Imidazo[2,1-i]purin-5-ones and Related Tricyclic Water-Soluble Purine Derivatives:  Potent A_2A- and A₃-Adenosine Receptor Antagonists^†

A series of tricyclic imidazo[2,1-i]purinones and ring-enlarged analogues derived from xanthine derivatives have been prepared as adenosine receptor (AR) antagonists. In comparison with xanthines, the tricyclic compounds exhibit increased water solubility due to a basic nitrogen atom, which can be protonated under physiological conditions. Substituents were introduced that confer high affinity for A_2A or A₃ ARs, respectively. A new capillary electrophoresis method was developed for the determination of the enantiomeric purity of selected chiral products using native and modified β-cyclodextrins as chiral discriminators. The compounds were investigated in radioligand binding assays at rat brain A₁ and A_2A ARs. Selected compounds were additionally investigated in radioligand binding assays at human recombinant A₃ ARs and in functional studies (adenylate cyclase assays) at A₁ ARs of rat fat cell membranes, A_2A ARs of rat PC 12 cell membranes, and mouse A_2B ARs of NIH 3T3 cell membranes. Structure−activity relationships were similar to those of corresponding xanthine derivatives. The 2-styrylimidazopurinones were less potent at A_2A ARs as compared to 8-styrylxanthine derivatives. The most potent compound at A_2A ARs was (S)-1,4-dimethyl-8-ethyl-2-styryl-imidazo[2,1-i]purinone (S-25) exhibiting a K_i value of 424 nM at rat A_2A ARs. The compound was highly selective for A_2A receptors vs A₁ and A₃ ARs. Selectivity vs A_2B ARs, however, was low. Among the 1-unsubstituted 2-phenyl-imidazo[2,1-i]purin-5-one derivatives, very potent and highly selective antagonists for human A₃ ARs were identified. The most potent A₃ antagonist of the present series was (R)-4-methyl-8-ethyl-2-phenyl-imidazo[2,1-i]purin-5-one (R-24) exhibiting a K_i value of 2.3 nM and high selectivity for A₃ receptors vs all other AR subtypes.