The role of neutral lipid nanospheres in Plasmodium falciparum haem crystallization

Effects of highly active novel artemisinin-chloroquinoline hybrid compounds on β-hematin formation, parasite morphology and endocytosis in Plasmodium falciparum

4-Aminoquinolines were hybridized with artemisinin and 1,4-naphthoquinone derivatives via the Ugi-four-component condensation reaction, and their biological activities investigated. The artemisinin-containing compounds 6a-c and its salt 6c-citrate were the most active target compounds in the antiplasmodial assays. However, despite the potent in vitro activities, they also displayed cytotoxicity against a mammalian cell-line, and had lower therapeutic indices than chloroquine. Morphological changes in parasites treated with these artemisinin-containing hybrid compounds were similar to those observed after addition of artemisinin. These hybrid compounds appeared to share mechanism(s) of action with both chloroquine and artemisinin: they exhibited potent β-hematin inhibitory activities; they caused an increase in accumulation of hemoglobin within the parasites that was intermediate between the increase observed with artesunate and chloroquine; and they also appeared to inhibit endocytosis as suggested by the decrease in the number of transport vesicles in the parasites. No cross-resistance with chloroquine was observed for these hybrid compounds, despite the fact that they contained the chloroquinoline moiety. The hybridization strategy therefore appeared to be borrowing the best from both classes of antimalarials.

Use of the NP-40 detergent-mediated assay in discovery of inhibitors of beta-hematin crystallization

The protozoan parasite responsible for malaria affects over 500 million people each year. Current antimalarials have experienced decreased efficacy due to the development of drug-resistant strains of Plasmodium spp., resulting in a critical need for the discovery of new antimalarials. Hemozoin, a crystalline by-product of heme detoxification that is necessary for parasite survival, serves as an important drug target. The quinoline antimalarials, including amodiaquine and chloroquine, act by inhibiting the formation of hemozoin. The formation of this crystal does not occur spontaneously, and recent evidence suggests crystallization occurs in the presence of neutral lipid particles located in the acidic digestive vacuole of the parasite. To mimic these conditions, the lipophilic detergent NP-40 has previously been shown to successfully mediate the formation of β-hematin, synthetic hemozoin. Here, an NP-40 detergent-based assay was successfully adapted for use as a high-throughput screen to identify inhibitors of β-hematin formation. The resulting assay exhibited a favorable Z' of 0.82 and maximal drift of less than 4%. The assay was used in a pilot screen of 38,400 diverse compounds at a screening concentration of 19.3 μM, resulting in the identification of 161 previously unreported β-hematin inhibitors. Of these, 48 also exhibited ≥ 90% inhibition of parasitemia in a Plasmodium falciparum whole-cell assay at a screening concentration of 23 μM. Eight of these compounds were identified to have nanomolar 50% inhibitory concentration values near that of chloroquine in this assay.

The hydroxyl functionality and a rigid proximal N are required for forming a novel non-covalent quinine-heme complex

Quinoline antimalarial drugs bind both monomeric and dimeric forms of free heme, with distinct preferences depending on the chemical environment. Under biological conditions, chloroquine (CQ) appears to prefer to bind to μ-oxo dimeric heme, while quinine (QN) preferentially binds monomer. To further explore this important distinction, we study three newly synthesized and several commercially available QN analogues lacking various functional groups. We find that removal of the QN hydroxyl lowers heme affinity, hemozoin (Hz) inhibition efficiency, and antiplasmodial activity. Elimination of the rigid quinuclidyl ring has similar effects, but elimination of either the vinyl or methoxy group does not. Replacing the quinuclidyl N with a less rigid tertiary aliphatic N only partially restores activity. To further study these trends, we probe drug-heme interactions via NMR studies with both Fe and Zn protoporphyrin IX (FPIX, ZnPIX) for QN, dehydroxyQN (DHQN), dequinuclidylQN (DQQN), and deamino-dequinuclidylQN (DADQQN). Magnetic susceptibility measurements in the presence of FPIX demonstrate that these compounds differentially perturb FPIX monomer-dimer equilibrium. We also isolate the QN-FPIX complex formed under mild aqueous conditions and analyze it by mass spectrometry, as well as fluorescence, vibrational, and solid-state NMR spectroscopies. The data elucidate key features of QN pharmacology and allow us to propose a refined model for the preferred binding of QN to monomeric FPIX under biologically relevant conditions. With this model in hand, we also propose how QN, CQ, and amodiaquine (AQ) differ in their ability to inhibit Hz formation.

Linear free energy relationships predict coordination and π-stacking interactions of small molecules with ferriprotoporphyrin IX

In order to better understand the interaction of antimalarial compounds with ferriprotoporphyrin IX (Fe(III)PPIX), association constants of pyridines, imidazoles, amines and phenolates with Fe(III)PPIX and protoporphyrin IX (PPIX) have been measured spectrophotometrically in 40% (v/v) aq. DMSO at pH 7.4. The pH independent log association constants for coordination of nitrogen donor ligands exhibit a linear free energy relationship (LFER) with the pK(a) of the donor atom. Association through π-stacking interactions (log K(π)) with PPIX and Fe(III)PPIX increases with the number of π-electrons in the aromatic ring system. These findings indicate that in the aqueous milieu of the malaria parasite digestive vacuole, coordination to the Fe(III) center of the porphyrin is necessarily very weak, while π-stacking interactions will be much stronger. On the other hand, in environments in which proton competition is absent, coordination will dominate, with the most basic donor atoms forming the strongest complexes with Fe(III)PPIX. The lipid nanospheres within the digestive vacuole which are now known to be the location of conversion of Fe(III)PPIX to hemozoin could possibly be such an environment, making both types of interaction relevant to the design of new hemozoin inhibitors.

