8-azapurine nucleus: a versatile scaffold for different targets.

-Azapurine nucleus is a bioisoster of the purine nucleus. Variously substituted 8-azapurines have been synthesised and studied for their interactions with many enzymes and receptors and for their antitumor and antiviral activity. In this paper the main results of the studies made in these last years on this topic are reported.

2-Alkyloxyalkylthiohypoxanthines as new potent inhibitors of xanthine oxidase.

The title compounds were prepared and tested as xanthine oxidase (XO) inhibitors. Results evidenced that potency was related to the position of the oxygen atom in the 2-linear chain and that it grew with distance from the sulfur atom until it became equipotent to 2-n-hexylthiohypoxanthine. Enzymatic oxidation on C(2) occurred in the 8-alkylthiohypoxanthines. On the contrary, oxidation on C(8) did not occur in the 2-alkythioderivatives, demonstrating that the chain forced these molecules to form a complex with molybdenum(VI) involving only the N(3) and N(9) nitrogen atoms.

New 5-substituted-1-(2-hydroxybenzoyl)-benzotriazoles, potassium channel activators. IV.

This paper reports the synthesis of a series of new 5-substituted-1-(2-hydroxybenzoyl)-benzotriazoles, which have been tested for their activity as possible activators of potassium channels. In rat aortic rings, the 'opened' derivatives 1a-f, intermediates of synthesis, showed vasorelaxing properties, with appreciable values of potency. However, the most remarkable effects were recorded for the 2-hydroxybenzoylbenzotriazoles 3a-f, which showed full vasorelaxing efficacy and high potency values. The introduction of a 2-hydroxybenzyl substituent in the 1 position of the benzotriazole ring (compound 7) strongly decreased the activity, showing the importance of the electron-acceptor carbonyl function. The best compound, 3b, was further investigated, in order to evaluate the possible mechanism of action involved in the vasodilator activity. In the vascular model, different potassium channel blockers inhibited the effects of the compound, and an increase of the levels of membrane depolarisation induced a significant reduction of the recorded responses. Compound 3b was also tested in a model of isolated rat heart, retroperfused through the aorta and submitted to a global ischemia/reperfusion cycle. In such an experimental condition, 3b showed an interesting cardioprotective activity. All the above observations are in agreement with the hypothesis of a mechanism linked to the activation of potassium channels.

Some structural changes on triazolyl-benzotriazoles and triazolyl-benzimidazolones as potential potassium channel activators. III.

This paper reports the synthesis and pharmacological evaluation of some compounds, obtained by structural modifications of 1,2,3-triazolyl-benzotriazoles and 1,2,3-triazolyl-benzimidazolones, which had shown activity as potential activators of the big-conductance calcium-activated potassium channels (BK(Ca)). Changes have concerned the introduction of a hinderer substituent in the 5-position of the benzimidazolone (4a, b) and benzotriazole (5a, b) rings, opening of the benzimidazolone ring (7) and substitution of the 1,2,3-triazole ring with a 2-hydroxyphenyl ring (10). Furthermore a series of 3-aryl-benzotriazin-4-one derivatives (13a-e) has been studied, which appears as a modification and/or combination of the benzimidazolone and benzotriazole rings. Only compound 10 shows interesting activity, while the other structural modifications either do not increase (compounds 4 and 5) or reduce (compounds 7 and 13) the pharmacological activity. However, these results provide useful information about structure-activity relationships.

New 2-(2'-phenyl-9'-benzyl-8'-azapurin-6'-ylamino)-carboxylic acid methylesters as ligands for A1 adenosine receptors.

Synthesis of a series of new 2-phenyl-9-benzyl-8-azaadenines bearing on N6 an alkyl or aralkyl chain having a carbonyloxymethyl group on the carbon bound to N6 were reported. The ester group could assure to the molecule a better water-solubility than the 8-azaadenines 2, 6 and 9 substituted with lipophilic groups synthesised in the past. Compounds synthesised demonstrated only little capability of binding A1 adenosine receptors.

Triazolyl-benzimidazolones and triazolyl-benzotriazoles: new potential potassium channel activators. II.

