Miltefosine-resistant Leishmania infantum strains with an impaired MT/ROS3 transporter complex retain amphotericin B susceptibility

Objectives Increasing numbers of miltefosine treatment failures in visceral leishmaniasis therapy and reports of miltefosine resistance in the Indian subcontinent resulted in the recommendation to use liposomal amphotericin B as first-line therapy. Cross-resistance between miltefosine and amphotericin B has recently been documented, suggesting a role of mutations in the miltefosine transporter, a complex encoded by the MT and ROS3 genes. This study aimed to further explore the putative role of MT/ROS3 defects in the molecular basis of amphotericin B cross-resistance. Methods The susceptibility profiles of different miltefosine-resistant Leishmania infantum strains with well-characterized mutations in the transporter complex and the corresponding episomally restored susceptible parasite lines were determined using both the routine extracellular promastigote assay and the intracellular amastigote assay. Results In vitro amastigote and promastigote susceptibility testing of the two miltefosine-resistant and the episomally reconstituted L. infantum lines revealed full susceptibility to amphotericin B, despite the variable miltefosine susceptibility profile. Conclusions Mutations present in either the MT and/or ROS3 gene are not sufficient to elicit higher tolerance to amphotericin B. Additional synergistic adaptations may be responsible for the miltefosine/amphotericin B cross-resistance described earlier.