Development and validation of a simple, fast, and sensitive LC/MS/MS method for the quantification of oxfendazole in human plasma and its application to clinical pharmacokinetic study

The most popular standard treatments for soil transmitted helminths in humans including mebendazole, albendazole, levamisole, and pyrantel pamoate, show greatly variable efficacy against different species of parasites and have unfavorable pharmacokinetic characteristics, such as short half-life. The transition of oxfendazole, a potent broad-spectrum anthelmintic with long half-life, from veterinary medicine to human use has been considered as a promising approach. However, analytical methods for the quantitative detection of oxfendazole in human matrix are very limited and lack sensitivity. In this study, we have developed a high-performance liquid chromatography-tandem mass spectrometry (LC/MS/MS) method for the quantification of oxfendazole in human plasma using albendazole as an internal standard. The established method was fully validated with lower limit of quantitation (LLOQ) of 0.5 ng/mL and linearity in the range of 0.5-1000 ng/mL; intra-day and inter-day accuracies ranged from 2.6 to 9.5% for 3 quality control levels (1.5 ng/mL, 75 ng/mL, and 750 ng/mL) and LLOQ; intra-day and inter-day precision was ≤13.6% for quality controls and ≤15.1% for LLOQ; matrix factor and extraction recovery were consistent with coefficient of variation of less than 15.0%. Other parameters including matrix selectivity, injection carryover, reinjection reproducibility, hemolysis effect, interference of analyte with internal standard, dilution integrity, freeze/thaw stability, whole blood stability, and stock solution stability were also validated and met the acceptance criteria. The assay was successfully applied to quantify oxfendazole plasma concentration in healthy adult volunteers after the administration of multiple oral doses of oxfendazole.

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.

Single-Ascending-Dose Pharmacokinetic Study of Tribendimidine in Opisthorchis viverrini-Infected Patients

Praziquantel is the only drug available for the treatment of Opisthorchis viverrini infections. Tribendimidine has emerged as a potential treatment alternative; however, its pharmacokinetic (PK) properties have not been sufficiently studied to date. Via two phase IIa dose-finding studies, 68 O. viverrini patients were treated with 25- to 600-mg doses of tribendimidine using 50- and 200-mg tablet formulations. Plasma, blood, and dried blood spots (DBS) were sampled at selected time points. The two main metabolites of tribendimidine, active deacetylated amidantel (dADT) and acetylated dADT (adADT), were analyzed in plasma, blood, and DBS. PK parameters were estimated by noncompartmental analysis. An acceptable agreement among plasma and DBS concentrations was observed, with a mean bias of ≤10%, and 60% dADT and 74% adADT concentrations being within ±20% margins. We found that 200-mg tribendimidine tablets possess immediate floating characteristics, which led to variable time to maximal concentration of drug (Tmax) values (2 to 24 h) between individuals. Dose proportionality was observed for dADT from 25 to 200 mg using 50-mg tablets, but at higher dosages (200 to 600 mg), saturation occurred. The median ratio of the area under the plasma concentration-time curve from 0 to 24 h (AUC0-24) of dADT to the AUC0- 24 of adADT ranged from 0.8 to 26.4, suggesting substantial differences in acetylation rates. Cure rates ranged from 11% (25-mg dose) to 100% (400-mg dose). Cured patients showed significantly higher dADT maximal serum concentrations (Cmax) and AUC0-24 values than uncured patients. Tribendimidine is a promising drug for the treatment of opisthorchiasis. However, the tablet formulation should be optimized to achieve consistent absorption among patients. Further studies are warranted to assess the large differences between individuals in the rate of metabolic turnover of dADT to adADT. (This study has been registered with the ISRCTN Registry under no. ISRCTN96948551.).

Pharmacokinetic Study of Praziquantel Enantiomers and Its Main Metabolite R-trans-4-OH-PZQ in Plasma, Blood and Dried Blood Spots in Opisthorchis viverrini-Infected Patients

BACKGROUND

Praziquantel (PZQ) is the treatment of choice for infections with the liver fluke Opisthorchis viverrini, a major health problem in Southeast Asia. However, pharmacokinetic (PK) studies investigating the disposition of PZQ enantiomers (R- and S-PZQ) and its main metabolite, R-trans-4-OH-PZQ, in diseased patients are lacking. The implementation of a dried blood spot (DBS) sampling technique would ease the performance of PK studies in remote areas without clinical facilities. The aim of the present study is to provide data on the disposition of PZQ enantiomers and R-trans-4-OH-PZQ in opisthorchiasis patients and to validate the use of DBS compared to plasma and blood sampling.

METHODOLOGY/PRINCIPAL FINDINGS

PZQ was administered to nine O. viverrini-infected patients at 3 oral doses of 25 mg/kg in 4 h intervals. Plasma, blood and DBS were simultaneously collected at selected time points from 0 to 24 h post-treatment. PK parameters were determined using non-compartmental analysis. Drug concentrations and areas under the curve (AUC0-24h) measured in the 3 matrices were compared using Bland-Altman analysis. We observed plasma AUC0-24hs of 1.1, 9.0 and 188.7 μg/ml*h and half-lives of 1.1, 3.3 and 6.4 h for R-PZQ, S-PZQ and R-trans-4-OH, respectively. Maximal plasma concentrations (Cmax) of 0.2, 0.9 and 13.9 μg/ml for R-PZQ, S-PQZ and R-trans-4-OH peaked at 7 h for PZQ enantiomers and at 8.7 h for the metabolite. Individual drug concentration measurements and patient AUC0-24hs displayed ratios of blood or DBS versus plasma between 79-94% for R- and S-PZQ, and between 108-122% for R-trans-4-OH.

CONCLUSIONS/SIGNIFICANCE

Pharmacodynamic (PD) in vitro studies on PZQ enantiomers and R-trans-4-OH-PZQ are necessary to be able to correlate PK parameters with efficacy. DBS appears to be a valid alternative to conventional venous sampling for PK studies in PZQ-treated patients.

Pharmacokinetics and residue depletion of praziquantel in rice field eels Monopterus albus

We investigated the pharmacokinetic characteristics of praziquantel (PZQ) in rice field eels Monopterus albus. Pharmacokinetic parameters were determined following a single intravenous administration (5 mg kg(-1) body weight [bw]) and a single oral administration (10 mg kg(-1) bw) at 22.0 ± 0.7°C. We also evaluated residue depletion in tissues following daily administration of PZQ (10 mg kg(-1) bw) that was given orally for 3 consecutive days at 22.0 ± 0.7°C. Following intravenous treatment, the plasma concentration-time curve was best described by a 3-compartment open model, with distribution half-life (t(1/2α)), elimination half-life (t(1/2β)), and area under the concentration-time curve (AUC) of 0.54 h, 17.10 h, and 14505.12 h µg l(-1), respectively. After oral administration, the plasma concentration-time curve was best described by a 1-compartment open model with first-order absorption, with absorption half-life (t(1/2Ka)), elimination half-life (t(1/2Ke)), peak concentration (C(max)), time-to-peak concentration (T(max)), and AUC estimated to be 2.28 h, 6.66 h, 361.29 µg l(-1), 5.36 h, and 6065.46 h µg l(-1), respectively. The oral bioavailability (F) was 20.9%. With respect to residue depletion of PZQ, the t(1/2β) values of muscle, skin, liver, and kidney were 20.2, 28.4, 14.9, and 54.1 h, respectively. Our results indicated rapid absorption, rapid elimination, and low bioavailability of PZQ in rice field eels at the tested dosing conditions.