This paper reports the synthesis and pharmacological evaluation of a series of 5-substituted-triazolyl benzotriazoles (2a-f) and the corresponding series of 5-substituted-triazolyl-benzimidazolones (6a-f), as potential activators of the big-conductance calcium-activated potassium channels (BK(Ca)). The synthesis and structure demonstration of the stock compounds of the two series have been described in our previous works, as well as the common starting compounds 4-carboxamido-5-(4-substituted-2amino-anilino)-1,2,3-triazoles (1a-f). The triazolyl-benzotriazoles were obtained by diazotization, while the triazolyl-benzimidazolones were obtained by thermal intramolecular cyclization of ethoxycarbonylamino derivatives or directly with phosgene. Benzimidazolone compounds generally showed little effect whilst the compounds with a benzotriazole ring showed full efficacy, with vasorelaxing properties and potency parameters a little lower than that of the reference compound NS 1619. These effects were significantly reduced by an increased membrane depolarization. This depolarization-sensitive response is in agreement with the pharmacodynamic hypothesis of activation of potassium channels.

5-(4'-Substituted-2'-nitroanilino)-1,2,3-triazoles as new potential potassium channel activators. I.

By the hypothesised correlation with the large conductance Ca(++)-activated potassium channel (BK(Ca)) openers NS 004 and NS 1619, bearing a benzimidazolone ring, a series of new 5-(4'-substituted-2'-nitroanilino)-1,2,3-triazoles were synthesised and tested on in vitro isolated vascular preparation. The compounds were prepared starting from the appropriately substituted 2-nitro-phenylazides by 1,3-dipolar cycloaddition reaction to cyanoacetamide and following Dimroth isomerisation of the corresponding 1-arylsubstituted-5-amino-1,2,3-triazoles. The analogous 5-(4'-substituted-2'-amino-anilino)-1,2,3-triazoles were also prepared to assess the role of the nitro group in the pharmacophoric model. Almost all the nitro compounds showed a vasorelaxant activity on endothelium-denuded rat aortic rings with a potency comparable to that recorded for the reference compound NS 1619. Such a vasorelaxing activity was significantly reduced by the increase of the level of membrane depolarisation and by the potassium channel blocker 4-aminopyridine with a pharmacodynamic behaviour consistent with a potassium channel activation.

Substituted 1,2,3-triazolo[1,5-a]quinazolines: synthesis and binding to benzodiazepine and adenosine receptors.

This paper reports the synthesis and evaluation of the biological affinity towards benzodiazepine and A(1) and A(2A) adenosine receptors of some 3-ethoxycarbonyl or 3-phenyl-substituted 1,2, 3-triazolo[1,5-a]quinazolines. Starting from the appropriate chloro-substituted phenylazides, the series of 7 or 8 chloro-substituted triazoloquinazolines were prepared. Nitration reactions of the triazoloquinazoline ring and chlorination reactions of the hydroxyl group in the 5 position of the same ring are also reported. By nucleophilic displacement of halogen, the corresponding 5-amino derivatives and some analogous derivatives bearing cyclohexylamino and p-toluidino substituents were obtained. The binding assays showed a generalized decrease in the affinity towards the benzodiazepine receptors and confirmed a moderate affinity towards the A(1) adenosine receptors in comparison with the previously studied triazoloquinazoline derivatives.

New 1,2,3-triazole derivatives (ureas, amides, urethanes) with a potential biological interest.

This paper reports the preparation of three series of 1,2,3-triazole derivatives bearing an ureido substituent (compounds 5a-h), a carboxamido substituent (compounds 6a-f) or an urethane substituent (compounds 7a-l). Some compounds were submitted to in vitro functional tests on guinea-pig isolated intestinal preparations and/or to in vitro antitumor and antiviral screening. The tested compounds showed no activity on guinea-pig ileum, except compound 7g, provided of contracturant properties; similarly, no anticancer or antiviral significant activity was found.

1,2,3-Triazolo[1,5-a][1,4]- and 1,2,3-triazolo[1,5-a]-[1,5]benzodiazepine derivatives: synthesis and benzodiazepine receptor binding.

This paper reports the synthesis of new 1,2,3-triazolo[1,4]benzodiazepine and 1,2,3-triazolo[1,5]benzodiazepine derivatives and their evaluation toward benzodiazepine receptors. Receptor affinity gradually and remarkably increases by moving the nitrogen atom of the central ring from position 3 through 4 to position 5, to give the most effective compound 6a (Ki = 150 nM). N-methylation of the diazepine ring (7a) lowers receptorial binding. Introduction of a chlorine atom on the benzene ring doubles the Ki value (6b) which remains unaltered by the N-methylation (7b).

