Adenosine Amine Congener Research Articles

Purinergic Signaling and Aminoglycoside Ototoxicity: The Opposing Roles of P1 (Adenosine) and P2 (ATP) Receptors on Cochlear Hair Cell Survival.

Purinergic signaling regulates important physiological processes and the homeostatic response to stress in the cochlea via extracellular nucleosides (adenosine) and nucleotides (ATP, UTP). Using a previously established organotypic culture model, the current study investigated the effect of purinergic P1 (adenosine) and P2 (ATP) receptor activation on the survival of the sensory hair cell population in the cochlea exposed to the ototoxic aminoglycoside neomycin. Organ of Corti explants were obtained from C57BL/6 mice at postnatal day 3 (P3) and maintained in normal culture medium (with or without purine receptor agonists or analogs) for 19.5 h prior to neomycin exposure (1 mM, 3 h) followed by a further incubation for 19.5 h in culture medium. The cochlear explants were then fixed in 4% paraformaldehyde (PFA) and sensory hair cells labeled with Alexa 488-phalloidin. Neomycin induced a substantial loss of the sensory hair cells, mostly in the middle segment of the cochlea. This neomycin-induced ototoxicity was unaffected by the addition of P2 receptor agonists (ATP and UTP) in the culture medium, whilst the addition of their slowly-hydrolyzable analogs (ATPγS, UTPγS) aggravated neomycin-induced sensory hair cell loss. In contrast, the activation of P1 receptors by adenosine or adenosine amine congener (ADAC) conferred partial protection from neomycin ototoxicity. This study demonstrates a pro-survival effect of P1 receptor stimulation, whilst prolonged activation of P2 receptors has an opposite effect. Based on these findings, we postulate that P1 and P2 receptors orchestrate differential responses to cochlear injury and that the balance of these receptors is important for maintaining cochlear homeostasis following ototoxic injury.

Pharmacokinetic Properties of Adenosine Amine Congener in Cochlear Perilymph after Systemic Administration.

Noise-induced hearing loss (NIHL) is a global health problem affecting over 5% of the population worldwide. We have shown previously that acute noise-induced cochlear injury can be ameliorated by administration of drugs acting on adenosine receptors in the inner ear, and a selective A1 adenosine receptor agonist adenosine amine congener (ADAC) has emerged as a potentially effective treatment for cochlear injury and resulting hearing loss. This study investigated pharmacokinetic properties of ADAC in rat perilymph after systemic (intravenous) administration using a newly developed liquid chromatography-tandem mass spectrometry detection method. The method was developed and validated in accordance with the USA FDA guidelines including accuracy, precision, specificity, and linearity. Perilymph was sampled from the apical turn of the cochlea to prevent contamination with the cerebrospinal fluid. ADAC was detected in cochlear perilymph within two minutes following intravenous administration and remained in perilymph above its minimal effective concentration for at least two hours. The pharmacokinetic pattern of ADAC was significantly altered by exposure to noise, suggesting transient changes in permeability of the blood-labyrinth barrier and/or cochlear blood flow. This study supports ADAC development as a potential clinical otological treatment for acute sensorineural hearing loss caused by exposure to traumatic noise.

Adenosine amine congener as a cochlear rescue agent.

We have previously shown that adenosine amine congener (ADAC), a selective A1 adenosine receptor agonist, can ameliorate noise- and cisplatin-induced cochlear injury. Here we demonstrate the dose-dependent rescue effects of ADAC on noise-induced cochlear injury in a rat model and establish the time window for treatment. Methods. ADAC (25–300 μg/kg) was administered intraperitoneally to Wistar rats (8–10 weeks old) at intervals (6–72 hours) after exposure to traumatic noise (8–16 kHz, 110 dB sound pressure level, 2 hours). Hearing sensitivity was assessed using auditory brainstem responses (ABR) before and 12 days after noise exposure. Pharmacokinetic studies investigated ADAC concentrations in plasma after systemic (intravenous) administration. Results. ADAC was most effective in the first 24 hours after noise exposure at doses >50 μg/kg, providing up to 21 dB protection (averaged across 8–28 kHz). Pharmacokinetic studies demonstrated a short (5 min) half-life of ADAC in plasma after intravenous administration without detection of degradation products. Conclusion. Our data show that ADAC mitigates noise-induced hearing loss in a dose- and time-dependent manner, but further studies are required to establish its translation as a clinical otological treatment.

