Synthesis and structure-activity relationships of pyrazolo[4,3-d]pyrimidin-7-ones as adenosine receptor antagonists

A series of 21 1,3-dialkylpyrazolo[4,3-d]pyrimidin-7-ones substituted in the 5-position with various phenyl substituents has been synthesized and found to have affinity for the adenosine A1 receptor. The potency pattern due to substituents of the phenyl ring was found to parallel that found in a previously reported 1,3-dialkyl-8-phenylxanthine series. A quantitative structure-activity relationship was developed between these two series that correctly predicted the potencies of six additional 5-substituted pyrazolo[4,3-d]pyrimidines that were synthesized during the course of the analysis. With use of the correlation as a guide, one additional 5-phenylpyrazolo[4,3-d]pyrimidine containing a 4-[[(dimethylamino)ethyl]amino]sulfonyl substituent to improve aqueous solubility was prepared. On the basis of the high correlation between adenosine binding affinities of analogously substituted xanthines and pyrazolo-[4,3-d]pyrimidines and the close superposition of the heterocyclic rings and substituents that is apparent from molecular models of these two series, it is hypothesized they fit the receptor in an analogous fashion.     

Quantitative structure-activity relationships of inhibitors of immune complex-induced inflammation: 1-phenyl-3-aminopyrazoline derivatives

Quantitative structure-activity relationships (QSAR) of the 1-phenyl-3-aminopyrazoline analogues as inhibitors of immune complex-induced inflammation have been studied. The correlation suggests that the overall size of the phenyl substituents are of importance, and bulky groups have negative effects on potency. The negative steric effects are gradually increased from ortho to meta to para positions. The negative steric effects were sometimes altered by the electronic effects of the substituents. Electron-releasing groups on the phenyl ring increased potency, while electron-withdrawing groups decreased it. Ortho substituents, however, have unaccounted for additional deleterious effects described here with an indicator variable. The octanol-water partition coefficient (log P) and dissociation constants (pKa) of the 1-(m-trifluoromethylphenyl)-3-aminopyrazoline analogue have been experimentally determined.     

N6,9-disubstituted adenines: potent, selective antagonists at the A1 adenosine receptor

N6-Substituted 9-methyladenines are potent antagonists of the activation of A1 adenosine receptors. The present study assessed the effect of N6 and N-9 substituents on the binding of adenines to the A1 and A2 receptors, respectively, of rat brain cortex and striatum and also on the antagonism of the A2 receptor mediated stimulation of the adenylate cyclase of PC12 cells by N-ethyladenosine-5'-uronamide. The potency ranking of 9-substituted adenines varied directly with the hydrophobicity of the substituent: cyclopentyl greater than phenyl greater than tetrahydrofuryl greater than ethyl greater than methyl greater than 2-hydroxyethyl. The 9-substituted adenines showed little selectivity for either receptor and the R enantiomer of N6-(1-phenyl-2-propyl)-9-methyladenine was only 4-fold more potent than the S enantiomer at the A1 receptor. An N6-cyclopentyl substituent increased potency at the A1 receptor and decreased potency at the A2 receptor, resulting in selectivity for the A1 receptor of up to 39-fold. The N6-cyclopentyl group completely overshadowed the effect of the hydrophobicity of the 9-substituent. A 2-chloro substituent did not alter the potency of an N6-substituted 9-methyladenine.     

8-Aryl-and 8-cycloalkyl-1,3-dipropylxanthines: further potent and selective antagonists for A1-adenosine receptors

A series of 1,3-dipropylxanthines were prepared with a variety of substituents at the 8-position. These included 8-aryl and 8-cycloalkyl groups. Polar carboxylate and carboxamide moieties were introduced as aryl substituents to increase water solubility. 1,3-Dipropyl-8-[2-hydroxy-4-[(carboxymethyl)oxy]phenyl]xanthine provided a functionalized congener with high potency (Ki = 37 nM) and selectivity (54-fold) for A1-adenosine receptors. This congener was used for preparation of a series of other analogues, some with higher potency and some with higher selectivity. 8-Cyclopentyl- and 8-cyclohexyl-1,3-dipropylxanthines were both very potent (Ki = 1-1.5 nM) and selective for A1 receptors, while 8-cycloalkylmethyl analogues were 10-fold less potent, but still very selective for A1 receptors. 8-Piperidinyl and 8-pyrazinyl analogues had very low activities as adenosine receptor antagonists.     

