5'-Carbamoyl derivatives of 2'-C-methyl-purine nucleosides as selective A1 adenosine receptor agonists: affinity, efficacy, and selectivity for A1 receptor from different species

A series of 5'-carbamoyl and 5'-thionocarbamoyl derivatives of 2'-C-methyl analogues of the A(1) adenosine receptor (A(1)AR) full agonists N(6)-cyclopentyladenosine (CPA), 2-chloro-N(6)-cyclopentyladenosine (CCPA), N(6)-[3-(R)-tetrahydrofuranyl]adenosine (tecadenoson), and 2-chloro analogue (2-Cl-tecadenoson) was synthesized and evaluated for their affinity for adenosine receptor subtypes from bovine, porcine, and human species. In the N(6)-cyclopentylamino series, the 5'-substituted derivatives showed a reduced affinity at the bovine A(1)AR compared to the parent compounds; however, the selectivity for A(1) versus A(2A) receptor was retained or increased. The corresponding N(6)-3-(R)-tetrahydrofuranylamino analogues displayed a very low affinity toward the bovine A(1)AR. The 5'-methylthionocarbamoyl derivative of 2'-Me-CCPA showed the best affinity at porcine A(1)AR with a K(i) value of 13 nM. At human AR subtypes tecadenoson derivatives showed 2.3- to 5-fold lower affinity at A(1)AR and very low affinity at the other subtypes (A(2A), A(2B), and A(3)) compared to the corresponding N(6)-cyclopentyl analogues. The 5'-carbamoyl and 5'-thionocarbamoyl derivatives of 2'-Me-CCPA 3, 4, 7 and tecadenoson derivative 12 were found to be partial A(1) agonists at the porcine receptor. Docking studies explained the lower affinity of N(6)-3-(R)-tetrahydrofuranyl-substituted compounds at bovine A(1)AR compared to that of N(6)-cyclopentyl analogues, showing that the oxygen of the tetrahydrofuranyl ring establishes unfavorable electrostatic interactions with the CO oxygen of Asn254. The low binding affinity of the 2'-C-methyl-N(6)-3-(R)-tetrahydrofuranyl adenosine analogues at human A(1)AR may be ascribed to the presence of unfavorable interactions between the hydrophilic tetrahydrofuranyl ring and the surrounding hydrophobic residues Leu250 (TM6) and Ile274 (TM7).     

Synthesis,characterization, and reactions of 2′-, 3′-, and 5′-(2-bromoethyl)-AMP: Potential affinity labels for nucleotide binding sites in enzymes

Two new adenosine analogs, 2′-(2-bromoethyl) adenosine monophosphate and 3′-(2-bromoethyl) adenosine monophosphate, were synthesized, purified by semipreparative high-pressure liquid chromatography, and completely characterized. A new synthesis of 5′-(2-bromoethyl) adenosine monophosphate is presented which facilitates the preparation of radioactive reagent with label either in the ethyl group or the purine ring of the nucleotide derivative. The reactive moiety of these derivatives, a bromoalkyl group, has the ability to react with the nucleophilic side chains of several amino acids. The second-order, pH-independent rate constants for reaction with the side chains of the amino acids cysteine, lysine, histidine, and tyrosine were determined as 3×10^?4, 6×10^?6, 3×10^?7, and <1×10^?7 M^?1 sec^?1, respectively. These data could be use in estimating the rate enhancement observed in modification of a protein by these affinity-labeling reagents. 5′-(S-(2-hydroxyethyl)cysteine) adenosine monophosphate, the derivative expected from exhaustive digestion of protein in which a cysteinyl residue is modified by 5′-(2-bromoethyl) adenosine monophosphate, and S-2-hydroxyethyl)cysteine, the derivative anticipated upon acid hydrolysis of such a modified protein, were synthesized, characterized, and their elution positions from an amino acid analyzer determined. These bromoethyl AMP derivatives are potential affinity labels for enzymes that bind 2′-, 3′-, or 5′-nucleotides such as TPN, coenzyme A, or ADP, respectively.     

