RESUMEN
A key step in drug discovery, common to many disease areas, is preclinical demonstration of efficacy in a mouse model of disease. However, this demonstration and its translation to the clinic can be impeded by mouse-specific pathways of drug metabolism. Here, we show that a mouse line extensively humanized for the cytochrome P450 gene superfamily ("8HUM") can circumvent these problems. The pharmacokinetics, metabolite profiles, and magnitude of drug-drug interactions of a test set of approved medicines were in much closer alignment with clinical observations than in wild-type mice. Infection with Mycobacterium tuberculosis, Leishmania donovani, and Trypanosoma cruzi was well tolerated in 8HUM, permitting efficacy assessment. During such assessments, mouse-specific metabolic liabilities were bypassed while the impact of clinically relevant active metabolites and DDI on efficacy were well captured. Removal of species differences in metabolism by replacement of wild-type mice with 8HUM therefore reduces compound attrition while improving clinical translation, accelerating drug discovery.
Asunto(s)
Enfermedades Transmisibles , Descubrimiento de Drogas , Ratones , Animales , Interacciones Farmacológicas , Modelos Animales de Enfermedad , Sistema Enzimático del Citocromo P-450/metabolismo , AceleraciónRESUMEN
Visceral leishmaniasis causes considerable mortality and morbidity in many parts of the world. There is an urgent need for the development of new, effective treatments for this disease. Here we describe the development of an anti-leishmanial drug-like chemical series based on a pyrazolopyrimidine scaffold. The leading compound from this series (7, DDD853651/GSK3186899) is efficacious in a mouse model of visceral leishmaniasis, has suitable physicochemical, pharmacokinetic and toxicological properties for further development, and has been declared a preclinical candidate. Detailed mode-of-action studies indicate that compounds from this series act principally by inhibiting the parasite cdc-2-related kinase 12 (CRK12), thus defining a druggable target for visceral leishmaniasis.
Asunto(s)
Quinasas Ciclina-Dependientes/antagonistas & inhibidores , Leishmania donovani/efectos de los fármacos , Leishmania donovani/enzimología , Leishmaniasis Visceral/tratamiento farmacológico , Leishmaniasis Visceral/parasitología , Terapia Molecular Dirigida , Pirazoles/farmacología , Pirimidinas/farmacología , Animales , Quinasa 9 Dependiente de la Ciclina/química , Quinasas Ciclina-Dependientes/química , Quinasas Ciclina-Dependientes/metabolismo , Modelos Animales de Enfermedad , Humanos , Ratones , Simulación del Acoplamiento Molecular , Proteoma/efectos de los fármacos , Proteómica , Pirazoles/química , Pirazoles/uso terapéutico , Pirimidinas/química , Pirimidinas/uso terapéutico , Reproducibilidad de los Resultados , Especificidad por SustratoRESUMEN
Visceral leishmaniasis (VL), caused by the protozoan parasites Leishmania donovani and Leishmania infantum, is one of the major parasitic diseases worldwide. There is an urgent need for new drugs to treat VL, because current therapies are unfit for purpose in a resource-poor setting. Here, we describe the development of a preclinical drug candidate, GSK3494245/DDD01305143/compound 8, with potential to treat this neglected tropical disease. The compound series was discovered by repurposing hits from a screen against the related parasite Trypanosoma cruzi Subsequent optimization of the chemical series resulted in the development of a potent cidal compound with activity against a range of clinically relevant L. donovani and L. infantum isolates. Compound 8 demonstrates promising pharmacokinetic properties and impressive in vivo efficacy in our mouse model of infection comparable with those of the current oral antileishmanial miltefosine. Detailed mode of action studies confirm that this compound acts principally by inhibition of the chymotrypsin-like activity catalyzed by the ß5 subunit of the L. donovani proteasome. High-resolution cryo-EM structures of apo and compound 8-bound Leishmania tarentolae 20S proteasome reveal a previously undiscovered inhibitor site that lies between the ß4 and ß5 proteasome subunits. This induced pocket exploits ß4 residues that are divergent between humans and kinetoplastid parasites and is consistent with all of our experimental and mutagenesis data. As a result of these comprehensive studies and due to a favorable developability and safety profile, compound 8 is being advanced toward human clinical trials.
