Pentamidine uptake in Leishmania donovani and Leishmania amazonensis promastigotes and axenic amastigotes

Evaluation of the Leishmania Inositol Phosphorylceramide Synthase as a Drug Target Using a Chemical and Genetic Approach

The lack of effective vaccines and the development of resistance to the current treatments highlight the urgent need for new anti-leishmanials. Sphingolipid metabolism has been proposed as a promising source of Leishmania-specific targets as these lipids are key structural components of the eukaryotic plasma membrane and are involved in distinct cellular events. Inositol phosphorylceramide (IPC) is the primary sphingolipid in the Leishmania species and is the product of a reaction mediated by IPC synthase (IPCS). The antihistamine clemastine fumarate has been identified as an inhibitor of IPCS in L. major and a potent anti-leishmanial in vivo. Here we sought to further examine the target of this compound in the more tractable species L. mexicana, using an approach combining genomic, proteomic, metabolomic and lipidomic technologies, with molecular and biochemical studies. While the data demonstrated that the response to clemastine fumarate was largely conserved, unexpected disturbances beyond sphingolipid metabolism were identified. Furthermore, while deletion of the gene encoding LmxIPCS had little impact in vitro, it did influence clemastine fumarate efficacy and, importantly, in vivo pathogenicity. Together, these data demonstrate that clemastine does inhibit LmxIPCS and cause associated metabolic disturbances, but its primary target may lie elsewhere.

Quantum Dynamics and Bi Metal Force Field Parameterization Yielding Significant Antileishmanial Targets

Amid emerging drug resistance to metal inhibitors, high toxicity, and onerous drug delivery procedures, the computational design of alternate formulations encompassing functional metal-containing compounds greatly relies on large-scale atomistic simulations. Simulations particularly with Au(I), Ag, Bi(V), and Sb(V) pose a major challenge to elucidate their molecular mechanism due to the absence of force field parameters. This study thus quantum mechanically derives force field parameters of Bi(V) as an extension of the previous experimental study conducted on heteroleptic triorganobismuth(V) biscarboxylates of type [BiR3(O2CR')2]. We have modeled two organo-bismuth(V) carboxylates, which are optimized and parameterized along with the famous pentavalent antimonial drug: meglumine antimoniate using quantum mechanics original Seminarian methods with the SBKJC effective core potential (ECP) basis set. Furthermore, molecular dynamics (MD) simulations of bismuth- and antimony-containing compounds in complex with two enzymes, trypanothione synthetase-amidase (TSA) and trypanothione reductase, are performed to target the (T(SH)2) pathway at multiple points. MD simulations provide novel insights into the binding mechanism of TSA and highlight the role of a single residue Arg569 in modulating the ligand dynamics. Moreover, the presence of an ortho group in a ligand is emphasized to facilitate interactions between Arg569 and the active site residue Arg313 for higher inhibitory activity of TSA. This preliminary generation of parameters specific to bismuth validated by simulations in replica will become a preamble of future computational and experimental research work to open avenues for newer and suitable drug targets.

Treatment of Cutaneous Leishmaniasis and Insights into Species-Specific Responses: A Narrative Review

Cutaneous leishmaniasis (CL) is a complex skin infection that has imposed a heavy burden on many developing countries and is caused by more than 20 Leishmania species. This disease is predominantly associated with disfiguring scars and major social stigma upon infection. The severity of the disease seemingly depends on many factors including the species of parasite, the host, region of endemicity, socio-economic status and the accessibility to health facilities. Despite myriad studies that have been performed on current and novel therapies, the treatment outcomes of CL remain contentious, possibly because of the knowledge gaps that still exist. The differential responses to the current CL therapies have become a major drawback in disease control, and the dearth of information on critical analyses of outcomes of such studies is a hindrance to the overall understanding. On the basis of currently available literature on treatment outcomes, we discuss the most effective doses, drug susceptibilities/resistance and treatment failures of the Leishmania genus for both monotherapy and combination therapy. This review focuses on the available treatment modalities for CL caused by different Leishmania species, with insights into their species-specific efficacies, which would inform the selection of appropriate drugs for the treatment and control of leishmaniasis.

Diminazene resistance in Trypanosoma congolense is not caused by reduced transport capacity but associated with reduced mitochondrial membrane potential

Trypanosoma congolense is a principal agent causing livestock trypanosomiasis in Africa, costing developing economies billions of dollars and undermining food security. Only the diamidine diminazene and the phenanthridine isometamidium are regularly used, and resistance is widespread but poorly understood. We induced stable diminazene resistance in T. congolense strain IL3000 in vitro. There was no cross-resistance with the phenanthridine drugs, melaminophenyl arsenicals, oxaborole trypanocides, or with diamidine trypanocides, except the close analogs DB829 and DB75. Fluorescence microscopy showed that accumulation of DB75 was inhibited by folate. Uptake of [³ H]-diminazene was slow with low affinity and partly but reciprocally inhibited by folate and by competing diamidines. Expression of T. congolense folate transporters in diminazene-resistant Trypanosoma brucei brucei significantly sensitized the cells to diminazene and DB829, but not to oxaborole AN7973. However, [³ H]-diminazene transport studies, whole-genome sequencing, and RNA-seq found no major changes in diminazene uptake, folate transporter sequence, or expression. Instead, all resistant clones displayed a moderate reduction in the mitochondrial membrane potential Ψm. We conclude that diminazene uptake in T. congolense proceed via multiple low affinity mechanisms including folate transporters; while resistance is associated with a reduction in Ψm it is unclear whether this is the primary cause of the resistance.

An Overview on the Therapeutics of Neglected Infectious Diseases-Leishmaniasis and Chagas Diseases

Neglected tropical diseases (NTDs) as termed by WHO include twenty different infectious diseases that are caused by bacteria, viruses, and parasites. Among these NTDs, Chagas disease and leishmaniasis are reported to cause high mortality in humans and are further associated with the limitations of existing drugs like severe toxicity and drug resistance. The above hitches have rendered researchers to focus on developing alternatives and novel therapeutics for the treatment of these diseases. In the past decade, several target-based drugs have emerged, which focus on specific biochemical pathways of the causative parasites. For leishmaniasis, the targets such as nucleoside analogs, inhibitors targeting nucleoside phosphate kinases of the parasite’s purine salvage pathway, 20S proteasome of Leishmania, mitochondria, and the associated proteins are reviewed along with the chemical structures of potential drug candidates. Similarly, in case of therapeutics for Chagas disease, several target-based drug candidates targeting sterol biosynthetic pathway (C14-ademethylase), L-cysteine protease, heme peroxidation, mitochondria, farnesyl pyrophosphate, etc., which are vital and unique to the causative parasite are discussed. Moreover, the use of nano-based formulations towards the therapeutics of the above diseases is also discussed.

