Multigram-scale syntheses, stability, and photoreactions of A2A adenosine receptor antagonists with 8-styrylxanthine structure: potential drugs for Parkinson's disease

1,3-Dialkyl-substituted tetrahydropyrimido[1,2-f]purine-2,4-diones as multiple target drugs for the potential treatment of neurodegenerative diseases

Adenosine receptors and monoamine oxidases are drug targets for neurodegenerative diseases such as Parkinson's and Alzheimer's disease. In the present study we prepared a library of 55 mostly novel tetrahydropyrimido[2,1-f]purinediones with various substituents in the 1- and 3-position (1,3-dimethyl, 1,3-diethyl, 1,3-dipropyl, 1-methyl-3-propargyl) and broad variation in the 9-position. A synthetic strategy to obtain 3-propargyl-substituted tetrahydropyrimido[2,1-f]purinedione derivatives was developed. The new compounds were evaluated for their interaction with all four adenosine receptor subtypes and for their ability to inhibit monoamine oxidases (MAO). Introduction of mono- or di-chloro-substituted phenyl, benzyl or phenethyl residues at N9 of the 1,3-dimethyl series led to the discovery of a novel class of potent MAO-B inhibitors, the most potent compound being 9-(3,4-dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)-dione (21g, IC(50) human MAO-B: 0.0629 μM), which displayed high selectivity versus the other investigated targets. Potent dually active A1/A2A adenosine receptor antagonists were identified, for example, 9-benzyl-1-methyl-3-propargyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)dione (19f, Ki, human receptors, A1: 0.249 μM, A2A: 0.253 μM). Several compounds showed triple-target inhibition, the best compound being 9-(2-methoxybenzyl)-1-methyl-3-(prop-2-ynyl)-6,7,8,9-tetrahydro pyrimido [1,2-f]purine-2,4(1H,3H)-dione (19g, Ki A1: 0.605 μM, Ki A2A: 0.417 μM, IC(50) MAO-B: 1.80 μM). Compounds inhibiting several different targets involved in neurodegeneration may exhibit additive or even synergistic effects in vivo.

Synthesis, biological activity and molecular modelling studies of tricyclic alkylimidazo-, pyrimido- and diazepinopurinediones

Syntheses and biological activities of imidazo-, pyrimido- and diazepino[2,1-f]purinediones containing N-alkyl substituents (with straight, branched or unsaturated chains) are described. Tricyclic derivatives were synthesized by the cyclization of 8-bromo-substituted 7-(2-bromoethyl)-, 7-(3-chloropropyl)- or 7-(4-bromobutyl)-theophylline with primary amines under various conditions. Compound 22 with an ethenyl substituent was synthesized by dehydrohalogenation of 9-(2-bromoethyl)-1,3-dimethyltetrahydropyrimido[2,1-f]purinedione. The obtained derivatives (5-35) were initially evaluated for their affinity at rat A1 and A2A adenosine receptors (AR), showing moderate affinity for both adenosine receptor subtypes. The best ligands were diazepinopurinedione 28 (K i = 0.28 μM) with fivefold A2A selectivity and the non-selective A1/A2A AR ligand pyrimidopurinedione 35 (K i A1 = 0.28 μM and K i A2A = 0.30 μM). The compounds were also evaluated for their affinity at human A1, A2A, A2B and A3 ARs. All of the obtained compounds were docked to the A2A AR X-ray structure in complex with the xanthine-based, potent adenosine receptor antagonist-XAC. The likely interactions of imidazo-, pyrimido- and diazepino[2,1-f]purinediones with the residues forming the A2A binding pocket were discussed. Furthermore, the new compounds were tested in vivo as anticonvulsants in maximal electroshock, subcutaneous pentylenetetrazole (ScMet) and TOX tests in mice (i.p.). Pyrimidopurinediones showed anticonvulsant activity mainly in the ScMet test. The best derivative was compound 11, showing 100 % protection at a dose of 100 mg/kg without symptoms of neurotoxicity. Compounds 6, 7, 8 and 14 with short substituents showed neurotoxicity and caused death. In rat tests (p.o.), 9 was characterized by a high protection index (>13.3). AR affinity did not apparently correlate with the antiepileptic potency of the compounds.

Dual targeting of adenosine A(2A) receptors and monoamine oxidase B by 4H-3,1-benzothiazin-4-ones

Blockade of A2A adenosine receptors (A2AARs) and inhibition of monoamine oxidase B (MAO-B) in the brain are considered attractive strategies for the treatment of neurodegenerative diseases such as Parkinson's disease (PD). In the present study, benzothiazinones, e.g., 2-(3-chlorophenoxy)-N-(4-oxo-4H-3,1-benzothiazin-2-yl)acetamide (13), were identified as a novel class of potent MAO-B inhibitors (IC50 human MAO-B: 1.63 nM). Benzothiazinones with large substituents in the 2-position, e.g., methoxycinnamoylamino, phenylbutyrylamino, or chlorobenzylpiperazinylbenzamido residues (14, 17, 27, and 28), showed high affinity and selectivity for A2AARs (Ki human A2AAR: 39.5-69.5 nM). By optimizing benzothiazinones for both targets, the first potent, dual-acting A2AAR/MAO-B inhibitors with a nonxanthine structure were developed. The best derivative was N-(4-oxo-4H-3,1-benzothiazin-2-yl)-4-phenylbutanamide (17, Ki human A2A, 39.5 nM; IC50 human MAO-B, 34.9 nM; selective versus other AR subtypes and MAO-A), which inhibited A2AAR-induced cAMP accumulation and showed competitive, reversible MAO-B inhibition. The new compounds may be useful tools for validating the A2AAR/MAO-B dual target approach in PD.

