Retina and ocular toxicity to ocular application of drugs

Eduardo Büchele Rodrigues, MD, Fernando M. Penha, MD, PhD,

Gustavo B. Melo, MD, Juliana Mantovani Bottós, MD, and Michel Eid Farah, MD

CHAPTER

15

INTRODUCTION

The periocular and intraocular route of delivery have proved an effective way of providing therapeutic levels of drugs to the retina. Among the local routes for drug delivery, the intravitreal delivery of drugs remains the most important route for pharmacotherapy, as there has been an expanding repertoire of drugs available in the market. However, some disadvantages of intravitreal delivery of drugs and chemicals include the potential for complications from a surgically invasive procedure, short half-lives of many drugs, and risk of retinal toxicity.1

The intravitreal delivery of drugs to the retina should be primarily evaluated in terms of toxicity issues. A key step to evaluate preclinical retinal biocompatibility of a new drug for human therapy concerns cell culture and animal studies. Cell culture models with retinal pigment epithelial or neuroretinal cells, i.e., R28 cells, may allow systematic evaluation of drug toxicity in a controlled fashion. Preclinical toxicity studies in animals may be designed to expose a limited number of animals to the highest possible dose of a compound to determine potential toxic effects. In that method, visual electrophysiological techniques and histology examination may identify unwanted adverse effects of therapies of the posterior segment, thereby playing an important role in these early stages of drug development.2

Even with the availability of cell culture and electrophysiology tests, there have been problems in evaluating the safety of such drugs and compounds by the lack of a standard agreed-on methodology. Some pharmacologic agents are often off-label use of compounds that have been developed for systemic use, not as primary intraocular agents. With the expanding use of intraocular drugs there has been a long list of conflicting publications in the scientific literature about their safety, including clinical studies in patients, in vivo studies in animals, and in vitro studies. The multiple methods of evaluation of drugs or compounds to be used in the eye lead to much confusion about their toxicity. For example, in a cell culture system a chemical agent may be found to be safe at certain concentrations, while when in vivo evaluation systems have been used, specific signs of damage may arise. These observations may result from the complexity and difficulty of the intraocular milieu to replicate in a cell or tissue culture system. Moreover, some investigators have attributed different results of toxicity evaluation to factors unrelated to the drug or chemical, such as pH, osmolarity, or the preservative of the solution carrying the drug as well as interspecies variability in the preclinical evaluation of drug retinal toxicity.3,4 In this chapter the retina toxicity issues of local ocular delivery of common drugs used for therapy of retinal diseases will be presented.

HISTORY

In 1911, Ohm first reported the technique of intravitreal injection to administer air for the repair of rhegmatogenous retinal detachment. The fear of harming the retina with intravitreal drug application hampered further developments and clinical use of such methods for several decades. However, later in the 1970s, intravitreal injections again arose as a common approach in the therapy of sight-threatening endophthalmitis. At this stage, the safety, clearance, and efficacy of

various antibiotics such as aminoglycosides and cephalosporins were demonstrated using electrophysiology and histopathology on laboratory and clinical studies. In 1987, the first case report described benefit from delivery of ganciclovir sodium to a patient with cytomegalovirus retinitis secondary to acquired immunodeficiency syndrome (AIDS).5 Later, in 1998, fomivirsen sodium (Vitravene) was licensed by the US Food and Drug Administration (FDA) by intravitreal injection in the treatment of cytomegalovirus retinitis. Further studies were performed in the 1980s and 1990s of intravitreal administration of 5-fluorouracil for patients with proliferative retinopathy, dexamethasone to diabetic retinopathy after vitrectomy, and tissue plasminogen activator for the management of submacular hemorrhage.6,7 Although the safe dosages of 5-fluorouracil, dexamethasone, and tissue plasminogen activator have been found after numerous reports on animal experiments, only intravitreal injection of tissue plasminogen activator was considered safe and of benefit in those initial clinical studies.

ANIMAL TESTING FOR DRUG

DELIVERY SAFETY

Assessing retinal drug toxicity with animal models has become increasingly important in retinal pharmacotherapy. Investigation of animal models using the standard tools of light microscopy and histopathological analysis are critical benchmarks for the study of development and disease. In animals, since behavioral assessment of visual function is not always easily obtained, the electroretinogram may be used to quantify possible functional damage of pharmacologic drugs.8 When using animal models, one of the great limitations is the interspecies differences in retinal anatomy and physiology. For instance, the quantity and ratio of retinal cells including photoreceptor populations may vary from species to species and should be considered when designing the experiment protocol and for its interpretation. Also, most rodents used in toxicology are essentially nocturnal species, whereas the nonrodent species (e.g., dogs, pigs and monkeys) are usually diurnal.

The rat eye has become increasingly important as an animal model because its anatomy shows great similarities to the human eye.9 The development and adult structure of the eye are similar in rodents and primates. However, the rodent retina is rod-dominant without a cone-rich macula; the electroretinogram and therapeutic mechanisms of the drugs in rodent model may be for instance different than the human retina.10 On the other hand, in the mouse eye, their relative small size makes clinical and laboratory examination more challenging than for larger animals. However, useful genetic tools and resources as well as microimaging technologies have emerged to meet those needs in mouse research. In addition, the search for mouse models of ocular diseases has been productive, and experimental studies in mice are proving useful in understanding retinal toxicity mechanisms.11

The rabbit eye is a great animal model to study the pharmacokinetics and toxicity of drugs injected intravitreally into the eye. Toxicological studies to evaluate the effect on the retina of subretinal drug injection may also use the rabbits as animal model.3 However, there are differences in the anatomy of the rabbit eye that have to be considered when