Essential Oil from Chenopodium ambrosioides as a Promising Antileishmanial Agent

Chenopodium ambrosioides has been used traditionally against parasitic diseases. The essential oil of the plant is a complex mixture of compounds with a rich structural diversity. This review focuses on recent evaluation of the essential oil from C. ambrosioides as a promising antileishmanial agent. The tested product showed activity against promastigotes and amastigotes of Leishmania amazonensis and L. donovani. An optimal dose of 30 mg/Kg was effective by intraperitoneal and oral routes in experimental cutaneous leishmaniasis. The chenopodium oil had a moderate toxicity against peritoneal macrophages of BALB/c mice and no side effects were detected in animals treated by the oral route. Isolates of L. amazonensis from treated mice were susceptible to the essential oil. Synergic effects were observed when the essential oil was incubated in conjunction with pentamidine on L. amazonensis promastigote cultures. Future studies focusing on formulation, toxicity and mechanism of action may help in the development of chenopodium oil as a new antileishmanial drug.

Do bismuth complexes hold promise as antileishmanial drugs?

Even after 70 years, pentavalent antimonials sodium stibogluconate and meglumine antimoniate remain the most important and cost-effective antileishmanial drugs. However, the drugs cannot be delivered orally and treatment involves intravascular or intramuscular injections for 28 days under strict medical monitoring due to the toxicity of Sb(III). The main alternatives, amphotericin B, pentamidine and miltefosine, are expensive and not without their own problems. Bismuth sits below antimony in the periodic table and is considered to be relatively nontoxic to humans while being capable of providing powerful antimicrobial activity. This review describes recent efforts into developing antileishmanial Bi(III) and Bi(V) drugs, which may resemble Sb analogs in effect and mode-of-action while providing lower mammalian cell toxicity and opportunities of oral delivery. Within the last 10 years, various studies concerning bismuth-based compounds as potential antileishmanial agents have been published. This review seeks to summarize the relevant studies and draw a conclusion as to whether bismuth complexes have the potential to be effective drugs.

Arginase Is Essential for Survival of Leishmania donovani Promastigotes but Not Intracellular Amastigotes

Studies of Leishmania donovani have shown that both ornithine decarboxylase and spermidine synthase, two enzymes of the polyamine biosynthetic pathway, are critical for promastigote proliferation and required for maximum infection in mice. However, the importance of arginase (ARG), the first enzyme of the polyamine pathway in Leishmania, has not been analyzed in L. donovani. To test ARG function in intact parasites, we generated Δarg null mutants in L. donovani and evaluated their ability to proliferate in vitro and trigger infections in mice. The Δarg knockout was incapable of growth in the absence of polyamine supplementation, but the auxotrophic phenotype could be bypassed by addition of either millimolar concentrations of ornithine or micromolar concentrations of putrescine or by complementation with either glycosomal or cytosolic versions of ARG. Spermidine supplementation of the medium did not circumvent the polyamine auxotrophy of the Δarg line. Although ARG was found to be essential for ornithine and polyamine synthesis, ornithine decarboxylase appeared to be the rate-limiting enzyme for polyamine production. Mouse infectivity studies revealed that the Δarg lesion reduced parasite burdens in livers by an order of magnitude but had little impact on the numbers of parasites recovered from spleens. Thus, ARG is essential for proliferation of promastigotes but not intracellular amastigotes. Coupled with previous studies, these data support a model in which L. donovani amastigotes readily salvage ornithine and have some access to host spermidine pools, while host putrescine appears to be unavailable for salvage by the parasite.

Comparative analysis of the omics technologies used to study antimonial, amphotericin B, and pentamidine resistance in leishmania

Leishmaniasis is a serious threat in developing countries due to its endemic nature and debilitating symptoms. Extensive research and investigations have been carried out to learn about the mechanism of drug resistance in Leishmania but results obtained in the laboratory are not in agreement with those obtained from the field. Also the lack of knowledge about the mode of action for a number of drugs makes the study of drug resistance more complex. A major concern in recent times has been regarding the role of parasitic virulence in drug resistance for Leishmania. Researchers have employed various techniques to unravel the facts about resistance and virulence in Leishmania. With advent of advanced and more specific means of detection, further hints about probable mechanisms of conferring resistance are expected. This review aims to provide a consolidated picture along with a comparative account of the work done so far to study the mechanism of antimony, amphotericin B, and pentamidine resistance using various techniques.

Evaluation of the Leishmanicidal Activity of Rutaceae and Lauraceae Ethanol Extracts on Golden Syrian Hamster (Mesocricetus auratus) Peritoneal Macrophages

Traditional medicine has provided a number of therapeutic solutions for the control of infectious agents, cancers, and other diseases. After screening a wide variety of Colombian plant extracts, we have identified promising antileishmanial activity in ethanol extracts from Ocotea macrophylla (Lauraceae) and Zanthoxyllum monophyllum (Rutaceae). In this study, we evaluated the in vitro activity of two ethanol extracts, one from Ocotea macrophylla and the other from Zanthoxyllum monophyllum and one alkaloid fraction of ethanol extract of Zanthoxyllum monophyllum, on peritoneal macrophages isolated from golden Syrian hamsters (Mesocricetus auratus) infected with Leishmania panamensis and Leishmania major promastigotes. All of the extracts studied displayed promising (≥2) selectivity indices (S/I), the most significant of which were for ethanol extract of Zanthoxyllum monophyllum against Leishmania panamensis (S/I=12) and alkaloid fraction of ethanol extract of Zanthoxyllum monophyllum against Leishmania major (S/I=11). These results support the use of ethanol extracts and alkaloid fractions isolated from Ocotea macrophylla and Zanthoxyllum monophyllum, respectively; as therapeutic options for cutaneous leishmaniasis.