Asunto(s)
Antiprotozoarios/administración & dosificación , Leishmania donovani/efectos de los fármacos , Leishmania infantum/efectos de los fármacos , Leishmaniasis Visceral/diagnóstico por imagen , Inhibidores de Proteasoma/administración & dosificación , Proteínas Protozoarias/antagonistas & inhibidores , Animales , Antiprotozoarios/química , Sitios de Unión , Modelos Animales de Enfermedad , Evaluación Preclínica de Medicamentos , Humanos , Leishmania donovani/química , Leishmania donovani/enzimología , Leishmania infantum/química , Leishmania infantum/enzimología , Leishmaniasis Visceral/parasitología , Masculino , Ratones , Complejo de la Endopetidasa Proteasomal/química , Complejo de la Endopetidasa Proteasomal/metabolismo , Inhibidores de Proteasoma/química , Conformación Proteica , Proteínas Protozoarias/química , Proteínas Protozoarias/metabolismoRESUMEN
Malaria and cryptosporidiosis, caused by apicomplexan parasites, remain major drivers of global child mortality. New drugs for the treatment of malaria and cryptosporidiosis, in particular, are of high priority; however, there are few chemically validated targets. The natural product cladosporin is active against blood- and liver-stage Plasmodium falciparum and Cryptosporidium parvum in cell-culture studies. Target deconvolution in P. falciparum has shown that cladosporin inhibits lysyl-tRNA synthetase (PfKRS1). Here, we report the identification of a series of selective inhibitors of apicomplexan KRSs. Following a biochemical screen, a small-molecule hit was identified and then optimized by using a structure-based approach, supported by structures of both PfKRS1 and C. parvum KRS (CpKRS). In vivo proof of concept was established in an SCID mouse model of malaria, after oral administration (ED90 = 1.5 mg/kg, once a day for 4 d). Furthermore, we successfully identified an opportunity for pathogen hopping based on the structural homology between PfKRS1 and CpKRS. This series of compounds inhibit CpKRS and C. parvum and Cryptosporidium hominis in culture, and our lead compound shows oral efficacy in two cryptosporidiosis mouse models. X-ray crystallography and molecular dynamics simulations have provided a model to rationalize the selectivity of our compounds for PfKRS1 and CpKRS vs. (human) HsKRS. Our work validates apicomplexan KRSs as promising targets for the development of drugs for malaria and cryptosporidiosis.
Asunto(s)
Criptosporidiosis , Cryptosporidium parvum/enzimología , Inhibidores Enzimáticos/farmacología , Lisina-ARNt Ligasa/antagonistas & inhibidores , Malaria Falciparum , Plasmodium falciparum/enzimología , Proteínas Protozoarias/antagonistas & inhibidores , Animales , Criptosporidiosis/tratamiento farmacológico , Criptosporidiosis/enzimología , Modelos Animales de Enfermedad , Inhibidores Enzimáticos/química , Humanos , Lisina-ARNt Ligasa/metabolismo , Malaria Falciparum/tratamiento farmacológico , Malaria Falciparum/enzimología , Ratones SCID , Proteínas Protozoarias/metabolismoRESUMEN
The clinical efficacy and safety of a drug is determined by its activity profile across many proteins in the proteome. However, designing drugs with a specific multi-target profile is both complex and difficult. Therefore methods to design drugs rationally a priori against profiles of several proteins would have immense value in drug discovery. Here we describe a new approach for the automated design of ligands against profiles of multiple drug targets. The method is demonstrated by the evolution of an approved acetylcholinesterase inhibitor drug into brain-penetrable ligands with either specific polypharmacology or exquisite selectivity profiles for G-protein-coupled receptors. Overall, 800 ligand-target predictions of prospectively designed ligands were tested experimentally, of which 75% were confirmed to be correct. We also demonstrate target engagement in vivo. The approach can be a useful source of drug leads when multi-target profiles are required to achieve either selectivity over other drug targets or a desired polypharmacology.
