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

Phenylethyl-substituted pyrimido[2,1-f]purinediones and related compounds: structure-activity relationships as adenosine A(1) and A(2A) receptor ligands

The synthesis of N-(un)substituted-phenylalkylpyrimido[2,1-f]purinediones was performed starting with 7-(3-chloropropyl)-8-bromotheophylline and 7-(3-chloropropyl)-8-bromo-1,3-dipropylxanthine. Compounds with unsubstituted or substituted ethylene spacer to an aromatic ring were synthesized. Additionally variations in the spacer-elongation of the linker containing more than two atoms, introduction of a double bond or heteroatoms were performed. Physicochemical properties of the synthesized compounds were described. The obtained compounds envisaged as sterically fixed and configurationally stable analogs of 8-styrylxanthines, were evaluated for their affinity to adenosine A(1) and A(2A) receptors, the receptor subtypes that are predominant in the brain. Selected compounds were also investigated for the affinity to the A(2B) and A(3) receptor subtypes. It was stated that phenylethyl pyrimido[2,1-f]purinediones and their analogs with variations of the ethylene spacer (substituted or extended) exhibit micromolar or submicromolar affinity for A(2A) ARs (adenosine receptors); for example compound 2Ac with p-hydroxy substituent displayed a K(i) value of 0.23 microM at the rat A(2A) receptor. In comparison to the previously obtained phenyl and benzyl pyrimido[2,1-f]purinediones compounds with a shorter spacer, phenethyl derivatives were optimal for A(2A) AR. The kind of substituent at the aromatic ring was important for the affinity. Oxygen and nitrogen atoms in the spacer resulted frequently in a slight decrease of the A(2A) AR affinity, introduction of more heteroatoms into the spacer-in carbamates-caused distinctly negative effect on the activity. In this series of compounds more frequently the adenosine A(1) activity was observed, also in submicromolar range as for dipropyl derivative 2Ba with K(i) value of 0.62 microM at the rat A(2A) AR. 3D-QSAR models were developed for the compounds presented in this paper as well as in the previous publications showing activity at adenosine A(1) and A(2A) ARs. It was concluded that for the activity at adenosine A(1) and A(2A) receptors lipophilicity, steric effects along with the molecule's electrostatic surface properties had greatest value. Chosen compounds were evaluated in vivo as anticonvulsants in MES, scMet tests and examined for neurotoxicity. Contrary to previously obtained phenyl and benzyl pyrimido[2,1-f]purinediones, all tested compounds were inactive as anticonvulsants.

Novel ergopeptides as dual ligands for adenosine and dopamine receptors

Multivalent ligands are promising pharmacological tools that may be more efficacious for several diseases than highly selective single-target drugs. A combined therapy using dopaminergic agonists and adenosinergic antagonists is currently being evaluated for the treatment of Parkinson's disease. [(a) Kanda, T.; et al. Exp. Neurol. 2000, 162, 321-327. (b) Jenner, P. Expert Opin. Invest. Drugs 2005, 14, 729-738. (c) Kase, H.; et al. Neurology 2003, 61 (Suppl 6), S97-S100.] Here we prepared dual ligands acting on adenosine and dopamine receptors by applying a combinatorial approach based on the ergolene privileged structure. The potency and efficacy of these novel compounds were determined by radioligand binding studies and intracellular cAMP production assays in cells expressing adenosine and dopamine receptors. Selected compounds displayed dual dopamine agonist and adenosine antagonist activity. Molecules with this pharmacological profile are potentially useful for studying dopamine-adenosine cross-talk in the central nervous system and for testing the therapeutic potential of multivalent drugs for Parkinson's disease.