7-Deaza-2-phenyladenines: structure-activity relationships of potent A1 selective adenosine receptor antagonists

A series of derivatives of 7-deazapurines with varying substituents in the 2-, 6-, and 9-position was synthesized in an attempt to improve the adenosine receptor affinity and A1 or A2 selectivity. The adenosine receptor affinities were assessed by measuring the inhibition of [3H]-(R)-N6-(phenylisopropyl) adenosine (R-PIA) binding to rat brain A1 and inhibition of [3H]-5'-(N-ethylcarboxamido)adenosine (NECA) binding to rat striatum A2 adenosine receptors. A selected set of compounds representing the main structural variations was further examined in adenosine receptor coupled adenylate cyclase assays. All tested compounds antagonized the inhibition of adenylate cyclase elicited by interaction of R-PIA with A1 receptors in rat fat cell membranes and the activation of adenylate cyclase elicited by interaction of NECA with A2 receptors of pheochromocytoma PC12 cell membranes. The results indicate that 7-deazahypoxanthines have a potential for A2 selectivity, while all 7-deazaadenines are A1 selective. Introduction of a phenyl residue in the 2-position of 7-deazaadenines increases A1 activity tremendously. 2-(p-Chlorophenyl)-7,8-dimethyl-9-phenyl-7-deazaadenine (29) is potent and specific for the A1 receptors of rat brain (Ki = 122 nM), having no affinity for the A2 receptors of rat striatum. The compound has low activity at the A2 receptors of rat PC12 cell membranes where it appears to act as a noncompetitive inhibitor. A 1-phenylethyl substituent at the 9-position was found to be superior to a phenyl residue in terms of A1 affinity. The most potent A1 antagonist in the present series is the highly A1 selective (790-fold) (R)-7,8-dimethyl-2-phenyl-9-(1-phenylethyl)-7-deazaadenine (31, Ki = 4.7 nM), which is 30-35 times more potent at A1 receptors than its S enantiomer. The solubility of six of the potent 7-deaza-2-phenyladenines was determined by means of an A1 binding assay. Chloro substitution of the 2-phenyl ring appeared to improve the solubility as well as the solubility over A1 affinity ratio of 9-phenyl- and 9-(1-phenylethyl)-substituted 7-deazadenines.     

Synthesis, biological activity, and molecular modeling investigation of new pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives as human A(3) adenosine receptor antagonists

A new series of pyrazolotriazolopyrimidines bearing different substitutions on the phenylcarbamoyl moieties at the N5 position, being highly potent and selective human A(3) adenosine receptor antagonists, is described. The compounds represent an extension and an improvement of our previous work on this class of compounds (J. Med. Chem. 1999, 42, 4473-4478; J. Med. Chem. 2000, 43, 4768-4780). All the synthesized compounds showed A(3) adenosine receptor affinity in the subnanomolar range and high levels of selectivity in radioligand binding assays at the human A(1), A(2A), A(2B), and A(3) adenosine receptors. In particular, the effect of the substitution and its position on the phenyl ring have been studied. From binding data, it is evident that the unsubstituted derivatives on the phenyl ring (e.g., compound 59, hA(3) = 0.16 nM, hA(1)/hA(3) = 3713, hA(2A)/hA(3) = 2381, hA(2B)/hA(3) = 1388) showed the best profile in terms of affinity and selectivity at the human A(3) adenosine receptors. The introduction of a sulfonic acid moiety at the para position on the phenyl ring was attempted in order to design water soluble derivatives. However, this substitution led to a dramatic decrease of affinity at all four adenosine receptor subtypes. A computer-generated model of the human A(3) receptor was built and analyzed to better interpret these results, demonstrating that steric control, in particular at the para position on the phenyl ring, plays a fundamental role in the receptor interaction. Some of the synthesized compounds proved to be full antagonists in a specific functional model, where the inhibition of cAMP-generation by IB-MECA was measured in membranes of CHO cells stably transfected with the human A(3) receptor with IC(50) values in the nanomolar range, with a statistically significative linear relationship with the binding data.     