Nucleus accumbens neurotransmission and effort-related choice behavior in food motivation: effects of drugs acting on dopamine, adenosine, and muscarinic acetylcholine receptors

Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Although nucleus accumbens (NAc) DA depletions or antagonism leave aspects of appetite and primary food motivation intact, rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements, and instead select less effortful food-seeking behaviors. Previous work showed that adenosine A2A antagonists can reverse the effects of DA D2 antagonists on effort-related choice, and that stimulation of adenosine A2A receptors produces behavioral effects that are similar to those induced by DA antagonism. The present review summarizes the literature on the role of NAc DA and adenosine in effort-related processes, and also presents original data on the effects of local stimulation of muscarinic acetylcholine receptors in NAc core. Local injections of the muscarinic agonist pilocarpine directly into NAc core produces shifts in effort-related choice behavior similar to those induced by DA antagonism or A2A receptor stimulation, decreasing lever pressing but increasing chow intake in rats responding on a concurrent fixed ratio/chow feeding choice task. In contrast, injections into a neostriatal control site dorsal to the NAc were ineffective. The actions of pilocarpine on this task were attenuated by co-administration of the muscarinic antagonist scopolamine. Thus, drugs that act on DA, adenosine A2A, and muscarinic receptors regulate effort-related choice behavior, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, fatigue or anergia that can be observed in depression and other disorders.

Effect of subtype-selective adenosine receptor antagonists on basal or haloperidol-regulated striatal function: studies of exploratory locomotion and c-Fos immunoreactivity in outbred and A(2A)R KO mice

Behavioral activation is regulated by dopamine (DA) in striatal areas. At low doses, while typical antipsychotic drugs produce psychomotor slowing, psychostimulants promote exploration. Minor stimulants such as caffeine, which act as adenosine receptor antagonists, can also potentiate behavioral activation. Striatal areas are rich in adenosine and DA receptors, and adenosine A2A receptors are mainly expressed in the striatum where they are co-localized with DA D2 receptors. Adenosine antagonists with different receptor-selectivity profiles were used to study spontaneous or haloperidol-impaired exploration and c-Fos expression in different striatal areas. Because A2A antagonists were expected to be more selective for reversing the effects of the D2 antagonist haloperidol, A2A receptor knockout (A2ARKO) mice were also assessed. CD1 and A2ARKO male mice were tested in an open field and in a running wheel. Only the A1/A2A receptor antagonist theophylline (5.0-15.0 mg/kg) and the A2A antagonist MSX-3 (2.0 mg/kg) increased spontaneous locomotion and rearing. Co-administration of theophylline (10.0-15.0 mg/kg), and MSX-3 (1.0-3.0 mg/kg) reversed haloperidol-induced suppression of locomotion. The A1 antagonist CPT was only marginally effective in reversing the effects of haloperidol. Although adenosine antagonists did not affect c-Fos expression on their own, theophylline and MSX-3, but not CPT, attenuated haloperidol induction of c-Fos expression. A2ARKO mice were resistant to the behavioral effects of haloperidol at intermediate doses (0.1 mg/kg) in the open field and in the running wheel. A2A receptors are important for regulating behavioral activation, and interact with D2 receptors in striatal areas to regulate neural processes involved in exploratory activity.

Psychostimulant pharmacological profile of paraxanthine, the main metabolite of caffeine in humans

Caffeine induces locomotor activation by its ability to block adenosine receptors. Caffeine is metabolized to several methylxanthines, with paraxanthine being the main metabolite in humans. In this study we show that in rats paraxanthine has a stronger locomotor activating effect than caffeine or the two other main metabolites of caffeine, theophylline and theobromine. As previously described for caffeine, the locomotor activating doses of paraxanthine more efficiently counteract the locomotor depressant effects of an adenosine A(1) than an adenosine A(2A) receptor agonist. In drug discrimination experiments in rats trained to discriminate a maximal locomotor activating dose of caffeine, paraxanthine, unlike theophylline, generalized poorly to caffeine suggesting the existence of additional mechanisms other than adenosine antagonism in the behavioral effects of paraxanthine. Pretreatment with the nitric oxide inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) reduced the locomotor activating effects of paraxanthine, but not caffeine. On the other hand, pretreatment with the selective cGMP-preferring phosphodiesterase PDE9 inhibitor BAY 73-6691, increased locomotor activity induced by caffeine, but not paraxanthine. Ex vivo experiments demonstrated that paraxanthine, but not caffeine, can induce cGMP accumulation in the rat striatum. Finally, in vivo microdialysis experiments showed that paraxanthine, but not caffeine, significantly increases extracellular levels of dopamine in the dorsolateral striatum, which was blocked by l-NAME. These findings indicate that inhibition of cGMP-preferring PDE is involved in the locomotor activating effects of the acute administration of paraxanthine. The present results demonstrate a unique psychostimulant profile of paraxanthine, which might contribute to the reinforcing effects of caffeine in humans.