Asunto(s)
Diseño de Fármacos , Ligandos , Animales , Automatización , Sistemas de Liberación de Medicamentos , Femenino , Masculino , Ratones , Ratones Endogámicos C57BL , Modelos Teóricos , Fenómenos Farmacológicos , Reproducibilidad de los ResultadosRESUMEN
The novel nitroimidazopyran agent (S)-PA-824 has potent antibacterial activity against Mycobacterium tuberculosis in vitro and in vivo and is currently in phase II clinical trials for tuberculosis (TB). In contrast to M. tuberculosis, where (R)-PA-824 is inactive, we report here that both enantiomers of PA-824 show potent parasiticidal activity against Leishmania donovani, the causative agent of visceral leishmaniasis (VL). In leishmania-infected macrophages, (R)-PA-824 is 6-fold more active than (S)-PA-824. Both des-nitro analogues are inactive, underlining the importance of the nitro group in the mechanism of action. Although the in vitro and in vivo pharmacological profiles of the two enantiomers are similar, (R)-PA-824 is more efficacious in the murine model of VL, with >99% suppression of parasite burden when administered orally at 100 mg kg of body weight(-1), twice daily for 5 days. In M. tuberculosis, (S)-PA-824 is a prodrug that is activated by a deazaflavin-dependent nitroreductase (Ddn), an enzyme which is absent in Leishmania spp. Unlike the case with nifurtimox and fexinidazole, transgenic parasites overexpressing the leishmania nitroreductase are not hypersensitive to either (R)-PA-824 or (S)-PA-824, indicating that this enzyme is not the primary target of these compounds. Drug combination studies in vitro indicate that fexinidazole and (R)-PA-824 are additive whereas (S)-PA-824 and (R)-PA-824 show mild antagonistic behavior. Thus, (R)-PA-824 is a promising candidate for late lead optimization for VL and may have potential for future use in combination therapy with fexinidazole, currently in phase II clinical trials against VL.
Asunto(s)
Antiprotozoarios/farmacología , Antiprotozoarios/uso terapéutico , Antituberculosos/farmacología , Antituberculosos/uso terapéutico , Leishmaniasis Visceral/tratamiento farmacológico , Animales , Antiprotozoarios/química , Antituberculosos/química , Femenino , Ratones , Ratones Endogámicos BALB C , Nitroimidazoles/química , Nitroimidazoles/farmacología , Nitroimidazoles/uso terapéutico , EstereoisomerismoRESUMEN
While treatment options for human African trypanosomiasis (HAT) have improved significantly, there is still a need for new drugs with eradication now a realistic possibility. Here, we report the development of 2,4-diaminothiazoles that demonstrate significant potency against Trypanosoma brucei, the causative agent of HAT. Using phenotypic screening to guide structure-activity relationships, potent drug-like inhibitors were developed. Proof of concept was established in an animal model of the hemolymphatic stage of HAT. To treat the meningoencephalitic stage of infection, compounds were optimized for pharmacokinetic properties, including blood-brain barrier penetration. However, in vivo efficacy was not achieved, in part due to compounds evolving from a cytocidal to a cytostatic mechanism of action. Subsequent studies identified a nonessential kinase involved in the inositol biosynthesis pathway as the molecular target of these cytostatic compounds. These studies highlight the need for cytocidal drugs for the treatment of HAT and the importance of static-cidal screening of analogues.
