with oxyclozanide were oxfendazole, fenbendazole sulphone, and fenbendazole, respectively. When oxfendazole was administered alone and co-administered with oxyclozanide, the AUC_FBZ /AUC_OFZ was 0.26 and 0.23, respectively, and the AUC_FBZSO2 /AUC_OFZ was 0.35 and 0.32, respectively. The volume of distribution (Vz/F) of oxfendazole was large in both groups. Oxyclozanide did not change the plasma disposition of oxfendazole. When the oxyclozanide tablet formulation was administered alone, the elimination half-life (21.35 h) and the Vz/F (940.17 ml/kg) were long and large, respectively. The area under the curve (AUC) and the maximum plasma concentration of oxyclozanide were significantly larger and higher, respectively, in the oxyclozanide plus oxfendazole group (1146.61 h × μg/ml and 29.80 μg/ml) compared with the oxyclozanide group (491.44 h × μg/ml and 14.24 μg/ml) while a significant decrease in apparent Vz/F (940.17 vs 379.14 ml/kg) and total clearance (30.52 vs 13.08 ml/h/kg) was detected. In conclusion, co-administration with oxfendazole causing an increase in the plasma profile of oxyclozanide may increase the antiparasitic activity of oxyclozanide.

Tegumental alterations and resistance of Fasciola gigantica adult worms exposed to flukicides in Egypt

Background

The current study was designed to investigate the in vitro effect of commercially available fasciolicides; albendazole (40 and 400 µg/ml), triclabendazole, rafoxanide and nitroxynil (50 and 100 µg/ml, each) against Fasciola gigantica adult worms. For all, worms were incubated for 3 h. Worm's motility was macroscopically and microscopically detected. Reduction of egg deposition was estimated. Alterations of worm's cuticle post-treatments were recorded using scanning electron microscopy (SEM).

Results

Nitroxynil had the most flukicidal effect with mild movement quickly disappeared within 15 min post-treatment. It showed the highest egg reduction (88.3% and 95% at concentrations of 50 and 100 µg/ml, respectively). Findings of SEM showed severe furrowing and destruction of spines. In rafoxanide-treated group, the motility disappeared 75 min post-treatment, and the egg deposition was significantly (P ≤ 0.05) reduced to 70% and 85% at the same concentrations. Teguments showed thickening, moderate furrowing and destruction of the spines. Albendazole showed the lowest effect: the motility of the worms was observed till 160 min post-treatment and the egg reduction was 43% and 75% at the same concentrations. Interestingly, in albendazole-treated flukes, the tegument had severe furrowing and spines were completely sloughed. Similarly, in triclabendazole-treated flukes, worms motility was observed till 160 min post-treatment and the egg reduction was 76.6% and 88.3%. The tegument showed swelling and mild furrowing with moderately damaged spines.

Conclusions

Nitroxynil was the most potent flukicide inducing evidential cuticular changes. Although albendazole induced the most potent cuticular damage, it showed the lowest flukicidal effect. Further in vivo study to investigate resistance/susceptibility of Fasciola species in cattle and buffaloes will be carried out.

Experimental and computational study of rafoxanide radioiodination via isotopic exchange reaction

The current study is an attempt to confirm the possibility of using rafoxanide (Raf) for diagnostic or radiotherapeutic purpose based on the radioiodine used in the labeling process. The isotopic exchange reaction was performed to radiolabel Raf. The maximum radiochemical yield of [125I]Raf (90%) was obtained when 20 μL of Na[125I]I (7.4 MBq (200 μCi)) in the reaction flask was added to 100 μg of Raf (0.799 mM) within 20 min at 140 °C. High pressure liquid chromatography was used to purify the labeled product of [125I]Raf. The activation energy was calculated experimentally in both ethyl acetate and methanol as reaction medium and found to be 22.82 kJ/mol and 24.43 kJ/mol, respectively. Furthermore, Gaussian 09 used the density function theory (DFT) to calculate the activation energy of the reaction in the two solvents.

