Potentiation of the anthelmintic activity of oxfendazole by parbendazole

Benzimidazole based hybrids against complex diseases: A catalogue of the SAR profile

The fused heterocyclic ring system has been recognized as a privileged structure that is used as a template in medicinal chemistry for drug discovery. Benzimidazole is one of the common scaffolds found in several natural products such as histidine, purines, and an integral part of vitamin B12. This hetero-aromatic bicyclic ring system acts as a pharmacophore in various drugs of therapeutic interest and has a broad spectrum of activity. Literature reports suggest that diversely substituted benzimidazoles possess distinct pharmacological profiles with multi-targeting potential, thereby, an indispensable anchor for the development of novel therapeutic agents against complex diseases such as cancer, malaria, inflammatory disorders, microbial diseases, hypertension, etc. Thus, lots of efforts have been diverted towards exploring the therapeutic potential of benzimidazoles. Despite great efforts made by the research community, still, some multi-factorial diseases continue to progress due to their complex pathophysiology. Under these sets of circumstances, there is a need to explore this nucleus for hybrid designing with multi-targeting potential against complex diseases. Benzimidazole-based hybrids have been reported to treat multifactorial diseases, making it a scaffold of interest for various pharmaceutical companies and research groups. In this write-up, we shed light on the recent pharmacological profiles, various designing strategies, and structure-activity relationships (SAR) of different benzimidazole-based hybrids.

Modulations of bovine hepatic microsomal metabolism of benzimidazoles by secondary plant metabolites

The study was aimed to estimate the effect of plant secondary metabolites present in ruminants diet and phytogenic feed additives on liver microsomal metabolism of albendazole and fenbendazole. The selected phytocompounds comprised of flavonoids (apigenin, quercetin) and saponins (hederagenin, medicagenic acid). The experiments were performed on liver microsomal fraction obtained from routinely slaughtered cows. The intensity of albendazole and fenbendazole metabolism in the presence of flavonoids and saponins was analyzed in equimolar concentration (100 μM). The obtained results revealed that both flavonoids and saponins intensify the metabolism of albendazole and fenbendazole in bovine microsomes. In the case of albendazole, apigenin and quercetin doubled the amount of degraded drug and the amount of produced albendazole sulfoxide. Additionally, both flavonoids increased the amount of produced albendazole sulfone. Saponins, hederagenin, and medicagenic acid intensified the degradation of albendazole (1.8-fold) and the production of albendazole sulfoxide (twofold). Medicagenic acid inhibited the production of albendazole sulfone. In the case of fenbendazole, the degradation of the drug and the production of oxfendazole were increased four and five times in the presence of saponins and flavonoids, respectively. The enhancement of benzimidazoles' metabolism caused by the studied plant metabolites could change pharmacokinetics and the efficacy of benzimidazoles' treatment in cattle.

Efficacy of pharmacokinetic interactions between piperonyl butoxide and albendazole against gastrointestinal nematodiasis in goats

To test the hypothesis that modulation of hepatic microsomal sulphoxidation and sulphonation by the cytochrome P450 inhibitor piperonyl butoxide could increase bioavailability of albendazole, the present study was undertaken to understand the pharmacokinetics of albendazole in goats at a dose of 7.5 mg kg− 1 body weight with and without co-administration with piperonyl butoxide at 63.0 mg kg− 1 body weight. Plasma albendazole sulphoxide metabolite, the anthelmintically active moiety, reached its maximum concentration of 0.322 ± 0.045 μg ml− 1 and 0.384 ± 0.013 μg ml− 1 at 18 h and 24 h after administration of albendazole alone and co-administration of albendazole with piperonyl butoxide, respectively. Analysis of the data revealed statistically increased albendazole sulphoxide levels at 24 (P <  0.001), 30 (P <  0.001) and 36 h (P <  0.01) after administration of albendazole with piperonyl butoxide, with statistically increased levels of albendazole sulphone at 24, 30 and 48 h after administration. No significance difference (P > 0.05) in values of maximum concentration (normal and calculated) could be observed between groups of goats. However, values of time to reach the concentration maximum (normal and calculated), area under the concentration–time curve (0–∞ and calculated), minimum residence time, distribution half-life, elimination half-life and total area under the first movement of plasma drug concentration–time curve were significantly higher (P <  0.05) in plasma levels of albendazole sulphoxide in goats following single oral co-administration of albendazole with piperonyl butoxide. The faecal egg count reduction and lower 95% confidence limit for the group treated with albendazole alone were 97 and 68%, while for co-administration of albendazole and piperonyl butoxide the values were 99 and 97%, respectively. The ED50 for egg hatch was 0.196, indicating suspected resistance to benzimidazole anthelmintics. The drug combination proved efficacious against an albendazole-resistant nematode parasite population in goats.

Diet-induced modulation of pharmacokinetics of albendazole in Sahiwal cattle

The influence of diet type and pre-treatment fasting on the kinetic disposition of albendazole was evaluated in Sahiwal heifers following oral and intra-ruminal administration of the drug. The anthelmintically active moiety albendazole sulphoxide appeared early and was eliminated early in cattle offered green fodder, with decreased maximum concentration (Cmax) and area under concentration–time curve (AUC) when the drug was administered both through oral and intra-ruminal routes. Further, the elimination half-life (t½β) revealed significantly increased values for albendazole sulphoxide in cattle administered albendazole through the intra-ruminal route. An increased AUC and t½β is reflective of increased bioavailability of albendazole in animals offered dry fodder. Increased values (P <  0.05) of Cmax, time to Cmax (Tmax), AUC and t½β for albendazole sulphoxide occurred in cattle with a pre-treatment 24-h fast, resulting in its increased bioavailability. Extrapolation of data of the active metabolite albendazole sulphoxide levels in terms of drug–parasite contact revealed increased exposure of parasites to the drug in cattle administered albendazole through the intra-ruminal route and with 24-h pre-treatment fasting.

The detection of anthelmintic resistance by the faecal egg count reduction test: An examination of some of the factors affecting performance and interpretation

This study examines and compares the possible effects of several procedural variants with those of a currently recommended faecal egg count reduction test for the detection of anthelmintic resistance. The results suggest that the failure of an anthelmintic to reduce the arithmetic mean egg counts of 10-15 animals by at least 90%, from either their pre-treatment levels or from those of an untreated control group 5-10 days later, is likely to be an appropriate procedure. It is recommended that such evaluations of drench performance be regularly conducted on farms. Some suggestions as to how this might be encouraged are made, and the role of larval cultures both in increasing the sensitivity and in enhancing the value of the faecal egg count reduction test are discussed.

