Enhanced absorption of pour-on ivermectin formulation in rats by co-administration of the multidrug-resistant-reversing agent verapamil

The effect of verapamil, a multidrug-resistance (Mdr)-reversing agent on th e absorption of a pour-on formulation of ivermectin was evaluated in rats. Absorption of ivermectin was effectively enhanced (40%) by the presence of verapamil, suggesting that absorption of ivermectin involves Mdr-P-glycopro tein and that verapamil should act as a competitive inhibitor for the trans port and extrusion of ivermectin by P-glycoprotein. This hypothesis is cons istent with other studies describing verapamil as a blocking agent of P-gly coprotein involved in the efflux of ivermectin in a resistant strain of Hae monchus contortus.