Susceptibility of dyskinetoplastic Trypanosoma evansi and T. equiperdum to isometamidium chloride.

Isometamidium chloride (ISM) is an effective trypanocide with curative and prophylactive activity in husbandry animals. The mode of action of ISM against pathogenic trypanosomes is not fully understood, but there is evidence in the literature that kinetoplastic topoisomerase type II is selectively inhibited by the drug. This prompted the hypothesis that dyskinetoplastic trypanosomes would express a reduced level of susceptibility to ISM. From parental Trypanosoma evansi and T. equiperdum populations we generated clones containing only dyskinetoplastic organisms and clones containing organisms with kinetoplasts. The susceptibility of these clones to ISM was quantified by in vitro assays. The susceptibility of all clones was in the same range. Minor differences in drug susceptibility found between the clones showed that the dyskinetoplastic T. evansi and T. equiperdum clones were even more susceptible to ISM than their kinetoplastic counterparts. Thus, the hypothesis that the dyskinetoplastic trypanosomes would be less susceptible to or tolerant of ISM was clearly disproved. The previously demonstrated inhibition of kinetoplastic topoisomerase type II by ISM cannot be the primary toxic effect of the drug on trypanosomes, and the mode of action of ISM needs to be reassessed.