In vitro antibacterial activity of AM-715, a new nalidixic acid analog

AM-715 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinolinecarboxylic acid] is a new nalidixic acid analog. AM-715 has a broad spectrum of antibacterial activity against gram-positive and gram-negative bacteria. The antibacterial activity of AM-715 was greater than those of pipemidic acid and nalidixic acid. AM-715 had higher antibacterial activity against Pseudomonas aeruginosa than did gentamicin. Most nalidixic acid-resistant bacteria were susceptible to AM-715, and cross-resistance was not observed between AM-715 and various antibiotics. The minimal concentration of AM-715 required to inhibit the growth of 75% of the total number of clinical isolates was as follows: Escherichia coli, 0.04 μg/ml; Klebsiella pneumoniae, 0.1 μg/ml; Serratia marcescens, 0.88 μg/ml; Enterobacter spp., 0.076 μg/ml; Staphylococcus aureus, 1.10 μg/ml; P. aeruginosa, 0.38 μg/ml; and nalidixic acid-resistant strains of gram-negative bacteria, 0.62 μg/ml. AM-715 at minimal inhibitory concentrations or at slightly higher concentrations had bactericidal activity against various species of bacteria. The effect of inoculum sizes on minimal inhibitory concentrations and minimal bactericidal concentrations of AM-715 against gram-negative bacteria was smaller than on those of pipemidic acid and nalidixic acid. The dose-response curve of AM-715 indicated a steep gradient, and the 50% inhibited doses of AM-715 were 0.014 μg/ml against E. coli ML4707 and 0.21 μg/ml against P. aeruginosa NC-5.