In vitro activity of fluoroquinolones against chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum.

The in vitro activity of three fluoroquinolones--ciprofloxacin, norfloxacin and ofloxacin--was studied on four laboratory-adapted strains (one chloroquine-resistant) and one fresh isolate of P. falciparum from Delhi by the schizont maturation inhibition microtest. The IC50 concentrations (mean +/- SD) were found to be as: ciprofloxacin 6.38 +/- 1.34 micrograms/ml, norfloxacin 11.24 +/- 1.27 micrograms/ml, and ofloxacin 22.3 +/- 3.11 micrograms/ml, while the MIC values were 32 micrograms/ml, 64 micrograms/ml and 128 micrograms/ml for the three drugs in the same order. The IC50 and MIC values for chloroquine-resistant strain were not significantly different from those for the chloroquine-sensitive strains. We conclude that there is little interstrain variability in the in vitro susceptibility of P. falciparum to fluoroquinolones, and that there is no cross resistance between them and chloroquine. The reported variability in clinical response of falciparum malaria to fluoroquinolones is not likely to be due to variation in parasite sensitivity.