Synthesis and structure-activity relationship study of potent trypanocidal thio semicarbazone inhibitors of the trypanosomal cysteine protease cruzain

Trypanocidal drugs: mechanisms, resistance and new targets

The protozoan parasitesTrypanosoma bruceiandTrypanosoma cruziare the causative agents of African trypanosomiasis and Chagas disease, respectively. These are debilitating infections that exert a considerable health burden on some of the poorest people on the planet. Treatment of trypanosome infections is dependent on a small number of drugs that have limited efficacy and can cause severe side effects. Here, we review the properties of these drugs and describe new findings on their modes of action and the mechanisms by which resistance can arise. We further outline how a greater understanding of parasite biology is being exploited in the search for novel chemotherapeutic agents. This effort is being facilitated by new research networks that involve academic and biotechnology/pharmaceutical organisations, supported by public–private partnerships, and are bringing a new dynamism and purpose to the search for trypanocidal agents.

Computational discovery of novel trypanosomicidal drug-like chemicals by using bond-based non-stochastic and stochastic quadratic maps and linear discriminant analysis

Herein we present results of a quantitative structure-activity relationship (QSAR) studies to classify and design, in a rational way, new antitrypanosomal compounds by using non-stochastic and stochastic bond-based quadratic indices. A data set of 440 organic chemicals, 143 with antitrypanosomal activity and 297 having other clinical uses, is used to develop QSAR models based on linear discriminant analysis (LDA). Non-stochastic model correctly classifies more than 93% and 95% of chemicals in both training and external prediction groups, respectively. On the other hand, the stochastic model shows an accuracy of about the 87% for both series. As an experiment of virtual lead generation, the present approach is finally satisfactorily applied to the virtual evaluation of 9 already synthesized in house compounds. The in vitro antitrypanosomal activity of this series against epimastigote forms of Trypanosoma cruzi is assayed. The model is able to predict correctly the behaviour for the majority of these compounds. Four compounds (FER16, FER32, FER33 and FER 132) showed more than 70% of epimastigote inhibition at a concentration of 100 microg/mL (86.74%, 78.12%, 88.85% and 72.10%, respectively) and two of these chemicals, FER16 (78.22% of AE) and FER33 (81.31% of AE), also showed good activity at a concentration of 10 microg/mL. At the same concentration, compound FER16 showed lower value of cytotoxicity (15.44%), and compound FER33 showed very low value of 1.37%. Taking into account all these results, we can say that these three compounds can be optimized in forthcoming works, but we consider that compound FER33 is the best candidate. Even though none of them resulted more active than Nifurtimox, the current results constitute a step forward in the search for efficient ways to discover new lead antitrypanosomals.

Synthesis and evaluation of 4-acyl-2-thiazolylhydrazone derivatives for anti-Toxoplasma efficacy in vitro

A new series of 4-acyl-2-thiazolylhydrazone derivatives was synthesized and screened for its in vitro activity against Toxoplasma gondii. We evaluated parasite growth inhibition and cytotoxicity, inhibition of replication, and inhibition of parasite invasion of host cells. The biological results indicated that some substances had an antiproliferative effect against intracellular T. gondii tachyzoites cultivated in vitro.

Antimalarial activity of thiosemicarbazones and purine derived nitriles

Malaria is a devastating illness caused by multiple species of the Plasmodium genus. The parasite's falcipain proteases have been extensively studied as potential drug targets. Here we report the testing of two established cysteine protease inhibitor scaffolds against both chloroquine sensitive and chloroquine resistant parasites. A subset of purine derived nitriles killed the parasite with moderate potency, and these inhibitors do not seem to exert their antiproliferative effects as cysteine protease inhibitors. Compound potency was determined to be similar against both parasite strains, indicating a low probability of cross resistance with chloroquine. These compounds represent a novel antimalarial scaffold, and a potential starting point for the development new inhibitors.