The Pattern of Blood-Milk Exchange for Antiparasitic Drugs in Dairy Ruminants

Simple Summary

This review article is focused on the description of the plasma–milk partition coefficients for different antiparasitic drug classes in dairy ruminants, and it contributes to rational pharmaco-therapy in lactating dairy animals, which is critical to understand the pattern of drug excretion in milk as well as the residual concentration patterns in dairy products elaborated by processing milk from drug-treated animals.

Abstract

The prolonged persistence of milk residual concentration of different antiparasitic drugs in lactating dairy animals should be considered before recommending their use (label or extra-label) for parasite control in dairy animals. The partition blood-to-milk ratio for different antiparasitic compounds depends on their ability to diffuse across the mammary gland epithelium. The high lipophilicity of some of the most widely used antiparasitic drugs explains their high partition into milk and the extended persistence of high residual concentrations in milk after treatment. Most of the antiparasitic drug compounds studied were shown to be stable in various milk-related industrial processes. Thus, the levels of residues detected in raw milk can be directly applicable to estimating consumer exposure and dietary intake calculations when consuming heat-processed fluid milk. However, after milk is processed to obtain milk products such as cheese, yogurt, ricotta, and butter, the residues of lipophilic antiparasitic drugs are higher than those measured in the milk used for their elaboration. This review article contributes pharmacokinetics-based information, which is useful to understand the relevance of rational drug-based parasite control in lactating dairy ruminants to avoid undesirable consequences of residual drug concentrations in milk and derived products intended for human consumption.

Current Challenges for Fasciolicide Treatment in Ruminant Livestock

Pharmacological treatment remains essential to control fasciolosis in areas where infection is endemic. However, there are major constraints to treating food-producing animals. Of particular concern is the lack of flukicides for treating early Fasciola infections in ruminant livestock in some countries. In addition, the information provided in package leaflets, particularly regarding withdrawal periods, is often incomplete, confusing, and/or contradictory. International regulatory bodies should harmonize the use of flukicides in livestock in favor of fairer, safer international trade. In addition, monitoring the efficacy of fasciolicides on farms is also essential to minimize the spread of drug-resistant populations of Fasciola. The current situation regarding flukicide formulations in the European Union and other, non-European countries is analyzed in this review paper.

The anthelmintic oxyclozanide restores the activity of colistin against colistin-resistant Gram-negative bacilli

Due to the significant increase in antimicrobial resistance in Gram-negative bacilli (GNB), development of non-antimicrobial therapeutic alternatives, which can be used together with the few and non-optimal available antimicrobial agents such as colistin, has become an urgent need. In this context, dysregulation of the bacterial cell wall could be a therapeutic adjuvant to the activity of colistin. The aim of this study was to analyse the activity of oxyclozanide, an anthelmintic drug, in combination with colistin against colistin-susceptible (Col-S) and colistin-resistant (Col-R) GNB. Three Col-S reference strains and 13 clinical isolates (1 Col-S, 12 Col-R) of Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae were studied. Microdilution assays and time-kill curves were performed to examine the activity of oxyclozanide in combination with colistin. The outer membrane protein (OMP) profile, membrane permeability and cell wall structure of Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide were assessed by SDS-PAGE, fluorescence microscopy and transmission electron microscopy, respectively. Oxyclozanide in combination with colistin increased the activity of colistin against Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae. Time-kill curves showed synergistic activity between oxyclozanide and colistin against these bacterial isolates. Moreover, Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide presented greater permeability and disruption of their cell wall than Col-S strains, without modification of their OMP profile. These data suggest that combination of oxyclozanide and colistin may be a new alternative for the treatment of Col-R GNB infections.

