Interaction of antiglaucomatous drugs and melanin granule

The author investigated the effects of several antiglaucomatous drugs and prostaglandins (E2 and F2 alpha) on intraocular pressure responses in pigmented and albino rabbits, and spectrophotometrically assessed the binding of these drugs to synthetic melanin. Topical application of 0.5% timolol, 1% epinephrine and 3% pilocarpine had greater ocular hypotensive effects in albino rabbits than in pigmented rabbits. However no such differences were seen with application of prostaglandins. Melanin binding of drugs was higher in the order of befunolol, carteolol, timolol, epinephrine and pilocarpine. At higher concentrations of the drugs, the degree of binding increased. Prostaglandins had no binding ability. Thus drugs with lower hypotensive effects in pigmented rabbits than in albino rabbits had high melanin binding ability, whereas drugs with similar effects in pigmented and albino rabbits had no melanin binding ability. It is speculated that some antiglaucomatous drugs bind to melanin, resulting in decreased pharmacological action.