Pyrazolo[1,5-a]pyrimidines:  Estrogen Receptor Ligands Possessing Estrogen Receptor β Antagonist Activity

In our search for novel subtype-selective estrogen receptor (ER) ligands, we have examined various heterocyclic units as core structural elements. Here, we have investigated the fused, bicyclic pyrazolo[1,5-a]pyrimidine core, which is a system that allows for analogues to be readily assembled in a library-like fashion. This series of pyrazolo[1,5-a]pyrimidine ER ligands provided us with a new pharmacological profile for an ER ligand:  compounds that are passive on both ERs, with a distinct potency selectivity in favor of ERβ. The most distinctive ligand in this series, 2-phenyl-3-(4-hydroxyphenyl)-5,7-bis(trifluoromethyl)-pyrazolo[1,5-a]pyrimidine, was 36-fold selective for ERβ in binding. Curiously, on the basis of molecular modeling, the ERβ binding selectivity of compounds in this series appears to be derived from differing orientations that they adapt in the ligand binding pockets of ERα vs ERβ. In transcription assays this pyrazolopyrimidine was fully effective as an ERβ antagonist while exhibiting no significant activity on ERα. Thus, this ligand functions as a potency- and efficacy-selective ERβ antagonist that would abrogate estrogen action through ERβ with minimal effects on its activity through ERα; as such, it could be used to study the biological function of ERβ.