In vitro drug susceptibility testing with the
malaria parasite has been used to assess the
antimalarial activities of new compounds and to monitor drug resistance in field isolates. We investigated the validity of a
SYBR green I fluorescent-based assay under various culture conditions and compared the assay results to those of previously published
histidine-rich
protein II (HRPII)
enzyme-linked
immunosorbent assay (ELISA) methods. Reference strains of Plasmodium falciparum were cultured in vitro by using standard conditions in complete medium with and without
phenol red before they were dispensed into 96-well plates predosed with
chloroquine,
mefloquine, or
quinine. Following incubation, the culture supernatants were divided and the 50% inhibitory concentrations (IC50s) were determined by using a
SYBR green I-based method and the HRPII capture ELISA method. There were no significant differences in IC50 values when
phenol red was included in the medium. The IC50s and the IC90s of the
antimalarials tested by both methods were similar or identical for each of the reference strains. Fresh clinical isolates of P. falciparum collected from imported cases of
malaria in Lyon, France, were tested for in vitro resistance to
chloroquine and
mefloquine by using the validated
SYBR green I and HRPII ELISA methods. The
SYBR green I-based method was able to calculate IC50 and IC90 values similar or identical to those calculated by the HRPII assay with fresh clinical samples without removal of white blood cells. The
SYBR green I-based method for determination of drug sensitivity levels produced results comparable to those produced by other methods, showing that this method can be used routinely to conduct surveillance for drug resistance in P. falciparum with fresh or cultured parasites.