Combined moxidectin-levamisole treatment against multidrug-resistant gastrointestinal nematodes: A four-year efficacy monitoring in lambs

Nematicide combinations may be a valid strategy to achieve effective nematode control in the presence of drug resistance. The goal of the current trial was to evaluate the pharmaco-parasitological performance of the moxidectin (MOX) and levamisole (LEV) combination after four years of continuous use in lambs naturally parasitized with multi-resistant gastrointestinal nematodes. At the beginning of the trial, 40 lambs were divided into four groups (n = 10), which were untreated (control) or subcutaneously treated with MOX (0.2 mg/kg), LEV (8 mg/kg) or with the combination MOX + LEV (administered separately at 0.2 and 8 mg/kg, respectively). Blood samples were collected at different times post-treatment and LEV and MOX plasma concentrations were measured by HPLC. The clinical efficacy of the continuous use of MOX + LEV combination was assessed with the controlled efficacy test (CET), performed at the beginning and end of the study, and with the faecal egg count reduction (FECR) test, performed over the four-year study period. No significant adverse pharmacokinetic changes were observed either for MOX or LEV after their co-administration to infected lambs. The CET (first year) showed efficacies of 84.3 % (Haemonchus contortus), 100 % (Teladorsagia circumcincta and Trichostrongylus axei), and 97.4 % (T. colubriformis). After the repetitive use of the combined treatment for four years, those efficacies remained high (100 %) and only decreased to 58 % against T. colubriformis. The evaluation of the FECR over the study period showed fluctuations in the performance of the combined administration. The initial FECR (2014) was 99 % (MOX), 85 % (LEV) and 100 % (MOX + LEV). The co-administration of MOX + LEV during the four-year experimental period resulted in a significantly higher anthelmintic effect (87 %) than that of MOX (42 %) or LEV (69 %) given alone. The combined use of MOX + LEV to control resistant gastrointestinal nematodes appears to be a valid strategy under specific management conditions. A high initial therapeutic response to the combination would be a relevant feature for the success of this tool.

Pharmacokinetic-pharmacodynamic assessment of the ivermectin and abamectin nematodicidal interaction in cattle

In a context of nematodicidal resistance, anthelmintic combinations have emerged as a reliable pharmacological strategy to control gastrointestinal nematodes in grazing systems of livestock production. The current work evaluated the potential drug-drug interactions following the coadministration of two macrocyclic lactones (ML) ivermectin (IVM) and abamectin (ABM) to parasitized cattle using a pharmacokinetic/pharmacodynamic (PK/PD) approach. The kinetic behavior of both compounds administered either separately or coadministered was assessed and the therapeutic response of the combination was evaluated under different resistance scenarios. In the pharmacological trial, calves received a single subcutaneous (s.c.) injection of IVM (100 μg/Kg); a single s.c. injection of ABM (100 μg/Kg) or IVM + ABM (50 μg/Kg each) administered in different injection sites to reach a final ML dose of 100 μg/Kg (Farm 1). Plasma samples were taken from those animals up to 20 days post-treatment. IVM and ABM plasma concentrations were quantified by HPLC. A parasitological trial was carried out in three farms with different status of nematodes resistance to IVM. Experimental animals received IVM (200 μg/Kg), ABM (200 μg/Kg) or IVM + ABM (100 μg/Kg each) in Farm 2, and IVM + ABM (200 μg/Kg each) in Farms 3 and 4. The anthelmintic efficacy was determined by fecal egg count reduction test (FECRT). PK analysis showed similar trends for IVM kinetic behavior after coadministration with ABM. Conversely, the ABM elimination half-life was prolonged and the systemic exposure during the elimination phase was increased in the presence of IVM. Although IVM alone failed to control Cooperia spp., the combination IVM + ABM was the only treatment that achieved an efficacy higher than 95% against resistant Cooperia spp. in all farms. In fact, when Cooperia spp. was the main genus within the nematode population and Haemonchus spp. was susceptible or slightly resistant to ML (Farms 2 and 4), the total FECR for the combination IVM + ABM was higher than 90%. Instead, when the predominant nematode genus was a highly resistant Haemonchus spp. (Farm 3), the total FECR after the combined treatment was as low as the single treatments. Therefore, the rational use of these pharmacological tools should be mainly based on the knowledge of the epidemiology and the nematode susceptibility status in each cattle farm.

Pharmacokinetic assessment of the monepantel plus oxfendazole combined administration in dairy cows

Monepantel (MNP) is a novel anthelmintic compound launched into the veterinary pharmaceutical market. MNP is not licenced for use in dairy animals due to the prolonged elimination of its metabolite monepantel sulphone (MNPSO₂ ) into milk. The goal of this study was to evaluate the presence of potential in vivo drug-drug interactions affecting the pattern of milk excretion after the coadministration of the anthelmintics MNP and oxfendazole (OFZ) to lactating dairy cows. The concentrations of both parent drugs and their metabolites were measured in plasma and milk samples by HPLC. MNPSO₂ was the main metabolite recovered from plasma and milk after oral administration of MNP. A high distribution of MNPSO₂ into milk was observed. The milk-to-plasma ratio (M/P ratio) for this metabolite was equal to 6.75. Conversely, the M/P ratio of OFZ was 1.26. Plasma concentration profiles of MNP and MNPSO₂ were not modified in the presence of OFZ. The pattern of MNPSO₂ excretion into milk was also unchanged in animals receiving MNP plus OFZ. The percentage of the total administered dose recovered from milk was 0.09 ± 0.04% (MNP) and 2.79 ± 1.54% (MNPSO₂ ) after the administration of MNP alone and 0.06 ± 0.04% (MNP) and 2.34 ± 1.38% (MNPSO₂ ) after the combined treatment. The presence of MNP did not alter the plasma and milk disposition kinetics of OFZ. The concentrations of the metabolite fenbendazole sulphone tended to be slightly higher in the coadministered group. Although from a pharmacodynamic point of view the coadministration of MNP and OFZ may be a useful tool, the presence of OFZ did not modify the in vivo pharmacokinetic behaviour of MNP and therefore did not result in reduced milk concentrations of MNPSO₂ .

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.

Influence of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 on expression of P-glycoprotein and cytochrome P450 3A in sheep

In order to improve calcium and phosphorus balance, beef cattle and dairy cows can be supplemented with vitamin D. However, different vitamin D metabolites have been shown to increase expression of P-glycoprotein (P-gp, MDR1, ABCB1) and cytochrome P450 3A (CYP3A) in rodents as well as in cell culture systems. As such interferences might have an impact on pharmacokinetics of some drugs widely-used in veterinary medicine, we investigated the expression of P-gp, CYP3A, vitamin D receptor (VDR), pregnane X receptor (PXR) and retinoid X receptor α (RXRα) in sheep either treated orally with 6μg/kg body weight (BW) 25-hydroxyvitamin D₃ (OHD₃) for ten days before sacrifice or 12h after intravenous injection of 0.5μg/kg BW 1,25-dihydroxyvitamin D₃ (1,25- (OH)₂D₃). Down-regulation of ruminal, jejunal and hepatic, but not renal P-gp could be found with 25-OHD₃ supplementation. Interestingly, this effect on P-gp was not observed in tissues from 1,25-(OH)₂D₃-treated sheep. In contrast, 1,25-(OH)₂D₃ induced a significant up-regulation of renal and jejunal CYP3A expression, while 25-OHD₃ had no impact. Renal expression of VDR and PXR was also increased by treatment with 1,25-(OH)₂D₃, while jejunal PXR expression was only stimulated in sheep supplemented with 25-OHD₃. Either treatments increased renal, but not ruminal, jejunal or hepatic expression of RXRα. These results demonstrate that the impact of large doses of vitamin D metabolites on different target organs and potential interactions with other medications should be further investigated in vitro and in vivo to understand the effects of vitamin D metabolites on metabolism and excretion pathways in livestock.