Blood stage Plasmodium falciparum antigens induce T cell independent immunoglobulin production via B cell activation factor of the TNF family (BAFF) pathway

T independent (TI) antigens (Ags) activate monocytes to produce a cytokine, termed B cell activation factor (BAFF), involved in immunoglobulin (Ig) production. This study aimed to investigate whether the soluble schizont fraction of Plasmodium falciparum antigen (sPfAg) and hemozoin (HZ) could act as TI Ag to induce P. falciparum (Pf) specific Ig production via BAFF pathway. Co-cultures of monocytes and naïve B cells from 6 healthy donors were stimulated with sPfAg (10mg/ml) or HZ (10μM). At interval times, the expressions of BAFF on activated monocytes, BAFF receptor (BAFF-R) and proliferation nuclear Ag in activated B cells were determined by flow cytometry. The soluble BAFF (sBAFF), total and specific IgG levels in the supernatants were assessed by enzyme-linked immunosorbent assay (ELISA). The finding revealed both sPfAg and HZ could activate monocytes to express BAFF on surface and release sBAFF in the supernatant within 72h of stimulation. The B cells responded to specific activation, indicated by BAFF-R expression on the surface within 72h, marked proliferation on day 7, and final production of total and specific IgG during days 7-12. Comparing to sPfAg, HZ stimulated monocyte and B cell co-culture to express higher levels of BAFF and sBAFF during 24-48h, more BAFF-R on HZ activated B cells within 24h and induced marked proliferation of B cells with higher Pf specific IgG level. However, stimulation with sPfAg showed a more significant correlation between BAFF expression on the activated monocytes at 72h and the Pf specific IgG level on day 12 (r=0.961, p=0.039, Pearson Correlation). In conclusion, it is possible that both sPfAg and HZ stimulated B cells to produce specific IgG with BAFF involvement.

The neutral lipid composition present in the digestive vacuole of Plasmodium falciparum concentrates heme and mediates β-hematin formation with an unusually low activation energy

In eukaryotic cells, neutral lipids serve as major energy storage molecules; however, in Plasmodium falciparum, a parasite responsible for causing malaria in humans, neutral lipids may have other functions during the intraerythrocytic stage of the parasite life cycle. Specifically, experimental data suggest that neutral lipid structures behave as a catalyst for the crystallization of hemozoin, a detoxification byproduct of several blood-feeding organisms, including malaria parasites. Synthetic neutral lipid droplets (SNLDs) were produced by depositing a lipid blend solution comprised of mono- and diglycerides onto an aqueous surface. These lipid droplets are able to mediate the production of brown pigments that are morphologically and chemically identical to hemozoin. The partitioning of heme into these SNLDs was examined by employing Nile Red, a lipid specific dye. Soluble ferriprotoporphyrin IX was observed to spontaneously localize to the lipid droplets, partitioning in a pH-dependent manner with an estimated log P of 2.6. Interestingly, the pH profile of heme partitioning closely resembles that of β-hematin formation. Differential scanning calorimetry and kinetic studies demonstrated that the SNLDs provide a unique environment that promotes hemozoin formation. SNLD-mediated formation of the malaria pigment displayed an activation energy barrier lower than those of individual lipid components. In particular, lipid droplets composed of diglycerides displayed activation barriers lower than those composed of monoglycerides. This difference was attributed to the greater fluidity of these lipids. In conjunction with the known pattern of lipid body proliferation, it is suggested that neutral lipid structures within the digestive vacuole not only are the location of in vivo hemozoin formation but are also essential for the survival of the parasite by functioning as a kinetically competent and site specific mediator for heme detoxification.

Increase on the initial soluble heme levels in acidic conditions is an important mechanism for spontaneous heme crystallization in vitro

BACKGROUND

Hemozoin (Hz) is a heme crystal that represents a vital pathway for heme disposal in several blood-feeding organisms. Recent evidence demonstrated that β-hematin (βH) (the synthetic counterpart of Hz) formation occurs under physiological conditions near synthetic or biological hydrophilic-hydrophobic interfaces. This seems to require a heme dimer acting as a precursor of Hz crystals that would be formed spontaneously in the absence of the competing water molecules bound to the heme iron. Here, we aimed to investigate the role of medium polarity on spontaneous βH formation in vitro.

METHODOLOGY/PRINCIPAL FINDINGS

We assessed the effect of water content on spontaneous βH formation by using the aprotic solvent dimethylsulfoxide (DMSO) and a series of polyethyleneglycols (PEGs). We observed that both DMSO and PEGs (3.350, 6.000, 8.000, and 22.000) increased the levels of soluble heme under acidic conditions. These compounds were able to stimulate the production of βH crystals in the absence of any biological sample. Interestingly, the effects of DMSO and PEGs on βH formation were positively correlated with their capacity to promote previous heme solubilization in acidic conditions. Curiously, a short chain polyethyleneglycol (PEG 300) caused a significant reduction in both soluble heme levels and βH formation. Finally, both heme solubilization and βH formation strongly correlated with reduced medium water activity provided by increased DMSO concentrations.

