Pyrazolo[3,4-d]pyrimidines as sigma-1 receptor ligands for the treatment of pain. Part 1: 4-acylamino derivatives.

The synthesis of a new series of 4-acylaminopyrazolo[3,4-d]pyrimidines active on the sigma-1 receptor (σ1R) is reported. Compounds were efficiently prepared using a two to three step process starting from commercially available 1H-pyrazolo[3,4-d]pyrimidin-4-amine. A SAR study shows that the σ1R requires the presence of relatively highly lipophilic substituents at opposite sides of the central scaffold, while selectivity versus the σ2R can be improved by shortening the distance of the basic nitrogen to it. Compound 9a was among the most active and selective in vitro derivatives and exhibited potent antinociceptive properties in several pain models in mice, indicating its antagonistic behaviour.