Pharmacokinetic disposition of triclabendazole in cattle and sheep; discrimination of the order and the rate of the absorption process of its active metabolite triclabendazole sulfoxide

A comparative pharmacokinetic study was conducted to determine the order and the rate of absorption of triclabendazole (TCBZ) in cattle and sheep. A commercial suspension of TCBZ (Biofasiolex, Biogénesis S.A., Argentina) was administered at a dose rate of 10 mg/kg by the oral route to six Holstein female calves and six Corriedale female sheep. The plasma concentration profiles of the metabolites triclabendazole sulfoxide (TCBZ-SO) and triclabendazole sulfone (TCBZ-SO(2)) were analysed by means of the non-compartmental method. The order of the absorption process of the active metabolite, TCBZ-SO, was determined by construction of curves of cumulative absorbed fraction of the drug by means of the Wagner-Nelson method. The appearance of TCBZ-SO in plasma of cattle and sheep resembles the entry of a constant quantity of drug into the organism per unit time. This is explained by the reservoir effect of the rumen, which acts as a biological slow-release system for TCBZ-SO and its precursor TCBZ to the posterior digestive tract where they are absorbed. The plasma concentration profiles of TCBZ-SO in both species were well described by a one-compartment open model with zero-order process of absorption and first-order process of elimination. The values of AUC(0-infinity) and C(max) of TCBZ-SO did not differ between species, while other kinetic parameters except for lambda(z) had higher values in calves than in sheep. In the case of TCBZ-SO(2), t(max) was the only parameter that did not differ between species, while other kinetic parameters except for lambda(z) had higher values in calves than in sheep.

Veterinary anthelmintics: old and new

Between 1960 and 1980, extraordinary success was achieved in anthelmintic development for animals. In these 20 years, drugs with diverse structure, novel activity and enviable safety were produced for a global livestock industry leading to the productivity gains needed to support a human population that grew by 1.5 billion during the same period. The following 20 years have been spent refining existing molecules with niche activity (parasite and host specificity), improving delivery systems and worrying about the inexorable spread of drug resistance. The challenge for the next 20 years will be to use the technologies available to design and produce new drugs and biological controls. Then, to use the lessons of the past to ensure that the new drugs enjoy a longer useful lifespan and contribute to an animal health industry (livestock and companion) which enriches the lives of a global population. Old and new veterinary anthelmintics comprise a very large field, which could not be comprehensively reviewed in a short article. The present mini-review focuses on major chemical discoveries, formulation developments, administration strategies and new products.

Aspects of the pharmacokinetics of albendazole sulphoxide in sheep

The plasma disposition kinetics of albendazole sulphoxide (ABZSO), ((+)ABZSO and (-)ABZSO) and its sulphone metabolite (ABZSO2) were investigated in adult sheep. Six Corriedale sheep received albendazole sulphoxide by intravenous injection at 5 mg/kg live weight. Jugular blood samples were taken serially for 72 h and the plasma was analysed by high-performance liquid chromatography (HPLC) for albendazole (ABZ), ABZ sulphoxide (ABZSO) and albendazole sulphone (ABZSO2). Albendazole was not detected in the plasma at any time after the treatment, ABZSO and ABZSO2 being the main metabolites detected between 10 min and 48 h after treatment. A biexponential plasma concentration versus time curve was observed for both ABZSO and ABZSO2 following the intravenous treatment. The plasma AUC values for ABZSO and ABZSO2 were 52.0 and 10.8 (microg x h)/ml, respectively. The ABZSO2 metabolite was measurable in plasma between 10 min and 48 h after administration of ABZSO, reaching a peak concentration of 0.38 microg/ml at 7.7 h after treatment. Using a chiral phase-based HPLC method, a biexponential plasma concentration versus time curve was observed for both ABZSO enantiomers. The total body clearance was higher for the (-) than for the (+) enantiomer, the values being 270.6 and 147.75 (ml/h)/kg, respectively. The elimination half-life of the (-) enantiomer was shorter than that of the (+) enantiomer, the values being 4.31 and 8.33 h, respectively. The enantiomeric ratio (+)ABZSO/( )ABZSO at t0 was close to unity. However, the ratio in the plasma increased with time.

Effect of amphiphilic surfactant agents on the gastrointestinal absorption of albendazole in cattle

Albendazole (ABZ) is a widely used broad-spectrum benzimidazole (BZD) anthelmintic. Low hydrosolubility and poor/erratic gastrointestinal (GI) absorption play against the systemic availability and resultant clinical efficacy of BZD compounds. Different strategies are currently investigated to improve their bioavailability and efficacy in different animal species and humans. Surfactant agents facilitate dissolution of lipophilic drugs and increase membrane permeability. The influence of amphiphilic surfactants on the pattern of absorption and systemic availability of ABZ and its metabolites in cattle was characterized in the current trial. Twenty (20) parasite-free Holstein calves (100-120 kg) were randomly allocated into four groups and treated intraruminally (10 mg/kg) using one of the following ABZ suspensions: control without surfactant (75/25 dimetyl sulphoxide/saline solution) (group A), 5 mM sodium taurocholate (STC) in saline solution (group B), 8.27 mM sodium lauryl sulphate (SLS) in saline solution (group C) and a commercial formulation (Valbazen((R)), Pfizer Inc. SA) (group D). Jugular blood samples were taken over 72 h post-treatment and plasma analysed by HPLC. Albendazole sulphoxide (ABZSO) and sulphone were the metabolites found in plasma. STC did not affect ABZ absorption while increased ABZSO peak plasma concentration (C(max)) (158% higher, P<0.001) was observed following co-administration of ABZ plus SLS, compared to the control group without surfactant. ABZSO plasma availability was significantly greater after the ABZ-SLS (164%) co-administration compared to that obtained in the control group without surfactant. A similar ABZSO plasma availability was obtained following the treatments with the ABZ-SLS and the commercially available formulation. SLS-mediated enhanced dissolution and absorption of ABZ accounted for the observed increased systemic availability of the active ABZSO metabolite in cattle. These results should be considered among strategies to improve the use of BZD anthelmintics.