Asunto(s)
Citostáticos , Tripanocidas , Trypanosoma brucei brucei , Tripanosomiasis Africana , Animales , Humanos , Tripanosomiasis Africana/tratamiento farmacológico , Tripanocidas/uso terapéutico , Tripanocidas/farmacocinética , Citostáticos/uso terapéutico , Barrera HematoencefálicaRESUMEN
There is an urgent need for new treatments for Chagas disease, a parasitic infection which mostly impacts South and Central America. We previously reported on the discovery of GSK3494245/DDD01305143, a preclinical candidate for visceral leishmaniasis which acted through inhibition of the Leishmania proteasome. A related analogue, active against Trypanosoma cruzi, showed suboptimal efficacy in an animal model of Chagas disease, so alternative proteasome inhibitors were investigated. Screening a library of phenotypically active analogues against the T. cruzi proteasome identified an active, selective pyridazinone, the development of which is described herein. We obtained a cryo-EM co-structure of proteasome and a key inhibitor and used this to drive optimization of the compounds. Alongside this, optimization of the absorption, distribution, metabolism, and excretion (ADME) properties afforded a suitable compound for mouse efficacy studies. The outcome of these studies is discussed, alongside future plans to further understand the series and its potential to deliver a new treatment for Chagas disease.
Asunto(s)
Enfermedad de Chagas , Leishmaniasis Visceral , Tripanocidas , Trypanosoma cruzi , Ratones , Animales , Inhibidores de Proteasoma/farmacología , Inhibidores de Proteasoma/uso terapéutico , Complejo de la Endopetidasa Proteasomal , Enfermedad de Chagas/tratamiento farmacológico , Enfermedad de Chagas/parasitología , Leishmaniasis Visceral/tratamiento farmacológico , Tripanocidas/farmacología , Tripanocidas/uso terapéutico , Tripanocidas/químicaRESUMEN
Visceral leishmaniasis (VL) is a parasitic infection that results in approximately 26â¯000-65â¯000 deaths annually. The available treatments are hampered by issues such as toxicity, variable efficacy, and unsuitable dosing options. The need for new treatments is urgent and led to a collaboration between the Drugs for Neglected Diseases initiative (DNDi), GlaxoSmithKline (GSK), and the University of Dundee. An 8-hydroxynaphthyridine was identified as a start point, and an early compound demonstrated weak efficacy in a mouse model of VL but was hampered by glucuronidation. Efforts to address this led to the development of compounds with improved in vitro profiles, but these were poorly tolerated in vivo. Investigation of the mode of action (MoA) demonstrated that activity was driven by sequestration of divalent metal cations, a mechanism which was likely to drive the poor tolerability. This highlights the importance of investigating MoA and pharmacokinetics at an early stage for phenotypically active series.
Asunto(s)
Antiprotozoarios/química , Antiprotozoarios/farmacología , Diseño de Fármacos , Leishmania/efectos de los fármacos , Naftiridinas/química , Naftiridinas/farmacología , Animales , Concentración 50 Inhibidora , Ratones , Solubilidad , Relación Estructura-Actividad , Agua/químicaRESUMEN
Visceral leishmaniasis (VL) affects millions of people across the world, largely in developing nations. It is fatal if left untreated and the current treatments are inadequate. As such, there is an urgent need for new, improved medicines. In this paper, we describe the identification of a 6-amino-N-(piperidin-4-yl)-1H-pyrazolo[3,4-d]pyrimidine scaffold and its optimization to give compounds which showed efficacy when orally dosed in a mouse model of VL.
RESUMEN
The leishmaniases are diseases that affect millions of people across the world, in particular visceral leishmaniasis (VL) which is fatal unless treated. Current standard of care for VL suffers from multiple issues and there is a limited pipeline of new candidate drugs. As such, there is a clear unmet medical need to identify new treatments. This paper describes the optimization of a phenotypic hit against Leishmania donovani, the major causative organism of VL. The key challenges were to balance solubility and metabolic stability while maintaining potency. Herein, strategies to address these shortcomings and enhance efficacy are discussed, culminating in the discovery of preclinical development candidate GSK3186899/DDD853651 (1) for VL.
