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Absorption, Metabolism, and Excretion of [¹⁴C]MK-0524, a Prostaglandin D₂ Receptor Antagonist, in Humans

Departments of Drug Metabolism (B.K., M.M., S.B., E.S., D.S., A.J., B.D., G.D.) and Clinical Pharmacology (G.G., T.C., A.N., E.L.), Merck Research Laboratories, Rahway, New Jersey; and Department of Drug Metabolism, Merck Research Laboratories, West Point, Pennsylvania (L.W., R.D.)

[(3R)-4-(4-Chlorobenzyl)-7-fluoro-5-(methylsulfonyl)-1,2,3,4-tetrahydrocyclopentaindol-3-yl]acetic acid (MK-0524) is a potent orally active human prostaglandin D₂ receptor 1 antagonist that is currently under development for the prevention of niacin-induced flushing. The metabolism and excretion of [¹⁴C]MK-0524 in humans were investigated in six healthy human volunteers following a single p.o. dose of 40 mg (202 µCi). [¹⁴C]MK-0524 was absorbed rapidly, with plasma C_max achieved 1 to 1.5 h postdose. The major route of excretion of radioactivity was via the feces, with 68% of the administered dose recovered in feces. Urinary excretion averaged 22% of the administered dose, for a total excretion recovery of 90%. The majority of the dose was excreted within 96 h following dosing. Parent compound was the primary radioactive component circulating in plasma, comprising 42 to 72% of the total radioactivity in plasma for up to 12 h. The only other radioactive component detected in plasma was M2, the acyl glucuronic acid conjugate of the parent compound. The major radioactive component in urine was M2, representing 64% of the total radioactivity. Minor metabolites included hydroxylated epimers (M1/M4) and their glucuronic acid conjugates, which occurred in the urine as urea adducts, formed presumably during storage of samples. Fecal radioactivity profiles mainly comprised the parent compound, originating from unabsorbed parent and/or hydrolyzed glucuronic acid conjugate of the parent compound. Therefore, in humans, MK-0524 was eliminated primarily via metabolism to the acyl glucuronic acid conjugate, followed by excretion of the conjugate into bile and eventually into feces.