Pteridine reductase-1 (PTR1) is a promising drug target for the treatment of trypanosomiasis. We investigated the potential of a previously identified class of thiadiazole inhibitors of Leishmania major PTR1 for activity against Trypanosoma brucei (Tb). We solved crystal structures of several TbPTR1-inhibitor complexes to guide the structure-based design of new thiadiazole derivatives. Subsequent synthesis and enzyme- and cell-based assays confirm new, mid-micromolar inhibitors of TbPTR1 with low toxicity. In particular, compound 4m, a biphenyl-thiadiazole-2,5-diamine with IC50 = 16 μM, was able to potentiate the antitrypanosomal activity of the dihydrofolate reductase inhibitor methotrexate (MTX) with a 4.1-fold decrease of the EC50 value. In addition, the antiparasitic activity of the combination of 4m and MTX was reversed by addition of folic acid. By adopting an efficient hit discovery platform, we demonstrate, using the 2-amino-1,3,4-thiadiazole scaffold, how a promising tool for the development of anti-T. brucei agents can be obtained.

Keetia leucantha is a West African tree used in traditional medicine to treat several diseases among which parasitic infections. The dichloromethane extract of leaves was previously shown to possess growth-inhibitory activities on Plasmodium falciparum, Trypanosoma brucei brucei and Leishmania mexicana mexicana with low or no cytotoxicity (>100μg/ml on human normal fibroblasts) (Bero et al., 2009, Bero et al., 2011). In continuation of our investigations on the antitrypanosomal compounds from this dichloromethane extract, we analyzed by GC-FID and GC–MS the essential oil of its leaves obtained by hydrodistillation and the major triterpenic acids in this extract by LC–MS. Twenty-seven compounds were identified in the oil whose percentages were calculated using the normalization method. The essential oil, seven of its constituents and the three triterpenic acids were evaluated for their antitrypanosomal activity on Trypanosoma brucei brucei bloodstream forms (Tbb BSF) and procyclic forms (Tbb PF) to identify an activity on the glycolytic process of trypanosomes. The oil showed an IC50 of 20.9μg/ml on Tbb BSF and no activity was observed on Tbb PF. The best antitrypanosomal activity was observed for ursolic acid with IC50 of 2.5 and 6.5μg/ml respectively on Tbb BSF and Tbb PF. The inhibitory activity on a glycolytic enzyme of T. brucei, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), was also evaluated for betulinic acid, olenaolic acid, ursolic acid, phytol, α-ionone and β-ionone. The three triterpenic acids and β-ionone showed inhibitory activities on GAPDH with oleanolic acid being the most active with an inhibition of 72.63% at 20μg/ml. This paper reports for the first time the composition and antitrypanosomal activity of the essential oil of Keetia leucantha. Several of its constituents and three triterpenic acids present in the dichloromethane leaves extract showed a higher antitrypanosomal activity on bloodstream forms of Tbb as compared to procyclic forms, namely geranyl acetone, phytol, α-ionone, β-ionone, ursolic acid, oleanolic acid and betulinic acid. The four last compounds were proven to be inhibitors of trypanosomal GAPDH, which may in part explain these antitrypanosomal activities.

A new cytotoxic ent-kaurane-type diterpene named xylopioxyde (16,17-epoxy-15-oxo-ent-kauran-19-oic acid) has been isolated from the fruits of Xylopia aethiopica Dunal (Annonaceae) together with three known compounds, namely 15a-acetoxy-ent-kaur-16-en-19 oic acid (xylopic acid), 15-oxo-ent-kaur-16-en-19-oic acid and ent-kaur-16-en-19-oic acid. Xylopic acid, obtained in a good amount, has been successively converted in moderate to good yields into 15-hydroxy-ent-kaur-16-en-19-oic acid, 15-oxo-ent-kaur-16-en-19-oic acid and two new selective trypanocidal stereoisomers of 15-acetoxy-16,17-ent-epoxy-kauran-19-oic acid, respectively. All the compounds except the synthetic epoxides displayed cytotoxic effects on the mammalian fibroblast cell line MRC-5 as well as inhibitory effects on the growth of the bloodstream forms of Trypanosoma brucei brucei cells (strain 241). © 2013 Desire SOH et al.

