Intravitreal injection of gentamicin in rabbits. Effect of inflammation and pigmentation on half-life and ocular distribution.

Most regimens for intravitreal injection of antibiotics are based on studies in normal albino rabbits. We examined the effect of two variables, infection and pigmentation, on the ocular distribution of an intravitreal injection of 50 micrograms of gentamicin in rabbit eyes. The half-life of gentamicin in the vitreous of normal pigmented rabbits after intravitreal administration was 24 hr. Antibiotic levels in cornea and aqueous reached a peak 18 hr after injection; terminal half-lives in these sites were similar to those in vitreous. With inflammation, the half-life in the vitreous was decreased to 10 hr, and there was no accumulation of drug in anterior sites. To the extent that these data are applicable to humans, they suggest that the treatment of bacterial endophthalmitis by the intravitreal route may require more frequent injections than had been recommended on the basis of studies in normal eyes. Normal pigmented and albino rabbits showed similar levels of gentamicin in the cornea, aqueous, and vitreous; however, the drug was barely detectable in iris, choroid-retina, and sclera of pigmented animals, presumably on account of an interaction with melanin-containing tissues. This effect of pigment may explain the differences in the reported thresholds for toxicity of gentamicin with intravitreal injection.