PD 81723

Hippocampal injury and neurobehavioral deficits are improved by PD 81,723 following hyperglycemic cerebral ischemia.[Pubmed:12957503]

Exp Neurol. 2003 Sep;183(1):188-96.

We investigated the effects of PD 81,723, an allosteric enhancer for the adenosine A(1) receptor subtype, on hippocampal injury and Morris water maze (MWM) performance following hyperglycemic cerebral ischemia and reperfusion (4-VO, 10 min) in the rat. PD 81,723 (3 or 10 mg/kg) or the equivalent volume of saline was administered intraperitoneally 30 min prior to ischemia. Moderate hyperglycemia was achieved by administration of D-glucose (3g/kg, i.p.) 15 min prior to induction of ischemia. Morris water maze trials were performed on the 6th, 7th, and 8th days after the ischemic insult. The rat brains were sectioned (8 microm), stained with cresyl violet/acid fuchsin, and evaluated for hippocampal ischemic injury by an experimenter blinded to the treatment conditions. At the higher dose, PD 81,723 (10 mg/kg) had no effect on hippocampal injury or MWM performance following hyperglycemic ischemia compared to corresponding saline-treated animals. In contrast, a lower dose of PD 81,723 (3 mg/kg) resulted in significant (P < 0.05, n = 8) reduction of hippocampal injury following hyperglycemic ischemia. Furthermore, corresponding Morris water maze performance (latency, learning index, and cumulative distance swum) was significantly improved by PD 81,723 (P < 0.05, n = 8). The results of the present study suggest that, in the presence of PD 81,723, an A(1) allosteric enhancer, endogenously produced adenosine is sufficient to exert significant neuroprotection during hyperglycemic ischemia. Moreover, the present study provides further evidence for a neuromodulatory role of adenosine during hyperglycemic ischemia.

Effects of the adenosine A1 receptor allosteric modulators PD 81,723 and LUF 5484 on the striatal acetylcholine release.[Pubmed:12421644]

Eur J Pharmacol. 2002 Nov 15;454(2-3):177-82.

The objective of the present study was to characterize the adenosine A(1) receptor allosteric enhancing and antagonistic actions of (2-amino-4,5,6,7-tetrahydrobenzo[b]thiophen-3-yl)(3,4-dichlorophenyl)methanone (LUF 5484) and (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81,723) on striatal acetylcholine release. Upon local administration in conscious rats, LUF 5484 or PD 81,723 caused a concentration-dependent increase of extracellular acetylcholine levels of approximately 40%, which was similar to that obtained by the selective adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dimethylxanthine (8CPT) and N(6)-cyclopentyl-9-methyladenine (N0840). In interaction experiments, LUF 5484 or PD 81,723 did not change the inhibition of acetylcholine release by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA), whereas 8CPT caused an eightfold rightward shift. Acetylcholine concentrations were diminished with 62+/-3%, 48+/-11% and 56+/-9% by CPA, CPA+LUF 5484 and CPA+PD 81,723, respectively. In conclusion, the antagonistic action of LUF 5484 and PD 81,723 seems to counteract the putative allosteric actions with respect to the reduction of striatal acetylcholine release.

Differential effects of the adenosine A(1) receptor allosteric enhancer PD 81,723 on agonist binding to brain and adipocyte membranes.[Pubmed:10517954]

Brain Res. 1999 Sep 4;840(1-2):75-83.

The benzoylthiophene analog, PD 81,723, has been shown to allosterically enhance agonist binding and functional activation of the mammalian adenosine (ADO) A(1) receptor subtype by putatively maintaining the receptor in a high affinity state. The present studies were conducted to evaluate the ability of PD 81,723 to enhance the binding of [3H]cyclohexyladenosine ([3H]CHA) to A(1) receptors of neural (cerebral cortex) and non-neural (adipocyte) origin in three different species; rat, guinea pig and dog. PD 81, 723 (0.3-100 microM) produced a concentration-dependent enhancement of [3H]CHA binding to rat brain A(1) receptors. These effects were also species-dependent with larger enhancements (150-200% of control) observed in guinea pig and dog brain membranes as compared to the rat (120% of control). In contrast, PD 81,723 did not produce any enhancement of [3H]CHA binding to A(1) receptors in adipocyte membranes from any of the species examined. Additional binding studies were conducted using pharmacological manipulations that have previously been shown to enhance the allosteric effects of PD 81,723. In the presence of 1 mM GTP, the allosteric effects of PD 81,723 (15 microM) were increased in rat, guinea pig and dog brain membranes, however, in adipocyte membranes from each species, no significant alteration in agonist binding was observed. Similarly, the A(1) receptor selective antagonist 8-cyclopentyl-1, 3-dipropylxanthine (added to effectively reduce the intrinsic antagonist properties of PD 81,723) was found to enhance the allosteric effects of PD 81,723 (15 microM) in brain, but produce no alteration of agonist binding in adipocyte membranes from each species. Examination of the dissociation kinetics of [3H]CHA binding from rat brain and adipocyte membranes revealed that PD 81,723 (15 microM) differentially slowed agonist dissociation from brain, but not adipocyte, membranes. Taken together, the present data support the hypothesis that in tissues that are sensitive to PD 81,723, this benzyolthiophene functions to maintain the A(1) receptor in a high-affinity state and that the relative proportions of high-affinity A(1) receptors present in specific tissues may contribute, at least in part, to the apparent differential effects of PD 81,723 on agonist binding. The tissue specific modulation of A(1) receptor function by PD 81,723 also illustrates the possibility that the locus of allosteric modulation by PD 81,723 may be manifest via a specific, but indirect and tissue-dependent, interaction with the A(1) receptor.

