[H-3]AL-5848 ([H-3]9 beta-(+)-fluprostenol). Carboxylic acid of travoprost(AL-6221), a novel FP prostaglandin o study the pharmacology and autoradiographic localization of the FP receptor

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 autoradiogra phic location of the FP-receptor. Specific [H-3]AL-5848 binding (84% of tot al) was linearly related to bovine corpus luteum tissue concentration and r eached equilibrium within 275 min at 23 degrees C. Scatchard analysis of sa turation isotherms indicated interaction of [H-3]AL-5848 with a single clas s of high-affinity (dissociation constant, K-d, = 33.8 +/- 2.9 nM, n = 4) a nd saturable (B-max = 37.3 +/- 3.0 pmol (g wet weight tissue)(-1)) FP recep tor-binding sites in bovine corpus luteum. Specific [H-3]AL-5848 binding wa s potently inhibited by the FP-receptor ligands 16-phenoxyPGF(2 alpha) (inh ibition constant K-i = 17.3 nM); cloprostenol (K-i = 56.8 nM); 17-phenyl PG F(2 alpha) (K-i = 87.0 nM); AL-5848 (K-i = 52.1 nM); PGF(2 alpha) (K-i = 19 5 nM); PHXA85 (K-i = 223 nM); (n = 3-11) but very weakly by PGD(2), ZK11818 2, BW245C, PGE(2), PGI(2) and U-46619. The pharmacology of specific [H-3]AL -5848 binding correlated well with the pharmacology of [H-3]PGF(2 alpha) bi nding 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 mus cle cells (A7r5) (r = 0.96) expressing FP receptors. Autoradiographic studi es revealed high levels of specific FP-receptor binding with [H-3]AL-5848 o n 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 [H-3]AL-5848 to be a high-affinity agonist radioligand c apable of selectively labelling the FP prostaglandin receptor.