Developments in diagnosis and treatment of visceral leishmaniasis during the last decade and future prospects

Human visceral leishmaniasis (VL) continues to be a life-threatening neglected tropical disease, with close to 200 million people at risk of infection globally. Epidemics and resurgence of VL are associated with negligence by the policy makers, economic decline and population movements. Control of the disease is hampered by the lack of proficient vaccination, rapid diagnosis in a field setting and severe side effects of current drug therapies. The diagnosis of VL relied largely on invasive techniques of detecting parasites in splenic and bone marrow aspirates. rK39 and PCR, despite problems related to varying sensitivities and specificities and field adaptability, respectively, are considered the best options for VL diagnosis today. No single therapy of VL currently offers satisfactory efficacy along with safety. The field of VL research only recently shifted toward actively identifying new drugs for safe and affordable treatment. Oral miltefosine and safe AmBisome along with better use of amphotericin B have been rapidly implemented in the last decade. A combination therapy will substantially reduce the required dose and duration of drug administration and reduce the chance of the development of resistance. In addition, identification of asymptomatic cases, vector control and treatment of post-kala-azar dermal leishmaniasis would allow new perspectives in VL control and management.

Leishmaniasis: current status of available drugs and new potential drug targets

The control of Leishmania infection relies primarily on chemotherapy till date. Resistance to pentavalent antimonials, which have been the recommended drugs to treat cutaneous and visceral leishmaniasis, is now widespread in Indian subcontinents. New drug formulations like amphotericin B, its lipid formulations, and miltefosine have shown great efficacy to treat leishmaniasis but their high cost and therapeutic complications limit their usefulness. In addition, irregular and inappropriate uses of these second line drugs in endemic regions like state of Bihar, India threaten resistance development in the parasite. In context to the limited drug options and unavailability of either preventive or prophylactic candidates, there is a pressing need to develop true antileishmanial drugs to reduce the disease burden of this debilitating endemic disease. Notwithstanding significant progress of leishmanial research during last few decades, identification and characterization of novel drugs and drug targets are far from satisfactory. This review will initially describe current drug regimens and later will provide an overview on few important biochemical and enzymatic machineries that could be utilized as putative drug targets for generation of true antileishmanial drugs.

Uptake of T-2307, a novel arylamidine, in Candida albicans

OBJECTIVES

T-2307, a novel arylamidine synthesized at Toyama Chemical Co., Ltd, has in vitro and in vivo broad-spectrum activities against pathogenic fungi. T-2307 particularly exhibits potent in vitro and in vivo activity against Candida albicans, suggesting that its uptake might be mediated by a transport system. In this report, we studied the uptake of T-2307 in C. albicans.

METHODS

C. albicans cells and rat hepatocytes were exposed to 0.02 microM [(14)C]T-2307. After incubation, the reaction mixture was concentrated and layered on a silicon layer (mixture of silicon oil and liquid paraffin) inside a tube. The tube was then centrifuged to transfer cells into the bottom layer (sodium hydroxide) for solubilization. The bottom layer was neutralized and measured for radioactivity.

RESULTS

T-2307 was concentrated from the extracellular medium by C. albicans cells in 10 mM phosphate buffer solution supplemented with 1% glucose by 3200- to 5100-fold. The accumulation was approximately two orders of magnitude greater than that achieved with a rat hepatocyte preparation. T-2307 uptake was sensitive to temperature and extracellular pH, and was reduced in the presence of inhibitors of mitochondrial respiration, oxidative phosphorylation and plasma membrane proton pump, and by an uncoupler. Furthermore, T-2307 uptake was concentration dependent and an Eadie-Hofstee plot suggested the involvement of two transport systems.

CONCLUSIONS

The considerably higher concentrations of T-2307 were selectively accumulated in C. albicans via transporter-mediated systems, as compared with the concentrations in rat hepatocytes. This transporter-mediated uptake of T-2307 contributes to its potent anticandidal activity.

Arylimidamide DB766, a potential chemotherapeutic candidate for Chagas' disease treatment

Chagas' disease, a neglected tropical illness for which current therapy is unsatisfactory, is caused by the intracellular parasite Trypanosoma cruzi. The goal of this work is to investigate the in vitro and in vivo effects of the arylimidamide (AIA) DB766 against T. cruzi. This arylimidamide exhibits strong trypanocidal activity and excellent selectivity for bloodstream trypomastigotes and intracellular amastigotes (Y strain), giving IC(50)s (drug concentrations that reduce 50% of the number of the treated parasites) of 60 and 25 nM, respectively. DB766 also exerts striking effects upon different parasite stocks, including those naturally resistant to benznidazole, and displays higher activity in vitro than the reference drugs. By fluorescent and transmission electron microscopy analyses, we found that this AIA localizes in DNA-enriched compartments and induces considerable damage to the mitochondria. DB766 effectively reduces the parasite load in the blood and cardiac tissue and presents efficacy similar to that of benznidazole in mouse models of T. cruzi infection employing the Y and Colombian strains, using oral and intraperitoneal doses of up to 100 mg/kg/day that were given after the establishment of parasite infection. This AIA ameliorates electrocardiographic alterations, reduces hepatic and heart lesions induced by the infection, and provides 90 to 100% protection against mortality, which is similar to that provided by benznidazole. Our data clearly show the trypanocidal efficacy of DB766, suggesting that this AIA may represent a new lead compound candidate to Chagas' disease treatment.

Two novel nucleobase/pentamidine transporters from Trypanosoma brucei

African trypanosomes are unable to synthesize purines de novo and must salvage preformed purine nucleosides and nucleobases from their hosts. The Trypanosoma brucei genome project has identified 12 members of the equilibrative nucleoside transporter family, most of which have been characterized previously as nucleoside and/or nucleobase transporters. Here the 11th member of this family, TbNT11.1, has been functionally expressed in null mutants of Leishmania that are deficient in purine nucleoside or nucleobase uptake and identified as a high-affinity purine nucleobase transporter. Expression of TbNT11.1 in Xenopus oocytes revealed that it is also a transporter for the diamidine drug pentamidine that is the principal drug employed to treat early stage human African trypanosomiasis and may thus contribute to the uptake of this therapeutically important compound. In addition, characterization of the 12th member of the family, TbNT12.1, reveals that it is an adenine/pentamidine transporter.

Combined effect of the essential oil from Chenopodium ambrosioides and antileishmanial drugs on promastigotes of Leishmania amazonensis

To date, there are no vaccines against Leishmania, and chemotherapy remains the mainstay for the control of leishmaniasis. The drugs of choice used for leishmaniasis therapy are significantly toxic, expensive and with a growing frequency of refractory infections. Because of these limitations, a combination therapy is the better hope. This work demonstrates that the essential oil from Chenopodium ambrosioides shows a synergic activity after incubation in conjunction with pentamidine against promastigotes of Leishmania amazonensis. However, an indifferent effect has been found for combinations of meglumine antimoniate or amphotericin B and the essential oil.

