Design, synthesis and antimalarial activity of novel, quinoline-based, zinc metallo-aminopeptidase inhibitors

Spotlight on 4-substituted quinolines as potential anti-infective agents: Journey beyond chloroquine

Continued emerging resistance of pathogens against the clinically approved candidates and their associated limitations continuously demand newer agents having better potency with a more suited safety profile. Quinoline nuclei containing scaffolds of natural and synthetic origin have been documented for diverse types of pharmacological activities, and a number of drugs are clinically approved. In the present review, we unprecedentedly covered the biological potential of 4-substituted quinoline and elaborated a rationale for its special privilege to afford the significant number of approved clinical drugs, particularly against infectious pathogens. Compounds with 4-substituted quinoline are well documented for antimalarial activity, but in the last two decades, they have been extensively explored for activity against cancer, tuberculosis, and several other pathogens including viruses, bacteria, fungi, and other infectious pathogens. In the present study, the anti-infective spectrum of this scaffold is discussed against viruses, mycobacteria, malarial parasites, and fungal and bacterial strains, along with recent updates in this area, with special emphasis on the structure-activity relationship.

Metalloaminopeptidases of the Protozoan Parasite Plasmodium falciparum as Targets for the Discovery of Novel Antimalarial Drugs

Malaria poses a significant threat to approximately half of the world's population with an annual death toll close to half a million. The emergence of resistance to front-line antimalarials in the most lethal human parasite species, Plasmodium falciparum (Pf), threatens progress made in malaria control. The prospect of losing the efficacy of antimalarial drugs is driving the search for small molecules with new modes of action. Asexual reproduction of the parasite is critically dependent on the recycling of amino acids through catabolism of hemoglobin (Hb), which makes metalloaminopeptidases (MAPs) attractive targets for the development of new drugs. The Pf genome encodes eight MAPs, some of which have been found to be essential for parasite survival. In this article, we discuss the biological structure and function of each MAP within the Pf genome, along with the drug discovery efforts that have been undertaken to identify novel antimalarial candidates of therapeutic value.

Probing the Binding Requirements of Modified Nucleosides with the DNA Nuclease SNM1A

SNM1A is a nuclease that is implicated in DNA interstrand crosslink repair and, as such, its inhibition is of interest for overcoming resistance to chemotherapeutic crosslinking agents. However, the number and identity of the metal ion(s) in the active site of SNM1A are still unconfirmed, and only a limited number of inhibitors have been reported to date. Herein, we report the synthesis and evaluation of a family of malonate-based modified nucleosides to investigate the optimal positioning of metal-binding groups in nucleoside-derived inhibitors for SNM1A. These compounds include ester, carboxylate and hydroxamic acid malonate derivatives which were installed in the 5'-position or 3'-position of thymidine or as a linkage between two nucleosides. Evaluation as inhibitors of recombinant SNM1A showed that nine of the twelve compounds tested had an inhibitory effect at 1 mM concentration. The most potent compound contains a hydroxamic acid malonate group at the 5'-position. Overall, our studies advance the understanding of requirements for nucleoside-derived inhibitors for SNM1A and indicate that groups containing a negatively charged group in close proximity to a metal chelator, such as hydroxamic acid malonates, are promising structures in the design of inhibitors.

IgG antibody response against Plasmodium falciparum aminopeptidase 1 antigen in Gabonese children living in Makokou and Franceville

The search for novel chemical classes of anti-malarial compounds to cope with the current state of chemoresistance of malaria parasites has led to the identification of Plasmodium falciparum aminopeptidase 1 (PfA-M1) as a new therapeutic target. PfA-M1, known to be involved in the hemoglobin digestion cascade which helps to provide most of the amino acids necessary to the parasite's metabolism, is currently considered as a promising target for anti-malarial chemotherapy. However, its immunogenic properties have not yet been tested in the Gabonese population. In Gabon, the prevalence of malaria remains three times higher in semi-urban areas (60·12%) than in urban areas (17·06%). We show that malaria-specific PfA-M1 antibodies are present in children and increase with the level of infection. Children living in semi-urban areas have higher anti-PfA-M1 antibody titers (0·14 ± 0·02 AU) than those living in urban areas (0·08 ± 0·02 AU, P = 0·03), and their antibody titers increase with age (P < 0·0001). Moreover, anti-PfA-M1 antibody titers decrease in children with hyperparasitemia (0·027 ± 0·055 AU) but they remain high in children with low parasite density (0·21 ± 0·034 AU, P = 0·034). In conclusion, our results suggest that malaria-specific PfA-M1 antibodies may play an important role in the immune response of the host against P. falciparum in Gabonese children. Further studies on the role of PfA-M1 during anemia are needed.