Adenosine amine congener ameliorates cisplatin-induced hearing loss

Adenosine amine congener ameliorates cisplatin-induced hearing loss

Adenosine amine congener mitigates noise-induced cochlear injury

Hearing loss from noise exposure is a leading occupational disease, with up to 5% of the population at risk world-wide. Here, we present a novel purine-based pharmacological intervention that can ameliorate noise-induced cochlear injury. Wistar rats were exposed to narrow-band noise (8-12 kHz, 110 dB SPL, 2-24 h) to induce cochlear damage and permanent hearing loss. The selective adenosine A(1) receptor agonist, adenosine amine congener (ADAC), was administered intraperitoneally (100 microg/kg/day) at time intervals after noise exposure. Hearing thresholds were assessed using auditory brainstem responses and the hair cell loss was evaluated by quantitative histology. Free radical damage in the organ of Corti was assessed using nitrotyrosine immunohistochemistry. The treatment with ADAC after noise exposure led to a significantly greater recovery of hearing thresholds compared with controls. These results were upheld by increased survival of sensory hair cells and reduced nitrotyrosine immunoreactivity in ADAC-treated cochlea. We propose that ADAC could be a valuable treatment for noise-induced cochlear injury in instances of both acute and extended noise exposures.

Purinergic Signalling special issue on “Purinergic Signalling in the Inner Ear”; a commentary by Professor Geoffrey Burnstock, Editor-in-Chief

This special edition—“Purinergic Signalling in the Inner Ear”—assembled under the steerage of Guest Editors Gary Housley and Jonathan Gale, extends the recent series of issues that have highlighted emerging themes of purinergic signalling. This special edition reflects the breadth of understanding of the role of extracellular nucleoside and nucleotide signalling in the cochlea. The inner ear has revealed fascinating processes related to the development and function of hearing and balance with broad relevance to other systems. The ten fully peer-reviewed papers are provided by research leaders who have made sustained contributions to this field and have accepted the invitation to contribute highlights of their current research focus to this special edition. The issue opens with papers describing the dynamics of ATP release mechanisms and related inter-cellular synchronous communication between the sensory and supporting cells and neurons. These studies resolve connexin hemi-channel mediated ATP release, controlled by P2X and P2Y receptors, contributing to the establishment of auditory neurotransmission, and effector pathways, including Ca2+ and nitric oxide second messenger signalling, that sustain the function and integrity of the cochlea. Subsequent papers provide new data on the breadth of P1 and P2 receptor and NTPDase expression in the cochlea and investigate the dynamics of these systems with regard to hearing function with ageing and noise-stress. Of particular note is the final paper, by Vlajkovic et al, which provides an exciting glimpse at the translational potential of the field. This study reports what may be the first effective treatment of hearing loss after noise trauma; systemic treatment of rats with the A1 receptor agonist (adenosine amine congener—ADAC) significantly reduced noise-induced hearing loss. As this issue moves from immediate on-line access, to print, it is pleasing to report that this exciting finding has already attracted considerable media attention.

DIFFERENTIAL EFFECTS OF THE ADENOSINE A1RECEPTOR AGONIST ADENOSINE AMINE CONGENER ON RENAL, FEMORAL AND CAROTID VASCULAR CONDUCTANCE IN PRETERM FETAL SHEEP

1. Adenosine A(1) receptor activation is critical for endogenous neuroprotection from hypoxia-ischaemia, raising the possibility that treatment with A(1) receptor agonists may be an effective physiological protection strategy for vulnerable preterm infants. However, the A(1) receptor can mediate unwanted systemic effects, including vasoconstriction of the afferent glomerular arteriole. There is limited information on whether this occurs at doses that improve cerebral perfusion in the immature brain. 2. Therefore, in the present study, we examined whether infusion of the selective A(1) receptor agonist adenosine amine congener (ADAC) is associated with reduced renal perfusion in chronically instrumented preterm (0.7 gestation) fetal sheep. In the present study, ADAC was given in successive doses of 2.5, 5.0 and 15.0 microg, 45 min apart. 3. Treatment with ADAC was associated with a marked reduction in renal vascular conductance (and blood flow), whereas carotid conductance was increased and there was no significant effect on femoral conductance. In contrast with the stable effects of increasing ADAC dose on vascular conductance, there was a significant dose-related fall in fetal heart rate and blood pressure. 4. In conclusion, these short-term data support the concern that A(1) receptor agonist infusion can selectively impair renal perfusion, even at low doses.