Comparative pharmacokinetic and pharmacodynamic response of single and double intraruminal doses of ivermectin and moxidectin in nematode-infected lambs

AIMS

To compare the pharmacokinetics, distribution and efficacy (pharmacodynamic response) of intraruminal ivermectin (IVM) and moxidectin (MXD) administered at 0.2 and 0.4 mg/kg to naturally nematode-infected lambs, and to determine the ex vivo accumulation of these anthelmintics by Haemonchus contortus.

METHODS

Romney Marsh lambs, naturally infected with IVM-resistant H. contortus, were allocated to treatment groups based on faecal nematode egg counts. They received 0.2 or 0.4 mg/kg IVM or MXD (n=10 per group), or no treatment (Control; n=6), on Day 0. Samples from four animals from each treatment group, including abomasal parasites, were obtained on Day 1. Plasma samples were also collected from Day 0 to 14, and a faecal egg count reduction test (FECRT) and a controlled efficacy trial were carried out on Day 14. Concentrations of IVM and MXD in plasma, in abomasal and intestinal tissues and in H. contortus were evaluated by high-performance liquid chromatography. Additionally, the ex vivo drug accumulation of IVM and MXD by H. contortus was determined.

RESULTS

Peak plasma concentrations and the area under the concentration vs. time curve for both IVM and MXD were higher for 0.4 than 0.2 mg/kg treatments (p<0.05), but there were no differences for other parameters. Concentrations of IVM and MXD in the gastrointestinal target tissues and in H. contortus were higher compared to those measured in plasma. Concentrations of both drugs in H. contortus were correlated with those observed in the abomasal content (r=0.86; p<0.0001). The exposure of H. contortus to IVM and MXD was related to the administered dose. Mean FECRT and efficacy for removal of adult H. contortus was 0% for IVM at 0.2 and 0.4 mg/kg. For MXD, FECRT were >95% for both treatments, and efficacy against H. contortus was 85.1% and 98.1% for 0.2 and 0.4 mg/kg, respectively. The ex vivo accumulation of IVM and MXD in H. contortus was directly related to the drug concentration present in the environment and was influenced by the duration of exposure.

CONCLUSION

Administration of IVM and MXD at 0.4 compared with 0.2 mg/kg accounted for enhanced drug exposure in the target tissues, as well as higher drug concentrations within resistant nematodes. The current work is a further contribution to the evaluation of the relationship between drug efficacy and basic pharmacological issues in the presence of resistant parasite populations.

Intestinal drug transport: ex vivo evaluation of the interactions between ABC transporters and anthelmintic molecules

The family of ATP-binding cassette (ABC) transporters is composed of several transmembrane proteins that are involved in the efflux of a large number of drugs including ivermectin, a macrocyclic lactone (ML) endectocide, widely used in human and livestock antiparasitic therapy. The aim of the work reported here was to assess the interaction between three different anthelmintic drugs with substrates of the P-glycoprotein (P-gp) and the breast cancer resistance protein (BCRP). The ability of ivermectin (IVM), moxidectin (MOX) and closantel (CST) to modulate the intestinal transport of both rhodamine 123 (Rho 123), a P-gp substrate, and danofloxacin (DFX), a BCRP substrate, across rat ileum was studied by performing the Ussing chamber technique. Compared to the controls, Rho 123 efflux was significantly reduced by IVM (69%), CST (51%) and the positive control PSC833 (65%), whereas no significant differences were observed in the presence of MOX (30%). In addition, DFX efflux was reduced between 59% and 72% by all the assayed drug molecules, showing a higher potency than that observed in the presence of the specific BCRP inhibitor pantoprazole (PTZ) (52%). An ex vivo intestinal transport approach based on the diffusion chambers technique may offer a complementary tool to study potential drug interactions with efflux transporters such as P-gp and BCRP.

Accumulation of monepantel and its sulphone derivative in tissues of nematode location in sheep: pharmacokinetic support to its excellent nematodicidal activity

The amino-acetonitrile derivatives (AADs) are a new class of anthelmintic molecules active against a wide range of sheep gastrointestinal (GI) nematodes including those that are resistant to other anthelmintic families. The plasma disposition of monepantel (MNP) has been previously characterized in sheep. However, information on drug concentration profiles attained at tissues of parasite location is necessary to fully understand the pharmacological action of this novel compound. The current work aimed to study the relationship between the concentrations of MNP parent drug and its main metabolite monepantel sulphone (MNPSO₂), measured in the bloodstream and in different GI tissues of parasite location in sheep. Twenty two (22) uninfected healthy Romney Marsh lambs received MNP (Zolvix, Novartis Animal Health) orally administered at 2.5 mg/kg. Blood samples were collected from six animals between 0 and 14 days post-treatment to characterize the drug/metabolite plasma disposition kinetics. Additionally, 16 lambs were sacrificed at 8, 24, 48 and 96 h post-administration to assess the drug concentrations in the GI fluid contents and tissues. MNP and MNPSO₂ concentrations were determined by HPLC. MNP parent compound was rapidly oxidized into MNPSO₂. MNP systemic availability was significantly lower than that observed for MNPSO₂. The peak plasma concentrations were 15.1 (MNP) and 61.4 ng/ml (MNPSO₂). The MNPSO₂ to MNP plasma concentration profile ratio (values expressed in AUC) reached a value of 12. Markedly higher concentrations of MNP and MNPSO₂ were measured in both abomasal and duodenal fluid contents, and mucosal tissues compared to those recovered from the bloodstream. A great MNP availability was measured in the abomasal content with concentration values ranging between 2000 and 4000 ng/g during the first 48 h post-treatment. Interestingly, the metabolite MNPSO₂ was also recovered in abomasal content but its concentrations were significantly lower compared to MNP. The parent drug and its sulphone metabolite were detected in the different segments of the sheep intestine. MNPSO₂ concentrations in the different intestine sections sampled were significantly higher compared to those measured in the abomasum. Although MNP is metabolized to MNPSO₂ in the liver, the large concentrations of both anthelmintically active molecules recovered during the first 48 h post-treatment from the abomasum and small intestine may greatly contribute to the well-established pharmacological activity of MNP against GI nematodes.