Fenbendazole pharmacokinetics, metabolism, and potentiation in horses

The present study was designed to describe the pharmacokinetics and fecal excretion of fenbendazole (FBZ) and fenbendazole sulphoxide (FBZSO) and their metabolites in horses, to investigate the effects which concurrent feeding has on the absorption and pharmacokinetics of FBZ, and to determine the effect of coadministration of the metabolic inhibitor piperonyl-butoxide on the in vivo pharmacokinetics and in vitro liver microsomal metabolism of sulfide and sulfoxide benzimidazoles. The effect of piperonyl-butoxide on the enantiomeric genesis of the sulfoxide moiety was also investigated. Following administration of FBZSO and FBZ, the fenbendazole sulphone metabolite predominated in plasma, and the C(max) and area under the plasma curve (AUC) values for each moiety were larger (P < 0.001) following FBZSO than FBZ. In feces the administered parent molecule predominated. The combined AUC for active benzimidazole moieties following oral administration of FBZ (10 mg/kg) in horses was almost 4 times as high in unfed horses (2.19 microg x h/ml) than in fed horses (0.59 microg x h/ml), and coadministration of piperonyl-butoxide significantly increased the AUC and C(max) of active moieties following intravenous administration of FBZSO and oral administration of FBZ. When FBZSO was administered i.v. as a racemate, the first enantiomer of oxfendazole (FBZSO-1) predominated in plasma, however, following coadministration with piperonyl-butoxide, the second enantiomer of oxfendazole (FBZSO-2) predominated for 10 h. Piperonyl-butoxide significantly reduced the oxidative metabolism of FBZSO and FBZ in equine liver microsomes and altered the ratio of enantiomers FBZSO-1/FBZSO-2 from >4:1 to 1:1. It is concluded that in horses efficacy of FBZSO and FBZ could be improved by administration to unfed animals and coadministration with piperonyl-butoxide.

Uptake of albendazole and albendazole sulphoxide by Haemonchus contortus and Fasciola hepatica in sheep

The pattern of in vivo uptake of albendazole (ABZ) and its major metabolite, ABZ-sulphoxide (ABZSO), by Haemonchus contortus and Fasciola hepatica recovered from ABZ-treated sheep, was investigated. Concentration profiles of both compounds were simultaneously measured in target tissues/fluids from the same infected sheep. In addition, the proportion of the (+) and (-) ABZSO enantiomers was determined in plasma, bile and F. hepatica recovered from treated sheep. Sheep naturally infected with H. contortus were intraruminally (i.r.) treated with ABZ (micronized suspension, 7. 5mg/kg) and the plasma concentrations of ABZSO and ABZ-sulphone (ABZSO(2)) determined in addition to the concentration of ABZ and ABZSO in H. contortus, abomasal mucosa and fluid content samples. In addition, F. hepatica artificially infected sheep were treated i.r. with the same ABZ suspension (7.5mg/kg), and samples of blood, bile, liver tissue and adult flukes were collected and analysed by HPLC to determine the concentrations of ABZ and both enantiomers of ABZSO. ABZSO and ABZSO(2) were the analytes recovered in plasma with ABZ and ABZSO present in H. contortus. ABZ was the analyte recovered at the highest concentration in H. contortus and abomasal mucosa, whereas higher concentrations of ABZSO were measured in abomasal fluid content. Only low concentrations of ABZ were detected in F. hepatica and bile, but markedly higher concentrations of ABZ were measured in liver tissue. ABZSO was the main molecule recovered in F. hepatica, plasma and bile samples collected from ABZ-treated sheep. The (+) enantiomer of ABZSO was recovered at a higher proportion in plasma (75%), bile (78%) and F. hepatica (74%) after ABZ administration to infected sheep.

Comparative disposition of ricobendazole enantiomers after intravenous and subcutaneous administration of a racemic formulation to calves

The enantioselective disposition kinetics of the benzimidazole anthelmintic, ricobendazole (RBZ), have been characterized after its intravenous (iv) and subcutaneous (sc) administration as a racemic formulation to cattle. The (+) and (-) RBZ enantiomeric forms were recovered in plasma after iv and sc administration of the racemic RBZ formulation, using a chiral phase based HPLC method. A biexponential plasma concentration versus time curve was observed for both RBZ enantiomers following the iv treatment. Total body clearance was higher for (-) RBZ (150.4 mL/h. kg) compared with that obtained for the (+) RBZ antipode (78.1 mL/h. kg). The elimination half-life of the (-) RBZ enantiomer was shorter (T1/2beta: 2.67 h) compared with the (+) enantiomer (T1/2beta: 5.41 h). The plasma availability (expressed as AUC) was significantly higher for (+) RBZ compared with that obtained for the (-) antipode following both treatments. The enantiomeric ratio in plasma at T(0) was close to unity (50% of each enantiomer); the analysis of the concentration ratios (+) RBZ/(-) RBZ, demonstrated an increase in the proportion of (+) RBZ during the time course of the kinetics after both iv and sc treatments. The results presented herein show the enantioselective disposition kinetics of RBZ in cattle and are a further contribution to the understanding of the kinetic behaviour of these sulphoxide-containing benzimidazole anthelmintics in ruminants.

Enhanced plasma and target tissue availabilities of albendazole and albendazole sulphoxide in fasted calves: evaluation of different fasting intervals

The influence of different pre- and post-treatment fasting periods on the plasma availability and disposition kinetics of albendazole (ABZ) and its sulphoxide metabolite (ABZSO) in cattle was investigated. The effect of fasting on the distribution of ABZ and ABZSO to different target tissues/fluids was also characterised. In Experiment I, 35 parasite-free Holstein calves were divided into seven groups according to the following feeding conditions and treated intraruminally with ABZ (10 mg/kg): control group (fed ad libitum), 24 h fasting either prior to (24 h pre-) or post (24 h post-) treatment, 24 h fasting with either 6 (6 h pre + 18 h post) or 12 h (12 h pre + 12 h post-) of feed restriction prior to treatment, 12 h fasting either prior to (12 h pre-) or post (12 h post) treatment. In Experiment II, calves from the same pool of animals were subjected to a 24 h fasting period prior to the same ABZ treatment and killed (two animals) at either 24, 36 or 48 h post-administration to obtain samples of abomasal/intestinal mucosa and fluid contents, bile and lungs. Plasma (Experiment I) and tissues/fluids (Experiment II) samples were analysed by HPLC. All the fasting periods investigated induced marked changes to the plasma availability and disposition kinetics of the ABZSO metabolite. Enhanced plasma availability between 37 and 118%, delayed peak concentrations and extended mean residence times for ABZSO were observed in fasted compared to fed calves. The changes in plasma kinetics, reflecting an altered quantitative gastrointestinal absorption, were reflected in increased availability of ABZ and ABZSO in the target tissues/fluids of fasted calves. The availabilities of ABZ and ABZSO in the gastrointestinal mucosa and fluids in fasted calves were markedly greater than in those fed ad libitum.

