Increased pigmentation of iridial melanocytes in primates induced by a prostaglandin analogue

The melanocytes in the mammalian eye have been thought to produce melanin o nly during fetal development and in the very young individual. The recent d iscovery that latanoprost, a prostaglandin analogue used in the treatment o f glaucoma, causes increased pigmentation of the iris in monkeys and in hum ans indicates that the iridial melanocytes can produce melanin in adult ind ividuals. Using microautoradiography of HG-H-3-methimazole, a false melanin precursor, we observed in an earlier study that there seems to be an ongoi ng melanogenesis in adult mice in the iridial melanocytes and in the iridia l pigment epithelium. In the present study latanoprost (13,14-dihydro-17-phenyl-18,19,20-trinor-P GF(2 alpha)-isopropyl ester) was applied once daily to the right eye of sev en cynomolgus monkeys; the left eye served as an untreated control. Two ani mals developed clear-cut increased pigmentation of the iris in the treated eye during the first three months of treatment. These animals were injected intravenously with G-H-3-methimazole and were killed 24 hr after the injec tion. The eyes were removed, fixed in 4% formalin supplemented with 10% ace tic acid and embedded in paraffin or Polybed 812. Sections from the eyes we re used for microautoradiography and light microscopic examination. A high uptake of radioactivity was observed in a few melanocytes in the iris of th e untreated eye. There were also a low uptake in the melanocytes in the str oma of the ciliary body and the choroid. No accumulation was observed in th e iridial or retinal pigment epithelium. In the iris of the treated eye the only observed difference from the untreated eye was an increased amount of melanin in the iridial melanocytes and an increased uptake of radioactivit y in a great number of these cells. Thus it seems likely that treatment wit h latanoprost in some individuals causes an increase of the low normal mela nin synthesis in iridal melanocytes. (C) 1999 Academic Press.