Adenosine A2A agonist reduces paralysis after spinal cord ischemia: correlation with A2A receptor expression on motor neurons

BACKGROUND

The adenosine A2A agonist ATL-146e ameliorates reperfusion inflammation, reducing subsequent paralysis and neuronal apoptosis after spinal cord ischemia. We hypothesized that neuroprotection with ATL-146e involves inducible neuronal adenosine A2A receptors (A2A-R) that are upregulated after ischemia.

METHODS

Eighteen rabbits underwent laparotomy, and 14 sustained spinal cord ischemia from cross-clamping the infrarenal aorta for 45 minutes. One group (ischemia-reperfusion [I/R] + ATL) received ATL-146e intravenously for 3 hours during spinal cord reperfusion. A second group (I/R) received equivolume intravenous saline solution for 3 hours and served as an ischemic control, and a third group (Sham) underwent sham laparotomy. At 48 hours, all subjects were assessed for motor impairment using the Tarlov scoring system (0 to 5). Lumbar spinal cord sections were immunolabeled for A2A-R and graded in a blinded fashion using light microscopy.

RESULTS

There was a significant improvement in Tarlov scores in I/R + ATL animals compared with the I/R group. Sham-operated animals demonstrated no A2A-R immunoreactivity. There was a dramatic increase in A2A-R immunoreactivity in neurons of lumbar spinal cord sections from I/R compared with I/R + ATL and sham-operated animals.

CONCLUSIONS

Reduction in paralysis in animals receiving ATL-146e correlates with the new finding of A2A-R expression on lumbar spinal cord motor neurons after ischemia. Adenosine A2A agonists may exert neuroprotective effects by binding to inducible neuronal A2A-R that are upregulated during spinal cord reperfusion, and reduced in response to administration of an A2A-R-specific agonist.

Adenosine A2A analogue improves neurologic outcome after spinal cord trauma in the rabbit

BACKGROUND

ATL-146e, an adenosine A2A agonist, reduces paralysis after spinal cord ischemia-reperfusion. We hypothesized that systemic ATL-146e could improve neurologic outcome after blunt spinal cord trauma.

METHODS

Twenty rabbits survived a thoracic spinal cord impact of 30 g-cm. One group received 0.06 microg/kg/min ATL-146e for the first 3 hours after impact (A2A group), whereas a second group received saline carrier (T/C group). Neurologic outcome was measured using the Tarlov scale (0-5). Histologic sections from the A2A and T/C groups were compared for neuronal viability.

RESULTS

There was significant improvement in Tarlov scores of A2A animals compared with T/C animals at 12 hours (p = 0.007), with a trend toward improvement at 36 (p = 0.08) and 48 (p = 0.09) hours after injury. There was decreased neuronal attrition in A2A animals (p = 0.06).

CONCLUSION

Systemic ATL-146e given after spinal cord trauma results in improved neurologic outcome. Adenosine A2A agonists may hold promise as a rapidly acting alternative to steroids in the early treatment of the spinal cord injured patient.

Assessment of myocardial inflammation produced by experimental coronary occlusion and reperfusion with 99mTc-RP517, a new leukotriene B4 receptor antagonist that preferentially labels neutrophils in vivo

BACKGROUND

99mTc-RP517 is a new leukotriene B4 (LTB4) receptor antagonist developed for imaging acute inflammation or infection. A unique property of 99mTc-RP517 is its ability to label white blood cells in vivo after intravenous injection. The goals of this study were to determine relative 99mTc-RP517 binding to human leukocyte subtypes and the 99mTc-RP517 uptake pattern in canine myocardium where inflammation was induced by either coronary occlusion and reperfusion or tumor necrosis factor alpha (TNFalpha) injection.

METHODS AND RESULTS

Fluorescence-activated cell sorter analysis was performed on whole human blood (n=2) and isolated neutrophils (n= 4) with a fluorescent analog of 99mTc-RP517, [F]-RP517. In whole blood, [F]-RP517 (500 nmol/L) preferentially labeled neutrophils. On isolated neutrophils, [F]-RP517 (10 nmol/L) binding was inhibited by 44% when LTB4 (400 nmol/L) was added. 99mTc-RP517 was injected intravenously in anesthetized, open-chest dogs before coronary occlusion (90 minutes) and reperfusion (120 minutes) (n=9) or before intramyocardial TNFalpha injection (n=3). Ex vivo images of heart slices were acquired. The left ventricle was divided into 72 segments for flow and 99mTc-RP517 uptake analysis. There was an inverse exponential relationship between 99mTc-RP517 uptake and occlusion flow (r=0.73). In the same 15 segments, 99mTc-RP517 uptake was highly correlated with the neutrophil enzyme myeloperoxidase (r=0.91). Ex vivo images revealed tracer uptake in the reperfused area (ischemic to normal count ratio=2.7+/-0.2).

CONCLUSIONS

RP517 binds to the neutrophil LTB4 receptor after intravenous injection. After reperfusion, 99mTc-RP517 uptake correlated with myeloperoxidase and was observed on ex vivo images, indicating that this tracer may have potential as an inflammation-imaging agent.

2-Aminothiazoles: a new class of agonist allosteric enhancers of A(1) adenosine receptors

This report describes the synthesis and structure-activity relationships of a new class of A(1) adenosine receptor agonist allosteric enhancers, 2-aminothiazolium salts. The EC(50) of compounds 6a, 6b, 7, and 8 were 0.3, 4.5, 3.8, and 1.2 microM, substantially lower than that of the 'Gold Standard' 2-amino-3-benzoyl thiophene (PD 81,723), which has an EC(50) of 38 microM.

Structure-activity relationships at human and rat A2B adenosine receptors of xanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions

In the search for improved selective antagonist ligands of the A2B adenosine receptor, which have the potential as antiasthmatic or antidiabetic drugs, we have synthesized and screened a variety of alkylxanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions. Competition for 125I-ABOPX (125I-3-(4-amino-3-iodobenzyl)-8-(phenyl-4-oxyacetate)-1-propylxanthine) binding in membranes of stably transfected HEK-293 cells revealed uniformly higher affinity (<10-fold) of these xanthines for human than for rat A2B adenosine receptors. Binding to rat brain membranes expressing A1 and A2A adenosine receptors revealed greater A2B selectivity over A2A than A1 receptors. Substitution at the 1-position with 2-phenylethyl (or alkyl/olefinic groups) and at N-3 with hydrogen or methyl favored A2B selectivity. Relative to enprofylline 2b, pentoxifylline 35 was equipotent and 1-propylxanthine 3 was >13-fold more potent at human A2B receptors. Most N-7 substituents did not enhance affinity over hydrogen, except for 7-(2-chloroethyl), which enhanced the affinity of theophylline by 6.5-fold to 800 nM. The A2B receptor affinity-enhancing effects of 7-(2-chloroethyl) vs 7-methyl were comparable to the known enhancement produced by an 8-aryl substitution. Among 8-phenyl analogues, a larger alkyl group at the 1-position than at the 3-position favored affinity at the human A2B receptor, as indicated by 1-allyl-3-methyl-8-phenylxanthine, with a K(i) value of 37 nM. Substitution on the 8-phenyl ring indicated that an electron-rich ring was preferred for A2B receptor binding. In conclusion, new leads for the design of xanthines substituted in the 1-, 3-, 7-, and 8-positions as A2B receptor-selective antagonists have been identified.

Adenosine A2A analogue ATL-146e reduces systemic tumor necrosing factor-alpha and spinal cord capillary platelet-endothelial cell adhesion molecule-1 expression after spinal cord ischemia

OBJECTIVE

Inflammation is likely a major contributor to spinal cord reperfusion injury after aortic reconstruction. Systemic 4-(3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl)-cyclohexanecarboxylic acid methyl ester (ATL-146e), a selective adenosine A(2A) agonist, has been shown to reduce paralysis after spinal cord ischemia. We hypothesized that ATL-146e reduces cytokine production during spinal cord reperfusion, curtailing inflammation and decreasing spinal cord capillary platelet-endothelial cell adhesion molecule-1 (PECAM-1) expression.

STUDY DESIGN

New Zealand White rabbits sustained spinal cord ischemia with 45-minute cross-clamping of the infrarenal aorta. One group of animals received intravenous ATL-146e at 0.06 microg/kg/min for 3 hours during reperfusion, beginning after 30 minutes of ischemia. A second group received saline solution vehicle alone for 3 hours, serving as an ischemic control. A third group served as sham-operated animals, undergoing laparotomy with anesthesia. Serum was assayed with enzyme-linked immunosorbent assay for tumor necrosing factor-alpha (TNF-alpha). Animals were allowed to recover for 48 hours and were evaluated for hind-limb motor function with the Tarlov (0 to 5) scoring system. At necropsy, animals from each group yielded spinal cords for immunohistochemical staining for PECAM-1. Data are expressed as mean +/- standard error of the mean, with statistical analysis with Student t test and Kruskal-Wallis nonparametric test.

RESULTS

Markedly improved Tarlov scores were seen in rabbits with ATL-146e (P <.001) during spinal cord reperfusion as compared with ischemic control animals. A significant reduction was found in TNF-alpha in the sera of rabbits with ATL-146e infusion (P <.01) as compared with ischemic control animals. Significantly reduced endothelial PECAM-1 staining intensity (P <.05) was seen in microscopic spinal cord sections from rabbits with ATL-146e.

CONCLUSION

ATL-146e, an adenosine A(2A) agonist, reduces spinal cord reperfusion injury. The mechanism of the protection may involve a reduction in circulating TNF-alpha during a critical 3-hour reperfusion interval and reduction in spinal cord endothelial PECAM-1 upregulation.