Synergistic combinations of anthelmintic salicylanilides oxyclozanide, rafoxanide, and closantel with colistin eradicates multidrug-resistant colistin-resistant Gram-negative bacilli

Repurposing nonantibiotic drugs for antimicrobial therapy presents a viable approach to drug discovery. Development of therapeutic strategies that overcome existing resistance mechanisms is important especially against those bacterial infections in which treatment options are limited, such as against multidrug-resistant Gram-negative bacilli. Herein, we provide in vitro data that suggest the addition of anthelmintic salicylanilides, including oxyclozanide, rafoxanide, and closantel, in colistin therapy to treat multidrug-resistant colistin-susceptible but more importantly colistin-resistant Gram-negative bacilli. As a stand-alone agent, the three salicylanilides suffered from limited outer membrane permeation in Pseudomonas aeruginosa, with oxyclozanide also susceptible to efflux. Synergy was apparent for the combinations against multidrug-resistant clinical isolates of P. aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, and Enterobacter cloacae. Susceptibility breakpoints for colistin, but also with polymyxin B, were reached upon addition of 1 µg ml^-1 of the corresponding salicylanilide against colistin-resistant Gram-negative bacilli. Furthermore, enhanced bacterial killing was observed in all combinations. Our data corroborate the repositioning of the three salicylanilides as adjuvants to counter resistance to the antibiotic of last resort colistin. Our findings are timely and relevant since the global dissemination of plasmid-mediated colistin resistance had been realized.

The Anthelmintic Drug Niclosamide Synergizes with Colistin and Reverses Colistin Resistance in Gram-Negative Bacilli

There is an urgent need for new therapies to overcome antimicrobial resistance especially in Gram-negative bacilli (GNB). Repurposing old U.S. Food and Drug Administration-approved drugs as complementary agents to existing antibiotics in a synergistic combination presents an attractive strategy. Here, we demonstrate that the anthelmintic drug niclosamide selectively synergized with the lipopeptide antibiotic colistin against colistin-susceptible but more importantly against colistin-resistant GNB, including clinical isolates that harbor the mcr-1 gene. Breakpoints for colistin susceptibility in resistant Gram-negative bacilli were reached in the presence of 1 μg/ml (3 μM) niclosamide. Reversal of colistin resistance was also observed in combinations of niclosamide and polymyxin B. Enhanced bacterial killing was evident for the combination, in comparison to colistin monotherapy, against resistant Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, and Enterobacter cloacae Accumulating evidence in the literature, along with our results, strongly suggests the potential for the combination of niclosamide and colistin to treat colistin-resistant Gram-negative bacillary infections. Our finding is significant since colistin is an antibiotic of last resort for multidrug-resistant Gram-negative bacterial infections that are nonresponsive to conventional treatments. With the recent global dissemination of plasmid-encoded colistin resistance, the addition of niclosamide to colistin therapy may hold the key to overcome colistin resistance.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Understanding the main route of drug entry in adult Fasciola hepatica: Further insights into closantel pharmacological activity

Closantel (CLS) is highly effective against adult liver flukes after its oral or subcutaneous (sc) administration in ruminants. Trans-tegumental diffusion and oral ingestion are the two potential routes available for the entry of drugs into Fasciola hepatica. The work reported here contributes to improve the understanding of CLS pharmacology. The main goals of were: I) to determine the pattern of in vivo CLS accumulation into adult F. hepatica and relevant tissues in CLS-treated sheep; II) to investigate the influence of the physicochemical composition of the incubation medium on the CLS diffusion process into adult F. hepatica; III) to assess the ovicidal activity of CLS against F. hepatica eggs; and IV) to investigate the in vivo effect of CLS treatment on glutathione S-transferases activity in adult liver flukes exposed to CLS. Fourteen healthy sheep were each orally infected with 75 F. hepatica metacercariae. Sixteen (16) weeks after infection, animals were treated with CLS by oral (n = 6, 10 mg/kg) or sub-cutaneous (sc) (n = 6, 5 mg/kg) route. At 12, 24 and 36 h post-treatment, animals were sacrificed (n = 2) and samples of blood, bile and adult F. hepatica were collected. In addition, flukes recovered from non-treated sheep (n = 2) were ex vivo incubated (60 min) in the presence of CLS in either RPMI or bile as incubation medium. CLS concentration was measured by HPLC. The ovicidal activity of CLS was investigated using eggs obtained from the bile of untreated sheep. Finally, glutathione S-transferase activity in F. hepatica recovered from untreated and CLS-treated sheep was assessed. In the in vivo studies, the highest CLS concentrations were measured in plasma and adult liver flukes. A positive correlation was observed between CLS concentration in plasma and in F. hepatica. Results obtained in the current work indicate that the in vivo accumulation of CLS into adult liver flukes occurs mainly by the oral route. After ex vivo incubation, the uptake of CLS by the parasite was markedly diminished in the presence of bile compared with that observed in the presence of RPMI as incubation medium. CLS lacks ovicidal activity at therapeutically relevant concentrations. Lastly, CLS significantly increased glutathione S-transferase activity in flukes recovered at 12 h (oral treatment) and 24 h (sc treatment), compared to the control liver flukes.

