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R F Bruns

Publications and source records attributed to R F Bruns.

At least 37 records · Page 2Linked to original sources
PubMed1991

N6-substituted adenosine receptor agonists: potential antihypertensive agents.

Adenosine is known to exert a wide range of pharmacological effects including hypotension. This effect of adenosine suggested that modified analogues of adenosine might provide useful antihypertensive agents. Thus, we prepared a series of novel N6-benzocycloalkyladenosines and studied their receptor binding and antihypertensive activity. The structure-activity relationship study shows that the adenosine analogues having the hydrophobic phenyl moiety one carbon away from the C6-nitrogen have modest affinity and selectivity for the A1 receptor, whereas those with the phenyl moiety two carbons away from the C6-nitrogen have excellent affinity and selectivity for the A1 receptor. Many of these analogues showed excellent antihypertensive activity with a wide range of effects on heart rate. There is no direct correlation between the receptor binding affinities and antihypertensive activity; however, it is more closely associated with A1 than A2 affinity. The bradycardic effect of these agonists seems to be due to the A1 affinity. From this set, compound 3 was further evaluated in secondary antihypertensive screens. It lowered the blood pressure dose dependently with effects lasting for over 20 h following administration of a 30 mg/kg dose. Compound 3 was also effective in lowering blood pressure in a renal hypertensive rat model. Thus, appropriately modified N6-substituted adenosines represent a novel class of antihypertensive agents.

Adenosine↗
PubMed1990

Allosteric enhancement of adenosine A1 receptor binding and function by 2-amino-3-benzoylthiophenes.

Several 2-amino-3-benzoylthiophenes were found to increase the binding of [3H]N6-cyclohexyladenosine to A1 adenosine receptors in rat brain membranes. Concentration-response curves were bell-shaped, with up to 45% stimulation of binding at 10 microM followed by inhibition at higher concentrations. Because these compounds originated from a series of nonxanthine adenosine antagonists, the inhibition of binding was attributed to the presence of interfering adenosine antagonist activity. The compounds stimulated binding of several A1 agonist ligands but only inhibited binding of the A1 antagonist ligand [3H]8-cyclopentyl-1,3-dipropylxanthine, indicating that enhancement was specific for the agonist conformation of the receptor. The enhancement was also specific for the A1 receptor, because agonist binding to A2 adenosine, M2 muscarinic, alpha 2 adrenergic, and delta opiate receptors showed little or no enhancement. Uncoupling of the A1 receptor from the inhibitory guanine nucleotide-binding protein did not prevent enhancement. The enhancers slowed the dissociation of [3H]N6-cyclohexyladenosine from the A1 receptor, implying an allosteric mechanism of action. The inhibition of forskolin-stimulated cyclic AMP accumulation in FRTL-5 cells was employed as a functional index of A1 receptor activation. The enhancers caused up to 19-fold leftward shifts in the concentration-response curve for N6-cyclopentyladenosine and also caused up to 55% inhibition of cyclic AMP accumulation in the absence of agonist. The binding and functional results are consistent with a model in which the enhancers bind preferentially to the agonist conformation of the A1 receptor, thereby shifting the receptor equilibrium in favor of agonist binding. Adenosine enhancers may be useful for ischemia and other conditions involving local energy deficits. More generally, allosteric enhancers may provide a means for strengthening physiological control circuits in a variety of receptor systems.

Adenosine↗
PubMed1990

Structure-activity relationships for enhancement of adenosine A1 receptor binding by 2-amino-3-benzoylthiophenes.