Human A(2A) adenosine receptors: high-affinity agonist binding to receptor-G protein complexes containing Gbeta(4)

Agonists bind with higher affinity to G protein-coupled heptahelical receptors than to uncoupled receptors. Recombinant A(1) and A(3) adenosine receptors couple well to G(i/o), but recombinant human A(2A) adenosine receptors (hA(2A)AR) couple poorly to G(s) and bind agonists with K(i) values in binding assays that are much higher than ED(50) values for functional responses such as coronary dilation and inhibition of neutrophil oxidative burst. In this study, we produced hA(2A)AR-G protein complexes in membranes derived from Sf9 cells quadruply infected with receptors and heterotrimeric G protein subunits. The composition of G(beta) markedly influences coupling such that A(2A)AR-alpha(s)beta(1)gamma(2) are 8 +/- 2% coupled whereas equivalently expressed A(2A)AR-alpha(s)beta(4)gamma(2) are 40 +/- 2% coupled. Hence, we were able for the first time to accurately measure high-affinity agonist binding to hA(2A)AR. The agonist 2-[2-(4-amino-3-[(125)I]iodophenyl)ethylamino]adenosine binds to coupled and uncoupled hA(2A)AR with K(D) values of 0.46 nM and 26 nM, respectively, a difference in affinity of 57-fold. The addition of GTPgammaS converts all receptors to the low-affinity state. A(2A)AR coupling does not influence binding of antagonists including, (125)I-4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol ((125)I-ZM241385), K(D) = 0.5 nM. Based on a comparison of high-affinity binding sites, N(6)-3-iodo-2-chlorobenzyladenosine-5'-N-methyluronamide is only 8-fold A(3) selective (A(2A Ki, H) = 18.3 +/- 3.2 nM; A(3 Ki, H) = 2.4 +/- 0.3 nM) and 2-chloro-N(6)-cyclopentyladenosine is only 33-fold A(1) selective (A(2A Ki, H) = 11.0 +/- 1.9; A(1 Ki, H) = 0.3 +/- 0.1). We conclude that recombinant hA(2A)AR can form a high-affinity receptor-G protein complex with alpha(s)beta(4)gamma(2) that is useful for determining receptor selectivity.

2-Amino-3-aroyl-4,5-alkylthiophenes: agonist allosteric enhancers at human A(1) adenosine receptors

2-Amino-3-benzoylthiophenes are allosteric enhancers (AE) of agonist activity at the A(1) adenosine receptor. The present report describes syntheses and assays of the AE activity at the human A(1)AR (hA(1)AR) of a panel of compounds consisting of nine 2-amino-3-aroylthiophenes (3a-i), eight 2-amino-3-benzoyl-4,5-dimethylthiophenes (12a-h), three 3-aroyl-2-carboxy-4,5-dimethylthiophenes (15a-c), 10 2-amino-3-benzoyl-5,6-dihydro-4H-cyclopenta[b]thiophenes (17a-j), 14 2-amino-3-benzoyl-4,5,6,7-tetrahydrobenzo[b]thiophenes (18a-n), and 15 2-amino-3-benzoyl-5,6,7,8-tetrahydro-4H-cyclohepta[b]thiophenes (19a-o). An in vitro assay employing the A(1)AR agonist [(125)I]ABA and membranes from CHO-K1 cells stably expressing the hA(1)AR measured, as an index of AE activity, the ability of a candidate AE to stabilize the agonist-A(1)AR-G protein ternary complex. Compounds 3a-i had little or no AE activity, and compounds 12a-h had only modest activity, evidence that AE activity depended absolutely on the presence of at least a methyl group at C-4 and C-5. Compounds 17a-c lacked AE activity, suggesting the 2-amino group is essential. Polymethylene bridges linked thiophene C-4 and C-5 of compounds 17a-j, 18a-n, and 19a-o. AE activity increased with the size of the -(CH(2))(n)()- bridge, n = 3 < n = 4 < n = 5. The 3-carbethoxy substituents of 17a, 18a, and 19a did not support AE activity, but a 3-aroyl group did. Bulky (or hydrophobic) substituents at the meta and para positions of the 3-benzoyl group and also 3-naphthoyl groups greatly enhanced activity. Thus, the hA(1)AR contains an allosteric binding site able to accommodate 3-aroyl substituents that are bulky and/or hydrophobic but not necessarily planar. A second region in the allosteric binding site interacts constructively with alkyl substituents at thiophene C-4 and/or C-5.

Adenosine-mediated cardioprotection in ischemic-reperfused mouse heart

We investigated the roles of A1, A2A, or A3 receptors and purine salvage in cardioprotection with exogenous adenosine, and tested whether A2A -mediated reductions in perfusion pressure modify post-ischemic recovery. Treatment with 10(-5) or 5 x 10(-5) M adenosine improved contractile recovery from 20 min ischemia 45 min reperfusion in isolated mouse hearts. Protection was attenuated by adenosine kinase inhibition (10(-5) M iodotubercidin) and receptor antagonism (5 x 10(-5) M 8-rho-sulfophenyltheophylline, 8-SPT). Enzyme efflux mirrored contractile recoveries. A 3 agonism with 10(-7) M 2-chloro- N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) improved ischemic tolerance whereas A1 agonism with 5 x 10(-8) M N6-cyclopentyladenosine (CPA) and A2A agonism with 10(-9) M 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) or 2 x 10(-8) M methyl-4-(3-[9-[4S,5S,2R,3R)-5-(N-ethylcarbamoyl)-3,4-dihydroxyoxolan-2-yl]-6-aminopurin-2-yl)]prop-2-ynyl) cyclohexane-carboxylate (ATL-146e) were ineffective. Protection via A1 receptor overexpression was enhanced by adenosine, but unaltered by A1 or A2A agonists. Finally, post-ischemic dysfunction in hearts perfused at constant flow was dependent on coronary pressure, with A2A AR-mediated reductions in pressure reducing diastolic contracture, and elevated perfusion pressure worsening contracture. Data indicate that cardioprotection with exogenous adenosine in asanguinous hearts involves purine salvage and activation of A3 but not A1 or A2A receptors.

International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

Systemic adenosine A2A agonist ameliorates ischemic reperfusion injury in the rabbit spinal cord

BACKGROUND

The adenosine A2A agonist ATL-146e (4-[3-[6-Amino-9-(5-ethylcarbamoyl-3,4-dihydroxytetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl]-cyclohexanecarboxylic acid methyl ester) has been shown to prevent reperfusion injury in multiple organ systems through inhibition of activated leukocyte-endothelial interaction. We hypothesized that systemic ATL-146e could reduce spinal cord reperfusion injury after aortic clamping.

METHODS

Twenty-six rabbits underwent cross-clamping of the infrarenal aorta for 45 minutes. One group received intravenous ATL-146e for 3 hours during reperfusion. A second cohort received only vehicle and served as controls. Animals were assessed at 24 and 48 hours using the Tarlov (0 to 5) scoring system for hind limb function. To evaluate neuronal attrition, immunostaining of lumbar spinal cord sections was performed using anti-SMI 33 antibody against neurofilament.

RESULTS

Systemic ATL-146e was tolerated without hemodynamic lability. Animals that received ATL-146e had significantly improved neurologic outcomes 24 and 48 hours after spinal cord ischemia (p < 0.001). There was preservation of neuronal architecture in the ventral horn of spinal cord sections from animals receiving ATL-146e compared with control animals.

CONCLUSIONS

Intravenous ATL-146e given during reperfusion is tolerated without hemodynamic lability, and results in substantially improved spinal cord function after ischemia by preservation of ventral horn neurons.

Targeted deletion of A(3) adenosine receptors improves tolerance to ischemia-reperfusion injury in mouse myocardium

A(3) adenosine receptors (A(3)ARs) have been implicated in regulating mast cell function and in cardioprotection during ischemia-reperfusion injury. The physiological role of A(3)ARs is unclear due to the lack of widely available selective antagonists. Therefore, we examined mice with targeted gene deletion of the A(3)AR together with pharmacological studies to determine the role of A(3)ARs in myocardial ischemia-reperfusion injury. We evaluated the functional response to 15-min global ischemia and 30-min reperfusion in isovolumic Langendorff hearts from A(3)AR(-/-) and wild-type (A(3)AR(+/+)) mice. Loss of contractile function during ischemia was unchanged, but recovery of developed pressure in hearts after reperfusion was improved in A(3)AR(-/-) compared with wild-type hearts (80 +/- 3 vs. 51 +/- 3% at 30 min). Tissue viability assessed by efflux of lactate dehydrogenase was also improved in A(3)AR(-/-) hearts (4.5 +/- 1 vs. 7.5 +/- 1 U/g). The adenosine receptor antagonist BW-A1433 (50 microM) decreased functional recovery following ischemia in A(3)AR(-/-) but not in wild-type hearts. We also examined myocardial infarct size using an intact model with 30-min left anterior descending coronary artery occlusion and 24-h reperfusion. Infarct size was reduced by over 60% in A(3)AR(-/-) hearts. In summary, targeted deletion of the A(3)AR improved functional recovery and tissue viability during reperfusion following ischemia. These data suggest that activation of A(3)ARs contributes to myocardial injury in this setting in the rodent. Since A(3)ARs are thought to be present on resident mast cells in the rodent myocardium, we speculate that A(3)ARs may have proinflammatory actions that mediate the deleterious effects of A(3)AR activation during ischemia-reperfusion injury.

Pharmacological stress myocardial perfusion imaging with the potent and selective A(2A) adenosine receptor agonists ATL193 and ATL146e administered by either intravenous infusion or bolus injection

BACKGROUND

Adenosine (Ado) and dipyridamole are alternatives to exercise stress for myocardial perfusion imaging. Though generally safe, side effects frequently occur that cause patient discomfort and sometimes lead to premature termination of the study or require aminophylline administration. Recently, a new class of A(2A) Ado receptor agonists was synthesized. ATL193 and ATL146e are 2-propynylcyclohexyl-5'-N-ethylcarboxamido derivatives of Ado. The study goals were to evaluate the potency and selectivity of these new compounds on recombinant canine Ado receptors and to evaluate their hemodynamic properties in dogs to assess their usefulness as vasodilators for myocardial perfusion imaging.

