Experimental studies on the evolution of antimony-resistant phenotype during the in vitro life cycle of Leishmania infantum: implications for the spread of chemoresistance in endemic areas

Pentavalent antimonial unresponsiveness is an emerging problem in endemic a reas and information on factors which could modulate the transmission of dr ug-resistant phenotypes and parasites during life cycle are warranted. Usin g axenic amastigotes resistant to potassium antimonyl tartrate (Sb(III)) we investigated tile modualtion of antimonyl resistance during the in vitro l ife cycle. We assessed: (i) the stability of the drug-resistant phenotype d uring the in vitro life cycle, (ii) the transmission of drug-resistant clon es when mixed with a wild-type clone at different susceptible/chemoresistan t ratios (50/50, 90/10, 10/90) after one or two in vitro life cycles. We de monstrate that: (i) mutants which were 12, 28, 35 and 44 fold more resistan t to Sb(III)-antimonial than their parental wild-type, were Glucantime (R) Sb(V)-resistant when growing in THP-1 cells; (ii) the drug-resistant phenot ype was partially retained during long-term in vitro culture (3 months) in drug free medium; (iii) the antimonyl-resistant phenotype was retained afte r one or more in vitro life cycles. However, when drug-resistant parasites were mixed with susceptible, mutants could not be detected in the resulting population, after one or two in vitro life cycles, whatever the initial wi ld-type: chemoresistant ratio. These results could be explained by the lowe r capacity of drug-resistant amastigotes to undergo the amastigote-promasti gote differentiation process, leading probably to their sequential eliminat ion during life cycle. Taken together, these observations demonstrate that different factors could modulate the transmission of Leishmania drug resist ance during the parasite's life cycle. (C) 2001 Elsevier Science B.V. All r ights reserved.