The structural requirements for stimulation of adenosine A1 agonist binding by 2-amino-3-benzoylthiophenes and related compounds were investigated. Slowing of the dissociation of [3H]N6cyclohexyladenosine binding was used as a specific measure of the allosteric effects of these compounds. The thiophene ring could be replaced with benzene but not with several nitrogen-containing heterocycles. The 2-amino group was required, and at least one hydrogen on the amino group appeared to be necessary for activity. The keto carbonyl was also essential. Alkyl substitution at the 4-position of the thiophene ring increased activity, whereas 5-position substitution appeared to have little effect. Activity was also increased by various substitutions on the phenyl ring, with 3-(trifluoromethyl) showing optimal activity. The phenyl ring could be replaced with cyclohexyl without major loss of activity. 1-Aminofluoren-9-one, a conformationally locked derivative, was active. Based in part in the latter observation, the active conformation is proposed to have an intramolecular hydrogen bond between the amino nitrogen and the carbonyl oxygen. Because the 2-amino-3-benzoylthiophenes showed competitive adenosine antagonism as well as allosteric enhancement, their affinities as competitive inhibitors of [3H]8-cyclopentyl-1,3-dipropylxanthine binding to A1 receptors were also assessed. Structure-activity relationships for competitive antagonism were distinct from those for allosteric enhancement, with ratios between the two activities varying by more than 1000-fold. Of the analogs tested, (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81,723) had the most favorable ratio of enhancement to antagonism.

Adenosine↗
PubMed1990

Subclasses of angiotensin II binding sites and their functional significance.

Specific binding sites for angiotensin II were identified in the rabbit adrenal gland and uterus and in the rat liver and were divided into two subclasses based on inhibition by nonpeptide ligands. Peptide ligands affected binding equally in all three tissues. However, the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II binding to adrenal and liver homogenates at nanomolar concentrations but exerted only a minimal effect on binding to uterine homogenates. The nonpeptide PD 123319 potently blocked angiotensin II binding to uterine homogenates but had no effect on adrenal or liver homogenates at concentrations up to 10 microM. Further analysis of angiotensin II binding in uterus showed that both sites are present, with the PD 123319-sensitive site predominating. Additionally, the nonhydrolyzable GTP analogue 5'-guanylyl-imidodiphosphate was able to modulate binding to liver and to the Dup 753-sensitive site in uterus but not that to the PD 123319-sensitive site. Saralasin and the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II-stimulated accumulation of inositol phosphates in cultured Clone 9 cells and also relaxed aortic rings previously contracted with angiotensin II. In contrast, PD 123319 had no effect on either angiotensin II-stimulated inositol phosphate accumulation or vasoconstriction. Saralasin and Exp 6803, but not PD 123319, lowered blood pressure in renal hypertensive rats following intravenous administration. These results suggest the existence of two subclasses of angiotensin II binding sites, which differ in their tissue distribution and affinity for the nonpeptide ligands Dup 763, Exp 6803, and PD 123319. Although no functional role for the PD 123319-sensitive subclass has yet been identified, the Dup 753/Exp 6803-sensitive subclass plays an important role in mediating inositol phosphate metabolism, vascular contractile activity, and blood pressure regulation.

Angiotensin II↗
PubMed1989

Comparison of the behavioral effects of adenosine agonists and dopamine antagonists in mice.

The adenosine agonists 5'-N-ethylcarboxamideadenosine (NECA), 2-chloroadenosine (2-CLA), N6-cyclohexyladenosine (CHA), N6-cyclopentyladenosine (CPA), 2-(phenylamino)adenosine (CV-1808) and R and S isomers of N6-phenylisopropyladenosine (R-PIA and S-PIA) decreased spontaneous locomotor activity in mice and, except for CPA, did so at doses that did not impair motor coordination, a profile shared by dopamine antagonists. CV-1808, the only agent with higher affinity for A2 as compared with A1 adenosine receptors, displayed the largest separation between locomotor inhibitory and ataxic potency. Like dopamine antagonists, NECA and CV-1808 also decreased hyperactivity caused by d--amphetamine at doses that did not cause ataxia whereas A1-selective adenosine agonists reduced amphetamine's effects only at ataxic doses. Unlike dopamine antagonists, adenosine agonists inhibited apomorphine-induced cage climbing only at doses that caused ataxia. Involvement of central adenosine receptors in these effects was suggested by the significant correlation obtained between potency for locomotor inhibition after IP and ICV administration. Affinity for A1 but not A2 adenosine receptors was significantly correlated with potency for inducing ataxia. These results suggest that the behavioral profile of adenosine agonists in mice is related to their affinity for A1 and A2 adenosine receptors and indicate that adenosine agonists produce certain behavioral effects that are similar to those seen with dopamine antagonists.

