(-)-(9S)-9-(3-bromo-4-fluorophenyl)-2,3,5,6,7,9-hexahydrothieno[3, 2-b]quinolin-8(4H)-one 1,1-dioxide (A-278637): A novel ATP-sensitive potassium channel opener efficacious in suppressing urinary bladder contractions. I. In vitro characterization

Alterations in the myogenic activity of the bladder smooth muscle are thought to serve as a basis for the involuntary detrusor contractions associated with the overactive bladder. Activation of ATP-sensitive K⁺ (K_ATP) channels has been recognized as a potentially viable mechanism to modulate membrane excitability in bladder smooth muscle. In this study, we describe the preclinical pharmacology of (-)-(9S)-9-(3-bromo-4-fluorophenyl)-2,3,5,6,7,9-hexahydrothieno[3, 2-b]quinolin-8(4H)-one 1,1dioxide (A-278637), a novel 1,4-dihydropyridine K_ATP channel opener (KCO) that demonstrates enhanced bladder selectivity for the suppression of unstable bladder contractions in vivo relative to other reference KCOs. A-278637 activated K_ATP channels in bladder smooth muscle cells in a glyburide (glibenclamide)-sensitive manner as assessed by fluorescence membrane potential assays using bis-(1,3-dibutylbarbituric acid)trimethine oxonol (EC₅₀ = 102 nM) and by whole cell patch clamp. Spontaneous (myogenic) phasic activity of pig bladder strips was suppressed (IC₅₀ = 23 nM) in a glyburide-sensitive manner by A-278637. A-278637 also inhibited carbachol- and electrical fieldstimulated contractions of bladder strips, although the respective potencies were 8- and 13-fold lower compared with inhibition of spontaneous phasic activity. As shown in the accompanying article [Brune ME, Fey TA, Brioni JD, Sullivan JP, Williams M, Carroll WA, Coghlan MJ, and Gopalakrishnan M (2002) J Pharmacol Exp Ther 303:387-394], A-278637 suppressed myogenic contractions in vivo in a model of bladder instability with superior selectivity compared with other KCOs, WAY-133537 [(R)-4-[3,4-dioxo-2-(1,2,2-trimethyl-propylamino)cyclobut-1 -enylamino]-3-ethylbenzonitrile] and ZD6169 [(S)-N-(4-benzoylphenyl)3,3,3-trifluro-2hydroxy-2-methyl-priopionamide]. A-278637 did not interact with other ion channels, including L-type calcium channels or other neurotransmitter receptor systems. The pharmacological profile of A-278637 represents an attractive basis for further investigations of selective K_ATP channel openers for the treatment of overactive bladder via myogenic etiology.