Further evidence for the interaction of the antimalarial drug amodiaquine with ferriprotoporphyrin IX

Applications of Mössbauer Spectroscopy in Biomedical Research

A brief review on the applications of Mössbauer spectroscopy in biomedical research discusses the results of more than fifty years of experience in this field. Basing on the numerous results the main directions of biomedical applications of Mössbauer spectroscopy are considered as follows: 1) studies of the quantitative changes of iron-containing biomolecules related to pathological processes; 2) studies of the qualitative changes in iron-containing biomolecules related to pathological processes; 3) studies of the effect of various environmental factors (physical, chemical, and biological) on iron-containing biomolecules; 4) studies of metabolic processes by means of analysis of the Mössbauer nuclides pathways in organisms; 5) studies of dynamic processes; 6) studies of pharmaceutical compounds and blood substitutes containing Mössbauer nuclides; 7) miscellaneous studies. Some examples of biomedical research using ⁵⁷Fe, ⁵⁷Co, ¹¹⁹Sn, ¹⁵³Sm, and ¹⁹⁷Au Mössbauer nuclides are presented.

Interactions of the antimalarial amodiaquine with lipid model membranes

A detailed molecular description of the mechanism of action of the antimalarial drug amodiaquine (AQ) is still an open issue. To gain further insights on that, we studied the interactions of AQ with lipid model membranes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) by spin labeling electron spin resonance (ESR) and differential scanning calorimetry (DSC). Both techniques indicate a coexistence of an ordered DPPS-rich domain with a disordered DPPC-rich domain in the binary DPPC/DPPS system. We found that AQ slightly lowered the melting transition temperatures associated to both domains and significantly increased the enthalpy change of the whole DPPC/DPPS phase transition. DSC and ESR data also suggest that AQ increases the number of DPPC molecules in the DPPC-rich domains. AQ also causes opposing ordering effects on different regions of the bilayer: while the drug increases the ordering of the lipid acyl chains from carbon 7 to 16, it decreases the order parameter of the lipid head group and of carbon 5. The gel phase was mostly affected by the presence of AQ, suggesting that AQ is able to influence more organized lipid domains. Moreover, the effects of AQ and cholesterol on lipid acyl chain ordering and mobility were compared at physiological temperature and, in a general way, they are similar. Our results suggest that the quinoline ring of AQ is located completely inside the lipid bilayers with its phenol ring and the tertiary amine directed towards the head group region. The nonspecific interaction between AQ and DPPC/DPPS bilayers is a combination of electrostatic and hydrophobic interactions.

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.

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.

Mechanism-based design of parasite-targeted artemisinin derivatives: synthesis and antimalarial activity of new diamine containing analogues

The potent antimalarial activity of chloroquine against chloroquine-sensitive strains can be attributed, in part, to its high accumulation in the acidic environment of the heme-rich parasite food vacuole. A key component of this intraparasitic chloroquine accumulation mechanism is a weak base "ion-trapping" effect whereupon the basic drug is concentrated in the acidic food vacuole in its membrane-impermeable diprotonated form. By the incorporation of amino functionality into target artemisinin analogues, we hoped to prepare a new series of analogues that, by virtue of increased accumulation into the ferrous-rich vacuole, would display enhanced antimalarial potency. The initial part of the project focused on the preparation of piperazine-linked analogues (series 1 (7-16)). Antimalarial evaluation of these derivatives demonstrated potent activity versus both chloroquine-sensitive and chloroquine-resistant parasites. On the basis of these observations, we then set about preparing a series of C-10 carba-linked amino derivatives. Optimization of the key synthetic step using a newly developed coupling protocol provided a key intermediate, allyldeoxoartemisinin (17) in 90% yield. Further elaboration, in three steps, provided nine target C-10 carba analogues (series 2 (21-29)) in good overall yields. Antimalarial assessment demonstrated that these compounds were 4-fold more potent than artemisinin and about twice as active as artemether in vitro versus chloroquine-resistant parasites. On the basis of the products obtained from biomimetic Fe(II) degradation of the C-10 carba analogue (23), we propose that these analogues may have a mode of action subtly different from that of the parent drug artemisinin (series 1 (7-16)) and other C-10 ether derivatives such as artemether. Preliminary in vivo testing by the WHO demonstrated that four of these compounds are active orally at doses of less than 10 mg/kg. Since these analogues are available as water-soluble salts and cannot form dihydroartemisinin by P450-catalyzed oxidation, they represent useful leads that might prove to be superior to the currently used derivatives, artemether and artesunate.