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.

Anthelmintic closantel enhances bacterial killing of polymyxin B against multidrug-resistant Acinetobacter baumannii

Polymyxins, an old class of antibiotics, are currently used as the last resort for the treatment of multidrug-resistant (MDR) Acinetobacter baumannii. However, recent pharmacokinetic and pharmacodynamic data indicate that monotherapy can lead to the development of resistance. Novel approaches are urgently needed to preserve and improve the efficacy of this last-line class of antibiotics. This study examined the antimicrobial activity of novel combination of polymyxin B with anthelmintic closantel against A. baumannii. Closantel monotherapy (16 mg l(-1)) was ineffective against most tested A. baumannii isolates. However, closantel at 4-16 mg l(-1) with a clinically achievable concentration of polymyxin B (2 mg l(-1)) successfully inhibited the development of polymyxin resistance in polymyxin-susceptible isolates, and provided synergistic killing against polymyxin-resistant isolates (MIC ⩾4 mg l(-1)). Our findings suggest that the combination of polymyxin B with closantel could be potentially useful for the treatment of MDR, including polymyxin-resistant, A. baumannii infections. The repositioning of non-antibiotic drugs to treat bacterial infections may significantly expedite discovery of new treatment options for bacterial 'superbugs'.

Pharmacokinetics of a novel closantel/ivermectin injection in cattle

Two studies are described on the pharmacokinetics of a combination anthelmintic consisting of ivermectin and closantel for use in cattle. In the first, the pharmacokinetics of both active drugs in the combination were compared with the formulation with either ivermectin or closantel removed. No differences in the pharmacokinetics were observed, indicating that neither the absorption nor distribution of ivermectin or closantel in the combination were influenced by the presence of the other. In the second study the pharmacokinetics of ivermectin and closantel in the combination product were compared with control formulations of each. No difference was found between the closantel formulations. With ivermectin it was noted that absorption and excretion were more rapid and Cmax higher in the combination, although the AUC of both formulations were not significantly different.

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

Three experiments were carried out in vitro to determine the roles of the tegument and gut of Fasciola hepatica in the uptake of the flukicidal drug, clorsulon. Changes to the two surfaces were assessed by scanning electron microscopy. In the first experiment, the flukes were ligatured to prevent the oral ingestion of drug and treated for 24 h in clorsulon (10 microg/ml). The gastrodermal surface remained normal and few changes to the tegumental surface were observed. In the second experiment, flukes were fed for 24 h on red blood cells isolated from rats dosed with clorsulon at 12.5 mg/kg body weight; this experiment was designed to prevent the exposure of the tegumental surface to the drug. The gastrodermal surface was severely disrupted and the gut lamellae were disorganised and necrotic. Swelling of the tegument and blebbing on the tegumental surface were evident, but the changes were not severe. More severe swelling of the tegument was observed in the third experiment, in which flukes were incubated for 24 h in clorsulon (10 microg/ml), with both absorptive surfaces being available for drug uptake. The gastrodermal surface was badly disrupted and the gut lamellae were disorganised and necrotic. Taking the results of the three experiments together, the gastrodermal surface was more affected than the tegument and the greatest disruption to the two surfaces was seen when both routes of entry were available to the fluke. The data support a previous study which indicated that entry of clorsulon into the fluke in vivo is principally by the oral ingestion of drug bound to the red blood cells.