Connexin 43 gap junction proteins are up‐regulated in remyelinating spinal cord

Alterations in the expression of gap junction proteins have previously been observed in several diseases affecting the central nervous system; however, the status of connexin 43 (Cx43) has not yet been reported in spinal cord remyelination. We studied Cx43 expression in demyelination and remyelination by using a chronic guinea pig model of experimental allergic encephalomyelitis (EAE). Hartley guinea pigs were immunized with homogenized whole CNS and complete Freund's adjuvant. Animals became chronically ill by day 40 postimmunization, and animals with paralysis were entered into the study. Animals were treated on days 40-60 postimmunization with either saline or drugs that promote remyelination: an adenosine amine congener (100 mug/kg), an anti-alpha4-integrin blocker (CT301; ELN 69299; 30 mg/kg), or a combination of both drugs. Remyelination was induced in all drug-treated groups. Cx43 expression was virtually absent in demyelinated lesions of saline-treated controls compared with healthy tissue and normal appearing white matter (P < 0.001), whereas Cx43 was considerably increased (300-500%) in remyelinating lesions of all treatment groups (P < 0.001), most notably in CT301-treated animals. These changes in Cx43 expression indicate that Cx43 may beimportant for recovery from neuroinflammation.

A1 adenosine receptor upregulation and activation attenuates neuroinflammation and demyelination in a model of multiple sclerosis.

The neuromodulator adenosine regulates immune activation and neuronal survival through specific G-protein-coupled receptors expressed on macrophages and neurons, including the A1 adenosine receptor (A1AR). Here we show that A1AR null (A1AR-/-) mice developed a severe progressive-relapsing form of experimental allergic encephalomyelitis (EAE) compared with their wild-type (A1AR+/+) littermates. Worsened demyelination, axonal injury, and enhanced activation of microglia/macrophages were observed in A1AR-/- animals. In addition, spinal cords from A1AR-/- mice demonstrated increased proinflammatory gene expression during EAE, whereas anti-inflammatory genes were suppressed compared with A1AR+/+ animals. Macrophages from A1AR-/- animals exhibited increased expression of the proinflammatory genes, interleukin-1beta, and matrix metalloproteinase-12 on immune activation when matched with A1AR+/+ control cells. A1AR-/- macrophage-derived soluble factors caused significant oligodendrocyte cytotoxicity compared with wild-type controls. The A1AR was downregulated in microglia in A1AR+/+ mice during EAE accompanied by neuroinflammation, which recapitulated findings in multiple sclerosis (MS) patients. Caffeine treatment augmented A1AR expression on microglia, with ensuing reduction of EAE severity, which was further enhanced by concomitant treatment with the A1AR agonist, adenosine amine congener. Thus, modulation of neuroinflammation by the A1AR represents a novel mechanism that provides new therapeutic opportunities for MS and other demyelinating diseases.

Effects of adenosine and ATP on the membrane potential and synaptic transmission in neurons of the rat locus coeruleus.

Effects of adenosine (Ado) and adenosine 5'-triphosphate (ATP) on the membrane potential and synaptic transmission in neurons of the rat locus coeruleus (LC) were examined, in vitro. Ado (30-300 microM) produced a hyperpolarizing response and inhibited spontaneous firing activity in neurons of the rat LC. Ado decreased input resistance of LC neurons. The Ado-induced hyperpolarization reversed polarity near the equilibrium potential of K+ (EK). Ado (100-300 microM) depressed both excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP). Ado (300 microM) did not alter the hyperpolarization induced by norepinephrine (30 microM). N6-Cyclopentyladenosine (CPA, 100 microM), an A1 receptor agonist, also produced a hyperpolarizing response and depressed both the EPSP and IPSP. Another A1 receptor agonist, adenosine amine congener (ADAC, 30 microM) also produced a hyperpolarizing response and consistently depressed the EPSP and IPSP. Application of ATP (100 microM) to LC neurons caused a depolarizing response associated with an increase in the firing rate of spontaneous action potential in LC neurons. The ATP-induced depolarization was accompanied by an increased input resistance and reversed polarity at--91 mV. ATP (100 microM) consistently depressed the IPSP, while it did not change the amplitude of the EPSP in a majority of neurons. alpha, beta-Methylene ATP (alpha, beta-meATP, 30 micro/M), a P2 receptor agonist, mimicked these effects of ATP. Adenosine 5'-(beta, gamma-imido) triphosphate (AMP-PNP, 100 microM), a non-metabolizable analogue of ATP, produced a depolarizing response in LC neurons, but it produced no obvious depression of the EPSP and IPSP. These results suggest that Ado and ATP cause inhibitory and excitatory modulation, respectively, of neuronal activity and synaptic transmission in the rat LC.