CONCLUSIONS

The data presented here support the notion that reduction of the water activity is an important mechanism to support spontaneous heme crystallization, which depends on the previous increase of soluble heme levels.

Cryo transmission X-ray imaging of the malaria parasite, P. falciparum

Cryo transmission X-ray microscopy in the "water window" of photon energies has recently been introduced as a method that exploits the natural contrast of biological samples. We have used cryo tomographic X-ray imaging of the intra-erythrocytic malaria parasite, Plasmodium falciparum, to undertake a survey of the cellular features of this important human pathogen. We examined whole hydrated cells at different stages of growth and defined some of the structures with different X-ray density, including the parasite nucleus, cytoplasm, digestive vacuole and the hemoglobin degradation product, hemozoin. As the parasite develops from an early cup-shaped morphology to a more rounded shape, puncta of hemozoin are formed; these coalesce in the mature trophozoite into a central compartment. In some trophozoite stage parasites we observed invaginations of the parasite surface and, using a selective permeabilization process, showed that these remain connected to the RBC cytoplasm. Some of these invaginations have large openings consistent with phagocytic structures and we observed independent endocytic vesicles in the parasite cytoplasm which appear to play a role in hemoglobin uptake. In schizont stage parasites staggered mitosis was observed and X-ray-dense lipid-rich structures were evident at their apical ends of the developing daughter cells. Treatment of parasites with the antimalarial drug artemisinin appears to affect parasite development and their ability to produce the hemoglobin breakdown product, hemozoin.

Synthesis, antimalarial activity, and cellular toxicity of new arylpyrrolylaminoquinolines

A set of nine new arylpyrrolyl derivatives of 7-chloro-4-aminoquinoline, characterized by different substituents on the phenyl ring or different distance between the pyrrolic nitrogen and the 4-aminoquinoline, has been synthesized and tested for their activity against D-10 (CQ-S) and W-2 (CQ-R) strains of Plasmodium falciparum. All compounds exhibited activity against the CQ-S strain in the low nM range, comparable to that of chloroquine. Some of them were also highly active against the CQ-R strain and not toxic against normal cells. The antimalarial activity of this new class of compounds seems to be related to the inhibition of heme detoxification process of parasites, as in the case of chloroquine.

Innate inflammatory response to the malarial pigment hemozoin

Malaria is an infectious disease caused by parasites of the genus Plasmodium. This intraerythrocytic protozoan produces hemozoin (HZ), an insoluble crystalline metabolite resulting from the heme detoxification mechanism. This review will focus on HZ biosynthesis and synthetic preparation, but in particular on its effect on host's innate inflammatory responses.

Bioorganometallic Chemistry and Malaria

This chapter summarizes recent developments in the design, synthesis, and structure–activity relationship studies of organometallic antimalarials. It begins with a general introduction to malaria and the biology of the parasite Plasmodium falciparum, with a focus on the heme detoxification system. Then, a number of metal complexes from the literature are reported for their antiplasmodial activity. The second half of the chapter deals with the serendipitous discovery of ferroquine, its mechanism(s) of action, and the failure to induce a resistance. Last, but not least, we suggest that the bioorganometallic approach offers the potential for the design of novel therapeutic agents.

Chemical genetics of Plasmodium falciparum

Malaria caused by Plasmodium falciparum is a catastrophic disease worldwide (880,000 deaths yearly). Vaccine development has proved difficult and resistance has emerged for most antimalarials. In order to discover new antimalarial chemotypes, we have employed a phenotypic forward chemical genetic approach to assay 309,474 chemicals. Here we disclose structures and biological activity of the entire library, many of which exhibited potent in vitro activity against drug resistant strains, and detailed profiling of 172 representative candidates. A reverse chemical genetic study identified 19 new inhibitors of 4 validated drug targets and 15 novel binders among 61 malarial proteins. Phylochemogenetic profiling in multiple organisms revealed similarities between Toxoplasma gondii and mammalian cell lines and dissimilarities between P. falciparum and related protozoans. One exemplar compound displayed efficacy in a murine model. Overall, our findings provide the scientific community with new starting points for malaria drug discovery.

Plasmodium falciparum: nitric oxide modulates heme speciation in isolated food vacuoles

Nitric oxide (NO) and NO-derived reactive nitrogen species (RNS) are present in the food vacuole (FV) of Plasmodium falciparum trophozoites. The product of PFL1555w, a putative cytochrome b(5), localizes in the FV membrane, similar to what was previously observed for the product of PF13_0353, a putative cytochrome b(5) reductase. These two gene products may contribute to NO generation by denitrification chemistry from nitrate and/or nitrite present in the erythrocyte cytosol. The possible coordination of NO to heme species present in the food vacuole was probed by resonance Raman spectroscopy. The spectroscopic data revealed that in situ generated NO interacts with heme inside the intact FVs to form ferrous heme nitrosyl complexes that influence intra-vacuolar heme solubility. The formation of heme nitrosyl complexes within the FV is a previously unrecognized factor that could affect the equilibrium between soluble and crystallized heme within the FV in vivo.

