Integrated assessment of ivermectin pharmacokinetics, efficacy against resistant Haemonchus contortus and P-glycoprotein expression in lambs treated at three different dosage levels

- Assessment of IVM pharmacokinetic and efficacy against resistant-H. contortus.
- IVM demonstrated a dose proportionality after both, SC and IR administration.
- The dose increment determines an improved anthelmintic efficacy.
- An enhanced parasite exposure to the drug may explain the improved efficacy.

The main goals of the current work were: (a) to assess the ivermectin (IVM) systemic exposure and plasma disposition kinetics after its administration at the recommended dose, x5 and x10 doses to lambs, (b) to compare the clinical efficacy of the same IVM dosages in lambs infected with an IVM-resistant isolate of Haemonchus contortus, and (c) to assess the expression of the transporter protein P-glycoprotein (P-gp) in H. contortus recovered at 14 days after administration of the IVM dose regimens. There were two separated trials where IVM was administered either subcutaneously (SC, Experiment I) or intraruminally (IR, Experiment II). Each experiment involved twenty-four (24) lambs artificially infected with a highly resistant H. contortus isolate. Animals were allocated into 4 groups (n = 6) and treated with IVM at either 0.2 (IVMx1), 1 (IVMx5) or 2 mg/kg (IVMx10). Plasma samples were collected up to 12 days post-treatment and analysed by HPLC. An untreated-control Group was included to assess the comparative anthelmintic efficacy of the different treatments. The level of expression of Pgp in H. contortus specimens obtained from lambs both untreated and IR treated with the different IVM doses was quantified by real time PCR. Parametric and non-parametric tests were used to compare the statistical significance of the results (P < 0.05). After the SC treatment, the IVM plasma area under the concentration-time curve (AUC0-LOQ) increased from 41.9 (IVMSCx1) up to 221 (IVMSCx5) and 287 (IVMSCx10)ng.day/mL and after the IR treatment from 20.8 (IVMIRx1) up to 121 (IVMIRx5) and 323 (IVMIRx10)ng.day/mL. Dose-adjusted AUC0-LOQ and Cmax were similar among doses, demonstrating dose proportionality for IVM after both SC and IR administration at the three different doses. The efficacies against resistant H. contortus after the SC treatment were 42% (IVMSC1), 75% (IVMSCx5) and 75% (IVMSCx10). However, the IR IVM treatment reached clinical efficacies ranging from 48% (IVMIRx1) up to 96% (IVMIRx5) and 98% (IVMIRx10). None of the IR IVM treatments increased the expression of P-gp in adult H. contortus at 14 days post-treatment compared to samples collected from the untreated control group. An enhanced parasite exposure of the drug at the abomasum may explain the improved efficacy against this recalcitrant H. contortus isolate observed only after the IR administration at 5- and 10-fold the IVM therapeutic dosage.