Synthesis, spectroscopic, and antimicrobial studies on bivalent zinc and mercury complexes of 2-formylpyridine thiosemicarbazone

A series of metal complexes of Zn(II) and Hg(II) having the general composition [M(L)₂]X₂ [where L = 2-formylpyridine thiosemicarbazone; M = Zn(II) and Hg(II); X = Cl⁻, NO₃⁻ and 1/2SO₄²⁻] have been prepared and characterized by elemental chemical analysis, molar conductance, and spectral (IR and mass) studies. The IR spectral data suggests the involvement of sulphur and azomethane nitrogen in coordination to the central metal ion. On the basis of spectral studies, a tetrahedral geometry has been assigned for Zn(II) and Hg(II) complexes. The free ligand and its metal complexes have been tested in vitro against a number of microorganisms in order to assess their antimicrobial properties.

Quantitative structure-activity relationships for a series of inhibitors of cruzain from Trypanosoma cruzi: molecular modeling, CoMFA and CoMSIA studies

Human parasitic diseases are the foremost threat to human health and welfare around the world. Trypanosomiasis is a very serious infectious disease against which the currently available drugs are limited and not effective. Therefore, there is an urgent need for new chemotherapeutic agents. One attractive drug target is the major cysteine protease from Trypanosoma cruzi, cruzain. In the present work, comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) studies were conducted on a series of thiosemicarbazone and semicarbazone derivatives as inhibitors of cruzain. Molecular modeling studies were performed in order to identify the preferred binding mode of the inhibitors into the enzyme active site, and to generate structural alignments for the three-dimensional quantitative structure-activity relationship (3D QSAR) investigations. Statistically significant models were obtained (CoMFA, r2=0.96 and q2=0.78; CoMSIA, r2=0.91 and q2=0.73), indicating their predictive ability for untested compounds. The models were externally validated employing a test set, and the predicted values were in good agreement with the experimental results. The final QSAR models and the information gathered from the 3D CoMFA and CoMSIA contour maps provided important insights into the chemical and structural basis involved in the molecular recognition process of this family of cruzain inhibitors, and should be useful for the design of new structurally related analogs with improved potency.

Microwave assisted tandem reactions for the synthesis of 2-hydrazolyl-4-thiazolidinones

A tandem method for the synthesis of 2-hydrazolyl-4-thiazolidinones (5) from commercially available materials in a 3 component reaction has been developed. The reaction connects aldehydes, thiosemicarbazides and maleic anhydride, effectively assisted by microwave irradiation. The synthesis of a new type of compound, 2-hydrazolyl-5,5-diphenyl-4-thiazolidinone (7), obtained by treatment of thiosemicarbazone with benzil in basic media is also reported. HOMO/LUMO energies, orbital coefficients and charge distribution were used to explain the proposed reaction mechanism.

Synthesis, cytotoxicity, and anti-Trypanosoma cruzi activity of new chalcones

Synthesis of a cytotoxic dihydrochalcone, first isolated from a traditional Amazonian medicinal plant Iryanthera juruensis Warb (Myristicaceae), followed by a comprehensive SAR analysis of saturated and unsaturated chalcone synthetic intermediates, led to the identification of analogues with selective and significant in vitro anti- Trypanosoma cruzi activity. Further SAR studies were undertaken with the synthesis of 21 new chalcones containing two allyloxy moieties that resulted in the discovery of 2',4'-diallyloxy-6'-methoxy chalcones with improved selectivity against this parasite at concentrations below 25 microM, four of which exhibited a selectivity index greater than 12.

Cruzain Inhibition by Terpenoids: A Molecular Docking Analysis

A molecular docking analysis has been carried out using monoterpene and sesquiterpene hydrocarbons and triterpenoids that have shown enzyme inhibitory activity as ligands for the cysteine protease cruzain. The binding energies of the docked ligands roughly correlate with their inhibitory activities. The orientations of the docked ligands are consistent with a mechanism whereby these hydrophobic compounds dock into a hydrophobic pocket near the active site, thereby blocking binding of the protein target to the protease.

New trypanocidal hybrid compounds from the association of hydrazone moieties and benzofuroxan heterocycle

Hybrid compounds containing hydrazones and benzofuroxan pharmacophores were designed as potential Trypanosoma cruzi-enzyme inhibitors. The majority of the designed compounds was successfully synthesized and biologically evaluated displaying remarkable in vitro activity against different strains of T. cruzi. Unspecific cytotoxicity was evaluated using mouse macrophages, displaying isothiosemicarbazone 10 and thiosemicarbazone 12 selectivity indexes (macrophage/parasite) of 21 and 27, respectively. In addition, the mode of anti-trypanosomal action of the derivatives was investigated. Some of these derivatives were moderate inhibitors of cysteinyl active site enzymes of T. cruzi, cruzipain and trypanothione reductase. ESR experiments using T. cruzi microsomal fraction suggest that the main mechanism of action of the trypanocidal effects is the production of oxidative stress into the parasite.

