Pharmacological profile of TH-142177, a novel orally active AT1-receptor antagonist

Summary— The pharmacological properties of TH-142177 (N-n-butyl-N-[2'-(1-H-tetrazole-5-yl)biphenyl-4-yl]-methyl-(N-carboxymethyl-benzylamino)-acetamide), a novel antagonist of the angiotensin II (AII) AT₁ receptor, were studied in vitro and in vivo, and compared to those of losartan. In the rat isolated aorta, TH-142177 produced parallel shifts to the right of the concentration-response curves for AII-induced contractions without affecting the maximal response (pA₂ = 9.07). The inhibitory potency of TH-142177 in the aorta was about three times greater than that of losartan. TH-142177 completely inhibited the specific binding of [¹²⁵I]AII to AT₁ receptor in rat aortic membranes (K_i = 1.6 × 10⁻⁸ M), whereas specific [¹²⁵I]AH binding to AT₂ receptor in bovine cerebellum and human myocardium was not affected by concentrations of TH-142177 up to 10⁻⁵ M. Losartan also inhibited the [¹²⁵I] AII binding to rat aortic membranes ( K _i = 2.2 × 10⁻⁸ M). Following the intravenous administration to anesthetized normotensive rats, TH-142177 dose-dependently inhibited the increase in systolic blood pressure induced by an intravenous bolus injection of AII that was 1.5 times less potent than losartan. Furthermore, the oral administration of TH-142177 to conscious renal hypertensive rats exerted a dose-dependent reduction of systolic blood pressure without significantly effecting the heart rate. TH-142177 was at least three times more potent than losartan. These results demonstrate that TH-142177 is a potent and selective antagonist of AT₁ receptors and by oral administration has a long-lasting antihypertensive activity.     

In vitro pharmacological characterization of UP 269-6, a novel nonpeptide angiotensin II receptor antagonist

Summary— The in vitro pharmacology of UP 269-6, a novel nonpeptide angiotensin II antagonist, was examined in radioligand binding and functional isolated tissue assays. UP 269-6 bound selectively to AT₁ receptors as evidenced by the inhibition of specific [¹²⁵I] Sar¹, Ile⁸-AII binding in rat adrenal membranes (IC₅₀ = 35.8 nM) and in cultured vascular smooth muscle cells (IC₅₀ = 23.8 nM). UP 269-6 displayed a very high selectivity for the AT₁ compared to the AT₂ receptor subtype (IC₅₀ > 10,000 nM). UP 269-6 inhibited the AII-induced contraction of isolated rabbit aortic strips. The pattern of AII antagonism suggested competitive antagonism at low concentrations (10⁻¹⁰, 3 × 10⁻¹⁰, 10⁻⁹ M) of UP 269-6 and insurmountable antagonism at higher concentrations (3 × 10⁻⁹, 10⁻⁸, 3 × 10⁻⁸ M). Based on the calculated p A₂ values, UP 269-6 (9.86 ± 0.25) was an angiotensin II receptor antagonist as potent as L-158,809 (9.82 ± 0.37) and much more potent than losartan (7.96 ± 0.38). UP 269-6 was devoid of affinity (IC₅₀ > 10,000 nM) for many other receptors, ion channels and uptake sites, demonstrating its high specificity for AII receptors. Furthermore, this compound did not affect the contractile response to KCl or phenylephrine in rabbit aorta and exhibited no effect on angiotensin converting enzyme activity. These data demonstrate that UP 269-6 is a highly potent, selective and specific AT₁ receptor antagonist.     