Differential effects of 4-aminoquinoline-containing antimalarial drugs on hemoglobin digestion in Plasmodium falciparum-infected erythrocytes

Several reports suggest that the antimalarial mode of action of quinoline drugs may differ in their mechanistic details. The malaria parasite Plasmodium falciparum was treated in culture with chloroquine, amodiaquine, quinine and mefloquine in a dose- and time-dependent fashion. After removal of the drug, the viability of the parasites and their hemoglobin content were determined. Whereas in the presence of chloroquine and amodiaquine, there was a correlation between parasite killing and accumulation of hemoglobin, with quinine and mefloquine parasite killing was not associated with the accumulation of hemoglobin. Mefloquine inhibited the chloroquine-dependent accumulation of hemoglobin. It is suggested that whereas chloroquine and amodiaquine inhibit the digestion of hemoglobin, mefloquine and possibly quinine inhibit the ingestion of host cell hemoglobin by interfering with the ingestion process. These results may explain the demonstrable antagonism between chloroquine and mefloquine and their antipodal sensitivity to these drugs.

Novel aryl-bis-quinolines with antimalarial activity in-vivo

Three rationally designed isomeric aryl-bridged bis-quinolines, N1,Nx-bis(7-chloroquinolin-4-yl)phenylene-1,x-diamines, where x=2, 3 or 4, i.e. o-, m- and p-substituted analogues respectively, were synthesized and evaluated against Plasmodium berghei in-vivo. The compound with x=2 had an ID50 of 30 mg kg(-1), whereas the p-substituted analogue (x=4) was not statistically schizonticidal at either of the two dose levels tested in olive oil-dimethylsulphoxide (5 and 25 mg kg(-1), ID50=60 mg kg(-1) approx.). When the delivery vehicle was changed to saline-DMSO, antimalarial potency increased for the p-substituted compound (ID50 17 mg kg(-1)). In contrast, the m-substituted analogue had marked antimalarial activity (ID50 1.2 mg kg(-1)), which compares favourably with that of chloroquine diphosphate (ID50 = 4.3 mg kg(-1)). The data presented show that the aminomethylene side chain in amodiaquine can be successfully replaced by a 7-halo-4-aminoquinoline, establishing that carbon bridges containing less than four contiguous carbon atoms can be present within highly active aryl-substituted 4-aminoquinoline antimalarials. These results confirm that the presence of an OH group in the aryl bridge is not necessary for antimalarial activity and substantiate the view that, despite the appearance of resistant strains, new and existing aminoquinolines still have an important role in treating malaria.

An assessment of drug-haematin binding as a mechanism for inhibition of haematin polymerisation by quinoline antimalarials

Chloroquine is thought to exert its antimalarial activity by preventing the polymerisation of toxic haematin released during proteolysis of haemoglobin in the Plasmodium digestive vacuole. However, the molecular mechanisms by which this inhibition occurs and the universality of this mechanism for other quinoline antimalarials remain to be established. We demonstrate here a correlation for eight antimalarial quinolines between inhibition of haematin polymerisation in vitro and inhibition of P. falciparum growth in culture, confirming haematin polymerisation as the likely target of quinoline blood schizonticides. Furthermore, using isothermal titration microcalorimetry, a correlation was observed between the haematin binding constant of these compounds and their ability to inhibit haematin polymerisation, suggesting that these compounds mediate their activity through binding to haematin. It was also observed that the compounds bind primarily to the mu-oxo dimer form of haematin rather than the monomeric form. It is postulated that this binding inhibits haematin polymerisation by shifting the haematin dimerisation equilibrium to the mu-oxo dimer, thus reducing the availability of monomeric haematin for incorporation into haemozoin. These data reconcile the haematin polymerisation theory with the Fitch hypothesis, which states that chloroquine mediates its activity through binding to haematin.

4-Aminoquinolines--past, present, and future: a chemical perspective

The 4-aminoquinoline chloroquine (1) can be considered to be one of the most important synthetic chemotherapeutic agents in history. Since its discovery, chloroquine has proved to be a highly effective, safe, and well-tolerated drug for the treatment and prophylaxis of malaria. However, the emergence of chloroquine-resistant strains of the malarial parasite has underlined the requirement for a synthetic alternative to chloroquine. This review describes structure-activity relationships for the 4-aminoquinolines, along with views on the mechanism of action and parasite resistance. A description of drug metabolism and toxicity also is included, with a brief description of potential approaches to the design of new synthetic derivatives.