The binding of closantel to ovine serum albumin, and homogenate fractions of Haemonchus contortus

Closantel binds to the serum proteins of the host and affects blood sucking parasites when they ingest the blood of treated hosts. Closantel binds specifically to ovine serum albumin (K(a) of 9. 3x10(6)M(-1)) at site I, the warfarin/phenylbutazone binding site of albumin Closantel also binds to invertebrate haemocyanin and haemolymph. The strongest binding of closantel in homogenates of H. contortus is found in fractions containing soluble proteins. This binding is of low affinity and, because the site itself is not fully denaturable, it may not be proteinaceous. There is no detectable difference in binding affinity between homogenate fractions from closantel susceptible and resistant isolates of adult or larval worms suggesting that closantel resistance is not due to changes in the closantel receptor or carrier.

Fasciolicides: efficacy, actions, resistance and its management

The modes of action of fasciolicides are described. Closantel and other salicylanilides interfere with energy metabolism by uncoupling oxidative phosphorylation in the fluke. Other fasciolicides are believed to have a metabolic action-halogenated phenols (via uncoupling) and clorsulon (via inhibition of glycolysis)-but direct evidence is lacking. Benzimidazoles (in particular, triclabendazole) bind to fluke tubulin and disrupt microtubule-based processes. Diamphenethide inhibits protein synthesis in the fluke. Other potential drug actions may contribute to overall drug efficacy. In particular, a number of fasciolicides-salicylanilides, phenols, diamphenethide-induce a rapid paralysis of the fluke, so their action may have a neuromuscular basis, although the actions remain ill-defined. Resistance to salicylanilides and triclabendazole has been detected in the field, although drug resistance does not appear to be a major problem yet. Strategies to minimize the development of resistance include the use of synergistic drug combinations, together with the design of integrated management programmes and the search for alternatives to drugs, in particular, vaccines.

Modes of action of anthelmintic drugs

Modes of action of anthelmintic drugs are described. Some anthelmintic drugs act rapidly and selectively on neuromuscular transmission of nematodes. Levamisole, pyrantel and morantel are agonists at nicotinic acetylcholine receptors of nematode muscle and cause spastic paralysis. Dichlorvos and haloxon are organophosphorus cholinesterase antagonists. Piperazine is a GABA (gamma-amino-butyric acid) agonist at receptors on nematode muscles and causes flaccid paralysis. The avermectins increase the opening of glutamate-gated chloride (GluCl) channels and produce paralysis of pharyngeal pumping. Praziquantel has a selective effect on the tegument of trematodes and increases permeability of calcium. Other anthelmintics have a biochemical mode of action. The benzimidazole drugs bind selectively to beta-tubulin of nematodes, cestodes and fluke, and inhibit microtubule formation. The salicylanilides: rafoxanide, oxyclozanide, brotianide and closantel and the substituted phenol, nitroxynil, are proton ionophores. Clorsulon is a selective antagonist of fluke phosphoglycerate kinase and mutase. Diethylcarbamazine blocks host, and possibly parasite, enzymes involved in arachidonic acid metabolism, and enhances the innate, nonspecific immune system.

Comparative efficacies of closantel, ivermectin, oxfendazole, thiophanate and levamisole against thiabendazole resistant Haemonchus contortus in sheep

Forty-nine sheep artificially exposed to a thiabendazole (TBZ) resistant isolate of Haemonchus contortus were assigned to 7 groups of 7 animals each and used to conduct a controlled anthelmintic trial. One group of sheep served as untreated infected controls and 6 groups were treated as follows: closantel, 5.0 mg kg-1; ivermectin, 0.2 mg kg-1; oxfendazole, 5.0 mg kg-1; thiophanate, 50 mg kg-1, levamisole, 7.5 mgkg-1 and thiabendazole, 66 mg kg-1. Eggs per gram of faeces were determined on days 21, 24 and day 34 (10 days post-treatment) after infection and all animals were necropsied for residual worm counts. The calculated efficacies of the treatments against H. contortus as indicated by worm reduction were closantel (100%), ivermectin (99.3%), oxfendazole (35.2%), thiophanate (56.7%), levamisole (98.6%) and thiabendazole (24.3%). The data therefore indicate that the TBZ-resistant isolate of H. contortus used was highly resistant to the 2 benzimidazole (BZ) anthelmintics, oxfendazole and thiophanate. This is the first report in Kenya of a field strain of H. contortus resistant to thiophanate.