Nonpeptide angiotensin II receptor antagonists: insurmountable angiotensin II antagonism of EXP3892 is reversed by the surmountable antagonist DuP 753

Effects of 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphen yl-4- yl)methyl]imidazole, potassium salt (DuP 753), a surmountable angiotensin II (AII) receptor antagonist, on the insurmountable AII antagonism induced by 2-n-propyl-4-trifluoromethyl-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4- yl)methyl]imidazole-5-carboxylic acid (EXP3892) were examined. In the rabbit aorta, EXP3892 exhibited selective and insurmountable AII antagonism. DuP 753 at 10(-6) M, added before or after EXP3892, reversed partially the depressed AII maximal response caused by 10(-9) M EXP3892. Repeated washing of the rabbit aorta created with DuP 753 at 10(-6) M or EXP3892 at 10(-9) M did not restore completely the sensitivity to AII for at least 2 hr. In the pithed rat, EXP3892 showed selective and insurmountable AII antagonism. DuP 753 at 0.1 to 3 mg/kg i.v., given before or after EXP3892, reversed the reduced AII-maximal response induced by EXP3892 at 0.1 mg/kg i.v. We propose that DuP 753 by binding to the AII receptor induces conformational changes resulting in a reduction of the affinity of the receptor for coupling factors/transducer proteins, which causes surmountable antagonism. EXP3892 would diminish the binding capacity for coupling factors accounting for insurmountable antagonism. As DuP 753 and EXP3892 compete for the same AII receptor, the reduced AII-maximal response by EXP3892 may be reversed by DuP 753.     

Identification of functional GRP-preferring bombesin receptors on human melanoma cells

Bombesin was originally isolated from amphibian skin, wherease its mammalian counterpart, gastrin-releasing peptide (GRP), was first identified in the nervous system of the gastrointestinal tract. Whether GRP is present in the human skin is not known. Bombesin-like peptides are also known to modulate growth. We therefore investigated whether human melanoma cell lines express functional GRP-preferring bombesin receptors and whether they alter growth or other specific cellular functions of these tumour cells. GRP receptor mRNA was found in HBL, D-10, Me-28 and A375-6 cell lines, but only A375-6 cells express a large number of high-affinity binding sites for [¹²⁵I]-[Tyr⁴] bombesin ( K _d 0.31 ± 0.04 nmol L⁻¹, 3880 ± 429 binding sites per cell). Bombesin dose-dependently increased cytosolic calcium, but did not alter interleukin (IL) 1β-induced reduction of cell viability or IL-6 secretion, both A375-6-specific cell functions. Growth of A375-6 cells was not altered by bombesin or the specific GRP receptor antagonist BIM26226 as measured by [³H]-thymidine incorporation or methylene blue assay, whereas insulin alone or in combination with other potential growth factors dose-dependently stimulated growth of these cells. The newly characterized GRP-preferring bombesin receptors on highly malignant human melanoma cells could initiate studies of growth effects on solid tumours or in vivo scanning using radiolabelled tracers.     

Pharmacology of DuP 532, a selective and noncompetitive AT1 receptor antagonist

DuP 532 (2-propyl-4-pentafluoroethyl-1-[(2'-(1H-tetrazol-5-yl)bip hen yl- 4-yl)methyl]imidazole-5-carboxylic acid) inhibited the specific binding of [125I]angiotensin II (AII) for the subtype receptor AT1 in rat adrenal cortical membranes with an IC50 of 3.1 X 10(-9) M, but not the [125I]AII binding for the subtype AT2 sites in rat adrenal medulla tissues. It inhibited the contractile response to AII selectively and noncompetitively in the isolated rabbit aorta with a KB value of 1.1 X 10(-10) M. The selective AII antagonism was confirmed in the guinea pig ileum and the pithed rat. In conscious rats, DuP 532 inhibited the AII-induced pressor effect, aldosterone secretion, and water drinking induced by AII. In conscious renal hypertensive rats, DuP 532 decreased blood pressure with i.v. and p.o. ED30 of 0.02 and 0.21 mg/kg, respectively. The antihypertensive effect of DuP 532 at 0.3 to 3 mg/kg p.o. lasted for at least 24 hr. In conscious spontaneously hypertensive rats, DuP 532 given i.v. or p.o. at 0.3 to 3 mg/kg reduced blood pressure dose-dependently. DuP 532, at doses up to 100 mg/kg i.v., did not cause a pressor response in conscious normotensive rats, suggesting lack of agonism. DuP 532 exerted selective AII antagonism in conscious dogs. In conscious furosemide-treated dogs, DuP 532 given either at 0.3 and 1 mg/kg i.v. or at 1 to 10 mg/kg p.o. decreased blood pressure. As the AT1 receptors are responsible for AII-induced vasoconstriction, aldosterone secretion, and water drinking, our study indicates that DuP 532 is a potent, orally active, selective, and noncompetitive AT1 receptor antagonist and antihypertensive agent.     

