In Vitro Susceptibilities of Leishmania donovani Promastigote and Amastigote Stages to Antileishmanial Reference Drugs: Practical Relevance of Stage-Specific Differences

The in vitro susceptibilities of the reference strain Leishmania donovani MHOM/ET/67/L82 to sodium stibogluconate, amphotericin B, miltefosine, and the experimental compound PX-6518 were determined for extracellular log-phase promastigotes, established axenic amastigotes, fresh spleen-derived amastigotes, and intracellular amastigotes in primary mouse peritoneal macrophages. Susceptibility to amphotericin B did not differ across the various axenic models (50% inhibitory concentrations [IC50], 0.6 to 0.7 μM), and amphotericin B showed slightly higher potency against intracellular amastigotes (IC50, 0.1 to 0.4 μM). A similar trend was observed for miltefosine, with comparable efficacies against the extracellular (IC50, 0.4 to 3.8 μM) and intracellular (IC50, 0.9 to 4.3 μM) stages. Sodium stibogluconate, used either as Pentostam or as a crystalline substance, was inactive against all axenic stages (IC50, >64 μg SbV/ml) but showed good efficacy against intracellular amastigotes (IC50, 22 to 28 μg SbV/ml); the crystalline substance was about two to three times more potent (IC50, 9 to 11 μg SbV/ml). The activity profile of PX-6518 was comparable to that of sodium stibogluconate, but at a much higher potency (IC50, 0.1 μg/ml). In conclusion, the differential susceptibility determines which in vitro models are appropriate for either drug screening or resistance monitoring of clinical field isolates. Despite the more complex and labor-intensive protocol, the current results support the intracellular amastigote model as the gold standard for in vitro Leishmania drug discovery research and for evaluation of the resistance of field strains, since it also includes host cell-mediated effects. Axenic systems can be recommended only for compounds for which no cellular mechanisms are involved, for example, amphotericin B and miltefosine.