Interaction of benzimidazole anthelmintics with Haemonchus contortus tubulin: Binding affinity and anthelmintic efficacy

Abstract

The ability of various benzimidazoles (BZs) to bind tubulin under different conditions was assessed by determining their IC 50 values (the concentration of unlabeled drug required to inhibit 50% of the labeled drug binding), K a (the apparent equilibrium association constant) and B max (the maximum binding at infinite [BZ]= [drugreceptor]). The ability of unlabeled benzimidazoles—fenbendazole, mebendazole (MBZ), oxibendazole (OBZ), albendazole (ABZ), rycobendazole (albendazole sulfoxide, ABZSO), albendazole sulfone, oxfendazole (OFZ), and thiabendazole—to bind tubulin was determined from their ability to inhibit the binding of [ 3H]MBZ or [ 3H]OBZ to tubulin in supernatants derived from unembryonated eggs or adult worms of Haemonchus contortus. The binding constants (IC 50, K a , and B max) correlated with the known anthelmintic potency (recommended therapeutic doses) of the BZ compounds except for OFZ and ABZSO whose K a values were lower than could be expected from anthelmintic potency. The binding of [ 3H]ABZ or [ 3H]OFZ to tubulin in supernatants derived from BZ-susceptible and BZresistant H. contortus was compared. [ 3H]ABZ demonstrated saturable high-affinity binding but [ 3H]OFZ bound with low affinity. The high-affinity binding of [ 3H]ABZ was reduced for the R strain. Tubulin bound BZ drugs at 4 °C with lower apparent K a than at 37 °C.