Falcipain-2 inhibitors

Malaria, particularly that one caused by Plasmodium falciparum, remains a serious health problem in Africa, South America, and many parts of Asia where it afflicts about 500 million people and is responsible for the death of more than one million children each year. The main reasons for the persistence of malaria are the emergence of resistance to common antimalarial drugs, inadequate control of mosquito vectors, and the lack of effective vaccines. Therefore, the identification and characterization of new targets for antimalarial chemotherapy are of urgent priority. This review is focused on inhibitors of falcipain-2, a cysteine protease from P. falciparum, which represents one of the most promising targets for antimalarial drug design. Falcipain-2 is a key enzyme in the life cycle of P. falciparum since it degrades hemoglobin, at the early trophozoite stage, and cleaves ankyrin and protein 4.1, the cytoskeletal elements vital to the stability of red cell membrane, at the schizont stage. The main classes of falcipain-2 inhibitors are peptides or peptidomimetics bearing the most popular pharmacophores of cysteine protease inhibitors, such as vinyl sulfones, halomethyl ketones, and aldehydes. Furthermore, many other chemotypes have been identified as inhibitors of falcipain-2, such as isoquinolines, thiosemicarbazones, and chalcones. These inhibitors represent all classes, which, to the best of our knowledge, have been disclosed in journal articles to date.

On the physico-chemical and physiological requirements of hemozoin formation promoted by perimicrovillar membranes in Rhodnius prolixus midgut

Triatomine insects are obligatory blood-feeders that detoxify most of the hemoglobin-derived heme through its crystallization into hemozoin (Hz). Previous evidence demonstrates the key role of midgut perimicrovillar membranes (PMVM) on heme crystallization in triatomines. Here, we investigated some of the physico-chemical and physiological aspects of heme crystallization induced by Rhodnius prolixus PMVM. Hz formation in vitro proceeded optimally at pH 4.8 and 28 degrees C, apparently involving three kinetically distinct mechanisms along this process. Furthermore, the insect feeding status and age affected PMVM-induced heme crystallization whereas pharmacological blockage of PMVM formation by azadirachtin, reduced hemoglobin digestion and Hz formation in vivo. Mössbauer spectrometry analyses of R. prolixus midgut showed that Hz represents the only measurable iron species found four days after a blood meal. Autocatalytic heme crystallization to Hz is revealed to be an inefficient process and this conversion is further reduced as the Hz concentration increases. Also, PMVM-derived lipids were able to induce rapid Hz formation, regardless of the diet composition. These results indicate that PMVM-driven Hz formation in R. prolixus midgut occurs at physiologically relevant physico-chemical conditions and that lipids derived from this structure play an important role in heme crystallization.

The lipid moiety of haemozoin (Malaria Pigment) and P. falciparum parasitised red blood cells bind synthetic and native endothelin-1

Endothelin1 (ET-1) is a 21-amino acid peptide produced by the vascular endothelium under hypoxia, that acts locally as regulator of vascular tone and inflammation. The role of ET-1 in Plasmodium falciparum malaria is unknown, although tissue hypoxia is frequent as a result of the cytoadherence of parasitized red blood cell (pRBC) to the microvasculature. Here, we show that both synthetic and endothelial-derived ET-1 are removed by parasitized RBC (D10 and W2 strains, chloroquine sensitive, and resistant, resp.) and native haemozoin (HZ, malaria pigment), but not by normal RBC, delipidized HZ, or synthetic beta-haematin (BH). The effect is dose dependent, selective for ET-1, but not for its precursor, big ET-1, and not due to the proteolysis of ET-1. The results indicate that ET-1 binds to the lipids moiety of HZ and membranes of infected RBCs. These findings may help understanding the consequences of parasite sequestration in severe malaria.

Crystallization of synthetic haemozoin (beta-haematin) nucleated at the surface of lipid particles

The mechanism of formation of haemozoin, a detoxification by-product of several blood-feeding organisms including malaria parasites, has been a subject of debate; however, recent studies suggest that neutral lipids may serve as a catalyst. In this study, a model system consisting of an emulsion of neutral lipid particles was employed to investigate the formation of beta-haematin, the synthetic counterpart of haemozoin, at the lipid-water interface. A solution of monoglyceride, either monostearoylglycerol (MSG) or monopalmitoylglycerol (MPG), dissolved in acetone and methanol was introduced to an aqueous surface. Fluorescence, confocal and transmission electron microscopic (TEM) imaging and dynamic light scattering analysis of samples obtained from beneath the surface confirmed the presence of homogeneous lipid particles existing in two major populations: one in the low micrometre size range and the other in the hundred nanometre range. The introduction of haem (Fe(iii)PPIX) to this lipid particle system under biomimetic conditions (37 degrees C, pH 4.8) produced beta-haematin with apparent first-order kinetics and an average half life of 0.5 min. TEM of monoglycerides (MSG or MPG) extruded through a 200 nm filter with haem produced beta-haematin crystals aligned and parallel to the lipid-water interface. These TEM data, together with a model system replacing the lipid with an aqueous organic solvent interface using either methyl laurate or docosane demonstrated that the OH and C[double bond, length as m-dash]O groups are apparently necessary for efficient nucleation. This suggests that beta-haematin crystallizes via epitaxial nucleation at the lipid-water interface through interaction of Fe(iii)PPIX with the polar head group. Once nucleated, the crystal grows parallel to the interface until growth is terminated by the curvature of the lipid particle. The hydrophobic nature of the mature crystal favours an interior transport resulting in crystals aligned parallel to the lipid-water interface and each other, strikingly similar to that seen in malaria parasites.