Selection for anthelmintic resistant Teladorsagia circumcincta in pre-weaned lambs by treating their dams with long-acting moxidectin injection

Administration of long-acting anthelmintics to pregnant ewes prior to lambing is a common practice in New Zealand. Today, most of these products contain macrocyclic lactone (ML) actives, which because of their lipophilic nature, are detectable in the milk of treated animals and in the plasma of their suckling offspring. This study was conducted to confirm the transfer of ML actives to lambs in the ewe's milk, and to assess whether this could result in selection for ML resistant nematodes in the lamb. Ninety, twin bearing Romney ewes were treated before lambing with a long-acting injectable formulation of moxidectin, a 100-day controlled release capsule (CRC) containing abamectin and albendazole, or remained untreated. After lambing, seven ewes from each treatment group were selected for uniformity of lambing date and, along with their twin lambs, relocated indoors. At intervals, all ewes and lambs were bled, and samples of ewe's milk were collected, for determination of drug concentrations. Commencing 4 weeks after birth all lambs were dosed weekly with 250 infective larvae (L3) of either an ML-susceptible or –resistant isolate of Teladorsagia circumcinta. At 12 weeks of age all lambs were slaughtered and their abomasa recovered for worm counts. Moxidectin was detected in the plasma of moxidectin-treated ewes until about 50 days after treatment and in their lambs until about day 60. Abamectin was detected in the plasma of CRC-treated ewes until the last sample on day 80 and in the plasma of their lambs until about day 60. Both actives were detectable in milk of treated ewes until day 80 after treatment. Establishment of resistant L3 was not different between the treatment groups but treatment of ewes with moxidectin reduced establishment of susceptible L3 by 70%, confirming the potential of drug transfer in milk to screen for ML-resistance in the suckling lamb.

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Long-acting anthelmintics were administered to pregnant ewes.

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Moxidectin and abamectin were detected in ewe's milk for >60 days.

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Moxidectin and abamectin were detected in plasma of lambs for >60 days.

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Abamectin treatment had not effect on establishment of larvae in the lambs.

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Moxidectin treatment reduced establishment of susceptible, but not resistant larvae.

Dexamethasone treatment interferes with the pharmacokinetics of ivermectin in young cattle

An experiment was carried out to study the possible interaction between dexamethasone (DXM) treatment and the efficacy of ivermectin (IVM) treatment in young cattle. Two groups, each of seven calves, were experimentally inoculated with an equal mixture containing 15,000 third stage larvae of Cooperia oncophora and Ostertagia ostertagi each, and with no history of being resistant to any anthelmintics. However, in this study C. oncophora was unexpectedly classified as IVM-resistant according to the outcome from the faecal egg count reduction test (FECRT). Blood parameters and faecal egg counts (FEC) were monitored from 0 to 35 days post infection (d.p.i.). The calves in one group received intramuscular injections of short and long-term acting DXM at 22 and 24 d.p.i., respectively. The other group remained as a control. Three days post patency (24 d.p.i.) both groups were injected subcutaneously with IVM (Merial) at the recommended dose (0.2mg/kg). A significant difference (p<0.001) in FEC patterns was observed between groups. Although both groups still excreted eggs (100-200 eggs per gram faeces) 11 days post anthelmintic treatment, the control group had a significantly higher reduction between 23 and 35 d.p.i. (p=0.025). After 35 days, four animals per group were euthanized, and worms in the gastrointestinal tract were counted. No O. ostertagi were found in the abomasums, but low to high numbers (800-6200) of C. oncophora remained in the small intestines in both groups. Overall, these findings indicated that there was an interaction between the efficacy of IVM and DXM treatment. As significantly lower plasma levels of IVM were observed in the DXM group, we conclude that the impaired efficacy of ivermectin was most likely due to the altered pharmacokinetics.

Pharmacokinetics and tissue residues of hydrochloric acid albendazole sulfoxide and its metabolites in crucian carp (Carassius auratus) after oral administration

The pharmacokinetics and residues elimination of hydrochloric acid albendazole sulfoxide (ABZSO) and its metabolites were studied in healthy crucian carp (Carassius auratus, 250 ± 30 g) kept at water temperatures of 10 °C and 25 °C. The concentrations of ABZSO and its metabolites concentration in plasma and tissues were determined using high-performance liquid chromatography (HPLC) using an ultraviolet detector. The results revealed that the plasma concentration of ABZSO in plasma was significantly higher than that of albendazole sulfone (ABZSO(2)), whereas albendazole-2-aminosulfone (ABZ-SO(2)NH(2)) was not detected. The plasma concentrations of ABZSO and its main metabolite ABZSO(2) concentration-time data were fitted using a single-compartment model at 10 °C and 25 °C. The absorption half-life (t₁/₂ka) of ABZSO was 3.86 h at 10 °C and 1.29 h at 25 °C, whereas the elimination half-life (t₁/₂ke) was 16.34 h at 10 °C and 6.72 h at 25 °C; the maximum plasma concentration (C(max)) and the time-point of maximum plasma concentration (T(p)) were calculated as 3.20 μg mL(-1) and 10.58 h at 10 °C, 4.39 μg mL(-1) and 3.80 h at 25 °C. The distribution volume (V(d)/F) of ABZSO was estimated to be 1.99 L kg(-1) at 10 °C and 1.53 L kg(-1) at 25 °C; the total body clearance (CL(b)) of ABZSO were computed as 0.08 and 0.19 L/(h kg) at 10 and 25 °C, respectively; the areas under the concentration-time curve (AUC) was 118.22 μg mL(-1)h at 10 °C and 63.12 μg mL(-1)h at 25 °C. The [Formula: see text] of ABZSO(2) was found to be 6.39 °C at 10 °C and 3.73 h at 25 °C, whereas the [Formula: see text] was 12.86 h at 10 °C and 6.56 h at 25 °C; the C(max) and T(p) of ABZSO(2) was calculated as 0.78 μg mL(-1) and 12.82 h at 10 °C, 1.03 μg mL(-1) and 7.04 h at 25 °C, respectively; the V(d)/F of ABZSO(2) were estimated to be 6.43 L kg(-1) at 10 °C and 4.61 Lkg(-1) at 25 °C; the CL(b) of ABZSO(2) were computed as 0.34 and 0.49 L/(h kg) at 10 °C and 25 °C, respectively; the AUC of ABZSO(2) were 28.86 μg mL(-1)h at 10 °C and 20.52 μg mL(-1)h at 25 °C. It was demonstrated that ABZSO(2) was the main metabolite of ABZSO. The concentrations of ABZSO and its main metabolite (ABZSO(2)) were detected in muscle, skin, liver and kidney, whereas ABZ-SO(2)NH(2) was only detected in liver and kidney. The ABZSO and it metabolite (ABZSO(2)) could still be detected at 4 d time-point after administration at both temperatures in all tissues. The results revealed that the depletion of ABZSO and its metabolite (ABZSO(2)) in crucian carp was slower with a long half-life time, especially at lower water temperature.

Simultaneous determination of benzimidazoles and their metabolites in plasma using high-performance liquid chromatography/tandem mass spectrometry: application to pharmacokinetic studies in rabbits

An HPLC/MS/MS method was developed for the simultaneous determination of the following benzimidazole anthelmintics and metabolites in plasma: flubendazole, albendazole, fenbendazole, mebendazole, thiabendazole, hydrolyzed flubendazole, albendazole sulfoxide, albendazole sulfone, albendazole aminosulfone, oxfendazole, fenbendazole sulfone, aminomebendazole, hydroxymebendazole, and 5-hydroxythiabendazole. The sample preparation process involved a pH-dependent extraction of the analytes. Chromatographic separation was performed on a C18 column with a mobile phase gradient starting with methanol-water (20 + 80, v/v) containing 0.1% formic acid. The overall average recoveries of the analytes based on a matrix-matched calibration ranged from 75.0 to 120.0%, with RSD values of <20.0%. The LODs ranged from 0.08 to 2.0 microg/kg and the LOQs from 0.3 to 5.0 microg/kg. The validated method was used in pharmacokinetic studies of benzimidazole compounds in rabbits, and the elimination of the metabolites was measured quantitatively.

The effect of oral anthelmintics on the survivorship and re-feeding frequency of anthropophilic mosquito disease vectors

In the Tropics, there is substantial temporal and spatial overlap of diseases propagated by anthropophilic mosquito vectors (such as malaria and dengue) and human helminth diseases (such as onchocerciasis and lymphatic filariasis) that are treated though mass drug administrations (MDA). This overlap will result in mosquito vectors imbibing significant quantities of these drugs when they blood feed on humans. Since many anthelmintic drugs have broad anti-invertebrate effects, the possibility of combined helminth control and mosquito-borne disease control through MDA is apparent. It has been previously shown that ivermectin can reduce mosquito survivorship when administered in a blood meal, but more detailed examinations are needed if MDA is to ever be developed into a tool for malaria or dengue control. We examined concentrations of drugs that follow human pharmacokinetics after MDA and that matched with mosquito feeding times, for effects against the anthropophilic mosquito vectors Anopheles gambiae s.s. and Aedes aegypti. Ivermectin was the only human-approved MDA drug we tested that affected mosquito survivorship, and only An. gambiae s.s. were affected at concentrations respecting human pharmacokinetics at indicated doses. Ivermectin also delayed An. gambiae s.s. re-feeding frequency and defecation rates, and two successive ivermectin-spiked blood meals following human pharmacokinetic concentrations compounded mortality effects compared to controls. These findings suggest that ivermectin MDA in Africa may be used to decrease malaria transmission if MDAs were administered more frequently. Such a strategy would broaden the current scope of polyparasitism control already afforded by MDAs, and which is needed in many African villages simultaneously burdened by many parasitic diseases.

