To determine whether bimatoprost is hydrolyzed to its free acid after topical application in humans in vivo. Prospective, masked, and vehicle controlled. Thirty-one eyes of 31 patients with cataracts. Beginning 7 days before scheduled cataract surgery, one eye of each patient was treated with bimatoprost 0.03% or vehicle once daily, with the last drop administered 2 to 12 hours before anterior chamber paracentesis before cataract surgery. In a masked fashion, aqueous humor specimens were assayed for bimatoprost and its free acid by high-pressure liquid chromatography and mass spectrometry. Detection of the free acid of bimatoprost in aqueous humor. Aqueous humor concentrations of the free acid of bimatoprost were 22.0±7.0 nmol/l (mean ± standard error of the mean, n = 12) and 7.0±4.6 nmol/l (n = 8) at 2 and 12 hours, respectively, and below the limit of detection after vehicle (n = 10). Concentrations of bimatoprost (amide) were 5.7±1.4 and 1.1±0.4 nmol/l at 2 and 12 hours, respectively, and undetectable after vehicle. After topical application of bimatoprost in humans, a sufficient concentration of its free acid, a potent FPprostanoid receptor agonist, is found in the aqueous humor to account for its ability to reduce intraocular pressure.
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