Alpha-glucosidase promotes hemozoin formation in a blood-sucking bug: an evolutionary history

BACKGROUND

Hematophagous insects digest large amounts of host hemoglobin and release heme inside their guts. In Rhodnius prolixus, hemoglobin-derived heme is detoxified by biomineralization, forming hemozoin (Hz). Recently, the involvement of the R. prolixus perimicrovillar membranes in Hz formation was demonstrated.

METHODOLOGY/PRINCIPAL FINDINGS

Hz formation activity of an alpha-glucosidase was investigated. Hz formation was inhibited by specific alpha-glucosidase inhibitors. Moreover, Hz formation was sensitive to inhibition by Diethypyrocarbonate, suggesting a critical role of histidine residues in enzyme activity. Additionally, a polyclonal antibody raised against a phytophagous insect alpha-glucosidase was able to inhibit Hz formation. The alpha-glucosidase inhibitors have had no effects when used 10 h after the start of reaction, suggesting that alpha-glucosidase should act in the nucleation step of Hz formation. Hz formation was seen to be dependent on the substrate-binding site of enzyme, in a way that maltose, an enzyme substrate, blocks such activity. dsRNA, constructed using the sequence of alpha-glucosidase gene, was injected into R. prolixus females' hemocoel. Gene silencing was accomplished by reduction of both alpha-glucosidase and Hz formation activities. Insects were fed on plasma or hemin-enriched plasma and gene expression and activity of alpha-glucosidase were higher in the plasma plus hemin-fed insects. The deduced amino acid sequence of alpha-glucosidase shows a high similarity to the insect alpha-glucosidases, with critical histidine and aspartic residues conserved among the enzymes.

CONCLUSIONS/SIGNIFICANCE

Herein the Hz formation is shown to be associated to an alpha-glucosidase, the biochemical marker from Hemipteran perimicrovillar membranes. Usually, these enzymes catalyze the hydrolysis of glycosidic bond. The results strongly suggest that alpha-glucosidase is responsible for Hz nucleation in the R. prolixus midgut, indicating that the plasticity of this enzyme may play an important role in conferring fitness to hemipteran hematophagy, for instance.

Recent advances in the discovery of haem-targeting drugs for malaria and schistosomiasis

Haem is believed to be the target of some of the historically most important antimalarial drugs, most notably chloroquine. This target is almost ideal as haem is host-derived and the process targeted, haemozoin formation, is a physico-chemical process with no equivalent in the host. The result is that the target remains viable despite resistance to current drugs, which arises from mutations in parasite membrane transport proteins. Recent advances in high-throughput screening methods, together with a better understanding of the interaction of existing drugs with this target, have created new prospects for discovering novel haem-targeting chemotypes and for target-based structural design of new drugs. Finally, the discovery that Schistosoma mansoni also produces haemozoin suggests that new drugs of this type may be chemotherapeutic not only for malaria, but also for schistosomiasis. These recent developments in the literature are reviewed.

Novel antimalarial aminoquinolines: heme binding and effects on normal or Plasmodium falciparum-parasitized human erythrocytes

Two new quinolizidinyl-alkyl derivatives of 7-chloro-4-aminoquinoline, named AM-1 and AP4b, which are highly effective in vitro against both the D10 (chloroquine [CQ] susceptible) and W2 (CQ resistant) strains of Plasmodium falciparum and in vivo in the rodent malaria model, have been studied for their ability to bind to and be internalized by normal or parasitized human red blood cells (RBC) and for their effects on RBC membrane stability. In addition, an analysis of the heme binding properties of these compounds and of their ability to inhibit beta-hematin formation in vitro has been performed. Binding of AM1 or AP4b to RBC is rapid, dose dependent, and linearly related to RBC density. Their accumulation in parasitized RBC (pRBC) is increased twofold compared to levels in normal RBC. Binding of AM1 or AP4b to both normal and pRBC is higher than that of CQ, in agreement with the lower pKa and higher lipophilicity of the compounds. AM1 or AP4b is not hemolytic per se and is less hemolytic than CQ when hemolysis is accelerated (induced) by hematin. Moreover, AM-1 and AP4b bind heme with a stoichiometry of interaction similar to that of CQ (about 1:1.7) but with a lower affinity. They both inhibit dose dependently the formation of beta-hematin in vitro with a 50% inhibitory concentration comparable to that of CQ. Taken together, these results suggest that the antimalarial activity of AM1 or AP4b is likely due to inhibition of hemozoin formation and that the efficacy of these compounds against the CQ-resistant strains can be ascribed to their hydrophobicity and capacity to accumulate in the vacuolar lipid (elevated lipid accumulation ratios).

Molecular and physiologic basis of quinoline drug resistance in Plasmodium falciparum malaria

30 years before the discovery of the pfcrt gene, altered cellular drug accumulation in drug-resistant malarial parasites had been well documented. Heme released from catabolized hemoglobin was thought to be a key target for quinoline drugs, and additional modifications to quinoline drug structure in order to improve activity against chloroquine-resistant malaria were performed in a few laboratories. However, parasite cell culture methods were still in their infancy, assays for drug susceptibility were not well standardized, and the power of malarial genetics was decades away. The last 10 years have witnessed explosive progress in elucidation of the biochemistry of chloroquine resistance. This review briefly summarizes that progress, and discusses where additional work is needed.