Asunto(s)
Leishmaniasis Visceral/tratamiento farmacológico , Morfolinas/uso terapéutico , Pirazoles/uso terapéutico , Pirimidinas/uso terapéutico , Tripanocidas/uso terapéutico , Animales , Femenino , Células Hep G2 , Humanos , Leishmania donovani/efectos de los fármacos , Masculino , Ratones Endogámicos BALB C , Estructura Molecular , Morfolinas/síntesis química , Morfolinas/toxicidad , Pruebas de Sensibilidad Parasitaria , Inhibidores de Proteínas Quinasas/síntesis química , Inhibidores de Proteínas Quinasas/uso terapéutico , Inhibidores de Proteínas Quinasas/toxicidad , Pirazoles/síntesis química , Pirazoles/toxicidad , Pirimidinas/síntesis química , Pirimidinas/toxicidad , Ratas Sprague-Dawley , Relación Estructura-Actividad , Tripanocidas/síntesis química , Tripanocidas/toxicidadRESUMEN
Crystallography has guided the hybridization of two series of Trypanosoma brucei N-myristoyltransferase (NMT) inhibitors, leading to a novel highly selective series. The effect of combining the selectivity enhancing elements from two pharmacophores is shown to be additive and has led to compounds that have greater than 1000-fold selectivity for TbNMT vs HsNMT. Further optimization of the hybrid series has identified compounds with significant trypanocidal activity capable of crossing the blood-brain barrier. By using CF-1 mdr1a deficient mice, we were able to demonstrate full cures in vivo in a mouse model of stage 2 African sleeping sickness. This and previous work provides very strong validation for NMT as a drug target for human African trypanosomiasis in both the peripheral and central nervous system stages of disease.
Asunto(s)
Aciltransferasas/antagonistas & inhibidores , Barrera Hematoencefálica/efectos de los fármacos , Diseño de Fármacos , Tripanocidas/química , Tripanocidas/farmacología , Trypanosoma brucei brucei/efectos de los fármacos , Tripanosomiasis Africana/tratamiento farmacológico , Animales , Supervivencia Celular , Femenino , Humanos , Ratones , Ratones Endogámicos , Modelos Moleculares , Estructura Molecular , Conformación Proteica , Relación Estructura-Actividad , Tripanosomiasis Africana/microbiologíaRESUMEN
Methionyl-tRNA synthetase (MetRS) has been chemically validated as a drug target in the kinetoplastid parasite Trypanosoma brucei. In the present study, we investigate the validity of this target in the related trypanosomatid Leishmania donovani. Following development of a robust high-throughput compatible biochemical assay, a compound screen identified DDD806905 as a highly potent inhibitor of LdMetRS (Ki of 18 nM). Crystallography revealed this compound binds to the methionine pocket of MetRS with enzymatic studies confirming DDD806905 displays competitive inhibition with respect to methionine and mixed inhibition with respect to ATP binding. DDD806905 showed activity, albeit with different levels of potency, in various Leishmania cell-based viability assays, with on-target activity observed in both Leishmania promastigote cell assays and a Leishmania tarentolae in vitro translation assay. Unfortunately, this compound failed to show efficacy in an animal model of leishmaniasis. We investigated the potential causes for the discrepancies in activity observed in different Leishmania cell assays and the lack of efficacy in the animal model and found that high protein binding as well as sequestration of this dibasic compound into acidic compartments may play a role. Despite medicinal chemistry efforts to address the dibasic nature of DDD806905 and analogues, no progress could be achieved with the current chemical series. Although DDD806905 is not a developable antileishmanial compound, MetRS remains an attractive antileishmanial drug target.