Adenosine A(2A) receptor blockade reverts hippocampal stress-induced deficits and restores corticosterone circadian oscillation

Maternal separation (MS) is an early life stress model that induces permanent changes in the central nervous system, impairing hippocampal long-term potentiation (LTP) and spatial working memory. There are compelling evidences for a role of hippocampal adenosine A(2A) receptors in stress-induced modifications related to cognition, thus opening a potential window for therapeutic intervention. Here, we submitted rats to MS and evaluated the long-lasting molecular, electrophysiological and behavioral impairments in adulthood. We then assessed the therapeutic potential of KW6002, a blocker of A(2A) receptors, in stress-impaired animals. We report that the blockade of A(2A) receptors was efficient in reverting the behavior, electrophysiological and morphological impairments induced by MS. In addition, this effect is associated with restoration of the hypothalamic-pituitary-adrenal axis (HPA-axis) activity, as both the plasma corticosterone levels and hippocampal glucocorticoid receptor expression pattern returned to physiological-like status after the treatment. These results reveal the involvement of A(2A) receptors in the stress-associated impairments and directly in the stress response system by showing that the dysfunction of the HPA-axis as well as the long-lasting synaptic and behavioral effects of MS can be reverted by targeting adenosine A(2A) receptors. These findings provide a novel evidence for the use of adenosine A(2A) receptor antagonists as potential therapy against psychopathologies.

The novel adenosine A(2A) antagonist prodrug MSX-4 is effective in animal models related to motivational and motor functions

Adenosine A(2A) and dopamine D2 receptors interact to regulate diverse aspects of ventral and dorsal striatal functions related to motivational and motor processes, and it has been suggested that adenosine A(2A) antagonists could be useful for the treatment of depression, parkinsonism and other disorders. The present experiments were performed to characterize the effects of MSX-4, which is an amino acid ester prodrug of the potent and selective adenosine A(2A) receptor antagonist MSX-2, by assessing its ability to reverse pharmacologically induced motivational and motor impairments. In the first group of studies, MSX-4 reversed the effects of the D2 antagonist eticlopride on a concurrent lever pressing/chow feeding task that is used as a measure of effort-related choice behavior. MSX-4 was less potent after intraperitoneal administration than the comparison compound, MSX-3, though both were equally efficacious. With this task, MSX-4 was orally active in the same dose range as MSX-3. MSX-4 also reversed the locomotor suppression induced by eticlopride in the open field, but did not induce anxiogenic effects as measured by the relative amount of interior activity. Behaviorally active doses of MSX-4 also attenuated the increase in c-Fos and pDARPP-32(Thr34) expression in nucleus accumbens core that was induced by injections of eticlopride. In addition, MSX-4 suppressed the oral tremor induced by the anticholinesterase galantamine, which is consistent with an antiparkinsonian profile. These actions of MSX-4 indicate that this compound could have potential utility as a treatment for parkinsonism, as well as some of the motivational symptoms of depression and other disorders.

Adenosine A2A receptor antagonism and genetic deletion attenuate the effects of dopamine D2 antagonism on effort-based decision making in mice

Brain dopamine (DA) and adenosine interact in the regulation of behavioral activation and effort-related processes. In the present studies, a T-maze task was developed in mice for the assessment of effort-related decision making. With this task, the two arms of the maze have different reinforcement densities, and a vertical barrier is positioned in the arm with the higher density (HD), presenting the animal with an effort-related challenge. Under control conditions mice prefer the HD arm, and climb the barrier to obtain the larger amount of food. The DA D(2) receptor antagonist haloperidol decreased selection of the HD arm and increased selection of the arm with the low density of reinforcement. However, the HD arm was still the preferred choice in haloperidol-treated mice trained with barriers in both arms. Pre-feeding the mice to reduce food motivation dramatically increased omissions, an effect that was distinct from the actions of haloperidol. Co-administration of theophylline, a nonselective adenosine receptor antagonist, partially reversed the effects of haloperidol. This effect seems to be mediated by the A(2A) receptor but not the A(1) receptor, since the A(2A) antagonist MSX-3, but not the A(1) antagonist CPT, dose dependently reversed the effects of haloperidol on effort-related choice and on c-Fos expression in the dorsal striatum and nucleus accumbens. In addition, adenosine A(2A) receptor knockout mice were resistant to the effects of haloperidol on effort-related choice in the maze. These results indicate that DA D(2) and adenosine A(2A) receptors interact to regulate effort-related decision making and effort expenditure in mice.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Pharmacological evidence for different populations of postsynaptic adenosine A2A receptors in the rat striatum

