Characterization of chloroquine-hematin mu-oxo dimer binding by isothermal titration calorimetry

Spectrally Resolved and Highly Parallelized Raman Difference Spectroscopy for the Analysis of Drug-Target Interactions between the Antimalarial Drug Chloroquine and Hematin

Malaria is a severe disease caused by cytozoic parasites of the genus Plasmodium, which infiltrate and infect red blood cells. Several drugs have been developed to combat the devastating effects of malaria. Antimalarials based on quinolines inhibit the crystallization of hematin into hemozoin within the parasite, ultimately leading to its demise. Despite the frequent use of these agents, there are unanswered questions about their mechanisms of action. In the present study, the quinoline chloroquine and its interaction with the target structure hematin was investigated using an advanced, highly parallelized Raman difference spectroscopy (RDS) setup. Simultaneous recording of the spectra of hematin and chloroquine mixtures with varying compositions enabled the observation of changes in peak heights and positions based on the altered molecular structure resulting from their interaction. A shift of (-1.12 ± 0.05) cm-1 was observed in the core-size marker band ν(CαCm)asym peak position of the 1:1 chloroquine-hematin mixture compared to pure hematin. The oxidation-state marker band ν(pyrrole half-ring)sym exhibited a shift by (+0.93 ± 0.13) cm-1. These results were supported by density functional theory (DFT) calculations, indicating a hydrogen bond between the quinolinyl moiety of chloroquine and the oxygen atom of ferric protoporphyrin IX hydroxide (Fe(III)PPIX-OH). The consequence is a reduced electron density within the porphyrin moiety and an increase in its core size. This hypothesis provided further insights into the mechanism of hemozoin inhibition, suggesting chloroquine binding to the monomeric form of hematin, thereby preventing its further crystallization to hemozoin.

Potent Antimalarial Activity of Two Arenes Linked with Triamine Designed To Have Multiple Interactions with Heme

Based on the idea that compounds designed to exhibit high affinity for heme would block hemozoin formation, a critical heme-detoxification process for malarial parasites, we synthesized a series of compounds with two π-conjugated moieties at terminal amino groups of triamine. These compounds exhibited moderate to high antimalarial activities in vitro toward both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. In a P. berghei-infected mouse model, 3a and 12a showed potent antimalarial activities compared to artesunate, as well as a prolonged duration of antimalarial effect. We found a good correlation between protective activity against hemin degradation and antimalarial activity. Compounds 8b and 3a strongly inhibited hemozoin formation catalyzed by heme detoxification protein.

Deconstructing Quinoline-Class Antimalarials to Identify Fundamental Physicochemical Properties of Beta-Hematin Crystal Growth Inhibitors

A versatile approach to control crystallization involves the use of modifiers, which are additives that interact with crystal surfaces and alter their growth rates. Elucidating a modifier's binding specificity to anisotropic crystal surfaces is a ubiquitous challenge that is critical to their design. In this study, we select hematin, a byproduct of malaria parasites, as a model system to examine the complementarity of modifiers (i.e., antimalarial drugs) to β-hematin crystal surfaces. We divide two antimalarials, chloroquine and amodiaquine, into segments consisting of a quinoline base, common to both drugs, and side chains that differentiate their modes of action. Using a combination of scanning probe microscopy, bulk crystallization, and analytical techniques, we show that the base and side chain work synergistically to reduce the rate of hematin crystallization. In contrast to general observations that modifiers retain their function upon segmentation, we show that the constituents do not act as modifiers. A systematic study of quinoline isomers and analogues shows how subtle rearrangement and removal of functional moieties can create effective constituents from previously ineffective modifiers, along with tuning their inhibitory modes of action. These findings highlight the importance of specific functional moieties in drug compounds, leading to an improved understanding of modifier-crystal interactions that could prove to be applicable to the design of new antimalarials.