Pharmacokinetics of eprinomectin in plasma and milk following subcutaneous administration to lactating dairy cattle

Eprinomectin is only available as a topically applied anthelmintic for dairy cattle. To determine whether eprinomectin can be applied as an injectable formulation in dairy cattle, a novel injectable formulation was developed and was subcutaneously delivered to four lactating dairy cattle at a dose rate of 0.2 mg/ kg. Plasma and milk samples were collected. The concentrations of eprinomectin in all samples were determined by HPLC. The peak plasma concentration (C(max))of 44.0+/-24.2 ng/ml occurred 39+/-19.3 h after subcutaneous administration, equivalent to the C(max) (43.76+/-18.23 ng/ml) previously reported for dairy cattle after a pour-on administration of 0.5 mg/kg eprinomectin. The area under the plasma concentration-time curve (AUC) after subcutaneous administration was 7354+/-1861 (ng h)/ml, higher than that obtained after pour-on delivery (5737.68+/-412.80 (ng h)/ml). The mean residence time (MRT) of the drug in plasma was 211+/-55.2 h. Eprinomectin was detected in the milk at the second sampling time. The concentration of drug in milk was parallel to that in plasma, with a milk to plasma ratio of 0.16+/-0.01. The highest detected concentration of eprinomectin in milk was 9.0 ng/ml, below the maximum residue limit (MRL) of eprinomectin in milk established by the Joint FAO/WHO Expert Committee on Food Additives in 2000. The amount of eprinomectin recovered in the milk during this trial was 0.39%+/-0.08% of the total administered dose. This study demonstrates that subcutaneous administration of eprinomectin led to higher bioavailability and a lower dose than a pour-on application, and that an injectable formulation of eprinomectin may be applied in dairy cattle with a zero withdrawal period.

Sensitive high performance liquid chromatographic assay for nitazoxanide metabolite in plasma

A sensible and specific HPLC analytical method for the determination of tizoxanide (TZO), the active metabolite of nitazoxanide (NTZ) in rat plasma was developed and validated. Samples of 200 microL were efficiently deproteinized with acetonitrile. Assay was performed using a C18 CC with a ternary gradient elution of 50 mmol x L(-1) KH2PO4 : acetonitrile : methanol and UV/Vis detection at 416 nm. The analytical method was linear in a range of 10-1280 ng x mL(-1), precise (RSD % > 2.2), accurate (RE % < 7.8) and with high recovery (% > 95%). Stability studies showed that TZO was stable in plasma for short and long-time period (45 days) and proved to be suitable for pharmacokinetic studies of NTZ in rats. The method was also evaluated using human plasma samples and no statistical differences were found in the response-curve between rat and human samples.

Estimation of absolute oral bioavailability of moxidectin in dogs using a semi-simultaneous method: influence of lipid co-administration

Moxidectin is a long-acting anthelmintic drug for which little is known about its kinetic behaviour in dogs and its oral absolute bioavailability has never been reported. We studied the pharmacokinetics of moxidectin in dogs, with a special emphasis on oral bioavailability and the influence of lipid co-administration, by using a semi-simultaneous method of administration. Ten Beagle dogs were dosed orally and then intravenously (i.v.) with 0.2 mg/kg moxidectin. The oral application was conducted with or without corn oil co-administration. Moxidectin concentration-time profiles in plasma were analysed using a compartmental modelling approach, designed to fit the oral and i.v. kinetic disposition curves simultaneously. In contrast to what happens in other species, our study indicates that the bioavailability of orally given moxidectin in dogs is nearly total (90.2 +/- 7.4%), and is not enhanced by lipid co-administration. The clearance, the volume of distribution, the mean residence time and the terminal half-life were similar to what was already described for other species. Finally our trial suggests that the body condition (degree of obesity) is likely to be a major determinant of moxidectin kinetics in dogs because of its modulation of the volume of distribution that indirectly controls the terminal half-life of the drug.

Comparative plasma disposition of fenbendazole, oxfendazole and albendazole in dogs

The plasma disposition of fenbendazole (FBZ), oxfendazole (OFZ) and albendazole (ABZ); and the enantiospecific disposition of OFZ, and ABZSO produced were investigated following an oral administration (50 mg/kg) in dogs. Blood samples were collected from 1 to 120 h post-administration. The plasma samples were analysed by high performance liquid chromatography (HPLC). The plasma concentration of FBZ, OFZ, ABZ and their metabolites were significantly different from each other and depended on the drug administered. The sulphone metabolite (FBZSO2) of FBZ was not detected in any plasma samples and the parent molecule ABZ did not reach quantifiable concentrations following FBZ and ABZ administration, respectively. OFZ and its sulphone metabolite attained a significantly higher plasma concentration and remained much longer in plasma compared with FBZ and ABZ and their respective metabolites. The maximum plasma concentrations (Cmax), area under the concentration time curve (AUC) and mean residence time (MRT) of parent OFZ were more than 30, 68 and 2 times those of FBZ, respectively. The same parameters for ABZSO were also significantly greater than those of FBZSO. The ratio for total AUCs of both the parent drug and the metabolites were 1:42:7 for following FBZ, OFZ and ABZ administration, respectively. The enantiomers were never in racemic proportions and (+) enantiomers of both OFZ and ABZSO were predominant in plasma. The AUC of (+) enantiomers of OFZ and ABZSO was, respectively more than three and seven times larger than that of (-) enantiomers of both molecules. It is concluded that the plasma concentration of OFZ was substantially greater compared with FBZ and ABZ. The data on the pharmacokinetic profile of OFZ presented here may contribute to evaluate its potential as an anthelmintic drug for parasite control in dogs.

Pharmacokinetics of albendazole sulfoxide enantiomers administered in racemic form and separately in rats

The pharmacokinetic behaviour of albendazole sulfoxide (ABZSO) enantiomers was studied in rats after the oral administration of 10 mg/kg of rac-ABZSO, 5 mg/kg of (-)-ABZSO or 5 mg/kg of (+)-ABZSO. The disposition profiles of ABZSO enantiomers were similar in all treatments, but the calculated area under the curve for the (-)-ABZSO was higher in all cases compared with (+)-ABZSO. The results suggest that there is no chiral inversion of ABZSO enantiomers. After the administration of rac-ABZSO, 17.2% of the total dose was recovered in urine as albendazole ABZ (0.1%), albendazole sulfone ABZSO(2) (0.3%), albendazole 2-aminosulfone (ABZ-SO(2)NH(2)) (3.1%) and ABZSO (13.7%). The ratio (+) to (-) was similar in urine (1.6) and blood (1.7).

The effect of diet type on the plasma disposition of triclabendazole in goats

The effect of two different diet types (concentrate feed+hay and grazing) on the pharmacokinetic profiles of triclabendazole following oral administration in goats was investigated. A total of 12 goats were randomly allocated into two groups which were either indoor and fed concentrate + hay ration (housed group) or were grazing on pasture (grazing group). Triclabendazole was administered orally to animals in two groups at 10 mg/kg bodyweight. Blood samples were collected from 1 h to 192 h post-treatment and analyzed by high performance liquid chromatography (HPLC). Feeding with different diets significantly effected the plasma disposition of triclabendazole sulphoxide. Maximum plasma concentration (C(max): 13.22+/-2.81 microg/ml), time to reach maximum plasma concentration (t(max): 18.4+/-2.19 h), area under the curve (AUC: 613+/-137 microg h/ml), half-life (t(1/2): 24.77+/-1.94 h) and mean resident time (MRT: 40.22+/-4.36 h) of triclabendazole sulphoxide in housed group were significantly different from those of grazing group (C(max): 10.17+/-1.51 microg/ml, t(max): 14.0+/-2.19 h, AUC: 406+/-98 microg h/ml), t(1/2): 16.16+/-1.17 h and MRT: 34.48+/-4.40 h). It is concluded that anthelmintically more active sulphoxide metabolite has higher plasma concentration when triclabendazole is administered to goats fed with concentrate feed + hay compared to grazing goats.