METHODS AND RESULTS

In assays of recombinant canine Ado receptors, ATL-193 and ATL-146e were highly selective for the A(2A) over the A(1) and A(3) receptors and were more potent than MRE-0470 and CGS-21680. In 16 anesthetized dogs, the agonists were administered by infusion (ATL-193; n=7 normal) or bolus injection (ATL-146e; n=9 critical left anterior descending coronary artery stenosis), and hemodynamic responses were compared with those of Ado. Both agonists produced dose-dependent coronary flow (CF) elevation without provoking the hypotension observed with Ado. After an ATL-146e bolus, the CF increase was sustained for several minutes, providing ample time for injection and myocardial uptake of (99m)Tc-sestamibi, and CF returned to baseline within 20 minutes. The CF increase was completely blocked by the selective A(2A) antagonist ZM241385 (3 microgram. kg(-1). min(-1)).

CONCLUSIONS

ATL-193 and ATL-146e are highly potent and selective Ado A(2A) receptor agonists with excellent potential for use as vasodilators for myocardial perfusion imaging. An important advantage of ATL-146e is the ability to administer it by bolus injection.

An adenosine A2A agonist, ATL-146e, reduces paralysis and apoptosis during rabbit spinal cord reperfusion

BACKGROUND

We hypothesized that systemic ATL-146e, an adenosine A(2A) agonist, would decrease spinal cord reperfusion inflammatory stress and inhibit apoptosis and that these effects would correlate with improved neurologic functional outcome.

METHODS

Thirty rabbits underwent cross-clamping of the infrarenal aorta for 45 minutes. One group of animals (n = 14) received 0.06 microg/kg per minute of ATL-146e infused intravenously for 3 hours, beginning 15 minutes before reperfusion. A second group of animals (n = 16) underwent spinal cord ischemia with saline vehicle alone and served as ischemic controls. Animals (n = 9, 11) from each group survived for 48 hours and assessed for neurologic impairment with the Tarlov (0-5) scoring system. Four animals from each group were humanely killed at the end of the 3-hour treatment period, and the remainder killed after 48 hours' survival. In all animals, lumbar spinal cord tissue specimens were frozen for subsequent Western blot analysis of heat shock protein 70 (HSP 70), and for the p85 fragment of poly (ADP-ribose) polymerase (PARP). Neuronal viability indices were determined at 48 hours with hematoxylin and eosin staining.

RESULTS

There was improvement in neurologic function in rabbits receiving ATL-146e (P <.001) compared with ischemic controls. At the end of the 3-hour treatment period there was a 46% (P <.05) decrease in HSP 70 expression in the ATL-146e group compared with the control group, but no difference in PARP expression. At 48 hours, there was no difference between control and ATL-146e groups in HSP 70 expression, but there was a 65% (P <.05) reduction in PARP in the spinal cords of animals that had received ATL-146e. There was a significant improvement in neuronal viability indices in animals receiving ATL-146e compared with ischemic controls (P <.05).

CONCLUSIONS

Systemic ATL-146e infusion during reperfusion after spinal cord ischemia results in preservation of hindlimb motor function. There is evidence of decreased spinal cord inflammatory stress immediately after treatment with ATL-146e as indicated by reduced HSP 70 induction. Treatment with ATL-146e is associated with a reduction in neuronal apoptosis as suggested by a substantial decrease in the fragmentation of PARP at 48 hours. These results suggest that inflammation during reperfusion and subsequent apoptosis contribute to paralysis after restoration of blood flow to the ischemic spinal cord.

Adenosine analogue reduces spinal cord reperfusion injury in a time-dependent fashion

BACKGROUND

We hypothesized that inflammation during spinal cord reperfusion worsens ischemic injury. ATL-146e, an adenosine A(2A) agonist with known anti-inflammatory properties, was used to test this hypothesis at varied intervals to determine the time course of reperfusion injury.

METHODS

Forty rabbits underwent cross-clamping of the infrarenal aorta for 45 minutes. One group (n = 14 animals) received 0.06 microg/kg/min systemic ATL-146e over 3 hours, beginning after 30 minutes of ischemic time. A second group (n = 6 animals) received ATL-146e over 1.5 hours. A third group (n = 3 animals) received ATL-146e over 1 hour, and a fourth group (n = 17 animals) received saline solution. All animals were assessed at 48 hours for hind limb motor function (Tarlov scale, 0-5).

RESULTS

Animals that received ATL-146e for 3 hours (Tarlov score, 4.3 +/- 0.22; P <.001) or 1.5 hours (Tarlov score, 2.7 +/- 0.6; P <.05) had improved neurologic outcomes compared with rabbits that received saline solution (Tarlov score, 0.6 +/- 0.29). Animals that received ATL-146e for 1 hour (Tarlov score, 0.7 +/- 0.8) were not significantly different from those animals that received saline solution.

CONCLUSIONS

Systemic ATL-146e, given during reperfusion, results in time-dependent improvement in spinal cord function after ischemia. This implies that the mechanism of spinal reperfusion injury includes leukocyte-mediated inflammation at a critical post-ischemic time interval.

Adenosine A(2A) receptor stimulation reduces inflammation and neointimal growth in a murine carotid ligation model

Endothelial activation and leukocyte recruitment are early events in atherosclerosis and the vascular response to injury. Adenosine has anti-inflammatory effects on leukocytes and endothelial cells mediated through its A(2A) receptor. We tested the hypothesis that A(2A) activation would reduce inflammation and neointimal formation in a murine carotid ligation model. Before injury, mice were randomized to a 7-day subcutaneous infusion of a specific A(2A) receptor agonist (ATL-146e, 0.004 microg/kg per minute), vehicle control, ATL-146e plus ZM241385 (a selective A(2A) antagonist), or ZM241385 alone. Leukocyte recruitment and adhesion molecule expression were assessed at early time points, and the neointimal area was measured at 14 and 28 days after injury. Compared with control mice, ATL-146e-treated mice had significantly less neutrophil and macrophage recruitment and vascular cell adhesion molecule-1, intercellular adhesion molecule-1, and P-selectin expression in the first 7 days after injury. Neointimal area was markedly and persistently reduced by 80% at 14 and 28 days, despite termination of ATL infusion at 7 days. ATL-146e+ZM241385-treated and ZM241385-treated animals had neointimal areas similar to those of control animals, confirming that the observed effects of ATL-146e were mediated specifically by the A(2A) receptor. These data demonstrate that novel stimulation of adenosine A(2A) receptors can inhibit early inflammatory processes that are important in neointimal formation after vascular injury.

Selective A(2A) adenosine receptor activation reduces skin pressure ulcer formation and inflammation

Activation of A(2A) adenosine receptors (A(2A)-AR) by ATL-146e (formerly DWH-146e) prevents inflammatory cell activation and adhesion. Recurrent ischemia-reperfusion (I/R) of the skin results in pressure ulcer formation, a major clinical problem. ATL-146e was evaluated in a novel reproducible rat model of pressure ulcer. A 9-cm(2) region of dorsal rat skin was cyclically compressed at 50 mmHg using a surgically implanted metal plate and an overlying magnet to generate reproducible tissue necrosis. Osmotic minipumps were implanted into 24 rats divided into four equal groups to infuse vehicle (control), ATL-146e (0.004 microg x kg(-1) x min(-1)), ATL-146e plus an equimolar concentration of A(2A) antagonist, ZM-241385, or ZM-241385 alone. Each group received 10 I/R cycles. In non-I/R-treated skin, ATL-146e has no effect on blood flow. I/R-treated skin of the ATL-146e group compared with the vehicle group had 65% less necrotic area, 31% less inhibition of average skin blood flow, and fewer extravasated leukocytes (23 +/- 3 vs. 49 +/- 6 per 500 microm(2)). These data suggest that ATL-146e, acting via an A(2A)-AR, reduces leukocyte infiltration and is a potent prophylactic for I/R injury in skin.

Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection

Adenosine accumulation during ischemia and inflammation protects tissues from injury. In ischemic tissues adenosine accumulates due to inhibition of adenosine kinase, and in inflamed tissues adenosine is formed from adenine nucleotides that are released from many cells including platelets, mast cells, nerves, and endothelium. Nucleotides are rapidly converted to adenosine by a family of ecto-nucleotidases including CD39 and CD73. Activation of A(1) and possibly A(3) adenosine receptors (ARs) protects heart and other tissues by preconditioning through a pathway including protein kinase C and mitochondrial K(ATP) channels. Activation of A(2A) receptors limits reperfusion injury by inhibiting inflammatory processes in neutrophils, platelets, macrophages and T cells. Adenosine produces proinflammatory responses mediated by receptors that vary among species; A(3) and A(2B) receptors mediate degranulation of rodent and human or canine mast cells, respectively. Novel adenosine receptor subtype-selective ligands have recently been developed. These include MRS1754 (A(2B) blocker), MRS1220 (A(3) blocker), MRE 3008F20 (human A(3) blocker), MRS1523 (rat A(3) blocker), and ATL146e (A(2A) agonist). These new pharmacologic tools will help investigators to sort out how adenosine protects tissues from injury and to identify new therapeutic agents that hold promise for the treatment of inflammatory and ischemic diseases.

Enhanced protection from renal ischemia-reperfusion [correction of ischemia:reperfusion] injury with A(2A)-adenosine receptor activation and PDE 4 inhibition

BACKGROUND

We previously demonstrated in rats and mice that agonists of A(2A)-adenosine receptors (A(2A)-ARs) reduce renal injury following ischemia-reperfusion. We now extend these studies and examine the effects of ATL-146e (formerly DWH-146e), an A(2A)-AR agonist, and rolipram, a type IV phosphodiesterase (PDE 4) inhibitor, on murine renal injury following ischemia-reperfusion.

METHODS

C57BL/6 mice were treated with rolipram, ATL-146e, or both compounds combined and were subjected to renal ischemia for 32 minutes and reperfusion for 24 to 48 hours. In vitro studies were performed on suspended and adhering human neutrophils.

