Benextramine acts as an irreversible noncompetitive antagonist of U46619-mediated contraction of the rat small mesenteric artery

Potent, long-acting cyclopentane-1,3-Dione thromboxane (A2)-receptor antagonists

A series of derivatives of the known thromboxane A2 prostanoid (TP) receptor antagonists, 3-(6-((4-chlorophenyl)sulfonamido)-5,6,7,8-tetrahydronaphthalen-1-yl)propanoic acid and 3-(3-(2-((4-chlorophenyl)sulfonamido)ethyl)phenyl) propanoic acid, were synthesized in which the carboxylic acid functional group was replaced with substituted cyclopentane-1,3-dione (CPD) bioisosteres. Characterization of these molecules led to the discovery of remarkably potent new analogues, some of which were considerably more active than the corresponding parent carboxylic acid compounds. Depending on the choice of the C2 substituent of the CPD unit, these new derivatives can produce either a reversible or an apparent irreversible inhibition of the human TP receptor. Given the potency and the long-lasting inhibition of TP receptor signaling, these novel antagonists may comprise promising leads for the development of antithromboxane therapies.

Phenoxybenzamine and benextramine, but not 4-diphenylacetoxy-N-[2-chloroethyl]piperidine hydrochloride, display irreversible noncompetitive antagonism at G protein-coupled receptors

Many irreversible antagonists have been shown to inactivate G protein-coupled receptors (GPCRs) and used to study agonists and spare receptors. Presumably, they bind to primary (agonist) binding sites on the GPCR, although noncompetitive mechanisms of antagonism have been demonstrated but not thoroughly investigated. We studied noncompetitive antagonism by phenoxybenzamine and benextramine at alpha(2A)-adrenoceptors in stably transfected Chinese hamster ovary cells, benextramine and 4-diphenylacetoxy-N-[2-chloroethyl]piperidine hydrochloride (4-DAMP mustard) at endogenous muscarinic acetylcholine (mACh) receptors in human neuroblastoma SH-SY5Y cells, and benextramine at serotonin 5-HT(2A) receptors in stably transfected SH-SY5Y cells. Primary binding sites were protected by reversible competitive antagonists during pretreatment with irreversible antagonists. We conducted appropriate radioligand binding assays by measuring remaining primary binding sites and agonist affinity, functional assays to evaluate agonist-induced responses, and constitutive guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS)-Galpha(o) binding assays to determine remaining G protein activity. Phenoxybenzamine (100 microM; 20 min) and benextramine (10 or 100 microM; 20 min) at alpha(2A)-adrenoceptors, but not 4-DAMP mustard (100 nM; 120 min) at mACh receptors, displayed irreversible noncompetitive antagonism in addition to their known irreversible competitive antagonism. Although agonist binding affinity is not influenced, signal transduction is modulated in a G protein-dependent manner via allotopic interactions. Benextramine noncompetitively inhibits agonist-induced responses at three different GPCR types (alpha(2A), mACh, and 5-HT(2A) receptors) that signal via three families of G proteins (G(i/o), G(s), and G(q/11)). We conclude that, where irreversible antagonists are utilized to study drug-receptor interaction mechanisms, the presence of significant irreversible noncompetitive antagonism may influence the interpretation of results under the experimental conditions used.

A diffusible substance(s) mediates endothelium-dependent contractions in the aorta of SHR

A modified bioassay system was designed to demonstrate the diffusible nature of endothelium-derived contracting factor(s) released by acetylcholine in the aorta of spontaneously hypertensive rat. In "sandwich"-like layered preparation, isometric tension was recorded from a bioassay strip (without endothelium) in the presence of N(G)-nitro-L-arginine and tetrahydrobiopterin to selectively potentiate endothelium-dependent contractions. A donor strip (with or without endothelium) was stitched on the bioassay tissue so that it did not directly contribute to the recorded contractions. Acetylcholine induced contractions that occurred only when the donor strip was with endothelium. Superoxide dismutase did not affect but catalase and the combination of superoxide dismutase plus catalase significantly decreased the endothelium-dependent contraction. The contractions in the layered preparations were abolished when the donor strip with endothelium was treated previously with valeryl salicylate, an irreversible cyclooxygenase-1 inhibitor, but remained unaffected when the bioassay strip was treated with the compound. Previous treatment of the bioassay strip alone with S 18886 abolished the contractile response, whereas treatment of the donor strip with endothelium by the selective TP receptor antagonist only produced a moderate inhibition. These results indicate that in the aorta of spontaneously hypertensive rats, endothelium-dependent contractions to acetylcholine involve a diffusible substance(s) released by the endothelium. The production of this contracting factor(s) requires the activation of endothelial cyclooxygenase-1, and its action the activation of TP receptors on the vascular smooth muscle cells.