The antagonist properties of S 11978 on 5HT3 receptors in N1E-115 neuroblastoma cells

Summary— The effects of the novel antagonist S 11978 (Endo-7-[(8-methyl-8-azabicyclo[3,2,1]-3-octyl)oxycarbonyl] benzo[b] thiophene) on 5HT₃ receptors were examined in N1E-115 mouse neuroblastoma x rat glioma hybrid cells, with radioligand binding and whole cell patch clamp techniques. The 5HT₃ receptor ligand [³H] quipazine was displaced by ICS 205–930, GR 38032F and S 11978 with K_I values of 2.25 nM, 36.5 nM and 1.75 nM respectively. Electrophysiological studies showed that S 11978 is a potent 5HT₃ antagonist: IC₅₀ values for inhibition of 5HT-induced inward current by ICS 205–930, GR 38032F and S 11978 were 0.22 nM, 0.63 nM and 0.43 nM respectively at a holding potential of-65 mV. It is concluded that S 11978 is a potent, high affinity 5HT₃ receptor antagonist.     

Simultaneous Quantitative Measurement of 131I-Iodine and 99mTc-Pertechnetate Uptake by Human Salivary Glands Using Scintiscanning with Validation by Direct Estimation in Biopsy Samples

Abstract. ¹³¹I-iodide and ⁹⁹Tc-pertechnetate concentration in human salivary glands has been measured simultaneously in vivo by quantitative scintiscanning in 7 thyrotoxic subjects. The mid scan times were 5, 12 and 20 min. and the gland to plasma ratio (G/P) of ¹³¹I rose to 4.03±0.82 (s.e.) in the parotid and 10.7 ±3.1 (s.e.) in the submandibular glands. Corresponding values for G/P ^99mTcO₄-were 2.70 ±0.34 in the parotid and 5.3 ± 1.2 in the submandibular glands. Values obtained at parotidectomy 1 h after intravenous administration of a mixture containing ¹²⁵I^- and ^99mTcO₄^- to 6 patients were 4.13 ± 0.85 for G/P ¹²⁵I- and 2.50 ± 0.62 for G/P^99mTcO₄-. G/P¹³¹I- (or G/P¹²⁵I-) and G/P ^99mTeO₄-, derived by both methods, were significantly correlated in parotid and submandibular glands. There was a significant correlation between G/P ¹³¹I-/G/P ^99mTcO₄- in the parotid glands and G/P¹³¹I-/G/P^99mTcO₄- in the submandibular glands. -It is concluded that 1. salivary gland values of ¹³¹I^- and ^99mTcO₄,^-as measured by scintiscanning are very close to values obtained by direct counting of excised human salivary gland tissue and 2. that the secretion processes in both glands are physiologically related.     