Antiplasmodial activity of new 4-aminoquinoline derivatives against chloroquine resistant strain

Emergence and spread of multidrug resistant strains of Plasmodium falciparum has severely limited the antimalarial chemotherapeutic options. In order to overcome the obstacle, a set of new side-chain modified 4-aminoquinolines were synthesized and screened against chloroquine-sensitive (3D7) and chloroquine-resistant (K1) strains of P. falciparum. The key feature of the designed molecules is the use of methylpiperazine linked α, β(3)- and γ-amino acids to generate novel side chain modified 4-aminoquinoline analogues. Among the evaluated compounds, 20c and 30 were found more potent than CQ against K1 and displayed a four-fold and a three-fold higher activity respectively, with a good selectivity index (SI=5846 and 11,350). All synthesized compounds had resistance index between 1.06 and >14.13 as against 47.2 for chloroquine. Biophysical studies suggested that this series of compounds act on heme polymerization target.

Multiple spectroscopic and magnetic techniques show that chloroquine induces formation of the μ-oxo dimer of ferriprotoporphyrin IX

Interaction of the antimalarial chloroquine (CQ) with ferriprotoporphyrin IX, Fe(III)PPIX, was investigated in aqueous solution (pH7.4) and as a precipitate from aqueous medium at pH5.0. In solution, spectrophotometric titrations indicated strong association (logKobs 13.3±0.2) and a Job plot gave a stoichiometry of 1:2 CQ:Fe(III)PPIX. UV-visible absorbance and magnetic circular dichroism spectra of the complex were compared to various Fe(III)PPIX species. Close similarity to the spectra of the μ-oxo dimer, μ-[Fe(III)PPIX]2O, was revealed. The induction of this species by CQ was confirmed by magnetic susceptibility measurements using the Evans NMR method. The observed low-magnetic moment (2.25±0.02 μB) could only be attributed to antiferromagnetically coupled Fe(III) centers. The value was comparable to that of μ-[Fe(III)PPIX]2O (2.0±0.1 μB). In the solid-state, mass spectrometry confirmed the presence of CQ in the complex. Dissolution of this solid in aqueous solution (pH7.4) resulted in a solution with a UV-visible spectrum consistent with the same 1:2 stoichiometry observed in the Job plot. Magnetic susceptibility measurements made on the solid using an Evans balance produced a magnetic moment (2.3±0.1 μB) consistent with that in solution. Diffusion coefficients of CQ and its complex with Fe(III)PPIX were measured in aqueous solution (3.3±0.3 and 0.6±0.2×10(-10) m(2)·s(-1), respectively). The latter was used in conjunction with an empirical relationship between diffusion coefficient and molar volume to estimate the degree of aggregation. The findings suggest the formation of a 2:4 CQ:Fe(III)PPIX complex in aqueous solution at pH7.4.

Quinoline drug-heme interactions and implications for antimalarial cytostatic versus cytocidal activities

Historically, the most successful molecular target for antimalarial drugs has been heme biomineralization within the malarial parasite digestive vacuole. Heme released from catabolized host red blood cell hemoglobin is toxic, so malarial parasites crystallize heme to nontoxic hemozoin. For years it has been accepted that a number of effective quinoline antimalarial drugs (e.g., chloroquine, quinine, amodiaquine) function by preventing hemozoin crystallization. However, recent studies over the past decade have revealed a surprising molecular diversity in quinoline-heme molecular interactions. This diversity shows that even closely related quinoline drugs may have quite different molecular pharmacology. This paper reviews the molecular diversity and highlights important implications for understanding quinoline antimalarial drug resistance and for future drug design.

Design and synthesis of 3-[(7-chloro-1-oxidoquinolin-4-ylamino)alkyl]-1,3-thiazolidin-4-ones as antimalarial agents

A new series of quinoline analogs have been synthesized and found active against P. falciparum in vitro and P. yoelli in vivo. Compounds 8, 10 and 11 exhibited superior in vitro activity compared to chloroquine. Selected compounds 8, 10 and 11 exhibited significant suppression of parasitaemia in vivo assay. These analogs form a complex with hematin and inhibit the β-hematin formation, suggesting that this class of compounds act on a heme polymerization target. Further this study confirms that quinoline ring nitrogen is essential for both transportation of the molecule across the membrane as well as for tight binding to hematin.