N9-benzyl-substituted 1,3-dimethyl- and 1,3-dipropyl-pyrimido[2,1-f]purinediones: synthesis and structure-activity relationships at adenosine A1 and A2A receptors

Synthesis and physicochemical properties of N-benzyl pyrimido[2,1-f]purinediones are described. These derivatives were synthesized by the cyclization of 7-chloropropylo-8-bromo-1,3-dimethyl- or 1,3-dipropyl xanthine derivatives with corresponding (un)substituted benzylamines. Dipropyl derivatives were obtained under microwave irradiation conditions either. The obtained compounds (1-20) were evaluated for their affinity to adenosine A1 and A2A receptors, selected compounds were additionally investigated for affinity to the A3 receptor subtype. The results of the radioligand binding assays to A1 and A2A adenosine receptors showed that most of the 1,3-dimethyl-9-benzylpyrimidopurinediones exhibited selective affinity to A2A receptors at micromolar or submicromolar concentrations (for example, derivative 9 with o-methoxy substituent displayed a Ki value of 0.699 microM at rat A2A receptor with more than 36-fold selectivity). Contrary to previously described arylpyrimido[2,1-f]purinediones dipropyl derivatives (compounds 15-20) showed affinity to both kinds of receptors increased, however A1 affinity increased to a larger extent, with the result that A2A selectivity was abolished. The best adenosine A1 receptor ligand was m-chlorobenzyl derivative 18 (Ki=0.089 microM and 5-fold A1 selectivity). Structure-activity relationships were discussed with the analysis of lipophilic and spatial properties of the investigated compounds. Pharmacophore model of adenosine A1 receptor antagonist was adopted for this purpose.

Adenosine A(2A) receptor antagonism reverses the effects of dopamine receptor antagonism on instrumental output and effort-related choice in the rat: implications for studies of psychomotor slowing

RATIONALE

Organisms frequently make effort-related decisions based upon assessments of motivational value and response costs. Energy-related dysfunctions such as psychomotor slowing and apathy are critically involved in some clinical syndromes. Dopamine (DA), particularly in the nucleus accumbens, regulates effort-related processes. Dopamine antagonism and accumbens dopamine depletions cause rats performing on choice tasks to reallocate their behavior away from food-reinforced tasks that have high response requirements.

OBJECTIVE

There is evidence of a functional interaction between DA and adenosine A(2A) receptors in the neostriatum and nucleus accumbens. The present experiments were conducted to determine if adenosine A(2A) receptor antagonism could reverse the effects of dopamine receptor antagonism on instrumental behavior and effort-related choice.

MATERIALS AND METHODS

The adenosine A(2A) receptor antagonist MSX-3 was investigated for its ability to reverse the effects of the dopamine receptor antagonist haloperidol (0.1 mg/kg) on fixed ratio 5 instrumental lever-pressing and on response allocation using a concurrent lever-pressing/chow-feeding choice task.

RESULTS

Haloperidol significantly suppressed fixed ratio 5 responding, and with rats responding on the concurrent choice task, it altered choice behavior, significantly reducing lever-pressing for food and increasing chow intake. Injections of MSX-3 (0.5-2.0 mg/kg) produced a dose-related attenuation of the effects of 0.1 mg/kg haloperidol on both tasks. The high dose of MSX-3, when administered in the absence of haloperidol, did not significantly affect responding on either task.

CONCLUSIONS

Adenosine and dopamine systems interact to regulate instrumental behavior and effort-related processes, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing or anergia.

Injections of the selective adenosine A2A antagonist MSX-3 into the nucleus accumbens core attenuate the locomotor suppression induced by haloperidol in rats

There is considerable evidence of interactions between adenosine A2A receptors and dopamine D2 receptors in striatal areas, and antagonists of the A2A receptor have been shown to reverse the motor effects of DA antagonists in animal models. The D2 antagonist haloperidol produces parkinsonism in humans, and also induces motor effects in rats, such as suppression of locomotion. The present experiments were conducted to study the ability of the adenosine A2A antagonist MSX-3 to reverse the locomotor effects of acute or subchronic administration of haloperidol in rats. Systemic (i.p.) injections of MSX-3 (2.5-10.0 mg/kg) were capable of attenuating the suppression of locomotion induced by either acute or repeated (i.e., 14 day) administration of 0.5 mg/kg haloperidol. Bilateral infusions of MSX-3 directly into the nucleus accumbens core (2.5 microg or 5.0 microg in 0.5 microl per side) produced a dose-related increase in locomotor activity in rats treated with 0.5 mg/kg haloperidol either acutely or repeatedly. There were no overall significant effects of MSX-3 infused directly into the dorsomedial nucleus accumbens shell or the ventrolateral neostriatum. These results indicate that antagonism of adenosine A2A receptors can attenuate the locomotor suppression produced by DA antagonism, and that this effect may be at least partially mediated by A2A receptors in the nucleus accumbens core. These studies suggest that adenosine and dopamine systems interact to modulate the locomotor and behavioral activation functions of nucleus accumbens core.

