Leveraging a Low Affinity Diazaspiro Orthosteric Fragment to Reduce Dopamine D3 Receptor (D3R) Ligand Promiscuity Across Highly Conserved Aminergic G-Protein-Coupled Receptors (GPCRs).

Previously, we reported a 3-(2-methoxyphenyl)-9-(3-((4-methyl-5-phenyl-4H-1,2,4-triazol-3-yl)thio)-propyl)-3,9-diazaspiro[5.5]undecane (1) compound with excellent dopamine D-3 receptor (D3R) affinity (D3R K-i = 12.0 nM) and selectivity (D2R/D3R ratio = 905). Herein, we present derivatives of 1 with comparable D3R affinity (32, D3R K-i = 3.2 nM, D2R/D3R ratio = 60) and selectivity (30, D3R K-i = 21.0 nM, D2R/D3R ratio = 934). Fragmentation of 1 revealed orthosteric fragment 5a to express an unusually low D3R affinity (K-i = 2.7 mu M). Compared to piperazine congener 31, which retains a high-affinity orthosteric fragment (5d, D3R K-i = 23.9 nM), 1 was found to be more selective for the D3R among D-1- and D-2-like receptors and exhibited negligible off-target interactions at serotoninergic and adrenergic G-protein-coupled receptors (GPCRs), common off-target sites for piperazine-containing D3R scaffolds. This study provides a unique rationale for implementing weakly potent orthosteric fragments into D3R ligand systems to minimize drug promiscuity at other aminergic GPCR sites.