Adenosine A(2A) receptors (A(2A)Rs) are highly concentrated in the striatum. Two pharmacological different functional populations of A(2A)Rs have been recently described based on their different affinities for the A(2A)R antagonist SCH-442416. This compound has high affinity for A(2A)Rs not forming heteromers or forming heteromers with adenosine A(1) receptors (A(1)Rs) while showing very low affinity for A(2A)Rs forming heteromers with dopamine D(2) receptors (D(2)Rs). It has been widely described that striatal A(1)R-A(2A)R heteromers are preferentially localized presynaptically in the glutamatergic terminals that contact GABAergic dynorphinergic neurons, and that A(2A)R-D(2)R heteromers are localized postsynaptically in GABAergic enkephalinergic neurons. In the present study we provide evidence suggesting that SCH-442416 also targets postsynaptic A(2A)R not forming heteromers with D(2)R, which are involved in the motor depressant effects induced by D(2)R antagonists. SCH-442416 counteracted motor depression in rats induced by the D(2)R antagonist raclopride at a dose that did not produce motor activation or that blocked motor depression induced by an A(2A)R agonist. Furthermore, we re-evaluated the recently suggested key role of cannabinoid CB(1) receptors (CB(1)Rs) in the effects of A(2A)R antagonists acting at postsynaptic A(2A)Rs. By recording locomotor activity and monitoring striatal glutamate release induced by cortical electrical stimulation in rats after administration of A(2A)R and CB(1)R antagonists, we did not find evidence for any significant role of endocannabinoids in the post- or presynaptic effects of A(2A)R antagonists. The present results further suggest the existence of at least two functionally and pharmacologically different populations of striatal postsynaptic A(2A)Rs.

Reinforcing and neurochemical effects of cannabinoid CB1 receptor agonists, but not cocaine, are altered by an adenosine A2A receptor antagonist

Several recent studies suggest functional and molecular interactions between striatal adenosine A(2A) and cannabinoid CB(1) receptors. Here, we demonstrate that A(2A) receptors selectively modulate reinforcing effects of cannabinoids. We studied effects of A(2A) receptor blockade on the reinforcing effects of delta-9-tetrahydrocannabinol (THC) and the endogenous CB(1) receptor ligand anandamide under a fixed-ratio schedule of intravenous drug injection in squirrel monkeys. A low dose of the selective adenosine A(2A) receptor antagonist MSX-3 (1 mg/kg) caused downward shifts of THC and anandamide dose-response curves. In contrast, a higher dose of MSX-3 (3 mg/kg) shifted THC and anandamide dose-response curves to the left. MSX-3 did not modify cocaine or food pellet self-administration. Also, MSX-3 neither promoted reinstatement of extinguished drug-seeking behavior nor altered reinstatement of drug-seeking behavior by non-contingent priming injections of THC. Finally, using in vivo microdialysis in freely-moving rats, a behaviorally active dose of MSX-3 significantly counteracted THC-induced, but not cocaine-induced, increases in extracellular dopamine levels in the nucleus accumbens shell. The significant and selective results obtained with the lower dose of MSX-3 suggest that adenosine A(2A) antagonists acting preferentially at presynaptic A(2A) receptors might selectively reduce reinforcing effects of cannabinoids that lead to their abuse. However, the appearance of potentiating rather than suppressing effects on cannabinoid reinforcement at the higher dose of MSX-3 would likely preclude the use of such a compound as a medication for cannabis abuse. Adenosine A(2A) antagonists with more selectivity for presynaptic versus postsynaptic receptors could be potential medications for treatment of cannabis abuse.

Biophysical mapping of the adenosine A2A receptor

A new approach to generating information on ligand receptor interactions within the binding pocket of G protein-coupled receptors has been developed, called Biophysical Mapping (BPM). Starting from a stabilized receptor (StaR), minimally engineered for thermostability, additional single mutations are then added at positions that could be involved in small molecule interactions. The StaR and a panel of binding site mutants are captured onto Biacore chips to enable characterization of the binding of small molecule ligands using surface plasmon resonance (SPR) measurement. A matrix of binding data for a set of ligands versus each active site mutation is then generated, providing specific affinity and kinetic information (K(D), k(on), and k(off)) of receptor-ligand interactions. This data set, in combination with molecular modeling and docking, is used to map the small molecule binding site for each class of compounds. Taken together, the many constraints provided by these data identify key protein-ligand interactions and allow the shape of the site to be refined to produce a high quality three-dimensional picture of ligand binding, thereby facilitating structure based drug design. Results of biophysical mapping of the adenosine A(2A) receptor are presented.

Oral tremor induced by galantamine in rats: a model of the parkinsonian side effects of cholinomimetics used to treat Alzheimer's disease

Anticholinesterases are the most common treatment for Alzheimer's disease, and, in recent years, a new group of cholinesterase inhibitors (i.e. rivastigmine, galantamine, and donepezil) has become available. Although these drugs improve cognitive symptoms, they also can induce or exacerbate parkinsonian symptoms, including tremor. The present studies were conducted to determine if galantamine induces tremulous jaw movements, a rodent model of parkinsonian tremor, and to investigate whether these oral motor impairments can be reversed by co-administration of adenosine A(2A) antagonists. The first experiment demonstrated that systemic injections of galantamine (0.75-6.0 mg/kg I.P.) induced a dose-related increase in tremulous jaw movements in rats. In a second study, co-administration of the muscarinic antagonist scopolamine (0.0156-0.25 mg/kg I.P.) produced a dose dependent suppression of tremulous jaw movements induced by a 3.0 mg/kg dose of galantamine, indicating that galantamine induces these tremulous oral movements through actions on muscarinic acetylcholine receptors. In two additional studies, analyses of freeze-frame video and electromyographic activity recorded from the lateral temporalis muscle indicated that the local frequency of these galantamine-induced jaw movements occurs in the 3-7 Hz frequency range that is characteristic of parkinsonian tremor. In the final experiment, the adenosine A(2A) antagonist MSX-3 significantly attenuated the tremulous jaw movements induced by the 3.0mg/kg dose of galantamine, which is consistent with the hypothesis that co-administration of adenosine A(2A) antagonists may be beneficial in reducing parkinsonian motor impairments induced by anticholinesterase treatment.

