Pefloxacin is a broad-spectrum and synthetic fluoroquinolone antibiotic and a methylated derivative of norfloxacin. It is active against Gram-negative and Gram-positive bacteria, including clinical isolates of E. coli, K. pneumoniae, S. aureus, and G. vaginalis. Pefloxacin selectively inhibits E. coli topoisomerase I and DNA gyrase over calf thymus topoisomerase I in cell-free assays. In vivo, pefloxacin is active against systemic E. coli, K. pneumoniae, and P. aeruginosa infections in mice with 50% of efficacy maximum (P50) values of 6.5, 29.2, and 219 mg/kg, respectively.