Drug resistance in Leishmania: similarities and differences to other organisms

The main line of defense available against parasitic protozoa is chemotherapy. Drug resistance has emerged however, as a primary obstacle to the successful treatment and control of parasitic diseases. Leishmania spp., the causative agents of leishmaniasis, have served as a useful model for studying mechanisms of drug resistance in vitro. Antimonials and amphotericin B are the first line drugs to treat Leishmania followed by pentamidine and a number of other drugs. Parasites resistant against all these classes of drugs have been selected under laboratory conditions. A multiplicity of resistance mechanisms has been detected, the most prevalent being gene amplification and transport mutations. With the tools now available, it should be possible to elucidate the mechanisms that govern drug resistance in field isolates and develop more effective chemotherapeutic agents.

Flavonoid dimers as bivalent modulators for pentamidine and sodium stiboglucanate resistance in leishmania

Drug resistance by overexpression of ATP-binding cassette (ABC) transporters is an impediment in the treatment of leishmaniasis. Flavonoids are known to reverse multidrug resistance (MDR) in Leishmania and mammalian cancers by inhibiting ABC transporters. Here, we found that synthetic flavonoid dimers with three (compound 9c) or four (compound 9d) ethylene glycol units exhibited a significantly higher reversing activity than other shorter or longer ethylene glycol-ligated dimers, with approximately 3-fold sensitization of pentamidine and sodium stibogluconate (SSG) resistance in Leishmania, respectively. This modulatory effect was dosage dependent and not observed in apigenin monomers with the linker, suggesting that the modulatory effect is due to its bivalent nature. The mechanism of reversal activity was due to increased intracellular accumulation of pentamidine and total antimony in Leishmania. Compared to other MDR modulators such as verapamil, reserpine, quinine, quinacrine, and quinidine, compounds 9c and 9d were the only agents that can reverse SSG resistance. In terms of reversing pentamidine resistance, 9c and 9d have activities comparable to those of reserpine and quinacrine. Modulators 9c and 9d exhibited reversal activity on pentamidine resistance among LeMDR1(-/-), LeMDR1(+/+), and LeMDR1-overexpressed mutants, suggesting that these modulators are specific to a non-LeMDR1 pentamidine transporter. The LeMDR1 copy number is inversely related to pentamidine resistance, suggesting that it might be involved in importing pentamidine into the mitochondria. In summary, bivalency could be a useful strategy for the development of more potent ABC transporter modulators and flavonoid dimers represent a promising reversal agent for overcoming pentamidine and SSG resistance in parasite Leishmania.

The role of Leishmania enriettii multidrug resistance protein 1 (LeMDR1) in mediating drug resistance is iron-dependent

In parasitic protozoan Leishmania enriettii, the role of a multidrug resistance (mdr) gene LeMDR1 (L. enriettii multidrug resistance 1) in mediating vinblastine resistance has been previously demonstrated by association, transfection and "gene knockout" studies. LeMDR1 has been shown to be located intracellularly and it was proposed to mediate drug resistance by sequestering drugs into intracellular organelles rather than by active efflux. Here we compared LeMDR1 overexpressed cell lines (Vint3 and V160), wild type (Le) and LeMDR1 "double knockout" mutant (LeMDR1-/-) and demonstrated that LeMDR1 gene copy number was associated with (1) higher level of intracellular iron, (2) increased sensitivity to an iron-dependent antibiotic, streptonigrin and (3) increased enzyme activity of an iron-sulfur-containing mitochondrial enzyme, aconitase. This result suggests that the normal function of LeMDR1 is related to mitochondrial iron homeostasis. To test such hypothesis, we have used the LeMDR1-overexpressing mutant V160 and LeMDR1-/- mutant to determine how iron level can affect its resistance level to drugs targeting either cytosol (vinblastine) or mitochondria (rhodamine 123 and pentamidine). It was found that the resistance level of V160 to vinblastine can be increased by iron whereas resistance to both rhodamine 123 and pentamidine can be increased by iron depletion and vice versa. Iron treatment can potentiate rhodamine 123 and pentamidine accumulation whereas iron deprivation can cause the reduction of rhodamine 123 accumulation. Our result highly suggests that LeMDR1's function in mediating drug resistance is iron-dependent.

Anti-plasmodial and anti-leishmanial activity of conformationally restricted pentamidine congeners

A library of 52 pentamidine congeners in which the flexible pentyldioxy linker in pentamidine was replaced with various restricted linkers was tested for in-vitro activity against two Plasmodium falciparum strains and Leishmania donovani. The tested compounds were generally more effective against P. falciparum than L. donovani. The most active compounds against the chloroquine-sensitive (D6, Sierra Leone) and -resistant (W2, Indochina) strains of P. falciparum were bisbenzamidines linked with a 1,4-piperazinediyl or 1, 4-homopiperazinediyl moiety, with IC50 values (50% inhibitory concentration, inhibiting parasite growth by 50% in relation to drug-free control) as low as 7 nM based on the parasite lactate dehydrogenase assay. Seven piperazine-linked bisbenzamidines substituted at the amidinium nitrogens with a linear alkyl group of 3-6 carbons (22, 25, 27, 31) or cycloalkyl group of 4, 6 or 7 carbons (26, 32, 34) were more potent (IC50<40 nM) than chloroquine or pentamidine as anti-plasmodial agents. The most active anti-leishmanial agents were 4,4'-[1,4-phenylenebis(methyleneoxy)]bisbenzenecarboximidamide (2, IC50 approximately 0.290 microM) and 1,4-bis[4-(1H-benzimidazol-2-yl)phenyl] piperazine (44, IC50 approximately 0.410 microM), which were 10- and 7-fold more potent than pentamidine (IC50 approximately 2.90 microM). Several of the more active anti-plasmodial agents (e.g. 2, 31, 33, 36-38) were also potent anti-leishmanial agents, indicating broad antiprotozoal properties. However, a number of analogues that showed potent anti-plasmodial activity (1, 18, 21, 22, 25-28, 32, 43, 45) were not significantly active against the Leishmania parasite. This indicates differential modes of anti-plasmodial and anti-leishmanial actions for this class of compounds. These compounds provide important structure-activity relationship data for the design of improved chemotherapeutic agents against parasitic infections.

Aromatic diamidines as antiparasitic agents

Parasitic infections are widespread in developing countries and frequently associated with immunocompromised patients in developed countries. Consequently, such infections are responsible for a significant amount of human mortality, morbidity and economic hardship. A growing consensus has identified the urgent need for the development of new antiparasitic compounds, mostly due to the large number of drug-resistant parasites and the fact that currently available drugs are expensive, highly toxic, require long treatment regimens and frequently exhibit significantly reduced activity towards certain parasite strains and evolutive stages. In this context, the activity of aromatic diamidines has been explored against a widespread range of micro-organisms, and the authors' present aim is to review the current status of chemotherapy with these compounds against human parasitic infections.

