No abstract

No abstract

Novel classes of heterocyclic compounds as adenosine antagonists were developed based on a template approach. Structure-affinity relationships revealed insights for extended knowledge of the receptor-ligand interaction. We replaced the bicyclic heterocyclic ring system of earlier described isoquinoline and quinazoline adenosine A(3) receptor ligands by several monocyclic rings and investigated the influence thereof on adenosine receptor affinity. The thiazole or thiadiazole derivatives seemed most promising, so we continued our investigations with these two classes of compounds. The large difference between a pyridine and isoquinoline ring in binding adenosine A(1) and A(3) receptors showed the importance of the second ring of the isoquinoline ligands. We prepared several N-[4-(2-pyridyl)thiazol-2-yl]benzamides, and these compounds showed adenosine affinities in the micromolar range. Most surprising in the series of the N-[4-(2-pyridyl)thiazol-2-yl]amides were the retained adenosine affinities by introduction of a cylopentanamide instead of the benzamide. A second series of compounds, the thiadiazolobenzamide series of compounds, revealed potent and selective adenosine receptor antagonists, especially N-(3-phenyl-1,2,4-thiadiazol-5-yl)-4-hydroxybenzamide (LUF5437, 8h) showing a K(i) value of 7 nM at the adenosine A(1) receptor and N-(3-phenyl-1,2,4-thiadiazol-5-yl)-4-methoxybenzamide (LUF5417, 8e) with a K(i) value of 82 nM at the adenosine A(3) receptor. 4-Hydroxybenzamide 8h is the most potent adenosine A(1) receptor antagonist of this new class of compounds. Structure--affinity relationships showed the existence of a steric restriction at the para-position of the benzamide ring for binding adenosine A(1) and A(3) receptors. The electronic nature of the 4-substituents played an important role in binding the adenosine A(3) receptor. Cis- and trans-4-substituted cyclohexyl derivatives were made next to the 4-substituted benzamide analogues. We used them to study the proposed specific interaction between the adenosine A(1) receptor and the 4-hydroxy group of this class of thiadiazolo compounds, as well as a suggested special role for the 4-methoxy group in binding the A(3) receptor. Both the adenosine A(1) and A(3) receptor slightly preferred the trans-analogues over the cis-analogues, while all compounds showed low affinities at the adenosine A(2A) receptor. Our investigations provided the potent and highly selective adenosine A(1) antagonist N-(3-phenyl-1,2,4-thiadiazol-5-yl)-trans-4-hydroxycyclohexanamide (VUF5472, 8m) showing a K(i) value of 20 nM. A third series of compounds was formed by urea analogues, N-substituted with thiazolo and thiadiazolo heterocycles. The SAR of this class of compounds was not commensurate with the SAR of the previously described quinazoline urea. On the basis of these findings we suggest the existence of a special interaction between adenosine receptors and a region of high electron density positioned between the thia(dia)zole ring and phenyl(pyridyl) ring. Molecular electrostatic ...

No abstract

4 Displacement studies with histamine showed that a limited fraction (32 ± 6%) of the binding sites showed a high affinity for histamine (2 ± 1.2 gM); the shallow displacement curves were reflected by a Hill-coefficient significantly different from unity (nH = 0.58 ± 0.09). The addition of 100 ILM Gpp(NH)p resulted in a steepening of the displacement curve (nH = 0.79 ± 0.02) and a loss of high affinity sites for histamine. 5 Displacement studies with other agonists indicated that the recently developed specific H2 agonists, amthamine and amselamine, showed an approximately 4-5 fold higher affinity for the human H2 receptor than histamine. 6 Stimulation of CHOhumH2 cells with histamine resulted in a rapid rise of the intracellular cyclic AMP levels. After 10 min an approximately 10 fold increase in cyclic AMP could be measured. The EC50 value for this response was 7 ± 1 nM for histamine. This response was effectively blocked by tiotidine and cimetidine, resulting in Ki values of 8 ± 1 nM and 0.56 ± 0.24 LM respectively.

No abstract

No abstract

Isoquinoline and quinazoline urea derivatives were found to bind to human adenosine A 3 receptors. Series of N-phenyl-N′-quinazolin-4-ylurea derivatives and N-phenyl-N′-isoquinolin-1-ylurea derivatives were synthesized and tested in radioligand binding assays on their adenosine receptor affinities. A structure-affinity analysis indicated that on the 2-position of the quinazoline ring or the equivalent 3-position of the isoquinoline ring a phenyl or heteroaryl substituent increased the adenosine A 3 receptor affinity in comparison to unsubstituted or aliphatic derivatives. Furthermore, the structure-affinity relationship of substituted phenylurea analogues was investigated. Substituents such as electron-withdrawing or electron-donating groups were introduced at different positions of the benzene ring to probe electronic and positional effects of substitution. Substitution on the 3-or 4-position of the phenyl ring decreased the adenosine A 3 receptor affinity. Substitution at position 2 with an electron-donating substituent, such as methyl or methoxy, increased human adenosine A 3 receptor affinity, whereas substitution on the 2-position with an electron-withdrawing substituent did not influence affinity. Combination of the optimal substituents in the two series had an additive effect, which led to the potent human adenosine A 3 (VUF5574, 10a) showing a K i value of 4 nM and being at least 2500-fold selective vs A 1 and A 2A receptors. Compound 10a competitively antagonized the effect of an agonist in a functional A 3 receptor assay, i.e., inhibition of cAMP production in cells expressing the human adenosine A 3 receptor; a pA 2 value of 8.1 was derived from a Schild plot. In conclusion, compound 10a is a potent and selective human adenosine A 3 receptor antagonist and might be a useful tool in further characterization of the human A 3 receptor. receptor antagonist N-(2-methoxyphenyl)-N′-(2-(3-pyridyl)quinazolin-4-yl)urea