New efficacy testing systems towards novel antimalarials

Responsible for more than half a million deaths globally, malaria continues to be a global health crisis. In the face of diminishing efficacy of insecticides and artemisinin-based combination therapies, new antimalarials are urgently needed. Efficacy of candidate drugs is routinely being assessed in preclinical and clinical studies. Thereby, it is of utmost importance to distinguish viable and nonviable parasites, which is complicated by the fact that viable parasites can be metabolically inactive, whilst dying parasites can still be metabolically active and morphologically unaffected. In this Ph. D. thesis, I highlight that – in contrast to conventional readouts for the assessment of drug efficacy – parasite viability readouts can discriminate between these parasite populations. I have developed three efficacy testing systems that allow to measure parasite viability after treatment of in vitro cultures, in mice, and in humans with high sensitivity.
The first method is the in vitro parasite reduction ratio (PRR) assay V2, which is based on the limiting-dilution assay published by Sanz et al. (2012). The PRR assay V2 comes with a shorter assay duration, optimized quality controls, and a standardized analysis algorithm that systematically estimates pharmacodynamic parameters, which can be deployed by pharmaco-kinetic/pharmacodynamic models to aid and standardize lead selection, optimization, and dose prediction. The second method, the in vitro-in silico interaction-PRR assay, allows to assess parasite viability following treatment with a range of drug combinations, and uses the resulting data to predict therapeutic efficacy of that drug combination in humans in a PK/PD model. The method was validated on two drug combinations for which clinical data exist, showing that the interaction-PRR assay can successfully predict the endpoints of the corresponding clinical trials. This illustrates that the interaction-PRR has the potential to improve and streamline the development of new drug combinations for malaria. The third method allows to study parasite viability, i.e. drug efficacy, in samples collected from mice and humans treated with antimalarials. This method was validated on the basis of artesunate and using two independent approaches to estimate parasite viability. This method allowed us to observe drug effects that had not been detected when measuring overall parasitemia in the blood of mice or humans, and hence helps to better translate preclinical data from murine infections to those observed in human infections in the clinical stage.
I further investigated the antimalarial efficacy of Artemisia afra, a herbal remedy traditionally used in Africa, in mice to contribute to the resolution of the debate on whether a phytochemical (or several) present in the plant require(s) metabolic activation inside the mammalian host to unfold its antimalarial potential. In Plasmodium berghei-infected mice, suspensions of A. afra were not active; future experiments in P. falciparum-infected mice are pending.
Overall, this work improves the development of new and better antimalarials by providing new and highly accurate efficacy testing systems, and adds another piece of knowledge to the debate on whether A. afra is a potential source of such antimalarials or not.