The characterization of pharmacokinetic properties and evaluation of in vitro drug combination efficacies of novel antimalarial compounds

Doctoral Thesis


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Relief of the global malaria burden relies on the management and application of effective therapies. Unfortunately, the continuous development of resistance to therapies by the deadliest parasite strain, Plasmodium falciparum, has made the treatment and control of malaria much more difficult. Derivatives of the Chinese peroxidic antimalarial drug artemisinin primarily used in first-line combination therapy for treatment of P. falciparum malaria have proved to be highly effective. However, their use also is now compromised by the development of resistance by the parasite to the artemisinin derivative in the drug combination. This event emphasizes the need for ongoing development of new and effective drug combinations. This research aimed to identify efficacious combinations selected from a group of compounds known to induce oxidative stress by redox cycling combined with an artemisinin, which as an oxidant drug also induces oxidative stress but is unable to undergo redox cycling. Combination of the artemisinin with a redox-active compound is expected to both enhance and maintain oxidative stress within the parasite's proliferative environment. These combinations should be used together with a third drug with a completely different mode of action, such as a quinolone. Selected amino artemisinins and redox active phenothiazines, phenoxazines, thiosemicarbazones, and quinolone derivatives were screened for antimalarial activity and mammalian toxicity. These were found to be potently active (11 μM) to Chinese Hamster ovarian (CHO) cells. The compounds are thus highly selective for P. falciparum, as revealed by the selectivity indices (SI) of >270. The in vitro absorption, distribution, metabolism, and elimination (ADME) properties of the compounds were also determined through the application of specific assays. In vivo pharmacokinetic (PK) profiling was also carried out by intravenous and oral administration of the individual compounds to healthy C57BL/6 mice. Biological samples were analysed via liquid chromatography-tandem mass spectrometry (LC-MS/MS) bioanalytical methods, which were validated according to the fit-for purpose recommendations by the FDA. Evaluation of the in vitro and in vivo profiles thereby facilitated the identification of suitable combination candidates. The phenoxazine and phenothiazine derivatives were identified as the best potential redox partners and were each investigated in combination with the amino-artemisinin artemisone through fixed ratio isobole analysis. A substantial synergistic interaction was observed. Overall, the investigation enabled the identification of drug combinations that are potently active in vitro. This synergistic interaction strongly supports the redox cycling rationale for identifying new antimalarial therapies and further suggests that such combinations in chemotherapy may delay the onset of resistance to the new agents. The results strongly encourage further investigation of the in vivo pharmacokinetic and pharmacodynamic (PK/PD) relationships of these combinations in the humanized murine model of P. falciparum