Characterization of the potency, selectivity, and pharmacokinetic profile for six adenosine A2A receptor antagonists

Antagonists of adenosine A2A receptors (A2A -antagonists) with different chemical structures have been developed by several pharmaceutical companies for the potential treatment of Parkinson's disease. Pharmacological characterization of these antagonists was incomplete, and different assay conditions were used in different labs. Therefore, we characterized the potencies, selectivities, and pharmacokinetic profiles of six prototypical A2A -antagonists. Displacements of [3H]MSX-2 and of [3H]CGS21680 binding to the human cloned and rat A2A receptors were performed. The rank order of potency of antagonists to displace [(3)H]MSX-2 binding to the human A2A was SCH58261 > or = Biogen-34 > or = Ver-6623 > or = MSX-2 > KW-6002 > > DMPX. For the rat striatal A2A, the order of potency was Biogen-34 > or = SCH58261 > or = Ver-6623 > or = MSX-2 > or = KW-6002 > > DMPX. SCH58261 was the most potent antagonist of the human A2A with a K(i) value of 4 nM, whereas Biogen-34 was the most potent antagonist of the rat A2A with a K(i) value of 1.2 nM. Similar results were obtained from cAMP assays. Selectivities of A2A-antagonists were determined using radioligands [3H]DPCPX, [3H]ZM241385, and [125I]-AB-MECA for A1, A2B, and A3 receptors, respectively. KW-6002 and Biogen-34 exhibited the highest selectivity for A2A vs A1 (human and rat), respectively. The pharmacokinetic profiles of antagonists were evaluated in vivo in rats. DMPX and KW-6002 had the greatest oral bioavailability. In contrast, SCH58261, MSX-2, and Ver-6623 had low or poor oral bioavailability. In summary, SCH58261, Biogen-34, MSX-2, and Ver-6623 had high affinities for both human and rat A2A receptors, with reasonable selectivity for A2A over A1 and A2B receptors. They are suitable as A2A -antagonists for in vitro pharmacological studies. Among the six A2A-antagonists, KW-6002 is the best for use in in vivo animal studies, particularly for a CNS target, based on its bioavailability, half life, and brain penetration.

Microwave-assisted ring closure reactions: synthesis of 8-substituted xanthine derivatives and related pyrimido- and diazepinopurinediones

BACKGROUND

Poly-substituted xanthine derivatives are an important class of compounds in medicinal chemistry. Substitution at the 8-position of the purine ring is generally accessible by ring closure of a carboxamido-substituted uracil precursor. Although several procedures to accomplish this synthetic step have been suggested, it still remains difficult in many cases.

RESULTS

Ring closure reaction with hexamethyldisilazane was studied under microwave conditions. Reaction times were dramatically reduced by the application of microwaves in the syntheses of the 8-styrylxanthine derivative istradefylline, and in the preparation of 2-substituted pyrimido [1,2,3-cd]purines. Furthermore, the new procedure allowed the preparation of a previously unaccessible diazepino [1,2,3-cd]purine. Yields were generally improved by the new method. The addition of THF as a co-solvent proved to be crucial.

CONCLUSION

A new, fast, and efficient ring closure method for the imidazole ring of xanthine derivatives and related tricyclic compounds has been developed. Apart from improving the syntheses of known compounds, some of which are important pharmacological tools or in development as novel drugs, it allows the preparation of 2-substituted diazepino [1,2,3-cd]purines--a new class of tricyclic purine derivatives.