Synthesis and in vitro pharmacology of arpromidine and related phenyl(pyridylalkyl)guanidines, a potential new class of positive inotropic drugs

Replacement of the cimetidine moiety in impromidine (1,N1-[3-(1H-imidazol-4-yl)propyl]-N2-[2-[[(5-methyl-1H-imidazol-4- yl)methyl]thio]ethyl]guanidine) by more lipophilic H2-nonspecific pheniramine-like structures resulted in potent H2 agonists with up to 160 times the activity of histamine in the isolated, spontaneously beating guinea pig right atrium. Additionally, the compounds proved to be moderate H1 antagonists. Highest H2-agonistic potency was found in compounds characterized by a three-membered carbon chain connecting the aromatic rings and the guanidine group. The activity in the atrium was increased 2-4-fold by halogen substituents in the meta or para position of the phenyl ring. Highest H1-antagonistic potency resides in the group of para-halogenated compounds, p-F representing the optimal substituent in both receptor models. The corresponding guanidine 52 (arpromidine, N1-[3-(4-fluorophenyl)-3-pyridin-2-ylpropyl]-N2-[3-(1H-imidazol-4- yl)propyl]guanidine) combines about 100 times the activity of histamine at the H2 receptor with H1-antagonistic potency in the range of pheniramine. Further increase in the activity on the atrium was achieved by disubstitution with halogen on the phenyl ring, such as 3,4-F2, 3,5-F2, and 3,4-Cl2 (63-65). The 2-pyridyl group in arpromidine was replaced by 3-pyridyl without significant change in H2 agonistic activity, whereas the 4-pyridyl and phenyl analogues were less active. The rank order of potency in the atrium was in good agreement with the positive inotropic effects found in isolated, perfused guinea pig hearts, where 63-65 were the most potent compounds as well.     

Improvement of cold tolerance by selective A1 adenosine receptor antagonists in rats

Previously we have shown that the improvement of cold tolerance by theophylline is due to antagonism at adenosine receptors rather than inhibition of phosphodiesterase. Since theophylline is a nonselective adenosine receptor antagonist for both A1 and A2 receptors, the present study investigated the adenosine receptor subtype involved in theophylline's action. Acute systemic injection of selective A1 receptor antagonists (1,3-dialkyl-8-aryl or 1,3-dialkyl-8-cyclopentyl xanthine derivatives) significantly increased both the total and maximal heat production as well as cold tolerance. In contrast, injection of a relatively selective A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (compound No. 19), failed to significantly alter the thermogenic response of the rat under cold exposure. Further, the relative effectiveness of these compounds in increasing total thermogenesis was positively correlated with their potency in blocking the A1 adenosine receptor (r = .52, p less than 0.01), but not in A2 adenosine receptor (r = .20, p less than 0.2). It is likely that the thermally beneficial effects of adenosine A1 antagonists are due to their attenuation of the inhibitory effects of endogenously released adenosine on lipolysis and glucose utilization, resulting in increased substrate mobilization and utilization for enhanced thermogenesis.     

QSAR analyses of the substituted indanone and benzylpiperidine rings of a series of indanone-benzylpiperidine inhibitors of acetylcholinesterase.

QSAR analyses have been performed on the substituted indanone and benzylpiperidine ring substructures of a set of acetylcholinesterase, AChE, inhibitors of which 1-benzyl-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine hydrochloride is a potent in vitro and ex vivo inhibitor. The method of molecular decomposition-recomposition was used to define the sets of molecular substructures and corresponding in vitro inhibition databases. A QSAR involving the magnitude of the dipole moment, the highest occupied molecular orbital (HOMO) energy, and a specific pi-orbital wave function coefficient of the substituted indanone ring substructure was constructed and found to be significant. The absence of any molecular-shape or bulk term in the QSAR, coupled with some of the relatively large substituents used to construct the QSAR, suggests considerable space is available around the indanone ring during the inhibition process. A set of QSARs were constructed and evaluated for substituents on the aromatic ring of the benzylpiperidine substructure. The most significant QSAR involves a representation of molecular shape, the largest principal moment of inertia, and the HOMO of the substituted aromatic ring. It appears that upon binding the receptor "wall" is closely fit around the benzyl ring, especially near the para position. Overall, the QSAR analysis suggests inhibition potency can be better enhanced by substitution on the indanone ring, as compared to the aromatic sites of the benzylpiperidine ring. Moreover, inhibition potency can be rapidly diminished, presumably through steric interactions with the receptor surface of AChE, by substitution of moderate to large groups on the benzyl ring, particularly at the para position.     