Simultaneous determination of albendazole metabolites, praziquantel and its metabolite in plasma by high-performance liquid chromatography–electrospray mass spectrometry

The analysis of albendazole sulfoxide, albendazole sulfone, praziquantel and trans-4-hydroxypraziquantel in plasma was carried out by high-performance liquid chromatography-mass spectrometry ((LC-MS-MS). The plasma samples were prepared by liquid-liquid extraction using dichloromethane as extracting solvent. The partial HPLC resolution of drug and metabolites was obtained using a cyanopropyl column and a mobile phase consisting of methanol:water (3:7, v/v) plus 0.5% of acetic acid, at a flow rate of 1.0 mL/min. Multi reaction monitoring detection was performed by electrospray ionization in the positive ion mode, conferring additional selectivity to the method. Method validation showed relative standard deviation (precision) and relative errors (accuracy) lower than 15% for all analytes evaluated. The quantification limit was 5 ng/mL and the linear range was 5-2500 ng/mL for all analytes. The method was used for the determination of drug and metabolites in swine plasma samples and proved to be suitable for pharmacokinetic studies.

Pharmacokinetics assessment of moxidectin long-acting formulation in cattle

The plasma kinetics disposition of moxidectin following a subcutaneous administration with a long-acting formulation (Cydectin) 10%, Fort Dodge Animal Health, France) at the recommended dose of 1 mg kg(-1) body weight was evaluated in Charolais cattle breed (five females weighing 425-450 kg) for 120 days. Furthermore, its concentration was measured in hair for the same period. After plasma extraction and derivatization, samples were analysed by HPLC with fluorescence detection. Moxidectin was first detected at 1 h after treatment for plasma (2.00+/-1.52 ng ml(-1)) and at 2 days for hair (446.44+/-193.26 ng g(-1)). The peak plasma concentration (C(max)) was 55.71+/-15.59 ng ml(-1) and 444.44+/-190.45 ng g(-1) for plasma and hair, respectively. The mean calculated time of peak occurrence (T(max)) was 3.40+/-3.36 and 2 days for plasma and hair, respectively. The mean residence time (MRT) was 28.93+/-2.87 and 13.32+/-2.48 days for plasma and hair cattle. The area under concentration-time curve (AUC) was 1278.95+/-228.92 ng day ml(-1) and 2663.82+/-1096.62 ng day g(-1) for plasma and hair, respectively. At the last sampling time (120 days), the concentration was 1.91+/-0.26 ng ml(-1) and 0.69+/-0.52 ng g(-1) for plasma and hair, respectively. The bioavailability of this long-acting formulation of moxidectin is similar to that registered after subcutaneous administration of moxidectin in cattle at 0.2 mg kg(-1) body weight. For the first time the moxidectin pharmacokinetics parameters in hair after a subcutaneous administration was described. The moxidectin profile concentrations in hair reflected that registered in plasma. The previous studies of efficacy have to be correlated to the extended period of absorption and distribution by the LA formulation due to the fivefold higher dose rate in comparison with the 1% injectable formulation (0.2 mg kg(-1) body weight).

Ivermectin (3.15%) long-acting formulations in cattle: absorption pattern and pharmacokinetic considerations

Ivermectin (IVM) is a broad-spectrum antiparasitic drug extensively used in veterinary medicine. The composition of the pharmaceutical preparation affects IVM absorption and its systemic availability. After the introduction of the first approved IVM formulation (propylene glycol/glycerol formal 60:40) used at 200 microg/kg, different pharmaceutical modifications have been assayed to extend IVM persistent endectocide activity. Recently, IVM 3.15% long-acting (IVM-LA) preparations to be administered at 630 microg/kg to cattle were introduced into the veterinary pharmaceutical market. The work reported here was designed to evaluate the comparative IVM absorption pattern and plasma concentration profiles obtained after subcutaneous administration of the classic pioneer IVM formulation (1%) and two different commercially available IVM-LA preparations (3.15%) to cattle. Twenty-eight Holstein heifers were divided in four experimental groups (n=7) and treated subcutaneously as follows--Group A: IVM 1% given at 200 microg/kg, Group B: IVM 1% administered at 630 microg/kg, Group C: IVM-LA (A) injected at 630 microg/kg and Group D: IVM-LA (B) given at 630 microg/kg. Blood samples were taken between 0.5 and 90 days post-treatment and IVM plasma concentrations were determined by HPLC with fluorescence detection. There were no differences in the persistence of IVM plasma concentrations after the administration of IVM 1% formulation at the two used dose levels (200 and 630 microg/kg). Higher peak plasma concentration (C(max)) and shorter mean residence time (MRT) were obtained for IVM 1% given at 630 microg/kg (Group B) compared to the treatments with both IVM-LA preparations. The IVM-LA (A) formulation showed a more extended absorption process than IVM-LA (B) preparation, which accounted for a longer persistence of detectable IVM plasma concentrations. The parasitological implications of the observed differences in peak plasma concentrations (C(max) values) and in the IVM concentration levels measured from day 20, and afterwards until day 90 post-treatment, between the different preparations assayed need to be elucidated. The characterization of the absorption patterns and kinetic behaviour obtained after injection of these novel long-acting formulations used at three times the therapeutic dose recommended for the classic IVM preparation in cattle is a further contribution to the field.

Absorption and metabolism of albendazole after intestinal ischemia/reperfusion

Pathophysiological processes involving inflammatory response may affect absorption and biotransformation of some drugs, modifying their pharmacokinetic behaviour. Ischemia/reperfusion (I/R) injury has been used as a model for inflammatory processes. The aim of this work was to study the effect of intestinal I/R injury on the absorption and metabolism processes of one orally administered drug, albendazole that is anthelmintic drug, it undergoes intestinal bioconversion into albendazole sulfoxide by two enzymatic systems, cytochromes P450 (CYP450) and flavin-containing monooxygenase (FMO). Male Wistar rats were used to study the influence of I/R in the intestinal absorption and metabolism of albendazole, after 60 min of mesenteric occlusion and 30 min of reperfusion. The intestinal studies were performed in microsomal, and everted ring incubations. During in situ studies, the I/R group had faster disappearance of albendazole from the lumen. In addition, albendazole only appeared in blood samples of the I/R group, while albendazole sulfoxide appeared in both samples and was higher in the control group. These findings are supported by significant reductions of albendazole sulfoxide formation in intestinal everted ring assays and in microsomal incubations after the I/R process. Both metabolizing systems, CYP4503A and FMO, were affected by I/R. Our data indicate that I/R injury, considered as an inflammatory model, reduces absorption and metabolism processes of albendazole.

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.

Pharmacokinetics of a new long acting endectocide formulation containing 2.25% ivermectin and 1.25% abamectin in cattle

The objective of this study was to determine the kinetic parameters of a new formulation that contained 2.25% ivermectin combined with 1.25% abamectin in bovine plasma. The results for 2.25% ivermectin: Cmax (37.11 ng/mL +/- 7.42), Tmax (16 days +/- 5.29), T(1/2) (44.62 days +/- 53.89), AUC (928.2 ng x day/mL +/- 153.83) and MRT (36.73 days +/- 33.64), and for 1.25% abamectin: Cmax (28.70 ng/mL +/- 9.54), Tmax (14 days +/- 4.04), T(1/2) (15.40 days +/- 11.43), AUC (618.05 ng x day/mL +/- 80.27) and MRT (20.79 days +/- 8.43) suggest that this combination of 2.25% ivermectin + 1.25% abamectin possesses properties that give this pharmaceutical formula a longer activity time than two of the commercial products tested (1% ivermectin and 1% abamectin), and showed similarity to 3.15% ivermectin.

Ivermectin in Senegalese Peulh Sheep: Influence of Sex on Plasma Disposition

The plasma disposition kinetics of ivermectin following a single subcutaneous administration of 0.2 mg/kg was investigated in male and female Senegalese Peulh sheep. Ten clinically healthy animals (5 males and 5 females) weighing 38-45 kg were used in this trial. Blood samples were collected by jugular puncture at different times between 0.5 h and 30 days post treatment. After plasma extraction and derivatization, samples were analysed by HPLC with fluorescence detection. Computerized kinetic analysis was carried out and mean parameters were statistically compared with the Mann-Whitney U-test. The area under the concentration-time curve (AUC) was significantly higher (p < 0.0027) in females than in males. Although the differences in maximum concentration (C (max)), mean residence time (MRT) and half-life of elimination (t (1/2el)) between males and females did not achieve statistical significance, values tended to be higher in females. Sex differences may be parallel with the level of storage in fat. Further investigations are required to improve the use of ivermectin in Senegalese sheep and findings may be used to predict optimal anthelmintic strategies for management of African species depending on the parasites present in a production system.