Identification of a new class of nonpeptidic inhibitors of cruzain

Cruzain is the major cysteine protease of Trypanosoma cruzi, which is the causative agent of Chagas disease and is a promising target for the development of new chemotherapy. With the goal of developing potent nonpeptidic inhibitors of cruzain, the substrate activity screening (SAS) method was used to screen a library of protease substrates initially designed to target the homologous human protease cathepsin S. Structure-based design was next used to further improve substrate cleavage efficiency by introducing additional binding interactions in the S3 pocket of cruzain. The optimized substrates were then converted to inhibitors by the introduction of cysteine protease mechanism-based pharmacophores. Inhibitor 38 was determined to be reversible even though it incorporated the vinyl sulfone pharmacophore that is well documented to give irreversible cruzain inhibition for peptidic inhibitors. The previously unexplored beta-chloro vinyl sulfone pharmacophore provided mechanistic insight that led to the development of potent irreversible acyl- and aryl-oxymethyl ketone cruzain inhibitors. For these inhibitors, potency did not solely depend on leaving group p K a, with 2,3,5,6-tetrafluorophenoxymethyl ketone 54 identified as one of the most potent inhibitors with a second-order inactivation constant of 147,000 s (-1) M (-1). This inhibitor completely eradicated the T. cruzi parasite from mammalian cell cultures and consequently has the potential to lead to new chemotherapeutics for Chagas disease.

Discovery of trypanocidal thiosemicarbazone inhibitors of rhodesain and TbcatB

Human African trypanosomiasis (HAT) is caused by the protozoan parasite Trypanosoma brucei. The cysteine proteases of T. brucei have been shown to be crucial for parasite replication and represent an attractive point for therapeutic intervention. Herein we describe the synthesis of a series of thiosemicarbazones and their activity against the trypanosomal cathepsins TbcatB and rhodesain, as well as human cathepsins L and B. The activity of these compounds was determined against cultured T. brucei, and specificity was assessed with a panel of four mammalian cell lines.

Novel Pd(II) complexes of 1-N-substituted 3-phenyl-2-pyrazoline derivatives and evaluation of antiamoebic activity

Cyclization of Mannich base with N(4)-substituted thiosemicarbazides by different aliphatic, aromatic and cyclic amines afforded a series of new 1-N-substituted cyclised pyrazoline analogues of thiosemicarbazones (PYZ-TSC) 1-10. Reaction of [Pd(DMSO)(2)Cl(2)] with pyrazoline derivatives led to new palladium(II) complexes [Pd(PYZ-TSC)Cl(2)] 1a-10a. The structures of all the compounds were characterized by spectroscopic methods. It was concluded that the pyrazoline thiosemicarbazone derivatives have two chelating arms, one attached at the 2-position of the pyrazole ring (that is, N donor) and other (S donor) linked to the thiosemicarbazone branch. The determination of antiamoebic activity of all the compounds was done using HM1:IMSS strain of Entamoeba histolytica, among all the complexes, 8a showed the most promising IC(50)=0.37 microM vs. IC(50)=1.81 microM of metronidazole, the reference drug. MTT assay showed that the compounds are non-toxic to human kidney epithelial cell line.

Thiosemicarbazones active against Clostridium difficile

A set of closely related furylidene thiosemicarbazones was prepared and screened against various clinically important Gram-positive bacteria. One compound containing an ethylene spacer and a 5-nitrofuryl group was found to have promising activity against Clostridium difficile.