Selective inhibition of PfA-M1, over PfA-M17, by an amino-benzosuberone derivative blocks malaria parasites development in vitro and in vivo

Background

Plasmodium falciparum M1 family aminopeptidase is currently considered as a promising target for anti-malarial chemotherapy. Several series of inhibitors developed by various research groups display IC50/Ki values down to nM range on native PfA-M1 or recombinant forms and block the parasite development in culture at µM to sub-µM concentrations. A handful of these inhibitors has been tested on murine models of malaria and has shown anti plasmodial in vivo activity. However, most of these inhibitors do also target the other neutral malarial aminopeptidase, PfA-M17, often with lower Ki values, which questions the relative involvement and importance of each enzyme in the parasite biology.

Results

An amino-benzosuberone derivative from a previously published collection of chemicals targeting specifically the M1-aminopeptidases has been identified; it is highly potent on PfA-M1 (Ki = 50 nM) and devoid of inhibitory activity on PfA-M17 (no inhibition up to 100 µM). This amino-benzosuberone derivative (T5) inhibits, in the µM range, the in vitro growth of two P. falciparum strains, 3D7 and FcB1, respectively chloroquino-sensitive and resistant. Evaluated in vivo, on the murine non-lethal model of malaria Plasmodium chabaudi chabaudi, this amino-benzosuberone derivative was able to reduce the parasite burden by 44 and 40% in a typical 4-day Peters assay at a daily dose of 12 and 24 mg/kg by intraperitoneal route of administration.

Conclusions

The evaluation of a highly selective inhibitor of PfA-M1, over PfA-M17, active on Plasmodium parasites in vitro and in vivo, highlights the relevance of PfA-M1 in the biological development of the parasite as well as in the list of promising anti-malarial targets to be considered in combination with current or future anti-malarial drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-2032-4) contains supplementary material, which is available to authorized users.

Two cap residues in the S1 subsite of a Plasmodium falciparum M1-family aminopeptidase promote broad specificity and enhance catalysis

The aminopeptidase PfA-M1 is a key contributor to peptide catabolism in the human malaria parasite Plasmodium falciparum. PfA-M1 substrate specificity is shaped by the cylindrical S1 subsite, which accommodates the sidechain of the substrate P1 residue. At the top of the S1 subsite are two "cap" residues, E572 and M1034, that are positioned to influence S1 subsite specificity. In this study, we have mutated the cap residues, individually and together, and have evaluated the effects on PfA-M1 specificity and catalytic efficiency. When the P1 residue was too small to engage the cap residues, the mutations had no effect on catalysis. Hydrolysis of dipeptide substrates with a basic P1 residue was significantly impaired in the E572A mutant, most likely due to the loss of a stabilizing salt bridge between E572 and the P1 sidechain. With M1034A, a substantial reduction in catalytic efficiency was observed when the P1 sidechain was large and non-polar. The double E572A/M1034A exhibited significant decreases in catalytic efficiency for most substrates. This effect was not reversed with the polar substitutions E572N/M1034Q, which replaced the PfA-M1 cap residues with those of Escherichia coli aminopeptidase N. Both E572 and M1034 contributed to the binding of the competitive aminopeptidase inhibitor bestatin.

Proteases as antimalarial targets: strategies for genetic, chemical, and therapeutic validation

Malaria is a devastating parasitic disease affecting half of the world's population. The rapid emergence of resistance against new antimalarial drugs, including artemisinin-based therapies, has made the development of drugs with novel mechanisms of action extremely urgent. Proteases are enzymes proven to be well suited for target-based drug development due to our knowledge of their enzymatic mechanisms and active site structures. More importantly, Plasmodium proteases have been shown to be involved in a variety of pathways that are essential for parasite survival. However, pharmacological rather than target-based approaches have dominated the field of antimalarial drug development, in part due to the challenge of robustly validating Plasmodium targets at the genetic level. Fortunately, over the last few years there has been significant progress in the development of efficient genetic methods to modify the parasite, including several conditional approaches. This progress is finally allowing us not only to validate essential genes genetically, but also to study their molecular functions. In this review, I present our current understanding of the biological role proteases play in the malaria parasite life cycle. I also discuss how the recent advances in Plasmodium genetics, the improvement of protease-oriented chemical biology approaches, and the development of malaria-focused pharmacological assays, can be combined to achieve a robust biological, chemical and therapeutic validation of Plasmodium proteases as viable drug targets.