Plasma disposition and faecal excretion of netobimin metabolites and enantiospecific disposition of albendazole sulphoxide produced in ewes

Netobimin (NTB) was administered orally to ewes at 20 mg/kg bodyweight. Blood and faecal samples were collected from 1 to 120 h post-treatment and analysed by high-performance liquid chromatography (HPLC). Using a chiral phase-based HPLC, plasma disposition of albendazole sulphoxide (ABZSO) enantiomers produced was also determined. Neither NTB nor albendazole (ABZ) was present and only ABZSO and albendazole sulphone (ABZSO(2)) metabolites were detected in the plasma samples. Maximum plasma concentrations (C(max)) of ABZSO (4.1 +/- 0.7 microg/ml) and ABZSO(2) (1.1 +/- 0.4 microg/ml) were detected at (t(max)) 14.7 and 23.8 h, respectively following oral administration of netobimin. The area under the curve (AUC) of ABZSO (103.8 +/- 22.8 (microg h)/ml) was significantly higher than that ABZSO(2)(26.3 +/- 10.1 (microg h)/ml) (p < 0.01). (-)-ABZSO and (+)-ABZSO enantiomers were never in racemate proportions in plasma. The AUC of (+)-ABZSO (87.8 +/- 20.3 (microg h)/ml) was almost 6 times larger than that of (-)-ABZSO (15.5 +/- 5.1 (microg h)/ml) (p < 0.001). Netobimin was not detected, and ABZ was predominant and its AUC was significantly higher than that of ABZSO and ABZSO(2), following NTB administration in faecal samples (p > 0.01). Unlike in the plasma samples, the proportions of the enantiomers of ABZSO were close to racemic and the ratio of the faecal AUC of (-)-ABZSO (172.22 +/- 57.6 (microg h)/g) and (+)-ABZSO (187.19 +/- 63.4 (microg h)/g) was 0.92. It is concluded that NTB is completely converted to ABZ by the gastrointestinal flora and absorbed ABZ is completely metabolized to its sulphoxide and sulphone metabolites by first-pass effects. The specific behaviour of the two enantiomers probably reflects different enantioselectivity of the enzymatic systems of the liver that are responsible for sulphoxidation and sulphonation of ABZ.

A rapid reversed phase high-performance liquid chromatographic method for determination of sophoridine in rat plasma and its application to pharmacokinetics studies

A high-performance liquid chromatography (HPLC) method for determining sophoridine in rat plasma was developed for application in the pharmacokinetic studies. The plasma was deproteinized with acetonitrile that contained an internal standard (ephedrine) and was separated from the aqueous layer by adding sodium chloride and sodium carbonate. The HPLC assay was carried out using a YMC-ODS column. The mobile phase was methanol-ethanol-0.01 moll(-1) ammonium acetate buffer-triethylamine (10:0.5:89.5:0.03, v/v/v/v) (pH 6.80). The flow rate was 0.8 ml min(-1). The detection wavelength was set at 210 nm. The method was used to determine the concentration-time profiles of sophoridine in the plasma following oral administration or injection of sophoridine aqueous solution. The fractions of sophoridine reaching the systemic circulation were estimated for the first time by a deconvolution method.

Evaluation of a covered-rod silicone implant containing ivermectin for long-term prevention of heartworm infection in dogs

OBJECTIVE

To evaluate use of covered-rod (CR) silicone implants containing ivermectin for long-term prevention of infection with Dirofilaria immitisin dogs.

ANIMALS

145 adult male and female dogs.

PROCEDURES

Dogs received implants of different sizes, and ivermectin concentrations and serum ivermectin concentrations were monitored for 16, 57, and 56 weeks, respectively, in 3 preclinical dose selection studies. Ability of implants to prevent infection with D immitis was evaluated in 2 further studies; dogs were challenged with 50 infective third-stage larvae 52 weeks after implant administration and necropsied 145 days after challenge, and the total number of adult heartworms was counted. A field study was then undertaken in which client-owned dogs received an implant and plasma samples were collected at intervals until week 52 for ivermectin analysis and heartworm antigen determination.

RESULTS

Use of the implants resulted in maintenance of an ivermectin concentration > or = 0.2 ng/mL for 12 months. In challenge studies, no treated dogs had adult heartworms, in contrast to untreated dogs, which all had adult heartworms at necropsy. In the field study, dogs treated with an implant had negative results of heartworm antigen testing for 12 months.

CONCLUSIONS AND CLINICAL RELEVANCE

The CR silicone implant containing 7.3 mg of ivermectin was 100% effective in preventing experimental infection with D immitislarvae and resulted in negative results for heartworm antigen in a field trial. This product has the potential to alleviate poor owner compliance with monthly prevention regimens.

Pharmacokinetics of ricobendazole after its intravenous, intraruminal and subcutaneous administration in sheep

Ricobendazole (RBZ) was administered in sheep at the dose rate of 5 mg/kg by intravenous (i.v.) route as a 10% experimental solution, by the intraruminal (i.r.) route as a 10% experimental suspension, and by the subcutaneous (s.c.) route as a 10% commercial formulation available in Argentina. Blood samples were drawn during a 60 h period. Plasma concentrations of RBZ and its inactive metabolite albendazole sulphone (ABZSO2) were determined by high-performance liquid chromatography. The pharmacokinetic parameters were determined by compartmental analysis. The fitting of the data was done by weighted least-squares non-linear regression analysis. The pharmacokinetic parameters were estimated for every animal by simultaneous fitting of the plasma concentrations profiles of RBZ obtained after its administration by the three routes. The kinetic analysis of ABZSO2 was performed by a statistical moment approach. Ricobendazole bioavailability was poor after i.r. administration, whereas high and sustained plasma concentrations and higher bioavailability were obtained after s.c. administration. A simple two-compartment open model explains in a mechanical sense the pharmacokinetic behaviour of RBZ in sheep and allows us to estimate the real first-order constant rate of absorption and the loss of drug from the absorption site after its administration by s.c. and i.r. routes.

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.

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

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

Pharmacokinetics of a novel formulation of ivermectin after administration to goats

OBJECTIVE

To evaluate the pharmacokinetics of a novel commercial formulation of ivermectin after administration to goats.

ANIMALS

6 healthy adult goats.

PROCEDURE

Ivermectin (200 microg/kg) was initially administered IV to each goat, and plasma samples were obtained for 36 days. After a washout period of 3 weeks, each goat received a novel commercial formulation of ivermectin (200 microg/kg) by SC injection. Plasma samples were then obtained for 42 days. Drug concentrations were quantified by use of high-performance liquid chromatography with fluorescence detection.

RESULTS

Pharmacokinetics of ivermectin after IV administration were best described by a 2-compartment open model; values for main compartmental variables included volume of distribution at a steady state (9.94 L/kg), clearance (1.54 L/kg/d), and area under the plasma concentration-time curve (AUC; 143 [ng x d]/mL). Values for the noncompartmental variables included mean residence time (7.37 days), AUC (153 [ng x d]/mL), and clearance (1.43 L/kg/d). After SC administration, noncompartmental pharmacokinetic analysis was conducted. Values of the variables calculated by use of this method included maximum plasma concentration (Cmax; 21.8 ng/mL), time to reach Cmax (3 days), and bioavailability (F; 91.8%).

CONCLUSIONS AND CLINICAL RELEVANCE

The commercial formulation used in this study is a good option to consider when administering ivermectin to goats because of the high absorption, which is characterized by high values of F. In addition, the values of Cmax and time to reach Cmax are higher than those reported by other investigators who used other routes of administration.