Incorporation of basic side chains into cryptolepine scaffold: structure-antimalarial activity relationships and mechanistic studies

The synthesis of cryptolepine derivatives containing basic side-chains at the C-11 position and their evaluations for antiplasmodial and cytotoxicity properties are reported. Propyl, butyl, and cycloalkyl diamine side chains significantly increased activity against chloroquine-resistant Plasmodium falciparum strains while reducing cytotoxicity when compared with the parent compound. Localization studies inside parasite blood stages by fluorescence microscopy showed that these derivatives accumulate inside the nucleus, indicating that the incorporation of a basic side chain is not sufficient enough to promote selective accumulation in the acidic digestive vacuole of the parasite. Most of the compounds within this series showed the ability to bind to a double-stranded DNA duplex as well to monomeric hematin, suggesting that these are possible targets associated with the observed antimalarial activity. Overall, these novel cryptolepine analogues with substantially improved antiplasmodial activity and selectivity index provide a promising starting point for development of potent and highly selective agents against drug-resistant malaria parasites.

4-Aminoquinoline derived antimalarials: synthesis, antiplasmodial activity and heme polymerization inhibition studies

A new series of 4-aminoquinoline derivatives have been synthesized and found to be active against both susceptible and resistant strains of Plasmodium falciparum in vitro. Compound 1-[3-(7-chloro-quinolin-4-ylamino)-propyl]-3-cyclopropyl-thiourea (7) exhibited superior in vitro activity against resistant strains of P. falciparum as compared to chloroquine (CQ). All the compounds showed resistance factor between 0.59 and 4.31 as against 5.05 for CQ. Spectroscopic studies suggested that this class of compounds act on heme polymerization target.

Synthesis and antimalarial properties of new chloro-9H-xanthones with an aminoalkyl side chain

The synthesis and antimalarial properties of twelve new chlorinated 9H-xanthones, carrying a [2-(diethylamino)ethyl]amino group in position 1, are reported. All compounds were found to be active towards the chloroquine-susceptible and chloroquine-resistant strains 3D7 and Dd2, resp., of the protozoa parasite Plasmodium falciparum. Especially one compound, 6-chloro-1-{[2-(diethylamino)ethyl]amino}-9H-xanthen-9-one (1k), was found to exhibit significant in vitro activity (IC50 = 3.9 microM) towards the resistant Dd2 strain.

Interactions of quinoline antimalarials with hematin in solution

Quinoline antimalarial drugs such as chloroquine and related compounds are believed to act by targeting ferriprotoporphyrin IX (Fe(III)PPIX) in the form of hematin (H(2)O/HO-Fe(III)PPIX), its mu-oxo dimer ([Fe(III)PPIX](2)O) or crystalline beta-hematin ([Fe(III)PPIX](2)) in the malaria parasite. Fe(III)PPIX is formed when the parasite digests host hemoglobin during its intraerythrocytic blood stage. This has led to a number of studies on the interaction of Fe(III)PPIX with quinoline antimalarials and related compounds. This article reviews the spectroscopy, thermodynamics and structures of Fe(III)PPIX-quinoline complexes in solution.

Quinolines and artemisinin: chemistry, biology and history

Plasmodium falciparum is the most important parasitic pathogen in humans, causing hundreds of millions of malaria infections and millions of deaths each year. At present there is no effective malaria vaccine and malaria therapy is totally reliant on the use of drugs. New drugs are urgently needed because of the rapid evolution and spread of parasite resistance to the current therapies. Drug resistance is one of the major factors contributing to the resurgence of malaria, especially resistance to the most affordable drugs such as chloroquine. We need to fully understand the antimalarial mode of action of the existing drugs and the way that the parasite becomes resistant to them in order to design and develop the new therapies that are so urgently needed. In respect of the quinolines and artemisinins, great progress has been made recently in studying the mechanisms of drug action and drug resistance in malaria parasites. Here we summarize from a historical, biological and chemical, perspective the exciting new advances that have been made in the study of these important antimalarial drugs.

