Intravitreal dexamethasone effect on intravitreal vancomycin elimination in endophthalmitis

Objective: To determine whether intravitreal dexamethasone administration can alter the elimination of intravitreal vancomycin hydrochloride in rabbit eyes with experimental Streptococcus pneumoniae endophthalmitis. Methods: Albino rabbits were infected with an intravitreal inoculum of S pneumoniae (2 x 10³ colony-forming units) and randomized after 24 hours to treatment with intravitreal vancomycin hydrochloride (1 mg), alone or in combination with intravitreal dexamethasone (400 μg). For comparison, uninfected eyes were similarly treated. All eyes were enucleated 24, 48, or 72 hours after treatment, and vitreous levels of vancomycin were quantitated using a fluorescence polarizing immunoassay. Results: The half- life of intravitreal vancomycin in infected eyes was prolonged from 48 to 84 hours when eyes were treated with dexamethasone. Conversely, such treatment shortened the half-life in uninfected eyes from 56 to 42 hours. Conclusions: Intravitreal dexamethasone administration reduces the elimination of intravitreal vancomycin in rabbit eyes with pneumococcal endophthalmitis, whereas an opposite effect is noted in uninfected eyes. Clinical Relevance: In patients with eyes having endophthalmitis caused by virulent organisms, the elimination of intravitreal vancomycin may be reduced when intraocular inflammation is minimized with corticosteroid therapy. This may enhance the efficacy of intravitreal vancomycin therapy in treating the infection.