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.

Ivermectin induces P-glycoprotein expression and function through mRNA stabilization in murine hepatocyte cell line.

Ivermectin is widely used in human and veterinary medicine for the control of helminth infections. Ivermectin is known to interact with P-glycoprotein (P-gp/MDR1), being a good substrate and a potent inhibitor, however, the influence of ivermectin on the expression of the transporter has not been investigated. Expression of P-glycoprotein was investigated in cultured mouse hepatocytes acutely exposed to ivermectin. The two P-glycoprotein murine isoforms, Mdr1a and Mdr1b, mRNA levels were assessed by real-time RT-PCR. Ivermectin induced a clear time- and concentration-dependent up-regulation of Mdr1a and Mdr1b mRNA levels (as early as a 12-h exposure and up to 2.5-fold at 10µM). Moreover, ivermectin-treated cells displayed enhanced cellular efflux of the P-glycoprotein substrate calcein that was inhibited by the P-glycoprotein blocker valspodar, providing evidence that the ivermectin-induced P-glycoprotein was functional. The mechanisms underlying these effects were investigated. Ivermectin-mediated Mdr1 mRNA induction was independent of the two nuclear receptors CAR and PXR, which are known to be involved in drug transporters regulation. Moreover, by using reporter cell lines that detects specific ligand-activated transcription factors, we showed that ivermectin did not displayed CAR, PXR or AhR ligand activities. However, studies with actinomycin D revealed that the half-life of Mdr1a and Mdr1b mRNA were significantly prolonged by two-fold in ivermectin-treated cells suggesting a post-transcriptional mode of ivermectin regulation. This study demonstrates for the first time that ivermectin induces P-glycoprotein overexpression through post-transcriptional mRNA stabilization, thus offering insight into the mechanism of reduced therapeutic efficacy and development of ivermectin-resistant parasites.

Licking behaviour induces partial anthelmintic efficacy of ivermectin pour-on formulation in untreated cattle.

Licking behaviour in cattle has been reported to account for the disposition of topically administered macrocyclic lactones. However, its impact on anthelmintic efficacy remains to be established. Therefore, we evaluated the impact of ivermectin exchange between cattle on the reduction in the faecal egg count (FEC) after pour-on administration in a group of 10 heifers experimentally infected with Ostertagia ostertagi and Cooperia oncophora. Four treated (500 µg/kg, pour-on) and six untreated animals were put together after treatment and plasma and faecal exposure to ivermectin as well as the FECs were evaluated before and over 40 days after treatment. Ivermectin was detected in plasma and faeces of the six untreated heifers, with maximal exposures two- to three-fold lower than the minimal exposures in treated animals. The interindividual variability of exposure was very high in untreated animals, with a ten-fold difference between the upper and lower limits compared with treated heifers, where there was only a two-fold difference. Anthelmintic efficacy, expressed as an average reduction of the FECs over the experimental period, was maximal in the treated group. In untreated heifers, anthelmintic efficacies ranged from zero to maximal efficacy, with intermediary values between 30% and 80%. The use of a classical pharmacodynamic model demonstrated a clear relationship between exposure and efficacy and enabled us to define the critical plasma or faecal ivermectin concentrations delimiting an exposure window associated with partial anthelmintic efficacy. This range of ivermectin plasma concentrations (0.1-1 ng/mL) could be considered as a potential selection window for anthelmintic resistance. Finally, our results show that macrocyclic lactone exchange between cattle after pour-on administration, resulting from natural grooming behaviour, can significantly impact on anthelmintic efficacy. This raises several issues such as the design of comparative clinical trials and the occurrence of partial efficacy which is considered a risk factor for the development of anthelmintic resistance.

Interaction of anthelmintic drugs with P-glycoprotein in recombinant LLC-PK1-mdr1a cells.