The behaviour of doramectin in the gastrointestinal tract, its secretion in bile and pharmacokinetic disposition in the peripheral circulation after oral and intravenous administration to sheep

Sheep were 'compartmentalized' by surgically implanting cannulae in the rumen, abomasum and terminal ileum with a re-entrant cannula inserted between the cystic duct and the duodenum to monitor bile secretion. Doramectin, containing a trace of [3H]-doramectin, was administered both intravenously (i.v.) and intraruminally (i.r.) at a dosage of 150 microg/kg. The pharmacokinetic behaviour of [3H]-labelled products was determined in these pools, and also in peripheral plasma, urine and faeces. Parent doramectin was also determined in plasma, abomasal digesta fluid and bile. Following i.r. administration, [3H] compounds were almost entirely associated with particulate digesta. A 14.5 h half-life in the rumen prolonged the presence of [3H] in the abomasum. Doramectin appeared to be degraded in abomasal digesta because only 24% of abomasal [3H] was attributed to the parent drug. Absorption of doramectin resulted in a systemic availability of 35%, of which 1.6 and 23.6% of the dose was contained in urine and biliary secretions, respectively. Following i.v. administration, almost negligible quantities of [3H] were secreted into the rumen or abomasum and only 2.7% of the dose was excreted in urine, whereas 132% was secreted in bile. This indicated that approximately one-third of biliary metabolites were enterohepatically recycled with biliary metabolites, elevating the proportion of [3H] in fluid digesta in the small intestine. Passage of the i.r.-administered drug through the gastrointestinal tract (GIT) resulted in virtually complete faecal excretion of [3H] within 5 days, whereas the continued secretion of i.v.-administered [3H] in bile prolonged the presence of [3H] in the GIT, with faecal clearance not being complete for at least 10 days. This multi-compartmental study has provided more information on the behaviour of doramectin than can be obtained from examining drug disposition in the peripheral circulation alone. With this knowledge, it is anticipated that opportunities for improving drug performance will be identified.

Differences in plasma and abomasal kinetics of albendazole and its metabolites in calves grazed on pasture or fed a grain-based diet

We evaluated the comparative plasma and abomasal fluid disposition kinetics of albendazole (ABZ) and its metabolites in calves either grazing on pasture or fed a grain-based concentrate diet. Six male Holstein calves (weight 180 to 200 kg) were allowed to graze on lush pasture for three weeks before intraruminal administration of ABZ at 10 mg kg-1(pasture group). After a three-week wash-out period, the same animals were housed and fed on a grain-based concentrate diet for three weeks prior to receiving the same ABZ treatment (concentrate group). Jugular blood and abomasal fluid samples were collected over 120 hours post-treatment. Plasma and abomasal fluid samples were analysed by high performance liquid chromatography (HPLC). The digesta transit time was measured using cobalt (Co) as a fluid marker; abomasal fluid and faecal samples were collected and Co concentrations measured by atomic absorption spectrophotometry. Complementary studies of the in vitro dissolution of ABZ particles at different pH values were also conducted. The pH of abomasal fluid collected from animals kept under both feeding conditions was registered. Increased concentrations of ABZ sulphoxide (ABZSO) and sulphone (ABZSO2) in plasma, resulting in significantly higher Cmax and area under the curve (AUC) values for both metabolites, were obtained in calves fed on the concentrate diet compared to those grazing on pasture. Enhanced abomasal fluid levels of ABZ and ABZSO were observed in concentrate-fed calves. The mean retention time of the digestive fluid marker in the gastrointestinal (GI) tract was significantly longer in the animals fed the grain-based diet. The in vitro dissolution of ABZ at a pH value equivalent to that obtained in the abomasum of the concentrate-fed calves (1.75) was significantly greater than that obtained at the pH registered in pasture-fed animals (2.00). The characterisation of the kinetic/metabolic behaviours and the resultant efficacy of antiparasitic drugs in animals reared under different management conditions may be relevant in increasing parasite control in livestock.

Influence of diet type and pretreatment fasting on the disposition kinetics of albendazole in sheep

The influence of the quality and quantity of diets on the disposition kinetics of albendazole were studied in sheep in two different experiments. The plasma concentration profiles of albendazole sulphoxide and albendazole sulphone were measured following intraruminal administration of albendazole at 5.0 mg/ kg body weight in weaner sheep offered three different diets: 100% green Sorghum spp., 100% dry mature Cenchrus ciliaris hay and a 50:50 mix of these two diets. The peak plasma concentrations and the availability of the albendazole metabolites, as measured by the area under the concentration time curve, were significantly higher (p < 0.01) in the animals offered exclusively dry fodder compared to other diets. Changing the diet from dry to green fodder resulted in a significantly lower systemic availability of the drug metabolites. It is suggested that a decreased transit time of the digesta in the bowel on the green diet, with its high water content, limited the systemic availability of the drug by reducing the time available for gastrointestinal absorption. An experiment on the influence of different levels of pretreatment fasting on the pharmacokinetics of albendazole revealed significantly higher (p < 0.05) plasma concentrations of the anthelmintically active sulphoxide metabolite from 12 h onwards following administration of the drug in animals subjected to 24 h of pretreatment fasting compared to other groups with pretreatment fasting of 8, 12 or 18 h. The area under the concentration time curve and the minimum residence time of the drug metabolites were significantly greater (p < 0.05) in animals that had been fasted for 24 h. It is suggested that fasting induces a decrease in the flow of digesta through the gastrointestinal tract of ruminants and prolongs the duration of dissolution of the drug, resulting in enhancement of the absorption of albendazole and of the systemic availability of its metabolites.

Bioavailability of albendazole sulphoxide after netobimin administration in sheep: effects of fenbendazole coadministration

After oral co-administration of two dosages of netobimin (7.5 and 20 mg kg-1 with fenbendazole (1.1 mg kg-1) to Merino sheep, the AUC0-infinity of albendazole sulphoxide at the lower dosage of netobimin, was significantly increased (75.5 per cent) from control value (34.43 +/- 7.91 versus 60.33 +/- 11.93 microg h ml-1). The pharmacokinetic parameters MRT and T1/2 were also increased: 18.96 +/- 2.54 vs 26.44 +/- 4.69 h and 10.31 +/- 1.72 vs 22.28 +/- 6.75 h respectively. No data corresponding to the higher dosage of netobimin (20 mg kg-1) were statistically different from control values. It is concluded that fenbendazole increases the bioavailability of albendazole sulphoxide in sheep at the 7.5 mg kg-1 dosage, and this may produce a potentiated anthelmintic action.