Identification of the molecular target of small molecule inhibitors of HDL receptor SR-BI activity

Scavenger receptor, class B, type I (SR-BI), controls high-density lipoprotein (HDL) metabolism by mediating cellular selective uptake of lipids from HDL without the concomitant degradation of the lipoprotein particle. We previously identified in a high-throughput chemical screen of intact cells five compounds (BLT-1-5) that inhibit SR-BI-dependent lipid transport from HDL, but do not block HDL binding to SR-BI on the cell surface. Although these BLTs are widely used to examine the diverse functions of SR-BI, their direct target(s), SR-BI itself or some other component of the SR-BI pathway, has not been identified. Here we show that SR-BI in the context of a membrane lipid environment is the target of BLT-1, -3, -4, and -5. The analysis using intact cells and an in vitro system of purified SR-BI reconstituted into liposomes was aided by information derived from structure-activity relationship (SAR) analysis of the most potent of these BLTs, the thiosemicarbazone BLT-1. We found that the sulfur atom of BLT-1 was crucially important for its inhibitory activity, because changing it to an oxygen atom resulted in the isostructural, but essentially inactive, semicarbazone derivative BLT-1sc. SAR analysis also established the importance of BLT-1's hydrophobic tail. BLTs and their corresponding inactive compounds can be used to explore the mechanism and function of SR-BI-mediated selective lipid uptake in diverse mammalian experimental models. Consequently, BLTs may help determine the therapeutic potential of SR-BI-targeted pharmaceutical drugs.

Anti-tubercular agents. Part IV: Synthesis and antimycobacterial evaluation of nitroheterocyclic-based 1,2,4-benzothiadiazines

In continuation of our earlier work on benzothiadiazines, we have prepared a series of nitrofuran, nitrothiophene and arylfuran coupled benzothiadiazines and evaluated them for antimycobacterial and antibacterial activities. One of the compounds 2f has shown good in vitro antimycobacterial activity. All the synthesized compounds have shown moderate to good antibacterial activity.

Synthesis, Cruzain Docking, and in vitro Studies of Aryl-4-Oxothiazolylhydrazones AgainstTrypanosoma cruzi

Research in recent years has demonstrated that the Trypanosoma cruzi cysteine protease cruzain (TCC) is a valid chemotherapeutic target. Herein we describe a small library of aryl-4-oxothiazolylhydrazones that have been tested in assays against T. cruzi cell cultures. The docking studies carried out suggest that these compounds are potential ligands for the TCC enzyme. The most promising compound of this series, N-(4-oxo-5-ethyl-2'-thiazolin-2-yl)-N'-phenylthio-(Z)-ethylidenehydrazone (6 f), was shown to be very active at non-cytotoxic concentrations in in vitro assays with mammalian cells and has a potency comparable with reference drugs such as nifurtimox (Nfx) and benznidazole (Bdz).

A cysteine protease inhibitor cures Chagas' disease in an immunodeficient-mouse model of infection

Chagas' disease, caused by the parasite Trypanosoma cruzi, remains the leading cause of cardiopathy in Latin America with about 12 million people infected. Classic clinical manifestations derive from infection of muscle cells leading to progressive cardiomyopathy, while some patients develop megacolon or megaesophagus. A very aggressive clinical course including fulminant meningoencephalitis has been reported in patients who contract Chagas' disease in the background of immunodeficiency. This includes patients with human immunodeficiency virus infection as well as patients receiving immunosuppressive therapy for organ transplant. Currently, only two drugs are approved for the treatment of Chagas' disease, nifurtimox and benznidazole. Both have significant limitations due to common and serious side effects as well as limited availability. A promising group of new drug leads for Chagas' disease is cysteine protease inhibitors targeting cruzain, the major protease of T. cruzi. The inhibitor N-methyl-Pip-F-homoF-vinyl sulfonyl phenyl (N-methyl-Pip-F-hF-VS phi) is in late-stage preclinical development. Therefore, the question arose as to whether protease inhibitors targeting cruzain would have efficacy in Chagas' disease occurring in the background of immunodeficiency. To address this question, we studied the course of infection in recombinase-deficient (Rag1(-/-)) and normal mice infected with T. cruzi. Infections localized to heart and skeletal muscle in untreated normal animals, while untreated Rag1(-/-) mice showed severe infection in all organs and predominantly in liver and spleen. Treatment with the dipeptide N-methyl-Pip-F-hF-VS phi rescued immunodeficient animals from lethal Chagas' infection. The majority (60 to 100%) of inhibitor-treated Rag1(-/-) mice had increased survival, negative PCR, and normal tissues by histopathological examination.

