RO 23-6240 (AM-833), a new fluoroquinolone: in vitro antimicrobial activity and tentative disk diffusion interpretive criteria.

The susceptibility of over 7000 recent clinical bacterial isolates to RO 23-6240, a new trifluorinated quinolone, was determined at four medical centers. Over 99% of Enterobacteriaceae and 97% of staphylococci were inhibited by less than or equal to 2.0 micrograms/ml of RO 23-6240. Only 71% of Pseudomonas spp. were inhibited by this concentration. Streptococci and enterococci were resistant to RO 23-6240. Clinical isolates of Haemophilus spp., pathogenic Neisseria spp., and Branhamella catarrhalis were inhibited by less than or equal to 0.25 micrograms/ml of RO 23-6240. This drug's antibacterial activity was comparable with that of enoxacin and norfloxacin, but was less than that of ciprofloxacin against most species. Using less than or equal to 2.0 micrograms/ml and greater than or equal to 8.0 micrograms/ml as the susceptible and resistant MIC breakpoints for RO 23-6240, the regression analysis-derived disk diffusion zone diameter breakpoints for the 5 micrograms disk are: Susceptible greater than or equal to 19 mm intermediate 16-18 mm, and resistant less than or equal to 15 mm.