[3H]AL-5848 ([3H]9β-(+)-Fluprostenol). Carboxylic Acid of Travoprost (AL-6221), a Novel FP Prostaglandin o study the Pharmacology and Autoradiographic Localization of the FP Receptor

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Abstract
AL-5848 (5Z,13E)-(9 S,11R,15S)-9,11,15-trihydroxy-5,13-prostadienoic acid) is the carboxylic acid of travoprost (AL-6221), a single (+)-isomer of (±)-fluprostenol, an FP-class prostaglandin agonist which lowers intraocular pressure. We have prepared a radioligand from this selective prostaglandin and demonstrated its utility for studying the pharmacology and autoradiographic location of the FP-receptor. Specific [3H]AL-5848 binding (84% of total) was linearly related to bovine corpus luteum tissue concentration and reached equilibrium within 275 min at 23°C. Scatchard analysis of saturation isotherms indicated interaction of [3H]AL-5848 with a single class of high-affinity (dissociation constant, Kd, = 33.8 ± 2.9 nM, n = 4) and saturable (Bmax = 37.3 ± 3.0 pmol (g wet weight tissue)−1) FP receptor-binding sites in bovine corpus luteum. Specific [3H]AL-5848 binding was potently inhibited by the FP-receptor ligands 16-phenoxyPGF2α (inhibition constant Ki = 17.3 nM); cloprostenol (Ki = 56.8 nM); 17-phenyl PGF2α (Ki = 87.0 nM); AL-5848 (Ki = 52.1 nM); PGF2α (Ki = 195 nM); PHXA85 (Ki = 223 nM); (n = 3–11) but very weakly by PGD2, ZK118182, BW245C, PGE2, PGI2 and U-46619. The pharmacology of specific [3H]AL-5848 binding correlated well with the pharmacology of [3H]PGF2α binding in the bovine corpus luteum preparation (r = 0–98, n = 14, P < 0.0001) and also with functional responses in Swiss 3T3 and rat vascular smooth muscle cells (A7r5) (r = 0.96) expressing FP receptors. Autoradiographic studies revealed high levels of specific FP-receptor binding with [3H]AL-5848 on granulosa cells in the bovine corpus luteum sections, and on longitudinal ciliary muscle, the ciliary process, the iris sphincter and the retina in eye sections from man. These studies show [3H]AL-5848 to be a high-affinity agonist radioligand capable of selectively labelling the FP prostaglandin receptor.