KBE009: An antimalarial bestatin-like inhibitor of the Plasmodium falciparum M1 aminopeptidase discovered in an Ugi multicomponent reaction-derived peptidomimetic library

Malaria is a global human parasitic disease mainly caused by the protozoon Plasmodium falciparum. Increased parasite resistance to current drugs determines the relevance of finding new treatments against new targets. A novel target is the M1 alanyl-aminopeptidase from P. falciparum (PfA-M1), which is essential for parasite development in human erythrocytes and is inhibited by the pseudo-peptide bestatin. In this work, we used a combinatorial multicomponent approach to produce a library of peptidomimetics and screened it for the inhibition of recombinant PfA-M1 (rPfA-M1) and the in vitro growth of P. falciparum erythrocytic stages (3D7 and FcB1 strains). Dose-response studies with selected compounds allowed identifying the bestatin-based peptidomimetic KBE009 as a submicromolar rPfA-M1 inhibitor (K_i=0.4μM) and an in vitro antimalarial compound as potent as bestatin (IC₅₀=18μM; without promoting erythrocyte lysis). At therapeutic-relevant concentrations, KBE009 is selective for rPfA-M1 over porcine APN (a model of these enzymes from mammals), and is not cytotoxic against HUVEC cells. Docking simulations indicate that this compound binds PfA-M1 without Zn²⁺ coordination, establishing mainly hydrophobic interactions and showing a remarkable shape complementarity with the active site of the enzyme. Moreover, KBE009 inhibits the M1-type aminopeptidase activity (Ala-7-amido-4-methylcoumarin substrate) in isolated live parasites with a potency similar to that of the antimalarial activity (IC₅₀=82μM), strongly suggesting that the antimalarial effect is directly related to the inhibition of the endogenous PfA-M1. These results support the value of this multicomponent strategy to identify PfA-M1 inhibitors, and make KBE009 a promising hit for drug development against malaria.

Targeting the active sites of malarial proteases for antimalarial drug discovery: approaches, progress and challenges

Malaria is an infectious disease causing vast mortality and morbidity worldwide. Although antimalarial drugs are effective in several parts of the world, there is a serious threat to malaria control as malaria parasites are continuously developing widespread resistance against currently available antimalarial drugs, including artemisinin. Such widespread antimalarial drug resistance confirms the need to improve the efficacy of existing or new drugs as well as to develop alternative treatments through the identification of novel drug targets and the development of candidate drugs. Similar to proteases in other parasitic diseases such as leishmaniasis, schistosomiasis, Chagas disease and African sleeping sickness, malarial proteases constitute the major virulence factors in malaria. Malarial proteases belong to several classes and many of them have been targeted for the design and discovery of antimalarial agents. This review summarises the approaches, progress and challenges in the design of small-molecule inhibitors as antimalarial drugs targeting the inhibition of various malarial proteases.

Are Antimalarial Hybrid Molecules a Close Reality or a Distant Dream?

Emergence of drug-resistant Plasmodium falciparum strains has led to a situation of haste in the scientific and pharmaceutical communities. Hence, all their efforts are redirected toward finding alternative chemotherapeutic agents that are capable of combating multidrug-resistant parasite strains. In light of this situation, scientists have come up with the concept of hybridization of two or more active pharmacophores into a single chemical entity, resulting in "antimalarial hybrids." The approach has been applied widely for generation of lead compounds against deadly diseases such as cancer and AIDS, with a proven potential for use as novel drugs, but is comparatively new in the sphere of antimalarial drug discovery. A sudden surge has been evidenced in the number of studies on the design and synthesis of hybrids for treating malaria and may be regarded as proof of their potential advantages over artemisinin-based combination therapy (ACT). However, it is evident from recent studies that most of the potential advantages of antimalarial hybrids, such as lower toxicity, better pharmacokinetics, and easier formulation, have yet to be realized. A number of questions left unaddressed at present need to be answered before this approach can progress to the late stages of clinical development and prove their worth in the clinic. To the best of our knowledge, this compilation is the first attempt to shed light on the shortcomings that are surfacing as more and more studies on molecular hybridization of the active pharmacophores of known antimalarials are being published.

