Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites

With the continued loss of antimalarials to resistance, drug repositioning may have a role in maximising efficiency and accelerating the discovery of new antimalarial drugs. Bayesian statistics was previously used as a tool to virtually screen USFDA approved drugs for predicted β-haematin (synthetic haemozoin) inhibition and in vitro antimalarial activity. Here, we report the experimental evaluation of nine of the highest ranked drugs, confirming the accuracy of the model by showing an overall 93% hit rate. Lapatinib, nilotinib, and lomitapide showed the best activity for inhibition of β-haematin formation and parasite growth and were found to inhibit haemozoin formation in the parasite, providing mechanistic insights into their mode of antimalarial action. We then screened the USFDA approved drugs for binding to the β-haematin crystal, applying a docking method in order to evaluate its performance. The docking method correctly identified imatinib, lapatinib, nilotinib, and lomitapide. Experimental evaluation of 22 of the highest ranked purchasable drugs showed a 24% hit rate. Lapatinib and nilotinib were chosen as templates for shape and electrostatic similarity screening for lead hopping using the in-stock ChemDiv compound catalogue. The actives were novel structures worthy of future investigation. This study presents a comparison of different in silico methods to identify new haemozoin-inhibiting chemotherapeutic alternatives for malaria that proved to be useful in different ways when taking into consideration their strengths and limitations.

Acute toxicity of cyanide in aerobic respiration: Theoretical and experimental support for murburn explanation

The inefficiency of cyanide/HCN (CN) binding with heme proteins (under physiological regimes) is demonstrated with an assessment of thermodynamics, kinetics, and inhibition constants. The acute onset of toxicity and CN's mg/Kg LD50 (μM lethal concentration) suggests that the classical hemeFe binding-based inhibition rationale is untenable to account for the toxicity of CN. In vitro mechanistic probing of CN-mediated inhibition of hemeFe reductionist systems was explored as a murburn model for mitochondrial oxidative phosphorylation (mOxPhos). The effect of CN in haloperoxidase catalyzed chlorine moiety transfer to small organics was considered as an analogous probe for phosphate group transfer in mOxPhos. Similarly, inclusion of CN in peroxidase-catalase mediated one-electron oxidation of small organics was used to explore electron transfer outcomes in mOxPhos, leading to water formation. The free energy correlations from a Hammett study and IC50/Hill slopes analyses and comparison with ligands ( CO/ H 2 S/ N 3 - ) $\left( {\text{CO}}/{{{{\text{H}}_{2}}\text{S}}/{\text{N}_{3}^{\text{-}}}\;}\; \right)$ provide insights into the involvement of diffusible radicals and proton-equilibriums, explaining analogous outcomes in mOxPhos chemistry. Further, we demonstrate that superoxide (diffusible reactive oxygen species, DROS) enables in vitro ATP synthesis from ADP+phosphate, and show that this reaction is inhibited by CN. Therefore, practically instantaneous CN ion-radical interactions with DROS in matrix catalytically disrupt mOxPhos, explaining the acute lethal effect of CN.

Magnesium Lithospermate B Derived from Salvia miltiorrhiza Ameliorates Right Ventricle Remodeling in Pulmonary Hypertensive Rats via Inhibition of NOX/VPO1 Pathway

Right ventricle (RV) remodeling is a major pathological feature in pulmonary arterial hypertension (PAH). Magnesium lithospermate B (MLB) is a compound isolated from the roots of Salvia miltiorrhiza and it possesses multiple pharmacological activities such as anti-inflammation and antioxidation. This study aims to investigate whether MLB is able to prevent RV remodeling in PAH and the underlying mechanisms. In vivo, SD rats were exposed to 10% O₂ for 21 d to induce RV remodeling, which showed hypertrophic features (increases in the ratio of RV weight to tibia length, cellular size, and hypertrophic marker expression), accompanied by upregulation in expression of NADPH oxidases (NOX2 and NOX4) and vascular peroxidase 1 (VPO1), increases in hydrogen peroxide (H₂O₂) and hypochlorous acid (HOCl) production and elevation in phosphorylation levels of ERK; these changes were attenuated by treating rats with MLB. In vitro, the cultured H9c2 cells were exposed to 3% O₂ for 24 h to induce hypertrophy, which showed hypertrophic features (increases in cellular size and hypertrophic marker expression). Administration of MLB or VAS2870 (a positive control for NOX inhibitor) could prevent cardiomyocyte hypertrophy concomitant with decreases in NOX (NOX2 and NOX4) and VPO1 expression, H₂O₂ and HOCl production, and ERK phosphorylation. Based on these observations, we conclude that MLB is able to prevent RV remodeling in hypoxic PAH rats through a mechanism involving a suppression of NOX/VPO1 pathway as well as ERK signaling pathway. MLB may possess the potential clinical value for PAH therapy.

Novel carbazole aminoalcohols as inhibitors of β-hematin formation: Antiplasmodial and antischistosomal activities

Malaria and schistosomiasis are two of the most socioeconomically devastating parasitic diseases in tropical and subtropical countries. Since current chemotherapeutic options are limited and defective, there is an urgent need to develop novel antiplasmodials and antischistosomals. Hemozoin is a disposal product formed from the hemoglobin digestion by some blood-feeding parasites. Hemozoin formation is an essential process for the parasites to detoxify free heme, which is a reliable therapeutic target for identifying novel antiparasitic agents. A series of novel carbazole aminoalcohols were designed and synthesized as potential antiplasmodial and antischistosomal agents, and several compounds showed potent in vitro activities against Plasmodium falciparum 3D7 and Dd2 strains and adult and juvenile Schistosoma japonicum. Investigations on the dual antiparasitic mechanisms showed the correlation between inhibitory activity of β-hematin formation and antiparasitic activity. Inhibiting hemozoin formation was identified as one of the mechanisms of action of carbazole aminoalcohols. Compound 7 displayed potent antiplasmodial (Pf3D7 IC₅₀ = 0.248 μM, PfDd2 IC₅₀ = 0.091 μM) and antischistosomal activities (100% mortality of adult and juvenile schistosomes at 5 and 10 μg/mL, respectively) and exhibited low cytotoxicity (CC₅₀ = 7.931 μM), which could be considered as a promising lead for further investigation. Stoichiometry determination and molecular docking studies were also performed to explain the mode of action of compound 7.

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Carbazole aminoalcohol was confirmed as a novel antiplasmodial and antischistosomal scaffold.

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The mechanism of action relied on β-hematin formation inhibition.

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The carbazole aminoalcohols interacted with hematin through forming a 1:1 complex.

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Compound 7 showed potent antiplasmodial ability (Pf3D7 IC₅₀ = 0.248 μM, PfDd2 IC₅₀ = 0.091 μM).

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In vitro antischistosomal effect of 7 meets the WHO's criterion of “hit” for schistosomiasis control.

Bayesian models trained with HTS data for predicting β-haematin inhibition and in vitro antimalarial activity

A large quantity of high throughput screening (HTS) data for antimalarial activity has become available in recent years. This includes both phenotypic and target-based activity. Realising the maximum value of these data remains a challenge. In this respect, methods that allow such data to be used for virtual screening maximise efficiency and reduce costs. In this study both in vitro antimalarial activity and inhibitory data for β-haematin formation, largely obtained from publically available sources, has been used to develop Bayesian models for inhibitors of β-haematin formation and in vitro antimalarial activity. These models were used to screen two in silico compound libraries. In the first, the 1510 U.S. Food and Drug Administration approved drugs available on PubChem were ranked from highest to lowest Bayesian score based on a training set of β-haematin inhibiting compounds active against Plasmodium falciparum that did not include any of the clinical antimalarials or close analogues. The six known clinical antimalarials that inhibit β-haematin formation were ranked in the top 2.1% of compounds. Furthermore, the in vitro antimalarial hit-rate for this prioritised set of compounds was found to be 81% in the case of the subset where activity data are available in PubChem. In the second, a library of about 5000 commercially available compounds (Aldrich(CPR)) was virtually screened for ability to inhibit β-haematin formation and then for in vitro antimalarial activity. A selection of 34 compounds was purchased and tested, of which 24 were predicted to be β-haematin inhibitors. The hit rate for inhibition of β-haematin formation was found to be 25% and a third of these were active against P. falciparum, corresponding to enrichments estimated at about 25- and 140-fold relative to random screening, respectively.