Assessments of pharmacokinetic drug interactions and tolerability of albendazole, praziquantel and ivermectin combinations

The pharmacokinetic interactions and tolerability of albendazole, praziquantel and ivermectin combinations were assessed in 23 healthy Thai volunteers (12 males and 11 females). The study was an open, randomised, three-way crossover design in which each subject attended the study on three separate occasions (Phases I, II and III), of 4 d or 8 d each, with at least 1 or 2 weeks (but not longer than 2 months) between each phase. All subjects received the three study drug regimens as follows: regimen I, oral praziquantel (40 mg/kg body weight); regimen II, oral ivermectin (200 microg/kg body weight) given concurrently with an oral dose of albendazole (400 mg); and regimen III, oral ivermectin given concurrently with albendazole and praziquantel. All treatment regimens showed acceptable tolerability profiles. The incidence of overall drug-related adverse events was significantly higher following regimens I (12/23) and III (7/23) compared with that following regimen II (0/23). Six statistically significant changes in the pharmacokinetic parameters of albendazole sulphoxide (Cmax, AUC0-infinity, Vz/F, CL/F), praziquantel (Vz/F) and ivermectin (AUC0-infinity) were observed when the three drugs were given concurrently. However, based on US Food and Drug Administration criteria, these changes were not considered of clinical relevance.

Eprinomectin in dairy zebu Gobra cattle (Bos indicus): plasma kinetics and excretion in milk

The pharmacokinetics and mammary excretion of eprinomectin were determined in zebu Gobra following topical administration of 0.5 mg kg(-1). The kinetics of plasma and milk was analysed using a one-compartment model. The maximum plasma concentration of 8.83+/-2.15 ng ml(-1) occurred 1.30 days post-administration. The area under the plasma concentration-time curve was 30.63+/-5.56 ng day(-1) ml(-1) and the mean residence time was 3.38+/-0.60 days. Eprinomectin was detected in milk at the first sampling time and thereafter for at least 8 days. The systemic availability of eprinomectin was significantly lower than that for other breeds of cattle. Comparison of the milk and plasma data demonstrated the parallel disposition of the drug in the milk and plasma with a milk/plasma ratio of 0.094. The very low extent of mammary excretion supports the permitted use of eprinomectin in lactating zebu Gobra.

Contribution of lymphatic transport to the systemic exposure of orally administered moxidectin in conscious lymph duct-cannulated dogs

Moxidectin, a macrocyclic lactone (ML), is a potent parasiticide widely used in veterinary medicine and currently under development for use in humans. The contribution of the lymphatic route to the intestinal absorption and transport of moxidectin to the systemic circulation was evaluated in lymph duct-cannulated dogs. Beagle dogs were operated for lymph duct cannulation and were orally dosed with 38g of corn oil and moxidectin (0.2mg/kg, n=3). The lymph and plasma were collected over 24h and moxidectin and triglyceride concentrations were measured. Similarly, control dogs (n=5) were dosed orally with moxidectin and oil and subsequently with moxidectin intravenously. Pharmacokinetic parameters were calculated for moxidectin in the plasma of the dogs. Moxidectin readily accumulated in the lymph and reached a plateau 8h post-administration, paralleling triglyceride appearance. The percentage of moxidectin recovered in lymph was 22+/-3% of the total administered dose with 92% being associated with triglyceride-rich particles. The systemic bioavailability of oral moxidectin coadministered with lipid was only 40% in the lymph duct-cannulated dogs compared with 71% in the controls. Our data clearly indicate that the lymphatic transport process contributes significantly to the post-prandial intestinal absorption of moxidectin and subsequently to its systemic bioavailability. The lymphatic transport of moxidectin offers potential strategies based on lipid formulations to improve the bioavailability of MLs when administered orally.

Effects of formulation concentration on intravenous pharmacokinetics, chirality and in vitro solubility of oxfendazole and its metabolites in sheep

This study compared pharmacokinetic (PK) profiles in sheep dosed intravenously with three different concentrations of oxfendazole (OFZ). An in vitro plasma OFZ solubility study provided additional information on plasma saturation. For the PK study, 18 adult, parasite-free, female Suffolk cross sheep, allocated into three groups (n = 6), were treated intravenously, at a dose rate of 5 mg/kg bodyweight, with aqueous formulations containing at 4, 8 or 16% OFZ. Plasma drug concentrations were measured, for up to 72 h post-treatment, by a validated high performance liquid chromatography method with UV detection. OFZ and fenbendazole sulphone (FBZSO2) were the main metabolites detected in all three experimental groups. In animals given the 4% formulation, OFZ depleted according to a biexponential concentration vs. time curve. In contrast, those given 8 or 16% preparations produced atypical curves fitted by monoexponential equations. No statistically significant differences in area under concentration-time curves (AUC) were observed, but concentration-dependent differences in distribution and mean residence time (MRT) were evident. Compared with 4% OFZ, animals treated with 8 and 16% formulations had slower half-lives of metabolite formation, and lower AUC's, suggesting that OFZ sulphonation may have been modified. In vitro there was evidence of plasma saturation associated with 8 and 16% OFZ preparations. It is concluded that differences in PK profiles were related to OFZ solubility and/or tissue drug precipitation.

Development and optimization of a rapid HPLC method for analysis of ricobendazole and albendazole sulfone in sheep plasma

A simple, rapid and reliable high performance liquid chromatographic (HPLC) method has been developed and validated for simultaneous determination of ricobendazole (RBZ) and its main metabolite albendazole sulfone (ABZSO(2)) in sheep plasma using an isocratic system with UV detection. The method involved solid phase extraction followed by separation on a reversed phase C-18 column. Internal standard was selected by quantitative structure retention relationships (QSRRs) analysis. A method to optimize the composition of ternary components mobile phase with the assistance of multiple linear regression is described. Retention times were within 10 min. The calibration curves were linear over a concentration range of 10-1000 ng/ml for both RBZ and ABZSO(2) (r > 0.999). Intra-day relative standard deviation at low, medium and high concentration levels were <5.5% for RBZ and <4.6% for ABZSO(2); average accuracies were 98.3, 101.0 and 100.5% for RBZ and 101.0, 102.4 and 100.8% for ABZSO(2). The inter-day variations at the same concentrations were <5.9% for RBZ and <6.4% for ABZSO(2). The extraction recoveries at these concentrations for RBZ, ABZSO(2) and the internal standard were all over 96%. The limit of detection and limit of quantitation were 2.4 and 7.1 ng/ml, respectively for RBZ, and 10ng/ml for both analytes.

Doramectin concentration profiles in the gastrointestinal tract of topically-treated calves: Influence of animal licking restriction

Endectocide compounds are extensively used for broad-spectrum parasite control and their topical administration to cattle is widespread in clinical practice. Pour-on formulations of moxidectin, ivermectin, eprinomectin and doramectin (DRM) are marketed internationally for use in cattle. However, variability in antiparasitic efficacy and pharmacokinetic profiles has been observed. Although the tissue distribution pattern for different endectocide molecules given subcutaneously to cattle has been described, only limited information on drug concentration profiles in tissues of parasite location after topical treatment is available. Understanding the plasma and target tissue kinetics for topically-administered endectocide compounds is relevant to optimise their therapeutic potential. The current work was designed to measure the plasma and gastrointestinal (GI) concentration profiles of DRM following its pour-on administration to calves. The influence of natural licking behaviour of cattle on DRM concentration in mucosal tissue and luminal content of different GI sections was evaluated. The trial was conducted in two experimental phases. In Phase I, the DRM plasma kinetics was comparatively characterised in free-licking and in 2-day licking-restricted (non-licking) calves. The pattern of distribution of topical DRM to mucosal and luminal contents from abomasum, duodenum, ileum, caecum and spiral colon was assessed in free-licking and non-licking calves restricted over 10 days post-administration (Phase II). The prevention of licking caused marked changes on the plasma and GI kinetics of DRM administered pour-on. In 2-day licking restricted calves, DRM systemic availability was significantly lower (29%) than in free licking animals during the first 9 days post-treatment. Following a 10-day long licking restriction period, DRM concentrations profiles in both mucosal tissue and luminal contents of the GI tract were markedly higher in animals allowed to lick freely. This enhancement in drug concentrations in free-licking compared to non-licking calves, was particularly pronounced in the abomasal (38-fold higher) and duodenal (six-fold higher) luminal content. As shown earlier for ivermectin, licking behaviour may facilitate the oral ingestion of topically-administered DRM in cattle. This would be consistent with the marked lower drug concentration profiles measured in the bloodstream and GI tract of the animals prevented from licking. The work reported here provides relevant information on the pattern of DRM distribution to the GI tract after pour-on treatment, and contributes to understand the variability observed in the antiparasitic persistence of topically-administered endectocides in cattle. The implications of natural licking in topical treatments are required to be seriously assessed to achieve optimal parasite control and to design parasitological and pharmacological studies within the drug approval process.