Roles for mitochondria in pentamidine susceptibility and resistance in Leishmania donovani

Pentamidine resistant Leishmania donovani was raised in the laboratory by stepwise exposure to increasing drug pressure until a line capable of growth in 8 microM pentamidine (R8) had been selected. An IC(50) value of 40 microM was determined for this line, some 50-fold higher than that recorded for the parental wild-type line. The pentamidine resistant promastigotes were cross-resistant to other toxic diamidine derivatives but not to antimonials or substrates of multidrug resistance pumps. Decreased mitochondrial transmembrane potential was observed in pentamidine resistant promastigotes. A substantial net decrease in accumulation of [(3)H]-pentamidine accompanied the resistance phenotype. Inhibitors of P-glycoprotein pumps, including prochlorperazine and trifluoperazine, did not reverse this decreased drug uptake, which distinguishes the L. donovani resistant line studied here from L. mexicana promastigotes previously studied for pentamidine resistance. Kinetic analysis identified a carrier with an apparent K(m) value of 6 microM for pentamidine. No significant difference between wild-type and resistant parasites could be detected with respect to this transporter in rapid uptake experiments. However, in longer-term uptake experiments and also using concentrations of pentamidine up to 1mM, it was demonstrated that wild-type cells, but not resistant cells, could continue to accumulate pentamidine after apparent saturation via the measured transporter had been reached. Agents that diminish the mitochondrial membrane potential inhibited this secondary route. A fluorescent analogue of pentamidine, 2,5-bis-(4-amidophenyl)-3,4-dimethylfuran (DB99), accumulated in the kinetoplast of wild-type but not resistant parasites indicating that uptake of this cationic compound into mitochondria of wild-type cells was more pronounced than in the resistant line. These data together indicate that resistance to pentamidine in L. donovani is associated with alterations to the mitochondria of the parasites, which lead to reduced accumulation of drug.

Identification and Characterization of a Polyamine Permease from the Protozoan Parasite Leishmania major

The proteins that mediate polyamine translocation into eukaryotic cells have not been identified at the molecular level. To define the polyamine transport pathways in eukaryotic cells we have cloned a gene, LmPOT1, that encodes a polyamine transporter from the protozoan pathogen, Leishmania major. Sequence analysis of LmPOT1 predicted an unusual 803-residue polytopic protein with 9-12 transmembrane domains. Expression of LmPOT1 cRNA in Xenopus laevis oocytes revealed LmPOT1 to be a high affinity transporter for both putrescine and spermidine, whereas expression of LmPOT1 in Trypanosoma brucei stimulated putrescine uptake that was sensitive to inhibition by pentamidine and proton ionophores. Immunoblot analysis established that LmPOT1 was expressed predominantly in the insect vector form of L. major, and immunofluorescence demonstrated that LmPOT1 was localized predominantly to the parasite plasma membrane. To our knowledge this is the first molecular identification and characterization of a cell surface polyamine transporter in eukaryotic cells.

Possible mechanism of miltefosine-mediated death of Leishmania donovani

Miltefosine causes leishmanial death, but the possible mechanism(s) of action is not known. The mode of action of miltefosine was investigated in vitro in Leishmania donovani promastigotes as well as in extra- and intracellular amastigotes. Here, we demonstrate that miltefosine induces apoptosis-like death in L. donovani based on observed phenomena such as nuclear DNA condensation, DNA fragmentation with accompanying ladder formation, and in situ labeling of DNA fragments by the terminal deoxyribonucleotidyltransferase-mediated dUTP-biotin nick end labeling method. Understanding of miltefosine-mediated death will facilitate the design of new therapeutic strategies against Leishmania parasites.

Effect of amidine derivatives on nitric oxide production by Leishmania amazonensis promastigotes and axenic amastigotes

The effects of pentamidine isethionate (reference drug) and N,N'-diphenyl-4-methoxy-benzamidine (test compound) on NO. production by Leishmania amazonensis promastigotes and axenic amastigotes were investigated by measuring nitrite, a by-product of nitric oxide released into culture supernatants. The NO. production by infective promastigotes was inhibited by OCH(3)-amidine in about 23.53% and by pentamidine in only 3.78%. In axenic amastigotes, the inhibition of NO. production by OCH(3)-amidine was significantly higher (52.94%; p=0.01) than that by pentamidine, which inhibited this radical production nonsignificantly (25.29%; p=0.1). The mechanism of amidine derivatives, as an antimicrobial agent, is unknown. However, other amidines, such as a diamidine (pentamidine), contain chemical structures shared by the guanidino group of the nitric oxide synthase substrate L-arginine, suggesting the possibility of an interaction with this enzyme or electronic factors (substituent constant) that alter physical and chemical properties significant for biological activity.

QSAR and molecular modelling studies on B-DNA recognition of minor groove binders

Aromatic bisamidines have been proved to be efficient compounds against Leishmania spp. and Pneumocystis carinii. Although the mode of action is still not known, these molecules are supposed to be DNA minor groove binders (MGBs). This paper describes a molecular modelling study for a set of MGBs in order to rank them through their complementarity to the Dickerson Drew Dodecamer (DDD) according to their interaction energies with B-DNA. A comparative molecular field analysis (CoMFA) has shown the importance of relatively bulky positively charged groups attached to the MGB aromatic rings, and small and negatively charged substituents into the middle chain. Models were obtained for DNA denaturation related to H-bonding processes of binding modes. Validation of the model demonstrated the robustness of CoMFA in terms of independent test set of similar MGBs. GRID results allotted bioisosteric substitution of z.sbnd;Oz.sbnd; by z.sbnd;NHz.sbnd; in furan ring of furamidine and related compounds as being capable to enhance the binding to DDD.

