2970 J. Med. Chem. 1994,37, 2970-2975 8-Azaxanthine Derivatives as Antagonists of Adenosine Receptors Palmarisa Franchetti,? Lea Messini,+Loredana Cappellacci,? Mario Grifantini,*pt Antonio Lucacchini,* Claudia Martini,* and Generoso Senatore* Dipartimento di Scienze Chimiche, Universita di Camerino, via S. Agostino 1, 62032 Camerino, Italy, and Istituto Policattreda di Discipline Biologiche, Universitb di Pisa, via Bonanno 6, 56126 Pisa, Italy Received March 9, 1994@ A series of 1,3-dimethyl- and 1,3-dipropyl-8-azaxanthines, substituted at the N8 or N7 position with substituents which usually increase the affinity of the xanthines for the adenosine receptors, was synthesized and studied in radioligand binding experiments. The substitution of CH with N at the 8-position of both theophylline and caffeine dramatically reduced the affinity, as demonstrated by the fact that 8-azatheophylline and 8-azacaffeine were inert. The introduction of a methyl group at 8-position of 8-azatheophylline restored the antagonistic activity at A2 receptors, while a 8-cycloalkyl substituent increased the affinity for both receptor subtypes. A more favorable effect on affinity was produced by the substitution of the 7-methyl group in 8-azacaffeine with cycloalkyl groups. 7-Cyclopentyl-l,3-dimethyl-8-azaxanthine was 3 times more potent than caffeine at A1 receptors and 6 times less active at A2 receptors. On the contrary, the 7-cyclohexyl-l,3-dimethyl-8-azaxanthine was more potent than caffeine at AZ receptors. The substitution of 1- and 3-methyl groups with propyl in both 7- and 8-substituted 8-azatheophylline increased remarkably the affinity for A1 receptors. The 7-cyclopentyl-l,3-dipropyl-8-azaxanthine appears to be one of the most potent and selective among 7-alkyl-substituted xanthines at A1 receptors so far known. Because the 8-aza analogues of 8-substituted 1,3-dialkylxanthine were in any case less active than the corresponding xanthine derivatives, it was confirmed that the hydrogen atom at the 7-position of xanthines plays an important role in the binding to adenosine receptors. Introduction Adenosine is a neuromodulator which, acting through specific cell-surface receptors, elicits a range of physi- ological responses, including vasodilation, antilipolytic effect, inhibition of platelet aggregation, inhibition of lymphocyte functions, inhibition of insuline and poten- tiation of glucagon release, inhibition of neurotransmit- ter release from nerve endings, stimulation of steroido- genesis, potentiation of histamine release from mast cells, and erythropoietin pr0duction.l Potent and selec- tive agonists have been developed for these receptors, of which at least two subclasses are known (A1and Az).l All adenosine receptor agonists designed up to now are derivatives of adenosine. The most important class of adenosine antagonists are the xanthines.z The meth- ylxanthines theophylline and caffeine exhibit a variety of pharmacological actions, including facilitation of the atrioventricular conductions, renal vasodilation, stimu- lation of the central nervous system and bronchodila- tion, primarily through blockade of adenosine receptor^.^ However, they are virtually nonselective antagonists and have weak affinity for A1 and ASreceptors. Minity enhancement of xanthines can be obtained by modifica- tion of the 1-, 3-, and 8-position. Introduction of alkyl groups to positions 1 and 3 and substitution at the 8-position with aryl or cycloalkyl groups increases the affmity for both receptor subtype^.^-^ Substitution at the 8-position with phenyl or cycloalkyl substituents enhances not only the affinity but also the selectivity for A1 receptors.6,8 Recently, it was found that the p-trifluoromethyl substitution of certain 8-phenylxan- thines increases the potency at A2 receptor^.^ Some caffeine derivatives such as 3,7-dimethyl-1- propagyl-, 1,3-dimethy1-7-propyl-, and 1,3-dimethyl-7- @ Abstract published in Advance ACS Abstracts, July 1, 1994. 0022-262319411837-2970$04.50/0 allylxanthine and various 8-substituted caffeine deriva- tives, including 8-(trifluoromethyl)caffeine and 8-vinyl- caffeine, are somewhat selective for A2 recept~rs.~-ll 8-Cycloalkyl-l,3,7-trialkylxanthine~~ and 8-styryl-1,3,7- tria1kylxanthinesl2J3 have high affinity and selectivity for A2 receptors. The greater selectivity of many xan- thine derivatives structurally related to caffeine for the A2 receptors leads one to suppose that the xanthine N7- hydrogen is less involved in the binding to this subtype of adenosine receptors. In order to obtain further information about the role played by this hydrogen in the interaction between the ligand and the receptor, we have synthesized a series of 1,3-dialkyl-8-azaxanthine substituted at the 8-posi- tion with substituents which usually increase the af- finity for the adenosine receptors, such as the cycloalkyl- and 2-amino-4-chlorophenyl g r o u p ~ . ~ J ~ Such com- pounds may be considered analogues of 8-substituted theophylline derivatives in which the 7-hydrogen is lacking. Chemistry The 1,3-dimethyl-8-azaxanthines substituted at the 8-position with cyclopentyl, cyclohexyl, and endo-nor- bornyl groups were obtained as described in Scheme 1. Cycloalkylationof 1,3-dimethyl-8-azaxanthine15 (1) with cyclopentyl, cyclohexyl, or exo-2-norbornyl bromide in DMF in the presence of K2CO3 at 80-120 "C gave a mixture of N8 (2, 4, and 6) and N7 (3, 5, and 7) regio- isomers which were separated by chromatography on silica gel. The cycloalkylationposition was determined by comparison of the 13C NMR spectra. In Table 2 the chemical shift values are reported of C-4 and C-5 of compounds 2-17 and of 1,3,7-trimethyl-8-azaxanthine15 (8-azacaffeine) and 1,3,8-trimethyl-8-azaxanthine15 (8- methyl-8-azatheophylline) used as reference compounds. 0 1994 American Chemical Society