A Multilayer Network Approach for Guiding Drug Repositioning in Neglected Diseases

Drug development for neglected diseases has been historically hampered due to lack of market incentives. The advent of public domain resources containing chemical information from high throughput screenings is changing the landscape of drug discovery for these diseases. In this work we took advantage of data from extensively studied organisms like human, mouse, E. coli and yeast, among others, to develop a novel integrative network model to prioritize and identify candidate drug targets in neglected pathogen proteomes, and bioactive drug-like molecules. We modeled genomic (proteins) and chemical (bioactive compounds) data as a multilayer weighted network graph that takes advantage of bioactivity data across 221 species, chemical similarities between 1.7 10⁵ compounds and several functional relations among 1.67 10⁵ proteins. These relations comprised orthology, sharing of protein domains, and shared participation in defined biochemical pathways. We showcase the application of this network graph to the problem of prioritization of new candidate targets, based on the information available in the graph for known compound-target associations. We validated this strategy by performing a cross validation procedure for known mouse and Trypanosoma cruzi targets and showed that our approach outperforms classic alignment-based approaches. Moreover, our model provides additional flexibility as two different network definitions could be considered, finding in both cases qualitatively different but sensible candidate targets. We also showcase the application of the network to suggest targets for orphan compounds that are active against Plasmodium falciparum in high-throughput screens. In this case our approach provided a reduced prioritization list of target proteins for the query molecules and showed the ability to propose new testable hypotheses for each compound. Moreover, we found that some predictions highlighted by our network model were supported by independent experimental validations as found post-facto in the literature.

Neglected tropical diseases are human infectious diseases that are often associated with poverty. Historically, lack of interest from the pharmaceutical industry resulted in the lack of good drugs to combat the majority of the pathogens that cause these diseases. Recently, the availability of open chemical information has increased with the advent of public domain chemical resources and the release of data from high throughput screening assays. Our aim in this work was to make use of data from extensively studied organisms like human, mouse, E. coli and yeast, among others, to prioritize and identify candidate drug targets in neglected pathogen proteomes, and drug-like bioactive molecules to foster drug development against neglected diseases. Our approach to the problem relied on applying bioinformatics and computational biology strategies to model large datasets spanning complete proteomes and extensive chemical information from publicly available sources. As a result, we were able to prioritize drug targets and identify potential targets for orphan bioactive drugs.

Exploration of structural and physicochemical requirements and search of virtual hits for aminopeptidase N inhibitors

Aminopeptidase N (APN) inhibitors have been reported to be effective in treating of life threatening diseases including cancer. Validated ligand- and structure-based pharmacophore mapping approaches were combined with Bayesian modeling and recursive partitioning to identify structural and physicochemical requirements for highly active APN inhibitors. Based on the assumption that ligand- and structure-based pharmacophore models are complementary, the efficacy of 'multiple pharmacophore screening' for filtering true positive virtual hits was investigated. These multiple pharmacophore screening methods were utilized to search novel virtual hits for APN inhibition. The number of hits was refined and reduced by recursive partitioning, drug-likeliness, pharmacokinetic property prediction, and comparative molecular-docking studies. Four compounds were proposed as the potential virtual hits for APN enzyme inhibition.

Structure-activity relationships and blood distribution of antiplasmodial aminopeptidase-1 inhibitors

Malaria is a severe infectious disease that causes between 655,000 and 1.2 million deaths annually. To overcome the resistance to current drugs, new biological targets are needed for drug development. Aminopeptidase M1 (PfAM1), a zinc metalloprotease, has been proposed as a new drug target to fight malaria. Herein, we disclosed the structure-activity relationships of a selective family of hydroxamate PfAM1 inhibitors based on the malonic template. In particular, we performed a "fluoro-scanning" around hit 1 that enlightened the key positions of the halogen for activity. The docking of the best inhibitor 2 is consistent with in vitro results. The stability of 2 was evaluated in microsomes, in plasma, and toward glutathione. The in vivo distribution study performed with the nanomolar hydroxamate inhibitor 2 (BDM14471) revealed that it reaches its site of action. However, it fails to kill the parasite at concentrations relevant to the enzymatic inhibitory potency, suggesting that killing the parasite remains a challenge for potent and druglike catalytic-site binding PfAM1 inhibitors. In all, this study provides important insights for the design of inhibitors of PfAM1 and the validity of this target.