The disposition of antiparasitic drugs in relation to the development of resistance by parasites of livestock

The kinetic and dynamic disposition of endo- and ectoparasiticides in livestock in relation to development of resistance is examined. Based on the modes of action of antiparasitic drugs, maximum activity necessitates that the parasite be exposed to 'toxic' concentrations for as great a duration as possible. In contrast, exposure to non-lethal discriminating drug concentrations has a significant potential to promote the development of resistance. Orally administered anthelmintics quickly associate with particulate digesta in the rumen; their subsequent desorption from particulate matter as it vacates the rumen maintains the duration of metabolite availability. The flow rate of digesta increases with feed intake and the presence of gastrointestinal parasites, and together with other parasite-induced physiological changes to the gut, contributes to reduced duration of drug availability. The potential for orally administered drugs to bypass the rumen, due to closure of the oesophageal groove, exacerbates the effect. Once absorbed, the metabolite concentration with time profile progressively decreases, the rate depending upon the chemical nature of the drug and the type and condition of the host into which it was administered. The greater hepatic activity of goats speeds drug elimination, the lower dose equivalent availability increases the potential for generation of drug resistance in parasites of goats as compared to sheep. Parasites whose resistance is generated in goats may be then transferred to sheep. Similar distribution/elimination kinetics apply to topically administered insecticides of sheep. The progressively reducing concentrations expose ectoparasites to discriminating drug levels, again contributing to the development of resistance. It is anticipated that a greater understanding of the physiological/pharmacological effects which are described in this review will permit the more efficient use of existing and future antiparasitic drugs.

Comparative pharmacokinetic disposition of closantel in sheep and goats

The pharmacokinetic disposition of closantel was examined following intraruminal (i.r.) or intramuscular (i.m.) administration to adult Merino sheep and to adult and 3-month-old, suckling Angora goats. In adult goats the maximum concentration (Cmax) and area under the plasma concentration with time curve (AUC) following 3.75, 7.5 and 15.0 mg closantel/kg given i.r. increased with dose however the time of Cmax (Tmax = 2.6d) in plasma was unaffected by dose rate. The elimination phase (K10) of closantel was monoexponential with a half-life (t1/2) of 4.7d again unaffected by dose rate. Apart from a more rapid absorption phase and earlier Tmax following 3.75 mg closantel/kg i.m., pharmacokinetic behaviour was similar to that following i.r. administration at 3.75 or 7.5 mg/kg. Although absorption rate was more rapid in kids after i.r. administration at 7.5 mg/kg, pharmacokinetic disposition of closantel was otherwise similar to that in adult goats. No closantel was detected in milk of treated doses or in the plasma of their kids. I.R. closantel at 7.5 mg/kg was more slowly absorbed in goats than in sheep but Cmax was similar in both species. However, K10 t1/2 was significantly shorter in goats (4d) than in sheep (14d). Faster elimination resulted in an almost three-fold lowering of AUC in goats and could dramatically reduce the sustained action of closantel in this species compared with sheep.

Relationship between pharmacological properties and clinical efficacy of ruminant anthelmintics

The purpose of this review article is to establish a relationship between pharmacological properties and clinical efficacy of the most widely used broad-spectrum veterinary anthelmintics. The impact of drug pharmacokinetics/pharmacodynamics on anthelmintic efficacy and resistance is discussed. We review the clinical pharmacokinetics of the currently available anthelmintics used in livestock-imidazothiazoles, tetrahydropyrimidines, benzimidazoles/pro-benzimidazoles, salicylanilides (closantel) and avermectin-type compounds. Understanding the pharmacokinetic and metabolic behaviour of broad-spectrum anthelmintics in the host, and factors modulating that behaviour, is highly important for maximizing anthelmintic utility and efficacy.