Adenosine↗
PubMed1989

Characteristics of an adenosine A1 binding site in human placental membranes.

Binding sites were solubilized from human placental membrane using 1.5% sodium cholate and were assayed using polyethylene glycol precipitation. These soluble binding sites had properties of an adenosine A1 binding site. 2-[3H]Chloroadenosine and N-[3H]-ethylcarboxamidoadenosine (NECA) binding were time dependent and reversible. Scatchard plots indicate two classes of binding sites with Kd values of 6 and 357 nM for 2-chloro[8-3H]adenosine and 0.1 and 26 nM with [3H]NECA. The specificity of [3H]NECA binding was assessed by the ability of adenosine analogs to complete for binding sites. Using this approach the estimated IC50 values were 60 nM for (R-PIA), 160 nM for S-PIA, 80 nM for NECA, and 20 nM for 2-chloroadenosine. Binding of [3H]NECA to the soluble sites is inhibited to 48% of the control value by 100 microM guanylyl-5'-imidodiphosphate (Gpp(NH)p). The IC50 value for NECA binding to the soluble binding site was increased from 80 nM to 1500 by Gpp(NH)p. There was a shift of binding affinity from a mixture of high and low affinity to only low affinity with 100 microM Gpp(NH)p. Despite these alterations a NECA prelabeled molecular species of 150 kDa did not decrease in molecular weight upon the addition of 100 microM Gpp(NH)p during high-performance liquid chromatography on a Superose 12 column. Other evidence to support the concept of preferential solubilization and assay of a small population of A1 binding sites was obtained. Following solubilization adenosine A2-like binding sites could be detected only in reconstituted vesicles. The existence of small amounts of A1 binding sites in intact human placental membranes was directly demonstrated using the A1 agonist ligand N6-[3H]cyclohexyladenosine and the A1 antagonist ligand 8-[3H]cyclopentyl-1,3-dipropylxanthine. JAR choriocarcinoma cells have "A2-like" membrane binding sites. In contrast to placental membranes, only A2-like binding sites could be solubilized from JAR choriocarcinoma cells. These observations indicate that human placental membranes contain adenosine A1 binding sites in addition to A2-like binding sites. These sites are guanine nucleotide sensitive, but do not shift to a lower molecular weight form upon assumption of a low affinity state.

2-Chloroadenosine↗
PubMed1989

C2,N6-disubstituted adenosines: synthesis and structure-activity relationships.

Extracellular adenosine receptors have been divided into two major subtypes, called A1 and A2. Substitution of the adenosine molecule with appropriate groups at C2 or N6 is known to impart selectivity for the A2 receptor over the A1 receptor. In the present study, we investigated whether substitution at both C2 and N6 would have additive effects on the A2/A1 affinity ratio, thereby providing compounds with greater A2 selectivity than presently available agents. Disappointingly, additivity appeared to hold only when an A1-selective group was present at N6. For instance, 2-(phenylamino) substitution of the A1-selective agonist N6-cyclopentyladenosine resulted in a 70-fold shift in selectivity in favor of the A2 receptor, but the same substitution applied to the A2-selective agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine resulted in a 100-fold loss of affinity with no change in A2 selectivity.

Adenosine↗
PubMed1989

Evaluation and synthesis of aminohydroxyisoxazoles and pyrazoles as potential glycine agonists.

Except for structurally similar small amino acids, such as alanine, beta-alanine, and serine, compounds acting as glycine-receptor agonists are an unknown class of pharmacological agents. To investigate the potential of small, substituted heterocycles to act as glycine agonists, we have evaluated the similarities between glycine and a series of hydroxy- and amino-substituted pyrazoles and isoxazoles through complementary molecular modeling techniques. Using a "scorecard approach" to determine the overall similarity of projected agonist structures to glycine, we prioritized synthesis and subsequently prepared several novel derivatives. The biological activity of these compounds was compared to that of glycine by using a [3H]strychnine-mediated glycine receptor binding assay. Despite the close similarity in the calculated parameters when compared to glycine, no significant receptor-binding activity was observed for the targeted analogues. These results illustrate the structurally exacting nature of the glycine receptor.