RESULTS

Continuous delivery of rolipram or ATL-146e during reperfusion reduced renal injury in a dose-dependent manner. Maximal protection was observed when ATL-146e was infused for six hours during reperfusion. Elevated plasma creatinine and myeloperoxidase activity produced by ischemia-reperfusion were reduced by rolipram (0.1 ng/kg/min) and ATL-146e (10 ng/kg/min) by up to approximately 60% and 70%, respectively. Co-infusion of both compounds produced a maximum reduction of plasma creatinine of approximately 90% and myeloperoxidase activity. In vitro studies on suspended and adhering human neutrophils demonstrated that selective stimulation of A(2A)-ARs by ATL-146e increased cAMP accumulation, reduced oxidative activity of activated neutrophils, and decreased activated neutrophil adherence. These responses were potentiated by rolipram.

CONCLUSIONS

We conclude that the combined infusion of ATL-146e and rolipram leads to enhanced renal tissue protection from ischemia-reperfusion by mechanisms that may include reduced neutrophil adherence/recruitment and release of reactive oxygen species.

[3H]MRS 1754, a selective antagonist radioligand for A(2B) adenosine receptors

MRS 1754 [N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)-phenoxy]acetamide] is a selective antagonist ligand of A(2B) adenosine receptors. This is the least well-defined adenosine receptor subtype, and A(2B) antagonists have potential as antiasthmatic drugs. For use as a radioligand, MRS 1754, a p-cyanoanilide xanthine derivative, was tritiated on the propyl groups in a two-step reaction using a p-carboxamido precursor, which was dehydrated to the cyano species using trifluoroacetic anhydride. [3H]MRS 1754 (150 Ci/mmol) bound to recombinant human A(2B) adenosine receptors in membranes of stably transfected HEK-293 cells. Specific binding was saturable, competitive, and followed a one-site model, with a K(D) value of 1.13 +/- 0.12 nM and a B(max) value of 10.9 +/- 0.6 pmol/mg protein. Specific binding utilizing 0.7 nM [3H]MRS 1754 was > 70% of total binding. The affinity calculated from association and dissociation binding constants was 1.22 nM (N = 4). Binding to membranes expressing rat and human A(1) and A(3) adenosine receptors was not significant, and binding in membranes of HEK-293 cells expressing human A(2A) receptors was of low affinity (K(D) > 50 nM). The effects of cations and chelators were explored. Specific binding was constant over a pH range of 4.5 to 6.5, with reduced binding at higher pH. The pharmacological profile in competition experiments with [3H]MRS 1754 was consistent with the structure-activity relationship for agonists and antagonists at A(2B) receptors. The K(i) values of XAC (xanthine amine congener) and CPX (8-cyclopentyl-1,3-dipropylxanthine) were 16 and 55 nM, respectively. NECA (5'-N-ethylcarboxamidoadenosine) competed for [3H]MRS 1754 binding with a K(i) of 570 nM, similar to its potency in functional assays. Thus, [3H]MRS 1754 is suitable as a selective, high-affinity radioligand for A(2B) receptors.

Ultrastructural localization of adenosine A2A receptors suggests multiple cellular sites for modulation of GABAergic neurons in rat striatum

Activation of adenosine A2A receptors (A2AR) has been shown to antagonize the function of D2 dopaminergic regulation of striatal gamma-aminobutyric acid (GABA)-ergic output and, thus, locomotor activity. Adenosine A2A receptor immunoreactivity (A2A-LI) has been localized to rat striatum by light microscopy by using a previously characterized human A2AR monoclonal antibody. In this study, we evaluated the localization of A2A-LI and its colocalization with GABA immunoreactivity (GABA-LI) in dorsolateral rat striatum by immunoelectron microscopy to further characterize the potential mechanism of purinergic control of striatal output. Ultrastructural analysis demonstrated A2A-LI associated with the plasma membrane and cytoplasmic membranous structures of striatal neurons. A2A-LI was prevalent in dendrites and dendritic spines ( approximately 70% of total A2A-profiles counted) and less prevalent in axons and axon terminals (23%), soma (3%), and glia (3%). Cellular elements exhibiting both A2A-LI and GABA-LI comprised 23% of the total profiles counted; colabeling was most common in dendrites. A2A-LI was observed primarily at asymmetric synapses (n = 70) (both pre- and postsynaptically but predominantly in the postsynaptic element) and less frequently at symmetric synapses (n = 17). Of the 714 A2A-immunoreactive profiles examined, 37% were apposed to GABA-labeled profiles. The most common appositions were A2A-labeled dendrites apposed to GABA-immunoreactive dendrites (n = 132), axon terminals (n = 28), and somata (n = 22) and A2A-labeled axons apposed to GABA-labeled dendrites (n = 58), axon terminals (n = 14), and somata (n = 9). Our findings suggest that adenosine may play an important role in modulating excitatory input to striatal neurons and that A2AR may modulate GABAergic signaling at several cellular sites within the rat striatum.

Cyclic AMP-dependent inhibition of human neutrophil oxidative activity by substituted 2-propynylcyclohexyl adenosine A(2A) receptor agonists

Novel 2-propynylcyclohexyl-5'-N:-ehtylcarboxamidoadenosines, trans-substituted in the 4-position of the cyclohexyl ring, were evaluated in binding assays to the four subtypes of adenosine receptors (ARs). Two esters, 4-(3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl)-cyclohexanecarboxylic acid methyl ester (ATL146e) and acetic acid 4-(3-[6-amino-9-(5-ethylcarbamoyl-3, 4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl] -prop-2-ynyl)-cyclohexylmethyl ester (ATL193) were >50 x more potent than 2-[4-(2-carboxyethyl)phenethylamino]-5'-N:-ethylcarboxamidoadenosine (CGS21680) for human A(2A) AR binding. Human A(2A) AR affinity for substituted cyclohexyl-propynyladenosine analogues was inversely correlated with the polarity of the cyclohexyl side chain. There was a comparable order of potency for A(2A) AR agonist stimulation of human neutrophil [cyclic AMP](i), and inhibition of the neutrophil oxidative burst. ATL146e and CGS21680 were approximately equipotent agonists of human A(3) ARs. We measured the effects of selective AR antagonists on agonist stimulated neutrophil [cyclic AMP](i) and the effect of PKA inhibition on A(2A) AR agonist activity. ATL193-stimulated neutrophil [cyclic AMP](i) was blocked by antagonists with the potency order: ZM241385 (A(2A)-selective)>MRS1220 (A(3)-selective)>>N-(4-Cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754; A(2B)-selective) approximately 8-(N-methylisopropyl)amino-N(6)-(5'-endohydroxy-endonorbornyl)-9-methyladenine (WRC0571; A(1)-selective). The type IV phosphodiesterase inhibitor, rolipram (100 nM) potentiated ATL193 inhibition of the oxidative burst, and inhibition by ATL193 was counteracted by the PKA inhibitor H-89. The data indicate that activation of A(2A)ARs inhibits neutrophil oxidative activity by activating [cyclic AMP](i)/PKA.

Inhibiting the inflammatory response in joint sepsis

PURPOSE: We created a rabbit model of infectious arthritis to test the effects of WRC-0470 (2-cyclohexylmethylidenehydarazinoadenosine), an adenosine analogue, and rolipram, a type IV phosphodiesterase inhibitor, on intra-articular white blood cell recruitment. Type of Study: Randomized trial involving mature rabbits. METHODS: Intra-articular injections ranging from 0 to 2,000 ng of Escherichia coli lipopolysaccharide (LPS) were tested as the infectious stimulus. The optimal LPS amount was determined based on synovial fluid analysis for white blood cell counts. A separate cohort of rabbits then received various intravenous concentrations of either rolipram, WRC-0470, or a combination of the 2 medications. Synovial fluid aspirations after a 6-hour incubation were analyzed for white blood cell counts. RESULTS: Intra-articular injections of 200 ng of LPS reproducibly generated an inflammatory response of 4,000 cells/mL of synovial fluid, establishing the use of this dose of LPS for our septic arthritis model. Following infusions of 10 µg/kg/min, the average white blood cell count dropped to 800 cells/mL for WRC-0470 (P <.01) and 1,225 cells/mL for rolipram (P <.05). A synergistic effect was seen with the combination of both medications at just 1.0 µg/kg/min, with a mean white blood cell count of 1,090 cells/mL (P <.01). CONCLUSIONS: Septic arthritis is a common clinical entity that frequently results in major long-term morbidity. Although bacteria can directly damage the articular surface, the cytokine-mediated immune response to the infection can exacerbate the insult by promoting the release of proteolytic enzymes by white blood cells. Currently, no established intervention exists that will decrease the inflammatory response to infectious challenges. Our study shows that WRC-0470 and rolipram effectively reduce the intra-articular recruitment of white blood cells in a septic arthritis model. Future investigations of these drugs will determine their ultimate degree of efficacy at limiting the joint destruction associated with septic arthritis.

Home-based factor infusion therapy and hospitalization for bleeding complications among males with haemophilia

Information from the medical records of 2650 US males with haemophilia living in six states was used to examine the influence of infusing factor concentrate at home (home therapy) and other variables on rates of hospitalization for a haemorrhagic bleeding complication (HBC) over a 4-year period. Bleeding complications included actual and suspected haemorrhagic events but excluded elective admissions for procedures necessitated by haemorrhage (e.g. joint synovectomy). Other risk determinants considered in the analyses included age, race, employment status, health insurance type, care received in federally funded haemophilia treatment centres (HTCs), factor deficiency type and severity, amount of factor prescribed, prophylactic treatment, and presence of inhibitors at baseline. Survival analysis methods were used to evaluate relationships between baseline risk factors and subsequent hospitalization rates. During 8708 person years (PYs) of follow-up, 808 subjects (30.5%) had a total of 1847 bleeding-related hospitalizations; an overall rate of 21.2 admissions per 100 PYs. Using proportional hazards regression to adjust for all of the studied factors, we found that home therapy use (among residents of four of the states) and care in HTCs were independently associated with a decreased risk for a first HBC. Patients who had government-sponsored health insurance or who had no insurance, those of minority race or ethnicity, those with higher levels of factor use, and those with inhibitors were at increased HBC risk. We conclude that the use of home therapy and receipt of care in HTCs are each associated with a substantially lower risk for HBC among males with haemophilia.