Effect of the adenosine A2A receptor antagonist MSX-3 on motivational disruptions of maternal behavior induced by dopamine antagonism in the early postpartum rat

RATIONALE

Mesolimbic dopamine (DA), particularly in the nucleus accumbens, importantly regulates activational aspects of maternal responsiveness. DA antagonism and accumbens DA depletions interfere with early postpartum maternal motivation by selectively affecting most forms of active maternal behaviors, while leaving nursing behavior relatively intact. Considerable evidence indicates that there is a functional interaction between DA D2 and adenosine A(2A) receptors in striatal areas, including the nucleus accumbens.

OBJECTIVE

This study was conducted to determine if adenosine A(2A) receptor antagonism could reverse the effects of DA receptor antagonism on early postpartum maternal behavior.

METHODS

The adenosine A(2A) receptor antagonist MSX-3 (0.25-2.0 mg/kg, IP) was investigated for its ability to reverse the effects of the DA D2 receptor antagonist haloperidol (0.1 mg/kg, IP) on the maternal behavior of early postpartum female rats.

RESULTS

Haloperidol severely impaired the expression of active maternal components, including retrieval and grouping the pups at the nest site, pup licking, and nest building. Co-administration of MSX-3 (0.25-2.0 mg/kg, IP) with haloperidol produced a dose-related attenuation of the haloperidol-induced behavioral deficits in early postpartum females. Doses of MSX-3 that effectively reversed the effects of haloperidol (0.5, 1.0 mg/kg), when administered in the absence of haloperidol, did not affect maternal responding or locomotor activity.

CONCLUSIONS

Adenosine and DA systems interact to regulate early postpartum maternal responsiveness. This research may potentially contribute to the development of strategies for treatments of psychiatric disorders during the postpartum period, with particular emphasis in maintaining or restoring the mother-infant relationship.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Prodrug approaches for enhancing the bioavailability of drugs with low solubility

Low water solubility and low bioavailability are frequent problems in drug development, particularly in the area of central nervous system (CNS) drugs. This short review describes selected prodrug approaches which have been developed to enhance the bioavailability of drugs, especially that of poorly soluble drugs. Some of the most successful drugs on the market are prodrugs. With a better understanding of active-transport processes at cell membranes in the gut as well as at the blood-brain barrier, the importance of prodrug approaches will further increase in the future. Prodrug approaches will already be considered in the early phase of drug discovery.

Nucleus accumbens and effort-related functions: behavioral and neural markers of the interactions between adenosine A2A and dopamine D2 receptors

Nucleus accumbens dopamine (DA) is a critical component of the brain circuitry regulating work output in reinforcement-seeking behavior and effort-related choice behavior. Moreover, there is evidence of an interaction between DA D(2) and adenosine A(2A) receptor function. Systemic administration of adenosine A(2A) antagonists reverses the effects of D(2) antagonists on tasks that assess effort related choice. The present experiments were conducted to determine if nucleus accumbens is a brain locus at which adenosine A(2A) and DA D(2) antagonists interact to regulate effort-related choice behavior. A concurrent fixed ratio 5 (FR5)/chow feeding procedure was used; with this procedure, rats can choose between completing an FR5 lever-pressing requirement for a preferred food (i.e., high carbohydrate operant pellets) or approaching and consuming a freely available food (i.e., standard rodent chow). Rats trained with this procedure spend most of their time pressing the lever for the preferred food, and eat very little of the concurrently available chow. Intracranial injections of the selective DA D(2) receptor antagonist eticlopride (1.0, 2.0, 4.0 microg) into nucleus accumbens core, but not a dorsal control site, suppressed FR5 lever-pressing and increased consumption of freely available chow. Either systemic or intra-accumbens injections of the adenosine A(2A) receptor antagonist MSX-3 reversed these effects of eticlopride on effort-related choice. Intra-accumbens injections of eticlopride also increased local expression of c-Fos immunoreactivity, and this effect was attenuated by co-administration of MSX-3. Adenosine and DA systems interact to regulate instrumental behavior and effort-related processes, and nucleus accumbens is an important locus for this interaction. These findings may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, anergia and fatigue.

Oral tremor induced by the muscarinic agonist pilocarpine is suppressed by the adenosine A2A antagonists MSX-3 and SCH58261, but not the adenosine A1 antagonist DPCPX

Tremulous jaw movements in rats, which can be induced by dopamine (DA) antagonists, DA depletion, and cholinomimetics, have served as a useful model for studies of tremor. Although adenosine A(2A) antagonists can reduce the tremulous jaw movements induced by DA antagonists and DA depletion, there are conflicting reports about the interaction between adenosine antagonists and cholinomimetic drugs. The present studies investigated the ability of adenosine antagonists to reverse the tremorogenic effect of the muscarinic agonist pilocarpine. While the adenosine A(2A) antagonist MSX-3 was incapable of reversing the tremulous jaw movements induced by the 4.0mg/kg dose of pilocarpine, both MSX-3 and the adenosine A(2A) antagonist SCH58261 reversed the tremulous jaw movements elicited by 0.5mg/kg pilocarpine. Systemic administration of the adenosine A(1) antagonist DPCPX failed to reverse the tremulous jaw movements induced by either an acute 0.5mg/kg dose of the cholinomimetic pilocarpine or the DA D2 antagonist pimozide, indicating that the tremorolytic effects of adenosine antagonists may be receptor subtype specific. Behaviorally active doses of MSX-3 and SCH 58261 showed substantial in vivo occupancy of A(2A) receptors, but DPCPX did not. The results of these studies support the use of adenosine A(2A) antagonists for the treatment of tremor.