A pharmacology-based comparison of the activity of albendazole and flubendazole against Echinococcus granulosus metacestode in sheep

Cyst echinococcosis (CE) is a zoonotic disease caused by the larval stage of the Echinococcus granulosus helminth parasite. The work reported here aimed to compare the efficacy of albendazole (ABZ) and flubendazole (FLBZ) against CE in naturally infected sheep. Additionally, their comparative pharmacokinetic behaviour and the assessment of serum liver enzymes activities were studied. Twelve (12) naturally infected sheep were allocated to the following experimental groups: unmedicated control group, FLBZ-treated and ABZ-treated. Treatments were orally performed every 48 h, over 55 days at dose rate of 10 (FLBZ) and 8.5 (ABZ) mg/kg (equimolar dose rates). The efficacy of the drug treatments was based on protoscoleces' vitality/viability. The kinetic disposition assessment included the Initial and Final Kinetic Studies which implicated the collection of blood samples after both the first and the last drug administration. Blood samples were processed to measure drug concentrations by HPLC. The protoscoleces' vitality observed in the untreated control group (98%) was significantly reduced in the presence of both ABZ and FLBZ. 90% of mice inoculated with protoscoleces in the control group developed hydatid cysts in their peritoneal cavity (viability study). However, only 25% (FLBZ) and 33% (ABZ) of mice inoculated with protoscoleces recovered from treated sheep, developed hydatid cysts in their abdominal cavity. Reduced FLBZ (R-FLBZ) was the main metabolite recovered in the bloodstream after oral administration of FLBZ to sheep. Low plasma concentrations of FLBZ parent drug were measured up to 48 h post-administration. ABZ was not detected in plasma at any time post-treatment, being its metabolites ABZ sulphoxide (ABZSO) and ABZ sulphone (ABZSO₂) recovered in plasma. Hepatotoxicity due to the continued treatment with either ABZ or FLBZ was not observed. A 3-fold increase ethoxyresorufin O-deethylase activity, a cytochrome P450 1A (CYP1A)-dependent enzyme reaction, was observed in liver microsomes obtained from sheep receiving ABZ, compared to those of the unmedicated and FLBZ-treated animals. In conclusion, FLBZ is an available anthelmintic which may be developed into an effective and safe drug for the human CE treatment. Despite the low plasma concentrations measured by FLBZ/R-FLBZ, an important reduction in protoscoleces' vitality was observed in cysts located in sheep liver. Modern pharmaceutical technology may help to greatly improve FLBZ systemic exposure improving its efficacy against CE.

Effects of sub-lethal exposure of rats to the herbicide glyphosate in drinking water: glutathione transferase enzyme activities, levels of reduced glutathione and lipid peroxidation in liver, kidneys and small intestine

Glyphosate (GLP), the active ingredient of many weed killing formulations, is a broad spectrum herbicide compound. Wistar rats were exposed during 30 or 90 days to the highest level (0.7 mg/L) of GLP allowed in water for human consumption (US EPA, 2011) and a 10-fold higher concentration (7 mg/L). The low levels of exposure to the herbicide did not produce histomorphological changes. The production of TBARS was similar or tended to be lower compared to control animals not exposed to the herbicide. In rats exposed to GLP, increased levels of reduced glutathione (GSH) and enhanced glutathione peroxidase (GPx) activity may act as a protective mechanism against possible detrimental effects of the herbicide. Overall, this work showed certain biochemical modifications, even at 3-20-fold lower doses of GLP than the oral reference dose of 2mg/kg/day (US EPA, 1993). The toxicological significance of these findings remains to be clarified.

Cytochrome P450 3A expression and function in liver and intestinal mucosa from dexamethasone-treated sheep

The effects of repeated administrations of dexamethasone (DEX) (3 mg/kg/day by i.m. route for 7 days) on the gene expression profile of a cytochrome P450 (CYP) 3A28-like isoenzyme, on the expression of a CYP3A-immunoreactive protein and on CYP3A-dependent metabolic activities in sheep liver and small intestinal mucosa were evaluated in the current work. CYP 3A-dependent metabolic activities (erythromycin and triacetyl-oleandomycin N-demethylations) were assessed in microsomal fractions. The mRNA expression of CYP3A28-like, glucocorticoid receptor, constitutive androstane receptor, pregnane X receptor and retinoic X receptor alpha (RXRα) was determined by quantitative real-time PCR. The expression of a CYP3A-immunoreactive protein was measured by Western blot analyses. In the liver, DEX treatment increased CYP3A28-like mRNA levels (2.67-fold, P<0.01) and CYP3A apoprotein expression (1.34-fold, P<0.05) and stimulated CYP3A-dependent metabolism. High and significant correlation coefficients between CYP3A-dependent activities and CYP3A28-like gene (r=0.835-0.856, P<0.01) or protein (r=0.728-0.855, P<0.05) expression profiles were observed. Among the transcriptional factors, DEX only stimulated (2.1-fold, P<0.01) the mRNA expression of RXRα. In sheep small intestine, DEX caused a slight increment (34.6%, P<0.05) in erythromycin N-demethylase activity in the jejunal mucosa and a significant enhancement (P<0.05) of CYP3A apoprotein level in the duodenal mucosa.

Pretreatment with the inducers rifampicin and phenobarbital alters ivermectin gastrointestinal disposition

The goal of the study was to evaluate the effects of rifampicin (RFP) and phenobarbital (PBT) on the plasma and gastrointestinal disposition kinetics of ivermectin (IVM) subcutaneously administered to Wistar rats. Fifty seven rats were used. Animals in Group I were the noninduced (control) group. Those in Groups II and III received a treatment with RFP (160 mg/day) and PBT (35 mg/day), respectively, both given orally during eight consecutive days as induction regimen. The IVM pharmacokinetic study was started 24 h after the RFP and PBT last administration. Animals received IVM (200 microg/kg) by subcutaneous injection. Rats were sacrificed between 6 h and 3 days after IVM administration. Blood and samples of liver tissue, intestinal wall and luminal content of jejunum were collected from each animal. IVM concentrations were measured by high performance liquid chromatography. IVM disposition kinetics in plasma and tissues was significantly modified by the PBT treatment, but not by RFP. Despite the enhanced CYP3A activity observed after the pretreatment with RPF and PBT, there were no marked changes on the percentages of IVM metabolites recovered from the bloodstream in induced and noninduced animals. An enhanced P-glycoprotein-mediated intestinal transport activity in pretreated animals (particularly in PBT pretreated rats) may explain the drastic changes observed on IVM disposition.

Interference with P-glycoprotein improves ivermectin activity against adult resistant nematodes in sheep

The in vivo co-administration of ivermectin (IVM) with P-glycoprotein (P-gp) modulator agents has been shown to enhance its systemic availability. However, there is no sufficient evidence on the impact that this type of drug-drug interaction may have on the in vivo efficacy against resistant nematodes in ruminant species. The current work reports on the effects of loperamide (LPM), a P-gp modulating agent, on both IVM kinetic behaviour and anthelmintic activity in infected lambs. Eighteen (18) lambs naturally infected with IVM-resistant gastrointestinal nematodes were allocated into three (3) experimental groups. Group A remained as untreated control. Animals in Groups B and C received IVM (200mug/kg, subcutaneously) either alone or co-administered with LPM (0.2 mg/kg, twice every 12h), respectively. Individual faecal samples were collected from experimental animals at days -1 and 14 post-treatment to perform the faecal eggs count reduction test (FECRT). Blood samples were collected between 0 and 14 days post-treatment and IVM plasma concentrations were determined by HPLC. Additionally, at day 14 post-treatment, lambs from all experimental groups were sacrificed and adult gastrointestinal nematode counts were performed. FECRT values increased from 78.6 (IVM alone) to 96% (IVM+LPM). Haemonchus contortus was highly resistant to IVM. The IVM alone treatment was completely ineffective (0% efficacy) against adult H. contortus. This efficacy value increased up to 72.5% in the presence of LPM. The efficacy against Trichostrongylus colubriformis increased from 77.9% (IVM alone) to 96.3% (IVM+LPM). The described favorable tendency towards improved anthelmintic efficacy was in agreement with the enhanced IVM plasma availability (P<0.05) and prolonged elimination half-life (P<0.05) induced by LPM in infected lambs. A LPM-induced P-gp modulation increases IVM systemic exposure in the host but also it may reduce P-gp efflux transport over-expressed in target resistant nematodes.