Given the widespread use of formulations combining anthelmintics which are possible P-glycoprotein interfering agents, the understanding of drug interactions with efflux ABC transporters is of concern for improving anthelmintic control. We determined the ability of 14 anthelmintics from different classes to interact with abcb1a (mdr1a, P-glycoprotein, Pgp) by following the intracellular accumulation of rhodamine 123 (Rho 123), a fluorescent Pgp substrate, in LLC-PK1 cells overexpressing Pgp. The cytotoxicity of the compounds that are able to interfere with Pgp activity was evaluated in cells overexpressing Pgp and compared with parental cells using the MTS viability assay. Among all the anthelmintics used, ivermectin (IVM), triclabendazole (TCZ), triclabendazole sulfoxide (TCZ-SO), closantel (CLOS) and rafoxanide (RAF) increased the intracellular Rho 123 in Pgp overexpressing cells, while triclabendazole sulfone, albendazole, mebendazole, oxfendazole, thiabendazole, nitroxynil, levamisole, praziquantel and clorsulon failed to have any effect. The concentration needed to reach the maximal Rho 123 accumulation (E(max)) was obtained with 10 microM for IVM, 80 microM for CLOS, 40 microM for TCZ and TCZ-SO, and 80 microM for RAF. We showed that for these five drugs parental cell line was more sensitive to drug toxicity compared with Pgp recombinant cell line. Such in vitro approach constitutes a powerful tool to predict Pgp-drug interactions when formulations combining several anthelmintics are administered and may contribute to the required optimization of efficacy of anthelmintics.

Role of P-glycoprotein in the disposition of macrocyclic lactones: A comparison between ivermectin, eprinomectin, and moxidectin in mice.

Macrocyclic lactones (MLs) are lipophilic anthelmintics and substrates for P-glycoprotein (P-gp), an ATP-binding cassette transporter involved in drug efflux out of both host and parasites. To evaluate the contribution of P-gp to the in vivo kinetic disposition of MLs, the plasma kinetics, brain concentration, and intestinal excretion of three structurally different MLs (ivermectin, eprinomectin, and moxidectin) were compared in wild-type and P-gp-deficient [mdr1ab(-/-)] mice. Each drug (0.2 mg/kg) was administered orally, intravenously, or subcutaneously to the mice. Plasma, brain, and intestinal tissue concentrations were measured by high-performance liquid chromatography. The intestinal excretion rate after intravenous administration was determined at different levels of the small intestine by using an in situ intestinal perfusion model. P-gp deficiency led to a significant increase in the area under the plasma concentration-time curve (AUC) of ivermectin (1.5-fold) and eprinomectin (3.3-fold), whereas the moxidectin AUC was unchanged. Ivermectin and to a greater extent eprinomectin were both excreted by the intestine via a P-gp-dependent pathway, whereas moxidectin excretion was weaker and mostly P-gp-independent. The three drugs accumulated in the brains of the mdr1ab(-/-) mice, but eprinomectin concentrations were significantly lower. We concluded that eprinomectin disposition in mice is controlled mainly by P-gp efflux, more so than that of ivermectin, whereas moxidectin disposition appears to be mostly P-gp-independent. Given that eprinomectin and ivermectin have higher affinity for P-gp than moxidectin, these findings demonstrated that the relative affinity of MLs for P-gp could be predictive of the in vivo kinetic behavior of these drugs.

Efficacy of an in-feed formulation containing ivermectin for the control of intestinal strongyles in captive zebras (Equus burchelli (Gray, 1824)).

The efficacy of medicated food pellets (containing 10mg of ivermectin per kg, UNIFEED, Veronesi, Verona, Italy) was evaluated for the control of intestinal strongyles in a group of captive zebras (Equus burchelli) at the Safari Park, Fasano (Apulia region, Brindisi Province, southern Italy). The egg reappearance period and the faecal egg counts in terms of eggs per gram of faeces were investigated. From day 0 until day +9, 30 zebras were fed with medicated pellets and pooled faecal samples were collected from the floor of paddocks. The drug showed an efficacy of 100% for up to 78 days post-treatment, with one exception. Strongyle eggs collected prior to the treatment were used to perform coprocultures and larvae harvested were molecularly identified as Cylicostephanus minutus and Cylicocyclus leptostomum with a reverse line blot hybridization assay. The administration of ivermectin with medicated food pellets was effective in controlling intestinal strongylosis in captive zebras. The opportunity to treat captive ungulates with an in-feed anthelmintic is discussed in relation to the fractious nature of these animals, which often impairs helminth control programs in zoo-parks.

