Structural Specificity of Chloroquine−Hematin Binding Related to Inhibition of Hematin Polymerization and Parasite Growth

Abstract

Considerable data now support the hypothesis that chloroquine (CQ)-hematin binding in the parasite food vacuole leads to inhibition of hematin polymerization and parasite death by hematin poisoning. To better understand the structural specificity of CQ-hematin binding, 13 CQ analogues were chosen and their hematin binding affinity, inhibition of hematin polymerization, and inhibition of parasite growth were measured. As determined by isothermal titration calorimetry (ITC), the stoichiometry data and exothermic binding enthalpies indicated that, like CQ, these analogues bind to two or more hematin mu-oxo dimers in a cofacial pi-pi sandwich-type complex. Association constants (K(a)'s) ranged from 0.46 to 2.9 x 10(5) M(-1) compared to 4.0 x 10(5) M(-1) for CQ. Remarkably, we were not able to measure any significant interaction between hematin mu-oxo dimer and 11, the 6-chloro analogue of CQ. This result indicates that the 7-chloro substituent in CQ is a critical structural determinant in its binding affinity to hematin mu-oxo dimer. Molecular modeling experiments reinforce the view that the enthalpically favorable pi-pi interaction observed in the CQ-hematin mu-oxo dimer complex derives from a favorable alignment of the out-of-plane pi-electron density in CQ and hematin mu-oxo dimer at the points of intermolecular contact. For 4-aminoquinolines related to CQ, our data suggest that electron-withdrawing functional groups at the 7-position of the quinoline ring are required for activity against both hematin polymerization and parasite growth and that chlorine substitution at position 7 is optimal. Our results also confirm that the CQ diaminoalkyl side chain, especially the aliphatic tertiary nitrogen atom, is an important structural determinant in CQ drug resistance. For CQ analogues 1-13, the lack of correlation between K(a) and hematin polymerization IC(50) values suggests that other properties of the CQ-hematin mu-oxo dimer complex, rather than its association constant alone, play a role in the inhibition of hematin polymerization. However, there was a modest correlation between inhibition of hematin polymerization and inhibition of parasite growth when hematin polymerization IC(50) values were normalized for hematin mu-oxo dimer binding affinities, adding further evidence that antimalarial 4-aminoquinolines act by this mechanism.

Novel cinnamic acid/4-aminoquinoline conjugates bearing non-proteinogenic amino acids: Towards the development of potential dual action antimalarials

“…HEterocyclic-DIpeptide-CINnamic acid conjugates (HEDICINs) were designed to link, through a suitable spacer, (i) the CQ heterocyclic core, known as relevant to inhibit hemozoin formation [12,13], to a (ii) trans-cinnamic acid motif, as cinnamic acids have been described to exhibit both antimalarial activity [14] and inhibiting enzyme catalytic Cys residues [15]. Cinnamic acid derivatives, due to their a,b-unsaturated carbonyl moiety, can act as Michael acceptors and inhibit cysteine proteases through S-alkylation [9,16,17].…”

Does chloroquine really act through oxidative stress?

“…1B). This theory also explains the importance of chlorine in position 7 and the fact that the antimalarial activity is lost when the nitrogen atom (N 4 ) is alkylated [3,20]. Investigations to further support this hypothesis are underway.…”

Design, Synthesis and Evaluation of New Fluoroamodiaquine Analogues

“…Experimental and theoretical data indicates that the 4-aminoquinolines and the heme group interact mainly by π-stacking interactions between the quinolone and the porphyrin ring, and by a charge-assisted hydrogen bond interaction between a donor group in the 4-aminoquinolines and the heme carboxylate group. 21,[25][26][27][28] Moreover, Menezes et al 26) carried out a docking study between a series of pyrazolo-pyridine isosteres of mefloquine (4-methanol-quinolines) and the heme group, and the authors observed that the distance between the quinoline ligand moiety and the heme porphyrin rings was shorter for the most active compounds, correlating the π-stacking interaction with the inhibitory profile. They suggested an optimal distance value of approximately 3.0-4.0 Å.…”