Animals↗
PubMed1989

Linear and proximal benzo-separated alkylated xanthines as adenosine-receptor antagonists.

The linear and proximal benzo-separated derivatives of 8-phenyltheophylline, 1,3-diethyl-8-phenylxanthine, 1,3-dipropylxanthine, 1,3-dibutylxanthine, 3-isobutyl-1-methylxanthine, theophylline, caffeine, and isocaffeine have been synthesized and evaluated for affinity at the A1 and A2 adenosine receptors. Although structure-activity relationships in the benzo-separated series differed from the relationships in the simple xanthines, the most potent of the benzo-separated xanthines were about equal in affinity to the most potent of the corresponding xanthines. On the basis of the present results and the diverse structures reported in the literature as non-xanthine adenosine antagonists, it appears that the primary requirement for adenosine-receptor affinity in nonnucleosides is a flat, neutral, fused-ring heterocycle and that once this requirement is met there are numerous potential binding modes.

Animals↗
PubMed1989

Solubilities of adenosine antagonists determined by radioreceptor assay.

The practical use of many adenosine receptor antagonists is limited by poor aqueous solubility. In some cases, solubilities are so low that they are difficult to measure by conventional means. To determine solubilities of adenosine antagonists, a sensitive radioreceptor method has been developed. Solubilities in Tris buffer (pH 7.7) ranged from 141 nM for 8-(2-amino-4-chlorophenyl)-1,3-dipropylxanthine to 945 microM for the amino-substituted xanthine PD 113,297. Ratios between solubility and adenosine receptor affinity varied from 15.8 for the A2-selective antagonist HTQZ to 169,000 for PD 113,297. From literature data on functional activity, it is apparent that useful adenosine antagonist activity in-vivo is only seen in compounds with solubility/affinity ratios greater than 100.

Adenosine↗
PubMed1988

N6-(arylalkyl)adenosines. Identification of N6-(9-fluorenylmethyl)adenosine as a highly potent agonist for the adenosine A2 receptor.

Several N6-(arylalkyl)adenosines related to N6-benzyladenosine were synthesized, and their A1 and A2 adenosine receptor binding affinities were determined. The annulated derivative N6-(1-naphthylmethyl)adenosine resulted in a very potent A2 agonist (A1 Ki = 24 nM, A2 Ki = 9.1 nM), whereas N6-(9-anthracenylmethyl)adenosine was virtually inactive (A1 Ki = 9,000 nM, A2 Ki = 29,000 nM). Interestingly, the structurally similar N6-(9-fluorenylmethyl)adenosine was the most potent A2 agonist reported to date, with a Ki of 4.9 nM in A2 binding and 5.1 nM in A1 binding. The homologues N6-9-fluorenyladenosine and N6-[2-(9-fluorenyl)ethyl]adenosine showed little or no activity at either adenosine receptor. Effects of these agents on heart rate and coronary flow in the isolated rat heart paralleled their A1 and A2 binding affinities, respectively. These data suggest that for high affinity at the A2 receptor a planar hydrophobic function at a certain distance and angle from the N6 nitrogen is required.

Adenosine↗
PubMed1988

[1,2,4]Triazolo[4,3-a]quinoxalin-4-amines: a new class of A1 receptor selective adenosine antagonists.

Several [1,2,4]triazolo[4,3-a]quinoxalines that were reported as antidepressants in the patent literature were found to possess moderate affinity for the adenosine A1 and A2 receptors. On the basis of structural parallels with adenine and adenosine, the N-cyclopentyl derivative was synthesized and found to have improved affinity and selectivity for the A1 receptor. In the N-cyclopentyl series, affinity was optimal with trifluoromethyl substitution at the 1-position, resulting in a compound (9) with 7.3 nM A1 affinity and 138-fold selectivity for the A1 receptor.