Biochemical and electrophysiological properties of SR 57746A, a new, potent 5-HT1A receptor agonist

Summary— SR 57746A (1-[2-(naphth-2-yl) ethyl]-4-(3-trifluoromethylphenyl)-1, 2, 5, 6 tetra-hydropyridine hydrochloride) binds competitively, and with high affinity (K_i = 2.0 ± 0.7 nM) to 5-HT_1A receptors from rat hippocampus in vitro , but has much less affinity for other 5-HT receptor subtypes (IC₅₀ > 650 nM). SR 57746A produces a concentration-dependent inhibition of forskolin-stimulated adenylate cyclase activity in rat hippocampal homogenates, with a maximal effect identical to that of 8-OH-DPAT, suggesting that SR 57746A behaves as a full agonist in tthis experimental model. SR 57746A potently displaces [³H]8-OH-DPAT binding to rat hippocampal membranes ex vivo , with an ID₅₀ of 11.1 mg/kg po, 30 min after administration, and 2.8 mg/kg po, 2 h after administration. This effect of SR 57746A is long-lasting (at least 24 hours at 10 mg/kg po). SR 57746A does not modify the levels of 5-HT or DA in various brain areas, but decreases the concentrations of 5-HIAA, and increases those of DOPAC, HVA and 3-MT. Following iv administration, SR 57746A (0.095 to 0.25 mg/kg) inhibits the spontaneous firing of dorsal raphe neurones, but does not modify the activity of DA neurones in the substantia nigra or ventral tegmental area. Thus, SR 57746A is a potent, selective and full agonist at 5-HT_1A receptors in vitro and vivo.     

Platelet Alpha2-Adrenoceptor Binding in Migraine

SYNOPSIS
Platelet alpha 2 -adrenoceptor binding was determined in ten migraine patients and sixteen healthy controls using (³H)-yohimbine as radioligand. The mean value of maximum binding (^Bmax ) in the migraine patients was 133 ±55 binding sites/ platelet which is significantly lower than that of 217 ± 76 binding sites/platelet in controls (p<0.01). There was also a significant difference in the mean dissociation constant (K_D ); 2.05 ± 0.44 nM in migraineurs and 2.98 ± 1.15 nM in controls (p<0.05). Platelet alpha 2 -adrenoceptors may constitute valid models for certain characteristics of alpha 2 -adrenoceptors in the brain. An alteration in platelet alpha₂ -adrenoceptor binding may thus reflect similar alterations in central alpha₂ -adrenoceptors. It is also discussed whether the alpha₂ -adrenoceptor abberrations are intrinsic to migraine or a result of endogenous and/or pharmacologically induced modulation.     

Comparative effects of angiotensin II and angiotensin III in rabbit adrenal and aortic tissues

The addition of angiotensin II (AII) and angiotensin III (AIII) to isolated tissue baths produced the same maximal contractile response of rabbit aortic strips. AIII was about 10 times less potent, the slope of its concentration-response curve was less steep and its rate of onset slower than that of AII. The responses of both AII and AIII were inhibited with equal potency by the surmountable AII antagonist Phe4, Tyr8-AII and its unsurmountable analog Sar1, Leu8-AII but the kinetic patterns of inhibition by both were less well defined with the agonist AIII than with AII. The addition of AIII to tissues which had exhibited a maximal response to AII did not increase the level of contraction, in contrast to the case when norepinephrine was added to tissues contracted by AII. Both AII and AIII displaced [125I]AII binding from rabbit adrenal membranes; AIII was 6 times less potent than AII but displayed competitive kinetics as an inhibitor of [125I]AII binding. In further studies two binding sites for [125I]AII were identified in adrenal membranes, having KD values of 2.0 +/- 0.2 and 19.6 +/- 2.3 nM, respectively. Each site was inhibited by both AII and AIII and the ratio of the apparent Ki values for the two hormones was not significantly different. The Hill coefficient for the high affinity site was, however, lower for AIII than AII. We interpret our data to suggest that AII and AIII act on the same receptors. AIII apparently binds less efficiently than does AII in both rabbit adrenal membranes and rabbit aortic strips.     