Trypanocidal nitroimidazole derivatives: Relationships among chemical structure and genotoxic activity

Human American trypanosomiasis is resurgent in Latin Americans, and new drugs are urgently required as current medications suffer from a number of drawbacks. Some nitroheterocycles have been demonstrated to exert a potent activity against trypanosomes. However, host toxicity issues halted their development as trypanocides. As part of the efforts to develop new compounds in order to treat parasitic infections, it is important to define their structure-activity relationship. In this study, 5-nitromegazol and two of its analogues, 4-nitromegazol, and 1-methyl-5-nitro-2-imidazolecarboxaldehyde 5-nitroimidazole-thiosemicarbazone, were tested and compared for in vitro induction of DNA damage in human leukocytes by the comet assay, performed at different pHs to better identify the types of damage. Specific oxidatively generated damage to DNA was also measured by using the comet assay with endonucleases. DNA damage was found in 5-nitromegazol-treated cells: oxidative stress appeared as the main source of DNA damage. 4-Nitromegazol did not produce any significant effect, thus confirming that 4-nitroimidazoles isomers have no important biological activity. The 5-nitroimidazole-thiosemicarbazone induced DNA damage with a higher efficiency than 5-nitromegazol. The central role in the reduction process played by the acidic hydrazine proton present in the thiosemicarbazone group but not in the cyclic (thiadiazole) form can contribute to rationalise our results. Given its versatility, thiosemicarbazone moiety could be involved in different reactions with nitrogenous bases (nucleophilic and/or electrophilic attacks).

Drug discovery and development for neglected parasitic diseases

Diseases caused by tropical parasites affect hundreds of millions of people worldwide but have been largely neglected for drug development because they affect poor people in poor regions of the world. Most of the current drugs used to treat these diseases are decades old and have many limitations, including the emergence of drug resistance. This review will summarize efforts to reinvigorate the drug development pipeline for these diseases, which is driven in large part by support from major philanthropies. The organisms responsible for these diseases have a fascinating biology, and many potential biochemical targets are now apparent. These neglected diseases present unique challenges to drug development that are being addressed by new consortia of scientists from academia and industry.

Discovery of trypanocidal compounds by whole cell HTS of Trypanosoma brucei

Chemotherapy against human African trypanosomiasis relies on four drugs that cause frequent and occasionally severe side-effects. Because human African trypanosomiasis is a disease of poor people in Africa, the traditional market-driven pathways to drug development are not available. One potentially rapid and cost-effective approach to identifying and developing new trypanocidal drugs would be high throughput-screening of existing drugs already approved for other uses, as well as clinical candidates in late development. We have developed an ATP-bioluminescence assay that could be used to rapidly and efficiently screen compound libraries against trypanosomes in a high throughput-screening format to validate this notion. We screened a collection of 2160 FDA-approved drugs, bioactive compounds and natural products to identify hits that were cytotoxic to cultured Trypanosoma brucei at a concentration of 1 mum or less. This meant that any hit identified would be effective at a concentration readily achievable by standard drug dosing in humans. From the screen, 35 hits from seven different drug categories were identified. These included the two approved trypanocidal drugs, suramin and pentamidine, several other drugs suspected but never validated as trypanocidal, and 17 novel trypanocidal drugs.

Design, synthesis, and biochemical evaluation of novel cruzain inhibitors with potential application in the treatment of Chagas' disease

A series of compounds bearing tetrahydronaphthalene, benzophenone, propiophenone, and related rigid molecular skeletons functionalized with thiosemicarbazone or unsaturated carbonyl moieties were prepared by chemical synthesis and evaluated for their ability to inhibit the enzyme cruzain. As potential treatment agents for Chagas' disease, three compounds from the group demonstrate potent inhibition of cruzain with IC(50) values of 17, 24, and 80 nM, respectively.

Synthesis, docking, and in vitro activity of thiosemicarbazones, aminoacyl-thiosemicarbazides and acyl-thiazolidones against Trypanosoma cruzi

A novel series of thiosemicarbazone and aminoacyl-thiazolidones derivatives were synthesized. Their structure suggests that these compounds could have anti-Trypanosoma cruzi activity. Biological evaluation indicates that some of these compounds are able to inhibit the growth of T. cruzi in concentrations non-cytotoxic to mammalian cells. Docking studies were carried out in order to investigate the binding pattern of these compounds for the T. cruzi cruzain (TCC) protein, and these showed a significant correlation with experimental data.