A1 adenosine receptor allosteric enhancer PD-81723 protects against renal ischemia-reperfusion injury.[Pubmed:22759398]

Am J Physiol Renal Physiol. 2012 Sep;303(5):F721-32.

Activation of A(1) adenosine receptors (ARs) protects against renal ischemia-reperfusion (I/R) injury by reducing necrosis, apoptosis, and inflammation. However, extrarenal side effects (bradycardia, hypotension, and sedation) may limit A(1)AR agonist therapy for ischemic acute kidney injury. Here, we hypothesized that an allosteric enhancer for A(1)AR (PD-81723) protects against renal I/R injury without the undesirable side effects of systemic A(1)AR activation by potentiating the cytoprotective effects of renal adenosine generated locally by ischemia. Pretreatment with PD-81723 produced dose-dependent protection against renal I/R injury in A(1)AR wild-type mice but not in A(1)AR-deficient mice. Significant reductions in renal tubular necrosis, neutrophil infiltration, and inflammation as well as tubular apoptosis were observed in A(1)AR wild-type mice treated with PD-81723. Furthermore, PD-81723 decreased apoptotic cell death in human proximal tubule (HK-2) cells in culture, which was attenuated by a specific A(1)AR antagonist (8-cyclopentyl-1,3-dipropylxanthine). Mechanistically, PD-81723 induced sphingosine kinase (SK)1 mRNA and protein expression in HK-2 cells and in the mouse kidney. Supporting a critical role of SK1 in A(1)AR allosteric enhancer-mediated renal protection against renal I/R injury, PD-81723 failed to protect SK1-deficient mice against renal I/R injury. Finally, proximal tubule sphingosine-1-phosphate type 1 receptors (S1P(1)Rs) are critical for PD-81723-induced renal protection, as mice selectively deficient in renal proximal tubule S1P(1)Rs (S1P(1)R(flox/flox) PEPCK(Cre/-) mice) were not protected against renal I/R injury with PD-81723 treatment. Taken together, our experiments demonstrate potent renal protection with PD-81723 against I/R injury by reducing necrosis, inflammation, and apoptosis through the induction of renal tubular SK1 and activation of proximal tubule S1P(1)Rs. Our findings imply that selectively enhancing A(1)AR activation by locally produced renal adenosine may be a clinically useful therapeutic option to attenuate ischemic acute kidney injury without systemic side effects.

Adenosine A1 receptor-dependent and -independent effects of the allosteric enhancer PD 81,723.[Pubmed:9918544]

J Pharmacol Exp Ther. 1999 Feb;288(2):446-54.

The 2-amino-3-benzoylthiophene PD 81,723 has been shown to exhibit allosteric enhancement of adenosine A1 receptor binding and function. The aim of this study was to clarify the mechanism of this effect using membranes purified from rat brain and Chinese hamster ovary (CHO)-A1 cells that stably express the rat adenosine A1 receptor as well as intact CHO-A1 and nontransfected CHO cells. In membranes containing 100 microM magnesium, (2-amino-4, 5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81, 723) significantly increased the affinity of the adenosine A1 receptor agonist, cyclopentyladenosine, for the low-affinity receptor without affecting high-affinity binding or Bmax. In intact cells, PD 81,723 inhibited basal adenylyl cyclase (AC) activity as well as forskolin-, cholera toxin-, and pertussis toxin-stimulated AC activity in CHO-A1 and CHO cells. Basal AC activity was inhibited 49% in CHO and 82% in CHO-A1 cells by 30 microM PD 81,723. In CHO-A1 cells, half-maximal inhibition of forskolin-stimulated AC occurred at 5 microM PD 81,723 compared to 10 microM in CHO cells. Cholera toxin-stimulated AC was reduced 90% in both CHO and CHO-A1 cells by 30 microM PD 81,723. At the same concentration of PD 81,723, pertussis toxin-stimulated AC activity was reduced 86% (CHO-A1) and 77% (CHO). [3H]forskolin was displaced from purified rat liver AC by PD 81,723 with an IC50 of 96 microM. These results demonstrate that two mechanisms appear to contribute to the observed effects of PD 81, 723. One mechanism is allosteric enhancement of adenosine A1 receptor function. Results from transfected and nontransfected cells suggest that PD 81,723 also inhibits AC directly by binding to the catalytic unit at or near the forskolin-binding site.

