Alpha-1-acid glycoprotein inhibits the effect of quinine on the growth of Plasmodium falciparum in vitro

World Antimalarial Resistance Network (WARN) IV: clinical pharmacology

A World Antimalarial Resistance Network (WARN) database has the potential to improve the treatment of malaria, through informing current drug selection and use and providing a prompt warning of when treatment policies need changing. This manuscript outlines the contribution and structure of the clinical pharmacology component of this database. The determinants of treatment response are multi-factorial, but clearly providing adequate blood concentrations is pivotal to curing malaria. The ability of available antimalarial pharmacokinetic data to inform optimal dosing is constrained by the small number of patients studied, with even fewer (if any) studies conducted in the most vulnerable populations. There are even less data relating blood concentration data to the therapeutic response (pharmacodynamics). By pooling all available pharmacokinetic data, while paying careful attention to the analytical methodologies used, the limitations of small (and thus underpowered) individual studies may be overcome and factors that contribute to inter-individual variability in pharmacokinetic parameters defined. Key variables for pharmacokinetic studies are defined in terms of patient (or study subject) characteristics, the formulation and route of administration of the antimalarial studied, the sampling and assay methodology, and the approach taken to data analysis. Better defining these information needs and criteria of acceptability of pharmacokinetic-pharmacodynamic (PK-PD) studies should contribute to improving the quantity, relevance and quality of these studies. A better understanding of the pharmacokinetic properties of antimalarials and a more clear definition of what constitutes "therapeutic drug levels" would allow more precise use of the term "antimalarial resistance", as it would indicate when treatment failure is not caused by intrinsic parasite resistance but is instead the result of inadequate drug levels. The clinical pharmacology component of the WARN database can play a pivotal role in monitoring accurately for true antimalarial drug resistance and promptly correcting sub-optimal dosage regimens to prevent these contributing to the emergence and spread of antimalarial resistance.

In vitro susceptibility of Plasmodium falciparum to quinine: relation to parasite density and drug distribution in culture fractions

We have studied the importance of parasite density (2, 0.2, 0.02 and 0.002%) for the in vitro susceptibility of Plasmodium falciparum (F32 strain) to quinine. Shorter exposures (< or = 48 hours) only briefly inhibited parasites in wells with the highest initial density. Parasites reappeared after 3-5.5 days in wells with intermediate (0.2 and 0.02%) and lowest density (0.002%). Longer exposures (> or = 72 hours), however, inhibited them for much longer periods and parasites did not reappear in most of the wells with the lowest density during the 28 days of follow-up. The mean multiplication rate following reappearance was tenfold per parasite schizogony cycle. The mean elimination rate per schizogony cycle was calculated to be 99.91%. The elimination and multiplication rates were not correlated to initial parasite density. The mean ratio between quinine concentrations in erythrocytes and medium was 3.6 regardless of quinine concentrations and presence of parasites. Mean quinine-free fractions of 36 and 67% were found from total concentrations of 0.33 and 10.4 mumol/l. We conclude that initial parasite density determines the time to reappearance of parasites following quinine exposure while the elimination and multiplication rates are independent of the initial parasite density, and that quinine protein binding is concentration-dependent in vitro and lower than during treatment.