Intravitreal Ganciclovir Pharmacokinetics in Rabbits and Man

Cytomegalovirus (CMV) retinitis occurs in immunocompromised patients and can be treated by repeated intravenous or intravitreal injections of ganciclovir (GCV) or foscarnet. Due to toxicity and complications these modalities are not ideal. The development of alternative administration routes is hindered by a lack of pharmacokinetic data. Devices giving pseudo zero order release of GCV were implanted intravitreally first in rabbits and then in patients with AIDS associated CMV retinitis as part of a Phase I clinical trial. Steady state intravitreal GCV levels were obtained immediately after death and the elimination rate constants were calculated assuming first order pharmacokinetics. Normalizing for retinal surface area, distribution volume and anatomic volume, the retinal elimination rate constants were calculated. These were found to be 0.017 cm⁻²hr⁻¹ in rabbits and 0.015 cm⁻²hr⁻¹ in man. This indicates that the rabbit eye is a good model for studying intravitreal pharmacokinetics of ganciclovir and suggests a common elimination mechanism which may be trans-retinal.