Cinoxacin: in vitro antibacterial studies of a new synthetic organic acid

Cinoxacin (compound 64716) is a synthetic organic acid with antibacterial activity against most aerobic gram-negative bacilli. Minimal inhibitory concentrations of cinoxacin (agar-dilution method) were determined for 419 strains. Escherichia coli was the most susceptible group of organisms. The majority of Klebsiella sp., Enterobacter sp., Proteus sp., and Serratia marcescens were inhibited by 8 mug of cinoxacin per ml. Pseudomonas aeruginosa and all gram-positive isolates tested were resistant to 64 mug or less of cinoxacin per ml. Zones of inhibition using a 30-mug disk correlated well with agar-dilution minimal inhibitory concentrations (r = -0.9). Cinoxacin was bactericidal when tested with inocula of 5 x 10(6) organisms per ml. Resistance to cinoxacin was readily developed in all three strains tested by serial passage on drug-containing agar. The in vitro properties of this agent were similar to those of nalidixic acid.