Malarial hemozoin: from target to tool

BACKGROUND

Malaria is an extremely devastating disease that continues to affect millions of people each year. A distinctive attribute of malaria infected red blood cells is the presence of malarial pigment or the so-called hemozoin. Hemozoin is a biocrystal synthesized by Plasmodium and other blood-feeding parasites to avoid the toxicity of free heme derived from the digestion of hemoglobin during invasion of the erythrocytes.

SCOPE OF REVIEW

Hemozoin is involved in several aspects of the pathology of the disease as well as in important processes such as the immunogenicity elicited. It is known that the once best antimalarial drug, chloroquine, exerted its effect through interference with the process of hemozoin formation. In the present review we explore what is known about hemozoin, from hemoglobin digestion, to its final structural analysis, to its physicochemical properties, its role in the disease and notions of the possible mechanisms that could kill the parasite by disrupting the synthesis or integrity of this remarkable crystal.

MAJOR CONCLUSIONS

The importance and peculiarities of this biocrystal have given researchers a cause to consider it as a target for new antimalarials and to use it through unconventional approaches for diagnostics and therapeutics against the disease.

GENERAL SIGNIFICANCE

Hemozoin plays an essential role in the biology of malarial disease. Innovative ideas could use all the existing data on the unique chemical and biophysical properties of this macromolecule to come up with new ways of combating malaria.

Synthesis and evaluation of 7-chloro-4-(piperazin-1-yl)quinoline-sulfonamide as hybrid antiprotozoal agents

A new series of 4-aminochloroquinoline based sulfonamides were synthesized and evaluated for antiamoebic and antimalarial activities. Out of the eleven compounds evaluated (F1-F11), two of them (F3 and F10) showed good activity against Entamoeba histolytica (IC50 <5 μM). Three of the compounds (F5, F7 and F8) also displayed antimalarial activity against the chloroquine-resistant (FCR-3) strain of Plasmodium falciparum with IC50 values of 2 μM. Compound F7, whose crystal structure was also determined, inhibited β-haematin formation more potently than quinine. To further understand the action of hybrid molecules F7 and F8, molecular docking was carried out against the homology model of P. falciparum enzyme dihydropteroate synthase (PfDHPS). The complexes showed that the inhibitors place themselves nicely into the active site of the enzyme and exhibit interaction energy which is in accordance with our activity profile data. Application of Lipinski 'rule of five' on all the compounds (F1-F11) suggested high drug likeness of F7 and F8, similar to quinine.

Antimalarial activity and mechanisms of action of two novel 4-aminoquinolines against chloroquine-resistant parasites

Chloroquine (CQ) is a cost effective antimalarial drug with a relatively good safety profile (or therapeutic index). However, CQ is no longer used alone to treat patients with Plasmodium falciparum due to the emergence and spread of CQ-resistant strains, also reported for P. vivax. Despite CQ resistance, novel drug candidates based on the structure of CQ continue to be considered, as in the present work. One CQ analog was synthesized as monoquinoline (MAQ) and compared with a previously synthesized bisquinoline (BAQ), both tested against P. falciparum in vitro and against P. berghei in mice, then evaluated in vitro for their cytotoxicity and ability to inhibit hemozoin formation. Their interactions with residues present in the NADH binding site of P falciparum lactate dehydrogenase were evaluated using docking analysis software. Both compounds were active in the nanomolar range evaluated through the HRPII and hypoxanthine tests. MAQ and BAQ derivatives were not toxic, and both compounds significantly inhibited hemozoin formation, in a dose-dependent manner. MAQ had a higher selectivity index than BAQ and both compounds were weak PfLDH inhibitors, a result previously reported also for CQ. Taken together, the two CQ analogues represent promising molecules which seem to act in a crucial point for the parasite, inhibiting hemozoin formation.

Synthesis, antiprotozoal, antimicrobial, β-hematin inhibition, cytotoxicity and methemoglobin (MetHb) formation activities of bis(8-aminoquinolines)

In continuing our search of potent antimalarials based on 8-aminoquinoline structural framework, three series of novel bis(8-aminoquinolines) using convenient one to four steps synthetic procedures were synthesized. The bisquinolines were evaluated for in vitro antimalarial (Plasmodiumfalciparum), antileishmanial (Leishmaniadonovani), antimicrobial (a panel of pathogenic bacteria and fungi), cytotoxicity, β-hematin inhibitory and methemoglobin (MetHb) formation activities. Several compounds exhibited superior antimalarial activities compared to parent drug primaquine. Selected compounds (44, 61 and 79) when tested for in vivo blood-schizontocidal antimalarial activity (Plasmodiumberghei) displayed potent blood-schizontocial activities. The bisquinolines showed negligible MetHb formation (0.2-1.2%) underlining their potential in the treatment of glucose-6-phosphate dehydrogenase deficient patients. The bisquinoline analogues (36, 73 and 79) also exhibited promising in vitro antileishmanial activity, and antimicrobial activities (43, 44 and 76) against a panel of pathogenic bacteria and fungi. The results of this study provide evidence that bis(8-aminoquinolines), like their bis(4-aminoquinolines) and artemisinin dimers counterparts, are a promising class of antimalarial agents.

Interference with hemozoin formation represents an important mechanism of schistosomicidal action of antimalarial quinoline methanols

BACKGROUND

The parasitic trematode Schistosoma mansoni is one of the major causative agents of human schistosomiasis, which afflicts 200 million people worldwide. Praziquantel remains the main drug used for schistosomiasis treatment, and reliance on the single therapy has been prompting the search for new therapeutic compounds against this disease. Our group has demonstrated that heme crystallization into hemozoin (Hz) within the S. mansoni gut is a major heme detoxification route with lipid droplets involved in this process and acting as a potential chemotherapeutical target. In the present work, we investigated the effects of three antimalarial compounds, quinine (QN), quinidine (QND) and quinacrine (QCR) in a murine schistosomiasis model by using a combination of biochemical, cell biology and molecular biology approaches.

METHODOLOGY/PRINCIPAL FINDINGS

Treatment of S. mansoni-infected female Swiss mice with daily intraperitoneal injections of QN, and QND (75 mg/kg/day) from the 11(th) to 17(th) day after infection caused significant decreases in worm burden (39%-61%) and egg production (42%-98%). Hz formation was significantly inhibited (40%-65%) in female worms recovered from QN- and QND-treated mice and correlated with reduction in the female worm burden. We also observed that QN treatment promoted remarkable ultrastructural changes in male and female worms, particularly in the gut epithelium and reduced the granulomatous reaction to parasite eggs trapped in the liver. Microarray gene expression analysis indicated that QN treatment increased the expression of transcripts related to musculature, protein synthesis and repair mechanisms.

CONCLUSIONS

The overall significant reduction in several disease burden parameters by the antimalarial quinoline methanols indicates that interference with Hz formation in S. mansoni represents an important mechanism of schistosomicidal action of these compounds and points out the heme crystallization process as a valid chemotherapeutic target to treat schistosomiasis.

[Conditional inhibition of hemozoin formation by chloroquine in vitro]

OBJECTIVE

To study the characteristics of inhibition on hemozoin formation by chloroquine under in vitro condition.

