Zymar (Gatifloxacin 0.3%) shows excellent Gram-negative activity against Serratia marcescens and Pseudomonas aeruginosa in a New Zealand White rabbit keratitis model.

PURPOSE

Whereas gatifloxacin, a newer fluoroquinolone, was engineered to increase its Gram-positive potency, we assessed whether it still retained significant Gram-negative activity in vivo. Specifically, we compared the efficacy of Zymar (gatifloxacin 0.3%), Ciloxan (ciprofloxacin 0.3%), and fortified tobramycin (14 mg/mL) in the treatment of experimental Gram-negative bacterial infections of Serratia marcescens (SM) and Pseudomonas aeruginosa (PA) in the New Zealand White (NZW) rabbit keratitis model.

METHODS

A total of 30 NZW rabbits each were intrastromally inoculated in both eyes with approximately 1000 CFU of SM and PA. By E-test, the minimum inhibitory concentrations (MICs; microg/mL) for SM were gatifloxacin (0.125), ciprofloxacin (0.047), and tobramycin (1.5), and for PA were gatifloxacin (0.125), ciprofloxacin (0.19), and tobramycin (0.5). After 16 hours, the rabbits were divided into 4 treatment groups: (1) Zymar, (2) Ciloxan, (3) fortified tobramycin, and (4) saline control. One drop was instilled in both eyes every 15 minutes for 5 doses and then every 30 minutes for 14 doses. One hour after the final treatment, the animals were euthanized, and bacterial colony counts from the corneas were determined.

RESULTS

For SM, Zymar and Ciloxan significantly reduced (P < 0.001, ANOVA) the colony counts compared with tobramycin and saline control. Zymar was more effective than Ciloxan (P < 0.001, ANOVA). For PA, all antibiotics reduced equivalently the colony counts compared with the saline control (P = 0.005, ANOVA).

CONCLUSIONS

The enhanced Gram-positive activity of gatifloxacin is not associated with any decreased Gram-negative activity in vivo. Zymar may prove useful for SM and PA keratitis.