The Adenosine A1Receptor Agonist Adenosine Amine Congener Exerts a Neuroprotective Effect against the Development of Striatal Lesions and Motor Impairments in the 3-Nitropropionic Acid Model of Neurotoxicity

Huntington's disease is a genetic neurodegenerative disorder characterized clinically by both motor and cognitive impairments and striatal lesions. At present, there are no pharmacological treatments able to prevent or slow its development. In the present study, we report the neuroprotective effect of adenosine amine congener (ADAC), a specific A1 receptor agonist known to be devoid of any of the side effects that usually impair the clinical use of such compounds. Remarkably, in a rat model of Huntington's disease generated by subcutaneous infusion of the mitochondrial inhibitor 3-nitropropionic acid (3NP), we have observed that an acute treatment with ADAC (100 microg x kg(-1) x d(-1)) not only strongly reduces the size of the striatal lesion (-40%) and the remaining ongoing striatal degeneration (-30%), but also prevents the development of severe dystonia of hindlimbs. Electrophysiological recording on corticostriatal brain slices demonstrated that ADAC strongly decreases the field EPSP amplitude by 70%, whereas it has no protective effect up to 1 microm against the 3NP-induced neuronal death in primary striatal cultures. This suggests that ADAC protective effects may be mediated presynaptically by the modulation of the energetic impairment-induced striatal excitotoxicity. Altogether, our results indicate that A1 receptor agonists deserve further experimental evaluation in animal models of Huntington's disease.

Lack of central effects of peripherally administered adenosine A 1 agonists on synaptic transmission in the rat hippocampus

Peripheral administration of adenosine A 1 receptor selective agonists is generally thought to protect the hippocampus against ischemic damage via central actions. We examined the effects of two peripherally administered A 1 agonists, cyclohexyladenosine (CHA) and adenosine amine congener (ADAC), on synaptic transmission in the hippocampus and on indices of cardiovascular function. We conclude that the permeability of these agonists is not sufficient to result in concentrations necessary to activate central adenosine A 1 receptors within the hippocampus.

Vasodilator responses to adenosine and hyperemia are mediated by A(1) and A(2) receptors in the cat vascular bed.

Hemodynamic responses to adenosine, the A(1) receptor agonists N(6)-cyclopentyladenosine (CPA) and adenosine amine congener (ADAC), and the A(2) receptor agonist 5'-(N-cyclopropyl)-carboxamido-adenosine (CPCA) were investigated in the hindquarter vascular bed of the cat under constant-flow conditions. Injections of adenosine, CPA, ADAC, CPCA, ATP, and adenosine 5'-O-(3-thiotriphosphate) (ATPgamma S) into the perfusion circuit induced dose-related decreases in perfusion pressure. Vasodilator responses to the A(1) agonists were reduced by the A(1) receptor antagonists KW-3902 and CGS-15943, whereas responses to CPCA were reduced by the A(2) antagonist KF-17837. Vasodilator responses to adenosine were reduced by KW-3902, CGS-15943, and by KF-17837, suggesting a role for both A(1) and A(2) receptors. Vasodilator responses to ATP and the nonhydrolyzable ATP analog ATP gamma S were not attenuated by CGS-15943 or KF-17837. After treatment with the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester, the cyclooxygenase inhibitor sodium meclofenamate, or the ATP-dependent K(+) (K) channel antagonists U-37883A or glibenclamide, responses to adenosine and ATP were not altered. Responses to adenosine, CPA, and CPCA were increased in duration by rolipram, a type 4 cAMP phosphodiesterase inhibitor, but were not altered by zaprinast, a type 5 cGMP phosphodiesterase inhibitor. When blood flow was interrupted for a 30-s period, the magnitude and duration of the reactive vasodilator response were reduced by A(1) and A(2) receptor antagonists. These data suggest that vasodilator responses to adenosine and the A(1) and A(2) agonists studied are not dependent on the release of cyclooxygenase products, nitric oxide, or the opening of K channels in the regional vascular bed of the cat. The present data suggest a role for cAMP in mediating responses to adenosine and suggest that vasodilator responses to adenosine and to reactive hyperemia are mediated in part by A(1) and A(2) receptors in the hindquarter vascular bed of the cat.