Determination of rafoxanide and closantel in ovine plasma by high performance liquid chromatography

A high performance liquid chromatographic method has been developed for the determination of rafoxanide and closantel in ovine plasma. Acetonitrile and chloroform were used for the extraction. The mean recoveries were 78.69% and 80.59% for rafoxanide and closantel, respectively. This method was applied to the characterization of rafoxanide plasma kinetics following oral administration of therapeutic doses to sheep.

Effect of early treatment with rafoxanide on antipyrine clearance in sheep infected with Fasciola hepatica

1. The effect of the salicylanilide compound rafoxanide against immature stages of Fasciola hepatica was investigated in sheep using a series of antipyrine clearance tests and measuring glutamate dehydrogenase and gamma-glutamyl transferase activities in plasma. 2. Three animal groups were used. In two groups, sheep were infected with 200 metacercariae each. Four weeks after infection one of the two groups was treated with rafoxanide and the other was left untreated. The third group was unparasitized and also received rafoxanide. 3. Infection was confirmed by post-mortem examination of the livers of infected sheep. Infection did not alter the plasma disposition of rafoxanide. 4. The efficacy of rafoxanide, as assessed by the reduction in the number of adult flukes in treated animals compared with controls, was 85%. 5. Glutamate dehydrogenase activity fell dramatically 2 weeks after treatment but increased again at 12 weeks post-infection in the infected rafoxanide-treated group. 6. Antipyrine clearance decreased between 8 and 14 weeks post-infection in untreated sheep. In the infected rafoxanide-treated group plasma clearance of antipyrine remained unchanged until 10 weeks after rafoxanide administration when it decreased from the preinfection value of 5.09 to 3.90 ml.kg/min. Rafoxanide did not affect antipyrine disposition in uninfected sheep. 7. It was concluded that rafoxanide had an incomplete anthelmintic effect against immature stages of Fasciola hepatica, and that surviving parasites caused delayed liver damage which was reflected in an elevation in glutamate dehydrogenase activity and decreased plasma clearance of antipyrine.

Effects of dietary variations on plasma concentrations of oral flukicides in sheep

Groups of parasite-free lambs which were either housed and fed hay and concentrates or were grazing on pasture were dosed with the oral flukicides rafoxanide and triclabendazole and subsequent plasma concentrations monitored. Peak plasma concentrations and areas under curves (AUC) of both chemicals were significantly reduced in the grazing compared with the housed lambs. In order to investigate the observation similar groups of lambs were dosed orally with chromium EDTA and faecal throughput estimated. It was observed that the rate of throughput was greater in the grazing lambs, leading to the conclusion that the differences in plasma concentrations of the flukicides was caused by a reduction in their absorption in the grazing lambs. The implications on flukicide efficacy and dose rates are discussed.

Plasma protein binding of nitroxynil in several species

The binding of nitroxynil to total plasma proteins of cows, sheep and rabbits was characterized using equilibrium dialysis. The data indicate clearly that nitroxynil was highly (97-98%) bound to plasma protein of each animal. This linear binding would be due to the particular power exerted by serum albumin. The results are in good agreement with known pharmacokinetic properties of nitroxynil in domestic species.

Pharmacokinetic data of bromofenofos in calves

Bromofenofos (BF) was administered orally to six calves at a dose of 12 mg/kg and the concentrations of BF and its metabolite dephosphate bromofenofos (DBF) in plasma were determined using a high-performance liquid chromatographic method. DBF reached maximum concentration of 45.9 +/- 16.3 micrograms/ml, which occurred at 1.3 +/- 0.4 days. The elimination half life was calculated to be 1.8 +/- 0.3 days and the area under the plasma concentration versus time curve was 185.1 +/- 49.4 micrograms.day/ml. The ultrafiltrate of plasma samples revealed no free metabolite, indicating a binding of greater than 99% to plasma protein. DBF disappeared from plasma in all calves 21 days after administration. No BF was detected in plasma at any time.