Gene dose effect reveals no Gs-coupled A2A adenosine receptor reserve in murine T-lymphocytes: studies of cells from A2A-receptor-gene-deficient mice

Agonist binding to extracellular A2A adenosine receptors (A2ARs) inhibits the activation of virtually all tested functions of T-cells and can induce apoptosis in thymocytes. The evaluation of levels of expression of these immunosuppressive receptors is expected to clarify whether the absence of spare A2ARs (no 'receptor reserve') might be one of the mechanisms of attenuation of the effects of extracellular adenosine on T-cells. A2A transcript is found in T-cells and functional receptors can be demonstrated, but the density of receptor on T-cells is too low to be detected by radioligand binding. Studies of direct radioligand binding to murine brain with the selective A2AR agonist [3H]CGS21680 (2-(4-[(2-carboxyethyl)-phenyl]ethylamino)-5'-N-ethylcarboxamidoadenosine) established that striata levels of A2AR are virtually absent from A2A knock-out mice. Mice that are heterozygous (A2AR+/-) for the A2AR express significantly decreased levels of A2AR. To test for the presence of spare receptors in T-cells we took advantage of this gene dose effect and examined whether the decrease in the number of receptors in thymocytes from A2AR+/- mice was proportionately reflected in a decrease in the functional cAMP response of T-cells to adenosine. cAMP accumulation and apoptosis induced by adenosine and by A2AR agonist are of a lower magnitude in T-cells from A2AR+/- heterozygous mice than in T-cells from A2AR+/+ littermate control mice. These results indicate that there is no A2AR reserve in murine T-cells. Strongly decreased adenosine-triggered cAMP increases were detected in thymocytes from A2AR-/- mice, suggesting that A2B adenosine receptors cannot fully compensate for the loss of A2ARs in murine T-cells. We conclude that the number of A2ARs is the limiting factor in determining the maximal cAMP response of T-lymphocytes to extracellular adenosine, thereby minimizing the immunosuppressive effects of extracellular adenosine.

Design, synthesis, and evaluation of novel A2A adenosine receptor agonists

We have been interested in the design, synthesis, and evaluation of novel adenosine A2A agonists. Through the use of comparative molecular field analysis (CoMFA) we have generated a training model that includes 78 structurally diverse A2A agonists and correlated their affinity for isolated rat brain receptors with differences in their structural and electrostatic properties. We validated this model by predicting the activity of a test set that included 24 additional A2A agonists. Our CoMFA model, which incorporates the physiochemical property of dipole and selects against A1 receptor activity, generated a correlated final model (r2 = 0.891) that provides for enhanced A2A selectivity and predictability. Synthesis, pharmacological evaluation, and modeling of four novel ligands further validate the utility and predictive power (r2 = 0.626) of the CoMFA model.

A3 adenosine receptor activation triggers phosphorylation of protein kinase B and protects rat basophilic leukemia 2H3 mast cells from apoptosis

Adenosine accumulates to high levels in inflamed or ischemic tissues and activates A3 adenosine receptors (ARs) on mast cells to trigger degranulation. Here we show that stimulation of rat basophilic leukemia (RBL)-2H3 mast-like cells with the A3 AR agonists N6-(3-iodo)benzyl-5'-N-methylcarboxamidodoadenosine (IB-MECA; 10 nM) or inosine (10 microM) stimulates phosphorylation of protein kinase B (Akt). IB-MECA (1 microM) also causes a >50% reduction in apoptosis caused by exposure of RBL-2H3 cells to UV light. Akt phosphorylation is not stimulated by 100 nM N6-cyclopentyladenosine (A1-selective) or CGS21680 (A2A-selective) and is absent in cells pretreated with wortmannin or pertussis toxin. The KI values of the AR antagonists BW-1433 and 8-sulfophenyltheophylline (8-SPT) were determined in radioligand binding assays for all four subtypes of rat ARs: BW-1433 (A1, 5.8 +/- 1.0 nM; A2A, 240 +/- 37; A2B, 30 +/- 10; A3, 12,300 +/- 3, 700); 8-SPT (A1, 3.2 +/- 1.2 microM; A2A, 57 +/- 4; A2), 2.2 +/- 0.8; A3, >100). BW-1433 and the A3-selective antagonist MRS1523 (5 microM), but not 8-SPT (100 microM), block IB-MECA-induced protection from apoptosis, confirming the A3 AR as the mediator of the antiapoptotic response. The data suggest that adenosine and inosine activate Gi-coupled A3 ARs to protect mast cells from apoptosis by a pathway involving the betagamma subunits of Gi, phosphatidylinositol 3-kinase beta, and Akt. We speculate that activation of A3 ARs on mast cells or other cells that express A3 ARs (e.g., eosinophils) may facilitate their survival and accumulation in inflamed tissues.

Dominance of G(s) in doubly G(s)/G(i)-coupled chimaeric A(1)/A(2A) adenosine receptors in HEK-293 cells

A(1) adenosine receptors inhibit adenylate cyclase by activating G(i)/G(o), whereas A(2A) receptors activate G(s). We examined how regions of A(1) and A(2A) receptors regulate coupling to G-proteins by constructing chimaeras in which the third intracellular loops (3ICL or L) and/or the C-termini (or T) were switched. Pertussis toxin (PTX) was used in membrane radioligand binding assays to calculate the fraction of recombinant receptors coupled to G(i)/G(o) and in whole cells to differentially influence agonist-stimulated cAMP accumulation. Switching A(1)/A(2A) 3ICL domains results in receptors that maintain binding selectivity for ligands but are doubly coupled. Receptor chimaeras with an A(1) 3ICL sequence (A(2A)/A(1)L or A(2A)/A(1)LT) respond to agonist stimulation with elevated cAMP despite being coupled predominantly to G(i)/G(o). These chimaeras have basal cAMP levels lower than those of wild-type A(2A) receptors, similar to wild-type A(1) receptors. The A(1) C-terminus modulates the coupling of receptors with A(1) 3ICL such that A(2A)/A(1)LT is better coupled to G(i)/G(o) than A(2A)/A(1)L. The C-terminus has little impact on coupling to receptors containing A(2A) 3ICL sequence. Our results show that the C-terminus sequence selectively facilitates coupling to G(i)/G(o) mediated by A(1) 3ICL and not by other intracellular domains that favour G(i) coupling. The C-terminus sequence has little or no effect on coupling to G(s). For doubly G(s)/G(i)-coupled adenosine receptors in HEK-293 cells, G(s)-mediated stimulation predominates over G(i)/G(o)-mediated inhibition of adenylate cyclase. We discuss the signalling consequences of simultaneously activating opposing G-proteins within single cells.

A(2A) adenosine receptor-mediated inhibition of renal injury and neutrophil adhesion

We sought to determine the mechanisms responsible for the reduced renal tissue injury by agonists of A(2A) adenosine receptors (A(2A)-ARs) in models of ischemia-reperfusion (I/R) injury. DWH-146e, a selective A(2A)-AR agonist, was administered subcutaneously to Sprague-Dawley rats and C57BL/6 mice via osmotic minipumps, and animals were subjected to I/R. I/R led to an increase in plasma creatinine and kidney neutrophil infiltration. Infusion of DWH-146e at 10 ng. kg(-1). min(-1) produced a 70% reduction in plasma creatinine as well as a decrease in neutrophil density in outer medulla and cortex and myeloperoxidase activity in the reperfused kidney. Myeloperoxidase activity in kidney correlated with the degree of renal injury. P-selectin and intercellular adhesion molecule 1 (ICAM-1) immunoreactivity were most prominent in endothelial cells of peritubular capillaries and interlobular arteries of cortex and outer and inner medulla of vehicle-treated mice whose kidneys were subjected to I/R. DWH-146e treatment led to a pronounced decrease in P-selectin- and ICAM-1-like immunoreactivity. These data are consistent with our hypothesis that A(2A)-AR agonists limit I/R injury due to an inhibitory effect on neutrophil adhesion.

Differential expression of a(2a), A(1)-adenosine and D(2)-dopamine receptor genes in rat peripheral arterial chemoreceptors during postnatal development

The sensitivity of peripheral arterial chemoreceptors in the carotid body to hypoxia increases with postnatal maturation. Carotid sinus nerve activity is augmented by adenosine binding to A(2a)-adenosine receptors and attenuated by dopamine binding to D(2)-dopamine receptors. In this study, we used in situ hybridization histochemistry to determine the change in the levels of mRNA expression for A(2a) and A(1)-adenosine receptors and D(2)-dopamine receptors in the rat carotid body. We also investigated the cellular distribution and possible colocalization of these receptor mRNAs and tyrosine hydroxylase (TH) mRNAs during the first 2 weeks of postnatal development. By using immunohistocytochemistry, we detected A(2a)-adenosine receptor protein in the carotid body and petrosal ganglion. We found that A(2a)-adenosine receptor mRNA and protein are expressed in the carotid body in animals at 0, 3, 6 and 14 postnatal days. The level of A(2a)-adenosine receptor mRNA expression significantly decreased by 14 postnatal days (P<0.02 vs. day 0) while D(2)-dopamine receptor mRNA levels significantly increased by day 3 and remained greater than D(2)-dopamine receptor mRNA levels at day 0 (P<0.001 all ages vs. day 0). TH mRNA was colocalized in cells in the carotid body with A(2a) adenosine receptor and D(2)-dopamine receptor mRNAs. A(1)-adenosine receptor mRNA was not expressed in the carotid body at any of the ages examined. In the petrosal ganglion, A(1)-adenosine receptor mRNA was abundantly expressed in numerous cells, A(2a)-adenosine receptor mRNA was expressed in a moderate number of cells while D(2)-dopamine receptor mRNA was seen in a few cells in the rostral petrosal ganglion. In conclusion, using in situ hybridization histochemistry, we have shown that mRNA for both the excitatory, A(2a)-adenosine receptor, and the inhibitory, D(2)-dopamine receptor, is developmentally regulated in presumably type I cells in the carotid body which may contribute to the maturation of hypoxic chemosensitivity. Furthermore, the presence A(1)-adenosine receptor mRNAs in cell bodies of the petrosal ganglion suggests that adenosine might also have an inhibitory role in hypoxic chemotransmission.

