Drugs of Application Ocular to Toxicity• 15Ocularchapterand Retina

autoimmune diseases. Due to its large variety of effects, the agent could be applied in a large variety of ophthalmic diseases; however, so far methotrexate may be indicated mainly in the therapy of primary intraocular lymphoma as routine therapy.

Few consecutive researchers investigated the safety of methotrexate in animals. As early as in 1985, it has been shown that 600 g methotrexate added to other chemotherapeutic agents such as doxorubicin, 5- fluorouracil, and bleomycin enhanced the risk of retina toxicity. However, in one recent study, the ocular pharmacokinetics and retinal toxicity of intravitreal methotrexate sodium at 400 g were studied in New Zealand white rabbits.37 This research group reported that intravitreal methotrexate, combined with fluorouracil and dexamethasone, showed no evidence of retina toxicity determined by electroretinography and histologic examination. Frenkel et al. released their 10-year experience with frequent intravitreal injection of 400 g/0.1 ml methotrexate. The effective agent rarely induced adverse reactions, the most common being self-limited corneal epitheliopathy, while the vitreoretinal involvement of lymphoma can be controlled effectively.38 In another retrospective case series, complications that occurred during the period of treatment with intravitreal methotrexate included cataract (73%), corneal epitheliopathy (58%), maculopathy (42%), vitreous hemorrhage (8%), and optic atrophy (4%). However, no patient had irreversible loss of vision that could be definitely attributed to the intravitreal injection of methotrexate. Overall, the intravitreal use of methotrexate in dose up to 400 g seems to be a safe approach in clinical practice. Although no preclinical investigation has been performed to investigate the retinal biocompatibility of thalidomide for local ocular application, one case report demonstrated corneal endothelium toxicity after oral use of the powerful agent.39

Summary and Key poinTS

Ocular toxicity caused by the exposure of the retinal tissue to a high concentration of drug for a certain period of time remains an important topic in the approach of intravitreal injection in the therapy of retinal diseases. In the evaluation of drug safety for retinal pharmacotherapy, both exposure concentration and exposure time can be important, but the toxic effect may also be directly related to the transient drug distribution inside the vitreous body. The retina is subject to the risk of drug-induced toxicities owing to its rich blood supply, complex wellorganized neuroretinal organization, and lifelong exposure to focused light rays. To enable its physiological functions in the risk of harming, the retina is protected by specific defense mechanisms including the endogenous detoxifying system cytochrome p450 and the blood–retinal barrier. An understanding of these cellular and molecular principles is a key aspect in elucidating the pathological pathways leading to retinotoxicity.

Currently, there are several approved drugs for intravitreal use, such as ranibizumab (Lucentis, Genentech) and pegaptanib (Macugen, Pfizer). However, there are numerous off-label uses of drugs and substances injected in the eye, but the scientific literature remains confusing with regard to intraocular properties and toxicology of off-label drugs such as commercial triamcinolone, antibiotics, antivirals, and tissue plasminogen activator.

Due to the difficulties in evaluating the toxic effect of drugs in the human eye, animal data such as rabbit experiments are used by ophthalmologists as a guideline for designing an administration regime. The toxic level of drug is established by making histologic slides of eye tissues and observing morphologic changes of retinal tissue. In addition, visual electrophysiological recordings, performed at single-cell, tissue, whole-animal, and human patient levels, can provide important information at various stages in the development of new drugs. The evolution of electrophysiological techniques permits them to be used to conduct studies that follow the standards required for good laboratory practice. An expanding number of laboratories are using these procedures and are consequently acquiring the expertise necessary to establish procedures that follow these standards. Regulatory institutions may play a role in the evaluation of preclinical drug safety.

Progresses on standardized methods and conditions to evaluate such drugs are an important topic in retinal pharmacotherapy. In the future, such advances in ocular toxicity methods will have a substantial impact on the research of novel drugs in the treatment of vitreoretinal diseases.

ACKNOWLEDGMENTS

The authors acknowledge Dr. Harry Flynn for providing Figures 2A and 2b as well as Dr. Roberta Manzano and Prof. Peyman for providing figure 3 of this chapter.

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delivery drug retinal for routes and models Animal • 2 section