Sex-related differences in the gastrointestinal disposition of ivermectin in the rat: P-glycoprotein involvement and itraconazole modulation

Ivermectin (IVM), a macrocyclic lactone used as antiparasite agent, has been reported as a P-glycoprotein (P-gp) substrate. The participation of P-gp in the IVM excretion process has been previously demonstrated. Sex-related differences in the kinetic behaviour of some macrocyclic lactone compounds have been observed. The aim of this work was to characterize in-vivo the comparative gastrointestinal disposition of IVM in male and female rats. The sex-related influence on the itraconazole (ITZ) modulation of P-gp-mediated IVM intestinal transport was also assessed. Sixty Wistar rats (30 male, 30 female) received IVM alone or co-administered with ITZ. Rats were killed between 6 and 72h after treatment and blood, gastrointestinal tissues and lumen contents were collected. IVM concentrations were determined by high performance liquid chromatography. Substantial sex-related differences in the IVM disposition kinetics were observed. Higher IVM systemic availability was observed in female rats. The ITZ-mediated modulation of the IVM disposition kinetics had a differential impact between male and female rats. Co-administration with ITZ resulted in a marked increase in the IVM concentrations in the wall tissue from different portions of the gastrointestinal tract of male rats. The presence of ITZ induced drastic sex-related changes on the P-gp-mediated IVM gastrointestinal disposition.

Inhibition of cytochrome P450 activity enhances the systemic availability of triclabendazole metabolites in sheep

Understanding the disposition kinetics and the pattern of metabolism is critical to optimise the flukicidal activity of triclabendazole (TCBZ) in ruminants. TCBZ is metabolised by both flavin-monooxygenase (FMO) and cytochrome P450 (P450) in the liver. Interference with these metabolic pathways may be useful to increase the systemic availabilities of TCBZ metabolites, which may improve the efficacy against Fasciola hepatica. The plasma disposition of TCBZ metabolites was evaluated following TCBZ co-administration with FMO [methimazole (MTZ)] and P450 [piperonyl butoxyde (PB) and ketoconazole (KTZ)] inhibitors in sheep. Twenty (20) healthy Corriedale x Merino weaned female lambs were randomly allocated into four experimental groups. Animals of each group were treated as follow: Group A, TCBZ alone (5 mg/kg, IV route); Group B, TCBZ (5 mg/kg, IV) + MTZ (3 mg/kg, IV); Group C, TCBZ (5 mg/kg, IV) + PB (30 mg/kg, IV) and Group D, TCBZ (5 mg/kg, IV) + KTZ (10 mg/kg, orally). Blood samples were taken over 240 h post-treatment and analysed by HPLC. TCBZ sulphoxide and sulphone were the main metabolites recovered in plasma. MTZ did not affect TCBZ disposition kinetics. TCBZ sulphoxide Cmax values were significantly increased (P < 0.05) after the TCBZ + PB (62%) and TCBZ + KTZ (37%) treatments compared to those measured in the TCBZ alone treatment. TCBZ sulphoxide plasma AUCs were higher (P < 0.05) in the presence of both PB (99%) and KTZ (41%). Inhibition of TCBZ P450-mediated oxidation in the liver accounted for the increased systemic availability of its active metabolite TCBZ sulphoxide. This work contributes to the search of different strategies to improve the use of this flukicidal drug in ruminants.

Involvement of P-glycoprotein on ivermectin kinetic behaviour in sheep: itraconazole-mediated changes on gastrointestinal disposition

Different pharmacological approaches have been used in an attempt to increase the systemic availability of anthelmintic drugs. The comparative effect of the itraconazole (ITZ)-mediated modulation of P-glycoprotein (P-gp) activity on the in vivo kinetic behaviour of ivermectin (IVM) administered by the intravenous (i.v.) and intraruminal (i.r.) routes to sheep was assessed in the current work. Corriedale sheep received IVM (50 microg/kg) by the i.v. route either alone (group A) or co-administered with the P-gp modulator ITZ (100 mg orally three times every 12 h) (group B). Animals in groups C and D were intraruminally treated with IVM (50 microg/kg) alone or co-administered with ITZ (100 mg orally three times every 12 h) respectively. Jugular blood and gastrointestinal tissue samples (animals treated by the i.r. route) were collected. The samples were analysed by HPLC using fluorescence detection. The plasma disposition of IVM given intravenously was unaffected by the presence of ITZ. However, after the i.r. treatment the co-administration with ITZ resulted in markedly higher IVM plasma concentration profiles compared to the control group. Likewise, the presence of ITZ enhanced the IVM concentration profiles measured in the gastrointestinal mucosal tissues. An ITZ-induced reduction on the P-gp efflux activity at the intestinal lining may have accounted for the greater absorption and enhanced systemic availability observed for IVM in the intraruminally treated animals.

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.

Understanding triclabendazole resistance

Triclabendazole (TCBZ) has been the drug of choice to treat liver fluke infections in livestock for >20 years, due to its high activity against both adult and juvenile flukes. More recently, it has been used successfully to treat human cases of fascioliasis. Resistance to TCBZ first appeared in the field in Australia in the mid-1990s. Since then, resistance has been reported from a number of countries throughout Europe: Ireland, Scotland, Wales, Spain and The Netherlands. The heavy reliance on a single drug puts treatment strategies for fascioliasis at risk. Should resistance develop further, the prospect is an alarming one. This review will present an overview of progress in understanding the mechanism of resistance to TCBZ, examining possible changes in the target molecule, in drug influx/efflux mechanisms and in the metabolism of TCBZ by the fluke. The review will also consider ways to deal with resistance, covering drug-oriented options such as: the use of alternative drugs, drug combinations and the search for new compounds.

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.