Changes to oxfendazole chiral kinetics and anthelmintic efficacy induced by piperonyl butoxide in horses

REASONS FOR PERFORMING THE STUDY

The study of novel pharmacological strategies to control parasitism in horses is required since many parasite species have developed resistance to anthelmintic drugs.

OBJECTIVES

To evaluate the effects of piperonyl butoxide (PB) (a metabolic inhibitor) on the plasma availability and enantiomeric behaviour of oxfendazole (OFZ) given orally to horses, and to compare the clinical efficacy of OFZ given either alone or co-administered with PB in naturally parasitised horses.

METHODS

Fifteen naturally parasitised crossbred male ponies were allocated into 3 groups (n = 5) and treated orally as follows: Group I (control) received distilled water as placebo; Group II was dosed with OFZ (10 mg/kg bwt); and Group III was treated with OFZ (10 mg/kg bwt) co-administered with PB (63 mg/kg bwt). Jugular blood samples were obtained over 120 h post treatment. Three weeks after treatments, all experimental horses were subjected to euthanasia.

RESULTS

The observed maximum plasma concentration (Cmax) and area under the concentration vs. time curve (AUC) values for OFZ increased 3- and 5-fold, respectively, in the presence of PB. The plasma concentration profiles of fenbendazole (FBZ), a metabolite generated from OFZ, were significantly lower after the treatment with OFZ alone (AUC = 0.8 microg x h/ml) compared to those obtained after the OFZ + PB treatment (AUC = 2.7 microg x h/ml). The enhanced pharmacokinetic profiles correlated with increased anthelmintic efficacy. The combination OFZ + PB showed 100% efficacy against mature nematode parasites. The efficacy against cyathostome L3 larvae increased from 94% (Group II) to 98.7% (Group III). Consistently, the number of L4 larvae recovered from OFZ + PB treated horses (Group III) (n = 146) was significantly lower (P<0.05) than that recovered from Group II (n = 1397).

CONCLUSIONS

The use of PB as a metabolic inhibitor may be useful to enhance OFZ activity against mature and migrating larvae of different parasite species in horses.

POTENTIAL RELEVANCE

Metabolic inhibitors may be used to enhance the activity of benzimidazole anthelmintics and extend the effective lifespan of benzimidazole drugs in the face of increasing resistance.

Pour-on formulation of eprinomectin for cattle: fecal elimination profile and effects on the development of the dung-inhabiting Diptera Neomyia cornicina (L.) (Muscidae)

The plasma and fecal concentrations of eprinomectin were determined in cattle following topical administration at a dose rate of 0.5 mg kg(-1). The maximum plasma concentrations of 12.24 ng ml(-1) occurred 2 d after administration, and eprinomectin remained detectable in plasma 29 d after administration (0.10 ng ml(-1)). The maximum dung concentration of 350 ng g(-1) was observed 3 d after administration and thereafter for at least 29 d (4 ng g(-1)). The amount of drug recovered in dung during this period was 20.50%+/-4.31% of the total administered dose. The effects of eprinomectin against the nontarget dung-feeding Diptera Neomyia cornicina was assessed under laboratory conditions. Feces voided by cattle treated with eprinomectin were associated with high larval mortality during the first 12 d after treatment, with null emergence until day 7. The no-observed-effect concentration for N. cornicina was estimated to be close to 7+/-5 ng g(-1).

Pharmacokinetics of ivermectin in zebu Gobra (Bos indicus)

Plasma disposition kinetics of ivermectin was evaluated in a West African cattle breed. Five clinically healthy zebu Gobra cattle (Bos indicus) weighing 220-270 kg were treated (0.2 mg kg-1) with a commercially available ivermectin formulation for cattle. Blood samples were collected by jugular puncture at different times between 0.5 h and 40 days post-treatment. After plasma extraction and derivatization, samples were analysed by HPLC with fluorescence detection. Ivermectin was detected in plasma between 30 min and 20 days post-treatment. The observed peak plasma concentration (Cmax) was 46.3+/-13.8 ng ml-1 and the time to reach Cmax (t(max)) was 0.9+/-0.2 day. The values for the absorption half-life (t1/2ab) and the elimination half-life (t1/2el) were 0.3+/-0.2 and 2.8+/-0.7 days, respectively. The calculated area under the concentration-time curve (AUC) was 185.2+/-12.1 ng day ml-1 and the mean residence time (MRT) was 4.2+/-1.3 days. The availability of ivermectin is low in zebu Gobra in comparison to other breeds cattle but equivalent to that reported in the yak and is likely to be due to physiological characteristics of this breed.

[Determination of ivermectin in pig serum by high performance liquid chromatography]

OBJECTIVE

To establish a high performance liquid chromatographic method for the determination of micro-amount of ivermectin in pig serum.

METHODS

Ivermectin in pig serum was extracted with ethyl acetate after the serum protein was precipitated with 0.05% metaphosphoric acid-methanol in the ratio of 7:3 (V/V). Then the sample was centrifuged at 4000 r/min for 5 min and the supernate was evaporated to dryness with rotary vacuum evaporator. The residue was dissolved with 0.20 ml of methanol as the sample solution for HPLC analysis. HPLC column used was a phenomenex C18 (5 microm, 250 mm x 4.6 mm) with a same type of guard column. Mobile phase consisted of methanol and water in the ratio of 90:10 (V/V) and the flow rate was 1.0 ml/min. The detection wavelength was 245 nm.

RESULTS

The linear range of the method was found to be 0.010-20 mg/L and its detection limit was 0.010 mg/L. The relative standard deviation of the method was 0.78%-3.82% and the recoveries varied from 94.0% to 100.0%, with an average recovery of 97.3%.

CONCLUSION

This method is simple, reproducible, accurate and suitable to the determination of micro-amount of ivermectin in pig serum.

Integrated pharmacological assessment of flubendazole potential for use in sheep: disposition kinetics, liver metabolism and parasite diffusion ability1

Flubendazole (FLBZ) is a broad spectrum benzimidazole methylcarbamate anthelmintic widely used in poultry and swine. However, there is no information available on the pharmacological behaviour of FLBZ in ruminants. The work reported here was addressed to evaluate the potential of FLBZ for use in sheep. The integrated assessment included evaluation of FLBZ and metabolites plasma disposition kinetics, liver metabolism and ex vivo ability to diffuse into the cestode parasite Moniezia benedeni. In a cross-over kinetic study, six healthy Corriedale sheep were treated with FLBZ by intravenous (i.v.) (4% solution) and intraruminal (i.r.) (4% suspension) administrations at the same dosage (5 mg/kg) with a 21-day washout period between treatments. Blood samples were collected between 0 and 72 h post-treatments. Sheep liver microsomes were incubated with 40 microm FLBZ and specimens of the cestode parasite M. benedeni, collected from untreated animals, were incubated (5-120 min) with FLBZ and its reduced (R-FLBZ) metabolite (5 microm). Samples of plasma, microsomal incubations and parasite material were prepared and analyzed by high-performance liquid chromatography to measure FLBZ and its metabolites. FLBZ parent drug showed a fast disposition being detected in the bloodstream up to 36 h after its i.v. administration. Both R-FLBZ and hydrolyzed FLBZ (H-FLBZ) metabolites were recovered in plasma as early as 5 min after the i.v. treatment in sheep. The plasma AUC ratios for R-FLBZ and FLBZ (AUC(R-FLBZ)/AUC(FLBZ)) were 4.07 i.v. and 5.55 i.r., respectively. R-FLBZ achieved a significantly higher (P < 0.01) C(max) value (0.14 microg/mL at 17.3 h post-treatment) than that observed for the parent drug FLBZ (0.04 microg/mL at 14.4 h post-treatment). Low plasma concentrations of FLBZ parent drug were measured between 6 and 48 h, and only trace concentrations of H-FLBZ were detected during a short period of time after the i.r. treatment. Consistently, sheep liver microsomes metabolized FLBZ into its reduced metabolite at a rate of 9.46 +/- 2.72 nmol/mg/h. Both FLBZ and R-FLBZ demonstrated a similar ability to quickly diffuse through the tegument of the cestode parasite. The data on FLBZ pharmacological behaviour presented here contribute to evaluate its potential to be developed as an anthelmintic for broad spectrum parasite control in ruminants.