Discriminating the intraerythrocytic lifecycle stages of the malaria parasite using synchrotron FT-IR microspectroscopy and an artificial neural network

Synchrotron Fourier transform infrared (FT-IR) spectra of fixed single erythrocytes infected with Plasmodium falciparum at different stages of the intraerythrocytic cycle are presented for the first time. Bands assigned to the hemozoin moiety at 1712, 1664, and 1209 cm(-1) are observed in FT-IR difference spectra between uninfected erythrocytes and infected trophozoites. These bands are also found to be important contributors in separating the trophozoite spectra from the uninfected cell spectra in principal components analysis. All stages of the intraerythrocytic lifecycle of the malarial parasite, including the ring and schizont stage, can be differentiated by visual inspection of the C-H stretching region (3100-2800 cm(-1)) and by using principal components analysis. Bands at 2922, 2852, and 1738 cm(-1) assigned to the nu(asym)(CH(2) acyl chain lipids), nu(sym)(CH(2) acyl chain lipids), and the ester carbonyl band, respectively, increase as the parasite matures from its early ring stage to the trophozoite and finally to the schizont stage. Training of an artificial neural network showed that excellent automated spectroscopic discrimination between P. falciparum-infected cells and the control cells is possible. FT-IR difference spectra indicate a change in the production of unsaturated fatty acids as the parasite matures. The ring stage spectrum shows bands associated with cis unsaturated fatty acids. The schizont stage spectrum displays no evidence of cis bands and suggests an increase in saturated fatty acids. These results demonstrate that different phases of the P. falciparum intraerthyrocytic life cycle are characterized by different lipid compositions giving rise to distinct spectral profiles in the C-H stretching region. This insight paves the way for an automated infrared-based technology capable of diagnosing malaria at all intraerythrocytic stages of the parasite's life cycle.

Antimalarial drugs and heme in detergent micelles: An NMR study

Proton nuclear magnetic resonance relaxation times were measured for the protons of micelles formed by the detergents sodium dodecyl sulfate, dodecyltrimethyl ammonium bromide, and polyethylene glycol sorbitan monolaureate in the presence of ferriprotoporphyrin IX and the antimalarial drugs chloroquine, 7-chloro-4-quinolyl 4-N,N-diethylaminobutyl sulfide, and primaquine. Diffusion coefficients were extracted from pulsed gradient NMR experiments to evaluate the degree of association of these drugs with the detergent micelles. Results indicate that at low or neutral pH when the quinolyl N is protonated, chloroquine does not associate with neutral or cationic detergent micelles. For this reason, chloroquine's interaction with heme perturbs the partitioning of heme between the aqueous medium and detergent micelles.

Accumulation of artemisinin trioxane derivatives within neutral lipids of Plasmodium falciparum malaria parasites is endoperoxide-dependent

The antimalarial trioxanes, exemplified by the naturally occurring sesquiterpene lactone artemisinin and its semi-synthetic derivatives, contain an endoperoxide pharmacophore that lends tremendous potency against Plasmodium parasites. Despite decades of research, their mechanism of action remains unresolved. A leading model of anti-plasmodial activity hypothesizes that iron-mediated cleavage of the endoperoxide bridge generates cytotoxic drug metabolites capable of damaging cellular macromolecules. To probe the malarial targets of the endoperoxide drugs, we studied the distribution of fluorescent dansyl trioxane derivatives in living, intraerythrocytic-stage Plasmodium falciparum parasites using microscopic imaging. The fluorescent trioxanes rapidly accumulated in parasitized erythrocytes, localizing within digestive vacuole-associated neutral lipid bodies of trophozoites and schizonts, and surrounding the developing merozoite membranes. Artemisinin pre-treatment significantly reduced fluorescent labeling of neutral lipid bodies, while iron chelation increased non-specific cytoplasmic localization. To further explore the effects of endoperoxides on cellular lipids, we used an oxidation-sensitive BODIPY lipid probe to show the presence of artemisinin-induced peroxyl radicals in parasite membranes. Lipid extracts from artemisinin-exposed parasites contained increased amounts of free fatty acids and a novel cholesteryl ester. The cellular accumulation patterns and effects on lipids were entirely endoperoxide-dependent, as inactive dioxolane analogs lacking the endoperoxide moiety failed to label neutral lipid bodies or induce oxidative membrane damage. In the parasite digestive vacuole, neutral lipids closely associate with heme and promote hemozoin formation. We propose that the trioxane artemisinin and its derivatives are activated by heme-iron within the neutral lipid environment where they initiate oxidation reactions that damage parasite membranes.

