Ocular accumulation and toxicity of certain systemically administered drugs

Certain polycyclic compounds with a coplanar ring structure (phenothiazines, thioxanthenes, 4-aminoquinolines, and amitriptyline), monocyclic sympathomimetic amines, and other drugs become concentrated in the eyes of animals following acute or chronic systemic administration. Some are known to cross the placental barrier and accumulate in the fetal eye. Following drug withdrawal, these substances disappear relatively slowly from ocular tissues compared with other tissues. The main reason for the accumulation of these compounds seem to be their affinity for the melanin of the uveal tract and pigment epithelium and they therefore do not accumulate in the eyes of albino animals. The mechanism of uptake by melanin probably involves a "charge transfer" reaction involving the transfer of an electron from drug to melanin, which acts as an "electron trap" and in consequence binds the donor compound firmly. The accumulation of a nontoxic drug in the eye is not necessarily of clinical significance, but ocular damage can occur in patients on long-term tricyclic agents when the amount, duration, and frequency of dosage are sufficiently high. The most serious form of ocular damage is pigmentary retinopathy, which, if caused by chloroquine, is irreversible. In contrast, phenothiazine retinopathy is reversible. Lesions may also be produced in anterior structures of the eye, usually the cornea and lens, by both chloroquine and the phenothiazines, but they are of a relatively minor nature. Possible mechanisms for the oculotoxicity of the phenothiazines and antimalarials are discussed, particularly in relation to melanin.