Resistance to pentamidine in Leishmania mexicana involves exclusion of the drug from the mitochondrion

The uptake of [(3)H]pentamidine into wild-type and drug-resistant strains of Leishmania mexicana was compared. Uptake was carrier mediated. Pentamidine-resistant parasites showed cross-resistance to other toxic diamidine derivatives. A substantial decrease in accumulation of the drug accompanied the resistance phenotype, although the apparent affinity for pentamidine by its carrier was not altered when initial uptake velocity was measured. The apparent V(max), however, was reduced. An efflux of pentamidine could be measured in both wild-type and resistant cells. Only a relatively small proportion of the total accumulated pentamidine was available for efflux in wild-type cells, while in resistant cells the majority of loaded pentamidine was available for release. Pharmacological reagents which diminish the mitochondrial membrane potential reduced pentamidine uptake in wild-type parasites, and the mitochondrial membrane potential was shown to be reduced in resistant cells. A fluorescent analogue of pentamidine, 4',6'-diamidino-2-phenylindole, accumulated in the kinetoplast of wild-type but not resistant parasites. These data together indicate that diamidine drugs accumulate in the Leishmania mitochondrion and that the development of the resistance phenotype is accompanied by lack of mitochondrial accumulation of the drug and its exclusion from the parasites.

Promising therapeutic targets for antileishmanial drugs

Current treatments for the parasitic disease leishmaniasis are unsatisfactory due to their route of administration, toxicity and expense. Resistance is also developing to first-line antimonial drugs. Fortunately, a handful of antileishmanial agents, such as the orally available compound miltefosine, are currently in clinical trials. In addition, several promising drug targets and lead molecules are being studied with the goal of developing new antileishmanial agents. Drug candidates have been identified through the continued investigation of parasite sterol metabolism and parasite proteases. New antileishmanial molecules have also been discovered through the study of novel targets and pathways, such as the bisphosphonate inhibitors of isoprenoid biosynthesis. This review presents a synopsis of the drug targets and lead compounds that have been investigated over the last few years against leishmaniasis, gives a perspective on the chemotherapeutic potential of each and discusses some of the obstacles to antileishmanial drug development.

Antileishmanial activities of several classes of aromatic dications

Aromatic dicationic molecules possess impressive activity against a broad spectrum of microbial pathogens, including Pneumocystis carinii, Cryptosporidium parvum, and Candida albicans. In this work, 58 aromatic cations were examined for inhibitory activity against axenic amastigote-like Leishmania donovani parasites. In general, the most potent of the compounds were substituted diphenyl furan and thiophene dications. 2,5-Bis-(4-amidinophenyl)thiophene was the most active compound. This agent displayed a 50% inhibitory concentration (IC50) of 0.42 +/- 0.08 microM against L. donovani and an in vitro antileishmanial potency 6.2-fold greater than that of the clinical antileishmanial dication pentamidine and was 155-fold more toxic to the parasites than to a mouse macrophage cell line. 2,4-Bis-(4-amidinopheny)furan was twice as active as pentamidine (IC50), 1.30 +/- 0.21 microM), while 2,5-bis-(4-amidinopheny)furan and pentamidine were essentially equipotent in our in vitro antileishmanial assay. Carbazoles, dibenzofurans, dibenzothiophenes, and benzimidazoles containing amidine or substituted amidine groups were generally less active than the diphenyl furans and thiophenes. In all cases, aromatic dications possessing strong antileishmanial activity were severalfold more toxic to the parasites than to a cultured mouse macrophage cell line. These structure-activity relationships demonstrate the potent antileishmanial activity of several aromatic dications and provide valuable information for the future design and synthesis of more potent antiparasitic agents.

Flow cytometry, a useful tool for detecting the lethal effect of pentamidine on Leishmania viannia complex promastigote forms

Flow cytometry (FC), combined with propidium iodide as supravital colorant, was used to study Leishmania(subgenus Viannia)panamensis [L (V) panamensis] complex strain susceptibility to meglumine antimoniate, sodium stibogluconate and pentamidine. Despite all drugs examined being leishmanicidal to axenic forms in in vitro trials (in the presence of macrophages), axenic amastigotes directly exposed to these drugs were highly resistant under our experimental conditions. A direct lethal effect on promastigotes, detectable by FC, was only obtained with pentamidine after in vitro treatment of both promastigotes and axenic amastigotes with the drugs. Pentamidine's rapid lethal effect, as detected by FC, could be further confirmed in short- and long-term parasite cultures after exposure to a drug. FC's suitability for measuring L (V) panamensis complex's promastigote susceptibility to pentamidine, shortly after in vitro drug exposure, might be useful in monitoring clinical trials with this drug and facilitating rapid pentamidine-resistant natural isolate identification.

Diamidine compounds: selective uptake and targeting in Plasmodium falciparum

Extensive drug resistance in Plasmodium falciparum emphasizes the urgent requirement for novel antimalarial agents. Here we report potent antimalarial activity of a number of diamidine compounds. The lead compound pentamidine is concentrated 500-fold by erythrocytes infected with P. falciparum. Pentamidine accumulation can be blocked by inhibitors of hemoglobin digestion, suggesting that the drug binds to ferriprotoporphyrin IX (FPIX). All of the compounds bound to FPIX in vitro and inhibited the formation of hemozoin. Furthermore, inhibitors of hemoglobin digestion markedly antagonized the antimalarial activity of the diamidines, indicating that binding to FPIX is crucial for the activity of diamidine drugs. Pentamidine was not accumulated into uninfected erythrocytes. Pentamidine transport into infected cells exhibits an initial rapid phase, nonsaturable in the micromolar range and sensitive to inhibition by furosemide and glibenclamide. Changing the counter-ion in the order Cl(-) < Br(-) < NO(2)(-) < I(-) <SCN(-) markedly stimulated pentamidine transport. These data suggest that pentamidine is transported although a pore or ion channel with properties similar to those of the recently characterized 'induced permeability pathway' on the infected red cell membrane. In summary, the diamidines exhibit two levels of selectivity against P. falciparum. The route of entry and molecular target are both specific to malaria-infected cells and are distinct from targets in other protozoa. Drugs that target the hemoglobin degradation pathway of malaria parasites have a proven record of accomplishment. The employment of induced permeability pathways to access this target represents a novel approach to antiparasite chemotherapy and offers an additional level of selectivity.

Transporters in African trypanosomes: role in drug action and resistance

Sleeping sickness is an increasing problem in many parts of sub-Saharan Africa. The problems are compounded by the lack of new medication, and the increasing resistance against traditional drugs such as melarsoprol, berenil and isometamidium. Over the last few years, much progress has been made in understanding how drug action, and the development of resistance, is related to the mechanisms by which the parasite ingests the drugs. In some cases novel transporters have been identified. In other cases, transporters do not appear to be involved in drug uptake, and selectivity must lie with other parasite features, such as a specific target or activation of the drug. Lessons learned from studying the uptake of drugs currently in use may assist the design of a much needed new generation of trypanocides.

