By using broth microdilution methods, the in vitro activity of tosufloxacin (A-64730), a new quinolone, was compared with those of other agents, including five quinolones, against geographically diverse cystic fibrosis sputum isolates obtained from 26 cystic fibrosis centers in the United States. These included Pseudomonas aeruginosa, conventional as well as especially resistant (ceftazidime, aztreonam, gentamicin, and/or tobramycin) isolates: Escherichia coli; Pseudomonas cepacia; Staphylococcus aureus; and Haemophilus influenzae. Tosufloxacin MICs for 50 and 90% of isolates of standard P. aeruginosa were 0.5 and 2.0 mg/liter, for resistant P. aeruginosa they were 4.0 and greater than 16.0 mg/liter, for E. coli they were less than or equal to 0.016 mg/liter, for P. cepacia they were 4.0 and 8.0 mg/liter, for S. aureus they were 0.063 and 0.063 mg/liter, and for H. influenzae they were less than or equal to 0.016 and 0.032 mg/liter, respectively. Tosufloxacin activities against standard and resistant strains of P. aeruginosa were similar to those of comparative quinolones. Against E. coli, tosufloxacin activity was similar to those of other quinolones. Against S. aureus, tosufloxacin activity was similar to those of trimethoprim-sulfamethoxazole and cephalexin, but tosufloxacin was more active than other agents. Against H. influenzae, tosufloxacin activity was similar to those of other quinolones. There was minor diminution of activity at pH 8.2 but major diminution of activity at pH 5.2 and at inoculum sizes of greater than or equal to 10(7) CFU/ml. Activity was unaffected by sputum but was enhanced by serum and by the omission of cation supplementation. Tosufloxacin has consistent activity against common cystic fibrosis pathogens. Its high degree of activity against S. aureus with activity maintained against P. aeruginosa and other gram-negative bacteria of interest suggests that further in vitro studies and assessment of activity in in vivo models of cystic fibrosis pulmonary infections are warranted.
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