Cyclooctadiene Ru(II) complexes of thiophene-2-carboxaldehyde-derived thiosemicarbazones: synthesis, characterization and antiamoebic activity

Thiosemicarbazones (TSC) 1-10 were synthesized by condensing substituted thiosemicarbazide with thiophene-2-carboxaldehyde. These thiosemicarbazones were further reacted with [Ru(eta4-C8H12)(CH3CN)2Cl2] to form complexes of the type [Ru(eta4-C8H12)(TSC)Cl2] 1a-10a. Thiosemicarbazones exhibited antiamoebic activity in the range IC50=1.09-5.42 microM. In vitro assessment of antiamoebic activity indicated that the thiosemicarbazones 3, IC50=1.67 microM, 4, IC50=1.11 microM and 6, IC50=1.09 microM showed substantially less IC50 value than metronidazole (IC50=1.87 microM), a commonly used drug against amoebiasis. Cyclooctadiene Ru(II) complexes of thiosemicarbazones showed significant improvement in antiamoebic activity (IC50=0.30-1.39 microM). All the complexes possess noteworthy potencies and showed less IC50 values than metronidazole against HK-9 strain of Entamoeba histolytica. Among all the complexes, the most promising antiamoebic activities was shown by the complexes 4a and 6a (IC50=0.31 microM of 4a and IC50=0.30 microM of 6a versus metronidazole).

Novel antitrypanosomal agents

Trypanosomes are the causative agents of Chagas' disease in Central and South America and sleeping sickness in sub-Saharan Africa. The current chemotherapy of the human trypanosomiases relies on only six drugs, five of which were developed > 30 years ago. In addition, these drugs display undesirable toxic side effects and the emergence of drug-resistant trypanosomes has been reported. Therefore, the development of new drugs in the treatment of Chagas' disease and sleeping sickness is urgently required. This article summarises the recent progress in identifying novel lead compounds for antitrypanosomal chemotherapy. Particular emphasis is placed on those agents showing promising, selective antitrypanosomal activity.

New ligand-based approach for the discovery of antitrypanosomal compounds

The antitrypanosomal activity of 10 already synthesized compounds was in silico predicted as well as in vitro and in vivo explored against Trypanosoma cruzi. For the computational study, an approach based on non-stochastic linear fingerprints to the identification of potential antichagasic compounds is introduced. Molecular structures of 66 organic compounds, 28 with antitrypanosomal activity and 38 having other clinical uses, were parameterized by means of the TOMOCOMD-CARDD software. A linear classification function was derived allowing the discrimination between active and inactive compounds with a confidence of 95%. As predicted, seven compounds showed antitrypanosomal activity (%AE>70) against epimastigotic forms of T. cruzi at a concentration of 100mug/mL. After an unspecific cytotoxic assay, three compounds were evaluated against amastigote forms of the parasite. An in vivo test was carried out for one of the studied compounds.

Synthesis, characterization and antiamoebic activity of 1-(thiazolo[4,5-b]quinoxaline-2-yl)-3-phenyl-2-pyrazoline derivatives

A new series of 1-N-thiocarboxamide-3-phenyl-2-pyrazolines 1-6 was synthesized by cyclization of different Mannich bases with unsubstituted thiosemicarbazide. The reaction of cyclized pyrazoline derivatives 1-6 with 2,3-dichloroquinoxaline afforded the title compounds 7-12. The structures of the new compounds were confirmed by elemental analyses as well as (1)H, (13)C NMR, IR and electronic spectral data. The HM1:IMSS strain of Entamoeba histolytica parasite was cultured in vitro and the sensitivity of the parasite to the synthesized compounds was evaluated using the microdilution method. Among all the pyrazoline derivatives 1-6, none was found to be a better inhibitor as compared to the reference drug, metronidazole. The quinoxaline derivatives, 9, 11 and 12 were found to be potent inhibitors of E. histolytica.