To explore the structure–activity relationships of thiosemicarbazones on African trypanosome: Trypanosoma brucei brucei, a series of thirty-five thiosemicarbazones (1–35) have been synthesized and characterized by their 1H NMR, 13C NMR, and FT-IR spectra. All compounds were tested for trypanocidal activity using the method “Lilit alamar blue”. The comparison of trypanocidal power of thiosemicarbazones was performed considering their structures. This study that was done using acetophenone thiosemicarbazone (1) as basic model, showed that: (a) the presence of lipophilic substituents in para position on benzene ring, (b) substitution of benzene ring and (c) substitution of hydrogen of thioamide function by a phenyl, strongly influence trypanocidal activity. The various modifications to basic structure (1) allowed the synthesis of 1-(4-chlorophenyl) ethylidene-4-phenyl-thiosemicarbazide (34). With a trypanocidal activity of 3.97 μM, this compound is the most active of the series.

A characteristic, well-studied feature of the pathogenic protists belonging to the family Trypanosomatidae is the compartmentalisation of the major part of the glycolytic pathway in peroxisome-like organelles, hence designated glycosomes. Such organelles containing glycolytic enzymes appear to be present in all members of the Kinetoplastea studied, and have recently also been detected in a representative of the Diplonemida, but they are absent from the Euglenida. Glycosomes therefore probably originated in a free-living, common ancestor of the Kinetoplastea and Diplonemida. The initial sequestering of glycolytic enzymes inside peroxisomes may have been the result of a minor mistargeting of proteins, as generally observed in eukaryotic cells, followed by preservation and its further expansion due to the selective advantage of this specific form of metabolic compartmentalisation. This selective advantage may have been a largely increased metabolic flexibility, allowing the organisms to adapt more readily and efficiently to different environmental conditions. Further evolution of glycosomes involved, in different taxonomic lineages, the acquisition of additional enzymes and pathways - often participating in core metabolic processes - as well as the loss of others. The acquisitions may have been promoted by the sharing of cofactors and crucial metabolites between different pathways, thus coupling different redox processes and catabolic and anabolic pathways within the organelle. A notable loss from the Trypanosomatidae concerned a major part of the typical peroxisomal H(2)O(2)-linked metabolism. We propose that the compartmentalisation of major parts of the enzyme repertoire involved in energy, carbohydrate and lipid metabolism has contributed to the multiple development of parasitism, and its elaboration to complicated life cycles involving consecutive different hosts, in the protists of the Kinetoplastea clade.

The upregulation of pteridine reductase (PTR1) is a major contributor to antifolate drug resistance in Leishmania spp., as it provides a salvage pathway that bypasses dihydrofolate reductase (DHFR) inhibition. The structure-based optimization of the PTR1 inhibitor methyl-1-[4-(2,4-diaminopteridin-6-ylmethylamino)benzoyl]piperidine-4-carboxylate (1) led to the synthesis of a focused compound library which showed significantly improved selectivity for the parasite's folate-dependent enzyme. When used in combination with pyrimethamine, a DHFR inhibitor, a synergistic effect was observed for compound 5b. This work represents a step forward in the identification of effective antileishmania agents.

ETHNOPHARMACOLOGICAL RELEVANCE:The aim of the study was to evaluate the in vitro antitrypanosomal and antileishmanial activity of crude extracts of 10 plant species traditionally used in Benin to treat parasitic infections. MATERIALS AND METHODS:For each species, dichloromethane, methanol and aqueous extracts were tested. Their antitrypanosomal and antileishmanial activities were evaluated in vitro on Trypanosoma brucei brucei (strain 427) (Tbb) and on promastigotes of Leishmania mexicana mexicana (MHOM/BZ/84/BEL46) (Lmm). RESULTS:The best growth inhibition was observed with the dichloromethane extracts of aerial parts of Acanthospermum hispidum DC. (Asteraceae) (IC(50)=14.5 μg/ml on Tbb and 11.1 μg/ml on Lmm), twigs of Keetia leucantha (K. Krause) Bridson (syn. Plectronia leucantha Krause) (IC(50)=5.8 μg/ml on Tbb), aerial parts of Byrsocarpus coccineus Schumach. & Thonn (syn. Rourea coccinea (Schumach. & Thonn.) Hook.f.) (IC(50)=14.7 μg/ml on Tbb) and aerial parts of Carpolobia lutea G.Don. (IC(50)=18.3 μg/ml on Tbb). All these extracts had a low cytotoxicity. It is not the case for the methanolic and water extracts of roots of Anchomanes difformis (Blume) Engl. (IC(50)=14.7 and 13.8 μg/ml on Tbb) which were toxic at the same concentration range on WI38, human cells. A bio-guided fractionation of the most active extract of Keetia leucantha allowed to identify oleanolic acid and ursolic acid as responsible for the observed activities. CONCLUSION:Our study gives some justification for antiparasitic activity of some investigated plants.