A successful, sustainable and low cost control-programme for bovine hypodermosis in France.

Bovine hypodermosis affecting livestock performance and the leather industry was still widespread in France up to the nineties despite successive directives from the Ministry of Agriculture since 1941, encouraging livestock owners to treat, on a voluntary basis. In 1969 the French Ministry of Agriculture, asked the national Farmers' Animal Health Organisation (FAHO), to plan a durable hypodermosis control programme and a working group including all the partners in cattle production was set up, under the coordination of the national FAHO. Improved systems of hypodermosis control, including new treatment and surveillance methods were developed. Among the main benefits obtained from this original work, were (i) the identification of scientific data which allowed inexpensive and accurate immuno-surveillance procedures, and a highly effective low-cost treatment, Ivomec, administered at the micro dose rate (2 microg/kg), which is environmentally non-threatening, and (ii) the development of a new strategy to manage the control programmes progressively, on a regional basis, in two to three concentric zones over three successive years at a maximum. The current programme, coordinated at the National level since 1998, has been implemented in each region of France. The compulsory systemic winter treatments directed against the endo-parasitic stage, carried out by technicians and veterinarians involved the entire bovine population in controlled zones. As each zone reached a hypodermosis herd prevalence of under 5%, usually after two years, the treatments were suspended. However treatments of the infected farms and contiguous farms were maintained. An immuno-survey was carried out, each winter, to evaluate the prevalence of the disease and detect any residual foci or re-infestations. Since 2002, bovine hypodermosis in France is under control with immuno-surveillance maintained at a very low cost. In 2006 hypodermosis became a notifiable disease.

Plasma and milk kinetic of eprinomectin and moxidectin in lactating water buffaloes (Bubalus bubalis).

The pharmacokinetics and mammary excretion of moxidectin and eprinomectin were determined in water buffaloes (Bubalus bubalis) following topical administration of 0.5mgkg(-1). Following administration of moxidectin, plasma and milk concentrations of moxidectin increased to reach maximal concentrations (C(max)) of 5.46+/-3.50 and 23.76+/-16.63ngml(-1) at T(max) of 1.20+/-0.33 and 1.87+/-0.77 days in plasma and milk, respectively. The mean residence time (MRT) were similar for plasma and milk (5.27+/-0.45 and 5.87+/-0.80 days, respectively). The AUC value was 5-fold higher in milk (109.68+/-65.01ngdayml(-1)) than in plasma (23.66+/-12.26ngdayml(-1)). The ratio of AUC milk/plasma for moxidectin was 5.04+/-2.13. The moxidectin systemic availability (expressed as plasma AUC values) obtained in buffaloes was in the same range than those reported in cattle. The faster absorption and elimination processes of moxidectin were probably due to a lower storage in fat associated with the fact that animals were in lactation. Nevertheless, due to its high excretion in milk and its high detected maximum concentration in milk which is equivalent or higher to the Maximal Residue Level value (MRL) (40ngml(-1)), its use should be prohibited in lactating buffaloes. Concerning eprinomectin, the C(max) were of 2.74+/-0.89 and 3.40+/-1.68ngml(-1) at T(max) of 1.44+/-0.20 and 1.33+/-0.0.41 days in plasma and milk, respectively. The MRT and the AUC were similar for plasma (3.17+/-0.41 days and 11.43+/-4.01ngdayml(-1)) and milk (2.70+/-0.44 days and 8.49+/-3.33ngdayml(-1)). The ratio of AUC milk/plasma for eprinomectin was 0.76+/-0.16. The AUC value is 20 times lower than that reported in dairy cattle. The very low extent of mammary excretion and the milk levels reported lower than the MRL (20ngml(-1)) supports the permitted use of eprinomectin in lactating water buffaloes.