Influence of ruminal bypass on the pharmacokinetics and efficacy of benzimidazole anthelmintics in sheep

Oxfendazole, fenbendazole and albendazole were each administered at 5mgkg(-1) to sheep fitted with abomasal cannulae as a single bolus intra-ruminally or infused intra-abomasally at a declining exponential rate, with half-life equivalent to the rate of rumen fluid outflow. The pharmacokinetic disposition of parent compound and metabolites in plasma and abomasal fluid was determined by high performance liquid chromatography. Compared with intra-ruminal administration, intra-abomasal infusion of fenbendazole lowered the area under the concentration-time curve of drug in both plasma and abomasal fluid; intra-abomasal infusion of albendazole substantially increased maximum drug concentration and the concentration-time curve in abomasal fluid and lowered the plasma concentration time curve of the sulphoxide metabolite; intra-abomasal infusion of oxfendazole increased maximum concentration and the concentration-time curve of drug in plasma and abomasal fluid. The greater availability in abomasal fluid of oxfendazole and albendazole when given at commercial dose rates of 5 mg kg(-1) and 3.9 mg kg(-1), respectively, by intra-abomasal infusion correlated with increased efficacy of both drugs against benzimidazole-resistant Trichostrongylus colubriformis and of albendazole against benzimidazole-resistant Haemonchus contortus over that achieved by intra-ruminal administration as a single bolus.

Comparative disposition kinetics of albendazole in sheep following oral and intraruminal administration

The pharmacokinetics of albendazole was studied in sheep following single oral and intraruminal administration at nematocidal dose rates. The disposition curves of its metabolites indicated increased uptake of the drug in sheep following intraruminal as compared to oral dosing (p < 0.05). The increased bioavailability of benzimidazole anthelmintics given by the intraruminal route could be exploited for optimizing the use of anthelmintic for sustained parasite control in small ruminants.

Ricobendazole kinetics and availability following subcutaneous administration of a novel injectable formulation to calves

The plasma and abomasal fluid disposition kinetics of ricobendazole (RBZ) after subcutaneous (s.c.) administration of a novel injectable formulation to calves, and the comparative plasma availability after s.c. injection of RBZ and that obtained after oral treatment with albendazole (ABZ), were characterised. Six parasite-free Holstein calves received RBZ (solution 150 mg ml(-1)) by s.c. injection at 3.75 mg kg(-1) (Experiment 1). Experiment 2 was conducted in two experimental phases; in phase I, five calves (Group A) received RBZ by s.c. injection and five animals (Group B) were orally treated with ABZ (suspension 100 mg ml(-1)), at 5 mg kg(-1). Drug treatments were reversed for each group in phase II and given at 7.5 mg kg(-1). Samples of abomasal fluid (via cannula) and jugular blood were collected over 72 hours post-treatment and analysed by HPLC. RBZ and its sulphone metabolite were detected in plasma following its s.c. administration. RBZ was rapidly absorbed, reaching the plasma Cmax at 4.5 hours post-dosing. The sulphone metabolite followed a similar kinetic pattern. Both molecules were rapidly and extensively distributed into the abomasum, being detected in abomasal fluid between 30 minutes and 36 hours post-administration. An extensive plasma/abomasum exchange process, with ionic-trapping in the abomasum, accounted for the higher AUC value (>200 per cent) obtained for RBZ in abomasum compared with plasma. The s.c. treatment with RBZ formulated as a solution resulted in a significantly greater plasma availability (measured as ABZ sulphoxide) than the oral treatment with ABZ (suspension) given at the same dose rates.

The disposition kinetics of albendazole following the administration of single and divided doses to cattle and buffalo

Concentrations of albendazole sulphoxide and its sulphone metabolite in plasma in cattle and buffalo were measured by high-performance liquid chromatography after single and divided intraruminal administration of albendazole at the recommended nematocidal and fasciolicidal dose rates of 7.5 and 15.0 mg/kg body weight, respectively. No significant differences in the plasma concentrations of the metabolites or their pharmacokinetic parameters were observed between cattle or buffalo at either dose rate. Pharmacokinetic analysis and the disposition curve of the metabolites indicated increased uptake of the drug in both cattle and buffalo when the same total amount of the drug was given in divided doses compared to a single dose (p < 0.05). The divided dose schedules of administration could possibly be exploited to extend the life of the available benzimidazole anthelmintics.

Efficacy and pharmacokinetics of triclabendazole in buffalo with induced fasciolosis

A study was conducted to understand pharmacokinetics and flukicidal activity of intraruminal administration of triclabendazole (TCBZ) at 12.0, 24.0 and 36.0 mg kg-1 body weight in experimentally Fasciola gigantica-infected buffaloes on Week 2 and 10 post-infection. No fluke eggs in faeces and no flukes could be recovered from the liver of buffaloes following intraruminal administration of triclabendazole at 24.0 and 36.0 mg kg-1 body weight both on Weeks 2 and 10 post-infection, while the recommended therapeutic dose at 12.0 mg kg-1 body weight was 19-23% effective. Pharmacokinetic analysis of the data revealed a significantly higher (P < 0.05) concentration maximum of both the metabolites and area under concentration-time curve of TCBZ-SO2 in animals treated at 12.0 mg kg-1 body weight on Week 10 post-infection, whereas a significantly higher area under the concentration-curve and elimination half-life of both the metabolites and significantly higher concentration maximum and area under the concentration-time curve of both the metabolites were observed in animals treated on Week 10 post-infection at the dose rates of 24.0 and 36.0 mg kg-1 body weight, respectively. Bioavailability of triclabendazole was more in buffaloes with mature flukes than with immature flukes.

The efficacy and pharmacokinetics of long-term low-level intraruminal administration of triclabendazole in buffalo with induced fasciolosis

A study was conducted on the pharmacokinetics and therapeutic efficacy of triclabendazole at three low dose rates of 0.5, 1.0 and 1.5 mg/kg body weight in buffaloes experimentally infected with Fasciola gigantica. The pharmacokinetics were compared with the effects of a single intraruminal dose at 24.0 mg/kg body weight in uninfected buffaloes. At all three dose rates, an equilibrium between the absorption of triclabendazole and the disposition of its metabolites was observed by days 3 and 4 and remained almost unchanged thereafter. Continuous daily dosing at 1.5 mg/kg body weight proved to be efficacious against liver fluke infection in buffaloes.

Kinetic disposition of triclabendazole in buffalo compared to cattle

Concentrations of triclabendazole sulfoxide and its sulfone metabolite in plasma were measured in buffalo and cross-bred cattle after single intraruminal administration of triclabendazole at two different doses. Plasma concentrations of both metabolites were significantly lower in buffalo than cattle at both doses, which resulted in a smaller area under the concentration-time curve, a lower concentration maximum and a lower relative bioavailability. Thus, the recommended doses of 12 mg/kg body weight for the treatment of bubaline fascioliasis may not be valid for buffalo because of the substantially lower uptake of the drug in this species.