Caffeine and an adenosine A(2A) receptor antagonist prevent memory impairment and synaptotoxicity in adult rats triggered by a convulsive episode in early life

Seizures early in life cause long-term behavioral modifications, namely long-term memory deficits in experimental animals. Since caffeine and adenosine A(2A) receptor (A(2A)R) antagonists prevent memory deficits in adult animals, we now investigated if they also prevented the long-term memory deficits caused by a convulsive period early in life. Administration of kainate (KA, 2 mg/kg) to 7-days-old (P7) rats caused a single period of self-extinguishable convulsions which lead to a poorer memory performance in the Y-maze only when rats were older than 90 days, without modification of locomotion or anxiety-like behavior in the elevated-plus maze. In accordance with the relationship between synaptotoxicity and memory dysfunction, the hippocampus of these adult rats treated with kainate at P7 displayed a lower density of synaptic proteins such as SNAP-25 and syntaxin (but not synaptophysin), as well as vesicular glutamate transporters type 1 (but not vesicular GABA transporters), with no changes in PSD-95, NMDA receptor subunits (NR1, NR2A, NR2B) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunits (GluR1, GluR2) compared with controls. Caffeine (1 g/L) or the A(2A)R antagonist, KW6002 (3 mg/kg) applied in the drinking water from P21 onwards, prevented these memory deficits in P90 rats treated with KA at P7, as well as the accompanying synaptotoxicity. These results show that a single convulsive episode in early life causes a delayed memory deficit in adulthood accompanied by a glutamatergic synaptotoxicity that was prevented by caffeine or adenosine A(2A)R antagonists.

Autoradiographic comparison of in vitro binding characteristics of various tritiated adenosine A2A receptor ligands in rat, mouse and pig brain and first ex vivo results

The adenosine A(2A) receptor in the basal ganglia is involved in the control of movement and plays a role in movement disorders such as Parkinsonism. Developing ligands to evaluate that receptor by noninvasive methods such as positron emission tomography has a high priority. In vitro radioligand binding guides the selection of ligands for in vivo application. This study measured the binding of the adenosine A(2A) receptor antagonist [(3)H]MSX-2 (3-(3-hydroxypropyl)-8-m-methoxystyryl)-7-methyl-1-propargylxanthine) to rat, mouse and pig brain by autoradiography. Other studies measured binding to membranes from PC12 pheochromocytoma cells. Those binding parameters were compared to those of the adenosine A(2A) receptor antagonist [(3)H]ZM241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino)ethyl)phenol), the adenosine A(2A) receptor agonist [(3)H]CGS21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosine) and the unselective adenosine receptor agonist [(3)H]NECA (5'N-ethylcarboxamido)adenosine). The potency order (K(d)) in the three species was [(3)H]ZM241385<[(3)H]MSX-2<[(3)H]NECA<[(3)H]CGS21680. The density of [(3)H]MSX-2 binding sites was greater in the striatum than in the cortex. Preliminary ex vivo experiments showed that by 10min after iv injection, [(3)H]MSX-2 and [(3)H]CGS21680 crossed the blood-brain barrier to the extent of almost 1% ID/g brain tissue, but [(3)H]NECA and [(3)H]ZM241385 to only 0.2% ID/g. The prior administration of unlabeled ZM241385 significantly lowered brain uptake of [(3)H]MSX-2. In conclusion, [(3)H]MSX-2 has a high affinity and sufficient selectivity for the adenosine A(2A) receptor. It penetrates the blood-brain barrier. Sensitivity to photoisomerization is a limitation. Further investigations assess its suitability as a ligand for imaging the brain adenosine A(2A) receptor.

The adenosine A2A antagonist MSX-3 reverses the effects of the dopamine antagonist haloperidol on effort-related decision making in a T-maze cost/benefit procedure

RATIONALE

Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Research involving choice tasks has shown that rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements and instead select less effortful food-seeking behaviors.

OBJECTIVE

Previous work showed that adenosine A(2A) antagonism can reverse the effects of the DA antagonist haloperidol in an operant task that assesses effort-related choice. The present work used a T-maze choice procedure to assess the effects of adenosine A(2A) and A(1) antagonism.

MATERIALS AND METHODS

With this task, the two arms of the maze have different reinforcement densities (four vs. two food pellets), and a vertical 44 cm barrier is positioned in the arm with the higher density, presenting the animal with an effort-related challenge. Untreated rats strongly prefer the arm with the high density of food reward and climb the barrier in order to obtain the food.

