Efficacies of moxifloxacin, ciprofloxacin, and vancomycin against experimental endocarditis due to methicillin-resistant Staphylococcus aureus expressing various degrees of ciprofloxacin resistance

Abstract

The new 8-methoxyquinolone moxifloxacin was tested against two ciprofloxacin-susceptible Staphylococcus aureus strains (strains P8 and COL) and two ciprofloxacin-resistant derivatives of strain P8 carrying a single grlA mutation (strain P8-4) and double grlA and gyrA mutations (strain P8-128). All strains were resistant to methicillin. The MICs of ciprofloxacin and moxifloxacin were 0.5 and 0.125 mg/liter, respectively, for P8; 0.25 and 0.125 mg/liter, respectively, for COL; 8 and 0.25 mg/liter, respectively, for P8-4; and >or=128 and 2 mg/liter, respectively, for P8-128. In vitro, the rate of spontaneous resistance of P8 and COL was 10(-7) on agar plates containing ciprofloxacin at two times the MIC, whereas it was <or=10(-10) on agar plates containing moxifloxacin at two times the MIC. Rats with experimental aortic endocarditis were treated with doses of drugs that simulate the kinetics in humans: moxifloxacin, 400 mg orally once a day; ciprofloxacin, 750 mg orally twice a day; or vancomycin, 1 g intravenously twice a day. Treatment was started either 12 or 24 h after infection and lasted for 3 days. Moxifloxacin treatment resulted in culture-negative vegetations in a total of 20 of 21 (95%) rats infected with P8, 10 of 11 (91%) rats infected with COL, and 19 of 24 (79%) rats infected with P8-4 (P < 0.05 compared to the results for the controls). In contrast, ciprofloxacin treatment sterilized zero of nine (0%) vegetations infected with first-level resistant mutant P8-4. Vancomycin sterilized only 8 of 15 (53%), 6 of 11 (54%), and 12 of 23 (52%) of the vegetations, respectively. No moxifloxacin-resistant derivative emerged among these organisms. However, moxifloxacin treatment of highly ciprofloxacin-resistant mutant P8-128 failed and selected for variants for which the MIC increased two times in 2 of 10 animals. Thus, while oral moxifloxacin might deserve consideration as treatment for staphylococcal infections in humans, caution related to its use against strains for which MICs are borderline is warranted.

FIG. 1

Time-kill experiments performed with peak levels of moxifloxacin (3 mg/liter) (A) and vancomycin (40 mg/liter) (B) in serum against ciprofloxacin-susceptible MRSA isolate P8 and ciprofloxacin-resistant MRSA derivative P8-4 (a grlA mutant). Antibiotics were added to the cultures at various times of bacterial growth, and the number of surviving organisms over time was determined by counting the colonies on agar plates. The results are the means ± standard deviations of three independent experiments.

FIG. 2

Intravegetation growth curve (A) and therapeutic results (B) for experimental endocarditis due to ciprofloxacin-susceptible MRSA strain P8. Rats were treated for 3 days with moxifloxacin (Moxi; 400 mg orally once a day), ciprofloxacin (Cipro; 750 mg orally every 12 h), or vancomycin (Vanco; 1 g i.v. every 12 h) at doses that resulted in kinetics similar to those achieved in humans. Therapy was started either early (12 h) or relatively late (24 h) after bacterial challenge (arrows on the growth curve). Each dot represents the bacterial density in the vegetation of a single animal. P values of ≤0.05 indicate statistically significant differences when groups were compared by Fisher's exact test with Bonferroni's correction for multiple group comparisons.

FIG. 3

Intravegetation growth curve (A) and therapeutic results (B) for experimental endocarditis due to ciprofloxacin-resistant MRSA isolate P8-4 (grlA mutant). Therapy was started either early (12 h) or relatively late (24 h) after bacterial challenge (arrows on the growth curve). Details are as described in the legend to Fig. 2.

FIG. 4

Therapeutic results for experimental endocarditis due to ciprofloxacin-resistant MRSA isolate P8-128 carrying double grlA and gyrA mutations. Therapy was started early (12 h) after bacterial challenge. Open dots indicate the vegetations which grew staphylococcal derivatives for which the moxifloxacin MIC had increased twofold after 3 days of therapy. Details are as described in the legend to Fig. 2.

FIG. 5

Selection of resistance to moxifloxacin (MOXI) and ciprofloxacin (CIPRO) during exposure of ciprofloxacin-susceptible MRSA isolates COL and P8 and ciprofloxacin-resistant derivatives of P8, strains P8-4 and P8-128, to stepwise increasing concentrations of the drugs. Series of tubes containing twofold increasing drug concentrations were inoculated with a final concentration of 10⁶ CFU of the test organisms per ml and were further incubated for 24 h at 35°C, like for the MIC tests. On the next day, the tubes with the highest antibiotic concentration still showing turbidity were used to inoculate a new series of antibiotic-containing tubes. The procedure was repeated and the increase in the MIC was monitored over time.