In vitro activity of PD 117596-2, a broad-spectrum difluoroquinolone.

The activity of PD 117596-2, a novel quinolone, was compared to that of other quinolones, ceftazidime, imipenem and gentamicin. PD 117596-2 inhibited most Enterobacteriaceae at concentrations less than 0.25 micrograms/ml, being equal or superior in activity to ciprofloxacin and 2- to 4-fold more active than ofloxacin. It inhibited ceftazidime-resistant Enterobacter spp., Citrobacter spp. and Serratia marcescens. The MIC90 for Pseudomonas aeruginosa, including strains with imipenem MICs of 8 micrograms/ml and gentamicin MICs greater than 16 micrograms/ml, was 0.25 micrograms/ml. PD 117596-2 was more active than ciprofloxacin against Pseudomonas cepacia and Pseudomonas maltophilia, and it inhibited Neisseria gonorrhoeae and Haemophilus influenzae at less than 0.03 micrograms/ml. PD 117596-2 inhibited staphylococci at 0.5 micrograms/ml, being 2-fold superior to other quinolones, and with an MIC of 0.25 micrograms/ml was more active against group A, B, C and G streptococci and Streptococcus pneumoniae. MICs for Bacteroides spp. were 2 micrograms/ml compared to 8-32 micrograms/ml for other agents. The frequency of spontaneous resistance was low (less than 10(-10]. Differences in MICs and MBCs were within one dilution, and there was a minimal effect of inoculum size. Although PD 117596-2 was less active at pH 5.5, MICs were less than 0.5 micrograms/ml.