The role of inflammation in vascular diseases

When the body responds to an infectious insult, it initiates an immune response to eliminate the pathogen. The hallmark of the immune response is an inflammatory cascade that can also do extensive damage to host tissues. Inflammation is a major contributing factor to many vascular events, including atherosclerotic plaque development and rupture, aortic aneurysm formation, angiogenesis, and ischemia/reperfusion damage. The immune response is mediated by both circulating and resident leukocytes and the cells with which they interact (e.g., vascular endothelium and smooth muscle cells). The process is orchestrated by the activity of a changing series of released and displayed mediators. These include the expression of adhesion molecules on leukocytes and underlying vascular endothelium and the release of cytokines, chemokines, and tissue-destructive metalloproteases and reactive oxygen species. This review focuses on the causes, the inflammatory processes involved, and possible strategies for decreasing vascular disease through regulation of the inflammatory response.

Activation of adenosine A2 alpha receptors inhibits mast cell degranulation and mast cell-dependent vasoconstriction

OBJECTIVE

Adenosine and inosine accumulate in tissue during periods of ischemia and both molecules have been shown to degranulate mast cells in the hamster cheek pouch via activation of an A3 receptor. An A2-mediated inhibitory action of adenosine on mast cell degranulation has also been reported (16), and the objective of this research was to investigate the role of adenosine A2 receptors in modulating inosine-induced mast cell degranulation and subsequent vasoconstriction of microvessels.

METHODS

Cheek pouches of the Golden hamster were prepared for in vivo microscopy. Adenosine, inosine, and other agents were applied either globally in the superfusion solution or to selected regions of the tissue by pipette.

RESULTS

Micropipette application of 10(-4) M inosine to periarteriolar mast cells caused a vasoconstriction and an associated mast cell degranulation in 71% of the arterioles tested. The average diameter reduction was 29 +/- 5%. To establish a modulatory role for the A2 receptor, low doses of adenosine (100 nM and 10 nM) were applied globally via the superfusion prior to inosine stimulation. This adenosine pretreatment resulted in a decrease in the incidence of the inosine-induced vasoconstriction (17% and 31%), as well as smaller constrictions (0.5 +/- 1% and 7 +/- 3%). Mast cell degranulation was also reduced by pretreatment with adenosine, as evidenced by a decreased number of mast cells exhibiting ruthenium red dye uptake. The inhibitory effect of adenosine could be eliminated by pretreatment with the nonselective A1/A2 antagonist 8-(p-sulfophenyl) theophylline, which restored the inosine-induced responses to control values. To demonstrate that the effect was A2 alpha-mediated, vessels were pretreated with the selective A2 alpha agonist 2-[4-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). Following this treatment, constriction in response to microapplication of inosine (10(-4) M) occurred in only 11% of the vessels tested; the average constriction was reduced to 2 +/- 2% and no mast cell degranulation was observed.

CONCLUSIONS

We conclude that mast cell degranulation can be inhibited via activation of an adenosine A2 alpha receptor; which activation occurs at a lower concentration of adenosine than stimulatory A3 receptor activation. This finding may have implications for the pathology of ischemia.

Anilide derivatives of an 8-phenylxanthine carboxylic congener are highly potent and selective antagonists at human A(2B) adenosine receptors

No highly selective antagonists of the A(2B) adenosine receptor (AR) have been reported; however such antagonists have therapeutic potential as antiasthmatic agents. Here we report the synthesis of potent and selective A(2B) receptor antagonists. The structure-activity relationships (SAR) of 8-phenyl-1, 3-di-(n-propyl)xanthine derivatives in binding to recombinant human A(2B) ARs in HEK-293 cells (HEK-A(2B)) and at other AR subtypes were explored. Various amide derivatives of 8-[4-[[carboxymethyl]oxy]phenyl]-1,3-di-(n-propyl)xanthine, 4a, were synthesized. A comparison of aryl, alkyl, and aralkyl amides demonstrated that simple anilides, particularly those substituted in the para-position with electron-withdrawing groups, such as nitro, cyano, and acetyl, bind selectively to human A(2B) receptors in the range of 1-3 nM. The unsubstituted anilide 12 had a K(i) value at A(2B) receptors of 1.48 nM but was only moderately selective versus human A(1)/A(2A) receptors and nonselective versus rat A(1) receptors. Highly potent and selective A(2B) antagonists were a p-aminoacetophenone derivative 20 (K(i) value 1.39 nM) and ap-cyanoanilide 27 (K(i) value 1.97 nM). Compound 27 was 400-, 245-, and 123-fold selective for human A(2B) receptors versus human A(1)/A(2A)/A(3) receptors, respectively, and 8.5- and 310-fold selective versus rat A(1)/A(2A) receptors, respectively. Substitution of the 1,3-dipropyl groups with 1,3-diethyl offered no disadvantage for selectivity, and high affinities at A(2B) receptors were maintained. Substitution of the p-carboxymethyloxy group of 4a and its amides with acrylic acid decreased affinity at A(2B) receptors while increasing affinity at A(1) receptors. 1, 3-Di(cyclohexylmethyl) groups greatly reduced affinity at ARs, although the p-carboxymethyloxy derivative 9 was moderately selective for A(2B) receptors. Several selective A(2B) antagonists inhibited NECA-stimulated calcium mobilization in HEK-A(2B) cells.

Effect of triple therapy on eradication of canine gastric helicobacters and gastric disease

Nine helicobacter-positive pet dogs with upper gastrointestinal signs were studied to evaluate the effect of a triple therapy, normally applied to humans for the eradication of gastric helicobacters, on clinical signs and gastric histology, as well as the recurrence of helicobacters after eradication in an extended follow-up in four dogs. Endoscopy was performed at entry to the study and repeated after eradication therapies and additional treatments. If the triple therapy (amoxycillin, metronidazole and bismuth subcitrate) failed, tetracycline and omeprazole were prescribed. Additional therapies were instituted if clinical signs persisted after eradication therapies. Helicobacter status was verified from gastric biopsy specimens by the urease test and histological examination, and in a few dogs also by brush cytology. Triple therapy eradicated gastric helicobacters in 7/9 dogs; gastric helicobacters were also eradicated in one dog treated with tetracycline and omeprazole. Eradication of helicobacters resulted in significant improvement, but not total resolution, of clinical signs. Subsequent additional therapies resulted in further alleviation of clinical signs. Neither triple therapy nor additional therapies had a significant effect on gastric histological changes. Gastric helicobacters recurred in 4/4 dogs within three years of the eradication treatment. Because canine gastric helicobacters alone were not definitively shown to induce clinical signs, routine eradication therapy seems not to be warranted at present.

Neutrophil A2A adenosine receptor inhibits inflammation in a rat model of meningitis: synergy with the type IV phosphodiesterase inhibitor, rolipram

Bacterial meningitis is a disease worsened by neutrophil-induced damage in the subarachnoid space. In this study, the A2A adenosine receptors on human neutrophils were characterized, and the role of A2A receptors on the trafficking of leukocytes to the cerebrospinal fluid and on blood-brain barrier permeability (BBBP) was assessed in a rat meningitis model. Neutrophils bind the A2A selective antagonist, 125I-ZM241385 (Bmax=843 receptors/neutrophil; KD=0.125 nM). A selective A2A receptor agonist, WRC-0470 (2-cyclohexylmethylidene-hydrazinoadenosine; 0.03-1 microM), alone and synergistically with the type IV phosphodiesterase inhibitor, rolipram, increased neutrophil [cAMP]i and reduced cytokine-enhanced neutrophil adherence, superoxide release, and degranulation. These effects of WRC-0470 were reversed by ZM241385 (100 nM). In a lipopolysaccharide-induced rat meningitis model, WRC-0470 (0-0.9 microgram/kg/h), with or without rolipram (0-0.01 microgram/kg/h), inhibited pleocytosis and reduced the lipopolysaccharide-induced increase in BBBP, indicative of decreased neutrophil-induced damage.

Selective adenosine-A2A activation reduces lung reperfusion injury following transplantation

BACKGROUND

The adenosine-A2A receptor on the neutrophil is responsible for several anti-inflammatory actions. We hypothesized that DWH-146e, a selective adenosine-A2A agonist, would reduce lung reperfusion injury following transplantation.

METHODS

We used an isolated, whole blood-perfused, ventilated rabbit lung model. Donor rabbits underwent lung harvest after pulmonary arterial PGE1 injection and Euro-Collins preservation solution flush, and lungs were preserved for 18 hours at 4 degrees C. Group I lungs (n = 9) served as control subjects. Group II lungs (n = 9) were reperfused with whole blood that was first passed through a leukocyte-depleting filter. In group III (n = 9), DWH-146e was added to the blood reperfusate (25 microg/kg) immediately before reperfusion and was administered throughout the reperfusion period (1 microg/kg/min). All lungs were reperfused for 30 minutes.