METHODS

Under different concentrations (0.5-2 mol/L) of sodium acetate (NaAc) and at the pH range of 4.0-5.0, chloroquine was tested for inhibition of beta-hematin (hemozion) formation by using the HPIA (heme polymerization inhibitory activity) assay. The morphology of beta-hematin crystals was determined by light microscopy. Ultraviolet spectrophotometry was employed to measure beta-hematin content, and the size of beta-hematin crystal was analyzed by X-ray diffraction (XRD).

RESULTS

Chloroquine exhibited varied effect on beta-hematin formation, depending on pH value and Na+ concentration. When the NaAc concentration increased from 0.5 mol/L (pH 4.2) to 2 mol/L (pH 4.8), the chloroquine inhibitory effect also increased. Results suggested that there exists a threshold pH, below which the beta-hematin formation escalates and chloroquine inhibition declines, and at or above which chloroquine exerts a stronger inhibitory effect on beta-hematin formation. With the increase of pH from 4.4 to 4.8, the crystallinity and the size of crystal changed from 6.93% and 357 angstrom to 6.32% and 264 angstrom, respectively. When pH reached to 5, no more beta-hematin formed. Chloroquine could reduce the crystallinity and crystal size of beta-hematin at same pH value. Morphology analysis on the samples was consistent with the above results.

CONCLUSION

Chloroquine inhibits hemozoin formation only when the pH value is at or above threshold pH.

Bilirubin inhibits Plasmodium falciparum growth through the generation of reactive oxygen species

Free heme is very toxic because it generates highly reactive hydroxyl radicals ((.)OH) to cause oxidative damage. Detoxification of free heme by the heme oxygenase (HO) system is a very common phenomenon by which free heme is catabolized to form bilirubin as an end product. Interestingly, the malaria parasite, Plasmodium falciparum, lacks an HO system, but it forms hemozoin, mainly to detoxify free heme. Here, we report that bilirubin significantly induces oxidative stress in the parasite as evident from the increased formation of lipid peroxide, decrease in glutathione content, and increased formation of H(2)O(2) and (.)OH. Bilirubin can effectively inhibit hemozoin formation also. Furthermore, results indicate that bilirubin inhibits parasite growth and induces caspase-like protease activity, up-regulates the expression of apoptosis-related protein (Gene ID PFI0450c), and reduces the mitochondrial membrane potential. (.)OH scavengers such as mannitol, as well as the spin trap alpha-phenyl-n-tert-butylnitrone, effectively protect the parasite from bilirubin-induced oxidative stress and growth inhibition. These findings suggest that bilirubin, through the development of oxidative stress, induces P. falciparum cell death and that the malaria parasite lacks an HO system probably to protect itself from bilirubin-induced cell death as a second line of defense.

Trioxaquines and heme-artemisinin adducts inhibit the in vitro formation of hemozoin better than chloroquine

Trioxaquines, potential antimalarial agents, and heme-artemisinin adducts, resulting from the alkylation of heme by artemisinin, were evaluated as inhibitors of beta-hematin formation in 10 M acetate medium at pH 5.

Activity of piperaquine and other 4-aminoquinoline antiplasmodial drugs against chloroquine-sensitive and resistant blood-stages of Plasmodium falciparum

Chloroquine (CQ), a 4-aminoquinoline, accumulates in acidic digestive vacuoles of the malaria parasite, preventing conversion of toxic haematin to beta-haematin. We examine how bis 4-aminoquinoline piperaquine (PQ) and its hydroxy-modification (OH-PQ) retain potency on chloroquine-resistant (CQ-R) Plasmodium falciparum. For CQ, PQ, OH-PQ and 4 and 5, representing halves of PQ, beta-haematin inhibitory activity (BHIA) was assayed, while potency was determined in CQ-sensitive (CQ-S) and CQ-R P. falciparum. From measured pK(a)s and the pH-modulated distribution of base between water and lipid (logD), the vacuolar accumulation ratio (VAR) of charged drug from plasma water (pH 7.4) into vacuolar water (pH 4.8) and lipid accumulation ratio (LAR) were calculated. All agents were active in BHIA. In CQ-S, PQ, OH-PQ and CQ were equally potent while 4 and 5 were 100 times less potent. CQ with two basic centres has a VAR of 143,482, while 4 and 5, with two basic centres of lower pK(a)s have VARs of 1287 and 1966. In contrast PQ and OH-PQ have four basic centres and achieve VARs of 104,378 and 19,874. This confirms the importance of VAR for potency against CQ-S parasites. Contrasting results were seen in CQ-R. 5, PQ and OH-PQ with LARs of 693; 973,492 and 398,118 (compared with 8.25 for CQ) showed similar potency in CQ-S and CQ-R. Importance of LAR for potency against CQ-R parasites probably reflects ability to block efflux by hydrophobic interaction with PfCRT but may relate to beta-haematin inhibition in vacuolar lipid.

Naturally occurring cobalamins have antimalarial activity

The acquisition of resistance by malaria parasites towards existing antimalarials has necessitated the development of new chemotherapeutic agents. The effect of vitamin B(12) derivatives on the formation of beta-haematin (synthetic haemozoin) was determined under conditions similar to those in the parasitic food vacuole (using chloroquine, a known inhibitor of haemozoin formation for comparison). Adenosylcobalamin (Ado-cbl), methylcobalamin (CH(3)-cbl) and aquocobalamin (H(2)O-cbl) were approximately forty times more effective inhibitors of beta-haematin formation than chloroquine, cyanocobalamin (CN-cbl) was slightly more inhibitory than chloroquine, while dicyanocobinamide had no effect. It is proposed that the cobalamins exert their inhibitory effect on beta-haematin formation by pi-interactions of their corrin ring with the Fe(III)-protoporphyrin ring and by hydrogen-bonding using their 5,6-dimethylbenzimidazole/ribose/sugar side-chain. The antimalarial activity for the cobalamins (Ado-cbl>CH(3)-cbl>H(2)O-cbl>CN-cbl) was found to be less than that for chloroquine or quinine. Ado-cbl, CH(3)-cbl and CN-cbl do not accumulate in the parasite food vacuole by pH trapping, but H(2)O-cbl does. Unlike humans, the malaria parasite has only one enzyme that uses cobalamin as a cofactor, namely methionine synthase, which is important for growth and metabolism. Thus cobalamins in very small amounts are necessary for Plasmodium falciparum growth but in larger amounts they display antimalarial properties.

Synthesis and antimalarial activity of side chain modified 4-aminoquinoline derivatives

A new series of side-chain modified 4-aminoquinolines have been synthesized and found active against P. falciparum in vitro and P. yoelli in vivo. Compounds 6, 11, 12, and 19 exhibited superior in vitro activity compared to chloroquine. Selected compounds 6, 12, and 19 exhibited significant suppression in the in vivo assay. These analogs form a complex with hematin and inhibit the beta-hematin formation, suggesting that this class of compounds act on a heme polymerization target.

Antimalarial drugs inhibiting hemozoin (β-hematin) formation: A mechanistic update

Digestion of hemoglobin in the food vacuole of the malaria parasite produces very high quantities of redox active toxic free heme. Hemozoin (beta-hematin) formation is a unique process adopted by Plasmodium sp. to detoxify free heme. Hemozoin formation is a validated target for most of the well-known existing antimalarial drugs and considered to be a suitable target to develop new antimalarials. Here we discuss the possible mechanisms of free heme detoxification in the malaria parasite and the mechanistic details of compounds, which offer antimalarial activity by inhibiting hemozoin formation. The chemical nature of new antimalarial compounds showing antimalarial activity through the inhibition of hemozoin formation has also been incorporated, which may help to design future antimalarials with therapeutic potential against multi-drug resistant malaria.

