P-gp modulating drugs greatly potentiate the in vitro effect of ivermectin against resistant larvae of Haemonchus placei

•Eight P-gp modulating drugs were associated to ivermectin in vitro.•Cyclosporin A caused the highest reduction in IVM EC50.•The effect of P-gp modulating drugs depends of ivermectin concentration.•The modulator drugs increased ivermectin efficacy against Haemonchus placei.

Since its production in the 1980s, ivermectin (IVM) has been used indiscriminately and the selection pressure to which bovine gastrointestinal nematodes have been exposed has been intense, resulting in considerable economic losses due to parasitic resistance. One possibility for the control of resistant parasites is the use of P-glycoprotein (P-gp) modulators, because one of the main biochemical changes in ivermectin-resistant parasites is the increased activity of membrane proteins responsible for the efflux of drugs and xenobiotics. This study aimed to evaluate the in vitro effect of eight P-gp modulating drugs to potentiate IVM efficacy against an IVM-resistant field isolate of Haemonchus placei (Nematoda: Trichostrongylidae). The association of IVM with cyclosporin-A, ceftriaxone, dexamethasone, diminazene aceturate, quercetin, trifluoperazine, verapamil, or vinblastine resulted in increased IVM (10−4 M) efficacy of 5.1%, 49.06%, 76.42%, 3.31%, 28.85%, 13.74%, 45.64% and 43.61%, respectively, and reduced the IVM half maximal effective concentration (EC50) from 4.381 × 10−6 M to 9.877 × 10−8, 2.739 × 10−7, 1.240 × 10−6, 1.651 × 10−6, 2.710 × 10−7, 1.159 × 10−7, 1.026 × 10−6 and 7.136 × 10−7 M, respectively. Only diminazene aceturate did not significantly reduce the number of migrating larvae when associated with IVM (P > 0.05). The effect of P-gp modulating drugs depended on IVM concentration, with greater potentiating effect at lower IVM concentrations. The in vitro application of trifluoperazine, dexamethasone, quercetin, verapamil, cyclosporin A, vinblastine, and ceftriaxone potentiated IVM efficacy against an IVM-resistant field isolate of H. placei, resulting in higher efficacy and lower IVM EC50.