Potent and orally bioavailable 8-bicyclo[2.2.2]octylxanthines as adenosine A1 receptor antagonists

In the search for a selective adenosine A1 receptor antagonist with greater aqueous solubility than the compounds currently in clinical trials as diuretics, a series of 1,4-substituted 8-cyclohexyl and 8-bicyclo[2.2.2]octylxanthines were investigated. The binding affinities of a variety of cyclohexyl and bicyclo[2.2.2]octylxanthines for the rat and human adenosine A1, A2A, A2B, and A3 receptors are presented. Bicyclo[2.2.2]octylxanthine 16 exhibited good pharmaceutical properties and in vivo activity in a rat diuresis model (ED50=0.3 mg/kg po). Optimization of the bridgehead substituent led to propionic acid 29 (BG9928), which retained high potency (hA1, Ki=7 nM) and selectivity for the adenosine A1 receptor (915-fold versus adenosine A2A receptor; 12-fold versus adenosine A2B receptor) with improved oral efficacy in the rat diuresis model (ED50=0.01 mg/kg) as well as high oral bioavailability in rat, dog, and cynomolgus monkey.     

Subtype-selective N-methyl-D-aspartate receptor antagonists: synthesis and biological evaluation of 1-(arylalkynyl)-4-benzylpiperidines.

A search of our compound library for compounds with structural similarity to ifenprodil (5) and haloperidol (7) followed by in vitro screening revealed that 4-benzyl-1-(4-phenyl-3-butynyl)piperidine (8) was a moderately potent and selective antagonist of the NR1A/2B subtype of NMDA receptors. Substitution on the benzyl group of 8 did not significantly affect NR1A/2B potency, while addition of hydrogen bond donors in the para position of the phenyl group enhanced NR1A/2B potency. Addition of a hydroxyl moiety to the 4-position of the piperidine group slightly reduced NR1A/2B potency while reducing alpha-1 adrenergic and dopamine D2 receptor binding affinities substantially, resulting in improved overall selectivity for NR1A/2B receptors. Finally, the butynyl linker was replaced with propynyl or pentynyl. When the phenyl was para substituted with amine or acetamide groups, the NR1A/2B potency order was butynyl > pentynyl > propynyl. For the para methanesulfonamide or hydroxyl groups, the order was butynyl approximately propynyl > pentynyl. The hydroxyl propyne (48) and butyne (23) were among the most potent NR1A/2B antagonists from this study. They both potentiated the effects of L-DOPA in the 6-hydroxydopamine-lesioned rat, a model of Parkinson's disease, dosed at 10 mg/kg ip, but 48 was not active at 30 mg/kg po.     

Synthesis and biological studies of a new series of 5-heteroarylcarbamoylaminopyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidines as human A3 adenosine receptor antagonists. Influence of the heteroaryl substituent on binding affinity and molecular modeling investigations

Some pyrazolotriazolopyrimidines bearing different heteroarylcarbamoylamino moieties at the N5-position are described. We previously reported the synthesis of a water soluble compound with high potency and selectivity versus the human A3 adenosine receptor as antagonist, and herein we present an enlarged series of compounds related to the previously mentioned one. These compounds showed A3 adenosine receptor affinity in the nanomolar range and different levels of selectivity evaluated in radioligand binding assays at human A1, A2A, A2B, and A3 adenosine receptors. In particular, the effect of the heteroaryl substituents at the N5 position has been analyzed. This study allows us to recognize that the presence of a pyridinium moiety in this position not only increases water solubility but also improves or retains potency and selectivity at the human A3 adenosine receptors. In contrast, replacement of pyridine with different heterocycles produces loss of affinity and selectivity at the human A3 adenosine receptors. A molecular modeling study has been carried out with the aim to explain these various binding profiles.     

