Multicenter in vitro evaluation of lomefloxacin (NY-198, SC-47111), including tests against nearly 7,000 bacterial isolates and preliminary recommendations for susceptibility testing.

Lomefloxacin (NY-198 or SC-47111) is a difluoro-quinolone derivative having a C-methyl at the 3-position of the piperazine ring, thus minimizing its metabolic alteration in vivo. In our research, its antimicrobial activity was most similar to that of difloxacin, enoxacin, fleroxacin, and norfloxacin but usually less than that of ciprofloxacin and ofloxacin against most species. Lomefloxacin shared cross-resistance with other 4-quinolones but remained very active against ceftazidime-resistant organisms, including stably derepressed beta-lactamase producing Gram-negative bacilli. Lower pH increased the lomefloxacin MICs. MBCs were usually identical to the measured MIC, and the lomefloxacin MICs were not significantly increased by high inoculum concentrations. The Enterobacteriaceae were found to have a very low rate of spontaneous mutation to lomefloxacin resistance (10(-8)-10(-9). In vitro tests by 5-micrograms and 10-micrograms lomefloxacin disks and dilution methods were correlated, and the 10-micrograms disk was recommended for clinical trials using a less than or equal to 4 micrograms/ml susceptible breakpoint. The quality assurance guidelines for dilution tests were determined by a multilaboratory study.