In vitro antagonistic properties of a new angiotensin type 1 receptor blocker, azilsartan, in receptor binding and function studies

The angiotensin II (AII) antagonistic action of azilsartan (AZL) [2-ethoxy-1-{[2'-(5-oxo-4,5-dihydro-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methyl}-1H-benzimidazole-7-carboxylic acid] was investigated in radioligand binding and function studies. AZL inhibited the specific binding of 12?I-Sar1-Ile?-AII to human angiotensin type 1 receptors with an IC?? of 2.6 nM. The inhibitory effect of AZL persisted after washout of the free compound (IC(50) value of 7.4 nM). Olmesartan, telmisartan, valsartan, and irbesartan also inhibited the specific binding with IC?? values of 6.7, 5.1, 44.9, and 15.8 nM, respectively. However, their inhibitory effects were markedly attenuated with washout (IC?? values of 242.5, 191.6, >10,000, and >10,000 nM). AZL also inhibited the accumulation of AII-induced inositol 1-phosphate (IP1) in the cell-based assay with an IC?? value of 9.2 nmol; this effect was resistant to washout (IC?? value of 81.3 nM). Olmesartan and valsartan inhibited IP1 accumulation with IC?? values of 12.2 and 59.8 nM, respectively. The activities of these compounds were markedly reduced after washout (IC?? value of 908.5 and 22,664.4 nM). AZL was defined as an inverse agonist in an experiment by using a constitutively active mutant of human angiotensin type 1 receptors. In isolated rabbit aortic strips, AZL reduced the maximal contractile response to AII with a pD'? value of 9.9. The inhibitory effects of AZL on contractile responses induced by AII persisted after the strips were washed; these inhibitory effects were more potent than those of olmesartan. These results suggest that AZL is a highly potent and slowly dissociating AII receptor blocker. Its tight receptor binding might be expected to produce potent and long-lasting antihypertensive effects in preclinical and clinical settings.     

Lineage-specific overexpression of the P2Y1 receptor induces platelet hyper-reactivity in transgenic mice

Summary.  In order to investigate the role of the platelet P2Y₁ receptor in several aspects of platelet activation and thrombosis, transgenic (TG) mice overexpressing this receptor specifically in the megakaryocytic/platelet lineage were generated using the promoter of the tissue-specific platelet factor 4 gene. Studies of the saturation binding of [³³P]2MeSADP in the presence or absence of the selective P2Y₁ antagonist MRS2179 indicated that wild-type (WT) mouse platelets bore 150 ± 31 P2Y₁ receptors and TG platelets 276 ± 34, representing an 84% increase in P2Y₁ receptor density. This led to a well defined phenotype of platelet hyper-reactivity in vitro, as shown by increased aggregations in response to adenosine 5'-diphosphate (ADP) and low concentration of collagen in TG as compared with WT platelets. Moreover, overexpression of the P2Y₁ receptor enabled ADP to induce granule secretion, unlike in WT platelets, which suggests that the level of P2Y₁ expression is critical for this event. Our results further suggest that the weak responses of normal platelets to ADP are due to a limited number of P2Y₁ receptors rather than to activation of a specific transduction pathway. TG mice displayed a shortened bleeding time and an increased sensitivity to in vivo platelet aggregation induced by infusion of a mixture of collagen and epinephrine. Overall, these findings emphasize the importance of the P2Y₁ receptor in hemostasis and thrombosis and suggest that variable expression levels of this receptor on platelets might play a role in thrombotic states in human, which remains to be assessed.     

Mack Forster Award Lecture Receptor nuclear medicine: vasointestinal peptide and somatostatin receptor scintigraphy for diagnosis and treatment of tumour patients