Novel Intracellular SbV reducing activity correlates with antimony susceptibility in Leishmania donovani

The standard treatment of human visceral leishmaniasis involves the use of pentavalent antimony (Sb(V)). Its mechanism of action is unknown because of the limited information available about intracellular antimony metabolism and about the genes that regulate these processes. Herein, flow injection-inductively coupled plasma mass spectrometry (ICP-MS), flow injection hydride generation ICP-MS, and ion chromatography ICP-MS were used to measure antimony accumulation and intracellular metabolism in the human protozoan parasite Leishmania donovani. Amastigotes (the intracellular form) and promastigotes (the extracellular form) accumulate Sb(V) and Sb(III) via separate transport systems. Stage-specific intracellular Sb(V) reducing activity was apparent in amastigotes, which reduced the negligibly toxic Sb(V) to highly toxic Sb(III). This amastigote-specific reducing activity was deficient in the Pentostam-resistant mutant L. donovani Ld1S.20. These data indicate that parasite susceptibility to Sb(V) correlates with its level of Sb(V) reducing activity. Also, in promastigotes of both wild-type L. donovani and the Pentostam-resistant mutant L. donovani Ld1S.20, Sb(V) inhibited the toxicity of Sb(III) but not of As(III). Both Sb(V) and Sb(III) were toxic to wild-type amastigotes. However, as observed in promastigotes, in mutant amastigotes Sb(V) inhibits Sb(III) but not As(III) activity. Anion exchange chromatography showed that intracellular antimony metabolism occurred in both promastigotes and amastigotes. These data demonstrate that the interaction between the two antimony oxidation states occurs intracellularly, within the parasite. The results also indicate that Sb(V) anti-leishmanial activity is dependent on its reduction to Sb(III). The mechanism of this novel intracellular Sb(V) reduction has yet to be identified, and it may or may not be enzymatic. This is the first description of intracellular Sb(V) reducing activity in Leishmania as well as in any prokaryotic or eukaryotic cell.

Cellular effects of leishmanial tubulin inhibitors on L. donovani

To aid our investigation of tubulin as an antileishmanial drug target, the effects of the mammalian antimicrotubule agents ansamitocin P3, taxol, and hemiasterlin on Leishmania donovani promastigotes were described. These drugs affected the assembly of purified leishmanial tubulin and inhibited the growth of L. donovani promastigotes at micromolar concentrations. When promastigotes were treated with these agents, mitotic partitioning of nuclear DNA and cytokinesis were usually inhibited. The spatial orientation of kinetoplasts was often disturbed, suggesting a role for microtubules in the segregation of these organelles during mitosis. Aberrant cell types produced in drug-treated cultures included parasites with one nucleus and two geometrically distinct kinetoplasts, parasites with multiple kinetoplasts, and cytoplasts containing a kinetoplast but no nucleus. A subset of unique cell types, parasites containing two nuclei, a spindle fiber, and two geometrically distinct kinetoplasts, were observed in hemiasterlin-treated cultures. Flow cytometric analysis of L. donovani promastigotes treated with these three drugs indicated a dramatic shift toward the G2 + M phase of the cell cycle, with some cells containing four times the amount of DNA present in G1. These results were used to evaluate the cellular effects of WR85915, an aromatic thiocyanate with in vitro antileishmanial and anti-tubulin activity, on L. donovani. Treatment of parasites with WR85915 did not produce the unusual cell types described above and did not cause the accumulation of parasites in G2 + M, suggesting that WR85915 acts on target(s) in Leishmania in addition to tubulin. These studies validate tubulin as a suitable antileishmanial drug target and provide criteria to assess the cellular mechanism of action of new candidate antileishmanial agents.

Putrescine and spermidine transport in Leishmania

The transport of putrescine and spermidine into Leishmnania donovani promastigotes and Leishmania mexicana promastigotes and amastigotes has been characterised. Polyamine transport was shown to be saturable and temperature-sensitive for both developmental stages of Leishmania. Transport was pH-dependent with pH optima of 7.4 and 5.5 for promastigotes and amastigotes, respectively. The uptake process was independent of extracellular Na+, but inhibited by protonophores and H+-ATPase inhibitors. Kinetic analyses of polyamine transport showed that Km and Vmax differed between promastigotes of the two species and between promastigotes and amastigotes of L. mexicana. Inhibition data suggest that putrescine and spermidine use different transporters. The aromatic diamidine pentamidine, the drug of choice for treatment of antimonial-resistant cases of leishmaniasis, inhibited both putrescine and spermidine transport non-competitively.

Developmental regulation of proline transport in Leishmania donovani

Leishmania donovani are the causative agents of kala azar in humans. These organisms cycle between the proline-rich environment of the sand fly vector (extracellular promastigotes) and the sugar-rich condition in the mammalian host (intracellular amastigotes). Parasites have adapted to these extreme changes in proline concentrations: promastigotes utilize proline as a carbon source, whereas amastigotes utilize sugars and fatty acids. Previous studies have suggested that promastigotes and amastigotes express distinct proline transporters. However, the information available on these transporters is limited. In this work, proline transport was investigated in axenic L. donovani cultures. Three transport systems were identified: cation-dependent and -independent proline transporters in promastigotes (systems A and B, respectively) and a single cation-independent transporter in amastigotes (system C). Systems A and C have broad specificity to almost all amino acids and obtain optimum activity at acidic pH ranges (pH 6 and 5, respectively). System B is more specific to proline, as it is inhibited by only five amino acids. Temperature response analyses indicated that the transporters of both promastigotes and amastigotes perform best at 37 degrees C. The activity of system A during parasite differentiation was assessed. The transport activity of system A disappeared 3 days after promastigotes were induced to differentiate into amastigotes. In these cells, elevated temperature and acidic pH each suppressed the activity of system A. When amastigotes were induced to differentiate back into promastigotes, system A resumed its activity 24 h after differentiation was initiated. In conclusion, L. donovani obtain proline transport systems that are stage specific, regulated by both pH and temperature. This paper constitutes the first investigation of amino acid transport in axenic L. donovani.

