The anaphylatoxin C3a is a potent chemotactic peptide and inflammatory mediator released during complement activation which binds to and activates a G-protein-coupled receptor. Molecular cloning of the C3aR has facilitated studies to identify nonpeptide antagonists of the C3aR. A chemical lead that selectively inhibited the C3aR in a high throughput screen was identified and chemically optimized. The resulting antagonist, N(2)-[(2,2-diphenylethoxy)acetyl]-L-arginine (SB 290157), functioned as a competitive antagonist of (125)I-C3a radioligand binding to rat basophilic leukemia (RBL)-2H3 cells expressing the human C3aR (RBL-C3aR), with an IC(50) of 200 nM. SB 290157 was a functional antagonist, blocking C3a-induced C3aR internalization in a concentration-dependent manner and C3a-induced Ca(2+) mobilization in RBL-C3aR cells and human neutrophils with IC(50)s of 27.7 and 28 nM, respectively. SB 290157 was selective for the C3aR in that it did not antagonize the C5aR or six other chemotactic G protein-coupled receptors. Functional antagonism was not solely limited to the human C3aR; SB 290157 also inhibited C3a-induced Ca(2+) mobilization of RBL-2H3 cells expressing the mouse and guinea pig C3aRS: It potently inhibited C3a-mediated ATP release from guinea pig platelets and inhibited C3a-induced potentiation of the contractile response to field stimulation of perfused rat caudal artery. Furthermore, in animal models, SB 290157, inhibited neutrophil recruitment in a guinea pig LPS-induced airway neutrophilia model and decreased paw edema in a rat adjuvant-induced arthritis model. This selective antagonist may be useful to define the physiological and pathophysiological roles of the C3aR.

The pharmacologic profile of the novel β-adrenoceptor antagonist/vasodilator, carvedilol, has been investigated in vitro. Carvedilol produced competitive antagonism of the β₁-adrenoceptor mediated positive chronotropic response to isoproterenol in guinea pig atria, and the β₂-adrenoceptor mediated relaxation to isoproterenol in carbachol (1 μmol/l) precontracted guinea pig trachea, with a dissociation constant (KB) for β₁-adrenoceptors of 0.8 nmol/l and β₂-adrenoceptors of 1.3 nmol/l. At slightly higher concentrations, carvedilol produced competitive inhibition of the α₁-adrenoceptor mediated contractile response to norepinephrine in rabbit aorta with a K_b of 11 nmol/l. Carvedilol had no significant effect on the contractile response to angiotensin II in rabbit aorta at concentrations up to 10 μmol/l, thus demonstrating the lack of nonspecific vasodilator actions in arteries. In canine saphenous vein, carvedilol produced noncompetitive blockade of α₂-adrenoceptor mediated vasoconstriction, indicative of some additional activity. In estrogen-primed rat uterus precontracted by depolarization with KC1 (70 mmol/l), carvedilol produced concentration-dependent relaxation (IC50 of 7.6 μmol/l), consistent with the notion that carvedilol may be a calcium channel antagonist. Support for this hypothesis was obtained in KCl (70 mmol/l) depolarized rabbit aorta where carvedilol (10 μmol/l) produced a 10-fold parallel rightward shift in the concentration-response curve to calcium chloride. These studies demonstrate that carvedilol is a potent β₁, β₂- and α₁-adrenoceptor antagonist, and a moderately potent calcium channel antagonist. These multiple activities of carvedilol may contribute to the antihypertensive activity of the compound.

Most alpha 2-adrenoceptor antagonists do not discriminate between pre- and postjunctional alpha 2-adrenoceptors, and this has led to the commonly held belief that pre- and postjunctional alpha 2-adrenoceptors may represent one homogeneous population of receptors. SK&F 104078 has been shown to be a potent antagonist at postjunctional alpha 2-adrenoceptors at concentrations that do not block prejunctional alpha 2-adrenoceptors. Thus, SK&F 104078 is a competitive postjunctional alpha 2-adrenoceptor antagonist in canine and rabbit saphenous veins, canine saphenous artery and human platelet with a dissociation constant of approximately 100 nmol/l. Conversely, SK&F 104078 is inactive as a prejunctional alpha 2-adrenoceptor antagonist in atria from dog, guinea pig, rabbit and rat, and in guinea-pig ileum at concentrations up to 10,000 nmol/l. Likewise, SK&F 104078 has the ability to block postjunctional arterial alpha 2-adrenoceptors in vivo in the pithed rat at doses that do not inhibit prejunctional alpha 2-adrenoceptors in the same model. The results suggest that pre- and postjunctional alpha 2-adrenoceptors may not represent one homogeneous class, but rather are discrete subtypes of the alpha 2-adrenoceptor that may be differentiated by SK&F 104078.

The stereoselectivity of carvedilol, a novel beta-adrenoceptor antagonist and vasodilator with one asymmetric carbon atom, was examined at alpha 1- and beta 1-adrenoceptors in vitro and in vivo. (-)-(S)-Carvedilol is a potent, competitive antagonist of the beta 1-adrenoceptor-mediated positive chronotropic response to isoproterenol in guinea pig atrium, with a dissociation constant (KB) of 0.4 nM. (+)-(R)-Carvedilol was more than 100-fold less potent than the (-)-S-enantiomer as an antagonist of beta 1-andrenoceptors, having a KB of approximately 45 nM. Consistent with these findings (-)-(S)-carvedilol (0.1 mg/kg, i.v.) produced a 25-fold rightward shift in the beta 1-adrenoceptor-mediated positive chronotropic response to isoproterenol in pithed rats, whereas the (+)-R-enantiomer had no beta 1-adrenoceptor blocking activity in vivo at this dose. In contrast to the marked degree of stereoselectivity observed at beta 1-adrenoceptors, both (-)-(S)- and (+)-(R)-carvedilol produced equal antagonism of the alpha 1-adrenoceptor-mediated vasoconstrictor response to norepinephrine in rabbit aorta, with KB values of 14 and 16 nM, respectively. Furthermore, in the pithed rat, the alpha 1-adrenoceptor-mediated pressor dose-response curve to cirazoline was shifted approximately 6-fold to the right by both the (+)-R- and (-)-S-enantiomers of carvedilol at a dose of 1 mg/kg, i.v. In anesthetized spontaneously hypertensive rats, (-)-(S)-carvedilol was 6-fold more potent as an antihypertensive than (+)-(R)-carvedilol.(ABSTRACT TRUNCATED AT 250 WORDS)

Vinylogous (Groups III and V) and acetylenologous (Group IV) analogs of the classical beta-adrenergic agents--stimulants and blockers--were prepared in order to evaluate the effect of degree of saturation, position of unsaturation and rigidity of the chain linking the aromatic ring and the amino containing functional group on biological activity. Derivatives from Group III, which represent 4-aryl-3-butenyl-2-ol-amine analogs of Group II, retained beta 1-adrenoceptor antagonist activity albeit substantially less potent (50-200-fold) than that possessed by their aryloxy counterparts. Consistent with the SAR for Group II compounds, substitution at position 2 of the aromatic ring yielded the most potent antagonists (5a, 5d, 5g), with KB's ranging from 73-93 nM while 3,4-dichloro substitution (5e) markedly reduced antagonist potency (KB = 2,400 nM). Agonist activity was also noted for 5b and 5d, suggesting that these compounds may be best classified as partial agonists. Representatives from Groups IV and V were inactive as antagonists at the beta 1-adrenoceptor confirming the importance of the spatial relationship between the hydroxyl and the amino nitrogen.