Highly Potent 5-Aminotetrahydropyrazolopyridines: Enantioselective Dopamine D3 Receptor Binding, Functional Selectivity, and Analysis of Receptor−Ligand Interactions

Heterocyclic dopamine surrogates of types 5 and 7 were synthesized and investigated for their dopaminergic properties. The enantiomerically pure biphenylcarboxamide (S)-5a displayed an outstanding Ki of 27 pM at the agonist-labeled D3 receptor and significant selectivity over the D2 subtype. Measurement of [35S]GTPγS incorporation in the presence of a coexpressed PTX-insensitive Gα0−1 subunit indicated highly efficient G-protein coupling. Comparison of ligand efficacy data from cAMP accumulation and [3H]thymidine incorporation experiments revealed that ligand biased signaling is exerted by the test compound (S)-5a. Starting from the D3 crystal structure, a combination of homology modeling and site directed mutagenesis gave valuable insights into the binding mode and the intermolecular origins of stereospecific receptor recognition. According to these data, the superior affinity of the eutomer 5a is caused by the favorable binding energy that results from interaction between the ligand’s central ammonium u...