Simple and Inexpensive Fluorescence-Based Technique for High-Throughput Antimalarial Drug Screening

Abstract

Radioisotopic assays involve expense, multistep protocols, equipment, and radioactivity safety requirements which are problematic in high-throughput drug testing. This study reports an alternative, simple, robust, inexpensive, one-step fluorescence assay for use in antimalarial drug screening. Parasite growth is determined by using SYBR Green I, a dye with marked fluorescence enhancement upon contact with Plasmodium DNA. A side-by-side comparison of this fluorescence assay and a standard radioisotopic method was performed by testing known antimalarial agents against Plasmodium falciparum strain D6. Both assay methods were used to determine the effective concentration of drug that resulted in a 50% reduction in the observed counts (EC 50 ) after 48 h of parasite growth in the presence of each drug. The EC 50 s of chloroquine, quinine, mefloquine, artemisinin, and 3,6-bis--(N,N-diethylamino)-amyloxyxanthone were similar or identical by both techniques. The results obtained with this new fluorescence assay suggest that it may be an ideal method for highthroughput antimalarial drug screening.The global scope of malaria and the spread of drug-resistant Plasmodium falciparum make the need for improved therapy undeniable (4). Assessment of both existing drugs and new antimalarials, alone or in combination, requires reliable methods for high-throughput testing. For decades, antimalarial drug effects have been measured in vitro by quantifying parasite uptake of radioactive substrates as a measure of growth and viability in the presence of the test drug (2, 3). [3 H]hypoxanthine is the most widely used radiolabel, but because it requires purine starvation prior to the assay, the [ 3 H]ethanolamine assay is favored by our laboratory. While these methods are accurate and reliable, they rely on relatively expensive radioisotopes and multistep procedures that become increasingly problematic and impractical as the volume of testing increases. We report on the development of an alternative, fluorescencebased in vitro assay for use for the high-throughput screening of the activities of drugs against P. falciparum. The goals for the assay included simplicity, cost savings, robust performance, applicability to automated analysis, and speed.The principle behind the assay is the contrast between host erythrocytes, which lack DNA and RNA, and the malaria parasites, which do not and which are thus readily stained with dyes that show enhanced fluorescence in the presence of nucleic acids (8). Fluorescence-based in vitro antimalarial assays have been developed previously (1, 12) but have required complex, multistep protocols or additional equipment not amenable to rapid, high-throughput use. This report establishes and validates a facile, one-step method that uses the same principle. MATERIALS AND METHODSChloroquine diphosphate, quinine sulfate, artemisinin, and saponin were purchased from Sigma Aldrich Chemical Company (St. Louis, Mo.); mefloquine was obtained from Walter Reed Army Institute for Research (Silver Spring, Md.); and 3,6-b...

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