Improving Potency, Selectivity, and Water Solubility of Adenosine A1 Receptor Antagonists: Xanthines Modified at Position 3 and Related Pyrimido[1,2,3-cd]purinediones

The structure-activity relationships of xanthine derivatives related to the adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 1,3-dipropyl-8-(3-noradamantyl)xanthine (KW3902) were investigated by focusing on variations of the 3-substituent. Aromatic residues were well tolerated by the A(1) receptor in that position. A moderate effect of stereochemistry was found for the 3-(1-phenylethyl)-substituted analogue of DPCPX (S>R) at A(1) and A(3) receptors, whereas the opposite stereoselectivity was observed at the A(2) receptor subtypes. A 3-hydroxypropyl substituent was found to be optimal for high A(1) affinity and selectivity. The most potent compound of the present series was 1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (10 c), which exhibits a K(i) value of 0.124 nM at rat, and 0.7 nM at human adenosine A(1) receptors, combined with high selectivity (>>200-fold) versus the other receptor subtypes. The similarly potent 8-cyclopentyl-3-(3-hydroxypropyl)-1-propylxanthine was converted into a water-soluble phosphate prodrug, which may become a useful pharmacological tool for in vivo studies. 8-Alkyl-2-(3-noradamantyl)pyrimido[1,2,3-cd]purine-8,10-diones, which can be envisaged as xanthine analogues with a fixed 3-propyl substituent, were identified as a new class of potent, selective adenosine A(1) receptor antagonists. For example, compound 14 (8-butyl-substituted) exhibits a K(i) value of 13.8 nM at human A(1) receptors. A selection of the most potent compounds was investigated in [(35)S]GTPgammaS binding assays and showed inverse agonistic activity. Their efficacy was generally lower than that of the full inverse agonist DPCPX, and depended on subtle structural changes. Some of the new compounds belong to the most potent and selective A(1) antagonists described to date.

Purinergic receptors involved in the immunomodulatory effects of ATP in human blood

We recently showed that the physiological compound ATP simultaneously inhibited TNF-alpha and stimulated IL-10 release in LPS-PHA stimulated blood. The purpose of the present study was to determine the mechanism involved in the concerted modulatory effect of ATP on TNF-alpha and IL-10. Incubation of blood with ATP in the presence of selective P2 receptor antagonists showed that the stimulatory effect of ATP on IL-10 release was completely annihilated by both 2-MeSAMP (a P2Y12/13 receptor antagonist) and PSB-0413 (a P2Y12 receptor antagonist). On the other hand, the inhibitory effect of ATP on TNF-alpha release was completely reversed by 5'-AMPS (a P2Y11 receptor antagonist) as well as by H-89, an inhibitor of cAMP-activated PKA. The concerted inhibition by ATP of TNF-alpha release via P2Y11 activation and stimulation of IL-10 release via P2Y12 activation implicates a novel approach towards immunomodulation by altering the balance among pro- and anti-inflammatory cytokines.

Synthesis and biological activity of tricyclic aryloimidazo-, pyrimido-, and diazepinopurinediones

