Efficacy of ciprofloxacin in experimental arthritis caused by Escherichia coli--in vitro-in vivo correlations.

Ciprofloxacin, a new carboxyquinolone, has potent in vitro bactericidal activity against the major aerobic, gram-negative bacillary pathogens that cause human pyoarthroses. We investigated the in vivo efficacy of ciprofloxacin in a rabbit model of septic arthritis due to Escherichia coli. Animals received either ciprofloxacin (80 mg/kg per day) or gentamicin (5 mg/kg per day). Ciprofloxacin was rapidly bactericidal in vivo and was significantly more effective in reducing the numbers of E. coli in synovial tissue than was gentamicin at days 10 and 17 of therapy (P less than .0005 and P less than .05, respectively). Similarly, ciprofloxacin was significantly more active than was gentamicin in reducing the numbers of E. coli in joint fluid on day 10 of therapy (P less than .0005); however, by day 17 of therapy, the numbers of E. coli in joint fluid were not significantly different in the two therapy groups. Neither regimen was effective in preventing the development of postinfectious inflammatory synovitis. There was no in vivo development of resistance to either antibiotic during therapy. Ciprofloxacin therapy was associated with significantly higher bactericidal titers in serum and joint fluid than were observed with gentamicin therapy (P less than .0005). Ciprofloxacin warrants further in vivo evaluation in invasive E. coli infections.