Mutant prevention concentrations of fluoroquinolones for clinical isolatesof Streptococcus pneumoniae

The mutant prevention concentration (MPC) represents a threshold above whic h the selective proliferation of resistant mutants is expected to occur onl y rarely. A provisional MPC (MPCpr) was defined and measured for five fluor oquinolones with clinical isolates of Streptococcus pneumoniae. Based on th eir potential for restricting the selection of resistant mutants, the five fluoroquinolones, in descending order, were found to be moxifloxacin > trov afloxacin > gatifloxacin > grepafloxacin > levofloxacin. For several compou nds, 90% of about 90 clinical isolates that lacked a known resistance mutat ion had a value of MPCpr that was close to or below the serum levels that c ould be attained with a dosing regimen recommended by the manufacturers. Si nce MPCpr overestimates MPC, these data identify moxifloxacin and gatifloxa cin as good candidates for determining whether MPCpr can be used as a guide for choosing and eventually administering fluoroquinolones to significantl y reduce the development of resistance.