Application of the paramagnetic dipole field for solution NMR active site structure determination in low-spin, cyanide-inhibited ferric hemoproteins

The principles for the application of the paramagnetic dipolar field of low-spin, cyanide-inhibited ferrihemoproteins for determining active site structure are briefly described. The ubiquitous dipolar shifts for assigned residues, together with crystal coordinates of some appropriate structural homolog, allow determination of the orientation and anisotropies of the paramagnetic dipolar tensor. The orientation of chi uniquely defines the orientation of the Fe-CN unit, which is tilted variably and sensitively monitors distal steric and H-bond interactions. The mapped dipolar field, in turn, can be used to determine the orientation of mutated residues. Case studies involving unusual genetic variants and point mutants of myoglobins, human hemoglobins, horseradish peroxidase and its substrate complex of heme oxygenase are presented as examples.

Simple colorimetric inhibition assay of heme crystallization for high-throughput screening of antimalarial compounds

Current assays for screening new antimalarials need initiators of beta-hematin formation that require laborious preparation, special devices, and substrates. In this study, based on reduction of heme absorption in beta-hematin formation, we developed a simple colorimetric assay using Tween 20 as an initiator and a microplate reader for high-throughput screening of inhibitors of beta-hematin formation.

Targeting of hematin by the antimalarial pyronaridine

Pyronaridine, 2-methoxy-7-chloro-10[3',5'-bis(pyrrolidinyl-1-methyl-)4'hydroxyphenyl]aminobenzyl-(b)-1,5-naphthyridine, a new Mannich base schizontocide originally developed in China and structurally related to the aminoacridine drug quinacrine, is currently undergoing clinical testing. We now show that pyronaridine targets hematin, as demonstrated by its ability to inhibit in vitro beta-hematin formation (at a concentration equal to that of chloroquine), to form a complex with hematin with a stoichiometry of 1:2, to enhance hematin-induced red blood cell lysis (but at 1/100 of the chloroquine concentration), and to inhibit glutathione-dependent degradation of hematin. Our observations that pyronaridine exerted this mechanism of action in situ, based on growth studies of Plasmodium falciparum K1 in culture showing antagonism of pyronaridine in combination with antimalarials (chloroquine, mefloquine, and quinine) that inhibit beta-hematin formation, were equivocal.

Probing the Role of the Covalent Linkage of Ferrocene into a Chloroquine Template

A new therapeutic approach to malaria led to the discovery of ferroquine (FQ, SR97276). To assess the importance of the linkage of the ferrocenyl group to a 4-aminoquinoline scaffold, two series of 4-aminoquinolines, structurally related to FQ, were synthesized. Evaluation of antimalarial activity, physicochemical parameters, and the beta-hematin inhibition property indicate that the ferrocene moiety has to be covalently flanked by a 4-aminoquinoline and an alkylamine. Current data reinforced our choice of FQ as a drug candidate.

Purification and characterization of a catalase from photosynthetic bacterium Rhodospirillum rubrum S1 grown under anaerobic conditions

The photosynthetic bacterium, Rhodospirillum rubrum S1, when grown under anaerobic conditions, generated three different types of catalases. In this study, we purified and characterized the highest molecular weight catalase from the three catalases. The total specific catalase activity of the crude cell extracts was 88 U/mg. After the completion of the final purification step, the specific activity of the purified catalase was 1,256 U/mg. The purified catalase evidenced an estimated molecular mass of 318 kDa, consisting of four identical subunits, each of 79 kDa. The purified enzyme exhibited an apparent Km value of 30.4 mM and a Vmax of 2,564 U against hydrogen peroxide. The enzyme also exhibited a broad optimal pH (5.0-9.0), and remained stable over a broad temperature range (20 degrees C-60 degrees C). It maintained 90% activity against organic solvents (ethanol/chloroform) known hydroperoxidase inhibitors, and exhibited no detectable peroxidase activity. The catalase activity of the purified enzyme was reduced to 19% of full activity as the result of the administration of 10 mM 3-amino-1,2,4-triazole, a heme-containing catalase inhibitor. Sodium cyanide, sodium azide, and hydroxylamine, all of which are known heme protein inhibitors, inhibited catalase activity by 50% at concentrations of 11.5 microM, 0.52 microM, and 0.11 microM, respectively. In accordance with these findings, the enzyme was identified as a type of monofunctional catalase.

Inhibition assay of β-hematin formation initiated by lecithin for screening new antimalarial drugs

Measurement of heme crystallization provides a tool for screening new antimalarial drugs. Current assays for heme crystallization have employed initiators such as thermo, histidine-rich proteins, and lipids extracted from parasites and infected plasma. These initiators are unnatural or require laborious steps to prepare. In this study, we used a commercially available lipid, lecithin, a kind of phospholipid containing about 50% unsaturated fatty acids, as an initiator for heme crystal (beta-hematin) formation. We demonstrated that the inhibition of lecithin-based beta-hematin formation by antimalarial drugs is highly correlated with the preformed beta-hematin-based method. In addition, the lecithin-based assay is sensitive and convenient for large-scale screening of new novel antimalarials. We also indicated that dimethyl sulfoxide is an ideal solvent for preparation of heme stock solution, which is stable and can be used for 1 month.

Electrodes modified with the phase transition polymer and heme peptide: biocatalysis and biosensing with tunable activity and dynamic range

An electrode was modified with a phase transition polymer, poly(N-isopropylacrylamide), and the polymer was further modified with a peroxidase model compound, heme peptide (HP). As the polymer layer shrank at temperatures above 30-40 degrees C, the catalytic activity of the HP molecules for H(2)O(2) reduction improved, and simultaneously, the number of HP molecules that can communicate electrochemically with the electrode increased. As a result, the catalytic current for H(2)O(2) reduction in the shrunken state was 4 times larger than that in the swollen state. This reversible change was exploited for tuning the sensitivity and dynamic range of the HP electrode in H(2)O(2) biosensing. The dynamic range in inhibition-based biosensing of imidazole derivatives was also tunable.

Clotrimazole Inhibits Hemoperoxidase of Plasmodium falciparum and Induces Oxidative Stress

The mechanism of antimalarial activity of clotrimazole was studied placing emphasis on its role in inhibiting hemoperoxidase for inducing oxidative stress in Plasmodium falciparum. Clotrimazole, in the presence of H2O2, causes irreversible inactivation of the enzyme, and the inactivation follows pseudo-first order kinetics, consistent with a mechanism-based (suicide) mode. The pseudo-first order kinetic constants are ki = 2.85 microM, k(inact) = 0.9 min(-1), and t(1/2) = 0.77 min. The one-electron oxidation product of clotrimazole has been identified by EPR spectroscopy as the 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) adduct of the nitrogen-centered radical (aN = 15 G), and as DMPO protects against inactivation, this radical is involved in the inactivation process. Binding studies indicate that the clotrimazole oxidation product interacts at the heme moiety, and the heme-clotrimazole adduct has been dissociated from the inactivated enzyme and identified (m/z 1363) by mass analysis. We found that the inhibition of hemoperoxidase increases the accumulation of H2O2 in P. falciparum and causes oxidative stress. Furthermore, the inhibition of hemoperoxidase correlates well with the inhibition of parasite growth. The results described herein indicate that the antimalarial activity of clotrimazole might be due to the inhibition of hemoperoxidase and subsequent development of oxidative stress in P. falciparum.

Investigation of the relationship between protein-protein interaction and catalytic activity of a heme-regulated phosphodiesterase from Escherichia coli (Ec DOS) by protein microarray

Ec DOS, a heme-regulated phosphodiesterase from Escherichia coli, is composed of an N-terminal heme-bound PAS domain and a C-terminal phosphodiesterase domain. The heme redox state in the PAS domain regulates Ec DOS phosphodiesterase activity. Interestingly, the isolated heme-bound PAS fragment enhances phosphodiesterase activity of full-length Ec DOS. The enhancement is also regulated by the heme redox state of the isolated PAS domain. In the present study, we used a newly developed protein microarray system to examine the relationship between catalytic activity and the interaction of full-length Ec DOS and the isolated PAS fragment. Adenosine 3',5'-cyclic monophosphate (cAMP), a substrate of the Ec DOS phosphodiesterase, was found to be indispensable for the interaction between Ec DOS and the PAS fragment, and two phosphodiesterase inhibitors, 3-isobutyl-methyl-xanthine and etazolate hydrochloride, hindered the interaction. In addition, an enzyme with a mutation in the putative cAMP-binding sites (H590 and H594) was unable to interact with Ec DOS and lacked enzymatic activity. These results strongly suggest a close relationship between Ec DOS phosphodiesterase activity and interaction with the isolated PAS fragment. Therefore, this study provides insights into the mechanism of how the isolated PAS domain activates Ec DOS, which has important implications for the general role of the isolated PAS domain in cells. Moreover, we found that multiple microscale analyses using the protein microarray system had several advantages over conventional affinity column methods, including the quantity of protein needed, the sensitivity, the variability of immobilized protein, and the time required for the experiment.