RESULTS

Arterial oxygenation in group II and group III was significantly higher than that of group I after 30 minutes of reperfusion (514.27 +/- 35.80 and 461.12 +/- 43.77 vs 91.41 +/- 20.58 mm Hg, p < .001). Pulmonary vascular resistance was significantly reduced in group III (22,783 +/- 357 dynes x s x cm(-5)) compared to both group II and group I (31,057 +/- 1743 and 36,911 +/- 2173 dynes x s x cm(-5), p < .001). Airway compliance was improved in groups II and III when compared to group I (1.68 +/- 0.08 and 1.68 +/- 0.05 vs 1.36 +/- 0.13, p = .03). Microvascular permeability in group III was reduced to 106.82 +/- 17.09 compared with 165.70 +/- 21.83 ng Evans blue dye per gram of tissue in group I (p = .05). Group III myeloperoxidase activity was 39.88 +/- 4.87 compared with 88.70 +/- 18.69 deltaOD/g/min in group I (p = .03); group II myeloperoxidase activity was 56.06 +/- 7.46.

CONCLUSIONS

DWH-146e reduced lung neutrophil sequestration and dramatically improved pulmonary graft function. Neutrophils are important components of the inflammatory cascade of reperfusion injury and their source may include both the circulating blood and the lung graft itself. Selective adenosine-A2A activation interrupts the neutrophil-mediated inflammatory response and reduces lung reperfusion injury following transplantation.

Characterization of human A(2B) adenosine receptors: radioligand binding, western blotting, and coupling to G(q) in human embryonic kidney 293 cells and HMC-1 mast cells

Recombinant human A(2B) adenosine receptors (A(2B)ARs) and receptors extended on the amino terminus with hexahistidine and the FLAG epitope, DYKDDDDK (H/F-A(2B)) were stably overexpressed (to >20,000 fmol/mg protein) in human embryonic kidney 293 cells (HEK-A(2B)). By Western blotting, the H/F-A(2B) receptor runs as a 34.8-kDa glycoprotein. Pharmacological properties of A(2B)ARs were characterized with (125)I-3-aminobenzyl-8-phenyl-(4-oxyacetic acid)-1-propylxanthine (K(D), 36 nM). In competition binding assays, the affinity of agonists is reduced by substitution on either the N(6)- or the C-2 position of the adenine ring, whereas 5'-substitutions increase affinity, resulting in the potency order: 5'-N-ethylcarboxamidoadenosine (NECA) >> N(6)-aminobenzyl-NECA approximately 2-chloroadenosine > 2-[4-(2-carboxyethyl)phenethylamino]-NECA (CGS21680) > N(6)-aminobenzyladenosine. The A(2B)AR is potently blocked by the A(2A)-selective antagonist 4-(2-[7-amino-2-[2-furyl][1,2, 4]triazolo-[2,3-a][1,3,5] triazin-5-yl-amino]ethyl)phenol (ZM241385; K(I), 32 nM for A(2B), 1.4 nM for A(2A)) and the A(1) selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (K(I), 50.5 nM for A(2B); 2.5 nM for A(1)). The K(I) values for the antiasthmatic xanthines, theophylline (7.8 microM) and enprofylline (6.4 microM), are below their therapeutic plasma concentrations (20 to 50 microM), and agree with K(I) determinations for inhibition of NECA-stimulated cAMP accumulation in HEK-A(2B) cells. NECA or N(6)-(2-iodo)benzyl-5'-N-methylcarboxamidodoadenosine (IB-MECA) stimulate inositol trisphosphates and calcium accumulation in HEK-A(2B) or HEK-A(3) cells, respectively, but only the A(3) response is prevented by pertussis toxin. In human HMC-1 mast cells, A(2B)AR activation stimulates calcium mobilization and cAMP accumulation. We conclude that HEK-A(2B) cells and HMC-1 mast cells possess A(2B)AR glycoproteins that are coupled to both G(q/11) and G(s).

Palmitoylation of the recombinant human A1 adenosine receptor: enhanced proteolysis of palmitoylation-deficient mutant receptors

Palmitoylation of the recombinant human A(1) adenosine receptor (A(1)AR) expressed in HEK-293 cells is demonstrated by showing that hexahistidine (His(6))/Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys (FLAG) (H/F) A(1)ARs, purified to homogeneity from cells metabolically labelled with [(3)H]palmitate, incorporate tritium into a 38-42 kDa receptor glycoprotein. The amount of palmitoylation is not affected by incubation of cells with the A(1)AR-selective agonist N(6)-cyclopentyladenosine (CPA). A(1)AR palmitoylation is abolished by treatment with neutral hydroxylamine or by mutation of Cys-309 to Ala (C(309)-->A). Based on Western blotting and pulse-chase experiments with [(35)S]methionine, at least 90% of wild-type receptors are palmitoylated and turn over with a t1/2 of 6.4 h. Of the C(309)-->A mutated receptors, 40% appear to turn over like wild-type receptors, with a t1/2 of 7.1 h, and 60% appear to be rapidly cleaved to form a 25 kDa receptor fragment that turns over with a t1/2 of 0.8 h. In HEK-293 cell lines expressing similar numbers of wild-type or C(309)-->A mutant A(1)Rs, there is little difference in the kinetics of CPA-induced receptor internalization (1 h), down-regulation (24 h), inhibition of forskolin-stimulated cAMP accumulation, or activation of co-transfected G-protein-activated inward rectifier K(+)/cardiac inward rectifying K(+) (GIRK1/CIR K(+)) channels. Also unaffected by palmitoylation is guanosine 5'-[gamma-thio]-triphosphate ([S]GTP)-sensitive binding to membranes by the agonist (125)I-labelled aminobenzyladenosine. The results suggest that palmitoylation has little effect on receptor-effector coupling, agonist-induced internalization or down-regulation. We speculate that palmitoylation may divert newly synthesized A(1)ARs from a pathway leading to rapid degradation.

Selective A2A adenosine receptor activation reduces ischemia-reperfusion injury in rat kidney

A2A adenosine receptors (A2A-ARs) are known modulators of renal hemodynamics and potent inhibitors of inflammation. We sought to determine whether selective activation of A2A-ARs protects kidneys from ischemia-reperfusion injury. The ester derivative of DWH-146 (DWH-146e), a selective A2A agonist, was found to be more potent and selective for A2A-ARs than the prototype compound CGS-21680. Osmotic minipumps were implanted subcutaneously to infuse into rats either vehicle or DWH-146e (0.004 microg. kg(-1). min(-1)), during and after ischemia-reperfusion injury. Following 24 and 48 h of reperfusion, the rise in serum creatinine and blood urea nitrogen for vehicle-treated rats was substantially elevated compared with DWH-146e-treated rats. Histological examination revealed widespread tubular epithelial necrosis and vascular congestion in the outer medulla of vehicle-treated compared with DWH-146e-treated animals. ZM-241385, a selective A(2A) antagonist, blocked the protective effect of DWH-146e. Delaying administration of DWH-146e until the initiation of reperfusion also decreased serum creatinine. We conclude that 1) selective A2A-AR activation by DWH-146e reduces ischemia-reperfusion injury in rat kidneys, 2) the effect of DWH-146e is A2A receptor mediated, and 3) the protective effects are mediated by preventing injury during the reperfusion period.

ATP sensitive potassium channel and myocardial preconditioning

KATP channels play an important role in physiology and pathophysiology of many tissues. As in the pancreatic beta cells, they couple the change of blood glucose with insulin release. The data coming from Baukrowitz et al. and Shyng and Nichols gave the possible answers to the two old enigmas of KATP channels, i.e., different ATP sensitivity reported in the same tissue and how the channel opened under intracellular millimolar ATP concentration, in which they showed the lipids and lipid metabolites are essential for KATP channel regulation by altering ATP sensitivity. This new information rises several further considerations. How does PIP2 reduce the sensitivity of the channel to ATP? In order to clarify the possibility of direct competing or allosteric effect on the ATP binding site, competitive binding assay should be performed. Since the PIP2 theory seems to be the key event to determine the ATP sensitivity and thus control the channel open probability, then what is the resting concentration of PIP2 in the cell membrane? Is it sufficient to account for the difference in the ATP sensitivity of the intact cell and excised patch from different tissues? Quantitative studies either immunoblotting by PIP2 antibody or fluorescence-labeled lipid assay-may obtain some basic but useful data for further studies to answer these questions. Furthermore, the ATPi mediated restoration of activity was inhibited by antibodies against PIP2. The dualistic behavior of KATP channels to intracellular NDPs should be reexamined with respect to PIP2. The vast majority of preconditioning studies has been performed in intact animals in which myocardial infarct size was used as the end point to define the cardio-protective effect of ischemic PC. These results suggest a key role for the KATP channel as both a trigger and as an end effector of both acute and delayed ischemic PC. The persistent activation of KATP channels during the early reperfusion phase is essential for a smooth and full recovery of contractile function, as well as for maintenance of electrical stability in heart that has been exposed to ischemia. Though activate adenosine A1 receptor coupled with Gi protein can open the KATP channels, adenosine is quickly released during ischemia and exerts potent coronary vasodilatation to maintain coronary blood flow through A2 receptors. This adenosine-induced coronary vasodilatation could be coupled with KATP channels based on the evidence of the augmentation effect of KCOs. Nitric oxide may also play some role in both first and second window of myocardial protection. It is possible that rapid and reversible phosphorylation and activation of constitutive expressed myocardial NOS or by direct KATP channel phosphorylation and activation leads to the first window of myocardial protection. This hypothesis can be further investigated either by using site direct mutagenesis of iNOS or KATP channel, or by applying the dominant negative iNOS in the cell ischemic model, or by building the adenosine or iNOS knock-out mice to study the relationship of these possible mechanisms. Recently, Kontos further showed that KCOs need L-lysine or L-arginine to dilate cerebral arterioles. This suggests that there may be an amino acid binding site inside the KATP channel and nitric oxide can open the KATP channel either by direct acting on the channel protein or by modulating the affinity of the amino acid binding site for L-lysine or L-arginine. Other KATP channel openers in need of additional characterization are the Type III KCOs (nicorandiol). They open the KATP channel only in the presence of elevated intracellular NDPs, which may make them specifically target to the ischemic region, because the intracellular NDP increases mostly in ischemic region. It is possible that type III KCOs can selectively improve blood flow to ischemic areas without diverting blood away to non-ischemic region, and prevents the "steal phenomenon". (ABSTRACT TRUNCATED)