The multiple aspects of radioligand–receptor interactions do not only show a major impact of certain cell surface-bound receptors in the pathophysiology of human disease: the concept of radioligand–receptor interactions has also been extended to the clinic. In particular, naturally occurring peptides, when radiolabelled, are clinically useful for the imaging diagnosis of human disease and have future implications for the treatment of tumour expressing certain target receptors using radiolabelled peptide tracers. The finding that receptors for VIP (vasoactive intestinal peptide) and SST (somatostatin) are overexpressed on tumour cells presents a breakthrough into this direction. Recent data indicate that [¹²³I]-VIP receptor scintigraphy is clinically useful for the in vivo localization of small primary adenocarcinomas, liver metastases and certain endocrine tumours of the gastrointestinal tract. After the successful clinical introduction of the SST analogues [¹²³I]-Tyr³-octreotide and [¹¹¹In]-DTPA- d -Phe¹-octreotide for localization diagnosis of neuroendocrine tumours in 1989, P829, labelled with the more cost-effective radionuclide ^99mTc, nowadays promises to be a potential novel diagnostic imaging agent for tumours expressing SST/VIP receptors. Furthermore, the novel SST analogue [⁹⁰Y]-MAURITIUS is entering the clinic for treatment of VIP/SST receptor-expressing tumours.     

Phenylacetic acids and the structurally related non-steroidal anti-inflammatory drug diclofenac bind to specific γ-hydroxybutyric acid sites in rat brain

γ-Hydroxybutyric acid (GHB) is a proposed neurotransmitter or neuromodulator with a yet unresolved mechanism of action. GHB binds to both specific high-affinity GHB binding sites and to γ-aminobutyric acid subtype B (GABA_B) receptors in the brain. To separate specific GHB effects from GABA_B receptor effects, it is imperative to develop GHB selective and potent compounds. We generated the compound, 4-(biphen-4-yl)-4-hydroxybutyric acid, which is the 4-hydroxyl analogue of the non-steroidal anti-inflammatory drug (NSAID) fenbufen (referred to as γ-hydroxyfenbufen). When measured in a rat brain homogenate [³H]NCS-382 binding assay, γ-hydroxyfenbufen inhibited [³H]NCS-382 binding with a 10-fold higher affinity than GHB ( K _i 0.44 μ m ), thus establishing it as a novel lead structure. The active metabolite of fenbufen, 4-biphenylacetic acid inhibited [³H]NCS-382 binding with a twofold higher affinity than GHB. Measuring the affinities of structurally related NSAIDs for the [³H]NCS-382 site identified diclofenac, a clinically relevant NSAID (Voltaren^®, Diclon^®) of the phenylacetic acid (PAA) type, as a GHB ligand ( K _i value of 5.1 μ m ). Other non-NSAID PAAs also exhibited affinities similar to GHB. Our data raise the interesting possibility that the widely used over-the-counter drug compound, diclofenac, might affect GHB binding at relevant clinical dosages. Furthermore, the identification of PAAs as GHB ligands supplies new information about the structural preferences of the GHB ligand-binding site.     

Up-regulation of 5-HT2 Serotonin Receptor: A Possible Mechanism of Transformed Migraine

SYNOPSIS
Transformation of episodic migraine to chronic daily headache (so called transformed migraine) ia now a well recognized phenomenon. Although several factors, i.e. analgesic overuse, increasing age, psychiatric disorders are reported to play some roles in this transformation, the precise cascade is still unclear. Further suppression of an already abnormal antinociceptive system with up-regulation of post-synaptic receptors is one of the possible explanation. In order to understand the mechanism underlying this condition, 5-HT₂ serotonin receptors on platelets were assayed by the radioligand binding technique. Six transformed migraine patients (67.67 ± 1.52 years) and seven healthy controls (72.86 ± 1.82 years) were studied. [³H]-spiperone and ketanserin were used to determine the specific binding. We found a significant increase (P <0.05) in the maximal receptor numbers (B_max ) on platelet membrane of the migraine patients when compared to the controls (64.31 ± 11.06 end 39.96 ± 5.42 fmol/mg protein, respectively), whereas the dissociation equilibrium constant (K D ) values remained unchanged (3.63 ± 0.78 nM and 2.84 ± 0.48 nM for the migraine patients and controls, respectively). The up-regulation of serotonin receptors found in this study provided further support to the "serotonergic hypofunction" theory of migraine pathogenesis and may explain the unusual loss of episodicity seen in the transformed migraine patients.     

