The macrocyclic lactones are the most widely used broad-spectrum antiparasitic drugs in veterinary medicine. Their notorious popularity is related to a high efficacy against ecto and endo parasites (nematodes), high potency, persistent activity and low toxicity. IVM, the first commercially available macrocyclic lactone endectocide, was introduced in the pharmaceutical market in the early ´ 80s. Since the IVM patent protection expire, several “similar” (generic) products entered the veterinary market worldwide. Uruguay was not an exception, according official data more than 60 different IVM formulations are currently registered for use in veterinary medicine, from which thirteen are solutions for oral administration to be used in sheep. The large number of available commercial formulations, situation that is reflected in many other countries around the world, faces the problem of a lack of information on their absorption patterns, which seems to be critical considering the possibility of differences on manufacturing processes and quality of components that may exist among formulations. These differences may substantially affect drug dissolution and its consequent GI absorption, which in turns could affect drug effectiveness. The comparison of the systemic drug exposure (measured as plasma concentration profiles) after treatment with different IVM generic formulation is an initial approach to check their pharmacotechnical quality, which has been shown to drastically affect the systemic availability of other active ingredients (i.e. albendazole) . A RF and two generic IVM preparations were selected to be tested in the work reported here. The selection of the formulations did not respond to any particular interest to compare the quality among them. However, the comparison was done in order to simulate a real practical situation that could result useful to illustrate a market situation with a great impact on parasite control.
In order to assess the pharmacokinetic behaviour of different formulations, absorption related pharmacokinetic parameters must primarily be considered. The AUC, which reflects the extent to which the active drug is absorbed and is independent of the rate of the absorption process, and Cmax, which indicates the extent and the rate of drug absorption. Since differences in body condition, breed, gender, feeding, and parasitism substantially affect the plasma disposition kinetics of macrocyclic lactones (reviewed by ), the current study was conducted in lambs with similar characteristics, uniformly distributed among experimental groups. This is particularly important for studies conducted using a parallel design, since this experimental design has a lower power than the cross-over design for relative bioavailability . However, the use of a parallel design can provide useful information on gross deficiencies in the absorption process of different anthelmintic formulations . Similar (P> 0.05) IVM plasma AUC and Cmax (Table 1) were observed among formulations, suggesting a similar extent of absorption among the addressed reference and generic formulations. Tmax and Cmax/AUC did not show significant differences among the studied formulations, revealing a similar rate of the absorption process. Furthermore, our study showed similar values for other pharmacokinetic parameters (Table 1). Since a similar pharmacokinetic behavior was observed for IVM after the administration of the RF compared to both test formulations in animals grazed on pasture, it could be concluded that the assayed commercial preparations deliver IVM in an equivalent way which may indicated that manufacturing and overall pharmaceutical quality did not differ among them.
A lower IVM plasma drug exposure (expressed as Cmax and AUC) was observed in the current experiment, compared to that previously reported . This may be related to differences in some experiment-related factors (parasitism, breed, body condition, feed, etc.) which have shown to affect the pharmacokinetic behaviour of IVM . On the other hand and as it was previously reported, IVM plasma concentrations is higher after the SC compared to the IR administration [16, 17]. Although similar concentration profiles were measured in the abomasal mucosa after treatment by both routes, markedly lower IVM concentrations were recovered in the abomasal contents after its SC injection. While the active secretion of IVM from the bloodstream to the abomasal lumen is of little relevance , the adsorption of IVM to ruminal particulate material may account for its low oral bioavailability which was estimated in about 25% .
High prevalence of anthelmintic resistance has now been reported in all parts of the world for GI helminth parasites, being nematodes of sheep and goats commonly involved [5, 20, 21]. In Uruguay, the development of anthelmintic resistance in sheep nematodes is not an exception. Resistance to IVM in sheep nematodes increased from 1.2%  to 65%  between 1996 and 2002. The trial described here demonstrated that current IVM resistance situation at the farm in which the study was conducted, is dramatically serious. The initially high IVM efficacy against GI nematodes in sheep has now drastically fell down, with almost a complete therapeutic failure to control some GI nematodes. Efficacies (evaluated by means the FECRT) as low as 7.1, 5.8 and 0% were observed for the RF, T1 and T2 IVM preparations under assay, respectively.
The identification of adult worms in the untreated lambs permitted to establish that the lambs were infected with Haemonchus spp., T.circumcincta, Trichostrongylus spp., Nematodirus spp., Cooperia spp., Oesophagostomum spp. and T.ovis. Nematode resistance in the current experiment was mainly related to Haemonchusspp., where all the IVM formulations failed to control this abomasal parasite. However, the clinical efficacy study also revealed a resistance-mediated failure to control T. circumcincta, where only efficacies ≤ 57% were observed. Resistance of T. circumcincta to IVM in Uruguay is reported here for the first time, which it may be useful as an indicator of the complexity of the resistance development phenomenon and its impact on livestock production.
All the tested IVM formulations also failed to control Cooperiaspp. and Nematodirus spp. However, the low number of these parasites in the untreated control animals, limited the relevance of this finding. Contrarily, IVM demonstrated to maintain high efficacy against Trichostrongylus spp., Oesophagostomum spp. and T.ovis. Only Haemonchus spp. L3 larvae were recovered from the fecal cultures obtained from all the IVM treated groups. However, larvae obtained from fecal cultures are not necessarily related to parasites found at necropsies, since the high egg output observed in Haemonchus spp. may “mask” other nematodes.
Oppositely to what has been observed for other anthelmintics (such as the benzimidazole compounds), no significant differences on relative bioavailability/systemic exposure were observed among the tested IVM oral formulations in lambs. It is likely that any pharmaceutical/manufacturing change may more deeply affect the systemic availability of those compounds where GI absorption largely depends on the dissolution of low water soluble drug particles (suspension) in the abomasal lumen (i.e. albendazole), compared to the more lipophilic compounds such as IVM, but formulated as a mixed organic/aqueous solution. In spite of the fact that all the IVM formulations showed to reach an equivalent systemic exposure, all of them failed to control some common GI nematodes. The resistance status observed at the farm where the current trial was conducted is likely to be an indicator of the overall situation of the sheep flocks in Uruguay, and perhaps in many other regions of the world, where IVM completely failed to control H.contortus. This overall picture described in Uruguay, with resistance extended into other avermectin-type compounds, may be even worse if we consider that resistant T. circumcincta has been reported for the first time.