Water-soluble pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidines as human A₃ adenosine receptor antagonists

A relevant problem of the pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine nucleus, an attractive scaffold for the preparation of adenosine receptor antagonists, is the low water solubility. We originally functionalized the C(5) position with a salifiable 4-pyridylcarbamoyl moiety that conferred good water solubility at low pH (<4.0) but poor solubility at physiologic pH, indicative of the dissociation of the pyridinium species. Here we replaced the pyridin-4-yl moiety with a 1-(substituted)piperidin-4-yl ring to exploit the higher basicity of this nucleus and for the the possibility to generate stable, water-soluble salts. The hydrochloride salt of the 1-(cyclohexylmethyl)piperidin-4-yl derivative (10, K(i)(hA(3)) = 9.7 nM, IC(50)(hA(3)) = 30 nM, K(i)(hA(1)/hA(3)) = 351, K(i)(hA(2A)/hA(3)) > 515, IC(50)(hA(2B)) > 5 μM) showed a solubility of 8 mg/mL at physiological pH and gave a stable aqueous system suitable for intravenous infusion. Molecular modeling studies were helpful in rationalizing the available structure-activity relationships and the selectivity profile of the new ligands.