Comparative evaluation of two ivermectin injectable formulations against psoroptic mange in feedlot cattle.

A study was carried out to compare the efficacy of two injectable formulations of ivermectin, Ivomec,(1) Merial (IVM reference) and Ivogell,(2) Intervet (IVM generic) in the treatment of psoroptic mange (Psoroptes ovis) in Charollais feedlot cattle. A total of 22 animals were ranked in order of the severity of mange and allocated to 11 replicates of 2 animals each. Within each replicate, one animal was randomly allocated to IVM reference product treatment (Group 1) and one to IVM generic (Group 2). Animals were treated on Day 0 and on Day 8 at the recommended dosage of 200 microg ivermectin/kg bodyweight. The pharmacokinetics profiles (pK) of both IVM formulations were evaluated in plasma samples taken from 6 cattle randomly chosen per group on Day 0, before treatment, and then at 6, 12, 24 hours and daily from Day 2 to Day 7 after the treatment on Day 0. Additionally, the severity of mange lesions was assessed and mites were counted in skin scrapings on Days 0, 8, 15 and 25. Animals were weighed on Day 0 and 25 and body weight and average daily gains (ADG) were evaluated. No statistical differences were found between the cattle of the two groups in any pK parameters, although the mean IVM plasma concentrations in cattle treated with the IVM reference product were consistently higher than those found in cattle treated with the generic compound. By Day 25, all animals in Group 1 had recovered clinically and parasitologically from psoroptic mange while cattle from Group 2 still had mange lesions and, in two animals, living mites were found in the skin scrapings; these differences were significant (P<0.001). The mean body weight of the two groups was significantly different on Day 25 (P<0.01) when animals in Group 1 weighed 20 kg more than those in Group 2. In conclusion, despite similarities in their pharmacokinetic profiles and formulations, the clinical efficacy of the two injectable formulations of IVM differed significantly in their therapeutic efficacy against psoroptic mange in feedlot cattle up to 25 days after treatment: this difference in response was reflected in an incomplete clinical and parasitological response in Group 2 and a slower growth rate.

Ketoconazole increases the plasma levels of ivermectin in sheep.

The parasiticide ivermectin and the antifungal drug ketoconazole are drugs that interact with P-glycoprotein. We have tested the ability of ketoconazole at a clinical dose to modify the pharmacokinetics of ivermectin in sheep. Lacaune lambs were administered with a single oral dose of ivermectin alone at 0.2 mg/kg (n=5) or in combination with a daily oral dose of ketoconazole (10 mg/kg) given for 3 days before and 2 days after the ivermectin (n=5). The plasma kinetics of ivermectin and its metabolite were followed over 15 days by HPLC analysis. Co-administration of ketoconazole induced higher plasma concentrations of ivermectin, leading to a substantial increase in the overall exposure of the animals to the drug. Ketoconazole did not reduce the production of the main ivermectin metabolite but it may rather act by inhibiting P-glycoprotein, and thus increasing the absorption of ivermectin. The use of a P-gp reversing agent such as ketoconazole could be useful tool to optimize antiparasitic therapy in the face of the worldwide development of anthelmintic resistance.

ABC transporter modulation: a strategy to enhance the activity of macrocyclic lactone anthelmintics.

The emergence of parasites resistant to anthelmintic macrocyclic lactones (MLs) threatens to severely limit current parasite control strategies. Improving the current ML-based chemotherapy to perpetuate the efficacy of this broad-spectrum class of anthelmintics would be advantageous. In recent years it has become evident that the absorption, distribution and elimination of the MLs in hosts and parasites are under the control of multidrug resistance transporters (MDRs) such as P-glycoproteins. Theoretically, the inhibition of these transporters should result in an increase of the drug concentration in the organisms and higher treatment efficiency. This opinion article will discuss the recent findings in this research field and assess the possibilities of this approach being used in the field.