MODULATION OF THE P-GLYCOPROTEIN-MEDIATED INTESTINAL SECRETION OF IVERMECTIN: IN VITRO AND IN VIVO ASSESSMENTS

The everted gut sac method was used to assess the role of the P-glycoprotein (P-gp) on the intestinal secretion of ivermectin (IVM), an antiparasitic widely used in human and veterinary medicine. The work included the evaluation of two different P-gp modulators [itraconazole (ITZ) and valspodar (PSC833)] used at equimolar doses in the rat. Furthermore, the influence of both P-gp modulator agents on the disposition kinetics of IVM in plasma, liver, and gastrointestinal tissues was characterized. For the in vitro experiments, ileal sacs were incubated with IVM (3 microM) in the presence or absence of either ITZ (10 microM) or PSC833 (10 microM). In the in vivo experiments, male Wistar rats were randomly allocated to three groups (n=18) and subcutaneously treated with IVM (200 microg/kg-1), alone and coadministered with ITZ (5 mg, two doses) or PSC833 (8.6 mg, two doses). Animals were sacrificed between 6 and 96 h. Blood, liver, and gastrointestinal samples were collected. IVM concentrations were determined by high performance liquid chromatography. The rate of IVM accumulation in the intestinal wall of everted sacs was significantly higher after its incubation with ITZ (0.115 nmol/g/min) and PSC833 (0.238 nmol/g/min) than that obtained after the incubation without the P-gp modulators (0.016 nmol/g/min). In agreement with the in vitro experiment, the presence of ITZ and PSC833 induced an enhancement in the concentrations of IVM in plasma and gastrointestinal tissues. The results obtained in the current work, both under in vivo and in vitro conditions, confirm the relevance of P-gp-mediated transport to the intestinal secretion of IVM.

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.

Moxidectin and ivermectin metabolic stability in sheep ruminal and abomasal contents

The oral administration of macrocyclic lactones to sheep leads to poorer efficacy and shorter persistence of the antiparasitic activity compared to the subcutaneous treatment. Gastrointestinal biotransformation occurring after oral treatment to ruminant species has been considered as a possible cause of the differences observed between routes of administration. The current work was addressed to evaluate on a comparative basis the in vitro metabolism of moxidectin (MXD) and ivermectin (IVM) in sheep ruminal and abomasal contents. Both compounds were incubated under anaerobic conditions during 2, 6 and 24 h in ruminal and abomasal contents collected from untreated adult sheep. Drug concentrations were measured by high-performance liquid chromatography with fluorescence detection after sample clean up and solid phase extraction. Neither MXD nor IVM suffered metabolic conversion and/or chemical degradation after 24-h incubation in ruminal and abomasal contents collected from adult sheep. Unchanged MXD and IVM parent compounds represented between 95.5 and 100% of the total drug recovered in the ruminal and abomasal incubation mixtures compared with those measured in inactive control incubations. The partition of both molecules between the solid and fluid phases of both sheep digestive contents was assessed. MXD and IVM were extensively bound (>90%) to the solid material of both ruminal and abomasal contents collected from sheep fed on lucerne hay. The results reported here confirm the extensive degree of association to the solid digestive material and demonstrates a high chemical stability without evident metabolism and/or degradation for both MXD and IVM in ruminal and abomasal contents.

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

Triclabendazole (TCBZ) is an halogenated trematodicidal benzimidazole compound extensively used in veterinary medicine. It is active against immature and adult stages of the liver fluke Fasciola hepatica. Free and conjugated TCBZ metabolites have been identified in the bile of treated sheep. The experimental aims were to characterize the in vitro patterns of TCBZ biotransformation both in the animal host (sheep liver microsomes) and target parasite (F. hepatica microsomal preparation); and to compare the ex vivo diffusion of TCBZ parent drug and its oxidized metabolites (TCBZ sulphoxide [TCBZSO], TCBZ sulphone [TCBZSO2], and TCBZ-hydroxy derivatives) into F. hepatica. Additionally, the octanol-water partition coefficients for TCBZ and all its metabolites were estimated as an indicator of the relationship between drug lipophilicity and diffusion into the target parasite. Drug/metabolites concentrations were quantified by HPLC after sample clean up and a solvent-mediated chemical extraction. Sheep liver microsomes metabolized TCBZ into its sulphoxide and sulphone metabolites after 30 min of incubation. The rate of TCBZ sulphoxidation in the liver was significantly greater (p < 0.01) than that observed for the sulphonation of TCBZSO. The trematode parasite oxidized TCBZ into its sulphoxide metabolite after 60 min of incubation at a metabolic rate of 0.09 nmol min(-1) mg protein(-1). TCBZ and all its oxidized metabolic products were recovered from F. hepatica as early as 15 min after their ex vivo incubation in a Kreb's Ringer Tris buffer. However, the diffusion of the hydroxy-derivatives into the fluke was lower than that observed for TCBZ, TCBZSO and TCBZSO2. There was a high correlation (r=0.82) between drug lipophilicity (expressed as octanol-water partition coefficients) and drug availability measured within the parasite. Unlike the uptake pattern previously observed for albendazole, the parent TCBZ and its sulphoxide and sulphone metabolites showed a similar ability to penetrate into the trematode parasite. Understanding the relationship between TCBZ metabolism, the relative pharmacological potency of its metabolic products and their ability to reach the target parasite may be critical to optimize its flukicidal activity, particularly when TCBZ resistant flukes have been already isolated in the field.

Comparative hepatic and extrahepatic enantioselective sulfoxidation of albendazole and fenbendazole in sheep and cattle

The enantioselective sulfoxidation of the prochiral anthelmintic compounds albendazole (ABZ) and fenbendazole (FBZ) was investigated in liver, lung and small intestinal microsomes obtained from healthy sheep and cattle. The microsomal fractions were incubated with a 40 microM concentration of either ABZ or FBZ. Inhibition of the flavin-containing monooxygenase (FMO) system was carried out by preincubation with 100 microM methimazole (MTZ) either with or without heat pretreatment (2 min at 50 degrees C). ABZ and FBZ were metabolized to the (+) and (-) enantiomers of their sulfoxide metabolites, named albendazole sulfoxide (ABZSO) and oxfendazole (OFZ), respectively. ABZ sulfoxidation rates were higher (p < 0.001) than those observed for FBZ. The FMO-mediated liver sulfoxidation of ABZ was enantioselective (100%) toward the (+) ABZSO production in both species. Liver sulfoxidation of FBZ by FMO was also enantioselective toward (+) OFZ (sheep = 65%; cattle = 79%). Cytochrome P450 was found to be mainly involved in the production of (-) ABZSO in the liver. MTZ did not affect the sulfoxidation of ABZ by lung microsomes, which may indicate that FMO is not involved in the production of ABZSO in this tissue. A significant (p < 0.05) inhibition of (-) ABZSO production by liver microsomes was observed after ABZ incubation in the presence of erythromycin (cattle = 21%) and ketoconazole (sheep = 36%). Both CYP3A substrates induced a reduction in the production of (-) ABZSO (sheep = 67-78%, cattle = 50-78%) by lung microsomes. Overall, the results reported here contribute to the identification of the metabolic pathways involved in the biotransformation of benzimidazole anthelmintics extensively used for parasite control in ruminants.

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.