Identification of novel, water-soluble, 2-amino-N-pyrimidin-4-yl acetamides as A2A receptor antagonists with in vivo efficacy

Potent adenosine hA2A receptor antagonists are often accompanied by poor aqueous solubility, which presents issues for drug development. Herein we describe the early exploration of the structure-activity relationships of a lead pyrimidin-4-yl acetamide series to provide potent and selective 2-amino-N-pyrimidin-4-yl acetamides as hA2A receptor antagonists with excellent aqueous solubility. In addition, this series of compounds has demonstrated good bioavailability and in vivo efficacy in a rodent model of Parkinson's disease, despite having reduced potency for the rat A2A receptor versus the human A2A receptor.     

7-Substituted 5-amino-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as A2A adenosine receptor antagonists: a study on the importance of modifications at the side chain on the activity and solubility.

It was demonstrated in the early 1990s that adenosine exerts many physiological functions through the interaction with four different receptors, named A1, A2A, A2B, and A3. In the past few years, our group has been involved in the development of A2A antagonists, which led to the synthesis of SCH 58261 (1), the first potent and selective adenosine A2A antagonist, which has been widely used as a reference compound. In this paper, we present an extended series of pyrazolotriazolopyrimidines synthesized with the aim to investigate the influence of the substitutions on the pyrazole ring. The choice of the substituents was based on their capability to improve water solubility while retaining high affinity and selectivity at the human A2A adenosine receptor subtype. In this series, some structural characteristics that are important for activity, i.e., tricyclic structure, free amino group at 5-position, furan ring, and substituent at 7-position on the pyrazole moiety, have been maintained. We focused our attention on the nature of the phenyl ring substituent to improve water solubility. Following this strategy, we developed new compounds with good affinity and selectivity for A2A adenosine receptors, such as 8d (K(i) 0.12 nM; hA1/hA2A ratio = 1025; R(m) = 2.8), 8h (K(i) 0.22; hA1/hA2A ratio = 9818; R(m) = 3.4), 8i (K(i) 0.18 nM; hA1/hA2A ratio = 994; R(m) = 2.8), 8k (K(i) 0.13 nM; hA1/hA2A ratio = 4430; R(m) = 3.6), and 14b (K(i) 0.19 nM; hA1/hA2A ratio = 2273; R(m) = 2.7). All the new synthesized compounds have no significant interaction with either A2B or A3 receptor subtypes. This new series of compounds deeply enlightens some structural requirements to display high affinity and selectivity for the A2A adenosine receptor subtype, although our goal of identifying new compounds with increased water solubility was not completely achieved. On this basis, other strategies will be devised to improve this class of compounds with a profile that appears to be promising for treatment of neurodegenerative disorders, such as Parkinson's disease.     

1,3-Dialkyl-8-(p-sulfophenyl)xanthines: potent water-soluble antagonists for A1- and A2-adenosine receptors

A series of 8-(substituted phenyl) derivatives of theophylline and other 1,3-dialkylxanthines were evaluated for potency and selectivity as antagonists at A1- and A2-adenosine receptors in brain tissue. Theophylline has a similar potency (Ki = 14 microM) at both A1 and A2 receptors. 8-Phenyltheophylline is 25-35-fold more potent as an adenosine receptor antagonist than theophylline, while 8-phenylcaffeine is only 2-3-fold more potent than caffeine. A p-hydroxyaryl substituent enhances the potency of 8-phenyltheophylline as an adenosine antagonist. p-Carboxy- and p-sulfoaryl substituents reduce potency of 8-phenyltheophylline, yielding water-soluble adenosine antagonists, which are some 2-5-fold more potent than theophylline at adenosine receptors. None of the 8-(substituted phenyl)theophyllines are particularly selective as antagonists toward A1- and A2-adenosine receptors. 1,3-Dipropyl-8-phenylxanthine represents a potent and somewhat selective A1-receptor antagonist about 23-fold more potent at A1 receptors than at A2 receptors. A p-hydroxyaryl substituent further enhances potency of the 1,3-dipropyl-8-phenylxanthine at both A1 and A2 receptors. The 8-(2-amino-4-chlorophenyl)-1,3-dipropylxanthine is a very potent and selective antagonist for A1 receptors, being nearly 400-fold more potent at A1 than at A2 receptors. The water-soluble 8-(p-sulfophenyl)- and 8-(p-carboxyphenyl)-1,3-propylxanthines no longer exhibit marked selectivity. Both compounds are much more potent as adenosine antagonists than theophylline. The striking selectivity of 1-isoamyl-3-isobutylxanthine as an A1 antagonist is retained in the 8-phenyl derivative but is virtually lost in the 8-p-sulfophenyl derivative.     