Breed differences in the pharmacokinetics of ivermectin administered subcutaneously to Holstein and Belgian Blue calves.

Belgian Blue (BB) cattle are very sensitive to mange caused by Psoroptes ovis and, in contrast to the case in Holstein cattle, single treatments with ivermectin do not result in complete elimination of the parasite. The objective of the present study was to determine the concentration of ivermectin in plasma, skin and hair following subcutaneous administration to Holstein and BB calves and to assess the influence of breed on drug pharmacokinetics and availability. Two groups of six healthy female Holstein and BB calves were treated with ivermectin (SC formulation) at a dose of 0.2 mg/kg. Blood, skin and hair were collected before treatment and up to 21 days after treatment. Ivermectin was analyzed in plasma and tissue by high-performance liquid chromatography (HPLC). The peak concentrations (Cmax), time-peak concentrations (Tmax), the area under the plasma concentration-time curves (AUC) and the mean residence time (MRT) were determined. The patterns of plasma and tissue ivermectin concentrations were similar in the two breeds of animals, however, the AUC and Cmax levels for plasma and skin were significantly higher in the BB calves. In hair, ivermectin was detected later than in plasma and skin, with the Tmax ranging between 4 days (Holstein group) and 6 days (BB group). The possible reasons for the significantly higher levels in plasma and skin in BB calves compared to Holstein calves are discussed.

Multiple oral dosing of ketoconazole increases dog exposure to ivermectin.

PURPOSE: The parasiticide ivermectin and the antimicrobial drug ketoconazole are macrolides that interact with P-glycoprotein. We investigated the effects of ketoconazole at a clinical dose on the pharmacokinetics of ivermectin, a CYP3A substrate with low hepatic clearance. METHODS: Beagle dogs received a single subcutaneous injection of ivermectin at 0.05 mg/kg alone (n=6) or in combination with a daily oral dose of ketoconazole 10 mg/kg over 5 days before and after ivermectin administration (n=6). The plasma kinetics of ivermectin and its metabolite were followed over 15 days by HPLC analysis. RESULTS: Co-administered ketoconazole induced a higher plasma concentration and longer residence time of ivermectin in dogs, leading to a substantial increase in the overall exposure of the animal to the drug. Ketoconazole does not interfere with the production of the ivermectin metabolite but it may rather inhibit the elimination of the parental drug by interfering with P-gp transport. CONCLUSIONS: Multiple oral dosing of ketoconazole dramatically altered the pharmacokinetics of ivermectin in dogs leading to an increase in systemic exposure to the drug. Neurotoxicity of ivermectin means that inhibition of the P-gp function at the blood-brain barrier during polytherapy using P-gp inhibitors must be taken into consideration.

New screening test to predict the potential impact of ivermectin-contaminated cattle dung on dung beetles.

According to European Union recommendations, a test method has been developed to evaluate the effects of veterinary pharmaceuticals on dung feeding insects. This test method was evaluated with the dung beetle Aphodius constans by using fecal residues of ivermectin after a pour-on administration. Dung of different age (and thus containing different concentrations of ivermectin) as well as mixtures of highly-contaminated spiked dung with untreated control dung were studied in five test runs in two laboratories. The concentration of ivermectin (active substance; a.s.) in the dung samples was verified analytically. The main test endpoint was the survival of first instar larvae. The LC50 using dung directly obtained from treated cattle ranged from 470 to 692 microg a.s. kg(-1) dung (dry weight; d.w.) and 67 to 97 microg a.s. kg(-1) dung (fresh weight; f.w.). Using mixtures, the outcome of two tests was almost identical: 770 to 781 microg a.s. kg(-1) dung (d.w.); 109 to 132 microg a.s. kg(-1) dung (f.w.). In comparison to the LC50 values obtained when ivermectin was spiked in control dung at several concentrations (LC50 880-985 microg a.s. kg(-1) dung (d.w.)), the LC50 values were again very similar. Three conclusions can be drawn from these results. The proposed test method seems to be robust and allows for the initiation of an international validation process (including ringtesting). Because of only small differences found in tests in which the test substance was spiked into control dung and those in which dung from treated cattle was applied, the use of a standard test method is proposed. The effects of ivermectin on ecologically relevant dung beetles obtained in a standardised test method reflect the results from field studies and are in the range of environmentally relevant concentrations.