Studies on a modulatory role for adenosine in antigen and arachidonic acid induced contractions of guinea pig airways

The modulatory effects of adenosine and selected derivatives were examined on antigen and arachidonic acid (AA) induced contractions of indomethacin-treated tracheal spirals and lung parenchymal strips from actively sensitized guinea pigs. Adenosine (up to 2 X 10(-4) M) had no effect on antigen-induced contractions, but inhibited AA-induced contractions by 30-40% if added 30 min prior to challenge. The weak effect of adenosine suggests that endogenous adenosine may only have a limited modulatory role in allergic bronchospasm. 2-Chloroadenosine (10(-6)-10(-4) M) dose-dependently inhibited antigen- and AA-induced contractions of trachea, but was considerably less effective on parenchyma. The substituted adenosine derivatives, R-phenylisopropyladenosine (R-PIA) and 5'-(N-ethylcarboxamido)-adenosine (NECA), and the adenosine transport inhibitor, 6-[p-nitrobenzyl)thio]-9-beta-D-ribofuranosyl purine, were also active as modulators, but their activity was relatively weak and varied with the stimulus and the tissue. An order of potency for R-PIA, NECA, and 2-chloroadenosine could not be determined and 8-phenyltheophylline (10(-5) M) was not an effective inhibitor of the effects of adenosine or the adenosine derivatives. This suggests that adenosine and its derivatives may modulate cells through mechanisms other than activation of conventional A1 and A2 receptors. A lack of specificity for the adenosine derivatives must also be considered.     

Agarose linked adenosine and guanosine-5''-monophosphate; a new general method for the coupling of ribonucleotides to polymers through their cis-diols.

Condensation of the cis-diol of adenosine (1a) or guanosine (1b) with ethyl levulinate led to the acetal-esters 2a or 2b. Phosphorylation with phosphorous oxide trichloride converted them into their 5'-monophosphates. On alkaline hydrolysis the acetal-esters 2a,b as well as their 5'-monophosphates gave the carboxylic acid derivatives 3a,b and 4a,b, respectively. Condensation of these acid derivatives with aminohexyl-agarose (5) using water soluble N-ethyl-N'-(3'-dimethyl-aminopropyl)-carbodiimide hydrochloride as coupling reagent led to the new polymers 6a/6b and 7a/7b. This method of preparing resin linked adenosine and guanosine derivatives should be generally applicable to any ribonucleoside or ribonucleotide with a cis-diol. Because of the widespread occurence of these molecules, particularly as cofactors of enzymes, the new polymers might be useful tools for the purification of certain classes of enzymes by affinity chromatography.     

Novel adenosine A1 receptor antagonists. Synthesis and structure-activity relationships of a novel series of 3-(2-cyclohexenyl-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazolo[1,5 -a]pyridines

A novel series of 3-(2-cyclohexenyl-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazol o[1,5-a]pyridines was synthesized and evaluated for in vitro adenosine A1 and A2A receptor binding activities. Most of the cyclohexenyl derivatives (7a-e, 8a-s) were found to be potent adenosine A1 receptor antagonists. In a series of analogues of FR166124 (3a), alcohol 7c, nitrile 7e and amide derivatives (7d, 8c, 8r) were found to be more potent A1 antagonists with higher A2A/A1 selectivity than FR166124. Amongst them, 8r showed considerable water solubility (33.3 mg/mL), but lower than that of the sodium salt of FR166124 (> 200 mg/mL). Additionally, FR166124 had strong diuretic activity by both p.o. and iv administration in rats (minimum effective dose=0.1 and 0.032 mg/kg, respectively).     