Several 5-O-alkyl- and 5-C-alkyl-mannitol bis-phosphates were synthesized and comparatively assayed as inhibitors of fructose bis-phosphate aldolases (Fbas) from rabbit muscle (taken as surrogate model of the human enzyme) and from Trypanosoma brucei. A limited selectivity was found in several instances. Crystallographic studies confirm that the 5-O-methyl derivative binds competitively with substrate and the 5-O-methyl moiety penetrating deeper into a shallow hydrophobic pocket at the active site. This observation can lead to the preparation of selective competitive or irreversible inhibitors of the parasite Fba.

Glycolysis and glyconeogenesis play crucial roles in the ATP supply and synthesis of glycoconjugates, important for the viability and virulence, respectively, of the human-pathogenic stages of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. These pathways are, therefore, candidate targets for antiparasite drugs. The glycolytic/gluconeogenic enzyme enolase is generally highly conserved, with similar overall fold and identical catalytic residues in all organisms. Nonetheless, potentially important differences exist between the trypanosomatid and host enzymes, with three unique, reactive residues close to the active site of the former that might be exploited for the development of new drugs. In addition, enolase is found both in the secretome and in association with the surface of Leishmania spp. where it probably functions as plasminogen receptor, playing a role in the parasite's invasiveness and virulence, a function possibly also present in the other trypanosomatids. This location and possible function of enolase offer additional perspectives for both drug discovery and vaccination.

The thiosemicarbazones of six aromatic ketones were synthesized as well as their 1,3,4-thiadiazolines derivatives obtained by cyclization under acetylating condition with yields going from 40 to 90%. The products purity was confirmed by mass spectrometry coupled with high-performance liquid chromatography (LC/MS) and there were characterized using spectrometry IR, NMR 1H and 13C (nuclear magnetic resonance). These compounds were then tested in vitro on Trypanosoma brucei brucei according to the “LILIT, Alamar Blue” method for a comparison of their trypanocidal activity. Thus, all thiosemicarbazones appeared much more active than their corresponding 1,3,4-thiadiazolines. Thiosemicarbazone 6a (IC50 = 9.62 µM) was the most active of all thiosemicarbazones tested and it is the same for its thiadiazoline 6b (IC50 = 49.03 µM) among 1,3,4-thiadiazolines.

Six 1,3,4-thiadiazolines derivatives were synthesized by cyclization of thiosemicarbazones under acetylating condition with yields going from 27 to 94%. The products purity was confirmed by LC/MS (Mass Spectrometry Coupled with High-Performance Liquid Chromatography) and they were characterized using spectrometry IR, NMR 1H and 13C (Nuclear Magnetic Resonance). These compounds were then tested in vitro on Trypanosoma brucei brucei according to the “LILIT, Alamar Blue” method to estimate their trypanocidal activity. 1,3,4-thiadiazoline 6 (IC50 = 38,79 ìM) was the most active of all compounds.

In continuation of our study on medicinal plants of Cameroon, stem barks of Polyalthia suaveolens were phytochemically studied. This investigation yielded a new indolosesquiterpene alkaloid, named polysin (1) and four hitherto known alkaloids (2–5). Polysin (1) appeared as a competitive reversible inhibitor (Ki=10μM) of phosphofructo kinase (PFK) of Trypanosoma brucei with respect to fructose-6-phosphate (Ki/KM=0.05) and could be used in the design of new trypanocidal drugs. The other isolated compounds (2–5) also exhibited interesting inhibitory effects on selected glycolytic enzymes (PFK, glyceraldehyde-3-phosphate dehydrogenase and aldolase).