Acyl-Hydrazide Derivatives of a Xanthine Carboxylic Congener (XCC) as Selective Antagonists at Human A2B Adenosine Receptors

The structure-activity relationships (SAR) of 8-phenyl-1,3-dipropylxanthine derivatives in binding to recombinant human A_2B adenosine receptors were explored, in order to identify selective antagonists. Based on the finding of receptor selectivity in MRS 1204, containing an N-hydroxysuccinimide ester attached through the p-position of the 8-phenyl substituent [Jacobson et al. (1999): Drug Dev. Res., 47:45-53], a hydrazide and its more stable imide derivatives were synthesized. The hydrazide of XCC (8-[4-[[[carboxy]methyl]oxy]phenyl]-1,3-dipropylxanthine) was acylated with a variety of mono- and dicarboxylic acids. K_i values were determined in the adenosine receptor binding assays. At recombinant human A_2B receptors expressed in membranes of HEK-293 cells, antagonist radioligands used were the xanthine ¹²⁵I-ABOPX (¹²⁵I-3-(4-amino-3-iodobenzyl)-8-oxyacetate-1-propyl-xanthine) and the nonxanthine antagonist [³H]ZM 241385 ([³H]4-(2-[7-amino-2-{furyl}{1,2,4}triazolo{2,3-a}{1,3,5}triazin-5-ylamino-ethyl)phenol). The initial screening utilized rat A₁/A_2A receptors and human A₃ receptors, and selected compounds were examined at the human A₁/A_2A subtypes. A 1,2-dimethylmaleimide derivative, 14 (MRS 1595), bound to human A_2B receptors with a Ki of 19 nM and proved to be selective vs. human A₁/A_2A/A₃ receptors by 160-, 100-, and 35-fold, respectively. Enprofylline (3-propylxanthine) is slightly selective for A_2B receptors, suggesting removal of the 1-propyl group; however, combination of the 1-H-3-Pr and 8-phenyl substituents eliminated the selectivity. Other potent and moderately selective A_2B antagonists were a tetrahydrophthaloyl derivative 18b (MRS 1614, K_i value 10 nM) and amino acid conjugates of the XCC-hydrazide, i.e., the glutarimide 24b (MRS 1626, K_i value 13 nM), and protected dipeptide 27 (MRS 1615, K_i value 11 nM). Drug Dev. Res. 47:178-188, 1999.

Acyl-Hydrazide Derivatives of a Xanthine Carboxylic Congener (XCC) as Selective Antagonists at Human A2B Adenosine Receptors

The structure-activity relationships (SAR) of 8-phenyl-1,3-dipropylxanthine derivatives in binding to recombinant human A_2B adenosine receptors were explored, in order to identify selective antagonists. Based on the finding of receptor selectivity in MRS 1204, containing an N-hydroxysuccinimide ester attached through the p-position of the 8-phenyl substituent [Jacobson et al. (1999): Drug Dev. Res., 47:45-53], a hydrazide and its more stable imide derivatives were synthesized. The hydrazide of XCC (8-[4-[[[carboxy]methyl]oxy]phenyl]-1,3-dipropylxanthine) was acylated with a variety of mono- and dicarboxylic acids. K_i values were determined in the adenosine receptor binding assays. At recombinant human A_2B receptors expressed in membranes of HEK-293 cells, antagonist radioligands used were the xanthine ¹²⁵I-ABOPX (¹²⁵I-3-(4-amino-3-iodobenzyl)-8-oxyacetate-1-propyl-xanthine) and the nonxanthine antagonist [³H]ZM 241385 ([³H]4-(2-[7-amino-2-{furyl}{1,2,4}triazolo{2,3-a}{1,3,5}triazin-5-ylamino-ethyl)phenol). The initial screening utilized rat A₁/A_2A receptors and human A₃ receptors, and selected compounds were examined at the human A₁/A_2A subtypes. A 1,2-dimethylmaleimide derivative, 14 (MRS 1595), bound to human A_2B receptors with a Ki of 19 nM and proved to be selective vs. human A₁/A_2A/A₃ receptors by 160-, 100-, and 35-fold, respectively. Enprofylline (3-propylxanthine) is slightly selective for A_2B receptors, suggesting removal of the 1-propyl group; however, combination of the 1-H-3-Pr and 8-phenyl substituents eliminated the selectivity. Other potent and moderately selective A_2B antagonists were a tetrahydrophthaloyl derivative 18b (MRS 1614, K_i value 10 nM) and amino acid conjugates of the XCC-hydrazide, i.e., the glutarimide 24b (MRS 1626, K_i value 13 nM), and protected dipeptide 27 (MRS 1615, K_i value 11 nM). Drug Dev. Res. 47:178-188, 1999.

Urinary adenosine and aminoimidazolecarboxamide excretion in methotrexate-treated patients with psoriasis

BACKGROUND

We hypothesized that low-dose methotrexate treatment for patients with psoriasis would block purine biosynthesis at the step catalyzed by aminoimidazolecarboxamide (AICA) ribotide transformylase and would inhibit adenosine metabolism as evidenced by increased urinary levels of AICA and adenosine, respectively. Eight patients collected a 24-hour urine specimen on the day before their methotrexate dose and the next day during their methotrexate dose. Eight age- and sex-matched controls also collected a 24-hour urine sample. Urinary AICA and adenosine were assayed by spectrophotometric and radioimmune assays, respectively; means are reported as micromole per millimole of creatinine and were compared by the paired t test (1-tailed).

OBSERVATIONS

Mean AICA excretion increased from 1.30 micromol/mmol on the day before to 1.85 micromol/mmol on the day during methotrexate dosing (P<.01). Mean adenosine values increased from 0.68 to 1.07 micromol/mmol, (P<.03). Controls had mean AICA and adenosine levels of 1.29 and 0.50 micromol/mmol, respectively. During the day of methotrexate dosing, patients had higher mean AICA and adenosine levels when compared with controls (P<.01). Mean AICA levels increased from 1.36 to 2.06 micromol/mmol (P<.025), and mean adenosine levels increased from 0.72 to 1.25 micromol/mmol (P<.025) in 5 patients showing improvement in clinical disease activity. In contrast, 3 patients with no change or worsening in clinical disease activity had smaller increases.

CONCLUSIONS

Methotrexate treatment of patients with psoriasis inhibits AICA ribotide transformylase and adenosine metabolism. Since adenosine is a T-lymphocyte toxin, it may be partially responsible for the immunosuppressive effect.

1,3-Dialkylxanthine Derivatives Having High Potency as Antagonists at Human A2B Adenosine Receptors

The structure-activity relationships (SAR) of alkylxanthine derivatives as antagonists at the recombinant human adenosine receptors were explored in order to identify selective antagonists of A2B receptors. The effects of lengthening alkyl substituents from methyl to butyl at 1- and 3-positions and additional substitution at the 7- and 8-positions were probed. Ki values, determined in competition binding in membranes of HEK-293 cells expressing A2B receptors using 125I-ABOPX (125I-3-(4-amino-3-iodobenzyl)-8-(phenyl-4-oxyacetate)-1-propylxanthine), were approximately 10 to 100 nM for 8-phenylxanthine functionalized congeners. Xanthines containing 8-aryl, 8-alkyl, and 8-cycloalkyl substituents, derivatives of XCC (8-[4-[[[carboxy]methyl]oxy]phenyl]-1,3-dipropylxanthine) and XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]-oxy]phenyl]-1,3-dipropylxanthine), containing various ester and amide groups, including L- and D-amino acid conjugates, were included. Enprofylline was 2-fold more potent than theophylline in A2B receptor binding, and the 2-thio modification was not tolerated. Among the most potent derivatives examined were XCC, its hydrazide and aminoethyl and fluoroethyl amide derivatives, XAC, N-hydroxyethyl-XAC, and the L-citrulline and D-p-aminophenylalanine conjugates of XAC. An N-hydroxysuccinimide ester of XCC (XCC-NHS, MRS 1204) bound to A2B receptors with a Ki of 9.75 nM and was the most selective (at least 20-fold) in this series. In a functional assay of recombinant human A2B receptors, four of these potent xanthines were shown to fully antagonize the effects of NECA-induced stimulation of cyclic AMP accumulation.

Purification of A1 adenosine receptor-G-protein complexes: effects of receptor down-regulation and phosphorylation on coupling

We examined the effects of exposing A1 adenosine receptors (A1ARs) to an agonist on the stability and phosphorylation state of receptor-guanine nucleotide-binding regulatory protein (R-G-protein) complexes. Non-denatured recombinant human A1ARs extended on the N-terminus with hexahistidine (His6) and the FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) epitope (H/F) were purified to near homogeneity from stably transfected Chinese-hamster ovary (CHO)-K1 cells. Purified receptors have pharmacological properties similar to receptors in membranes. G-proteins were co-purified with 15+/-2% of H/F-A1AR unless receptor-G-protein (R-G) complexes were uncoupled by pre-treating cell membranes with GTP. By silver staining, purified A1AR-G-protein complexes contain receptors, G-protein alpha and beta subunits and an unidentified 97 kDa protein. Pretreating intact cells with N6-cyclopentyladenosine (CPA) for 24 h decreased both the total number of receptors measured in membranes and the number of purified A1ARs by about 50%. In contrast, pretreating cells with CPA decreased the number of R-G complexes measured in membranes (54+/-6%) significantly less than it decreased the number of purified R-G complexes (78+/-3%) as detected by 125I-N6-(4-aminobenzyl)adenosine binding or by Western blotting Gialpha2. The effect of CPA to decrease the fraction of receptors purified as R-G complexes was not associated with any change in low-level A1AR phosphorylation (found on serine), or low-level phosphorylation of G-protein alpha or beta subunits or the 97 kDa protein. These experiments reveal a novel aspect of agonist-induced down-regulation, namely a diminished stability of receptor-G-protein complexes that is manifested as uncoupling during receptor purification.