Exploring the antimalarial mechanism of action: Adsorption of diverse inhibitors to crystalline malaria pigment (haemozoin).

Muller, Ronel (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The crystallization of free haem (ferriprotoporphyrin IX) into haemozoin within the parasitic food vacuole is a vital detoxification process in the life cycle of the malaria parasite. Quinoline antimalarial drugs achieve their mechanism of action by adsorbing onto the fastest growing face of haemozoin, which brings about inhibition of further crystal growth. Therefore, an in-depth study of the adsorption of quinoline antimalarial drugs and structurally related non-quinoline compounds to β-haematin (synthetic haemozoin) was undertaken in order to assess this hypothesis. Adsorption of clinically-relevant quinoline drugs onto both the fastest and the second fastest growth faces of β-haematin was evaluated using molecular mechanics simulations. The adsorption geometries obtained were consistent with those previously reported in literature. When applying a multiple correlation that takes into account both the percentage speciation at pH 4.8 (pH in the food vacuole), as well as the adsorption onto both the fastest growing crystal faces, a significant linear correlation was found when comparing the calculated adsorption energies (Eads (kcal.mol-1)) and the determined β-haematin growth inhibition activities (Lipid BHIC50), where an R2 value of 0.78 and a P value of 0.004 are found. This observation greatly supported the above mentioned hypothesis. The study was further extended to evaluate the adsorption of a set of non-quinoline compounds onto the two fastest growing crystal faces. A new adsorption geometry was observed where the inhibitors were able to adsorb over two adjacent unit cells, thereby forming π-π interactions to two neighbouring porphyrin structures. This is the first time that this adsorption geometry has been observed. Therefore, following the successful synthesis of a small set of quinoline inhibitors, it was demonstrated that an improved adsorption and a subsequent enhanced β-haematin growth inhibition activity is dependent on the formation of a larger number of intermolecular interactions between the inhibitor and the crystal surface. Finally, by combining all compounds investigated (25 in total) and thereby evaluating a more comprehensive data set, an exponential relationship is found when comparing the calculated adsorption energy (Eads (kcal.mol-1)) and the determined β-haematin growth inhibition activity (NP40 BHIC50), when applying the multiple correlation equation, and upon considering both the percentage speciation at pH 4.8 as well as the adsorption onto both the fastest growing crystal faces. An R2 value of 0.73 with a P value of <0.0001 are obtained. These results add merit to the argument that antimalarial drugs adsorb onto β-haematin to inhibit further crystal growth. Moreover, by determining the necessary intermolecular interactions required for improved adsorption onto these crystal faces, this information could be applied in future drug development to rationally design novel and more active antimalarial drugs.

AFRIKAANSE OPSOMMING: Die kristallisering van vrye heem (ysterprotoporfirien IX) na hemozoin binne in die parasitiese voedsel vakuool, is ‘n belangrikke ontgiftigings proses in die lewenssiklus van die malaria parasiet. Kinolien anti-malaria geneesmiddelle verrig hul meganisme van aksie deur op die vinnigste groeiende kristalvlak van hemozoin te adsorbeer, wat dan lei tot die inhibisie van verdere kristal groei. Daarom, was ‘n in-diepte studie van die adsorpsie van kinolien en strukturele verwante nie-kinolien verbindings op β-hematien (sintetiese hemozoin) uitegevoer, om dus hierdie hipotese te ondersoek. Adsorpsie van klinies-relevante kinolien geneesmiddele op beide die vinnigste en die tweede vinnigste groei vlakke van β-hematien was geëvalueer deur gebruik te maak van molekulêre meganiese simulasies. Die adsorpsie geometrieë wat verkry was, is oorenstemmend met dié wat voorheen in die literatuur gepubliseer is. Met die toepassing van ‘n meervoudige korrelasie wat beide die persentasie spesiasie by ‘n pH van 4.8 (pH in die voedsel vakuool) sowel as die adsorpsie op beide die twee vinigste groeiende kristalvlakke in ag neem, was ‘n beduidende liniêre verband tussen die berekende adsorpsie energieë (Eads (kcal.mol-1)) en die bepaalde β-hematien groei inhibisie aktiwiteite (Lipied BHIC50) gevind, met ‘n R2 waarde van 0.78 en ‘n P waarde van 0.004. Hierdie observasie ondersteun dus die bogenoemde hipotese. Die studie was verder uitgebrei om die adsorpsie van ‘n stel nie-kinolien verbindings op die twee vinnigste groeiende kristalvlakke te ondersoek. ’n Nuwe adsorpsie geometrie was waargeneem, waar die verbindings oor twee aanligende seleenhede adsorbeer, en waar die vorming van π-π interaksies na die twee aanliggende porfirien strukture bevorder word. Dit is die eerste keer dat hierdie adsorpsie geometrie waargeneem was. Daarom, gevolg deur die sintese van ‘n klein stel kinolien verbindings, was dit gedemonstreer dat verbeterde adsorpsie en dus ook ‘n verbeterde β-hematien inhibisie aktiwiteit afhanklik is van die vorming van ‘n vermeerderde aantal intermolekulêre interaksies tussen die verbinding en die kristaloppervlakte. As ‘n finale bydrae, deur al die verbindings wat ondersoek was saam te evalueer (25 in totaal) en gevolglik ‘n meer voledige data stel te ondersoek, was ‘n eksponensiële verhouding verkry wanneer die berekende adsorpsie energieë (Eads (kcal.mol-1)) en die bepaalde β-hematien groei inhibisie aktiwiteite (NP40 BHIC50) vergelyk was deur die meervoudige korrelasie toe te pas, en sodoende ook die persentasie spesiasie by ‘n pH van 4.8 en die adsorpsie op beide die vinnigste groeiende kristalvlakke in ag te neem. ‘n R2 waarde van 0.73 met ‘n P waarde van <0.0001 was verkry. Hierdie resultate voeg meriete tot die argument dat anti-malaria geneesmiddelle adsorbeer op β-hematien wat lei tot die inhibisie van verdere kristal groei. Ook, deur die belangrikke intermolekulêre interaksies wat noodsaak is vir ‘n verbeterde adsorpsie te identifiseer, kan hierdie inliging toegepas word in verdere geneesmiddel ontwikkeling, waar nuwe anti-malaria geneesmiddelle met verbeterde aktiwiteit rasioneel ontwerp kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101372
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