Agonist-independent effect of an allosteric enhancer of the A1 adenosine receptor in CHO cells stably expressing the recombinant human A1 receptor.[Pubmed:9152383]

J Pharmacol Exp Ther. 1997 May;281(2):761-8.

The allosteric enhancer PD 81,723, a 2-amino-3-benzoylthiophene derivative, has been shown to potentiate agonist binding to A1 adenosine receptors (A1AdoRs) and to enhance the functional effects of adenosine and adenosine analogs. The objective of this study was to determine whether the apparent agonist-independent effect of PD 81,723 observed in CHO cells stably expressing the recombinant human A1AdoR was due to the potentiation of the action of endogenous adenosine, to the presence of constitutive receptor activity and/or to the binding of PD 81,723 to the agonist binding site of the A1AdoR. The allosteric enhancer PD 81,723, the A1AdoR agonist (R)-N6-(2-phenylisopropyl)adenosine and adenosine all significantly inhibited forskolin-stimulated cAMP accumulation in intact cells and increased [35S]-5'-(gamma-thio)triphosphate binding to cell membranes. The effects of adenosine on cAMP formation and [35S]-5'-(gamma-thio)triphosphate binding were attenuated by adenosine deaminase, but the effects of PD 81,723 were not. In the presence of ADA, the A1AdoR antagonist 8-cyclopentyl-1,3-dipropylxanthine increased forskolin-stimulated cAMP accumulation in cells expressing the recombinant human A1AdoR but not in nontransfected CHO cells. In binding experiments, the agonist (R)-N6-(2-phenylisopropyl)adenosine, but not PD 81,723, significantly displaced the specific binding of the A1AdoR agonist [3H]-N6-cyclohexyladenosine and the antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine. The results of this study demonstrate that in CHO cells stably expressing the recombinant human A1AdoR, the agonist-independent effect of PD 81,723 is not due to potentiation of the action of endogenous adenosine or mediated by the binding of the allosteric enhancer to the agonist binding site of the recombinant human A1AdoR. It is possible that these effects are due to potentiation of constitutive receptor activity by PD 81,723.

PD 81,723, an allosteric enhancer of the A1 adenosine receptor, lowers the threshold for ischemic preconditioning in dogs.[Pubmed:8781475]

Circ Res. 1996 Sep;79(3):415-23.

PD 81,723 (PD) acts allosterically to increase agonist binding to A1 adenosine receptors and to enhance functional A1 receptor-mediated responses in the heart and other tissues. To determine if PD lowers the threshold for ischemic preconditioning (PC), pentobarbital-anesthetized dogs were subjected to 60 minutes of left anterior descending coronary artery (LAD) occlusion and 3 hours of reperfusion. Ischemic PC was produced by either 2.5 or 5 minutes of LAD occlusion 10 minutes before the 60-minute occlusion. PD (100 micrograms/kg total dose, 5 to 50 mumol/L in coronary arterial blood) or vehicle was infused intracoronarily for 17.5 minutes before the 60-minute occlusion period in non-PC dogs or in dogs preconditioned with 2.5 minutes of ischemia. Myocardial infarct size was determined by triphenyltetrazolium staining and expressed as a percentage of the area at risk. Compared with the control group (26.3 +/- 3.6%, mean +/- SEM), infarct size was not significantly affected by 2.5 minutes of PC alone (23.4 +/- 4.2%) or by PD alone (26.5 +/- 1.7%) but was decreased by PD + PC (14.6 +/- 1.7%, P < .05) or by a longer period (5 minutes) of PC alone (12.5 +/- 3.3%). The intravenous administration of the selective antagonist of A1 adenosine receptors, 8-cyclopentyl-1,3-dipropylxanthine (1 mg/kg), or the ATP-sensitive K+ channel blocker, glibenclamide (0.3 mg/kg), for 15 minutes before PD + PC blocked the protection (23.6 +/- 2.3% or 25.9 +/- 3.3%, respectively). None of the compounds studied affected systemic hemodynamics, collateral blood flow, or AAR. To determine which subtypes of canine adenosine receptors were affected by 10 mumol/L PD, radioligand binding studies were conducted using membranes derived from COS-7 cells expressing recombinant canine receptors and agonist radioligands. PD enhanced the binding of [125I]N6-4-amino-3-iodobenzyladenosine (125I-ABA) to A1 receptors by increasing the t1/2 for dissociation by 2.18-fold, but PD had no effect on the dissociation kinetics of 125I-ABA from A3 receptors or [125I]-[2-(4-amino-3-iodo-phenyl)ethylamino] adenosine from A2A receptors. Glibenclamide at concentrations up to 10 mumol/L had no effect on the binding of radioligands to recombinant canine A1, A2A, or A3 receptors. These data suggest that PD reduces the amount of time required for ischemia to produce preconditioning by enhancing adenosine binding to its A1 receptor. Glibenclamide prevents the protection afforded by A1 receptor activation by a mechanism not involving adenosine receptor blockade.

Structure-activity relationships for enhancement of adenosine A1 receptor binding by 2-amino-3-benzoylthiophenes.[Pubmed:2250667]

Mol Pharmacol. 1990 Dec;38(6):950-8.

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.