RESULTS

Haloperidol produced a dose-related (0.05-0.15 mg/kg i.p.) reduction in the number of trials in which the rats chose the high-barrier arm. Co-administration of the adenosine A(2A) receptor antagonist MSX-3 (0.75, 1.5, and 3.0 mg/kg i.p.), but not the A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (0.75, 1.5, and 3.0 mg/kg i.p.), reversed the effects of haloperidol on effort-related choice and latency.

CONCLUSIONS

Adenosine A(2A) and D2 receptors interact to regulate effort-related decision making, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing or anergia that can be observed in depression, parkinsonism, and other disorders.

The adenosine A2A antagonist MSX-3 reverses the effort-related effects of dopamine blockade: differential interaction with D1 and D2 family antagonists

RATIONALE

Brain dopamine (DA) participates in the modulation of instrumental behavior, including aspects of behavioral activation and effort-related choice behavior. Rats with impaired DA transmission reallocate their behavior away from food-seeking behaviors that have high response requirements, and instead select less effortful alternatives. Although accumbens DA is considered a critical component of the brain circuitry regulating effort-related choice behavior, emerging evidence demonstrates a role for adenosine A(2A) receptors.

OBJECTIVE

Adenosine A(2A) receptor antagonism has been shown to reverse the effects of DA antagonism. The present experiments were conducted to determine if this effect was dependent upon the subtype of DA receptor that was antagonized to produce the changes in effort-related choice.

MATERIALS AND METHODS

The adenosine A(2A) receptor antagonist MSX-3 (0.5-2.0 mg/kg IP) was assessed for its ability to reverse the effects of the D1 family antagonist SCH39166 (ecopipam; 0.2 mg/kg IP) and the D2 family antagonist eticlopride (0.08 mg/kg IP), using a concurrent lever pressing/chow feeding procedure.

RESULTS

MSX-3 produced a substantial dose-related reversal of the effects of eticlopride on lever pressing and chow intake. At the highest dose of MSX-3, there was a complete reversal of the effects of eticlopride on lever pressing. In contrast, MSX-3 produced only a minimal attenuation of the effects of SCH39166, as measured by regression and effect size analyses.

CONCLUSIONS

The greater ability of MSX-3 to reverse the effects of D2 vs. D1 blockade may be related to the colocalization of D2 and adenosine A(2A) receptors on the same population of striatal neurons.

Antagonists of the human A(2A) adenosine receptor. 4. Design, synthesis, and preclinical evaluation of 7-aryltriazolo[4,5-d]pyrimidines

Antagonism of the human A(2A) receptor has been implicated as a point of therapeutic intervention in the alleviation of the symptoms associated with Parkinson's disease. This is thought to occur, at least in part, by increasing the sensitivity of the dopaminergic neurons to the residual, depleted levels of striatal dopamine. We herein describe a novel series of functionalized triazolo[4,5-d]pyrimidine derivatives that display functional antagonism of the A(2A) receptor. Optimization of these compounds has resulted in improvements in potency, selectivity, and the pharmacokinetic properties of key derivatives. These efforts have led to the discovery of 60 (V2006/BIIB014), which demonstrates strong oral activity in commonly used models of Parkinson's disease. Furthermore, this derivative has shown excellent preclinical pharmacokinetics and has successfully completed phase I clinical studies. This compound is presently undergoing further clinical evaluation in collaboration with Biogen Idec.

Recent developments in adenosine A2A receptor ligands

The development of potent and selective agonists and antagonists of adenosine receptors (ARs) has been a target of medicinal chemistry research for several decades, and recently the US Food and Drug Administration has approved Lexiscan, an adenosine derivative substituted at the 2 position, for use as a pharmacologic stress agent in radionuclide myocardial perfusion imaging. Currently, some other adenosine A(2A) receptor (A(2A)AR) agonists and antagonists are undergoing preclinical testing and clinical trials. While agonists are potent antiinflammatory agents also showing hypotensive effects, antagonists are being developed for the treatment of Parkinson's disease.However, since there are still major problems in this field, including side effects, low brain penetration (for the targeting of CNS diseases), short half-life, or lack of in vivo effects, the design and development of new AR ligands is a hot research topic.This review presents an update on the medicinal chemistry of A(2A)AR agonists and antagonists, and stresses the strong need for more selective ligands at the human A(2A)AR subtype, in particular in the case of agonists.

Highlights on the development of A(2A) adenosine receptor agonists and antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

2-Amino-6-furan-2-yl-4-substituted nicotinonitriles as A2A adenosine receptor antagonists

A 2A adenosine receptor antagonists usually have bi- or tricyclic N aromatic systems with varying substitution patterns to achieve desired receptor affinity and selectivity. Using a pharmacophore model designed by overlap of nonxanthine type of previously known A 2A antagonists, we synthesized a new class of compounds having a 2-amino nicotinonitrile core moiety. From our data, we conclude that the presence of at least one furan group rather than phenyl is beneficial for high affinity on the A 2A adenosine receptor. Compounds 39 (LUF6050) and 44 (LUF6080) of the series had K i values of 1.4 and 1.0 nM, respectively, with reasonable selectivity toward the other adenosine receptor subtypes, A 1, A 2B, and A 3. The high affinity of 44 was corroborated in a cAMP second messenger assay, yielding subnanomolar potency for this compound.