Novel iron complexes bearing N6-substituted adenosine derivatives: synthesis, magnetic, 57Fe Mössbauer, DFT, and in vitro cytotoxicity studies

Iron complexes (1-7) involving N6-benzyladenosine derivatives of the predominant composition [Fe(L(n))Cl(3)].H(2)O {where L(1)=N6-(2-fluorobenzyl)adenosine (1), L(2)=N6-(4-fluorobenzyl)adenosine (2), L(3)=N6-(2-trifluoromethylbenzyl)adenosine (3), L(4)=N6-(3-trifluoromethylbenzyl)adenosine (4), L(5)=N6-(4-trifluoromethylbenzyl)adenosine (5), L(6)=N6-(4-trifluoromethoxybenzyl)adenosine (6), and L(7)=N6-(4-chlorobenzyl)adenosine (7)} have been synthesized. The compounds have been characterized by elemental analysis, variable-temperature and in-field 57Fe M?ssbauer, ES+ MS, FTIR, 1H and 13C NMR spectroscopies, magnetochemical and conductivity measurements, thermal (TGA/DSC/DTA) analyses, and DFT calculations. It has been found that the organic molecule is coordinated to iron via N7 atom of the appropriate adenosine derivative and the products are represented by mixtures of complexes with various iron oxidation (Fe(III)/Fe(II)) and spin states (S=5/2, 4/2, 3/2, 2/2) and geometries (tetrahedral or trigonal bipyramidal). It is caused by the fact that partial redox processes proceed during the reactions due to the presence of a ribose moiety, which is oxidized to the corresponding 5'-ribotic acid, and simultaneously, a portion of Fe(III) cations is reduced to Fe(II) ones. Moreover, a significant effect of crystal water molecules on stereochemistry, and hence, on magnetic and spectral properties of the prepared complexes has been found. The compounds have been tested for their in vitro cytotoxicity against the following human cancer cell lines: malignant melanoma (G-361), osteogenic sarcoma (HOS), chronic myelogenous leukemia (K-562), and breast adenocarcinoma (MCF-7). The most important results have been obtained for complex 2 with IC(50) values 8-16 microM against HOS, K-562, and MCF-7 cell lines, and for complex 6 with IC(50) value 4 microM against MCF-7 cell line.     

Binding thermodynamics and intrinsic activity of adenosine A1 receptor ligands

A thermodynamic analysis of the binding to rat cortex adenosine A1 receptors of 5'-deoxyribose-N6-cyclopentyladenosine (full agonist) and several 8-alkylamino homologues of N6-cyclopentyladenosine (partial agonists) was performed. The intrinsic activity of the compounds was also evaluated by measuring the inhibition of forskolin-stimulated 3'-5'-cyclic adenosine monophosphate (c-AMP) levels in isolated epididymal rat adipocytes. Standard free energy (deltaG), enthalpy (deltaH ) and entropy (deltaS ) of the binding equilibrium were determined by affinity measurements carried out at different temperatures (0, 10, 20, 25, 30 degrees C). Affinity constants of drug-receptor interactions were obtained by displacement experiments in the presence of 1nM [3H]N6-cyclohexyladenosine. Levels of c-AMP were evaluated by performing competitive binding assays. As the affinity of the ligands was found to increase with temperature enhancement, the binding of full and partial agonists is therefore totally entropy-driven. Standard entropy values of a wide series of adenosine derivatives, including the compounds under examination, are strictly correlated to those of intrinsic activity. Similarly, deltaS values appear correlated to the in vivo ability of the adenosine derivatives to inhibit rat heart rate. Thermodymanic data of adenosine A1 receptor ligands are proposed as an indicator of their pharmacodynamics.     

Comparison of inhibitory activity of isomeric triazolopyridine derivatives towards adenosine receptor subtypes or do similar structures reveal similar bioactivities?

The synthesis of an array of 8-amino-2-aryl-[1,2,4]triazolo[1,5-a]pyridine-6-carboxyl amide derivatives is described for the first time. A subset of 20 derivatives were compared to their isomeric 5-amino-2-aryl-[1,2,4]triazolo[1,5-a]pyridine-7-carboxyl amide counterparts with regard to their potential to inhibit the human adenosine 2a (hA2a) receptor and their selectivity against the human adenosine 1 (hA1) receptor. Based on the analysis of H-bond donor/acceptor capabilities of the isomeric triazolopyridine pairs it can be concluded that the H-bond donor strength of the free amino functionality is the main determinant for hA2a inhibitory activity and hA1 selectivity.     