Fingerprinting the substrate specificity of M1 and M17 aminopeptidases of human malaria, Plasmodium falciparum

Background

Plasmodium falciparum, the causative agent of human malaria, expresses two aminopeptidases, PfM1AAP and PfM17LAP, critical to generating a free amino acid pool used by the intraerythrocytic stage of the parasite for proteins synthesis, growth and development. These exopeptidases are potential targets for the development of a new class of anti-malaria drugs.

Methodology/Principal Findings

To define the substrate specificity of recombinant forms of these two malaria aminopeptidases we used a new library consisting of 61 fluorogenic substrates derived both from natural and unnatural amino acids. We obtained a detailed substrate fingerprint for recombinant forms of the enzymes revealing that PfM1AAP exhibits a very broad substrate tolerance, capable of efficiently hydrolyzing neutral and basic amino acids, while PfM17LAP has narrower substrate specificity and preferentially cleaves bulky, hydrophobic amino acids. The substrate library was also exploited to profile the activity of the native aminopeptidases in soluble cell lysates of P. falciparum malaria.

Conclusions/Significance

This data showed that PfM1AAP and PfM17LAP are responsible for majority of the aminopeptidase activity in these extracts. These studies provide specific substrate and mechanistic information important for understanding the function of these aminopeptidases and could be exploited in the design of new inhibitors to specifically target these for anti-malaria treatment.

Engagement of the S1, S1' and S2' subsites drives efficient catalysis of peptide bond hydrolysis by the M1-family aminopeptidase from Plasmodium falciparum

The M1-family aminopeptidase PfA-M1 catalyzes the last step in the catabolism of human hemoglobin to amino acids in the Plasmodium falciparum food vacuole. In this study, the structural features of the substrate that promote efficient PfA-M1-catalyzed peptide bond hydrolysis were analyzed. X-Ala and Ala-X dipeptide substrates were employed to characterize the specificities of the enzyme's S1 and S1' subsites. Both subsites exhibited a preference for basic and hydrophobic sidechains over polar and acidic sidechains. The relative specificity of the S1 subsite was similar over the pH range 5.5-7.5. Substrate P1 and P1' residues affected both K(m) and k(cat), revealing that sidechain-subsite interactions not only drive the formation of the Michaelis complex but also influence the rates of ensuing chemical steps. Only a small fraction of the available binding energy was exploited in interactions between substrate sidechains and the S1 and S1' subsites, which indicates a modest level of complementarity. There was no correlation between S1 and S1' specificities and amino acid abundance in hemoglobin. Interactions between PfA-M1 and the backbone atoms of the P1' and P2' residues as well as the P2' sidechain further contributed to the catalytic efficiency of substrate hydrolysis. By demonstrating the engagement of multiple, broad-specificity subsites in PfA-M1, these studies provide insight into how this enzyme is able to efficiently generate amino acids from highly sequence-diverse di- and oligopeptides in the food vacuole.

Plasmodium falciparum PfA-M1 aminopeptidase is trafficked via the parasitophorous vacuole and marginally delivered to the food vacuole

Background

The Plasmodium falciparum PfA-M1 aminopeptidase, encoded by a single copy gene, displays a neutral optimal activity at pH 7.4. It is thought to be involved in haemoglobin degradation and/or invasion of the host cells. Although a series of inhibitors developed against PfA-M1 suggest that this enzyme is a promising target for therapeutic intervention, the biological function(s) of the three different forms of the enzyme (p120, p96 and p68) are not fully understood. Two recent studies using PfA-M1 transfections have also provided conflicting results on PfA-M1 localization within or outside the food vacuole. Alternative destinations, such as the nucleus, have also been proposed.

Methods

By using a combination of techniques, such as cellular and biochemical fractionations, biochemical analysis, mass-spectrometry, immunofluorescence assays and live imaging of GFP fusions to various PfA-M1 domains, evidence is provided for differential localization and behaviour of the three different forms of PfA-M1 in the infected red blood cell which had not been established before.

Results

The high molecular weight p120 form of PfA-M1, the only version of the protein with a hydrophobic transmembrane domain, is detected both inside the parasite and in the parasitophorous vacuole while the processed p68 form is strictly soluble and localized within the parasite. The transient intermediate and soluble p96 form is localized at the border of parasitophorous vacuole and within the parasite in a compartment sensitive to high concentrations of saponin. Upon treatment with brefeldin A, the PfA-M1 maturation is blocked and the enzyme remains in a compartment close to the nucleus.