Influence of diet type on the kinetic disposition of fenbendazole in cattle and buffalo

The plasma concentration profiles of fenbendazole (FBZ), FBZ-sulphoxide and FBZ-sulphone were measured following intraruminal administration of FBZ at 7.5 mg kg-1 body weight in cattle and buffalo offered 3 different diets: 100% dry mature sorghum hay, 100% green Pennisetum spp. and a 50:50 mix of these 2 diets. Changing the diet from dry to green fodder resulted in significantly lower systemic availability of FBZ and its metabolites in both species. Buffalo had a lower systemic availability of the drug than cattle on the dry diet and the difference between species increased when the diet included green fodder. It is suggested that decreased transit time of digesta on the green fodder reduced systemic concentrations by reducing the time available for gastrointestinal absorption of the drug.

The pharmacokinetics of albendazole metabolites following administration of albendazole, albendazole sulfoxide and netobimin to one-month- and eight-month-old sheep

The principal metabolites detected in plasma of sheep following oral administration of albendazole (ABZ), albendazole sulfoxide (ABSO) and netobimin (NTB) each at 5.0 mg kg-1 body weight were ABSO and albendazole sulfone (ABSO2). The areas under the plasma concentration-time curve (AUC) for ABSO and ABSO2 were significantly (P < 0.05) larger following administration for ABSO than NTB in 1-month- and 8-month-old sheep. The AUC for the ABSO and ABSO2 metabolites were larger following administration of ABZ than NTB in 1-month- but not 8-month-old sheep and the AUC of the ABSO and ABSO2 metabolites were greater following ABSO than ABZ as parent compound in 8-month-old sheep only. The larger AUC values for metabolites following administration of ABSO as the parent compound were generally coincident with significantly higher maximum (Cmax) concentrations and not with persistence in the body, since mean residence times (MRT) of the metabolites were not significantly different from those determined following ABZ and NTB as parent compounds. The lower metabolite concentration following administration of NTB may have been a feature of its requirement for metabolic conversion and its larger molecular weight. Correction of AUC values for molecular weight removed any significant differences between AUC values for either metabolite in 8-month-old lambs. The corrected metabolite AUCs following NTB were, however, significantly lower than those following ABSO administration in 1-month-old lambs, suggesting that immature metabolic processes in these animals contributed to the lower relative bioavailability of NTB in this age group. Age did not affect the disposition of metabolites following ABZ or ABSO but the AUC of the ABSO metabolite following NTB was significantly (P = 0.014) lower in 1-month- than in 8-month-old sheep.

A comparison of plasma metabolite levels in goats and sheep during continuous low-level administration of fenbendazole

Plasma levels of fenbendazole (FBZ) and its sulphoxide (OFZ) and sulphone (FBZ.SO2) metabolites were measured in goats and sheep during low-level administration of FBZ given by intraruminal infusion or formulated into a urea-molasses feed supplement block (UMB). In experiment 1, 6 goats and 6 sheep were offered UMB containing 0.5 g FBZ/kg (MUMB) and individual block consumption was measured daily for 18 days. In experiment 2, some of the same animals (n = 4 for each species) received FBZ by intraruminal infusion at 1, 1.5 and 3 mg/kg liveweight per day for 7 days at each dosage. FBZ, OFZ and FBZ.SO2 levels were determined in plasma collected every 3 days in experiment 1 and on days 4, 5 and 6 of each infusion period in experiment 2. In both experiments, higher equilibrium levels were observed for the three metabolites in sheep than in goats. Significant linear relationships were observed between the daily FBZ dosages and the plasma levels of the three metabolites in both species. The regression coefficients were significantly higher in sheep than in goats for FBZ and OFZ but not for FBZ.SO2, and they were also significantly higher during MUMB administration than during infusion for all three metabolites in both species. FBZ is a suitable anthelmintic for incorporation into a MUMB formulation for use in livestock production systems where responses to molasses urea supplementation have been demonstrated and gastrointestinal parasitism impairs productivity. The results indicate that target dose rates for goats should be 0.75 mg/kg per day compared with 0.5 mg/kg per day for sheep.

Influence of subclinical nematodosis on the kinetic disposition of fenbendazole in buffaloes

The disposition kinetics of fenbendazole was studied in buffaloes subclinically infected with gastrointestinal nematodes. There was significantly reduced uptake of the drug in infected animals compared to uninfected controls. The pH of the duodenal liquor was highly alkaline compared to the acidic pH in uninfected animals. The egg count in the faeces never became zero though the numbers were reduced to a great extent compared to pre-treatment values. The influence of the host's physiology on the reduced bioavailability of fenbendazole is discussed.

The effect of level of feed intake on the pharmacokinetic disposition of oxfendazole in sheep

Oxfendazole (OFZ) containing a trace of [14C]-OFZ was administered intraruminally and intravenously to sheep fitted with rumen and abomasal cannulae and which were being fed 800 and 400 g of 50:50 lucerne:wheaten chaff daily. The [14C] was extensively associated with rumen particulate digesta, the shorter residence time of digesta in sheep on high compared to low feed intake reduced the duration of OFZ absorption. Abomasal fluid flow was greater in sheep on high than low intake and was attributed to increased gastric secretions. At high intake a greater proportion of the [14C] dose flowed from the abomasum in digesta fluid, but its residence time in the abomasum was of shorter duration compared with low intake. The more rapid passage of digesta through the gastrointestinal tract in the former sheep reduced the duration for drug desorption from particulate material and absorption into the bloodstream. In these high intake sheep and availability of [14C]-compounds in plasma was lower and more rapidly cleared than in sheep on low feed intake. Concomitant with the reduced absorption a greater proportion of the dose was excreted in faeces, and a lower proportion in the urine of sheep on high compared with low feed intake. The extensive association of OFZ and its metabolites with rumen digesta, is a principal determinant of OFZ kinetics.

Chemotherapy and delivery systems--helminths

The milbemycins are the only novel broad spectrum anthelmintic chemicals to reach the market place in the last 10 years. Many new systems for delivery and strategies for rational use have, however, been introduced. Boluses which are retained by virtue of specific gravity and by variable geometry are now available. They contain benzimidazoles, morantel, ivermectin and levamisole. Their release mechanisms involve preferential corrosion of a retaining metal core, constant diffusion from a laminated ethylene acetate sandwich, and a hydrostatic pump driven by osmotic pressure. Some are biodegradable. Experimental delivery systems have been developed incorporating ear implants and liposomes. The anthelmintic efficacy of some drugs has been potentiated by the synergistic action of metabolic inhibitors and these combinations hold promise for the future. Much new information is now available on those factors which affect anthelmintic efficacy such as concurrent administration with food and the presence of the target parasites themselves. This knowledge provides a sound basis for the rational use of anthelmintic drugs.