Formation of the food vacuole in Plasmodium falciparum: a potential role for the 19 kDa fragment of merozoite surface protein 1 (MSP1(19))

Plasmodium falciparum Merozoite Surface Protein 1 (MSP1) is synthesized during schizogony as a 195-kDa precursor that is processed into four fragments on the parasite surface. Following a second proteolytic cleavage during merozoite invasion of the red blood cell, most of the protein is shed from the surface except for the C-terminal 19-kDa fragment (MSP1₁₉), which is still attached to the merozoite via its GPI-anchor. We have examined the fate of MSP1₁₉ during the parasite's subsequent intracellular development using immunochemical analysis of metabolically labeled MSP1₁₉, fluorescence imaging, and immuno-electronmicroscopy. Our data show that MSP1₁₉ remains intact and persists to the end of the intracellular cycle. This protein is the first marker for the biogenesis of the food vacuole; it is rapidly endocytosed into small vacuoles in the ring stage, which coalesce to form the single food vacuole containing hemozoin, and persists into the discarded residual body. The food vacuole is marked by the presence of both MSP1₁₉ and the chloroquine resistance transporter (CRT) as components of the vacuolar membrane. Newly synthesized MSP1 is excluded from the vacuole. This behavior indicates that MSP1₁₉ does not simply follow a classical lysosome-like clearance pathway, instead, it may play a significant role in the biogenesis and function of the food vacuole throughout the intra-erythrocytic phase.

Investigation of the redox behavior of ferroquine, a new antimalarial

Ferroquine (FQ or SR97193) is a unique ferrocene antimalarial drug candidate which just entered phase IIb clinical trials in autumn 2007. FQ is able to overcome the chloroquine (CQ) resistance problem, an important limit to the control of Plasmodium falciparum, the principal causative agent of malaria. However, as for other therapeutic agents such as chloroquine (CQ) and artemisin, its mechanism of action remains partially unknown. Most investigations have so far focused on comparing the activity of FQ to that of CQ in order to understand how the ferrocene core contributes to a stronger antiplasmodial activity. Studies have already shown that the ferrocene altered the shape, volume, lipophilicity, basicity and also electronic profile of the parent molecule and, hence, its pharmacodynamic behavior. However, few investigations have been undertaken to probe the real contribution of redox properties of the ferrocene (iron(II))/ferricinium (iron(III)) system in FQ as reported in this article. In our experimental and theoretical approach, we considered the redox profile of the ferrocene core of FQ in the specific conditions (acidic and oxidizing) of the parasitic digestive vacuole as a possible discriminating property from CQ in the antimalarial activity.

Quinine and chloroquine differentially perturb heme monomer-dimer equilibrium

Nuclear magnetic resonance (NMR) measurements of magnetic susceptibility have been utilized to study the equilibrium between two forms (high-spin monomer vs the antiferromagnetically coupled mu-oxo dimer) of ferriprotoporphyrin(IX) as a function of pH. The pH dependence of this equilibrium is significantly altered by the addition of either chloroquine or quinine. Chloroquine promotes the mu-oxo dimer whereas quinine promotes the monomer.

HDP-a novel heme detoxification protein from the malaria parasite

When malaria parasites infect host red blood cells (RBC) and proteolyze hemoglobin, a unique, albeit poorly understood parasite-specific mechanism, detoxifies released heme into hemozoin (Hz). Here, we report the identification and characterization of a novel Plasmodium Heme Detoxification Protein (HDP) that is extremely potent in converting heme into Hz. HDP is functionally conserved across Plasmodium genus and its gene locus could not be disrupted. Once expressed, the parasite utilizes a circuitous "Outbound-Inbound" trafficking route by initially secreting HDP into the cytosol of infected RBC. A subsequent endocytosis of host cytosol (and hemoglobin) delivers HDP to the food vacuole (FV), the site of Hz formation. As Hz formation is critical for survival, involvement of HDP in this process suggests that it could be a malaria drug target.

The crystal structure of halofantrine-ferriprotoporphyrin IX and the mechanism of action of arylmethanol antimalarials

The crystal structure of the complex formed between the antimalarial drug halofantrine and ferriprotoporphyrin IX (Fe(III)PPIX) has been determined by single crystal X-ray diffraction. The structure shows that halofantrine coordinates to the Fe(III) center through its alcohol functionality in addition to pi-stacking of the phenanthrene ring over the porphyrin. The length of the Fe(III)-O bond is consistent with an alkoxide and not an alcohol coordinating group. The iron porphyrin is five coordinate and monomeric. Changes in the electronic spectrum of Fe(III)PPIX upon addition of halofantrine base in acetonitrile solution are almost identical to those observed upon addition of quinidine free base in the same solvent. This suggests homologous binding. Molecular mechanics modeling of Fe(III)PPIX complexes of quinidine, quinine, 9-epiquinine and 9-epiquinidine based on this homology suggests that the antimalarially active quinidine and quinine can readily adopt conformations that permit formation of an intramolecular salt bridge between the protonated quinuclidine tertiary amino group and unprotonated heme propionate group, while the inactive epimers 9-epiquinidine and 9-epiquinine have to adopt high energy conformations in order to accommodate such salt bridge formation. We propose that salt bridge formation may interrupt formation of the hemozoin precursor dimer formed during the heme detoxification pathway and so account for the strong activity of the two active isomers.