Conclusions

The PfA-M1 trafficking/maturation scenario that emerges from this data indicates that PfA-M1, synthesized as the precursor p120 form, is targeted to the parasitophorous vacuole via the parasite endoplasmic reticulum/Golgi, where it is converted into the transient p96 form. This p96 form is eventually redirected into the parasite to be converted into the processed p68 form that is only marginally delivered to the parasite food vacuole. These results provide insights on PfA-M1 topology regarding key compartments of the infected red blood cells that have important implications for the development of inhibitors targeting this plasmodial enzyme.

Metallo-aminopeptidase inhibitors

Aminopeptidases are enzymes that selectively hydrolyze an amino acid residue from the N-terminus of proteins and peptides. They are important for the proper functioning of prokaryotic and eukaryotic cells, but very often are central players in the devastating human diseases like cancer, malaria and diabetes. The largest aminopeptidase group include enzymes containing metal ion(s) in their active centers, which often determines the type of inhibitors that are the most suitable for them. Effective ligands mostly bind in a non-covalent mode by forming complexes with the metal ion(s). Here, we present several approaches for the design of inhibitors for metallo-aminopeptidases. The optimized structures should be considered as potential leads in the drug discovery process against endogenous and infectious diseases.

Structural modifications of quinoline-based antimalarial agents: Recent developments

Antimalarial drugs constitute a major part of antiprotozoal drugs and have been in practice for a long time. Antimalarial agents generally belong to the class of quinoline which acts by interfering with heme metabolism. The recent increase in development of chloroquine-resistant strains of Plasmodium falciparum and failure of vaccination program against malaria have fuelled the drug discovery program against this old and widespread disease. Quinoline and its related derivative comprise a class of heterocycles, which has been exploited immensely than any other nucleus for the development of potent antimalarial agents. Various chemical modifications of quinoline have been attempted to achieve analogs with potent antimalarial properties against sensitive as well as resistant strains of Plasmodium sp., together with minimal potential undesirable side effects. This review outlines essentially some of the recent chemical modifications undertaken for the development of potent antimalarial agents based on quinoline.

Synthesis of 9-anilinoacridine triazines as new class of hybrid antimalarial agents

There is challenge and urgency to synthesize cost-effective chemotherapeutic agents for treatment of malaria after the widespread development of resistance to CQ. In the present study, we synthesized a new series of hybrid 9-anilinoacridine triazines using the cheap chemicals 6,9-dichloro-2-methoxy acridine and cyanuric chloride. The series of new hybrid 9-anilinoacridine triazines were evaluated in vitro for their antimalarial activity against CQ-sensitive 3D7 strain of Plasmodium falciparum and their cytotoxicity were determined on VERO cell line. Of the evaluated compounds, two compounds 17 (IC(50)=4.21 nM) and 22 (IC(50)=4.27 nM) displayed two times higher potency than CQ (IC(50)=8.15 nM). Most of the compounds showed fairly high selectivity index. The compounds 13 and 29 displayed >96.59% and 98.73% suppression, respectively, orally against N-67 strain of Plasmodium yoelii in swiss mice at dose 100 mg/kg for four days.

Chloroquine mediates specific proteome oxidative damage across the erythrocytic cycle of resistant Plasmodium falciparum

Resistance of Plasmodium falciparum to chloroquine hinders malaria control in endemic areas. Current hypotheses on the action mechanism of chloroquine evoke its ultimate interference with the parasite's oxidative defence systems. Through carbonyl derivatization by 2,4-dinitrophenylhydrazine and proteomics, we compared oxidatively modified proteins across the parasite's intraerythrocytic stages in untreated and transiently IC(50) chloroquine-treated cultures of the chloroquine-resistant P. falciparum strain Dd2. Functional plasmodial protein groups found to be most oxidatively damaged were among those central to the parasite's physiological processes, including protein folding, proteolysis, energy metabolism, signal transduction, and pathogenesis. While an almost constant number of oxidized proteins was detected across the P. falciparum life cycle, chloroquine treatment led to increases in both the extent of protein oxidation and the number of proteins oxidized as the intraerythrocytic cycle progressed to mature stages. Our data provide new insights into early molecular effects produced by chloroquine in the parasite, as well as into the normal protein-oxidation modifications along the parasite cycle. Oxidized proteins involved in the particular parasite drug-response suggest that chloroquine causes specific oxidative stress, sharing common features with eukaryotic cells. Targeting these processes might provide ways of combating chloroquine-resistance and developing new antimalarial drugs.