Comparative pharmacokinetics of fenbendazole in buffalo and cattle

Swamp buffalo (Bubalus bubalis) and Droughtmaster cattle (Bos indicus x B. taurus), fitted with gastrointestinal cannulae, were dosed intraruminally with fenbendazole at 7.5 mg/kg liveweight, together with a chromium oxide capsule and a pulse dose of NaCoEDTA, to estimate the flow dynamics of the digesta in the rumen and duodenum. The concentrations of fenbendazole (FBZ) metabolites were measured in plasma and duodenal fluid collected over 120 h. In plasma, significantly lower peak concentrations and earlier disappearance of FBZ and its sulphoxide (OFZ) metabolite were observed in buffalo, which considerably reduced systemic availability in comparison with cattle. The availability of OFZ in the duodenal fluid of buffalo was significantly lower, whereas FBZ disposition was similar to that in cattle. The turnover rate of fluid in the rumen was higher in buffalo than in cattle, while the flow parameters for other digesta were similar in the two species. It is concluded that the decreased absorption of drug in buffalo was attributable to the shorter residence time of the dose in the rumen, and probably in the entire gastrointestinal tract. This may reduce the efficacy of treatment and indicate the need for higher dose rates for benzimidazole anthelmintics in buffalo than in cattle.

The partition and fate of soluble and digesta particulate associated oxfendazole and its metabolites in the gastrointestinal tract of sheep

The disposition of oxfendazole (OFZ) containing a trace of [14C]OFZ was examined in the gastrointestinal tract and bloodstream of sheep fitted with rumen and abomasal cannulae. Within 2 h of intraruminal (IR) administration, OFZ and its metabolites were almost completely associated with rumen particulate digesta. The proportion of metabolites in digesta fluid increased with their passage from the rumen into the abomasum. To determine the fate of 14C-labelled metabolites after distribution throughout rumen digesta, the rumen particulate and fluid digesta phases from a donor sheep were separated and each transferred to the rumen of an untreated recipient sheep. The 14C-labelled metabolites which derived from the donor rumen fluid quickly associated with recipient rumen particulate material. The metabolites were then progressively desorbed, as were metabolites which were transferred already associated with rumen particulate digesta. Desorption occurred faster in the abomasum than in the rumen. There was no difference in uptake kinetics between administration routes, indicating rapid equilibrium. Consequently the disposition of [14C] OFZ and its metabolites in the bloodstream was similar in each group. It is suggested that the progressive desorption of particulate associated metabolites is a principal determinant of the duration of OFZ availability.

Pharmacokinetic behaviour of fenbendazole in buffalo and cattle

Concentrations of fenbendazole and of drug metabolites in plasma were measured in buffalo and cross-bred cattle after single intraruminal administration at two different doses. Plasma concentrations of the parent compound fenbendazole and the two metabolites, viz. oxfendazole and fenbendazole sulfone, were much lower in buffalo compared with cattle, at a dose of 7.5 mg/kg body weight as indicated by lower area under concentration curve and concentration maximum. At a dose of 15 mg/kg body weight there were corresponding increases in plasma metabolite concentrations in cattle. However, buffaloes did not show a similar corresponding increase.

Plasma levels of fenbendazole metabolites in buffalo and cattle after long-term intraruminal administration

Plasma metabolite levels were measured in buffalo and cross-bred cattle after daily intraruminal administration of fenbendazole at 0.5 mg/kg body weight. An equilibrium between the absorption of fenbendazole and disposition of its metabolites could be observed between days 3 to 6 and remained almost unchanged thereafter. The buffaloes had a lower uptake of the anthelmintic and lower plasma levels of its metabolites than in cattle. The findings were compared with those obtained after a single intraruminal therapeutic dose of fenbendazole.

Comparative kinetic disposition of oxfendazole in sheep and goats before and during infection with Haemonchus contortus and Trichostrongylus colubriformis

The kinetic disposition of [14C]-oxfendazole (OFZ) and its metabolites, fenbendazole (FBZ) and fenbendazole sulphone (FBZ.SO2), in plasma and abomasal fluid were determined in Merino sheep and Angora goats before and during infection with Trichostrongylus colubriformis and Haemonchus contortus. The systemic availability (area under the plasma curve, AUC) of OFZ was significantly lower in goats (13.5 micrograms.h/ml) than in sheep (22.2 micrograms.h/ml) and was reduced with infection in goats (5.6 micrograms.h/ml) and sheep (15.1 micrograms.h/ml). The elimination of plasma [14C] was faster in goats than in sheep. The responses observed for [14C] were a reflection of the behaviour of OFZ. The concentration of OFZ and metabolites in abomasal fluid were similar in both species in the absence or presence of infection. However, as the mean flow rate of abomasal fluid was slower in goats (240 ml/h) than in sheep (488 ml/h), only 7% of the dose passed the pylorus in abomasal fluid of goats compared with 14% in sheep. The presence of gastrointestinal nematodes generally increased abomasal fluid flow rate but neither species nor infection had any effect on the rate or extent of [14C] excretion in urine or faeces. It is suggested that goats possess a faster hepatic metabolism than sheep resulting in more rapid elimination of OFZ.

The uptake of fenbendazole by cattle and buffalo following long-term low-level administration in urea-molasses blocks: further studies on block formulations

Fenbendazole (Hoechst India Ltd.) was incorporated at 0.5 g/kg into urea molasses blocks made by two different processes. The proportion of the drug remaining in the blocks and the plasma concentrations of the parent compound and its metabolites were measured. Recovery of the drug in blocks made by the cold and the modified hot processes was 90% and 96%, respectively. The plasma metabolite profile revealed a plateau between days 4 and 6 of feeding in cattle and buffalo. However, the plasma concentrations of fenbendazole and its metabolites were low in buffalo compared to cattle.

The effect of feed intake on the rate of flow of digesta and the disposition and activity of oxfendazole in sheep

Within 3 h of intra-ruminal (IR) administration oxfendazole (OFZ) was extensively associated with digesta particulate material. This association appeared to be a passive adsorption/desorption since sheep given dose equivalent OFZ which was contained solely in rumen particulate digesta (obtained from OFZ treated donor sheep) exhibited similar OFZ disposition to the IR treated animals. The association of OFZ with rumen particulate digesta is a major determinant of the pharmacokinetic behaviour of OFZ. The kinetic disposition of OFZ was compared in sheep fed 400 g and 800 g day-1 of 50:50 lucerne/wheaten chaff. The flow rate of rumen and abomasal fluid and particulate digesta increased with feed intake whilst the availability of OFZ and its metabolites in the digestive tract and bloodstream decreased. Compared to untreated sheep fed 800 g day-1, OFZ did not affect benzimidazole (BZ) resistant Trichostrongylus colubriformis, but reduced the numbers of BZ-resistant Haemonchus contortus by 61%. When sheep were fed 400 g day-1 36 h before and after OFZ administration, 60% of T. colubriformis and 94% of H. contortus were removed. Elevated anthelmintic efficacy in sheep can therefore be obtained when gastric flow has been slowed by temporarily reducing feed intake.