Imidazole antibiotics inhibit the nitric oxide dioxygenase function of microbial flavohemoglobin

Flavohemoglobins metabolize nitric oxide (NO) to nitrate and protect bacteria and fungi from NO-mediated damage, growth inhibition, and killing by NO-releasing immune cells. Antimicrobial imidazoles were tested for their ability to coordinate flavohemoglobin and inhibit its NO dioxygenase (NOD) function. Miconazole, econazole, clotrimazole, and ketoconazole inhibited the NOD activity of Escherichia coli flavohemoglobin with apparent K(i) values of 80, 550, 1,300, and 5,000 nM, respectively. Saccharomyces cerevisiae, Candida albicans, and Alcaligenes eutrophus enzymes exhibited similar sensitivities to imidazoles. Imidazoles coordinated the heme iron atom, impaired ferric heme reduction, produced uncompetitive inhibition with respect to O(2) and NO, and inhibited NO metabolism by yeasts and bacteria. Nevertheless, these imidazoles were not sufficiently selective to fully mimic the NO-dependent growth stasis seen with NOD-deficient mutants. The results demonstrate a mechanism for NOD inhibition by imidazoles and suggest a target for imidazole engineering.

Synthesis and antimalarial activity of sulfonamide chalcone derivatives

A series of sulfonamide chalcone derivatives were synthesized and investigated for their abilities to inhibit beta-hematin formation in vitro and their activity against cultured Plasmodium falciparum parasites. Inhibition of beta-hematin formation was minimal in the aromatic ring of the chalcone moiety as it appeared for compounds 4b, 4d-f, and greatest with compounds 4g (IC50 0.48 microM) and 4k (IC50 0.50 microM) with a substitution of 3,4,5-trimethoxyl and 3-pyridinyl, respectively. In this study, the most active compound resulted 1[4'-N(2'',5''-dichlorophenyl) sulfonyl-amidephenyl]-3-(4-methylphenyl)-2-propen-1-one 4i, effective as antimalarial by the inhibition of cultured P. falciparum parasites (1 microM). These studies open up the novel possibility of development of sulfonamide derivatives as antimalarials that target beta-hematin formation and the inhibition of the development of cultured P. falciparum parasites, which should help delay the rapid onset of resistance to drugs acting at only a single site. Results with these assays suggest that chalcones exert their antimalarial activity via multiple mechanisms.

Synthesis of Some Cryptolepine Analogues, Assessment of Their Antimalarial and Cytotoxic Activities, and Consideration of Their Antimalarial Mode of Action

A series of analogues of cryptolepine (1) have been synthesized and evaluated for their in vitro antiplasmodial and cytotoxic properties. The IC(50) values of several compounds (11a, 11k-m, 11o, 13) against Plasmodium falciparum (strain K1) were <0.1 muM, 5-10-fold lower than that of 1 but their cytotoxicities were only 2-4 times greater than that of 1. Compounds with a halogen in the quinoline ring and a halogen or a nitro group in the indole ring have enhanced antiplasmodial activity. In mice infected with P. berghei, the 7-bromo-2-chloro (11k) and 2-bromo-7-nitro (13) derivatives of 1 suppressed parasitemia by >90% at doses of 25 mg kg(-1) day(-1) with no apparent toxicity to the mice. 2,7-Dibromocryptolepine (15) was evaluated at several dose levels, and a dose-dependent suppression of parasitemia was seen (ED(90) = 21.6 mg kg(-1) day(-1)). The antimalarial mode of action of 1 appears to be similar to that of chloroquine and involves the inhibition of hemozoin formation. A number of analogues were assessed for their effects on the inhibition of beta-hematin (hemozoin) formation, and the results were compared with their antiplasmodial activities having taken account of their predicted accumulation into the acidic parasite food vacuole. No correlation was seen (r(2) = 0.0781) suggesting that the potent antimalarial activity of compounds such as 15 involves other mechanisms in addition to the inhibition of hemozoin formation.

A colorimetric high-throughput β-hematin inhibition screening assay for use in the search for antimalarial compounds

Antimalarial drugs such as chloroquine are believed to act by inhibiting hemozoin formation in the food vacuole of the malaria parasite. We have developed a new assay for measuring and detecting inhibition of synthetic hemozoin (beta-hematin) formation. Aqueous pyridine (5% v/v, pH 7.5) forms a low-spin complex with hematin but not with beta-hematin. Its absorbance obeys Beer's law, making it useful for quantitating hematin concentration in hematin/beta-hematin mixtures, allowing compounds to be investigated for inhibition of beta-hematin formation. The assay is rapid (60 min incubation) and requires no centrifugation. The beta-hematin inhibition data show good agreement with alternative assay methods reported by four laboratories. The assay was adapted for high-throughput colorimetric screening, allowing visual identification of beta-hematin inhibitors. In this mode, the assay successfully detected all 18 beta-hematin inhibitors in a set of 47 compounds tested, with no false positive results. The quantitative in vitro antimalarial activities of a set of 13 aminoquinolines and quinoline methanols were found to correlate significantly with beta-hematin inhibition values determined using the assay.

Control of heme peptide activity by using phase transition polymers modified with inhibitors

Catalytic activity of a heme peptide (HP) modified-electrode for H(2)O(2) reduction was controlled by use of poly(N-isopropylacrylamide) modified with an inhibitory moiety, imidazole group. The polymers inhibited the catalytic activity below their lower critical solution temperature (LCST) where the polymers were dissolved and did not inhibit the activity above the LCST where the polymers were precipitated. A polymer with a longer side chain connecting with the imidazole group was more inhibitory than a polymer with a shorter side chain at temperatures below the LCST. Formation constants of dissolved HP-imidazole complexes were evaluated by spectroscopic means, and it was found that the polymers were more inhibitory than the corresponding monomers.

In vitro inhibition of β-haematin formation, DNA interactions, antiplasmodial activity, and cytotoxicity of synthetic neocryptolepine derivatives

Neocryptolepine, a minor alkaloid of Cryptolepis sanguinolenta, was investigated as a lead for new antiplasmodial agents, because of its lower cytotoxicity than cryptolepine, the major alkaloid. Synthetic 2- or 3-substituted neocryptolepine derivatives were evaluated for their biological activity. In addition to the antiplasmodial activity (Plasmodium falciparum chloroquine-sensitive and -resistant) also the cytotoxicity (MRC-5 cells) was determined. Several compounds such as 2-bromoneocryptolepine showing higher and more selective antiplasmodial activity than neocryptolepine were obtained. Several functional assays and in vitro tests were used to obtain additional information on the mechanism of action, i.e., the beta-haematin formation inhibitory assay (detoxification of haem) and the DNA-methylgreen displacement assay (interaction with DNA). It could be demonstrated that the 2- or 3-substituted neocryptolepine derivatives investigated here have about the same potency to inhibit the beta-haematin formation as chloroquine, indicating that inhibition of haemozoin formation makes at least an important contribution to their antiplasmodial activity, although their in vitro antiplasmodial activity is still less than chloroquine.