Asunto(s)
Antiprotozoarios/farmacología , Inhibidores Enzimáticos/farmacología , Leishmania donovani/enzimología , Metionina-ARNt Ligasa/antagonistas & inhibidores , Metionina-ARNt Ligasa/metabolismo , Descubrimiento de Drogas , Inhibidores Enzimáticos/química , Ensayos Analíticos de Alto Rendimiento , Leishmania donovani/efectos de los fármacos , Estructura MolecularRESUMEN
Chagas' disease, caused by the protozoan parasite Trypanosoma cruzi, is the most common cause of cardiac-related deaths in endemic regions of Latin America. There is an urgent need for new safer treatments because current standard therapeutic options, benznidazole and nifurtimox, have significant side effects and are only effective in the acute phase of the infection with limited efficacy in the chronic phase. Phenotypic high content screening against the intracellular parasite in infected VERO cells was used to identify a novel hit series of 5-amino-1,2,3-triazole-4-carboxamides (ATC). Optimization of the ATC series gave improvements in potency, aqueous solubility, and metabolic stability, which combined to give significant improvements in oral exposure. Mitigation of a potential Ames and hERG liability ultimately led to two promising compounds, one of which demonstrated significant suppression of parasite burden in a mouse model of Chagas' disease.
Asunto(s)
Enfermedad de Chagas/tratamiento farmacológico , Triazoles/química , Triazoles/uso terapéutico , Tripanocidas/química , Tripanocidas/uso terapéutico , Trypanosoma cruzi/efectos de los fármacos , Aminación , Animales , Enfermedad de Chagas/parasitología , Chlorocebus aethiops , Descubrimiento de Drogas , Femenino , Humanos , Ratones , Relación Estructura-Actividad , Triazoles/farmacocinética , Triazoles/farmacología , Tripanocidas/farmacocinética , Tripanocidas/farmacología , Células VeroRESUMEN
The antiplasmodial activity, DMPK properties, and efficacy of a series of quinoline-4-carboxamides are described. This series was identified from a phenotypic screen against the blood stage of Plasmodium falciparum (3D7) and displayed moderate potency but with suboptimal physicochemical properties and poor microsomal stability. The screening hit (1, EC50 = 120 nM) was optimized to lead molecules with low nanomolar in vitro potency. Improvement of the pharmacokinetic profile led to several compounds showing excellent oral efficacy in the P. berghei malaria mouse model with ED90 values below 1 mg/kg when dosed orally for 4 days. The favorable potency, selectivity, DMPK properties, and efficacy coupled with a novel mechanism of action, inhibition of translation elongation factor 2 (PfEF2), led to progression of 2 (DDD107498) to preclinical development.
Asunto(s)
Antimaláricos/farmacología , Descubrimiento de Drogas , Malaria/tratamiento farmacológico , Plasmodium falciparum/efectos de los fármacos , Quinolinas/farmacología , Animales , Antimaláricos/síntesis química , Antimaláricos/química , Modelos Animales de Enfermedad , Ratones , Estructura Molecular , Quinolinas/síntesis química , Quinolinas/química , Relación Estructura-ActividadRESUMEN
In this paper we describe the optimization of a phenotypic hit against Plasmodium falciparum, based on a trisubstituted pyrimidine scaffold. This led to compounds with good pharmacokinetics and oral activity in a P. berghei mouse model of malaria. The most promising compound (13) showed a reduction in parasitemia of 96% when dosed at 30 mg/kg orally once a day for 4 days in the P. berghei mouse model of malaria. It also demonstrated a rapid rate of clearance of the erythrocytic stage of P. falciparum in the SCID mouse model with an ED90 of 11.7 mg/kg when dosed orally. Unfortunately, the compound is a potent inhibitor of cytochrome P450 enzymes, probably due to a 4-pyridyl substituent. Nevertheless, this is a lead molecule with a potentially useful antimalarial profile, which could either be further optimized or be used for target hunting.