Dopamine/adenosine interactions related to locomotion and tremor in animal models: possible relevance to parkinsonism

Adenosine A(2A) antagonists can exert antiparkinsonian effects in animal models. Recent experiments studied the ability of MSX-3 (an adenosine A(2A) antagonist) to reverse the locomotor suppression and tremor produced by dopamine antagonists in rats. MSX-3 reversed haloperidol-induced suppression of locomotion, and reduced the tremulous jaw movements induced by haloperidol, pimozide, and reserpine. Infusions of MSX-3 into the nucleus accumbens core increased locomotion in haloperidol-treated rats, but there were no effects of infusions into the accumbens shell or ventrolateral neostriatum. In contrast, MSX-3 injected into the ventrolateral neostriatum reduced pimozide-induced tremulous jaw movements. Dopamine/adenosine interactions in different striatal subregions are involved in distinct aspects of motor function.

Adenosine A2A receptor antagonists exert motor and neuroprotective effects by distinct cellular mechanisms

OBJECTIVE

To investigate whether the motor and neuroprotective effects of adenosine A(2A) receptor (A(2A)R) antagonists are mediated by distinct cell types in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease.

METHODS

We used the forebrain A(2A)R knock-out mice coupled with flow cytometric analyses and intracerebroventricular injection to determine the contribution of A(2A)Rs in forebrain neurons and glial cells to A(2A)R antagonist-mediated motor and neuroprotective effects.

RESULTS

The selective deletion of A(2A)Rs in forebrain neurons abolished the motor stimulant effects of the A(2A)R antagonist KW-6002 but did not affect acute MPTP neurotoxicity. Intracerebroventricular administration of KW-6002 into forebrain A(2A)R knock-out mice reinstated protection against acute MPTP-induced dopaminergic neurotoxicity and attenuated MPTP-induced striatal microglial and astroglial activation.

INTERPRETATION

A(2A)R activity in forebrain neurons is critical to the control of motor activity, whereas brain cells other than forebrain neurons (likely glial cells) are important components for protection against acute MPTP toxicity.

Synthesis and properties of a new water-soluble prodrug of the adenosine A 2A receptor antagonist MSX-2

The compound L-valine-3-{8-[(E)-2-[3-methoxyphenyl)ethenyl]-7-methyl-1-propargylxanthine-3-yl}propyl ester hydrochloride (MSX-4) was synthesized as an amino acid ester prodrug of the adenosine A2A receptor antagonist MSX-2. It was found to be stable in artificial gastric acid, but readily cleaved by pig liver esterase.

Adenosine receptor subtype-selective antagonists in inflammation and hyperalgesia

In this study, we examined the effects of systemic and local administration of the subtype-selective adenosine receptor antagonists PSB-36, PSB-1115, MSX-3, and PSB-10 on inflammation and inflammatory hyperalgesia. Pharmacological blockade of adenosine receptor subtypes after systemic application of antagonists generally led to a decreased edema formation after formalin injection and, with the exception of A(3) receptor antagonism, also after the carrageenan injection. The selective A(2B) receptor antagonist PSB-1115 showed a biphasic, dose-dependent effect in the carrageenan test, increasing edema formation at lower doses and reducing it at a high dose. A(1) and A(2B) antagonists diminished pain-related behaviors in the first phase of the formalin test, while the second, inflammatory phase was attenuated by A(2B) and A(3) antagonists. The A(2B) antagonist was particularly potent in reducing inflammatory pain dose-dependently reaching the maximum effect at a low dose of 3 mg/kg. Inflammatory hyperalgesia was totally eliminated by the A(2A) antagonist MSX-3 at a dose of 10 mg/kg. In contrast to the A(1) antagonist, the selective antagonists of A(2A), A(2B), and A(3) receptors were also active upon local administration. Our results demonstrate that the blockade of adenosine receptor subtypes can decrease the magnitude of inflammatory responses. Selective A(2A) antagonists may be useful for the treatment of inflammatory hyperalgesia, while A(2B) antagonists have potential as analgesic drugs for the treatment of inflammatory pain.

Adenosine A2A receptor blockade prevents memory dysfunction caused by beta-amyloid peptides but not by scopolamine or MK-801

Adenosine A2A receptor antagonists alleviate memory deficits caused by aging or by administration of beta-amyloid peptides in rodents, which is in accordance with the beneficial effects of caffeine consumption (an adenosine receptor antagonist) on memory performance in aged individuals and in preventing Alzheimer's disease. We now tested if A2A receptor blockade affords a general beneficial effect in different experimental paradigms disturbing memory performance in rodents. The beta-amyloid fragment present in patients with Alzheimer's disease (Abeta1-42, 2 nmol, icv) decreased spontaneous alternation in the Y-maze after 15 days (29%) to an extent similar to the decrease of memory performance caused by scopolamine (2 mg/kg, ip) or MK-801 (0.25 mg/kg, ip) after 30 min (28% and 39%, respectively). The selective A2A receptor antagonist SCH58261 (0.05 mg/kg, ip every 24 h, starting 30 min before the noxious stimuli) prevented Abeta1-42-induced amnesia, but failed to modify scopolamine- or MK-801-induced amnesia. Similar conclusions were reached when testing another A2A receptor antagonist (KW6002, 3 mg/kg, ip). These results indicate that A2A receptors do not affect general processes of memory impairment but instead play a crucial role restricted to neurodegenerative conditions involving an insidious synaptic deterioration leading to memory dysfunction.