Novel 3-aralkyl-7-(amino-substituted)-1,2,3-triazolo[4,5-d]pyrimidines with high affinity toward A1 adenosine receptors.

Three series of several 1,2,3-triazolo[4,5-d]pyrimidine derivatives bearing various amino substituents at the 7 position and one of three lipophilic substituents at the 3 position (benzyl, phenethyl, or 2-chlorobenzyl) were prepared starting from the corresponding 7-chloro compounds, by nucleophilic substitution by the appropriate amine. Radioligand binding assays at bovine brain adenosine A1 and A2A receptors showed that some compounds possessed a high affinity and selectivity for the A1 receptor subtype. In particular the biological results suggested the compounds bearing cycloalkylamino (cyclopentyl- and cyclohexylamino) or aralkylamino (alpha-methylbenzyl- and 1-methyl-2-phenylethylamino or amphetamino) substituents at the 7 position were the most active derivatives. The best lipophilic substituent at the 3 position was the 2-chlorobenzyl (A1 affinity Ki < 50 nM) followed by the benzyl and then the phenethyl groups. This pattern of structure-activity relationship (SAR) was similar to that previously reported for analogous 1,2,3-triazolopyridazino derivatives (Biagi et al., 1994, 1995, 1996) except for the compounds bearing substituted aromatic amines which presented a generalized and strong decrease of the A1 receptor affinity. These facts allowed us to attribute to these molecules a binding mode within the A1 adenosine receptor analogous to that of the corresponding triazolopyridazines.

A 3D model of the human A1 adenosine receptor. An evaluation of the binding free-energy with ligands.

A model of A1 adenosine receptor was built on the basis of the prediction of transmembrane helices made by PHDtopology and forcing the rough initial model over the scaffold of the rhodopsin. Only helices were accurately modeled. Several complexes between the model of the A1 receptor and some ligands were built. The binding site was hypothesized on the basis of biochemical experiments (site directed mutagenesis). Ligands were selected so that their Kis range between millimolar to nanomolar. The validation of the model was carried out performing calculations of the binding free energy between ligands and the receptor model. The free energy calculations were accomplished by using the linear free energy approximation method (LIE). We could observe that the trend of the calculated delta delta Gs (differences in binding free energies between the antagonist 2, showing the lowest Ki, and the other antagonists analyzed) agreed with the one obtained from biological data.

Synthesis of 4,6-disubstituted- and 4,5,6-trisubstituted-2-phenyl-pyrimidines and their affinity towards A1 adenosine receptors.

Synthesis and assay of title compounds are reported. The results can support our hypothesis about the possibility that molecules characterized by great flexibility, as the title 2-phenyl-4,5,6-triaminopyrimidines, can better interact with the receptor sites compared with rigid molecules as 2,6,9-trisubstituted 8-azaadenines. Relatively low activity shown by pyrimidine derivatives demonstrated the importance of the bicyclic aromatic system in 8-azaadenines and adenines to give a favourable interaction between a hexogenous molecule and the A1 adenosine receptors.

New chiral inhibitors of induced platelet aggregation: the enantiomeric specificity of (R)- and (S)-methyl and ethyl esters of 1-(4-[(1-hydroxycarbonyl)-ethoxy]-benzyl)-1H-1,2,3-triazole as a tool for determining their biological target.

The synthesis of title enantiomers was accomplished and their biological behaviour as inhibitors of rabbit platelet aggregation process induced by ADP and arachidonic acid was determined. Structure-activity comparison with that of SM-12502 [(2R,5S)-(+) 3,5-dimethyl-2-(3-pyridyl)-thiazolidin-4-one hydrochloride] and Dazoxiben [4-[2-(1H-imidazol-1-yl)-ethoxy]-benzoic acid] allowed us to formulate the possible capability for the synthesized compounds to interact with the biological targets of the model molecules.

1,2,3-Triazolo [4,5-d] pyridazines--V. Preparation and adenosine receptor binding of new 4-amino derivatives.

