The in vitro and in vivo activities of a mixture of six oleane triterpene saponins, recovered from the methanolic extract of the leaves of the Vietnamese plant Maesa balansae (PX-6518), were evaluated against drug-sensitive visceral Leishmania strains. The in vitro 50% inhibitory concentration (IC 50 ) against intracellular Leishmania infantum amastigotes was 0.04 g/ml. The cytotoxic concentrations causing 50% cell death (CC 50 s) were about 1 g/ml in murine macrophage host cells and >32 g/ml in human fibroblasts (MRC-5 cell line). Evaluation in the Leishmania donovani BALB/c mouse model indicated that a single subcutaneous administration of 0.4 mg/kg at 1 day after infection reduced liver amastigote burdens by about 95% in all treated animals. If treatment was delayed until 14 days after infection, a dose of 1.6 mg/kg of body weight was required to maintain the same level of activity. Single 250-mg/kg doses of sodium stibogluconate (Pentostam) 1 and 14 days after infection produced comparable efficacies. A single dose of PX-6518 at 2.5 mg/kg administered 5 days before infection was still 100% effective in preventing liver infection, suggesting a particularly long residual action. Spleen and bone marrow could not be cleared by PX-6518 nor sodium stibogluconate. PX-6518 did not show activity after oral dosing at up to 200 mg/kg for 5 days. This study concludes that triterpenoid saponins from M. balansae show promising in vitro and in vivo antileishmanial potential and can be considered as new lead structures in the search for novel antileishmanial drugs.Leishmaniasis is a growing health problem in many parts of the world, with about 350 million people living in areas of disease endemicity and about 2 million new cases each year (10). As for the other neglected diseases of poverty (28), treatment options for leishmaniases are limited and rely on pentavalent antimonials as first-line and amphotericin B and pentamidine as second-line drugs (8, 13). However, the excessively high cost of liposomal amphotericin B precludes any practical use (41). An important step forward was the recent approval of miltefosine as the first oral treatment for visceral leishmaniasis (37).Given the prospect that antileishmanial vaccines may not become available in the near future (14), the search for better drugs should be continued, whereby natural products may offer an unlimited source of chemical diversity for identification of new drug templates (1, 5, 11).During a random drug screening program, the methanolic extract of the leaves of the Vietnamese plant Maesa balansae Mez. (Myrsinaceae) was found to possess strong antileishmanial potential (23), and pentacyclic triterpene saponins were identified as the active constituents (12). Saponins have been demonstrated in a large number of plant species, and very different biological activities have been described (16). Antileishmanial activity has been reported for Hedera (9, 31), Dracaena (26), and Yucca (29) saponins, but their value as drug candidates cannot be fully assessed, since in v...

The methanolic extract from the leaves of the Vietnamese medicinal plant Maesa balansae showed potent in vitro and in vivo activity against the tropical protozoal parasite Leishmania infantum. Bioassay-guided purification of the extract led to the identification of six triterpenoid saponins, maesabalides I-VI (1-6), each having a strong and specific anti-leishmanial activity. Maesabalide III (3) and IV (4) were the most potent with IC(50) values against intracellular amastigotes of about 7 and 14 ng/mL. In comparison, the IC(50) value of sodium stibogluconate, the reference drug for treatment of leishmaniasis, is only 5.6 microg/mL. No cytotoxicity was present on a human fibroblast (MRC-5) cell line (CC(50) > 32 microg/mL). In vivo evaluation in the BALB/C mouse model demonstrated that >90% reduction of liver amastigote burdens was obtained 1 week after a single subcutaneous dose at 0.2-0.4 mg/kg was administered. Several chemical derivatives of maesabalides I-VI were prepared in order to study the structure-activity relationship.

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Maesabalide III (MB-III), an oleane triterpene saponin isolated from the Vietnamese plant Maesa balansae, is a new antileishmanial lead compound whose activity against Leishmania donovani (MHOM/ET/67/L82) in groups of five golden hamsters was evaluated after administration of a single subcutaneous dose on either day 1 (prophylactic treatment) or day 28 (curative treatment) after infection. Liposomal amphotericin B (AmBisome), administered intravenously at 5 mg/kg of body weight, was used as the reference drug. Amastigote burdens in liver, spleen, and bone marrow were determined either 7 days (early effects) or 56 days (late effects) after treatment. Prophylactic administration of MB-III at 0.2 mg/kg reduced liver amastigote burdens by 99.8 and 83% within 7 and 56 days after treatment, respectively. In the latter group, however, all animals became ill and some died. Among other tropical diseases, insufficient resources are being allocated to tackle leishmaniasis (15). The emergence of resistance to the first-line drugs (19) and the fact that the drugs available at present do not adequately cover the prime requirements for effective disease control (4) explain why new drug discovery must remain a pivotal objective (9). We recently demonstrated that olenane triterpene saponins (PX-6518) are promising new antileishmanial lead structures (10). By using the Leishmania donovani-infected BALB/c mouse model, administration of a single subcutaneous dose at 0.4 mg/kg of body weight day 1 after infection reduced liver amastigote burdens by about 95% within 7 days of treatment. Although the BALB/c model is highly validated for drug screening purposes, it does not reproduce the progressive disease features that are seen in human visceral leishmaniasis (VL) (14). The golden hamster is regarded to be more sensitive to progressive VL and is a better model for prediction of the effects of drugs on progressive VL (11, 16) and was therefore selected as the experimental host in the present study. From advanced characterization of the individual constituents of PX-6518, maesabalide III (MB-III) (Fig. 1) was retained as a potential drug development candidate (10). Although multiple-dose treatment schedules are adopted in standard clinical practice, single-dose regimens for MB-III and liposomal amphotericin B (AmBisome) were envisaged in the present study because a high degree of efficacy is already obtained after administration of a single dose (10) and because this provided a more accurate basis for comparative assessment of their efficacies. Among the commercial liposomal formulations of amphotericin B, AmBisome was shown to have the best antileishmanial potential under experimental conditions (20) and in clinical practice as a low, single-dose treatment (18). MATERIALS AND METHODSParasite. Amastigotes of a drug-sensitive L. donovani strain (MHOM/ET/67/ L82) were harvested from the spleens of 6-week-infected donor hamsters (Mesocrisetus auratus). The inoculum for infection was prepared as described previously (10) and involved purif...

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