Adenosine A 1 receptor agonism in the immature rat brain and heart

We examined if the adenosine A 1 receptor agonist adenosine amine congener (ADAC, 100 μg/kg i.p.) is neuroprotective in 7-day-old rats subjected to hypoxic ischemia. Brain damage, evaluated as weight deficit and gross morphology, was not affected by ADAC treatment. Nonetheless, ADAC (100 μg/kg i.p.) reduced heart rate by 44% ( p<0.0001), indicating that the dose given was pharmacologically active. Adenosine A 1 receptors were determined by [ 3H] 1,3-dipropyl-8-cyclopentylxanthine (DPCPX)-binding and levels were 23% of the adult levels. GTP did not affect [ 3H] DPCPX-binding in the cerebral cortex at postnatal day 7 whereas there was strong enhancement of [ 3H] DPCPX-binding in the heart. This suggested a poor G-protein coupling at postnatal day 7 in the brain, which also was confirmed using GTP [γ- 35S]-binding in the presence of an adenosine A 1 receptor agonist. Thus, the lack of a neuroprotective effect of ADAC may be explained by the fact that adenosine A 1 receptors are not part of a functional unit in the 7-day-old rat brain.

Protection against ischemic damage by adenosine amine congener, a potent and selective adenosine A 1 receptor agonist

Although the selectivity and potency of adenosine amine congener (ADAC) at adenosine A 1 receptors are similar to other highly selective agonists at this receptor type, the chemical structure of the N 6 substituent is completely different. We now demonstrate that the characteristics of the therapeutic profile of ADAC are distinct from those observed during our previous studies of adenosine A 1 receptor agonist-mediated neuroprotection. Most significantly, chronic treatment with low microgram doses of ADAC (25–100 μg/kg) protects against both mortality and neuronal damage induced by 10 min bilateral carotid occlusion in gerbils. At higher chronic doses, the statistical significance of the protective effect is lost. Acute preischemic administration of the drug at 75–200 μg/kg also results in a statistically significant reduction of postischemic mortality and morbidity. These data indicate that, contrary to other adenosine A 1 receptor agonists whose chronic administration enhances postocclusive brain damage, ADAC may be a promising agent in treatment of both acute (e.g., cerebral ischemia) and chronic (seizures) disorders of the central nervous system in which adenosine A 1 receptors appear to be involved.

Adenosine prevents permeability increase in oxidant-injured endothelial monolayers.

Adenosine is thought to prevent or reduce the increase in permeability, which is a hallmark of oxidant injury to endothelium. However, the effect of adenosine on endothelial cells directly exposed to oxidant species has not been demonstrated in vitro. By measuring the passage of Evan's blue dye-labeled albumin across confluent monolayers, we demonstrated the ability of adenosine (0.1-100 microM) to lower basal permeability of human umbilical vein endothelial cells in a concentration-dependent fashion and prevent the permeability increase induced by exposure of the cells to xanthine plus xanthine oxidase (X/XO). Whereas pretreatment of monolayers for 10 min with adenosine (10 and 100 microM) prevented the X/XO-induced permeability increase, these same concentrations of adenosine failed to increase intracellular adenosine 3',5'-cyclic monophosphate in X/XO-exposed cells. The protective effect of adenosine on endothelial monolayers was mimicked by adenosine amine congener and 5'-(N-ethylcarboxamido)adenosine but not by other agonists examined. Hence, the protective effect of adenosine against oxidant injury may include an adenosine 3',5'-cyclic monophosphate-independent mechanism by direct action of adenosine at A1 receptors on endothelial cells.

A novel cardioprotective function of adenosine A1 and A3 receptors during prolonged simulated ischemia.