Quantitative structure-activity relationships in MAO-inhibitory 2-phenylcyclopropylamines: Insights into the topography of MAO-A and MAO-B

Ten (E)-and (Z)-isomers of 2-phenylcyclopropylamine (PCA), 1-Me-PCA, 2-Me-PCA, N-Me-PCA, and N, N-diMe-PCA and fifteeno ⁻, m⁻, p⁻ isomers of (E)-PCA with substituents of Me, Cl, F, OMe, OH were synthesized in this laboratory and tested for the inhibition of rat brain mitochondrial MAO-A and MAO-B. The effects of substituents, their positions, and stereochemistry on the inhibition were assessed for the compounds with substituents at cyclopropyl and amino groups and QSAR analyses were performed using the potency data of ring-substituted compounds. The best correlated QSAR equations are as follows: pI₅₀=0.804 Π² Blo−1.069 Blm+0.334 Lp−1.709 HDp+7.897 (r=0.945, s=0.211, F=16.691, p=0.000) for the inhibition of MAO-A; pI₅₀=1.815 π-0.825 Π² R+0.900 Es²+0.869 Es³+0.796 Es⁴−0.992 HDp+0.562 HAo+3.893 (r=0.982, s=0.178, F=23.351, p=0.000) for the inhibition of MAO-B. Based on the potency difference between stereoisomers of cyclopropylamine-modified compounds and on QSAR results, it is proposed that the active sites of MAO-A are composed of one deep hydrophobic cavity near para position, two hydrophobic cavities interacting with Me group, a hydrophobic area accomodating phenyl and cyclopropyl backbone, steric boundaries, a hydrogen-acceptor site near para position, and an amino group binding site and that in addition to the same two hydrophobic cavities, hydrophobic area, steric boundaries, hydrogen-acceptor site, and amino group binding site, another steric boundary near para position and a hydrogen donating site near ortho position constitute active sites of MAO-B.     

Quantitative structure-activity relationships of N2-phenylguanines as inhibitors of herpes simplex virus thymidine kinases.

Quantitative structure-activity relationships of the Hansch-type were developed to account for inhibition of thymidine kinases from Herpes simplex viruses types 1 and 2 (HSV1,2) by N2-phenylguanines. Derivatives with meta and/or para substituents on the phenyl ring display a wide range of overlapping, but not identical, potencies as inhibitors of the enzymes. IC50 values for 36 (HSV1) and 35 inhibitors (HSV2) were used to develop equations using hydrophobic (pi), electronic (sigma, R), and group size (MR) parameters. Equations 1 and 2 with correlation coefficients of 0.797 and 0.805, respectively, were obtained for inhibitors of the types 1 and 2 enzymes. Potencies were correlated positively with pi values of meta substituents but negatively with pi values of para substituents in the phenyl ring. Positive correlations were also obtained with the resonance parameter R of para substituents and with sigma constants of meta substituents. The most potent inhibitor of both enzymes was N2-[m-(trifluoromethyl)phenyl]guanine, although HSV2 thymidine kinase was more sensitive to certain compounds than the HSV1 enzyme.     

Cyclohexylcarbamic acid 3'- or 4'-substituted biphenyl-3-yl esters as fatty acid amide hydrolase inhibitors: synthesis, quantitative structure-activity relationships, and molecular modeling studies