A novel non-stochastic quadratic fingerprints-based approach for the 'in silico' discovery of new antitrypanosomal compounds

A non-stochastic quadratic fingerprints-based approach is introduced to classify and design, in a rational way, new antitrypanosomal compounds. A data set of 153 organic chemicals, 62 with antitrypanosomal activity and 91 having other clinical uses, was processed by a k-means cluster analysis to design training and predicting data sets. Afterwards, a linear classification function was derived allowing the discrimination between active and inactive compounds. The model classifies correctly more than 93% of chemicals in both training and external prediction groups. The predictability of this discriminant function was also assessed by a leave-group-out experiment, in which 10% of the compounds were removed at random at each time and their activity predicted a posteriori. In addition, a comparison with models generated using four well-known families of 2D molecular descriptors was carried out. As an experiment of virtual lead generation, the present TOMOCOMD approach was finally satisfactorily applied on the virtual evaluation of 10 already synthesized compounds. The in vitro antitrypanosomal activity of this series against epimastigotes forms of Trypanosomal cruzi was assayed. The model was able to predict correctly the behaviour of these compounds in 90% of the cases.

Electrochemical and ESR study of 5-nitrofuryl-containing thiosemicarbazones antiprotozoal drugs

Cyclic voltammetry and electron spin resonance (ESR) techniques were used in the investigation of several potential antiprotozoal thiosemicarbazones nitrofurane derivatives. A self-protonation process involving the protonation of the nitro group due to the presence of an acidic proton in the thiosemicarbazone moiety was observed in the first step of a CEE(rev) reduction mechanism of these derivatives. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. AM1 methodology was used to obtain the optimized geometries and UB3LYP calculations were performed to obtain the theoretical hyperfine coupling constants. The theoretical study exhibited an unusual assignment of the spin densities showing a free radical centered in the thiosemicarbazone moiety rather than the nitro which are in agreement with the experimental hyperfine pattern.

Design, synthesis and anti-plasmodial evaluation in vitro of new 4-aminoquinoline isatin derivatives

A new class of 4-aminoquinoline derivatives based on the natural product isatin scaffold were designed and synthesized for biological evaluation against three strains of the malaria parasite Plasmodium falciparum. These derivatives showed anti-plasmodial IC(50) values in the ranges of 1.3-0.079 and 2.0-0.050muM against a chloroquine-sensitive (D10) and two resistant (K1 and W2) strains of P. falciparum, respectively. In order to determine potential targets for this class of compounds in P. falciparum, selected compounds were also tested against the parasitic cysteine protease falcipain-2. In terms of further development of this class of isatin derivatives, two of the compounds based on a flexible alkyl chain linker and a thiosemicarbazone moiety warrant further investigation as potential anti-plasmodial leads. These two derivatives showed good in vitro activity against K1 and W2 with IC(50) values of 51 and 54nM, respectively, while retaining potency against the D10 strain with IC(50) values of 79 and 95nM, respectively. Generally speaking, the inhibitory potency of all compounds in the series against the parasites did not strongly correlate with inhibitory potency against falcipain-2 for selected compounds tested, which at best was weak to moderate, suggesting other mechanisms of inhibition may also be involved or compounds may be selectively taken up by Plasmodium falciparum.

Development of alpha-keto-based inhibitors of cruzain, a cysteine protease implicated in Chagas disease

Trypanosoma cruzi, a protozoan parasite, is the causative agent of Chagas disease, a major cause of cardiovascular disease in many Latin American countries. There is an urgent need to develop an improved therapy due to the toxicity of existing drugs and emerging drug resistance. Cruzain, the primary cysteine protease of T. cruzi, is essential for the survival of the parasite in host cells and therefore is an important target for the development of inhibitors as potential therapeutics. A novel series of alpha-ketoamide-, alpha-ketoacid-, alpha-ketoester-, and aldehyde-based inhibitors of cruzain has been developed. The inhibitors were identified by screening protease targeted small molecule libraries and systematically optimizing the P1, P2, P3, and P1' residues using specific structure-guided methods. A total of 20 compounds displayed picomolar potency in in vitro assays and three inhibitors representing different alpha-keto-based inhibitor scaffolds demonstrated anti-trypanosomal activity in cell culture. A 2.3A crystallographic structure of cruzain bound with one of the alpha-ketoester analogs is also reported. The structure and kinetic assay data illustrate the covalent binding, reversible inhibition mechanism of the inhibitor. Information on the compounds reported here will be useful in the development of new lead compounds as potential therapeutic agents for the treatment of Chagas disease and as biological probes to study the role that cruzain plays in the pathology. This study also demonstrates the validity of structure-guided approaches to focused library design and lead compound optimization.