Synthesis, SAR, and preliminary mechanistic evaluation of novel antiproliferative N⁶,5'-bis-ureido- and 5'-carbamoyl-N⁶-ureidoadenosine derivatives

We have developed efficient methods for the preparation of N(6),5'-bis-ureidoadenosine derivatives and their 5'-carbamoyl-N(6)-ureido congeners. Treatment of 5'-azido-5'-deoxy-N(6)-(N-alkyl or -arylurea)adenosine derivatives (6a-d) with H(2)/Pd-C or Ph(3)P/H(2)O, followed by N-methyl-p-nitrophenylcarbamate gave N(6),5'-bis-ureido products 7a-d in 49-78% yield. Analogous derivatives in the 5'-carbamoyl-N(6)-ureido series were prepared by treatment of 2',3'-bis-O-TBS-adenosine (11) with N-methyl-p-nitrophenylcarbamate followed by acylation with appropriate isocyanates which gave 13a-d in 45-69% yield. A more versatile route for obtaining potentially vast libraries of compounds from both series was achieved by treatment of 5'-N-methylureido- or 5'-N-methylcarbamoyladenosine derivatives with ethylchlorformate to give N(6)-ethoxycarbonyl derivatives (9 and 14) in 55-63% yields, respectively. Simple heating of 9 or 14 in the presence of primary alkyl- or arylamines gave the corresponding N(6),5'-bis-ureido- or 5'-carbamoyl-N(6)-ureidoadenosine derivatives in good yields (33-72% and 39-83%; 10a-e and 15a-e, respectively). Significant antiproliferative activities (IC(50)≈4-10 μg/mL) were observed for a majority of the N(6),5'-bis-ureido derivatives, whereas the 5'-carbamoyl-N(6)-ureido derivatives were generally less active (IC(50) >100 μg/mL). A 2',3'-O-desilylated derivative (5'-amino-5'-deoxy-5'-N-methylureido-N(6)-(N-phenylcarbamoyl)adenosine, 16) was shown to inhibit binding of 16 of 441 protein kinases to immobilized ATP-binding site ligands by 30-40% in a competitive binding assay at 10 μM. Compound 16 was also shown to bind to bone morphogenetic protein receptor 1b (BMPR1b) with a Kd=11.5 ± 0.7 μM.     

Synthesis of modified nucleosides for incorporation of formyletheno and carboxyetheno adducts of adenine nucleosides into oligonucleotides

Three protected derivatives of 1,N(6)-ethenoadenine nucleosides, viz. 3-[5-O-(4,4'-dimethoxytrityl) of 7-formyl-(1) and 7-(1,2-diacetyloxypropyl)-2'-deoxyadenosine (2), and 3-[5-O-(4,4'-dimethoxytrityl)-2-O-(tert-butyldimethylsilyl)-7-(ethoxycarbonyl)adenosine (3), expected to allow introduction of formyletheno and carboxyethenoadenine adducts into oligonucleotides by the conventional phosphoramidite chemistry, have been synthesized.     

Synthesis and structure-activity relationships of selective ligands for P3 purinoceptor-like protein (P3LP).

We examined the structure-activity relationship of various 5'-N-substituted-carboxamidoadenosine derivatives toward P3 purinoceptor-like protein (P3LP), which has affinity for both adenine nucleosides and nucleotides. We discovered a hydrophobic binding region near the 5'-N-substituted-carboxamide group. From the linear alkyl N-substituted derivatives, 1-(adenin-9-yl)-1-deoxy-N-n-pentyl-beta-D-ribofuranuronamide (6) was found to be the most potent ligand. In the series of the N-cycloalkyl derivatives, 1-(adenin-9-yl)-1-deoxy-N-cyclohexyl-beta-D-ribofuranuronamide (8) was the strongest ligand. We also examined the receptor selectivity for the selected nucleosides 6 and 8 with 1 (HAK2701) and N-ethylcarboxamidoadenosine (NECA) versus P1 purinoceptor subtypes, such as adenosine A1, A2A, A2B, and A3 receptors and found 8 is the most selective ligand for P3LP.     