Fatty acid amide hydrolase (FAAH) is a promising target for modulating endocannabinoid and fatty acid ethanolamide signaling, which may have important therapeutic potential. We recently described a new class of O-arylcarbamate inhibitors of FAAH, including the cyclohexylcarbamic acid biphenyl-3-yl ester URB524 (half-maximal inhibitory concentration, IC(50) = 63 nM), which have significant anxiolytic-like properties in rats. In the present study, by introducing a selected group of substituents at the meta and para positions of the distal phenyl ring of URB524, we have characterized structure-activity profiles for this series of compounds and shown that introduction of small polar groups in the meta position greatly improves inhibitory potency. Most potent in the series was the m-carbamoyl derivative URB597 (4i, IC(50) = 4.6 nM). Furthermore, quantitative structure-activity relationship (QSAR) analysis of an extended set of meta-substituted derivatives revealed a negative correlation between potency and lipophilicity and suggested that small-sized substituents may undertake polar interactions with the binding pocket of the enzyme. Docking studies and molecular dynamics simulations, using the crystal structure of FAAH, indicated that the O-biphenyl scaffold of the carbamate inhibitors can be accommodated within a lipophilic region of the substrate-binding site, where their folded shape mimics the initial 10-12 carbon atoms of the arachidonyl moiety of anandamide (a natural FAAH substrate) and methyl arachidonyl fluorophosphonate (a nonselective FAAH inhibitor). Moreover, substituents at the meta position of the distal phenyl ring can form hydrogen bonds with atoms located on the polar section of a narrow channel pointing toward the membrane-associated side of the enzyme. The structure-activity characterization reported here should help optimize the pharmacodynamic and pharmacokinetic properties of this class of compounds.     

Synthesis of thieno[2,3-b]pyridinones acting as cytoprotectants and as inhibitors of [3H]glycine binding to the N-methyl-D-aspartate (NMDA) receptor

The standard glycine site antagonist of the N-methyl-D-aspartate (NMDA) receptor, 3-phenyl-4-hydroxyquinolin-2(1H)-one (21), was used as a template for bioisostere benzene/thiophene exchange. Phenylacetylation of aminothiophene carboxylic acid methyl esters and subsequent cyclization delivered the three possible thienopyridinone isomers. 4-Hydroxy-5-phenylthieno[2,3-b]pyridin-6(7H)-one (3a), with the shortest distance between the sulfur and the nitrogen atom, was the most potent isomer (K(i) against the binding of [(3)H]glycine to rat membranes 16 microM), comparable in potency to the model quinolinone (21, 12 microM). Replacement of the phenyl substituent of 21 by a 2-thienyl residue resulted in a 2-5-fold loss in potency and was abandoned. In the thieno part of the thienopyridinone nucleus, the most successful substituents were halogen (Cl or Br) close to the sulfur atom and short alkyl chains at the other position, resulting in 7h, 8h, 8i, and 8m, with K(i) values between 5.8 and 10.5 nM. Introduction of a 3'-phenoxy moiety yielded several compounds with still higher potencies (18h, 18i, 18l, and 18m; K(i) between 1.1 and 2.0 nM). Quantitative structure-activity relationship (QSAR) calculations resulted in a consistent interpretation of the potencies of most compounds. Several of these 3'-phenoxy derivatives protected mouse fibroblast cell lines with transfected NMDA receptors from glutamate-induced toxicity. In addition, we report in vivo results for four of these compounds.     

1,2,4-Triazolo[5,1-i]purine derivatives as highly potent and selective human adenosine A(3) receptor ligands

A series of triazolopurines showed structural similarity to human adenosine A(3) receptor antagonist, 9-chloro-2-(2-furanyl)-5-[(phenylacetyl)amino][1,2,4]triazolo[1,5-c]quinazoline (MRS 1220, 1). In this study, we found novel 1,2,4-triazolo[5,1-i]purine derivatives (2) showing human adenosine A(3) receptor affinities. The compounds were obtained in two steps from 5-amino-4-cyanoimidazole (33). The affinity was determined in radioligand binding assays for the cloned human adenosine A(1), A(2A), A(2B), and A(3) receptors. After the structure-activity relationship was analyzed, we determined that there was a mild parabolic relationship between the length of alkyl groups at the 5-position and the affinities at the A(3) receptor and positive correlation between the length of the substituents on phenyl groups at the 8-position and the affinities at the A(2A) receptor. These investigations led to potent and selective human adenosine A(3) receptor ligands. The most potent A(3) receptor ligand (5-n-butyl-8-(4-methoxyphenyl)-3H-[1,2,4]triazolo[5,1-i]purine (27, K(i) = 0.18 nM) and the most selective A(3) receptor ligand against A(1), A(2A), and A(2B) receptors, (5-n-butyl-8-(4-n-propoxyphenyl)-3H-[1,2,4]triazolo[5,1-i]purine (29, >19 600), were discovered.