Effectiveness of a new tobramycin (0.3%) and dexamethasone (0.05%) formulation in the treatment of experimental Pseudomonas keratitis

Objective: To quantitatively determine, in a Pseudomonas keratitis model, the anti-inflammatory and bactericidal properties of a new formulation of tobramycin (0.3%) and dexamethasone (0.05%) that utilizes a xanthan gum vehicle. Research methods: In a randomized and masked fashion, rabbit corneas (n >= 16 eyes per group) were intrastromally injected with 10(3) colony-forming units (CFU) of P. aeruginosa. Eyes were untreated or were administered a single drop every 15 min between 16 and 17 h postinfection (PI) and then a single drop every 30 min between 17 and 22 h PI, a total of 15 drops of either 0.1% dexamethasone and 0.3% tobramycin (TobraDex*; Tdex) or a new formulation 0.3% tobramycin and 0.05% dexamethasone with xanthan gum (TobraDex ST dagger; ST). Slit lamp examination scores (SLE +/- SEM) were derived from grading seven parameters at 22 h PI. Rabbits were sacrificed at 23 h PI and the log CFU +/- SEM per cornea was determined. Results: Untreated eyes had SLE scores of 11.11 +/- 0.43 and had log CFU of 7.27 +/- 0.06. Eyes treated with Tdex, as compared to the untreated eyes, had significantly lower SLE scores (7.39 +/- 0.21, p < 0.0001) and significantly fewer bacteria (6.32 +/- 0.29 log CFU, p=0.0213). Eyes treated with ST had a SLE score (6.56 +/- 0.19) that was significantly lower than both the untreated eyes (p < 0.0001) and the eyes treated with Tdex (p=0.0124). Furthermore, eyes treated with ST had significantly fewer log CFU (5.78 +/- 0.30) than untreated eyes (p=0.0001) or eyes treated with Tdex (p = 0.0434). Conclusions: The ST formulation with xanthan gum demonstrated statistically superior anti-inflammatory and bactericidal properties as compared to Tdex. Limitations: Variations in inoculation procedures produced limited eye-to-eye differences in the infection.