AL-3138 antagonizes FP prostanoid receptor-mediated inositol phosphates generation: Comparison with some purported FP antagonists

The aim of this study was to pharmacologically characterize the antagonist properties of a novel prostaglandin F-2 alpha (PGF(2 alpha)) analogue (11-d eoxy-16-fluoro PGF(2 alpha); AL-3138) using a variety of second-messenger a ssays of prostaglandin receptor subtypes. A detailed comparison was made be tween AL-3138 and some purported FP receptor antagonists such as PGF(2 alph a) dimethylamine, PGF(2 alpha) dimethylamide, glibenclamide and phloretin u sing the FP receptor-mediated phosphoinositide turnover assay in A7r5 rat t horacic aorta smooth muscle cells and mouse Swiss 3T3 fibroblasts. The potency and efficacy of AL-3138 as an FP receptor agonist were: EC50=72 .2 +/- 17 .9 nM (E-max = 37%) (n = 3) in A7r5 cells and EC50 = 20 .5 +/- 2 .8 nM (E-max = 33%) (n = 5) in 3T3 cells. Being a partial agonist, the ant agonist potency of AL-3138 against fluprostenol in A7r5 cells was determine d to be: K-i = 296 +/- 17 nM (n = 3) and K-b = 182 +/- 44 nM (n = 5) (-log K-b = 6 . 79 +/- 0 .1). AL-3138 exhibited very minimal or no antagonistic e ffects at EP2, EP4, DP and TP prostaglandin receptors. Both PGF(2 alpha) di methylamide and PGF(2 alpha) dimethylamine were inactive as FP receptor ant agonists, whereas phloretin and glibenclamide were very weak and had -log K -b values of 5 . 28 +/- 0 . 09 (n = 3) and 3 . 58 +/- 0 . 32 (n = 3), respe ctively. However, phloretin antagonized functional responses of EP2 and DP prostanoid receptors, and also the V-1-vasopressin receptor. AL-3138 compet ed for [H-3]PGF(2 alpha) binding to FP receptors with a relatively high aff inity (IC50(high) = 312 +/- 95 nM) matching its functional antagonist poten cy. In conclusion, AL-3138 is a more potent and selective FP receptor antagonis t than glibenclamide, phloretin, PGF(2 alpha) dimethylamide and PGF(2 alpha ) dimethylamine and is therefore a unique and novel pharmacological tool to help characterize FP receptor-mediated functions.