Specific interaction of benzimidazole anthelmintics with tubulin: high-affinity binding and benzimidazole resistance in Haemonchus contortus.

The specific (high-affinity) binding of tritiated benzimidazole [(3H]BZ) anthelmintics-mebendazole [(3H]MBZ) and oxibendazole [(3H]OBZ) to, and the specific displacement (inhibition) of these radioligands by unlabelled BZs (oxibendazole, mebendazole, oxfendazole, albendazole, fenbendazole and thiabendazole) from crude tubulin extracts prepared from thiabendazole (TBZ)-susceptible (S) or TBZ-resistant (R) strains of adult Haemonchus contortus, have been examined. The most striking difference between R and S was that the drug specifically bound at infinite ligand concentration (Bmax), was markedly reduced for the R strain, with no apparent change in association constant (Ka). Thus, resistance was associated with a loss of high-affinity receptors. TBZ-resistance was not associated with a change in low-affinity binding. There was a greater loss of high affinity receptors for [3H]OBZ than for [3H]MBZ. Using the displacement data. BZs were ranked according to their Ka and IC50 (concentration of BZ inhibiting 50% of radioligand binding) values. The Ka and IC50 values and the rank order of the BZs were approximately independent of the radioligand displaced or source (S or R) of the tubulin extracts used. The results are consistent with tubulin binding being the primary mechanism of action for all of these BZs.