Thermodynamic factors controlling the interaction of quinoline antimalarial drugs with ferriprotoporphyrin IX

The interaction of a variety of quinoline antimalarial drugs as well as other quinoline derivatives with strictly monomeric ferriprotoporphyrin IX [Fe(III)PPIX] has been investigated in 40% aqueous DMSO solution. At an apparent pH of 7.5 and 25 degrees C, log K values for bonding are 5.52 +/- 0.03 (chloroquine), 5.39 +/- 0.04 (amodiaquine), 4.10 +/- 0.02 (quinine), 4.04 +/- 0.03 (9-epiquinine), and 3.90 +/- 0.08 (mefloquine). Primaquine, 8-hydroxyquinoline, 5-aminoquinoline, 6-aminoquinoline, 8-aminoquinoline, and quinoline exhibit no evidence of interaction with Fe(III)PPIX. The enthalpy and entropy changes for the interaction of quinolines with Fe(III)PPIX, as determined from the temperature dependence of the log K values, exhibit a compensation phenomenon that is suggestive of hydrophobic interaction. This is supported by the finding that the interactions of chloroquine and quinine with Fe(III)PPIX are weakened by increasing concentrations of acetonitrile. Interactions of chloroquine, quinine, and 9-epiquinine with Fe(III)PPIX are shown to remain strong at pH 5.6, the approximate pH of the food vacuole of the malaria parasite which is believed to be the locus of drug activity. Implications for the design of antimalarial drugs are briefly discussed.

Synthesis, antimalarial activity, and molecular modeling of tebuquine analogues

Tebuquine (5) is a 4-aminoquinoline that is significantly more active than amodiaquine (2) and chloroquine (1) both in vitro and in vivo. We have developed a novel more efficient synthetic route to tebuquine analogues which involves the use of a palladium-catalyzed Suzuki reaction to introduce the 4-chlorophenyl moiety into the 4-hydroxyaniline side chain. Using similar methodology, novel synthetic routes to fluorinated (7a, b) and a dehydroxylated (7c) analogue of tebuquine have also been developed. The novel analogues were subjected to testing against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. Tebuquine was the most active compound tested against both strains of Plasmodia. Replacement of the 4-hydroxy function with either fluorine or hydrogen led to a decrease in antimalarial activity. Molecular modeling of the tebuquine analogues alongside amodiaquine and chloroquine reveals that the inter-nitrogen separation in this class of drugs ranges between 9.36 and 9.86 A in their isolated diprotonated form and between 7.52 and 10.21 A in the heme-drug complex. Further modeling studies on the interaction of 4-aminoquinolines with the proposed cellular receptor heme revealed favorable interaction energies for chloroquine, amodiaquine, and tebuquine analogues. Tebuquine, the most potent antimalarial in the series, had the most favorable interaction energy calculated in both the in vacuo and solvent-based simulation studies. Although fluorotebuquine (7a) had a similar interaction energy to tebuquine, this compound had significantly reduced potency when compared with (5). This disparity is possibly the result of the reduced cellular accumulation (CAR) of fluorotebuquine when compared with tebuquine within the parasite. Measurement of the cellular accumulation of the tebuquine analogues and seven related 4-aminoquinolines shows a significant relationship (r = 0.98) between the CAR of 4-aminoquinoline drugs and the reciprocal of drugs IC50.

The interaction of the heme-octapeptide, N-acetylmicroperoxidase-8 with antimalarial drugs: solution studies and modeling by molecular mechanics methods

The equilibrium constants for the coordination of quinine (log K = 2.55 +/- 0.02) and 9-epiquinine (log K = 2.42 +/- 0.05) to form a 1:1 complex with the monomeric ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-9 have been determined in aqueous solution (pH 6.25, 0.1 M phosphate buffer) at 25 degrees C. The electronic spectra of the complexes suggest that coordination to Fe(III) occurs through the 9-alkoxide group, in agreement with previous NMR work on the interaction of quinine with urohaemin-I [I. Constantinidis and J. D. Satterlee, J. Amer. Chem. Soc. 110, 927 (1988)]. By contrast, two chloroquine molecules bind sequentially to N-acetylmicroperoxidase-8 under the same conditions (log K1 = 3.08 +/- 0.04; log K2 = 1.56 +/- 0.10) in an apparently pi- pi cofacial manner. Molecular mechanics techniques have been applied to a study of the structure of these species with an Fe(III)-porphine nucleus, and show that coordination of the 9-alkoxide group to Fe(III) is possible for both quinine and 9-epiquine, with the quinoline ring system virtually parallel to the porphyrin plane. The energy-minimized structure for the interaction of a single chloroquine molecule with Fe(III)-porphine has the quinoline rings parallel to the periphery of the porphyrin ring.

