Evaluation of pharmacokinetics and possible retinal toxicity of intravitreal injection of methotrexate in experimental rabbit model

ABSTRACT Evaluation Of Pharmacokinetics And Possible Retinal Toxicity Of Intravitreal Injection Of Methotrexate In Experimental Rabbit Model Pharmacokinetics and possible retinal damage due to intravitreal injection of methotrexate were investigated at early and late periods. In the first stage of the study intravitreal injection of 800 ug/0.1 ml methotrexate was done to twentyone of twentyfive New Zealand white rabbits. Vitreous concentrations of the drug was measured by fluorescein polarization immunoassay at 7 different times within a period of 30 minutes to 72 hours. In the second stage; early effects of injections on retina were evaluated at 1st, 2nd and 3rd days following the injection. Histopathologic changes in retina were evaluated 30 days after four injections with 7 days intervals by light and electron microscopy in two rabbits. The control group consisted of two rabbits. Volume of distribution was calculated 1,33ml following intravitreal injection of 800ug methotrexate. Vitreus concentrations of the drug was found to be decreasing related to the mathematical equation; drug concentration 1426,73 e -°>1182(tm,e> and remained over effective dose by 81 hours with a half life of 5,9 hours. In the second stage; edema, vacuolization and loss of internal mitochondrial membrane of some retinal cells were observed as early changes. Long term effects after serial injections were edema in photoreceptor cells, inner nuclear and ganglionic cell layers. Cellular disorganization was observed resembling cellular loss in ganglionic cell layer in light microscopic examination. Electron microscopic examination revealed vacuole formation in cells, degeneration of nucleus in several cells in outer nuclear layer, myelin figures on photoreceptor cell processes and membranous structures in nerve fiber layer. In conclusion; these findings evidenced that vitreous levels of methotrexate after 800ug intravitreal injections can be calculated by a mathematical equation at each time point. At early stages no major change was observed in cellular layers of the retina and minimal retinal damage was seen after serial injections of methotrexate. Key Words: Electron microscopy, intravitreal, methotrexate, pharmacokinetics. VIII