Enhancement of oral moxidectin bioavailability in rabbits by lipid co-administration

Moxidectin is a member of the macrocyclic lactone family of drugs widely used for the control of internal and external parasites. Because moxidectin is highly lipophilic, we suspect that lymphatic transport influences the intestinal absorption of oral formulations of the drug. We studied the influence of lipid co-administration on the pharmacokinetics of an oral formulation of moxidectin in rabbits. Ten rabbits were orally administered 0.3 mg kg(-1) moxidectin with or without sunflower oil. Moxidectin and triglyceride were analyzed in plasma over 23 days. Sunflower oil co-administration significantly increased the area under the plasma concentration-time curve of moxidectin (98%, P<0.05) and prolonged its mean residence time from 1.52 days to 2.12 days (P<0.04). Simultaneously, an increase in plasma triglyceride was observed in response to oil administration. It is suggested that lipid administration increases the systemic availability of oral moxidectin by enhancing the extent of intestinal lymphatic transport of the drug. Lipid-based formulations should improve the bioavailability of moxidectin in rabbits.

Validated capillary electrophoretic method for the analysis of ivermectin in plasma after intragastric administration in pigs and horses

A capillary electrophoretic (CE) method has been developed for the determination of ivermectin (CAS 70288-86-7), a new generation drug with antiparasitic activity, in pig and horse plasma. The method was statistically validated for its linearity, accuracy, precision and selectivity. The linear range was from 1 to 30 ng mL(-1) with correlation coefficients greater than 0.999. The limit of detection was 0.3 ng mL(-1), while the quantitative limit was 1 ng mL(-1), using a 0.5 mL sample size. The validated procedure was used to determination of pharmacokinetic parameters of ivermectin after ingestion of 0.1 mg for pigs and 0.2 mg dose per kg body weight for horses, respectively. Studies were performed on a group of eight pigs and six horses. There were no significant differences between pigs and horses in any of the estimated pharmacokinetic parameters.

Reversed-phase liquid chromatographic method with fluorescence detection for the simultaneous determination of albendazole sulphoxide, albendazole sulphone and albendazole 2-aminosulphone in sheep plasma

A rapid and sensitive HPLC method for the simultaneous quantification of albendazole sulphoxide (ABZ-SO), albendazole sulphone (ABZ-SO2) and albendazole 2-aminosulphone (ABZ-SO2NH2) in sheep blood plasma has been developed. Plasma samples were extracted with ethyl acetate under alkaline conditions. Separation was achieved on a C18 reversed-phase analytical column, in the presence of positively- (tetra-n-butylammonium hydrogen sulphate) and negatively-charged (octanesulphonate sodium) pairing ions, while detection was performed fluorometrically. Excitation and emission wavelengths were 290 and 320 nm, respectively. Limits of quantification were defined at 39 ng/ml for ABZ-SO, 4.95 ng/ml for ABZ-SO2 and 4 ng/ml for ABZ-SO2NH2. Accuracy data, in terms of recovery efficiency showed overall values (+/- S.E.M.) of 85.6 +/- 1.0% for ABZ-SO, 100.0 +/- 1.0% for ABZ-SO2 and 89.1 +/- 0.6% for ABZ-SO2NH2. The method was successfully applied to quantitatively determine the three albendazole metabolites in plasma samples collected from sheep that had been orally administered albendazole.

The effect of solubilization on the oral bioavailability of three benzimidazole carbamate drugs

The effect of solubilization by complexation with povidone on the oral bioavailability of three anthelmintic benzimidazole carbamate drugs: mebendazole (MBZ), albendazole (ABZ) and ricobendazole (RBZ), was studied in mice. The following in vitro characteristics of the initial raw materials and the drug-povidone complexes were evaluated: melting point (MP); mean dissolution time (MDT); solubility constants (Cs) in n-octanol, acid (pH 1.2) and neutral (pH 7.4) aqueous media; apparent partition coefficients (P) and capacity factors (k'W) determined by HPLC. The following in vivo parameters were also evaluated: AUC(0-infinity), C(max), T(max) and MRT. The possible relationship between in vitro characteristics and in vivo parameters was explored and it was found that an increase in solubility, especially in acidic medium, leads to an increase in AUC and C(max) and a decrease in T(max). Therefore, dissolution seems to be the absorption limiting step for these drugs. For the in vivo parameters related to the amount of absorbed drug (AUC and C(max)), the best correlation was obtained with the in vitro characteristics related to solubility which are Cs, MP and MDT. On the other hand, there were good linear correlations between T(max) which is an in vivo parameter related to the rate of drug absorption, and the lipophilia/hydrophilia (logP and log k'W) relation-parameters.

Depletion rates of injected and ingested ivermectin from blood serum of penned white-tailed deer, Odocoileus virginianus (Zimmermann) (Artiodactyla: Cervidae)

Penned female and male white-tailed deer, Odocoileus virginianus (Zimmerman), were administered ivermectin both by direct subcutaneous injection and by ingestion of ivermectin-medicated whole kernel corn. Depletion rates of ivermectin were determined by biweekly and weekly assays of blood serum. No statistical differences were observed between mean peak ivermectin serum concentrations in deer (data of sexes combined) from injection and ingestion studies, and ivermectin concentrations decreased to below detectable within 21 d after injection and 14 d after ingestion.

Albendazole sulphoxide concentrations in plasma of endemic normals from a lymphatic filariasis endemic region using liquid chromatography

A simple and sensitive reversed-phase isocratic HPLC method for the determination of albendazole and its metabolites has been developed. The mobile phase consisting of acetonitrile-water-perchloric acid (70%) (30:110:0.06 (v/v/v)) was pumped at a flow rate of 0.80 ml/min on a 5 microm, reverse phase, Discovery RPamide C16 column with UV detection at 290 nm. The calibration graphs were linear in the range of 0.05- 1 microg/ml for albendazole, albendazole sulphoxide and albendazole sulphone. The limit of quantification was 50 ng/ml for albendazole, 25 ng/ml for albendazole sulphoxide and 30 ng/ml for albendazole sulphone. The within-day and day-to-day coefficient of variation averaged 4.98 and 6.95% for albendazole, 3.83 and 6.83% for albendazole sulphoxide and 3.44 and 5.51% for albendazole sulphone, respectively. The mean extraction recoveries of albendazole, albendazole sulphoxide and albendazole sulphone were 79.25, 93.03 and 88.78%, respectively. The method was applied to determine the plasma levels of albendazole sulphoxide in endemic normals administered with albendazole during pharmacokinetic studies.

A detailed assessment of the pattern of moxidectin tissue distribution after pour-on treatment in calves

The use of topical (pour-on) administration of endectocide drugs in cattle has reached world-wide acceptance. However, only limited information is available on the kinetic behaviour for topically administered moxidectin (MXD). To improve our understanding of the relationship between pharmacokinetics and efficacy for pour-on preparations, MXD concentration profiles were measured in tissues of endo- and ectoparasites location over 35 days postadministration. MXD distribution to the fluid content and mucosal tissue of the abomasum and different intestinal sections (duodenum, ileum, caecum and colon) was assessed. The comparative patterns of MXD distribution to skin and hypodermic tissue from different anatomical sites (backline, rib cage, thigh and face) were also investigated following the pour-on administration. Wide tissue distribution and long residence time characterized the kinetics of topically administered MXD. MXD was recovered between 1 and 35 days post-treatment in all the tissues investigated. The highest MXD availabilities were observed in the skin layers at the site of administration (backline) and in the fat tissue. The fluid contents of different intestinal sections showed MXD concentrations higher than those measured in their respective mucosal tissues, particularly at day 1 post-treatment. MXD concentrations in the skin (epidermis + dermis) were higher than those measured in the hypodermic tissue. Large differences in the availability of MXD in skin from different anatomical regions (backline > rib cage > thigh > face) were observed. The low plasma and the high skin availability indicate the formation of a skin depot of the drug, being released slowly to the plasma and reaching concentrations in systemic tissues (abomasal mucosa, lungs, etc.) similar to those measured after subcutaneous administration. These findings demonstrate that target parasites may be exposed to markedly different drug concentrations according to their location sites, which is particularly relevant for ectoparasites located in different anatomical regions. Knowledge of the tissue distribution of topically administered endectocides contributes to understand the differences observed in efficacy and/or persistence of activity and to optimize their use in cattle.