Effect of the ionophore antibiotic monensin on the ruminal biotransformation of benzimidazole anthelmintics

The benzimidazole (BZD) anthelmintics, netobimin (NTB) pro-drug and albendazole sulphoxide (ABZSO) are reduced to albendazole (ABZ) by ruminal microflora. The aim of the current work was to evaluate the influence of the ionophore monensin (MON) on the in vitro biotransformation of NTB and ABZSO by sheep ruminal fluid. Ruminal fluid, collected from Corriedale sheep, was preincubated (24 h) either without (control) or with known MON concentrations (0.5, 1.5 and 3.0 microg/mL) at 38 degrees C under a CO2 atmosphere. Afterwards, aliquots from both MON-pretreated and control ruminal fluid samples were incubated (30 and 60 min) with 2 microg/mL of either NTB or ABZSO. Incubated samples were chemically extracted and analysed by High Performance Liquid Chromatography to quantify the metabolites formed. The rate of ABZ production after 30 min of NTB incubation with control ruminal fluid was 0.023 microg/min. Conversely, the rates of ABZ formation were significantly (P<0.05) lower (0.009, 0.011 and 0.013 microg/min) when NTB was incubated with ruminal fluid pretreated with MON (at 0.5, 1.5 and 3.0 microg/mL, respectively). After both incubation periods, the reduction of ABZSO to ABZ was 22 to 70% lower when the ruminal fluid was preincubated with the different MON concentrations. The lower ABZ production observed in the presence of MON may result in a modified availability of this molecule in the gastrointestinal (GI) tract and hence, on its anthelmintic efficacy against GI nematodes.

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.

Breed differences on the plasma availability of moxidectin administered pour-on to calves

The pharmacokinetic profile of avermectin and milbemycin compounds is affected by different drug- and host-related factors. This work reports the influence of cattle breeds on the plasma kinetics of moxidectin (MXD) after topical (pour-on) administration. Parasite-free Aberdeen Angus and Holstein calves were treated with a commercial MXD pour-on formulation at 500 microg/kg. Blood samples were collected over a period of 35 days post-treatment and the recovered plasma was analysed by high performance liquid chromatography using fluorescence detection. MXD was detected in plasma from two hours up to 35 days post-treatment in animals from both breeds. A slow MXD absorption and delayed peak plasma concentration were observed in Aberdeen Angus compared to Holstein calves. Significant lower systemic availability (expressed as AUC) (P<0.01) and peak plasma concentration (C(max)) (P<0.05) were also observed in Aberdeen Angus calves, although the plasma mean residence time (MRT) and elimination half-lives (T(1/2el)) of MXD in both breeds were similar. The pharmacokinetic differences observed between cattle breeds contribute to explain the variability in the pattern of clinical efficacy for pour-on administered endectocide compounds reported in different field trials.

Loperamide-induced enhancement of moxidectin availability in cattle

Moxidectin (MXD) is a milbemycin endectocide compound active at extremely low dosages against a wide variety of nematode and arthropod parasites. Different pharmacological approaches are currently being tested to delay the bile-faecal elimination and to obtain increased systemic availability for endectocide molecules in ruminants. Loperamide (LPM) is an opioid derivative, whose main pharmacological action is to abolish intestinal propulsive peristaltic waves. The influence of LPM on the pattern of faecal excretion of MXD and on its plasma disposition following intravenous (i.v.) and subcutaneous (s.c.) administrations to cattle was evaluated in the current work. Parasite-free calves were treated with MXD given either alone at 200 microg/kg by i.v. (Experiment 1) and s.c. (Experiment 2) administrations or coadministered with LPM subcutaneously injected at 0.4 mg/kg. Blood and faecal samples were collected over a period of 20 (Experiment 1) and 40 (Experiment 2) days post-treatment. The recovered plasma and faecal samples were extracted and analysed by high-performance liquid chromatography (HPLC) using fluorescence detection. Significantly higher MXD plasma concentrations were obtained after the coadministration of MXD + LPM compared with treatments with MXD alone by both routes. The higher MXD plasma concentration profiles measured after the coadministration with LPM accounted for the significantly higher AUC values obtained following the i.v. (> 46%) and s.c. (> 38%) treatments. A reduced MXD body clearance was observed in the presence of LPM. The appearance of MXD in faeces was significantly delayed after the i.v. and s.c. coadministrations of MXD with LPM (T(1/2app)=5.87 and 10.6 h, respectively) than that observed after the treatment with MXD alone (T(1/2app)=3.48 and 5.12 h). A delayed MXD peak concentration in faeces collected from MXD + LPM-treated animals compared with those receiving MXD alone, was observed. The delayed intestinal transit time caused by LPM and a potential competition between MXD and LPM for the P-glycoprotein-mediated bile/intestinal secretion processes, may account for the enhanced MXD systemic availability measured in cattle in the current work.

In vitro ruminal biotransformation of benzimidazole sulphoxide anthelmintics: enantioselective sulphoreduction in sheep and cattle

The comparative in vitro sulphoreduction of the (+) and (-) enantiomers of albendazole sulphoxide (ABZSO) and oxfendazole (OFZ) by ruminal fluid obtained from sheep and cattle, was investigated, under anaerobic conditions, in this study. Ruminal fluid samples were obtained from Holstein steers fitted with a permanent rumen fistula and from Corriedale lambs via an oesophageal tube. Albendazole sulphoxide, incubated as either the racemic (rac) mixture or as each individual enantiomeric form, was extensively sulphoreduced to form albendazole (ABZ) by ruminal fluid from both species. The concentrations of ABZ formed at different incubation times were between 55 and 158% greater after the incubation of cattle ruminal fluid with (+) ABZSO, compared with that produced when (-) ABZSO was the incubated substrate. Similarly, the concentrations of ABZ were 1.3--3.0-fold higher when (+) ABZSO was incubated with sheep ruminal fluid. Significantly higher rates of depletion were observed for the (+) enantiomeric form when ABZSO was incubated with ruminal fluid from both species. The rates of ABZ formation from both ABZSO enantiomeric forms were significantly higher in sheep compared with cattle ruminal fluid. Fenbendazole (FBZ) was the metabolite formed after the incubation of the racemic form of OFZ with ruminal fluid obtained from both species. The metabolic profile of both OFZ enantiomers followed a similar pattern to that observed for ABZSO enantiomers. A bi-directional chiral inversion of one enantiomer into its antipode was observed. The (+) enantiomer appeared in the incubation medium when (-) ABZSO was the incubated substrate, and also the (-) antipode was detected after (+) ABZSO incubation with ruminal fluid obtained from both species. The results reported here demonstrate an enantioselective ruminal sulphoreduction of ABZSO and OFZ (substrate enantioselectivity). These findings contribute to interpret the chiral behaviour of benzimidazole-sulphoxide anthelmintics.

