Antimicrobial activity and interaction of pefloxacin and its principal metabolites. Collaborative Antimicrobial Susceptibility Testing Group.

The in vitro antimicrobial activity of pefloxacin and its major metabolites was determined and the interaction of pefloxacin and N-demethyl pefloxacin (norfloxacin) assessed at the ratio naturally occurring in urine (1:2). Pefloxacin and N-demethyl pefloxacin had approximately the same spectrum but were markedly more active than N-oxide pefloxacin (MIC90s greater than or equal to 64 micrograms/ml) against 867 stock strains. When combined with N-demethyl pefloxacin, pefloxacin had greater potency and a broader spectrum in tests against 5869 fresh clinical organisms. For approximately 10% more strains pefloxacin MICs were less than or equal to 2 micrograms/ml when pefloxacin was combined with 2 parts (4 micrograms/ml) of N-demethyl pefloxacin. The most significant extension of the pefloxacin spectrum was to include non-enteric gram-negative bacilli (inhibition of 67% versus 88%) and enterococci-streptococci (inhibition of 33% versus 86%). These results are similar to those previously noted for enoxacin plus 3-oxo-enoxacin, and potentially achievable with other newer fluoro-quinolones undergoing significant metabolism.