The possible cardioprotective roles of adenosine A1 and A3 receptors were investigated in a cardiac myocyte model of injury. The adenosine A3 receptor is a novel cardiac receptor capable of mediating potentially important cardioprotective functions. Prolonged hypoxia with glucose deprivation was used to simulate ischemia and to induce injury in cardiac ventricular myocytes cultured from chick embryos 14 days in ovo. When present during the prolonged hypoxia, the adenosine A3 agonists N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA) and 2-chloro-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (CI-IB-MECA) caused a dose-dependent reduction in the extent of hypoxia-induced injury as manifested by a decrease in the amount of creatine kinase released and the percentage of myocytes killed. The adenosine A1 agonists 2-chloro-N6-cyclopentyladenosine (CCPA), N6-cyclohexyladenosine, and adenosine amine congener were also able to cause a decrease in the extent of myocyte injury. The A1 receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine blocked the cardioprotective effect of the A1 but not of the A3 agonists. Conversely, the selective A3 antagonists MRS-1191 and MRS-1097 blocked the protection induced by CI-IB-MECA but had minimal effect on that caused by CCPA. Thus the cardioprotective effects of A1 and A3 agonists were mediated by their respective receptors. This study defines a novel cardioprotective function of the cardiac A3 receptor and provides conclusive evidence that activation of both A1 and A3 receptors during hypoxia can attenuate myocyte injury.

Postischemic administration of adenosine amine congener (ADAC): analysis of recovery in gerbils

Although adenosine receptor-based treatment of cerebral ischemia and other neurodegenerative disorders has been frequently advocated, cardiovascular side effects and an uncertain therapeutic time window of such treatment have constituted major obstacles to clinical implementation. Therefore, we have investigated the neuroprotective effects of the adenosine A 1 receptor agonist adenosine amine congener (ADAC) injected after either 5 or 10 min ischemia at 100 μg/kg. When the drug was administered at either 6 or 12 h following 5 min forebrain ischemia, all animals were still alive on the 14th day after the occlusion. In both ADAC treated groups neuronal survival was approximately 85% vs. 50% in controls. Administration of a single dose of ADAC at times 15 min to 12 h after 10 min ischemia resulted in a significant improvement of survival in animals injected either at 15 or 30 min, or at 1, 2, or 3 h after the insult. In all 10 min ischemia groups, administration of ADAC resulted in a significant protection of neuronal morphology and preservation of microtubule associated protein 2 (MAP-2). However, postischemic Morris' water maze tests revealed full preservation of spatial memory and learning ability in animals injected at 6 h. On the other hand, the performance of gerbils treated at 12 h postischemia was indistinguishable from that of the controls. Administration of ADAC at 100 μg/kg in non-ischemic animals did not result in bradycardia, hypotension, or hypothermia. The data indicate that when ADAC is used postischemically, the most optimal level of protection is obtained when drugs are given at 30 min to 6 h after the insult. Although the mechanisms involved in neuroprotective effects of adenosine A 1 receptor agonists require further studies, the present results demonstrate the feasibility of their clinical applications.

Reduction of postischemic brain damage and memory deficits following treatment with the selective adenosine A 1 receptor agonist

Agonists of adenosine A 1 receptors have been frequently proposed as candidates for clinical development in treatment of cerebral ischemia and stroke. Numerous experimental studies have shown that pre- and postischemic administration of these drugs results in a very significant reduction of postischemic brain damage. However, only a few studies determined the impact of cerebral ischemia and drug treatment on postischemic recovery of spatial memory. The present paper demonstrates that preischemic i.p. administration of adenosine amine congener (ADAC) at 100 μg/kg in gerbils results in a significant ( P < 0.05) reduction of postischemic mortality and hippocampal, cortical and striatal morbidity. Postischemic Morris' water maze tests show that preischemic treatment with ADAC also leads to a very significant ( P < 0.001) reduction of postischemic spatial memory loss. Our results indicate feasibility of further consideration of adenosine A 1 receptor agonists as a clinically applicable acute treatment of brain ischemia. Recent development of neuroprotective adenosine A 1 receptor agonists that are free of cardiovascular side effects supports such development.

Adenosine analogs with covalently attached lipids have enhanced potency at A 1-adenosine receptors

Chemically functionalized congeners of N 6-phenyladenosine and 1,3-dipropyl-8-phenylxanthine have been covalently coupled to fatty acids, diglycerides, and a phospholipid. The lipid-drug conjugates inhibit R-[ 3H]-phenylisopropyladenosine binding to A 1-adenosine receptors in rat cerebral cortex membranes. A xanthine-phosphatidylethanolamine conjugate bound with a K i value of 19 nM. Various xanthine esters of low potency are potential prodrugs. Amides of an adenosine amine congener (ADAC) with 18-carbon fatty acids exhibited K i values at A 1-adenosine receptors of 70 pM, representing a 130-fold enhancement over the affinity of the corresponding acetyl amide. The very high affinity of adenosine-lipid conjugates may be due to stabilization of these adducts in the phospholipid microenvironment of the receptor protein.