CHANGES IN PLATELET FREE Ca2+ CONCENTRATION AFTER CHRONIC DIGOXIN TREATMENT

Summary— Cell Na+ and Ca²⁺ concentrations control each other by various mechanisms. In excitable cells from various origins, Ca²⁺ extrusion from the cell and its entry are dependent for a large part on the activity of the Na+, Ca²⁺-countertransport system. Cytosolic free Ca²⁺ concentration is also controlled by the Na+–H+ exchange activity. To analyze the changes in cytosolic Ca²⁺ concentration accompanying the reduction of the membrane Na+ gradient, cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) was measured by fluorescent dyes in platelets and erythrocytes from healthy subjects, before and during digoxin treatment (0.25 mg/day for 6 days). [Ca²⁺]_i was increased in platelets from 169±30 to 321±61 nmol/l ( n = 7, P <0.02) and unchanged in erythrocytes (121±6 and 104±7 nmol/l). This increase in platelet [Ca²⁺]_i was not accompanied by a change in serotonin content (5.43±0.67 vs 5.49±0.61 10⁻⁷ mol per 10¹¹ cells) and could not be reproduced by in vitro addition of 10⁻⁴ mol/l ouabain (198±33 vs 186±73 nmol/l). The enhanced [Ca²⁺]_i in platelets is thus not a short-term consequence of a reduced membrane Na+ gradient, but reflects either the overload of intracellular Ca²⁺ stores or an enhanced in vivo stimulation by hormones or neurotransmitters.     

Autoradiographic distribution of [3H]sumatriptan-binding sites in post-mortem human brain

The anatomical distribution of [³H]sumatriptan-binding sites was analysed in brain tissue sections from 11 subjects. Relevant concentrations of [³H]sumatriptan-binding sites were seen in areas such as visual cortex>locus niger>globus pallidus>layers IV-V of the frontal cortex>subiculum>entorhinal cortex>nucleus tractus solitarius>nucleus trigeminalis caudalis. This distribution of [³H]sumatriptan-binding sites in the human brain shows some differences when compared with that of 5HT_1D receptors, confirming that, besides 5HT_1D,sumatriptan also binds to 5HT_1F receptor subtype. Some species differences are evident between the distribution of [³H]sumatriptan-binding sites in the human brain and that reported for guinea-pig and rat brains, emphasizing that caution is needed in extrapolating experimental data from animals to humans. Furthermore, these data help to explain some of the therapeutic actions of sumatriptan. The remarkable levels of binding found in areas such as nucleus tractus solitarius and nucleus trigeminalis caudalis suggest that in migraine attacks sumatriptan could exert its specific anti-emetic effects and, partly at least, induce analgesia by directly acting over these brain nuclei.     

Pharmacological characterization of the nonpeptide angiotensin II receptor antagonist, SK&F 108566.

The angiotensin II (AII) antagonist activity of (E)-alpha-[[2-butyl-1-[(4-carboxyphenyl)methyl]-1H-imidazol-5- yl]methylene]-2-thiophenepropanoic acid (SK&F 108566), was examined in a number of in vitro and in vivo assays. In rat and human adrenal cortical membranes, SK&F 108566 displaced specifically bound [125I]AII with IC50 of 9.2 and 3.9 nM, respectively. SK&F 108566 also inhibited [125I]AII binding to human liver membranes (IC50 = 1.7 nM) and to rat mesenteric artery membranes (IC50 = 1.5 nM). In rabbit aortic smooth muscle cells, SK&F 108566 caused a concentration-dependent inhibition of AII-induced increases in intracellular Ca++ levels. In rabbit aortic rings, SK&F 108566 produced parallel rightward shifts in the AII concentration-response curve without affecting the maximal contractile response. Schild analysis of the data yielded a KB value of 0.26 nM and a slope not different from 1, indicative of competition antagonism. SK&F 108566 had no effect on the contractile responses to KCl, norepinephrine or endothelin in rabbit aorta. In conscious normotensive rats, i.v. administration of SK&F 108566 (0.01-0.3 mg/kg) produced dose-dependent parallel shifts in the AII pressor dose-response curve. Administration of SK&F 108566 (3-10 mg/kg) intraduodenally or intragastrically to conscious normotensive rats resulted in a dose-dependent inhibition of the pressor response to AII (250 ng/kg, i.v.). At 10 mg/kg, i.d., significant inhibition of the pressor response to AII was observed for 3 hr. In this same rat model, SK&F 108566 had no effect on base-line pressure or on the pressor response to norepinephrine or vasopressin. The data demonstrate that SK&F 108566 is a potent, highly selective, competitive nonpeptide AII antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vitro pharmacology of a nonpeptidic angiotensin II receptor antagonist, SC-51316.

