Macrocyclic lactones and their relationship to the SNPs related to benzimidazole resistance.

Haemonchus contortus is an abomasal nematode of ruminants that is widely present across the world. Its ability to cause death of infected animals and rapidly develop anthelmintic resistance makes it a dangerous pathogen. Ivermectin (IVM) and moxidectin (MOX) are macrocyclic lactones (MLs). They have been successfully used to treat parasitic nematodes over the last three decades. A genetic association between IVM selection and single nucleotide polymorphisms (SNPs) on the β-tubulin isotype 1 gene was reported in H. contortus. These SNPs result in replacing phenylalanine (F, TTC) with tyrosine (Y, TAC) at position 167 or 200 on the β-tubulin protein. Recently we reported a direct interaction of IVM with α- and β-tubulin. It had been hypothesized that the SNPs (F167Y and F200Y) may change tubulin dynamics and directly affect IVM binding. The goal of the current study was to observe the effects of SNPs (F167Y and F200Y) on tubulin polymerization and IVM binding. It was also of interest to evaluate the differences between IVM and MOX on tubulin polymerization. We conclude that the SNPs cause no difference in the polymerization of wild and mutant tubulins. Furthermore, neither of the SNPs reduced IVM binding. Varying results were obtained in the degree of polymerization of parasitic and mammalian tubulin for IVM and MOX, i.e., the extent of polymerization was greater for IVM compared with MOX, for H. contortus tubulin, and vice versa for mammalian tubulin. Molecular modeling showed that IVM and MOX docked into the taxane binding pocket of both mammalian and parasitic wild type and mutant tubulins. However the binding was stronger for mammalian tubulin as compared to parasitic tubulin.