Evaluation of pH during cytostomal endocytosis and vacuolar catabolism of haemoglobin in Plasmodium falciparum

The DV (digestive vacuole) of the malaria parasite, Plasmodium falciparum, is the site of Hb (haemoglobin) digestion and haem detoxification and, as a consequence, the site of action of CQ (chloroquine) and related antimalarials. However, the precise pH of the DV and the endocytic vesicles that feed it has proved difficult to ascertain. We have developed new methods using EGFP [enhanced GFP (green fluorescent protein)] to measure the pH of intracellular compartments. We have generated a series of transfectants in CQ-sensitive and -resistant parasite strains expressing GFP chimaeras of the DV haemoglobinase, plasmepsin II. Using a quantitative flow cytometric assay, the DV pH was determined to be 5.4-5.5. No differences were detected between CQ-sensitive and -resistant strains. We have also developed a method that relies on the pH dependence of GFP photobleaching kinetics to estimate the pH of the DV compartment. This method gives a pH estimate consistent with the intensity-based measurement. Accumulation of the pH-sensitive probe, LysoSensor Blue, in the DV confirms the acidity of this compartment and shows that the cytostomal vesicles are not measurably acidic, indicating that they are unlikely to be the site of Hb digestion or the site of CQ accumulation. We show that a GFP probe located outside the DV reports a pH value close to neutral. The transfectants and methods that we have developed represent useful tools for investigating the pH of GFP-containing compartments and should be of general use in other systems.

Haemozoin: from melatonin pigment to drug target, diagnostic tool, and immune modulator

Plasmodium spp produce a pigment (haemozoin) to detoxify the free haem that is generated by haemoglobin degradation. Haemozoin was originally thought to be an inert waste byproduct of the parasite. However, recent research has led to the recognition that haemozoin is possibly of great importance in various aspects of malaria. Haemozoin is the target of many antimalarial drugs, and the unravelling of the exact modes of action may allow the design of novel antimalarial compounds. The detection of haemozoin in erythrocytes or leucocytes facilitates the diagnosis of malaria. The number of haemozoin-containing monocytes and granulocytes has been shown to correlate well with disease severity and may hold the potential for becoming a novel, automated laboratory marker in the assessment of patients. Finally, haemozoin has a substantial effect on the immune system. Further research is needed to clarify these aspects, many of which are important in clinical practice.

Hemozoin: oil versus water

Because the quinolines inhibit heme crystallization within the malaria parasite much work has focused on mechanism of formation and inhibition of hemozoin. Here we review the recent evidence for heme crystallization within lipids in diverse parasites and the new implications of a lipid site of crystallization for drug targeting. Within leukocytes hemozoin can generate toxic radical lipid metabolites, which may alter immune function or reduce deformability of uninfected erythrocytes.

Identification of hydroxyeicosatetraenoic acid components of schistosomal hemozoin

During the late stages of Schistosoma mansoni infection, adult schistosomes catabolize host erythrocytic hemoglobin. In order to evade the toxic effects of free heme, the blood fluke biomineralizes dimeric heme into an inert crystalline pigment called hemozoin. In the present study, the chemical reactivity of schistosomal hemozoin (SmHz) toward lipid oxidation was examined, and the biological consequences of reactivity were investigated. Mass spectrometric analysis of polar lipid content associated with SmHz identified a variety of primary and secondary polyunsaturated fatty acid oxidation products, including hydroxyeicosatetraenoic acids. Furthermore, RAW 264.7 macrophage-like cells challenged with lipopolysaccharide prior to phagocytosis of SmHz experienced a decrease in nitric oxide production as compared to control experiments. The presence of these biologically active oxidation products suggests native SmHz is capable of modulating the innate immune response and may play a potential role in the pathogenesis of schistosomiasis.

2-tert-butyl-8-quinolinamines exhibit potent blood schizontocidal antimalarial activity via inhibition of heme crystallization

We have recently reported that the attachment of a bulky metabolically stable tert-butyl group at the C-2 position of a quinoline ring in primaquine results in a tremendous improvement in the blood schizontocidal antimalarial activity of 8-quinolinamine. Because free heme released from hemoglobin catabolism in a malarial parasite is highly toxic, the parasite protects itself mainly by crystallization of heme into insoluble nontoxic hemozoin. We now demonstrate the ability of 2-tert-butylprimaquine to inhibit in vitro beta-hematin formation, to form a complex with heme with a stoichiometry of 1:1, and to enhance heme-induced hemolysis. The results described herein indicate that a major improvement in the blood-schizontocidal antimalarial activity of 2-tert-butylprimaquine might be due to a disturbance of heme catabolism pathway in the malarial parasite.

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.

Extracellular lipid droplets promote hemozoin crystallization in the gut of the blood flukeSchistosoma mansoni

Hemozoin (Hz) is a heme crystal produced upon hemoglobin digestion as the main mechanism of heme disposal in several hematophagous organisms. Here, we show that, in the helminth Schistosoma mansoni, Hz formation occurs in extracellular lipid droplets (LDs). Transmission electron microscopy of adult worms revealed the presence of numerous electron-lucent round structures similar to LDs in gut lumen, where multicrystalline Hz assemblies were found associated to their surfaces. Female regurgitates promoted Hz formation in vitro in reactions partially inhibited by boiling. Fractionation of regurgitates showed that Hz crystallization activity was essentially concentrated on lower density fractions, which have small amounts of pre-formed Hz crystals, suggesting that hydrophilic-hydrophobic interfaces, and not Hz itself, play a key catalytic role in Hz formation in S. mansoni. Thus, these data demonstrate that LDs present in the gut lumen of S. mansoni support Hz formation possibly by allowing association of heme to the lipid-water interface of these structures.