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

Computational insights into the inhibition of β-haematin crystallization by antimalarial drugs

During the red blood cell phase of their life cycle, malaria parasites digest their host's haemoglobin, with concomitant release of potentially toxic iron(iii) protoporphyrin IX (FePPIX). The parasites' strategy for detoxification of FePPIX involves its crystallization to haemozoin, such that the build-up of free haem in solution is avoided. Antimalarial drugs of both historical importance and current clinical use are known to be capable of disrupting the growth of crystals of β-haematin, which is the synthetic equivalent of haemozoin. Hence, the disruption of haemozoin crystal growth is implicated as a possible mode of action of such drugs. However, the details of β-haematin crystal poisoning at the molecular level have yet to be fully elucidated. In this study, we have used a combination of density functional theory (DFT) and molecular modelling to examine the possible modes of action of ten different antimalarial drugs, including quinine-type aliphatic alcohols, amodiaquine-type phenols, and chloroquine-type aliphatic diamines. The DFT calculations indicate that each of the drugs can form at least one molecular complex with FePPIX. These complexes have 1 : 1 or 2 : 1 FePPIX : drug stoichiometries and all of them incorporate Fe-O bonds, formed either by direct coordination of a zwitterionic form of the drug, or by deprotonation of water. Most of the drugs can form more than one such complex. We have used the DFT model structures to explore the possible formation of a monolayer of each drug-haem complex on four of the β-haematin crystal faces. In all cases, the drug complexes can form a monolayer on the fast-growing {001} and {011} faces, but not on the slower growing {010} and {100} faces. Additional modelling of the chloroquine and quinidine complexes shows that individual molecules of these species can also obstruct the growth of new layers on other crystal faces. The implications of these observations for antimalarial drug development are discussed.

Identification and SAR Evaluation of Hemozoin-Inhibiting Benzamides Active against Plasmodium falciparum

Quinoline antimalarials target hemozoin formation causing a cytotoxic accumulation of ferriprotoporphyrin IX (Fe(III)PPIX). Well-developed SAR models exist for β-hematin inhibition, parasite activity, and cellular mechanisms for this compound class, but no comparably detailed investigations exist for other hemozoin inhibiting chemotypes. Here, benzamide analogues based on previous HTS hits have been purchased or synthesized. Only derivatives containing an electron deficient aromatic ring and capable of adopting flat conformations, optimal for π-π interactions with Fe(III)PPIX, inhibited β-hematin formation. The two most potent analogues showed nanomolar parasite activity, with little CQ cross-resistance, low cytotoxicity, and high in vitro microsomal stability. Selected analogues inhibited hemozoin formation in Plasmodium falciparum causing high levels of free heme. In contrast to quinolines, introduction of amine side chains did not lead to benzamide accumulation in the parasite. These data reveal complex relationships between heme binding, free heme levels, cellular accumulation, and in vitro activity of potential novel antimalarials.

Alkoxide coordination of iron(III) protoporphyrin IX by antimalarial quinoline methanols: a key interaction observed in the solid-state and solution

The quinoline methanol antimalarial drug mefloquine is a structural analogue of the Cinchona alkaloids, quinine and quinidine. We have elucidated the single crystal X-ray diffraction structure of the complexes formed between racemic erythro mefloquine and ferriprotoporphyrin IX (Fe(iii)PPIX) and show that alkoxide coordination is a key interaction in the solid-state. Mass spectrometry confirms the existence of coordination complexes of quinine, quinidine and mefloquine to Fe(iii)PPIX in acetonitrile. The length of the iron(iii)-O bond in the quinine and quinidine complexes as determined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy unequivocally confirms that coordination of the quinoline methanol compounds to Fe(iii)PPIX occurs in non-aqueous aprotic solution via their benzylic alkoxide functional group. UV-visible spectrophotometric titrations of the low-spin bis-pyridyl-Fe(iii)PPIX complex with each of the quinoline methanol compounds results in the displacement of a single pyridine molecule and subsequent formation of a six-coordinate pyridine-Fe(iii)PPIX-drug complex. We propose that formation of the drug-Fe(iii)PPIX coordination complexes is favoured in a non-aqueous environment, such as that found in lipid bodies or membranes in the malaria parasite, and that their existence may contribute to the mechanism of haemozoin inhibition or other toxicity effects that lead ultimately to parasite death. In either case, coordination is a key interaction to be considered in the design of novel antimalarial drug candidates.

Effects of the angiotensin II Ala-scan analogs in erythrocytic cycle of Plasmodium falciparum (in vitro) and Plasmodium gallinaceum (ex vivo)

The anti-plasmodium activity of angiotensin II and its analogs have been described in different plasmodium species. Here we synthesized angiotensin II Ala-scan analogs to verify peptide-parasite invasion preservation with residue replacements. The analogs were synthesized by 9-fluorenylmethoxycarbonyl (Fmoc) and tert-butyloxycarbonyl (t-Boc) solid phase methods, purified by liquid chromatography and characterized by mass spectrometry. The results obtained in Plasmodium falciparum assays indicated that all analogs presented some influence in parasite invasion, except [Ala(4)]-Ang II (18% of anti-plasmodium activity) that was not statistically different from control. Although [Ala(8)]-Ang II presented a lower biological activity (20%), it was statistically different from control. The most relevant finding was that [Ala(5)]-Ang II preserved activity (45%) relative to Ang II (47%). In the results of Plasmodium gallinaceum assays all analogs were not statistically different from control, except [Ala(6)]-Ang II, which was able to reduce the parasitemia about 49%. This approach provides insight for understanding the importance of each amino acid on the native Ang II sequence and provides a new direction for the design of potential chemotherapeutic agents without pressor activity.

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.

A computational study of ligand binding affinities in iron(iii) porphine and protoporphyrin IX complexes

In the context of antimalarial drug development, density functional theory has been used to model the interactions between a diverse set of 31 small ligands and the iron(iii) centre of ferriprotoporphyrin IX, as well as key events in the crystallization of this molecule by the malaria parasite.

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.

Differences in anti-malarial activity of 4-aminoalcohol quinoline enantiomers and investigation of the presumed underlying mechanism of action

Background

A better anti-malarial efficiency and lower neurotoxicity have been reported for mefloquine (MQ) (+)- enantiomer. However, the importance of stereoselectivity remains poorly understood as the anti-malarial activity of pure enantiomer MQ analogues has never been described. Building on these observations, a series of enantiopure 4-aminoalcohol quinoline derivatives has previously been synthesized to optimize the efficiency and reduce possible adverse effects. Their in vitro activity on Plasmodium falciparum W2 and 3D7 strains is reported here along with their inhibition of β-haematin formation and peroxidative degradation of haemin, two possible mechanisms of action of anti-malarial drugs.

Results

The (S)-enantiomers of this series of 4-aminoalcohol quinoline derivatives were found to be at least as effective as both chloroquine (CQ) and MQ. The derivative with a 5-carbon side-chain length was the more efficient on both P. falciparum strains. (R )-enantiomers displayed an activity decreased by 2 to 15-fold as compared to their (S) counterparts. The inhibition of β-haematin formation was significantly stronger with all tested compounds than with MQ, irrespective of the stereochemistry. Similarly, the inhibition of haemin peroxidation was significantly higher for both (S) and (R)-enantiomers of derivatives with a side-chain length of five or six carbons than for MQ and CQ.

Conclusions

The prominence of stereochemistry in the anti-malarial activity of 4-aminoalcohol quinoline derivatives is confirmed. The inhibition of β-haematin formation and haemin peroxidation can be put forward as presumed mechanisms of action but do not account for the stereoselectivity of action witnessed in vitro.