Definition of an electronic profile of compounds with inhibitory activity against hematin aggregation in malaria parasite

Malaria is one of the most important parasitic diseases, affecting almost half of the world and posing a threat to the other half. Xanthone derivatives can behave as antimalarial drugs in the same mechanistic way as chloroquine and other related quinolines. This action is due to the inhibition of the detoxification pathway of the parasite, responsible for the production of hemozoin. We report a study of the electronic properties of the xanthonic and quinolinic compounds based on DFT calculations, in order to determine a pattern that could be applied to the development of new potentially active antimalarial molecules. As a result, a new interpretation of structure-activity relationship of the quinoline antimalarial drugs, and of the active hydroxylated xanthones is proposed here. We conclude that electronic features rather than steric factors control primarily the inhibitory activity of the studied compounds against hematin aggregation, concurring to a potential antimalarial activity.

Computational studies of new potential antimalarial compounds--stereoelectronic complementarity with the receptor

One of the most important pharmacological mechanisms of antimalarial action is the inhibition of the aggregation of hematin into hemozoin. We present a group of new potential antimalarial molecules for which we have performed a DFT study of their stereoelectronic properties. Additionally, the same calculations were carried out for the two putative drug receptors involved in the referred activity, i.e., hematin mu-oxo dimer and hemozoin. A complementarity between the structural and electronic profiles of the planned molecules and the receptors can be observed. A docking study of the new compounds in relation to the two putative receptors is also presented, providing a correlation with the defined electrostatic complementarity.

Effects of solvent composition and ionic strength on the interaction of quinoline antimalarials with ferriprotoporphyrin IX

Enthalpy-entropy compensation in the interaction of quinoline antimalarials with ferriprotoporphyrin IX (Fe(III)PPIX) in 40% aqueous dimethyl sulfoxide (DMSO) has been compared with that in pure aqueous solution. The data indicate that the degree of desolvation and loss of conformational freedom is virtually identical in both systems. Taken together with previous findings showing that the molar free energies of association of these drugs with Fe(III)PPIX in both solvent systems are very similar, this suggests that the recognition site on the metalloporphyrin is comparable in both cases. This is despite the fact that Fe(III)PPIX exists as a dimer in aqueous solution, but is monomeric in 40% DMSO. Free energies of association of chloroquine, quinine and quinidine with Fe(III)PPIX are largely insensitive to the concentration of sodium perchlorate in 40% DMSO. This demonstrates that electrostatic interactions play only a minor role in the overall stability of these complexes under these conditions. Increasing DMSO concentration greatly weakens the interactions of chloroquine, amodiaquine, quinine, quinidine and 9-epiquinine with Fe(III)PPIX. This suggests that hydrophobic interaction plays a major role in the stability of these complexes. Further investigation of chloroquine has revealed that the free energy of association with Fe(III)PPIX also weakens as a function of decreasing solvent polarity in pure organic solvents. However, the free energies of association are weaker in the mixed aqueous solvent than in pure organic solvents. This indicates that dispersion and electrostatic interactions are relatively strong in the non-aqueous environment. The results demonstrate that any successful model of antimalarial drug-Fe(III)PPIX interactions will need to take both solvation and electrostatic factors into account.