Interaction of macrocyclic lactones with P-glycoprotein: structure-affinity relationship.

P-glycoprotein (P-gp) is involved in the ATP-dependant cellular efflux of a large number of drugs including ivermectin, a macrocyclic lactone (ML) endectocide, widely used in livestock and human antiparasitic therapy. The interactions of P-gp with ivermectin and other MLs were studied. In a first approach, the ability of ivermectin (IVM), eprinomectin (EPR), abamectin (ABA), doramectin (DOR), selamectin (SEL), or moxidectin (MOX) to inhibit the rhodamine123 efflux was measured in recombinant cells overexpressing P-gp. Then, the influence of these compounds on the P-gp ATPase activity was tested on membrane vesicles prepared from fibroblasts overexpressing P-gp. All the MLs tested increased the intracellular rhodamine123. However, the potency of MOX to inhibit P-gp function was 10 times lower than the other MLs. They all inhibited the basal and decreased the verapamil-stimulated P-gp ATPase activity. But SEL and MOX were less potent than the other MLs when competing with verapamil. According to the structural specificity of SEL and MOX, we conclude that the integrity of the sugar moiety is determinant to achieve the optimal interaction of macrocyclic lactones with P-gp. The structure-affinity relationship for interaction with P-gp is important information for improving ML bioavailability and reversal of multidrug resistance (MDR).

Influence of dyslipidemia on moxidectin distribution in plasma lipoproteins and on its pharmacokinetics.

PURPOSE: We studied the influence of dyslipemia on the distribution of moxidectin, a potent antiparasitic drug of the macrocyclic lactone (ML) family, in plasma lipoproteins and on its pharmacokinetic behaviour. MATERIALS AND METHODS: Plasma samples from normolipidemic or dyslipidemic subjects were spiked with moxidectin (20 ng/ml). Rabbits fed with standard (n = 5) or cholesterol-enriched diet (n = 5) were injected subcutaneously with moxidectin (300 microg/kg) and blood samples were collected over 32 days. Lipoproteins were separated from plasma samples by ultracentrifugation on density gradients. Moxidectin and lipids were measured in plasma and in lipoproteins and the pharmacokinetic parameters calculated. RESULTS: In normolipidemic subjects or rabbits, the drug bound preferentially to HDL. In hyperlipidemic samples, moxidectin shifted to the VLDL-LDL fraction. In addition, hyperlipidemic rabbits had a 2.8-fold higher area under the plasma concentration versus time curve (AUC) and a lower clearance and volume of distribution when compared with controls. CONCLUSION: Dyslipidemia led to major changes in moxidectin plasma distribution and in drug disposition. Therefore, a high variability in moxidectin disposition might be expected in humans or animals liable to develop dyslipidemia, with a possible impact on the efficacy and safety of this class of drugs.

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.

Interaction of ivermectin with multidrug resistance proteins (MRP1, 2 and 3).

