Evidence for a distinct Ca2+ antagonist receptor for the novel benzothiazinone compound HOE 166.
The pharmacological and binding properties of the novel enantiomerically pure benzothiazinone (R)-(+)-3,4-dihydro-2-isopropyl-4-methyl-2-[2-[4-[4-[2-(3,4,5-tri- methoxyphenyl)-ethyl]-piperazinyl]-butoxyl-phenyl]-2H-1,4- benzothiazine-3-one dihydrochloride (HOE 166), are described. HOE 166 stereoselectively inhibited KCl-but not noradrenaline-induced contractions of guinea-pig pulmonary arteries, rabbit aorta, rat mesenteric artery preparations and k-strophantin-induced enhancement of guinea-pig papillary muscle contraction in a dose-dependent manner. KCl-induced smooth muscle contraction was inhibited by HOE 166 with IC50-values of approximately 70 nM (5-11 times less potent than nifedipine, 2-16 times more potent than verapamil), the respective S-(-)-enantiomer being approximately 10-fold less potent. HOE 166 decreased the upstroke velocity of the slow action potential in partially depolarized guinea-pig papillary muscle at similar concentrations than nifedipine. To investigate possible interactions with the calcium channel, HOE 166 and its S-(-)-enantiomer were characterized by radioligand binding studies in heart, brain and skeletal muscle transverse-tubule membranes. HOE 166 was a 4-15 times more potent inhibitor of reversible (+)-[3H]PN200-110, (-)-[3H]desmethoxyverapamil and d-cis [3H]diltiazem binding compared to its pharmacologically less active (S)-(-)-enantiomer, with IC50 values in the low nanomolar range. Extensive equilibrium and kinetic studies suggest that HOE 166 exerts its Ca2+-antagonistic effect by binding to a Ca2+-channel-associated drug receptor which is distinct from the 1,4-dihydropyridine, phenylalkylamine or benzothiazepine-selective domain. This HOE 166-selective site is, however, allosterically linked to the other sites of the Ca2+ antagonist receptor complex. We conclude that HOE 166 is a novel calcium antagonist.
Striessnig
J
J
Institut für Biochemische Pharmakologie, Innsbruck, Austria.
Meusburger
E
E
Grabner
M
M
Knaus
H G
HG
Glossmann
H
H
Kaiser
J
J
Schölkens
B
B
Becker
R
R
Linz
W
W
Henning
R
R
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D002121:Calcium Channel Blockers / Q000378:metabolism*; D015220:Calcium Channels; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D017275:Isradipine; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D009132:Muscles / Q000187:drug effects / Q000378:metabolism; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism; D010069:Oxadiazoles / Q000378:metabolism; D010210:Papillary Muscles / Q000187:drug effects; D010880:Piperidines / Q000378:metabolism*; D011817:Rabbits; D011978:Receptors, Nicotinic / Q000378:metabolism*; D013152:Splanchnic Circulation / Q000187:drug effects; D013328:Strophanthins / Q000494:pharmacology; D013843:Thiazines / Q000378:metabolism*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010069:Oxadiazoles; D010880:Piperidines; D011978:Receptors, Nicotinic; D013328:Strophanthins; D013843:Thiazines; C055625:Hoe 166; C029082:strophanthin K; D017275:Isradipine