Relevance of drug-melanin interactions to ocular pharmacology and toxicology.

In melanocytes, the biosynthesis of L-dopa derived indole polymer, melanin, is accelerated by tyrosinase and related enzymes. The brown to black pigment is characterized by a stable free-radical property. In humans, a pigment dependent slow onset of ocular actions of ephedrine, atropine, cocaine, pilocarpine and related medications was observed. Extensive accumulation of drugs by melanin appears to be the most important factor governing the long term therapeutic/toxicological activities. Drugs crossing placental circulation are localized in the mouse fetal eye. Thus, drugs exhibit a high binding capacity for melanin containing tissues. Studies on synthetic melanin and melanin granules also indicated a high binding capacity of many therapeutic classes of drugs, including psychotropics. In addition to the liposoluble property of the molecule, there is a definite relationship between chemical structure and the affinity of drugs for melanin. For example, the affinity of chlorpromazine for melanin is higher than that of chlorprothixene. NMR studies, with soluble melanins indicate that there is a steric preference among ephedrine enantiomers. A high binding capacity indicates that more than two molecules of (-)-ephedrine may complex with one indole unit of melanin. Ocular drug development calls for the study of qualitative and quantitative aspects of drug-melanin interaction.