Asunto(s)
Antimaláricos/química , Antimaláricos/uso terapéutico , Malaria/tratamiento farmacológico , Plasmodium berghei/efectos de los fármacos , Plasmodium falciparum/efectos de los fármacos , Pirimidinas/química , Pirimidinas/uso terapéutico , Animales , Antimaláricos/farmacocinética , Antimaláricos/farmacología , Humanos , Malaria/parasitología , Malaria Falciparum/tratamiento farmacológico , Malaria Falciparum/parasitología , Ratones SCID , Parasitemia/tratamiento farmacológico , Parasitemia/parasitología , Pirimidinas/farmacocinética , Pirimidinas/farmacologíaRESUMEN
Trypanosoma brucei N-myristoyltransferase (TbNMT) is an attractive therapeutic target for the treatment of human African trypanosomiasis (HAT). From previous studies, we identified pyrazole sulfonamide, DDD85646 (1), a potent inhibitor of TbNMT. Although this compound represents an excellent lead, poor central nervous system (CNS) exposure restricts its use to the hemolymphatic form (stage 1) of the disease. With a clear clinical need for new drug treatments for HAT that address both the hemolymphatic and CNS stages of the disease, a chemistry campaign was initiated to address the shortfalls of this series. This paper describes modifications to the pyrazole sulfonamides which markedly improved blood-brain barrier permeability, achieved by reducing polar surface area and capping the sulfonamide. Moreover, replacing the core aromatic with a flexible linker significantly improved selectivity. This led to the discovery of DDD100097 (40) which demonstrated partial efficacy in a stage 2 (CNS) mouse model of HAT.
Asunto(s)
Aciltransferasas/antagonistas & inhibidores , Pirazoles/síntesis química , Sulfonamidas/síntesis química , Tripanocidas/síntesis química , Tripanosomiasis Africana/tratamiento farmacológico , Aminopiridinas/química , Animales , Barrera Hematoencefálica/efectos de los fármacos , Sistema Nervioso Central/efectos de los fármacos , Femenino , Humanos , Concentración 50 Inhibidora , Ratones , Pirazoles/farmacología , Pirazoles/uso terapéutico , Relación Estructura-Actividad , Sulfonamidas/química , Sulfonamidas/farmacología , Sulfonamidas/uso terapéutico , Tripanocidas/farmacología , Trypanosoma brucei brucei/efectos de los fármacosRESUMEN
Safer and more effective oral drugs are required to treat visceral leishmaniasis, a parasitic disease that kills 50,000 to 60,000 people each year in parts of Asia, Africa, and Latin America. Here, we report that fexinidazole, a drug currently in phase 1 clinical trials for treating African trypanosomiasis, shows promise for treating visceral leishmaniasis. This 2-substituted 5-nitroimidazole drug is rapidly oxidized in vivo in mice, dogs, and humans to sulfoxide and sulfone metabolites. Both metabolites of fexinidazole were active against Leishmania donovani amastigotes grown in macrophages, whereas the parent compound was inactive. Pharmacokinetic studies with fexinidazole (200 mg/kg) showed that fexinidazole sulfone achieves blood concentrations in mice above the EC(99) (effective concentration inhibiting growth by 99%) value for at least 24 hours after a single oral dose. A once-daily regimen for 5 days at this dose resulted in a 98.4% suppression of infection in a mouse model of visceral leishmaniasis, equivalent to that seen with the drugs miltefosine and Pentostam, which are currently used clinically to treat this tropical disease. In African trypanosomes, the mode of action of nitro drugs involves reductive activation via a NADH (reduced form of nicotinamide adenine dinucleotide)-dependent bacterial-like nitroreductase. Overexpression of the leishmanial homolog of this nitroreductase in L. donovani increased sensitivity to fexinidazole by 19-fold, indicating that a similar mechanism is involved in both parasites. These findings illustrate the potential of fexinidazole as an oral drug therapy for treating visceral leishmaniasis.