Steric and charge factors in the resistance of uridylyl(3' - 5')N6-(N-threonylcarbonyl)adenosine to venom phosphodiesterase hydrolysis.

The contribution of steric and negative charge factors to the resistance of uridylyl(3' - 5')N6-(N-threonylcarbonyl)adenosine to venom phosphodiesterase was investigated. The hydrolysis rates of uridylyl(3'-5')N6-(N-threonylcarbonyl)-adenosine, its model derivatives, methyl ester and O-benzyl ester, together with unmodified uridyly (3'-5')adenosine, were studied. It was found that the contribution of both factors is of the same order. The steric inhibition of digestion is distinctly higher than that confirmed by N6-(delta2-isopentenyl)adenosine [1], which is ascribed to the rigid conformation of the threonylcarbonyladenosine side chain.     

Synthesis of di- and tri-substituted adenosine derivatives and their affinities at human adenosine receptor subtypes

The synthesis of 2-(hex-1-ynyl)adenosine derivatives substituted at the N6- and/or 5'-position was carried out on the basis that 2-(hex-1-ynyl)adenosine-5'-N-ethyluronamide (HENECA, 2) showed good affinity and different degree of selectivity for rat adenosine receptors. All new compounds were tested in radioligand binding and adenylyl cyclase assays with recently cloned human A1, A2A, A2B, and A3 adenosine receptors.     

Fluorescence studies of ATP-diphosphohydrolase from Solanum tuberosum var. Desirée

Chemical modification of potato apyrase suggests that tryptophan residues are close to the nucleotide binding site. Kd values (+/- Ca2+) for the complexes of apyrase with the non-hydrolysable phosphonate adenine nucleotide analogues, adenosine 5'-(beta,gamma-methylene) triphosphate and adenosine 5'-(alpha,beta-methylene) diphosphate, were obtained from quenching of the intrinsic enzyme fluorescence. Other fluorescent nucleotide analogues (2'(3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate, 2'(3')-O-(2,4,6-trinitrophenyl) adenosine 5'-diphosphate. 1,N6-ethenoadenosine triphosphate and 1,N6-ethenoadenosine diphosphate) were hydrolysed by apyrase in the presence of Ca2+, indicating binding to the active site. The dissociation constants for the binding of these analogues were calculated from both the decrease of the protein (tryptophan) fluorescence and enhancement of the nucleotide fluorescence. Using the sensitised acceptor (nucleotide analogue) fluorescence method, energy transfer was observed between enzyme tryptophans and ethene-derivatives. These results support the view that tryptophan residues are present in the nucleotide-binding region of the protein, appropriately oriented to allow the energy transfer process to occur.     

Synthesis and biological evaluation of 2-aminothiazoles and their amide derivatives on human adenosine receptors. Lack of effect of 2-aminothiazoles as allosteric enhancers

A number of 2-aminothiazoles (2a-e) and their amide derivatives (4-10) were prepared. The 2-aminothiazoles themselves were tested as allosteric enhancers of agonist binding to human adenosine A(1) receptors. In a variety of experimental set-ups the compounds did not show any such effect, in contrast to earlier findings by another research group. Subsequently the 2-aminothiazoles were used as intermediates in the synthesis of a number of amide derivatives of either aromatic (4-6) or aliphatic nature (7-10). Some of the compounds emerged as moderately active antagonists on human adenosine A(1) and/or A(2A) receptors with lower or negligible potency at adenosine A(3) receptors.     

A Functional Screening of Adenosine Analogues at the Adenosine A2B Receptor: A Search for Potent Agonists

Abstract

Various adenosine analogues were tested at the adenosine A_2B receptor. Agonist potencies were determined by measuring the cyclic AMP production in Chinese Hamster Ovary cells expressing human A_2B receptors. 5′-.N-Substituted carboxamidoadenosines were most potent. 5′-N-Ethylcarboxamidoadenosine (NECA) was most active with an ECso value of 3.1 μM. Other ribose modified derivatives displayed low to negligible activity. Potency was reduced by substitution on the exocyclic amino function (N⁶) of the purine ring system. The most active N⁶-substituted derivative N⁶-methyl-NECA was 5 fold less potent than NECA. C8-and most C2-substituted analogues were virtually inactive. 1-Deaza-analogues had a reduced potency, 3-and 7-deazaanalogues were not active.     

