Abstract

We report two improved assays for in vitro and in vivo screening of chemicals with potential anti-malarial activity against the blood stages of the rodent malaria parasite Plasmodiumberghei. These assays are based on the determination of luciferase activity (luminescence) in small blood samples containing transgenic blood stage parasites that express luciferase under the control of a promoter that is either schizont-specific (ama-1) or constitutive (eef1alphaa). Assay 1, the in vitro drug luminescence (ITDL) assay, measured the success of schizont maturation in the presence of candidate drugs quantifying luciferase activity in mature schizonts only (ama-1 promoter). The ITDL assay generated drug-inhibition curves and EC(50) values comparable to those obtained with standard in vitro drug-susceptibility assays. The second assay, the in vivo drug-luminescence (IVDL) assay, measured parasite growth in vivo in a standard 4-day suppressive drug test, monitored by measuring the constitutive luciferase activity of circulating parasites (eef1alphaa promoter). The IVDL assay generates growth-curves that are identical to those obtained by manual counting of parasites in Giemsa-stained smears. The reading of luminescence assays is rapid, requires a minimal number of handling steps and no experience with parasite morphology or handling fluorescence-activated cell sorters, produces no radioactive waste and test-plates can be stored for prolonged periods before processing. Both tests are suitable for use in larger-scale in vitro and in vivo screening of drugs. The standard methodology of anti-malarial drug screening and validation, which includes testing in rodent models of malaria, can be improved by the incorporation of such assays.