Tolerance of Gambian Plasmodium falciparum to Dihydroartemisinin and Lumefantrine Detected by Parasite Survival Rate Assay.

Monitoring of sensitivity to antimalarial drugs in Africa is vital for malaria elimination. However, the commonly used / 50% inhibitory concentration (IC) test gives inconsistent results for several antimalarials, while the alternative ring-stage survival assay (RSA) for artemisinin derivatives has not been widely adopted. Here, we applied an alternative two-color flow cytometry-based parasite survival rate assay (PSRA) to detect antimalarial tolerance in isolates from The Gambia. The PSRA infers parasite viability by quantifying reinvasion of uninfected cells following 3 consecutive days of drug exposure (10-fold the IC of drug for field isolates). The drug survival rate is obtained for each isolate from the slope of the growth/death curve. We obtained parasite survival rates of 41 isolates for dihydroartemisinin (DHA) and lumefantrine (LUM) out of 51 infections tested by ring-stage survival assay (RSA) against DHA. We also determined the genotypes for known drug resistance genetic loci in the genes , , , , and The PSRA results determined for 41 Gambian isolates showed faster killing and lower variance after treatment with DHA than after treatment with LUM, despite a strong correlation between the two drugs. Four and three isolates were tolerant to DHA and LUM, respectively, with continuous growth during drug exposure. Isolates with the PfMDR1-Y184F mutant variant showed increased LUM survival, though the results were not statistically significant. Sulfadoxine/pyrimethamine (SP) resistance markers were fixed, while all other antimalarial variants were prevalent in more than 50% of the population. The PSRA detected antimalarial tolerance in Gambian This calls for its wider application and for increased vigilance against resistance to artemisinin combination therapies (ACTs) in this population.