Animals↗
PubMed1987

Correlation of adenosine receptor affinities and cardiovascular activity.

Binding affinities of 28 adenosine analogs at A1 adenosine receptors (rat whole brain membranes, [3H]N6-cyclohexyladenosine, CHA), and at A2 adenosine receptors (rat striatal membranes, [3H]NECA) were compared to their EC25 values for decreasing heart rate and increasing coronary flow in the isolated rat heart. Heart rate (an A1 response) correlated with A1 binding affinity (r2 = 0.71, p less than 0.0001) but not with A2 binding affinity (r2 = 0.007, n.s.); conversely, coronary flow (an A2 response) correlated with A2 binding affinity (r2 = 0.83, p less than 0.0001) but not with A1 binding affinity (r2 = 0.05, n.s.). These results confirm that the brain A1 and A2 receptors, studied by binding methods, bear close similarities to their respective counterparts in the heart, studied by means of functional responses.

Adenosine↗
PubMed1987

Binding of the A1-selective adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine to rat brain membranes.

8-Cyclopentyl-1,3-dipropylxanthine (PD 116,948) is a very potent, very A1-selective adenosine antagonist, with a Ki of 0.46 nM in 3H-CHA binding to A1 receptors in rat whole brain membranes and 340 nM in 3H-NECA binding to A2 receptors in rat striatal membranes. Its 740-fold A1-selectivity is the highest reported for an adenosine antagonist. 3H-PD 116,948 (117 Ci/mmol) was prepared by reduction of the diallyl analog. 3H-PD 116,948 bound to a single site in rat whole brain membranes, with a Bmax of 46 pmol/g wet weight and Kd of 0.42 nM. Nonspecific binding was extremely low, amounting to about 3% of total binding under standard conditions and less than 1% when higher tissue concentrations were used. Affinities of compounds for inhibition of 3H-PD 116,948 binding were highly consistent with an A1 adenosine receptor. Antagonists were equally potent in 3H-PD 116,948 binding and in 3H-CHA binding, while agonists were consistently about 12-fold more potent in 3H-CHA binding. Hill coefficients were 1.0 for antagonists and about 0.65 for agonists. 3H-PD 116,948 should be a useful antagonist ligand for adenosine A1 receptors.

Adenosine↗
PubMed1987

PD 115,199: an antagonist ligand for adenosine A2 receptors.

PD 115,199, N-[2-(dimethylamino)ethyl]-N-methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3- dipropyl-1H-purin-8-yl)benzenesulfonamide, was found to have high affinity for the A2 adenosine receptor labeled by 3H-NECA in rat striatal membranes (Ki 15.5 nM). Unlike other potent adenosine antagonists, which always showed some degree of selectivity for the A1 receptor, PD 115,199 had equal affinity at A1 and A2 receptors (Ki in 3H-CHA binding to A1 receptors 13.9 nM). 3H-PD 115,199 (126 Ci/mmol) was prepared by reduction of the diallyl analog, and binding experiments were performed with 0.5 nM 3H-PD 115,199 at 25 degrees C in rat striatal membranes. By nonlinear least-squares analysis of the concentration-inhibition curve for the highly A1-selective adenosine antagonist PD 116,948 (8-cyclopentyl-1,3-dipropylxanthine), it could be demonstrated that about 11% of specific 3H-PD 115,199 binding was to A1 receptors, and the remainder to A2 receptors. A 20 nM concentration of PD 116,948 was included in subsequent experiments to eliminate the A1 component of binding. The remaining binding had a Kd of 2.6 nM and Bmax of 56 pmol/g wet weight. Specific binding was about 79% of total binding. Affinities of compounds in the 3H-PD 115,199 assay were consistent with binding to a high-affinity A2 receptor: antagonists were consistently about three times more potent in 3H-PD 115,199 binding than in 3H-NECA binding, whereas agonists were consistently about fivefold less potent.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