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Two open-label, randomized, 2-way crossover studies (1 single-dose and 1 steady-state) were conducted in healthy volunteers to compare the pharmacokinetics and pharmacodynamics of a novel extended-release ciprofloxacin (ciprofloxacin ER; 500 mg once daily) and immediate-release ciprofloxacin (ciprofloxacin IR; 250 mg twice daily). For both studies, mean ciprofloxacin maximum concentration (Cmax) values after once-daily ciprofloxacin ER were significantly greater than those after the first daily dose of ciprofloxacin IR (P < .0001) but were lower than those after the second daily dose of ciprofloxacin IR (P < .05). The relative bioavailability of ciprofloxacin ER compared to ciprofloxacin IR was 93.8% in the single-dose study and 97.7% in the steady-state study. Mean urinary ciprofloxacin concentrations and excretion rates after either treatment were substantially greater than the minimum inhibitory concentrations (MICs) for susceptible uropathogens in both studies. The area under the concentration-time curve (AUC)/MIC, Cmax/MIC, amount excreted (Ae)/MIC, and Ae24/MIC ratios with ciprofloxacin ER were similar to or slightly greater than with ciprofloxacin IR for all susceptible organisms.