Interactions of quinoline antimalarial drugs with ferrihaem : structural and kinetic insights into the inhibition of malaria pigment formation

Gildenhuys, Johandie (2013-12)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: The work in this dissertation provides structural and kinetic insight into the mechanism of action of quinoline antimalarial drugs which may aid rational drug design. Quinoline antimalarial drug-ferrihaem (Fe(III)PPIX) complexes were investigated. Single crystal Xray diffraction (SCD) structures of the complexes formed between Fe(III)PPIX and the quinoline methanol antimalarials quinine, quinidine and mefloquine have been determined, and are the first observed structures of complexes formed between free Fe(III)PPIX and quinoline antimalarial compounds. Quinine, quinidine and mefloquine are shown to have a three-point binding mode to Fe(III)PPIX, which comprises direct coordination of the drug to the Fe(III) centre through its benzylic alcohol functionality, π-stacking between the drug and porphyrin aromatic systems, and intramolecular hydrogen bond formation between the porphyrin propionate group and the protonated quinuclidine nitrogen atom of the drug in the case of quinine and quinidine, and formation of an intramolecular hydrogen bonding network in the case of mefloquine. Extended X-ray absorption fine structure spectroscopy (EXAFS) was use to elucidate structural information of Fe(III)PPIX-drug complexes in solution, and indicates that coordination persists in solution. The protocol for lipid-mediated formation of β-haematin, where monopalmitic glycerol was used as a model lipid, was successfully modified to incorporate antimalarial drugs into the aqueous layer in order to investigate drug activity under biologically-relevant conditions. Four compounds were chosen, namely chloroquine and amodiaquine, both 4- aminoquinolines and quinine and quinidine. IC50 values for the inhibition of β-haematin formation show good correlation with biological activities determined against a chloroquine-sensitive Plasmodium falciparum strain. The lipid-water interface system was further used to investigate the effects of quinine, quinidine chloroquine and amodiaquine on the kinetics of β-haematin formation. The results led to the development of a kinetic model based on the Avrami equation and the Langmuir isotherm. The data strongly support a mechanism of antimalarial drug action by adsorption to the growing face of haemozoin, with precipitation of Fe(III)PPIX at high drug concentrations accounting for decreased yields. Adsorptions constants (log Kads) determined for each drug show a strong correlation with biological activity. Finally, the first SCD structure of the μ-propionato dimer of Fe(III)PPIX, the structural unit of haemozoin, has been determined as its DMSO solvate. EXAFS suggests that this species is only formed upon nucleation, with the π-π dimer species being favoured in solution.

AFRIKAANSE OPSOMMING: Die werk in die dissertasie verleen struktuur en kinetiese insig in the meganisme van aktiwiteit vir kinolien antimalariamiddels wat kan bydra tot die ontwikkeling van nuwe medisyne. Kinolien antimalariamiddel-ferriheem (Fe(III)PPIX) komplekse was ondersoek. Navorsing is gedoen op die enkelkristal X-straaldiffraksie strukture van die komplekse gevorm tussen Fe(III)PPIX en die kinolien metanol antimalaria middels kinien, kinidien en mefloquine. Die strukture is die eerste komplekse wat waargeneem is tussen vrye Fe(III)PPIX en kinolien antimalariamiddels. Kinien, kinidien en mefloquine het ʼn driepunt bindingsvorm, direkte koördinasie met die Fe(III) deur die bensielalkohol groep, ʼn π- stapel tussen die middel en die porfirien aromatiese sisteem, ʼn intramolekulêre waterstofbinding tussen the porfirienpropionaat funksie en die geprotoneerde kinuklidien stikstofatoom (kinien en kinidien) en ʼn netwerk van intramolekulêre waterstof bindings (mefloquine) insluit. Uitgebreide X-straal absorpsie fyn struktuur spektroskopie (EXAFS) is gebruik om inligting oor Fe(III)PPIX-middel komplekse in oplossing te verkry en het aangedui dat die koördinasie in oplossing voorkom. Deur gebruik te maak van monopalmitiengliserol as die lipid in the lipid-water interfase sisteem, waar antimalariamiddels suksesvol in die buffer geïnkorporeer was, was die middel se aktiwiteit onder biologiese kondisies geondersoek. Vier middels was gekies naamlik, chloroquine en amodiaquine, albei 4-aminokinoliene en kinien en kinidien om die IC50-waarde vir inhibisie van β-hematien vorming te bepaal. Die IC50 waardes het ʼn goeie korrelasie met biologiese aktiwiteite teen die chloroquine-sensitiewe Plasmodium falciparum stam gewys. Die lipid-water interfase-sisteem was ook gebruik om die effek van kinien, kinidien, chloroquine en amodiaquine op die kineties effek op die vorming van β-hematien te ondersoek. Die resultate het gelei to die ontwikkeling van die kinetiese model gebaseer op die Avrami vergelyking en die Langmuir isoterm. Die data ondersteun ʼn meganisme van middel aksie waar die middel teen die groeiende vlak van hemosoïen kristal adsorbeer. Die neerslag van Fe(III)PPIX wat vorm by hoë konsentrasies, het gelei tot laer opbrengste. Die adsorpsiekonstante (log Kads) bepaal vir elke middel, het goeie korrelasie met biologiese aktiwiteit getoon. Enkelkristal X-straaldiffraksie strukture van μ- propionatodimeer van Fe(III)PPIX, die struktuur eenheid van hemosoïen, was bepaal as ʼn DMSO solvaat. EXAFS het aangedui dat die spesie slegs by kernvorming ontstaan en dat die π-π dimeerspesie in oplossing voorkom.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/85621
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