Inhibition of heme aggregation by chloroquine reduces Schistosoma mansoni infection

Adult Schistosoma mansoni digest large amounts of host hemoglobin and release potentially toxic heme inside their guts. We have previously demonstrated that free heme in S. mansoni is detoxified through aggregation, forming hemozoin (Hz). Possible mechanisms of heme aggregation and the effects of chloroquine (CLQ) on formation of Hz and on the viability of this parasite have now been investigated. Different fractions isolated from S. mansoni, such as crude whole-worm homogenates, total lipid extracts, and Hz itself promoted heme aggregation in vitro in a CLQ-sensitive manner. Treatment of S. mansoni-infected mice with CLQ led to remarkable decreases in total protein, Hz content, and viability of the worms, as well as in parasitemia and deposition of eggs in mouse livers. These results indicate that inhibition of formation of Hz in S. mansoni, by CLQ, led to an important decrease in the overall severity of experimental murine schistosomiasis. Taken together, the results presented here suggest that formation of Hz is a major mechanism of heme detoxification and a potential target for chemotherapy in S. mansoni.

Synthesis and antimalarial evaluation of new 1,4-bis(3-aminopropyl)piperazine derivatives

Synthesis and evaluation of the activity of a new family of 1,4-bis(3-aminopropyl)piperazine derivatives against a chloroquine-resistant strain of Plasmodium falciparum, and as inhibitors of beta-hematin formation, are described. The highest antimalarial activities were obtained for compounds displaying the highest predicted vacuolar accumulation ratios and the best potencies as inhibitors of beta-hematin formation. The most potent compound displayed an activity 3-fold better than chloroquine for a comparable selectivity index upon MRC-5 cells. Therefore, in this series, the replacement of the 7-chloroquinoline group can constitute a strong rationale for further investigation.

Inhibition of heme crystal growth by antimalarials and other compounds: implications for drug discovery

During intraerythrocytic infection, Plasmodium falciparum parasites crystallize toxic heme released during hemoglobin catabolism. The proposed mechanism of quinoline inhibition of crystal growth is either by a surface binding or a substrate sequestration mechanism. The kinetics of heme crystal growth was examined in this work using a new high-throughput crystal growth determination assay based on the differential solubility of free vs. crystalline FP in basic solutions. Chloroquine (IC(50)=4.3 microM) and quinidine (IC(50)=1.5 microM) showed a previously not recognized reversible inhibition of FP crystal growth. This inhibition decreased by increasing amounts of heme crystal seed, but not by greater amounts of FP substrate. Crystal growth decreases as pH rises from 4.0 to 6.0, except for a partial local maxima reversal from pH 5.0 to 5.5 that coincides with increased FP solubility. The new crystal growth determination assay enabled a partial screen of existing clinical drugs. Nitrogen heterocycle cytochrome P450 inhibitors also reversibly blocked FP crystal growth, including the azole antifungal drugs clotrimazole (IC(50)=12.9 microM), econazole (IC(50)=19.7 microM), ketoconazole (IC(50)=6.5 microM), and miconazole (IC(50)=21.4 microM). Fluconazole did not inhibit. Both subcellular fractionation of parasites treated with subinhibitory concentrations of ketoconazole and in vitro hemozoin growth assays demonstrated copurification of hemozoin and ketoconazole. The chemical diversity of existing CYP inhibitor libraries that bind FP presents new opportunities for the discovery of antimalarial drugs that block FP crystal growth by a surface binding mechanism and possibly interfere with other FP-sensitive Plasmodium pathways.

The relationship of physico-chemical properties and structure to the differential antiplasmodial activity of the cinchona alkaloids

Background

The 8-amino and 9-hydroxy substituents of antimalarial cinchona alkaloids have the erythro orientation while their inactive 9-epimers are threo. From the X-ray structures a 90° difference in torsion angle between the N1-H1 and C9-O12 bonds in the two series is believed to be important. In order to kill the malaria parasite, alkaloids must cross the erythrocyte and parasite membranes to accumulate in the acid digestive vacuole where they prevent detoxication of haematin produced during haemoglobin breakdown.

Methods

Ionization constants, octanol/water distribution and haematin interaction are examined for eight alkaloids to explain the influence of small structural differences on activity.

Results

Erythro isomers have a high distribution ratio of 55:1 from plasma to the erythrocyte membrane, while for the more basic threo epimers this is only 4.5:1. This gives an increased transfer rate of the erythro drugs into the erythrocyte and thence into the parasite vacuole where their favourable conformation allows interaction with haematin, inhibiting its dimerization strongly (90 ± 7%) and thereby killing the parasite. The threo compounds not only enter more slowly but are then severely restricted from binding to haematin by the gauche alignment of their N1-H1 and C9-O12 bonds. Confirmatory molecular models allowed measurement of angles and bond lengths and computation of the electronic spectrum of a quinine-haematin complex.

Conclusion

Differences in the antiplasmodial activity of the erythro and threo cinchona alkaloids may therefore be attributed to the cumulative effects of lipid/aqueous distribution ratio and drug-haematin interaction. Possible insights into the mechanism of chloroquine-resistance are discussed.

Metalloporphyrin probes for antimalarial drug action

Metal-substituted protoporphyrin IXs (Co(III)PPIX (1), Cr(III)PPIX (2), Mn(III)PPIX (3), Cu(II)PPIX (4), Mg(II)PPIX (5), Zn(II)PPIX (6) and Sn(IV)PPIX (7)), phthalocyanine tetrasulfonates (PcS (8) and Ni(II)PcS (9)), and anionic and cationic porphyrins (meso-tetra(4-sulfonatophenyl)porphine (TPPS4, 10), meso-tetra(4-carboxyphenyl)porphine (TPPC4, 11), tetrakis(4-N-trimethylaminophenyl)porphine (TMAP, 12) and meso-tetra(N-methyl-4-pyridyl)porphine (TMPyP4, 13)) have been used as probes to compare two different assays for the inhibition of beta-hematin formation. The results demonstrate that the efficacy of these probes in either the beta-hematin inhibition assay (9, 7, 6, 5>4>11, 3>10, 8>2, 1; 12 and 13 did not inhibit.) or the bionucleating template assay (8>1>11>9, 2>4>3>7>10>5>6; 12 and 13 did not inhibit.) differ significantly. These differences are examined in light of possible interactions between the inhibitor probes, heme, beta-hematin and the bionucleating template. This detailed analysis highlights the fact that while dominant modes of interactions may be occasionally identified, the precise mechanism of inhibition undoubtedly consists of the interplay between multiple interactions.

Experimental conditions for testing the inhibitory activity of chloroquine on the formation of beta-hematin

Some antimalarial drugs act by inhibiting the process of ferriprotoporphyrin IX polymerization which protects the parasite against the noxious effect of this product of host cell hemoglobin digestion. As the quest for new drugs with a similar mode of action continues, high-throughput screening methods are needed. We demonstrate herein that such a recently described screening technique (Basilico et al., J. Antimicrob. Chemother. 42, 55-60, 1998) is considerably disturbed by certain ions. Thus, at the assay's pH 2.6, the phosphate ions are responsible for the inhibitory activity of chloroquine phosphate, rather than chloroquine itself. Using a combination of solubility tests and Fourier transform infrared spectrometry we also show that two alternative methods using higher pH's are also prone to salt effects and demonstrate that these can be minimized by extensive washing of the product with DMSO.