Amodiaquine accumulation in Plasmodium falciparum as a possible explanation for its superior antimalarial activity over chloroquine

Amodiaquine is a 4-aminoquinoline antimalarial whose structure is similar to chloroquine. In contrast to the wealth of information available about chloroquine accumulation and its relationship to activity, little is known about the uptake characteristics of amodiaquine, a drug that is inherently more active against malaria parasites. In this study we have investigated the accumulation of amodiaquine in Plasmodium falciparum in vitro, in order to gain an insight into the mechanisms responsible for its superior activity over chloroquine. The driving force for parasite accumulation of the 4-aminoquinolines is proposed to be a transmembrane proton gradient maintained by a vacuolar ATPase. In the present study, amodiaquine accumulation was greatly reduced, at steady state, in the absence of glucose and at 0 degrees C indicating a clear energy dependence of uptake. Amodiaquine accumulation in Plasmodium falciparum was shown to be 2- to 3-fold greater than chloroquine accumulation. This observation probably accounts for amodiaquine's greater inherent activity but is surprising given that amodiaquine is a weaker base than chloroquine. With this in mind we present evidence for an intraparasitic binding component in the accumulation of the 4-aminoquinolines. Differences in binding affinity of this 'receptor' for amodiaquine and chloroquine may partially explain the greater accumulation and in vitro potency of amodiaquine compared to chloroquine.

The iron environment in heme and heme-antimalarial complexes of pharmacological interest

Mössbauer spectroscopy has been utilized to probe the electronic environment of iron in a number of Ferriprotoporphyrin IX complexes of relevance to malaria. The markedly different iron environments found for the complexes of hemin with quinine, chloroquine, and the Chinese herbal antimalarial artesunate suggest that these compounds act by protecting the heme from polymerization to insoluble hemozoin, and by facilitating the transport of the protected heme to the food vacuole membrane where it is able to exercise its cytotoxic redox catalytic activity. Mössbauer parameters determined here for purified malaria pigment and synthetic beta-hematin confirm the chemical identical-ness of these species. The Mössbauer spectra of the complexes are discussed in light of the proposed structures of the complexes.

Mode of antimalarial effect of methylene blue and some of its analogues on Plasmodium falciparum in culture and their inhibition of P. vinckei petteri and P. yoelii nigeriensis in vivo

The antimalarial action of methylene blue (MB) was first noted by Paul Ehrlich in the late 19th century. Although it has only sporadically been adopted as a serviceable drug, the resolution of its antimalarial action seems warranted, as it is currently used for the treatment of various methemoglobinemias. In this work we have used MB, and its analogues Azures A (AZA), B (AZB), C (AZC), and thionin (TH), as well as the oxazine Celestine blue (CB) and azine Phenosaphranin (PS). All MB analogues inhibit the growth of various strains of Plasmodium falciparum in culture with IC50s in the 2 x 10(-9)-1 x 10(-7) M range, with the rank order MB approximately AZA > AZB > AZC > TH > PS > CB. The IC50s for a mammalian cell line were in the 3 x 10(-6)-4 x 10(-5) M range, and the rank order was TH approximately AZB > AZA approximately PS > AZC approximately CB > MB. As MB could affect cell growth through the oxidation of NADPH, we tested the action of the various compounds on the hexose-monophosphate shunt activity. Appreciable activation of the shunt was observed at 1 x 10(-5) M in both cell types, thus accounting for inhibition of growth of mammalian cells but not of parasites. All compounds were found to complex with heme in a rank order similar to their antimalarial effect. It is therefore suggested that MB and its congeners act by preventing the polymerization of heme, which is produced during the digestion of host cell cytosol in the parasite food vacuole, into hemozoin. In this respect, these compounds seem to act similarly to the 4-aminoquinoline antimalarials. All compounds effectively suppressed the growth of P. vinckei petteri in vivo with IC50 in the 1.2-5.2 mg/kg range, and MB and AZB suppressed P. yoelii nigeriensis in the 9-11 mg/kg range (i.e. at doses similar to those of chloroquine). The potential toxicity of these compounds may restrict their clinical use, but their impressive antimalarial activities suggest that the phenothiazine structure could serve as a lead compound for further drug development.

Quinoline anti-malarial drugs inhibit spontaneous formation of beta-haematin (malaria pigment)

Polymerisation of haematin to beta-haematin (haemozoin or malaria pigment) in acidic acetate solutions was studied using infrared spectroscopy. The reaction was found to occur spontaneously between 6 and 65 degrees C, in 0.1-4.5 M acetate and pH 4.2-5.0. The anti-malarial drugs quinine, chloroquine and amodiaquin were found to block spontaneous beta-haematin formation, while the anti-malarially inactive 9-epiquinine and 8-hydroxyquinoline had no effect on the reaction, as did primaquine, a drug which is active only against exo-erythrocytic stages of infection. It is argued that the intra-erythrocytically active anti-malarial agents act by binding to haematin, blocking beta-haematin formation and leaving toxic haematin in the parasite food vacuoles.