Aldolase (ALD) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) of Trypanosoma brucei are considered to be promising targets for chemotherapeutic treatment of African sleeping sickness, because glycolysis is the single source of ATP for the parasite when living in the human bloodstream. Moreover, these enzymes appeared to possess distinct kinetic and structural properties that have already been exploited for the discovery of effective and selective inhibitors with trypanocidal activity. Here we present an experimental, quantitative assessment of the importance of these enzymes for the glycolytic pathway. This was achieved by decreasing the concentrations of ALD and GAPDH by RNA interference. The effects of these knockdowns on parasite growth, levels of various enzymes and transcripts, enzyme activities and glucose consumption were studied. A partial depletion of ALD and GAPDH was already sufficient to rapidly kill the trypanosomes. An effect was also observed on the activity of some other glycolytic enzymes.

Folate analogue inhibitors of Leishmania major pteridine reductase (PTR1) are potential antiparasitic drug candidates for combined therapy with dihydrofolate reductase (DHFR) inhibitors. To identify new molecules with specificity for PTR1, we carried out a virtual screening of the Available Chemicals Directory (ACD) database to select compounds that could interact with L. major PTR1 but not with human DHFR. Through two rounds of drug discovery, we successfully identified eighteen drug-like molecules with low micromolar affinities and high in vitro specificity profiles. Their efficacy against Leishmania species was studied in cultured cells of the promastigote stage, using the compounds both alone and in combination with 1 (pyrimethamine; 5-(4-chlorophenyl)-6-ethylpyrimidine-2,4-diamine). Six compounds showed efficacy only in combination. In toxicity tests against human fibroblasts, several compounds showed low toxicity. One compound, 5c (riluzole; 6-(trifluoromethoxy)-1,3-benzothiazol-2-ylamine), a known drug approved for CNS pathologies, was active in combination and is suitable for early preclinical evaluation of its potential for label extension as a PTR1 inhibitor and antiparasitic drug candidate.

Trypanosoma brucei is the causative agent of human African trypanosomiasis (sleeping sickness) which is fatal if left untreated. This disease occurs in 36 African countries, south of the Sahara, where 60 million people are at risk of acquiring infection. The current chemotherapy relies on only four drugs, three of which were developed more than 60 years ago. These drugs have many limitations, ranging from oral inabsorption, acute toxicities, short duration of action and the emergence of trypanosomal resistance. Despite decades of use of most of the current trypanocides, little is known about their mode of action. That being said, African trypanosomes continue to be among the most extensively studied parasitic protists to date. Many of their intriguing biological features have been well documented and can be viewed as attractive targets for antitrypanosomal chemotherapy. A considerable number of natural products with diverse molecular structures have revealed antiparasitic potency in the laboratory and represent interesting lead compounds for the development of new and urgently needed antiparasitics. The major validated drug targets in T. brucei are discussed with particular emphasis on those known to be attacked by natural compounds.

Allosteric regulation provides a rate management system for enzymes involved in many cellular processes. Ligand-controlled regulation is easily recognizable, but the underlying molecular mechanisms have remained elusive. We have obtained the first complete series of allosteric structures, in all possible ligated states, for the tetrameric enzyme, pyruvate kinase, from Leishmania mexicana. The transition between inactive T-state and active R-state is accompanied by a simple symmetrical 6 degrees rigid body rocking motion of the A- and C-domain cores in each of the four subunits. However, formation of the R-state in this way is only part of the mechanism; eight essential salt bridge locks that form across the C-C interface provide tetramer rigidity with a coupled 7-fold increase in rate. The results presented here illustrate how conformational changes coupled with effector binding correlate with loss of flexibility and increase in thermal stability providing a general mechanism for allosteric control.

Polycarpol, sitosterol and sitosterol-3-O-beta-D-glucoside isolated for the first time from Piptostigma preussi (Annonaceae) occur regularly in some Annonaceae such as Piptostigma genus. Polycarpol exhibits interesting antitrypanosomal activity with an ED(50) value of 5.11 microM on Trypanosoma brucei cells. Moreover, it inhibits T. brucei glycolytic enzymes GAPDH and PFK with IC(50) values of 650 and 180 microM respectively.