RESUMO
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.