Effects of intermediates of methionine metabolism and nucleoside analogs on S-adenosylmethionine transport by Trypanosoma brucei brucei and a drug-resistant Trypanosoma brucei rhodesiense

The effects of purine nucleoside analogs, polyamines, and established trypanocidal agents on the uptake of [8-14C]adenosine and S-[methyl-3H]adenosylmethionine (AdoMet) by bloodform trypanosomes of drug-susceptible Trypanosoma brucei brucei and a drug-resistant Trypanosoma brucei rhodesiense clinical isolate were compared. AdoMet uptake was not antagonized by omithine or methionine (500 microM), adenosine (100 microM), or other purine nucleosides, including methylthioadenosine (MTA) at 500 microM. Hydroxyethylthioadenosine (HETA), a trypanocidal analog of methylthioadenosine, and sinefungin, an analog of AdoMet, were competitive with AdoMet transport in both isolates. Dipyridamole, an antagonist of the adenosine P2 transporter, also competed with AdoMet transport in both isolates. The trypanocidal diamidines pentamidine, Berenil, CGP 40215, and the decarboxylated S-adenosylmethionine (dAdoMet) analog MDL 73811 (5'-¿[(Z)-4-amino-2-butenyl]¿methyl-amino¿-5'-deoxyadenosine) competed with P2 adenosine transport but did not inhibit AdoMet transport at 100 microM. Methylglyoxalbis(guanylhydrazone) (MGBG), an analog of dAdoMet, was a strong competitive inhibitor of adenosine transport at 100 microM, but did not inhibit AdoMet transport. The polyamines putrescine, spermine, and spermidine (1 mM) were examined for competition with adenosine and AdoMet transport. Putrescine significantly inhibited P2 adenosine transport in both strains (in the presence of saturating inosine), but AdoMet transport was not affected by these polyamines. P2 adenosine transport in both strains was highly inhibited by melarsen oxide and melamine, its key organic component, whereas AdoMet uptake was not affected by these agents. These findings further characterize distinguishing features of the unique AdoMet transporter in African trypanosomes, and indicate that the P2 adenosine transporter remains functional in melarsen- and diamidine-resistant clinical isolates.

The pharmacology of leishmaniasis

The development of new strategies on chemotherapy of parasitic protozoan diseases is one of the most exciting research fields of recent years. World Health Organization (WHO) reports have recognized that the physiology and biochemistry of protozoan parasites and the host-parasite relation are the main targets for the design of new drugs that can be used in the future against these diseases.

Phenotypic characterization of Leishmania mexicana pentamidine-resistant promastigotes. Modulation of the resistance during in-vitro developmental life cycle

Two clones of Leishmania mexicana resistant to 5 microM (LmR5CL2) and 20 microM (LmR20CL1) pentamidine, derived from a parental wild-type clone (LmWTCL3) were selected in vitro using a continuous drug pressure protocol. Both resistant clones expressed a cross-resistance to diminazene aceturate. No differences in their in-vitro infectivity for mouse peritoneal macrophages between wild-type and pentamidine-resistant promastigotes were observed. During these experiments, promastigotes of LmR20CL1 derived from intramacrophagic amastigote forms reverted to the pentamidine-sensitive phenotype, unlike the lower resistant ones. In the same way, when a complete developmental sequence of L. mexicana was achieved in axenic cultures, LmR20CL1 promastigotes derived from axenically growing amastigotes expressed an IC50 value close to the wild-type one, whereas resulting LmR5CL2 promastigotes remained pentamidine resistant. This modulation of the chemoresistance during the developmental life cycle could be significant in the transmission of drug-resistant strains by Phlebotominae as well as in basic research to follow drug resistance during the in-vitro and in-vivo life cycle of Leishmania.

In vitro life cycle of pentamidine-resistant amastigotes: stability of the chemoresistant phenotypes is dependent on the level of resistance induced

Using a continuous drug pressure protocol, we induced pentamidine resistance in an active and dividing population of amastigote forms of Leishmania mexicana. We selected in vitro two clones with different levels of resistance to pentamidine, with clone LmPENT5 being resistant to 5 microM pentamidine, while clone LmPENT20 was resistant to 20 microM pentamidine. Resistance indexes (50% inhibitory concentration [IC50] after drug presure/IC50 before drug pressure) of 2 (LmPENT5) and 6 (LmPENT20) were determined after drug selection. Both resistant clones expressed significant cross-resistance to diminazene aceturate and primaquine. Pentamidine resistance was not reversed by verapamil, a calcium channel blocker known to reverse multidrug resistance (A. J. Bitonti, et al., Science 242:1301-1303, 1988; A. R. C. Safa et al., J. Biol. Chem. 262:7884-7888, 1987). No difference in the in vitro infectivity for resident mouse macrophages was observed between the wild-type clone (clone LmWT) and pentamidine-resistant clones. During in vitro infectivity experiments, when the life cycle was performed starting from the intramacrophagic amastigote stage, the drug resistance of the resulting LmPENT20 amastigotes was preserved even if the intermediate promastigote stage could not be considered resistant to 20 microM pentamidine. In the same way, when a complete developmental sequence of L. mexicana was achieved axenically by manipulation of appropriate culture conditions, the resulting axenically grown LmPENT20 amastigotes remained pentamidine resistant, whereas LmPENT5 amastigotes lost their ability to resist pentamidine, with IC50s and index of resistance values close to those for the LmWT clone. These results strongly indicate that the level of pentamidine tolerated by resistant amastigotes after the life cycle was dependent on the induced level of resistance. This fact could be significant in the in vivo transmission of drug-resistant parasites by Phlebotominae. Particular attention should be given to the finding that the emergence of parasite resistance is a potential risk of the use of inadequate doses as therapy in humans.

Axenically cultured amastigote forms as an in vitro model for investigation of antileishmanial agents

Using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide microassay, previously described as a means of quantifying Leishmania amazonensis in vitro at the amastigote stage (D. Sereno and J. L. Lemesre, Parisitol. Res., in press), we have compared the activities of seven drugs, including those currently used to treat leishmaniasis, against axenically grown amastigote and promastigote forms of three Leishmania species (L. amazonensis, L. mexicana, and L. infantum, responsible for diffuse cutaneous, cutaneous, and visceral leishmaniasis, respectively). The ability of axenically cultured amastigote organisms to be used in an investigation of antileishmanial agents was first evaluated. We have confirmed the toxicities of sodium stibogluconate (Pentostam), pentamidine, and amphotericin B to active and dividing populations of axenically cultured amastigotes. The toxicity of potassium antimonyl tartrate trihydrate, which is generally higher than that of Pentostam, seemed to indicate that pentavalent antimony can be metabolized in vivo to compounds, possibly trivalent in nature, which are more active against the amastigote organisms. When the drug susceptibilities of parasites at both stages were compared, great variations were found for all the drugs studied. These major differences, which show the specific chemosusceptibility of the parasite at the mammalian stage, demonstrate the potential of using cultured amastigotes instead of promastigotes in a drug-screening procedure for early detection. This in vitro model may help in the isolation of active compounds, particularly those with low-grade activities, against the mammalian stage of the parasite.