Fast in vitro methods to determine the speed of action and the stage-specificity of anti-malarials in Plasmodium falciparum.
BACKGROUND
Recent whole cell in vitro screening campaigns identified thousands of compounds that are active against asexual blood stages of Plasmodium falciparum at submicromolar concentrations. These hits have been made available to the public, providing many novel chemical starting points for anti-malarial drug discovery programmes. Knowing which of these hits are fast-acting compounds is of great interest. Firstly, a fast action will ensure rapid relief of symptoms for the patient. Secondly, by rapidly reducing the parasitaemia, this could minimize the occurrence of mutations leading to new drug resistance mechanisms.An in vitro assay that provides information about the speed of action of test compounds has been developed by researchers at GlaxoSmithKline (GSK) in Spain. This assay also provides an in vitro measure for the ratio between parasitaemia at the onset of drug treatment and after one intra-erythrocytic cycle (parasite reduction ratio, PRR). Both parameters are needed to determine in vitro killing rates of anti-malarial compounds. A drawback of the killing rate assay is that it takes a month to obtain first results.


METHODS
The approach described in the present study is focused only on the speed of action of anti-malarials. This has the advantage that initial results can be achieved within 4-7 working days, which helps to distinguish between fast and slow-acting compounds relatively quickly. It is expected that this new assay can be used as a filter in the early drug discovery phase, which will reduce the number of compounds progressing to secondary, more time-consuming assays like the killing rate assay.


RESULTS
The speed of action of a selection of seven anti-malarial compounds was measured with two independent experimental procedures using modifications of the standard [3H]hypoxanthine incorporation assay. Depending on the outcome of both assays, the tested compounds were classified as either fast or non-fast-acting.


CONCLUSIONS
The results obtained for the anti-malarials chloroquine, artesunate, atovaquone, and pyrimethamine are consistent with previous observations, suggesting the methodology is a valid way to rapidly identify fast-acting anti-malarial compounds. Another advantage of the approach is its ability to discriminate between static or cidal compound effects.
Claire
C
Le Manach
Christian
C
Scheurer
Sibylle
S
Sax
Sarah
S
Schleiferböck
Diego Gonzalez
DG
Cabrera
Yassir
Y
Younis
Tanya
T
Paquet
Leslie
L
Street
Peter
P
Smith
Xavier C
XC
Ding
David
D
Waterson
Michael J
MJ
Witty
Didier
D
Leroy
Kelly
K
Chibale
Parasite Chemotherapy Unit, Swiss Tropical and Public Health Institute, Basel, Switzerland. sergio.wittlin@unibas.ch.
Sergio
S
Wittlin
D000962:Antimalarials / Q000494:pharmacology*; D004353:Drug Evaluation, Preclinical / Q000379:methods; D020128:Inhibitory Concentration 50; D021261:Parasitic Sensitivity Tests / Q000379:methods; D010963:Plasmodium falciparum / Q000187:drug effects*; D013997:Time Factors
D000962:Antimalarials