Novel phenothiazine antimalarials: synthesis, antimalarial activity, and inhibition of the formation of beta-haematin

We report the synthesis of a series of novel phenothiazine compounds that inhibit the growth of both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. We found that the antimalarial activity of these phenothiazines increased with an increase in the number of basic groups in the alkylamino side chain, which may reflect increased uptake into the parasite food vacuole or differences in the toxicities of individual FP-drug complexes. We have examined the ability of the parent phenothiazine, chlorpromazine, and some novel phenothiazines to inhibit the formation of beta-haematin. The degree of antimalarial potency was loosely correlated with the efficacy of inhibition of beta-haematin formation, suggesting that these phenothiazines exert their antimalarial activities in a manner similar to that of chloroquine, i.e. by antagonizing the sequestration of toxic haem (ferriprotoporphyrin IX) moieties within the malaria parasite. Chlorpromazine is an effective modulator of chloroquine resistance; however, the more potent phenothiazine derivatives were more active against chloroquine-sensitive parasites than against chloroquine-resistant parasites and showed little synergy of action when used in combination with chloroquine. These studies point to structural features that may determine the antimalarial activity and resistance modulating potential of weakly basic amphipaths.

Optimization of xanthones for antimalarial activity: the 3,6-bis-omega-diethylaminoalkoxyxanthone series

Hydroxyxanthones have been identified as novel antimalarial agents. The compounds are believed to exert their activity by complexation to heme and inhibition of hemozoin formation. Modification of the xanthone structure was pursued to improve their antimalarial activity. Attachment of R-groups bearing protonatable nitrogen atoms was conducted to enhance heme affinity through ionic interactions with the propionate side chains of the metalloporphyrin and to facilitate drug accumulation in the parasite food vacuole. A series of 3,6-bis-omega-diethylaminoalkoxyxanthones with side chains ranging from 2 to 8 carbon atoms were prepared and evaluated. Measurement of heme affinity for each of the derivatives revealed a strong correlation (R(2) = 0.97) between affinity and antimalarial potency. The two most active compounds in the series contained 5- and 6-carbon side chains and exhibited low nanomolar 50% inhibitory concentration (IC(50)) values against strains of chloroquine-susceptible and multidrug-resistant Plasmodium falciparum in vitro. Both of these xanthones exhibit stronger heme affinity (8.26 x 10(5) and 9.02 x 10(5) M(-1), respectively) than either chloroquine or quinine under similar conditions and appear to complex heme in a unique manner.

Prooxidant activity of beta-hematin (synthetic malaria pigment) in arachidonic acid micelles and phospholipid large unilamellar vesicles

Intraerythrocytic malaria parasite has evolved a unique pathway to detoxify hemoglobin-derived heme by forming a crystal of Ferri-protoporphyrin IX dimers, known as hemozoin or "malaria pigment." The prooxidant activity of beta-hematin (BH), the synthetic malaria pigment obtained from hematin at acidic pH, was studied in arachidonic acid micelles and phospholipid Large Unilamellar Vesicles (LUVs) and compared to that of alpha-hematin (AH, Ferri-protoporphyrin IX-hydroxide) and hemin (HE, Ferri-protoporphyrin-chloride). Lipid peroxidation was measured as production of thiobarbituric acid reactive substances (TBARS). The extent of peroxidation induced by either AH or BH was strongly dependent upon the content of pre-existing hydroperoxides and efficiently inhibited by triphenylphosphine, a deoxygenating agent able to reduce hydroperoxides to hydroxides and by lipophilic scavengers. BH prooxidant activity was linearly related to the material, whereas that of AH seemed dependent on the aggregation state of the porphyrin. Maximal activity was observed when AH was present in concentration lower than 2 microM. In this case a shift of spectra in the Soret region, leading to the increase of the O.D. 400/385 nm ratio, suggested a transition toward a less aggregated state. BH prooxidant activity was significantly lower than that of monomeric AH, yet higher than that of AH aggregates. Differently from AH aggregates, BH-induced peroxidation was unaffected by GSH and inhibited rather than enhanced by acidic pH (5.7) and chloroquine. UV/Vis spectroscopy of AH aggregates at acidic pH, low GSH concentrations and chloroquine suggests a shift of AH aggregates toward the less aggregated state, more active as peroxidation catalyst.

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.