Albendazole sulphoxide enantiomeric ratios in plasma and target tissues after intravenous administration of ricobendazole to cattle

The comparative concentration profiles of the (+) and (-) albendazole sulphoxide (ABZSO) enantiomers obtained in plasma and in selected target tissues/fluids after intravenous (i.v.) administration of a racemic formulation of ricobendazole (RBZ) to cattle were characterised. Fourteen Holstein calves received RBZ (racemic solution, 150 mg/mL) by i.v. administration at 7.5 mg/kg. Jugular blood samples were collected over 48 h post-treatment (plasma kinetic trial) and two animals were sacrificed at either 4, 12, 20, 28 or 32 h post-treatment to obtain samples of abomasal/small intestine mucosal tissue, abomasal/small intestine fluids, bile, liver and lung tissue (tissue distribution study). The (-)ABZSO enantiomer was depleted significantly faster from plasma compared with the (+)ABZSO antipode. The plasma AUC for (+)ABZSO (38.3 microg. h/mL) was significantly higher (P < 0.05) compared with that obtained for (-)ABZSO (20.5 microg. h/mL). The (+)ABZSO enantiomer was the predominant antipode measured in bile, abomasal fluid and abomasal mucosa. For instance, at 12 h post-treatment the (+)/(-) concentration ratios were: 12.9 (plasma), 1.62 (abomasal mucosa), 13.0 (abomasal fluid), 2.92 (intestinal mucosa), 9.87 (intestinal fluid) and 21.5 (bile). No marked differences between the concentration profiles of both enantiomers were observed in the liver tissue. Albendazole (ABZ) was recovered from the liver, lung and gastrointestinal (GI) mucosal tissues of RBZ-treated calves up to 32 h post-treatment, probably produced by a GI microflora-mediated sulphoreduction of RBZ. An enantioselective kinetic behaviour may account both for the faster depletion of the (-) enantiomer and for the higher availabilities of the (+) antipode observed in plasma and in most of the tissues/fluids investigated. The simultaneous evaluation of the plasma kinetics and tissue concentration profiles of both enantiomeric forms reported here, may help to interpret the relationship between chiral behaviour and pharmacological action for sulphoxide derivatives of benzimidazole (BZD) methylcarbamate anthelmintics.

Depletion of moxidectin tissue residues in sheep

The pattern of tissue depletion of moxidectin (MXD) subcutaneously administered to sheep was characterized in this study. MXD concentration profiles were determined in muscle, fat, and liver and at the site of injection following administration of a formulation combining MXD (0.5% w/v) with a standard 6 in 1 clostridial vaccine. Thirty-five (35) parasite-free Lincoln sheep were treated with the MXD injectable formulation at a dose rate of 0.2 mg of MXD/kg of live weight, administered subcutaneously on the inner surface of the thigh. Treated animals were sacrificed in randomly selected groups of six sheep weekly from day 21 until day 49 post-treatment. Three nontreated animals were sacrificed to obtain blank tissue samples to validate the analytical methodology. MXD concentration profiles were determined by a validated HPLC analytical method using fluorescence detection. MXD has an adequate pattern of absorption, based on the low residual concentrations found in the injection site area at all sampling intervals. Muscle samples showed the lowest MXD concentrations throughout the study period. The highest MXD concentrations at all sampling times were measured in the adipose tissue, indicating that fat is a target tissue for MXD. MXD concentrations in all of the tissues analyzed were below the accepted maximum limit of residue at 21 days post-treatment.

Nutritional condition affects the hepatic antioxidant systems in steers

The objectives of this study were to evaluate the activity of antioxidant systems in the hepatic tissues of steers experimentally subjected to a restricted diet. Hence, the activities of superoxide dismutase (SOD) and reduced glutathione (GSH) and lipid peroxide levels were measured. Nine male Holstein steers were used. They were separated in two groups: three steers in group 1 (control) and six steers in group 2, which were subjected to a restricted diet that covered only 60% of the maintenance requirements. After 30 days, the animals in both groups were killed and studies were completed. Both the CuZn-SOD and the glucose-6-phosphate dehydrogenase activities were significantly decreased (p < 0.05) in group 2, reaching 68% and 60%, respectively, of the concentrations found in group 1. The concentration of GSH in group 2 was 6.71+/-0.9 nmol/mg protein, which was significantly lower (p < 0.01) than that of the controls, 25.7+/-2.4 nmol/mg protein. In addition, the lipid peroxide levels were significantly increased (p < 0.01) in group 2, being 50-60% higher than that in group 1. These results showed that the poor nutritional status caused modifications to the enzymatic antioxidant systems, with a lower ability to reduce oxidative compounds and a state of lipid peroxidation.

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.

Enantioselective liver microsomal sulphoxidation of albendazole in cattle: effect of nutritional status

1. The enantioselective liver microsomal sulphoxidation of the benzimidazole anthelmintic, albendazole (ABZ), by cattle liver microsomes has been investigated. The influence of nutritional condition on this biotransformation process was also characterized. 2. ABZ was oxidized to its sulphoxide metabolite (ABZSO) in a NADPH concentration-dependent reaction and the (+) and (-) ABZSO enantiomers formed were identified. 3. Vmax (0.27 nmol ABZSO formed per min x mg(-1) microsomal protein) and Km (15.10 microM) for ABZ sulphoxidation by cattle liver microsomes were obtained. Different Vmax (0.11 and 0.16 nmol x min(-1) x mg(-1)) and Km (9.40 and 26.70 microM) characterized the enantioselective formation of (+) and (-) ABZSO antipodes, respectively. 4. Free fatty acid (FFA) concentrations and beta-hydroxybutyrate concentrations (beta-OHB) in serum and liver homogenates were significantly higher in feed-restricted (poor nutritional condition) compared with control animals in an optimal nutritional status. Serum protein concentrations and liver cytosolic glucose 6-phosphate dehydrogenase (G6PD) activity were significantly lower in the feed-restricted compared with control calf. 5. Animal nutritional condition affected the pattern of ABZ sulphoxidation. A higher Km for (total) ABZSO and (+) ABZSO production was observed in the calf subjected to a period of undernutrition. 6. A nutritionally induced impairment in the affinity of microsomal mixed-function oxidases responsible of ABZ oxidation may be responsible for the observed changes in the liver microsomal sulphoxidation of ABZ in the feed-restricted calf. Furthermore, undernutrition may affect primarily the FMO-mediated formation of (+) ABZSO. These in vitro observations agree with the changes observed in vivo following the administration of ABZ to the calf subjected to a dietary restriction.

Comparative distribution of ivermectin and doramectin to parasite location tissues in cattle

Pharmacokinetic studies have been used traditionally to characterize drug concentration profiles achieved in the bloodstream. However, endectocide molecules exert their persistent and broad spectrum activity against parasites localized in many different tissues. The aim of this study was to compare the distribution of ivermectin (IVM) and doramectin (DRM) to different tissues in which parasites are found following subcutaneous administration to calves. Holstein calves weighing 120-140 kg were injected in the shoulder area with commercially available formulations of IVM (Ivomec 1% MSD AGVET, NJ, USA) (Group A) or DRM (Dectomax 1%, Pfizer, NY, USA) (Group B). Two treated calves were sacrificed at 1, 4, 8, 18, 28, 38, 48 or 58 days post-treatment. Plasma, abomasal and small intestinal fluids and mucosal tissues, bile, faeces, lung and skin samples were collected, extracted, derivatized and analyzed by high performance liquid chromatography (HPLC) with fluorescence detection to determine IVM and DRM concentrations. IVM and DRM were distributed to all the tissues and fluids analyzed. Concentrations >0.1 ng/ml (ng/g) were detected between 1 and 48 days post-treatment in all the tissues and fluids investigated. At 58 days post-treatment, IVM and DRM were detected only in bile and faeces, where large concentrations were excreted. Delayed Tmax values for DRM (4 days post-administration) compared to those for IVM (1 day) were observed in the different tissues and fluids. High IVM and DRM concentrations were measured in the most important target tissues, including skin. The highest IVM and DRM concentrations were measured in abomasal mucosa and lung tissue. Enhanced availabilities of both IVM (between 45 and 244%) and DRM (20-147%) were obtained in tissues compared to plasma. There was good correlation between concentration profiles of both compounds in plasma and target tissues (mucosal tissue, skin, and lung). Drug concentrations in target tissues remained above 1 ng/g for either 18 (IVM) or 38 (DRM) days post-treatment. The characterization of tissue distribution patterns contributes to our understanding of the basis for the broad-spectrum endectocide activity of avermectin-type compounds.