Syntheses and physicochemical properties of N-aryl-substituted imidazo-, pyrimido-, and 1,3-diazepino[2,1-f]purinediones are described. These derivatives were synthesized by the cyclization of 7-haloalkyl-8-bromo-1,3-dimethyl- or 1,3-dipropyl-xanthine derivatives with corresponding arylamines. The obtained compounds (1-40), which can be envisaged as sterically fixed and configurationally stable analogs of 8-styrylxanthines, were evaluated for their affinity to adenosine A(1) and A(2A) receptors, the receptor subtypes that are predominant in the brain. Selected compounds were additionally investigated for affinity to the A(2B) and A(3) receptor subtypes. Many of the compounds showed adenosine A(2A) receptor affinity at micromolar or submicromolar concentrations and were A(2A)-selective, for example, compound 23 with p-fluoro substituent displayed K(i) value of 0.147 microM at the rat A(2A) receptor and more than 170-fold-A(2A) selectivity, compound 17 with naphthyl substituent had K(i) value of 0.219 microM and a more than 114-fold-A(2A) selectivity. The compounds were somewhat weaker and less selective at the human receptor subtypes. Elongation of the dimethyl substituent to dipropyl in xanthine moiety improved affinity but reduced selectivity. 1,3-Dimethylimidazo-, pyrimido-, and diazepinopurinediones were evaluated in vivo as anticonvulsants in MES, ScMet, TTE tests and examined for neurotoxicity in mice (ip). Substances with pyrimido ring displayed protective activity in ScMet or in MES and ScMet tests, showing also neurotoxicity. The pyrimidine annelated ring is beneficial for both receptor affinity and anticonvulsant activity.

Suzuki-type Pd(0) coupling reactions in the synthesis of 2-arylpurines as Cdk inhibitors

A new series of 2-aryl-substituted purine derivatives has been synthesized by Suzuki Pd(0) coupling reactions. Moderate in vitro inhibitory activity against Cdk1 and Cdk5 was observed. These compounds are inactive against GSK3.

Progress in the pursuit of therapeutic adenosine receptor antagonists

Ever since the discovery of the hypotensive and bradycardiac effects of adenosine, adenosine receptors continue to represent promising drug targets. First, this is due to the fact that the receptors are expressed in a large variety of tissues. In particular, the actions of adenosine (or methylxanthine antagonists) in the central nervous system, in the circulation, on immune cells, and on other tissues can be beneficial in certain disorders. Second, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes (A1, A2A, A2B, and A3) have been cloned and pharmacologically characterized, all of which are G protein-coupled receptors. Adenosine receptors can be distinguished according to their preferred mechanism of signal transduction: A1 and A3 receptors interact with pertussis toxin-sensitive G proteins of the Gi and Go family; the canonical signaling mechanism of the A2A and of the A2B receptors is stimulation of adenylyl cyclase via Gs proteins. In addition to the coupling to adenylyl cyclase, all four subtypes may positively couple to phospholipase C via different G protein subunits. The development of new ligands, in particular, potent and selective antagonists, for all subtypes of adenosine receptors has so far been directed by traditional medicinal chemistry. The availability of genetic information promises to facilitate understanding of the drug-receptor interaction leading to the rational design of a potentially therapeutically important class of drugs. Moreover, molecular modeling may further rationalize observed interactions between the receptors and their ligands. In this review, we will summarize the most relevant progress in developing new therapeutic adenosine receptor antagonists.

The purines: potent and versatile small molecule inhibitors and modulators of key biological targets

The goal of this review is to highlight the wide range of biological activities displayed by purines, with particular emphasis on new purine-based agents which find potential application as chemical-biology tools and/or therapeutic agents. The expanding interest in the biological properties of polyfunctionalized purine derivatives issues, in large part, from the development of rapid high-throughput screening essays for new protein targets, and the corresponding development of efficient synthetic methodology adapted to the construction of highly diverse purine libraries. Purine-based compounds have found new applications as inducers of interferon and lineage-committed cell dedifferentiation, agonists and antagonists of adenosine receptors, ligands of corticotropin-releasing hormone receptors, and as inhibitors of HSP90, Src kinase, p38alpha MAP kinase, sulfotransferases, phosphodiesterases, and Cdks. The scope of application of purines in biology is most certainly far from being exhausted. Testing purine derivatives against the multitude of biological targets for which small molecule probes have not yet been found should thus be a natural reflex.