The properties of a novel nonpeptidic angiotensin II (AII) receptor antagonist, 2,5-dibutyl-2,4-dihydro-4-([2-(1H-tetrazol-5-yl)(1,1'-biphenyl) -4'-yl]methyl)-3H-1,2,4-triazol-3-one (SC-51316), are described. SC-51316 inhibited [125I]AII binding selectively to the AT1 receptor with IC50 values of 3.6 and 5.1 nM in rat adrenal cortical and rat uterine membrane preparations, respectively. The compound was a competitive and reversible antagonist of AII-mediated contraction of rabbit aortic rings with a pA2 of 8.86. In addition, SC-51316 inhibited AII-induced aldosterone release from rat adrenal zona glomerulosa cells and blocked inhibition of renin release by AII from rat kidney slices with pA2 values of 8.62 and 8.9, respectively. The agent (0.1 mM) did not inhibit angiotensin-converting enzyme or plasma renin activity. These data demonstrate that SC-51316 is a potent AII receptor antagonist which may prove to be useful as a pharmacologic tool for studying the role of the renin-angiotensin system in cardiovascular diseases.     

Nonpeptide angiotensin II receptor antagonists. III. Structure-function studies

A series of 1-benzylimidazole-5-acetate derivatives defining the critical substituents on the phenyl ring was synthesized in order to improve on the affinity of 2-butyl-4-chloro-1-(2-nitrobenzyl)imidazole-5-acetate, sodium (S-8308) for the angiotensin II (AII) receptor. The analogs, substituted with -1-(4-carboxybenzyl) (EXP6155),-1-[4-(2-carboxybenzamido)benzyl] (EXP6159) and the 5-methylacetate of EXP6159 (EXP6803), were found to inhibit the binding of [3H]AII to AII receptors in rat adrenal cortical microsomes with 9-, 35- and 107-fold higher affinity, respectively, than that of S-8308 (IC50, 15 X 10(-6) microM). Scatchard analysis of the [3H]AII binding revealed that in the presence of EXP6155 (10(-6) M), the dissociation constant for AII was increased from 1.2 to 3.9 X 10(-9) M, whereas the total number of binding sites remained unchanged, suggesting a competitive nature of antagonism. A similar order of affinity or potency (saralasin much greater than EXP6803 greater than EXP6159 greater than EXP6155 greater than S8308) was observed in various in vitro and in vivo assays: rat smooth muscle cells AII binding, 45Ca++ influx in rat aortic rings, contractile response in isolated rabbit aorta and AII-induced pressor response in anesthetized rats. Responses (45Ca++ and contraction) elicited by norepinephrine or by KCl were unaltered by these agents at concentrations of up to 10(-4) M. In addition, they exerted no direct effect on the activity of rabbit angiotensin converting enzyme and rat renin. In conscious renal artery-ligated rats, EXP6155, EXP6159 and EXP6803 were p.o. inactive, but caused a rapid decrease in mean arterial pressure when administered i.v.(ABSTRACT TRUNCATED AT 250 WORDS)