This paper reports the continuation of studies on the 4-aminosubstituted 1,2,3-triazolo [4,5-d] pyridazine derivatives which had shown binding affinity towards A1-adenosine receptors. 1-Benzyl derivatives bearing new amines and new 1-(substituted benzyl) derivatives were prepared and tested. The biological results showed that some compounds possessed high affinity (approximately 30-70 nM) and good selectivity toward the A1-adenosine receptors. Further information about structure-biological activity relationships was aquired.

N(9)-Substituted 2-phenyl-N(6)-benzyl-8-azaadenines: A1 adenosine receptor affinity. A comparison with the corresponding N(6)-substituted 2-phenyl-N(9)-benzyl-8-azaadenines.

The synthesis and the assay of title compounds were accomplished. Biological results indicated the general lack of activity among the tested disubstituted compounds bearing the benzyl group on N(6) and confirmed the activity of those with this group on N(9).

Synthesis and biological evaluation of new imidazole, pyrimidine, and purine derivatives and analogs as inhibitors of xanthine oxidase.

Several derivatives of 4,5-disubstituted imidazole, 2,4,5-trisubstituted pyrimidine, 2-substituted purine, thiazolo[3,2-alpha]purine, [1,3]thiazino[3,2-alpha]purine, thiazolo[2,3-i]purine, [1,3]thiazino-[2,3-i]purine, and 6-substituted pyrazolo[3,4-d]pyrimidine were synthesized and tested as inhibitors of the xanthine oxidase enzyme. Of those, some 4-(acylamino)-5-carbamoylimidazoles and 2-thioalkyl-substituted purines exhibited very good inhibitory activity, being at least 500 times more effective than allopurinol. The ineffectiveness of 6-n-alkylpyrazolo[3,4-d]pyrimidines is imputable to the alkyl chain which could hinder the coordination with molybdenum according to the known mechanism for the binding of the inhibitor allopurinol; the effectiveness of imidazole derivatives, by contrast with the ineffectiveness of 4,5-diamino-2-(thioalkyl)-6-hydroxypyrimidines, indicates the relative importance of the five-membered ring in the interaction with the enzyme. Moreover, the marked effectiveness of the angularly-cyclized [1,3]thiazino[2,3-i]purinones, which constitute an interesting new class of inhibitors, together with the weak activity of linearly-cyclized derivatives, allowed us to characterize more precisely the lipophilic region of the enzyme facing the N(1)-C(2) positions of the substrate hypoxanthine.

1,2,3-triazolo[1.5-a]quinazolines: synthesis, benzodiazepine receptor binding and theoretical calculations.

This paper reports the synthesis of 3-substituted-1,2,3-triazolo[1,5-a]quinazolin-5-ones prepared by 1,3-dipolar cycloaddition reaction of 2-azidobenzoic acid to methylenic compounds activated by a cyano group. The new derivatives were submitted to benzodiazepine receptor binding assays: the results indicated an interesting receptorial affinity of the 1,2,3-triazolo[1,5-a]quinazoline ring. On the basis of the biological results, theoretical calculations were performed, which suggested useful structural modifications.

N(6) or N(9) substituted 2-phenyl-8-azaadenines: affinity for A1 adenosine receptors. VII.

The A1 activities shown respectively by N(6) or N(9) substituted 8-azaadenines were compared. At least in some cases, the biological results indicated the ability of the receptor to accept the exogenous molecule in various arrangements, and an attempt to rationalizing these arrangements was made by means of a model with two different molecular orientations.

Xanthine oxidase (XO). 4(5)-aminosubstituted-5(4)-carboxyamido-1H-1,2, 3-triazoles: a new class of monocyclic triazole inhibitors.

The 4(5)-aminosubstituted-5(4)-carboxyamido-1H-1,2,3-triazoles constitute a new class of monocyclic compounds as effective inhibitors of XO. In the past to these compounds the structure of 9-unsubstituted-8-azahypoxanthines was wrongly attributed. However some 8-azahypoxanthines obtained via a new annulation reaction have been described in this paper. The inhibitory activity of the title triazoles resulted greater than that shown by the corresponding 8-azahypoxanthines. The inhibitory competitive activity of 4(5)-n-pentyloxyoxalylamino-5(4)-carbamoyl-1H-1,2,3-triazole toward the oxidation of 8-n-pentylhypoxanthine disclosed that only one lipophilic pocket is present within the enzyme.