Synthesis and pharmacology of pyrido[2,3-d]pyrimidinediones bearing polar substituents as adenosine receptor antagonists

Amino-substituted pyrido[2,3-d]pyrimidinediones have previously been found to bind to adenosine A1 and A2A receptors in micromolar concentrations. The present study was aimed at studying the structure-activity relationships of this class of compounds in more detail. Most of the investigated compounds were provided with polar substituents, such as ethoxycarbonyl groups and basic amino functions, in order to improve their water-solubility. The compounds were synthesized starting from 6-amino-1,3-dimethyluracil via different reaction sequences involving (cyano)acetylation, Vilsmeier formylation, or reaction with diethyl ethoxymethylenemalonate (EMME). The most potent and selective compound of the present series was 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-5-(2-naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (11c) with a Ki value of 5 nM at rat and 25 nM at human A1 receptors. The compound was more than 60-fold selective versus A3 and more than 300-fold selective versus A2A receptors. It showed an over 300-fold improvement with respect to the lead compound. In GTPgammaS binding studies at membranes of Chinese hamster ovary cells recombinantly expressing the human adenosine A1 receptor, 11c behaved as an antagonist with inverse agonistic activity. A regioisomer of 11c, 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-7-(2- naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (7a) in which the 2-naphthylmethylamino substituent at position 5 of 11c was moved to the 7-position, was a relatively potent (Ki=226 nM) and selective (>20-fold) A3 ligand. In the series of compounds lacking an electron-withdrawing ethoxycarbonyl or cyano substituent in the 6-position, compounds with high affinity for adenosine A2A receptors were identified, such as 1,2,3,4-tetrahydro-1,3-dimethyl-5-(1-naphthyl)aminopyrido[2,3-d]pyrimidine-2,4-dione 16b (Ki human A2A=81.3 nM, Ki human A1=153 nM, and Ki human A3>10,000 nM).

Solid-phase synthesis of 7-substituted 3H-imidazo[2,1-i]purines

A method for solid-supported synthesis of N,N-disubstituted (3H-imidazo[2,1-i]purin-7-yl)methyl amines has been developed. The key features of this library synthesis are: (i) immobilization of commercially available N6-benzoyl-5'-O-(4,4'-dimethoxytrityl)-2'-deoxyadenosine 3'-(2-cyanoethyl N,N-diisopropylphosphoramidite) by phosphitylation to a hydroxyl-functionalized support, (ii) quantitative conversion of the deprotected adenine base to 3H-imidazo[2,1-i]purine-7-carbaldehyde with bromomalonaldehyde in DMF in the presence of formic acid and 2,6-lutidine, (iii) reductive amination of the formyl group followed by N-alkylation or N-acylation, and (iv) release from the support by acidolytic cleavage of the N-glycosidic bond. Steps (ii) and (iii) have been optimized in some detail by using (adenin-9-yl)acetic acid anchored to a Phe-Wang resin as a model compound.

Development of spin-labeled probes for adenosine receptors

Functionalized xanthine derivatives bearing a nitroxide moiety at the 3- or 8-position were synthesized as electron paramagnetic resonance (EPR) probes. The 8-cyclopentyl-1-propylxanthine derivative 4, spin-labeled at N3 by substitution with a nitroxide-bearing dihydropyrrole moiety, was a potent and selective A(1) adenosine receptor antagonist (K(i) for A(1) 5.5 nM, 1600-fold selectivity vs A(2A), >200-fold vs A(2B), and 310-fold vs A(3) adenosine receptors). 8-(1-Oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)-1,3-dipropylxanthine 10 (K(i) for A(1) 8.2 nM) was similarly potent and selective, while 8-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1,3-dipropylxanthine 11 (K(i) for A(1) 160 nM) exhibited significantly lower affinity for A(1) adenosine receptors. 8-[4-(((1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)amino)-2-oxoethoxy)phenyl]-1-propylxanthine14, a 3-unsubstituted xanthine derivative, was found to be a potent A(2B) adenosine receptor antagonist (K(i) for A(2B) 48 nM) but also exhibited high affinity for A(1) receptors (K(i) for A(1) 15.7 nM). An X-ray structure of compound 10 was obtained, confirming the proposed structure. The novel spin-labeled A(1)-selective or A(1)/A(2B)-nonselective adenosine receptor antagonists may become useful probes for biophysicochemical investigations of adenosine receptors in their membrane environment.