5''-Terminal and internal methylated nucleosides in herpes simplex virus type 1 mRNA.

RNA labeled with [methyl-3H]methionine and/or [32P]orthophosphate was isolated from the polyribosomes of herpes simplex virus (HSV) types 1-infected cells and separated into polyadenylylated [poly(A+)]and non-polyadenylylated [poly(A-)] fractions. Virus-specific RNA was obtained by hybridization in liquid to either excess HSV DNA or filters containing immobilized HSV DNA. Analysis in denaturing sucrose gradients indicated that HSV-specific poly(A+) RNA sedimented in a broad peak, with a modal S value of 20. The ratio of [3H]methyl to 32P decreased with increasing size of RNA, suggesting that each RNA chain contains a similar sumber of methyl groups. Further analysis indicated an average of one RNase-resistant structure of the type m7G(5')pppNmpNp or m7G(5')pppNmpNmpNp per 2,780 nucleotides. The following components were identified in the 5'-terminal oligonucleotides of polyribosome-associated HSV-specific poly(A+) and poly(A-) RNA: 7-methylguanosine, N6,2'-O-dimethyladenosine, and the 2'-O-methyl derivatives of guanosine, adenosine, uridine, and denosine, and the 2'-O-methyl derivatives of guanosine, adenosine, uridine, and cytidine. The most common 5'-terminal sequences were m7G(5')pppm6Am and m7G(5')pppGm. An additional modified nucleoside, N6-methyladenosine, was present in an internal position of HSV-specific RNA.     

N6-substituted C5'-modified adenosines as A1 adenosine receptor agonists

Adenosines bearing 5'-modification in conjunction with an N6-substituent have previously been shown to act as partial agonists at the A1 adenosine receptor. Our current work investigates the effect of modifying the 5'-position in conjunction with efficacious bicyclic and tricyclic N6-substituents. Several highly potent agonists for the A1 adenosine receptor were identified; however, all of these compounds behaved as full agonists. In keeping with previous reports, 5'-halogen and 5'-sulfide derivatives of N6-(endo-norborn-2-yl)adenosine were, in general, low nanomolar agonists of the A1 adenosine receptor. The known partial agonist, N6-cyclopentyl-5'-deoxy-5'-ethylthioadenosine (2), also behaved as a full agonist in our assay.     

Structure-activity relationships of adenosines with heterocyclic N6-substituents

Two series of N⁶-substituted adenosines with monocyclic and bicyclic N⁶ substituents containing a heteroatom were synthesized in good yields. These derivatives were assessed for their affinity ([³H]CPX), potency, and intrinsic activity (cAMP accumulation) at the A₁ adenosine receptor in DDT₁ MF-2 cells. In the monocyclic series, the N⁶-tetrahydrofuran-3-yl and thiolan-3-yl adenosines (1 and 26, respectively) were found to possess similar activities, whereas the corresponding selenium analogue 27 was found to be more potent. A series of nitrogen containing analogues showed varying properties, N⁶-((3R)-1-benzyloxycarbonylpyrrolidin-3-yl)adenosine (30) was the most potent at the A₁AR; IC₅₀ = 3.2 nM. In the bicyclic series, the effect of a 7-azabicyclo[2.2.1]heptan-2-yl substituent in the N⁶-position was explored. N⁶-(7-Azabicyclo[2.2.1]heptan-2-yl)adenosine (38) proved to be a reasonably potent A₁ agonist (K_i = 51 nM, IC₅₀ = 35 nM) while further substitution on the 7″-nitrogen with tert-butoxycarbonyl (31, IC₅₀ = 2.5 nM) and 2-bromobenzyloxycarbonyl (34, IC₅₀ = 9.0 nM) gave highly potent A₁AR agonists.