Standardization of the physicochemical parameters to assess in vitro the beta-hematin inhibitory activity of antimalarial drugs

Intraerythrocytic plasmodia form hemozoin as a detoxification product of hemoglobin-derived heme. An identical substance, beta-hematin (BH), can be obtained in vitro from hematin at acidic pH. Quinoline-antimalarials inhibit BH formation. Standardization of test conditions is essential for studying the interaction of compounds with this process and screening potential inhibitors. A spectrophotometric microassay of heme polymerization inhibitory activity (HPIA) (Basilico et al., Journal of Antimicrobial Chemotherapy 42, 55-60, 1998) previously reported was used to investigate the effect of pH and salt concentration on BH formation. The yield of BH formation decreased with pH. Moreover, under conditions used in the above HPIA assay (18 h, 37 degrees C, pH = 2.7), several salts including chloride and phosphate inhibited the process. Aminoquinoline drugs formulated as salts (chloroquine-phosphate, primaquine-diphosphate), but not chloroquine-base, also inhibited the reaction. Interference by salts was highest at low pH and decreased at higher pH (pH 4). Here, we describe different assay conditions that eliminate these problems (BHIA, beta-hematin inhibitory activity). By replacing hematin with hemin as the porphyrin and NaOH solution with DMSO as solvent, the formation of BH was independent of pH up to pH 5.1. No interference by salts was observed over the pH range 2.7-5.1. Dose-dependent inhibition of BH formation was obtained with chloroquine-base, chloroquine-phosphate, and chloroquine-sulfate at pH 5.1. Primaquine was not inhibitory. The final product, characterized by solubility in DMSO, consists of pure BH by FT-IR spectroscopy. The BHIA assay (hemin in DMSO, acetate buffer pH 5 +/- 0.1, 18 h at 37 degrees C) is designed to screen for those molecules forming pi-pi interactions with hematin and thus inhibiting beta-hematin formation.

Nitric-oxide dioxygenase activity and function of flavohemoglobins. sensitivity to nitric oxide and carbon monoxide inhibition

Widely distributed flavohemoglobins (flavoHbs) function as NO dioxygenases and confer upon cells a resistance to NO toxicity. FlavoHbs from Saccharomyces cerevisiae, Alcaligenes eutrophus, and Escherichia coli share similar spectra, O(2), NO, and CO binding kinetics, and steady-state NO dioxygenation kinetics. Turnover numbers (V(max)) for S. cerevisiae, A. eutrophus, and E. coli flavoHbs are 112, 290, and 365 NO heme(-1) s(-1), respectively, at 37 degrees C with 200 microm O(2). The K(M) values for NO are low and range from 0.1 to 0.25 microm. V(max)/K(M)(NO) ratios of 900-2900 microm(-1) s(-1) indicate an extremely efficient dioxygenation mechanism. Approximate K(M) values for O(2) range from 60 to 90 microm. NO inhibits the dioxygenases at NO:O(2) ratios of > or =1:100 and makes true K(M)(O(2)) values difficult to determine. High and roughly equal second order rate constants for O(2) and NO association with the reduced flavoHbs (17-50 microm(-1) s(-1)) and small NO dissociation rate constants suggest that NO inhibits the dioxygenase reaction by forming inactive flavoHbNO complexes. Carbon monoxide also binds reduced flavoHbs with high affinity and competitively inhibits NO dioxygenases with respect to O(2) (K(I)(CO) = approximately 1 microm). These results suggest that flavoHbs and related hemoglobins evolved as NO detoxifying components of nitrogen metabolism capable of discriminating O(2) from inhibitory NO and CO.

A new class of potential chloroquine-resistance reversal agents for Plasmodia: syntheses and biological evaluation of 1-(3'-diethylaminopropyl)-3-(substituted phenylmethylene)pyrrolidines

1-(3'-Diethylaminopropyl)-3-(substituted phenylmethylene)pyrrolidines were synthesized and evaluated for CQ-resistant reversal activity. In general the compounds of the series elicit better biological response than their phenylmethyl analogues. The most active compound 4b has been evaluated in vivo in detail, and the results are presented. The possible mode of action of the compounds of this series is by inhibition of the enzyme heme oxygenase, thereby increasing the levels of heme and hemozoin, which are lethal to the parasite.

Metalloporphyrins inhibit beta-hematin (hemozoin) formation

Metal-substituted protoporphyrin IXs (Cr(III)PPIX (1), Co(III)PPIX (2), Mn(III)PPIX (3), Cu(II)PPIX (4), Mg(II)PPIX (5), Zn(II)PPIX (6), and Sn(IV)PPIX (7)) act as inhibitors to beta-hematin (hemozoin) formation, a critical detoxification biopolymer of malarial parasites. The central metal ion plays a significant role in the efficacy of the metalloprotoporphyrins to inhibit beta-hematin formation. The efficacy of these compounds correlates well with the water exchange rate for the octahedral aqua complexes of the porphyrin's central metal ion. Under these in vitro reaction conditions, metalloporphyrins 5, 6 and 7 are as much as six times more efficacious than the free ligand protoporphyrin IX in preventing beta-hematin formation and four times as efficacious as chloroquine, while metalloporphyrins 3 and 4 are three to four times more effective at preventing beta-hematin formation than the free protoporphyrin IX base. In contrast, the relatively exchange inert metalloporphyrins 1 and 2 are only as efficacious as the free ligand and only two-thirds as effective as chloroquine. Aggregation studies of the heme:MPPIX using UV-Vis and fluorescence spectroscopies are indicative of the formation of pi-pi hetero-metalloporphyrin assemblies. Thus, hemozoin inhibition is likely prevented by the formation of heme:MPPIX complexes through pi-stacking interactions. The ramifications of such hetero-metalloporphyrin assemblies, in the context of the emerging structural picture of hemozoin, are discussed.

A flavoprotein mechanism appears to prevent an oxygen-dependent inhibition of cGMP-associated nitric oxide-elicited relaxation of bovine coronary arteries

The redox state of the heme of soluble guanylate cyclase (sGC) may regulate the sensitivity of vascular tissue to nitric oxide (NO). In this study, diphenyliodonium (DPI) is used as an inhibitor of flavoprotein oxidoreductases to examine their potential role in the expression of NO-elicited cGMP-associated arterial relaxation and sGC stimulation. The relaxation of endothelium-removed bovine coronary arteries (BCAs) precontracted with 30 mmol/L KCl to the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) or to NO is markedly suppressed by 10 micromol/L DPI under an atmosphere of 21% O(2) (5% CO(2)). In contrast, DPI has minimal effects on the relaxation to SNAP under 95% N(2) (5% CO(2)). If BCAs are treated with DPI under 21% O(2) and then exposed to the hemoprotein reductant sodium dithionite (1 mmol/L) under N(2), there is a partial reversal of the inhibitory effects of DPI compared with BCAs that were not treated with dithionite. DPI did not inhibit relaxation elicited by 8-bromo-cGMP or forskolin. Increases in tissue cGMP levels stimulated by SNAP were eliminated by pretreatment of BCAs with DPI under 21% O(2) but not under N(2). Activation of sGC by SNAP in BCA homogenate was also eliminated when vessels were pretreated with 10 micromol/L DPI under 21% O(2), but DPI did not have an inhibitory effect when directly added to the assay of sGC activity. These observations are consistent with a flavoprotein-dependent oxidoreductase functioning to prevent the expression of a novel O(2)-dependent process from oxidizing the heme on sGC and inhibiting NO-elicited cGMP-mediated BCA relaxation.

Inhibition of the FixL sensor kinase by the FixT protein in Sinorhizobium meliloti

Nitrogen fixation in symbiotic rhizobia is subject to multiple levels of gene regulation. In Sinorhizobium meliloti, the alfalfa symbiont, the FixLJ two-component regulatory system plays a major role in inducing nitrogen fixation and respiration gene expression in response to the low ambient O(2) concentration of the nodule. Here we report on the mode of action of the FixT protein, a recently identified repressor of nitrogen fixation gene expression in S. meliloti. First, we provide evidence that FixT prevents transcription of the intermediate key regulatory genes nifA and fixK by counteracting the activity of the FixLJ two-component system under otherwise inducing microoxic conditions. Second, we demonstrate that FixT acts as an inhibitor of the sensor hemoprotein kinase FixL, preventing the production or the accumulation of its phosphorylated form. FixT is thus a new example of a regulatory protein that blocks signal transduction in two-component systems at the level of the sensor kinase.

The soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a] quinoxalin-1-one is a nonselective heme protein inhibitor of nitric oxide synthase and other cytochrome P-450 enzymes involved in nitric oxide donor bioactivation

Soluble guanylyl cyclase (sGC) is an important effector for nitric oxide (NO). It acts by increasing intracellular cyclic GMP (cGMP) levels to mediate numerous biological functions. Recently, 1H-[1,2, 4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) was identified as a novel and selective inhibitor of this enzyme. Therefore, ODQ may represent an important pharmacological tool for differentiating cGMP-mediated from cGMP-independent effects of NO. In the present study, we examined the inhibitory action of ODQ both functionally and biochemically. In phenylephrine-preconstricted, endothelium-intact, isolated aortic rings from the rat, ODQ, in a concentration-dependent manner, increased contractile tone and inhibited relaxations to authentic NO with maximal effects at 3 microM. Pretreatment of vascular rings with ODQ induced a parallel, 2-log-order shift to the right of the concentration-response curves (CRCs) to histamine, ATP, NO, the NO-donors S-nitrosoglutathione, S-nitroso-N-acetyl-D,L-penicillamine, and spermine NONOate [N-[4-[1-(3-amino propyl)-2-hydroxy-2-nitroso hydrazino]butyl]-1, 3-propane diamine], and the direct sGC-stimulant [3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole] YC-1 but did not affect relaxations induced by papaverine and atriopeptin II. Moreover, the rightward shift of the CRCs to Angeli's salt, peroxynitrite, and linsidomine was similar to that of NO. These results suggested that ODQ is specific for sGC. Furthermore, they indicate that NO can cause vasorelaxation independent of cGMP. Three interesting exceptions were observed to the otherwise rather uniform inhibitory effect of ODQ: the responses to acetylcholine, glycerol trinitrate, and sodium nitroprusside. The latter two agents are known to require metabolic activation, possibly by cytochrome P-450-type proteins. The 3- to 5-log-order rightward shift of their CRCs suggests that, in addition to sGC, ODQ may interfere with heme proteins involved in the bioactivation of these NO donors and the mechanism of vasorelaxation mediated by acetylcholine. In support of this notion, ODQ inhibited hepatic microsomal NO production from both glycerol trinitrate and sodium nitroprusside as well as NO synthase activity in aortic homogenates. The latter effect seemed to require biotransformation of ODQ. Collectively, these data reveal that ODQ interferes with various heme protein-dependent processes in vascular and hepatic tissue and lacks specificity for sGC.

Non-iron porphyrins inhibit beta-haematin (malaria pigment) polymerisation

Infrared spectroscopy was used to evaluate the effect of non-iron porphyrins (protoporphyrin IX and haematoporphyrin) on haematin polymerisation to beta-haematin at acidic pH. Both molecules effectively inhibited the reaction, with haematoporphyrin 6 times as active as protoporphyrin IX. We postulated that the interaction between the pi electron system of porphyrin rings leads to the formation of pi-pi adducts, which inhibit polymer elongation in the same way as antimalarial drugs (e.g., chloroquine); the presence of hydroxyl groups able to bind haem iron enhances activity.

Inhibition of hemozoin formation in Plasmodium falciparum trophozoite extracts by heme analogs: possible implication in the resistance to malaria conferred by the beta-thalassemia trait

BACKGROUND

Human falciparum malaria, caused by the intracellular protozoa Plasmodium falciparum, results in 1-2 million deaths per year. P. falciparum digests host erythrocyte hemoglobin within its food vacuole, resulting in the release of potentially toxic free heme. A parasite-specific heme polymerization activity detoxifies the free heme by cross-linking the heme monomers to form hemozoin or malaria pigment. This biochemical process is the target of the widely successful antimalarial drug chloroquine, which is rapidly losing its effectiveness due to the spread of chloroquine resistance. We have shown that chloroquine resistance is not due to changes in the overall catalytic activity of heme polymerization or its chloroquine sensitivity. Therefore, the heme polymerization activity remains a potential target for novel antimalarials. In this study, we investigated the ability of heme analogs to inhibit heme polymerization and parasite growth in erythrocytes.

MATERIALS AND METHODS

Incorporation of radioactive hemin substrate into an insoluble hemozoin pellet was used to determine heme polymerization. Incorporation of radioactive hypoxanthine into the nucleic acid of dividing parasites was used to determine the effects of heme analogs on parasite growth. Microscopic and biochemical measurements were made to determine the extent of heme analog entry into infected erythrocytes.

RESULTS

The heme analogs tin protoporphyrin IX (SnPP), zinc protoporphyrin IX (ZnPP), and zinc deuteroporphyrin IX, 2,4 bisglycol (ZnBG) inhibited polymerization at micromolar concentrations (ZnPP << SnPP < ZnBG). However, they did not inhibit parasite growth since they failed to gain access to the site of polymerization, the parasite's food vacuole. Finally, we observed high ZnPP levels in erythrocytes from two patients with beta-thalassemia trait, which may inhibit heme polymerization.

CONCLUSIONS

The heme analogs tested were able to inhibit hemozoin formation in Plasmodium falciparum trophozite extracts. The increased ZnPP levels found in thalassemic erythrocytes suggest that these may contribute, at least in part, to the observed antimalarial protection conferred by the beta-thalassemia trait. This finding may lead to the development of new forms of antimalarial therapy.

A salivary nitrophorin (nitric-oxide-carrying hemoprotein) in the bedbug Cimex lectularius

Salivary gland homogenate of the bedbug Cimex lectularius caused vasodilation of the preconstricted rabbit aortic ring in the absence of endothelium. Vasodilation was augmented in the presence of superoxide dismutase and inhibited in the presence of Methylene Blue. Utilization of the Griess reaction indicated the presence of reactive nitrogen equivalents of the order of 337 +/- 57 pg equivalent NO2- per pair of salivary glands (mean +/- S.E.M.; N = 3). Salivary gland homogenates have a nitrosyl-hemoprotein that releases nitric oxide in a pH-dependent manner. The fraction containing the NO-carrying hemoprotein, when separated by HPLC, caused vasodilation of the preconstricted rabbit aortic strip. Furthermore, the presence of a nitrosyl-hemoprotein in Cimex lectularius salivary gland was verified by electron paramagnetic resonance spectroscopy. It is proposed that, as in the case of Rhodnius prolixus (Triatominae), Cimex lectularius salivary glands contain a hemoprotein (nitrophorin) that carries NO from the glands to the host tissues. However, because Cimex lectularius and Rhodnius prolixus belong to different hemipteran families (Cimicidae and Reduvidae) and evolved independently to blood feeding, Cimex lectularius and Rhodnius prolixus nitrophorin may be a case of convergent evolution.

Photodynamic and non-photodynamic action of several porphyrins on the activity of some heme-enzymes

The action of porphyrins, uroporphyrin I and III (URO I and URO III), pentacarboxylic porphyrin I (PENTA I), coproporphyrin I and III (COPRO I and COPRO III), protoporphyrin IX (PROTO IX) and mesoporphyrin (MESO), on the activity of human erythrocytes delta-aminolevulinic acid dehydratase, porphobilinogenase, deaminase and uroporphyrinogen decarboxylase in the dark and under UV light was investigated. Both photoinactivation and light-independent inactivation was found in all four enzymes using URO I as sensitizer. URO III had a similar action as URO I on porphobilinogenase and deaminase and PROTO IX exerted equal effect as URO I on delta-aminolevulinic acid dehydratase and uroporphyrinogen decarboxylase. Photodynamic efficiency of the porphyrins was dependent on their molecular structure. Selective photodecomposition of enzymes by URO I, greater specificity of tumor uptake by URO I and enhanced porphyrin synthesis by tumors from delta-aminolevulic acid, with predominant formation of URO I, underline the possibility of using URO I in detection of malignant cells and photodynamic therapy.