Stage-specific activity of pentavalent antimony against Leishmania donovani axenic amastigotes

The standard treatment of human visceral leishmaniasis involves the use of pentavalent antimony (SbV) compounds. In recent Fears increasing numbers of clinical failures of treatment with SbV have been reported, probably due t o the development of parasite resistance to this compound. The mode of acti on and mechanisms of resistance to SbV have not been fully elucidated. In t he present study an axenic amastigote culture was used to study the in vitr o responses of Leishmania donovani to SbV. Susceptibility to both sodium st ibogluconate and meglumine antimoniate was found to be stage specific, Amas tigotes were 73 to 271 times more susceptible to SbV than were promastigote s. rls opposed to SbV, trivalent antimony (SbIII) was similarly toxic to bo th developmental stages. When promastigotes were transformed to amastigotes , susceptibility to meglumine antimoniate developed after 4 to 5 days, upon the completion of differentiation, In contrast, with transformation from a mastigotes to promastigotes, resistance to meglumine antimoniate was acquir ed rapidly, within 24 h, before the completion of differentiation. The cult ure of promastigotes at an acidic pH (5.5) or at an elevated temperature (3 7 degrees C) alone did not lead to the appearance of SbV susceptibility, em phasizing the requirement of both these environmental factors for the devel opment of SbV susceptibility, A previously isolated sodium stibogluconate ( Pentostam)-resistant L. donovani mutant (Ld1S.20) is also resistant to megl umine antimoniate, indicating cross-resistance to SbV-containing compounds. In contrast, no cross-resistance was found with SbIII, suggesting a mechan ism of SbV resistance different from that described in Leishmania tarentola e. These data show that L. donovani susceptibility to SbV is parasite intri nsic, stage specific, and macrophage independent.