Chemical target validation studies of aminopeptidase in malaria parasites using alpha-aminoalkylphosphonate and phosphonopeptide inhibitors

During its intraerythrocytic phase, the most lethal human malarial parasite, Plasmodium falciparum, digests host cell hemoglobin as a source of some of the amino acids required for its own protein synthesis. A number of parasite endopeptidases (including plasmepsins and falcipains) process the globin into small peptides. These peptides appear to be further digested to free amino acids by aminopeptidases, enzymes that catalyze the sequential cleavage of N-terminal amino acids from peptides. Aminopeptidases are classified into different evolutionary families according to their sequence motifs and preferred substrates. The aminopeptidase inhibitor bestatin can disrupt parasite development, suggesting that this group of enzymes might be a chemotherapeutic target. Two bestatin-susceptible aminopeptidase activities, associated with gene products belonging to the M1 and M17 families, have been described in blood-stage P. falciparum parasites, but it is not known whether one or both are required for parasite development. To establish whether inhibition of the M17 aminopeptidase is sufficient to confer antimalarial activity, we evaluated 35 aminoalkylphosphonate and phosphonopeptide compounds designed to be specific inhibitors of M17 aminopeptidases. The compounds had a range of activities against cultured P. falciparum parasites with 50% inhibitory concentrations down to 14 muM. Some of the compounds were also potent inhibitors of parasite aminopeptidase activity, though it appeared that many were capable of inhibiting the M1 as well as the M17 enzyme. There was a strong correlation between the potencies of the compounds against whole parasites and against the enzyme, suggesting that M17 and/or M1 aminopeptidases may be valid antimalarial drug targets.

Novel Selective Inhibitors of the Zinc Plasmodial Aminopeptidase PfA-M1 as Potential Antimalarial Agents

Proteases that are expressed during the erythocytic stage of Plasmodium falciparum are newly explored drug targets for the treatment of malaria. We report here the discovery of potent inhibitors of PfA-M1, a metallo-aminopeptidase of the parasite. These compounds are based on a malonic hydroxamic template and present a very good selectivity toward neutral aminopeptidase (APN-CD13), a related protease in mammals. Structure-activity relationships in these series are described. Further optimization of the best inhibitor yielded a nanomolar, selective inhibitor of PfA-M1. This inhibitor displays good physicochemical and pharmacokinetic properties and a promising antimalarial activity.

Recent advances in antimalarial drug development

Malaria caused by protozoa of the genus Plasmodium, because of its prevalence, virulence, and drug resistance, is the most serious and widespread parasitic disease encountered by mankind. The inadequate armory of drugs in widespread use for the treatment of malaria, development of strains resistant to commonly used drugs such as chloroquine, and the lack of affordable new drugs are the limiting factors in the fight against malaria. These factors underscore the continuing need of research for new classes of antimalarial agents, and a re-examination of the existing antimalarial drugs that may be effective against resistant strains. This review provides an in-depth look at the most significant progress made during the past 10 years in antimalarial drug development.

Synthesis and structure activity relationships of novel non-peptidic metallo-aminopeptidase inhibitors

Racemic derivatives of 3-amino-2-tetralone were synthesised and evaluated for their ability to inhibit metallo-aminopeptidase activities. New compounds substituted in position 2 by methyl ketone, substituted oximes or hydroxamic acids as well as heterocyclic derivatives were evaluated against representative members of zinc-dependent aminopeptidases: leucine aminopeptidase (E.C. 3.4.11.1), aminopeptidase-N (E.C. 3.4.11.2), Aeromonas proteolytica aminopeptidase (E.C. 3.4.11.10), and the aminopeptidase activity of leukotriene A(4) hydrolase (E.C. 3.3.2.6). Several compounds showed K(i) values in the low micromolar range against the 'one-zinc' aminopeptidases, while most of them were rather poor inhibitors of the 'two-zinc' enzymes. This interesting selectivity profile may guide the design of new, specific inhibitors of target mammalian aminopeptidases with one active site zinc.

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.