[Retinal toxicity of intravitreal doxycycline. A pilot study]

To assess the retinal toxicity of varying concentrations of intravitreally administered doxycycline, a member of tetracycline family.

Fourteen New Zealand albino rabbits, divided into 5 groups, were used for this study. The initial concentration of doxycycline (100 mg) was titrated using 5% dextrose solution to the following concentrations in a volume of 0.1 ml: 2000 microg, 1000 microg, 500 microg, 250 microg, 125 microg, and 62.5 microg. Each concentration was injected into 2 rabbit eyes. Two control eyes received 0.1 ml of 5% dextrose solution. All animals were examined before and after injection using indirect ophthalmoscopy and slit-lamp biomicroscopy. Electroretinography (ERG) was performed on all animals prior to the intravitreal injection and 2 weeks post-injection. The animals were re-examined at this time by indirect ophthalmoscopy and slit-lamp biomicroscopy and were then subjected to euthanasia. Their eyes were enucleated and examined using light microscopy.

The doxycycline injected group exhibited significant decreases in ERG of the eyes injected with 2000 microg, 1000 microg, 500 microg, and 250 microg/0.1 ml. No significant changes in the ERG were observed following the injection of lesser concentration levels. There were no signs of retinal toxicity on slit-lamp examination, indirect ophthalmoscopy, or light microscopy in all the eyes injected with doxycycline concentrations of 125 microg or lower.

Doxycycline injected intravitreally appeared safe at concentrations of 125 microg/0.1 ml or less in albino rabbits. Intravitreal doxycycline may be beneficial, and is an inexpensive alternative drug which could be used in the treatment of bacterial endophthalmitis particularly against resistant Staphylococcus aureus organisms.