With regard to eye drops combining an antibiotic and a corticosteroid, fixed combinations have several advantages over the use of single components, including better patient compliance, lower costs, and reduction of potential washout effects and ocular toxicity by reducing exposure to preservatives.
23–25 Based on our current results, the levofloxacin-prednisolone and ofloxacin-prednisolone combinations had stable antibacterial effects against Gram-negative bacteria, and the neomycin-prednisolone, ofloxacin-dexamethasone, ofloxacin-prednisolone, and polymyxin-dexamethasone combinations maintained a stable antibacterial effect against all of the Gram-positive bacterial strains we tested. In the case of the commercially available tobramycin and dexamethasone ophthalmic ointment, a fixed combination medication, we found a reduced antibacterial effect except in the case of
S. epidermidis. Notably, however, this reduced effect of this preparation was never more than 10%, and the intracellular tobramycin concentration was found in our analyses to be increased by 43%. There are complex interactions involved therefore, and these should be confirmed in further clinical studies. In clinical practice, neomycin and polymyxin B sulfates and dexamethasone ophthalmic ointment are applied with a pressure patch within 1 day after intraocular surgery. In our current analyses of these medications, we found that the dexamethasone reduced the antibacterial effects of neomycin by 4.95% against
P. aeruginosa and by 5.07% in the case of
S. aureus. In addition, dexamethasone reduced the antibacterial effects of polymyxin on
P. aeruginosa by 5.84%. Overall, therefore, dexamethasone did not reduce the effect of the two different antibiotics toward Gram-positive bacteria. The interaction of neomycin and polymyxin contained in eye drops may also be an important consideration in this regard. In previous studies, neomycin and polymyxin showed additive effects against
P. aeruginosa and
S. aureus, and a synergistic effect was reported for
Enterococcus faecalis, which is not used in this study, but can infect the cornea although with a low probability (0.9%).
18,26 In addition, another combination eye drop contains 1% prednisolone acetate, 0.5% gatifloxacin, and 0.075% bromfenac sodium.
26 The gatifloxacin-prednisolone combination had reduced antibacterial effects except against
S. pneumoniae.