Ivermectin disposition kinetics after subcutaneous and intramuscular administration of an oil-based formulation to cattle

Slight differences in formulation may change the plasma kinetics and ecto-endoparasiticide activity of endectocide compounds. This work reports on the disposition kinetics and plasma availability of ivermectin (IVM) after subcutaneous (SC) and intramuscular (IM) administration as an oil-based formulation to cattle. Parasite-free Aberdeen Angus calves (n = 24; 240-280 kg) were divided into three groups (n = 8) and treated (200 microg/kg) with either an IVM oil-based pharmaceutical preparation (IVM-TEST formulation) (Bayer Argentina S.A.) given by subcutaneous (Group A) and intramuscular (Group B) injections or the IVM-CONTROL (non-aqueous formulation) (Ivomec, MSD Agvet) subcutaneously administered (Group C). Blood samples were taken over 35 days post-treatment and the recovered plasma was extracted and analyzed by HPLC using fluorescence detection. IVM was detected in plasma between 12 h and 35 days post-administration of IVM-TEST (SC and IM injections) and IVM-CONTROL formulations. Prolonged IVM absorption half-life (p < 0.05) and delayed peak plasma concentration (p < 0.001) were obtained following the SC administration of the IVM-TEST compared to the IVM-CONTROL formulation. No differences in total plasma availability were observed among treatments. However, the plasma residence time and elimination half-life of IVM were significantly longer after injection of the IVM-TEST formulation. IVM plasma concentrations were above 0.5 ng/ml for 20.6 (CONTROL) and 27.5 days (IVM-TEST SC), respectively (p < 0.05). The modified kinetic behaviour of IVM obtained after the administration of the novel oil-based formulation examined in this trial, compared to the standard preparation, may positively impact on its strategic use in cattle.

Moxidectin in cattle: correlation between plasma and target tissues disposition

The time of parasite exposure to active drug concentrations determines the persistence of the antiparasitic activity of endectocide compounds. This study evaluates the disposition kinetics of moxidectin (MXD) in plasma and in different target tissues following its subcutaneous (s.c.) administration to cattle. Eighteen male, 10-month old Holstein calves weighing 120-140 kg were subcutaneously injected in the shoulder area with a commercially available formulation of MXD (Cydectin 1%, American Cyanamid, Wayne, NJ, USA) at 200 micrograms/kg. Two treated calves were killed at each of the following times post-treatment: 1, 4, 8, 18, 28, 38, 48, 58 and 68 days. Abomasal and small intestine mucosal tissue and fluids, bile, faeces, lung, skin and plasma samples were collected, extracted, derivatized and analysed to determine MXD concentrations by high performance liquid chromatography (HPLC) with fluorescence detection. MXD was extensively distributed to all tissues and fluids analysed, being detected (concentrations > 0.1 ng/g; ng/mL) between 1 and 58 days post-treatment. MXD peak concentrations were attained during the first sampling day. MXD maximum concentration (Cmax) values ranged from 52.9 (intestinal mucosa) up to 149 ng/g (faeces). The mean residence time (MRT) in the different tissues and fluids ranged from 6.8 (abomasal mucosa) up to 11.3 (bile) days. MXD concentrations in abomasal and intestinal mucosal tissue were higher than those detected in plasma; however, there was a high correlation between MXD concentrations observed in plasma and those detected in both gastrointestinal mucosal tissues. MXD concentrations were markedly greater in the mucosa than in its respective digestive fluid (P < 0.01). MXD concentrations in skin were higher than those found in plasma (P < 0.01). Drug concentrations recovered in the dermis were greater than those detected in the hypodermal tissue (P < 0.05). Large concentrations of MXD were excreted in bile and faeces. These findings may contribute to an understanding of the relationship between the kinetic behaviour and the persistence of the antiparasite activity of MXD against different ecto-endoparasites in cattle.

Persistence of ivermectin in plasma and faeces following administration of a sustained-release bolus to cattle

Six calves (weight 210 to 230 kg) were dosed with an intra-ruminal slow-release bolus prepared to deliver ivermectin at a low daily dosage for 135 days. Ivermectin concentrations in jugular blood 160 days post-treatment were determined by high performance liquid chromatography (HPLC) using fluorescence detection. Ivermectin plasma concentrations increased gradually to achieve the steady-state concentration (20 ng ml(-1)) at approximately four days post-treatment, which was maintained for 120 days. The ivermectin peak plasma concentration (28.5 ng ml(-1)) was attained at 15 days post-administration of the bolus. The faecal ivermectin concentration rose to a maximal concentration of 4.1 microg g(-1) at four days post-treatment, dropping to a steady-state concentration of around 1.18 microg g(-1) which was maintained up to 120 days post-treatment. Ivermectin was detected in both plasma (0.05 ng ml(-1)) and faeces (2.67 ng g(-1)) up to 160 days. The high levels of ivermectin recovered in faeces indicate that a large proportion of the dose released by the bolus (80 to 90 per cent) is excreted in faeces.

Bioequivalence of ivermectin formulations in pigs and cattle

The vehicle in which endectocide compounds are formulated plays a relevant role in their absorption kinetics and resultant systemic availability. The pharmaceutical bioequivalence and comparative plasma disposition kinetics of ivermectin (IVM), following the subcutaneous administration of two injectable formulations to pigs and cattle were investigated using parallel experimental designs. Sixteen parasite-free male Duroc Jersey-Yorkshire crossbred pigs (90-110 kg) (Expt 1) and 16 parasite-free male Holstein calves (100-120 kg) (Expt 2) were divided into two groups and treated subcutaneously at either 300 (pigs) or 200 (calves) microg/kg with two different propylene glycol/glycerol formal (60: 40) based IVM formulations; in both experiments pigs or calves in Group A received the test (IVM-TEST) formulation and those in Group B were treated with the reference formulation (IVM-CONTROL). Heparinized blood samples were taken from 0 h up to either 20 (pigs) or 30 (calves) days post-treatment and plasma was extracted, derivatized and analysed by high performance liquid chromatography (HPLC) using fluorescence detection. Early detection of IVM (12 h) with a peak plasma concentration (C(max)) between 33 and 39 ng/mL was observed in pigs. The drug was detected in plasma up to 20 days post-administration of either formulation, resulting in elimination half-lives between 3.47 and 3.80 days. There were no differences between the IVM-TEST and IVM-CONTROL formulations in the kinetic parameters (except t(max)) obtained in pigs. IVM was detected in plasma between 12 h and 30 days post-administration of both formulations under investigation in cattle. The plasma disposition kinetics of IVM in calves was similar following treatment with both formulations. C(max) values (between 40.5 and 46.4 ng/mL) were achieved at 2 days post-administration of both formulations. None of the estimated kinetic parameters were statistically different between drug formulations. The injectable IVM formulations investigated were bioequivalent after their subcutaneous administration to both pigs and calves at recommended dose rates.