Engineering the specificity of antibacterial fluoroquinolones: Benzenesulfonamide modifications at C-7 of ciprofloxacin change its primary target in Streptococcus pneumoniae from topoisomerase IV to gyrase

We have examined the antipneumococcal mechanisms of a series of novel fluor oquinolones that are identical to ciprofloxacin except for the addition of a benzenesulfonylamido group to the C-7 piperazinyl ring. A number of these derivatives displayed enhanced activity against Streptococcus pneumoniae s train 7785, including compound NSFQ-105, bearing a 4-(4-aminophenylsulfonyl )-1-piperazinyl group at C-7. which exhibited an MLC of 0.06 to 0.125 mu g/ ml compared with a ciprofloxacin MIC of 1 mu g/ml, Several complementary ap proaches established that unlike the case for ciprofloxacin (which targets topoisomerase IV), the increased potency of NSFQ-105 was associated with a target preference for gyrase: (i) parC mutants of strain 7785 that were res istant to ciprofloxacin remained susceptible to NSFQ-105, whereas by contra st, mutants bearing a quinolone resistance mutation in gyrA were four- to e ightfold more resistant to NSFQ-105 (MIC of 0.5 mu g/ml) but susceptible to ciprofloxacin; (ii) NSFQ-105 selected first-step gyrA mutants (MICs of 0.5 mu g/ml) encoding Ser-81-to-Phe or -Tyr mutations, whereas ciprofloxacin s elects parC mutants; and (iii) NSFQ-105 was at least eightfold more effecti ve than ciprofloxacin at inhibiting DNA supercoiling by S. pneumoniae gyras e in vitro but was fourfold less active against topoisomerase TV. These dat a show unequivocally that the C-7 substituent determines not only the poten cy but also the target preference of fluoroquinolones. The importance of th e C-7 substituent in drug-enzyme contacts demonstrated here supports one ke y postulate of the Shen model of quinolone action.