Ivermectin is a potent antiparasitic drug from macrocyclic lactone (ML) family, which interacts with the ABC multidrug transporter P-glycoprotein (Pgp). We studied the interactions of ivermectin with the multidrug resistance proteins (MRPs) by combining cellular and subcellular approaches. The inhibition by ivermectin of substrate transport was measured in A549 cells (calcein or 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, BCECF) and in HL60-MRP1 (calcein). Ivermectin induced calcein and BCECF retention in A549 cells (IC(50) at 1 and 2.5microM, respectively) and inhibited calcein efflux in HL60-MRP1 (IC(50)=3.8microM). The action of ivermectin on the transporters ATPase activity was followed on membranes from Sf9 cells overexpressing human Pgp, MRP1, 2 or 3. Ivermectin inhibited the Pgp, MRP1, 2 and 3 ATPase activities after stimulation by their respective activators. Ivermectin showed a rather good affinity for MRPs, mainly MRP1, in the micromolar range, although it was lower than that for Pgp. The transport of BODIPY-ivermectin was followed in cells overexpressing selectively Pgp or MRP1. In both cell lines, inhibition of the transporter activity induced intracellular retention of BODIPY-ivermectin. Our data revealed the specific interaction of ivermectin with MRP proteins, and its transport by MRP1. Although Pgp has been considered until now as the sole active transporter for this drug, the MRPs should be taken into account for the transport of ivermectin across cell membrane, modulating its disposition in addition to Pgp. This could be of importance for optimizing clinical efficacy of ML-based antiparasitic treatments. This offers fair perspectives for the use of ivermectin or non-toxic derivatives as multidrug resistance-reversing agents.

Assessment of a liposomal formulation of ivermectin in rabbit after a single subcutaneous administration.

Ivermectin is a member of the macrocyclic lactone family widely used in livestock, pets, and humans as a potent parasiticide. Slight differences in formulation may change the plasma kinetics and efficacy of these compounds. The aim of the study is to evaluate the ability of a liposomal formulation of ivermectin to generate an efficient exposure of the animal to the drug. Ten rabbits were subcutaneously administered with 0.3 mg kg(-1) of ivermectin using Ivomec (n=5) or a liposomal formulation (n=5). The areas under serum concentration-time curve were similar after both treatments, indicating the same bioavailability for the two formulations. However, the liposomal formulation gave a higher C(max) value (33.33 ng ml(-1)) compared with Ivomec (20.82 ng ml(-1)) and a significantly faster absorption as indicated by the T(max) of 0.23 days compared with 1.13 days for the Ivomec formulation. The use of liposomal formulation shows promise as this system improves the efficacy of ivermectin and related drugs.

Toxicity of four veterinary parasiticides on larvae of the dung beetle Aphodius constans in the laboratory.

The environmental risk assessment of veterinary pharmaceuticals for dung beetles is strongly hampered because no standardized test method is available so far. Therefore, a test with the temperate dung beetle species Aphodius constans was developed. The survival of beetle larvae was determined after exposure to four veterinary parasitical pharmaceuticals (ivermectin, moxidectin, dicyclanil, and praziquantel) representing different treatment regimes, modes of action, and effect levels. The test was performed in the laboratory (three week duration) with fresh dung, as well as formulated (dried, ground, and rewetted) dung as test substrate (i.e., at least one range-finding test, two definitive test runs per pharmaceutical). Ivermectin was the most toxic substance (median lethal concentration [LC50] = 0.88-0.98 mg of active substance per kilogram of dung dry weight [mg a.s./kg dung (dry wt)] followed by dicyclanil (LC50 = 1.5-6.0 mg a.s./kg dung [dry wt]) and moxidectin (LC50 = 4.0-5.4 mg a.s./kg dung [dry wt]), whereas praziquantel showed very low toxicity (LC50 > 1,000 mg a.s./kg dung [dry wt]). The toxicity in fresh and formulated dung differed by a factor of between 1.1 and 4. The comparison with literature data on toxic effects of these substances on dung beetles in the laboratory or in the field is difficult because no results for praziquantel and dicyclanil have been published so far. With the use of data from ivermectin and moxidectin, the test results are on the same order of magnitude as those known from other studies. On the basis of the experiments reported here, it is recommended that this test be standardized in an international ring test so that it can be incorporated into the risk assessment process as described in the respective international guidelines for the registration of veterinary pharmaceuticals.