Biliary secretion and enterohepatic recycling of fenbendazole metabolites in sheep

Fenbendazole (FBZ) was administered intraruminally at 5.0 mg/kg, containing a trace of [14C]-FBZ, to sheep fitted with a permanent bile duct cannula and the behaviour of FBZ and its metabolites examined in bile and plasma. Of the administered radiolabelled dose, 47% was secreted in bile of which 34% was accounted for as conjugated and 4% as unconjugated (free) metabolites. Hydroxylated oxfendazole (OH.OFZ) was the major biliary metabolite contributing 66%, and hydroxy-FBZ (OH.FBZ) 27%, of the total metabolites characterized. Small amounts of OFZ and hydroxy FBZ sulphone (OH.FBZ.SO2) were also present in bile. The rapid appearance of OH.OFZ in bile, even before maximum concentrations of OFZ occurred in plasma, indicated that sulphoxidation and hydroxylation was the major route of FBZ metabolism. Following intraduodenal infusion of free biliary metabolites, FBZ and its metabolites rapidly appeared in bile indicating absorption from the small intestine. When conjugated metabolites were infused they continued to appear in bile for a further 15-20 h after cessation of infusion indicating that absorption of hydroxylated metabolites occurred largely after bacterial deconjugation in the large intestine. Approximately 40% of biliary metabolites were estimated to undergo enterohepatic reabsorption but they contributed minimally to the metabolite content in plasma. It is suggested that during the process of recycling-biliary metabolites make substantial contact with parasites in the mucosa of the small and large intestine thereby contributing to the anti-helminthic activity of FBZ.

Comparative sulphoxidation of albendazole by sheep and cattle liver microsomes and the inhibitory effect of methimazole

1. The comparative rates of oxidation of the benzimidazole anthelmintic, albendazole (ABZ), by sheep and cattle liver microsomes, and inhibition by the antithyroid compound methimazole (MTZ) were investigated. 2. ABZ was oxidized to its sulphoxide metabolite (ABZSO) in an NADPH concentration-dependent reaction. Heat inactivation of the microsomal flavin-containing mono-oxygenase system significantly decreased the NADPH consumption of microsomes in the presence of ABZ, MTZ and thiourea. 3. Oxidation of ABZ, MTZ and thiourea by sheep liver microsomes consumed significantly more NADPH than oxidation by cattle microsomes. 4. Neither the pro-ABZ drug, netobimin, nor the ABZ sulphone metabolite (ABZSO2) was modified by incubation with either sheep or cattle liver microsomes. 5. ABZSO was oxidized into ABZSO2 at a very slow rate and only when a high microsomal protein concentration was used. 6. MTZ was a potent inhibitor of ABZ sulphoxidation and the inhibition was significantly lower in cattle than in sheep microsomes.

The uptake of fenbendazole by cattle and buffalo following long-term low-level administration in urea-molasses blocks

Fenbendazole (Panacur bolus, Hoechst India Ltd) was incorporated at a rate of 0.5 g/kg into urea-molasses blocks made by two different processes. The concentration of the drug in blocks and its bioavailability were measured using plasma oxfendazole as marker. The recovery of the drug in blocks made by a warm process was 68% and the plasma oxfendazole concentration remained fairly stable at 0.2 and 0.12 microgram/ml from day 6 of feeding in cattle and buffalo, respectively. The drug seemed to be inactivated in blocks made by a hot process, with reduced bioavailability. A low and sustained plasma concentration of the active metabolite of the drug could be maintained by self-medication using urea-molasses blocks as fenbendazole carrier.

The effect of an oil formulation on the systemic availability of oxfendazole

Pure oxfendazole (OFZ) suspended in peanut oil and the commercial formulation 'Synanthic' were each intraruminally administered to Merino weaners at 5 mg kg-1. Plasma was collected and concentrations of OFZ, fenbendazole (FBZ) and FBZ sulphone (FBZ.SO2) were determined by HPLC. The maximum concentrations and area under the plasma concentration curve (AUC) of OFZ and FBZ were significantly higher (P less than 0.05) when OFZ was suspended in peanut oil than when administered as the commercial formulation.

Effect of feed type on the pharmacokinetic disposition of oxfendazole in sheep

The plasma concentration profiles of oxfendazole (OFZ), fenbendazole (FBZ) and FBZ sulphone (FBZ.SO2) were measured followed intraruminal administration of OFZ at 5 mg kg-1 to Merino weaners fed either dry forage or grazed on pasture lucerne clover. Plasma concentrations of OFZ and FBZ were significantly lower in sheep given the dry forage.

The effect of co-administration of parbendazole on the disposition of oxfendazole in sheep

The effect of intraruminal administration of parbendazole (PBZ) on the flow rate of bile and the pharmacokinetic behaviour of oxfendazole (OFZ) was examined in sheep. PBZ given at 18, 9 and 4.5 mg/kg resulted in a dose-related reduction in bile flow rate which was also inversely related to changing concentration of PBZ and its metabolites in plasma. Co-administration of 4.5 mg PBZ/kg with 5.0 mg [14C]-OFZ/kg resulted in increased concentrations of fenbendazole (FBZ), OFZ and fenbendazole sulphone (FBZ-SO2) in plasma, although total 14C levels remained unchanged compared with that observed when OFZ alone was administered. The presence of PBZ also reduced biliary secretion of 14C by 22% and altered the relative proportions of OFZ metabolites in bile during the 72-h experimental period. The ratio of 4'-hydroxy-OFZ (OH-OFZ) to 4'-hydroxy-FBZ (OH-FBZ) changed from 7:1 in the absence of PBZ to approximately 1:1 in the presence of PBZ. There was no change in urinary or faecal 14C excretion. The PBZ-induced effects were temporary since the pharmacokinetic behaviour of OFZ given alone two weeks before was similar to that given two weeks after PBZ co-administration. It is suggested that the presence of PBZ temporarily slowed hepatic metabolism and biliary secretion of OFZ metabolites but concomitantly increased extra-biliary transfer of OFZ and/or its metabolites from plasma into the gastrointestinal tract. Elevated exposure of parasites in the gut wall to plasma-derived drug, coupled with higher concentrations of anthelmintically active OH-FBZ secreted in bile, could contribute to the previously reported increased efficacy of OFZ when co-administered with PBZ.