Dopamine D2-like Functionally Selective Agonists

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We have successfully identified highly D3R-selective compounds using a bitopic design, which includes a high-affinity primary pharmacophore (PP), such as 4-phenylpiperazine, connected to an extended aryl amide functional group that occupies a secondary binding pocket (SBP) to enhance subtype selectivity. We were curious to learn if this drug design approach might be directed toward the more ubiquitous D2 receptor subtype (D2R) and further, if structural modifications of a known D2-like agonist would provide clues toward functionally biased and potentially therapeutically useful new drug targets. In this study, we took our synthon approach using the known D2-preferential agonist, sumanirole, as the PP, adding structural complexity through alkylation at the 1 and N-5-positions, and creating the first bivalent analogues by extending an aryl amide from these positions with a butyl linking chain. In this way, we identified fully efficacious analogues and D2R-preferential ligands with extended aryl amide pharmacophores. Moreover, using molecular simulations with sumanirole and selected analogues, we began to computationally explore the molecular interactions between these ligands at D2R and D3R. Based on extensive radioligand binding and functional studies we confirmed the importance of probe dependency and determined that sumanirole is only 30-fold D2R selective. Importantly, we discovered that modifications at the N-5 position could improve D2R affinity, but not selectivity, even with the extended arylamide substituents that in the 4-phenypiperazines confer D3R-selectivity. These studies confirm that when sumanirole is the PP, the D3R SBP is not uniquely accessed. Moreover, substitution at the N-1-position appears to result in lower efficacy, especially at D3R, for these analogues, suggesting that substitution here affects the positioning of the PP in the OBS and there may be subtle difference here between subtypes. Molecular simulations confirmed this hypothesis and provided rationale for further investigating these two positions on the sumanirole template for potential bitopic and functionally biased agonists. More recently, we have expanded the SAR of this class of molecules toward novel and functionally biased D2 receptor selective compounds. By investigating drug-protein interactions at an atomistic level using small molecule SAR, computational modeling and cell-based functional assays, we have been able to uncover drug molecule poses in the orthosteric binding site (OBS) that result in a preference for D2 or D3 receptor selectivity as well as functional selectivity at D2R, despite high homology in this region of these receptor subtypes. Further investigation using this approach will unveil structure-function information that can be utilized for future drug design of molecules with improved therapeutic efficacy. We have recently investigated a novel structural template that combines the PP sumanirole with a positive allosteric modulator or other SPs with unique and chiral linkers. The resulting enantiopure analogues are Go-protein biased and in vivo studies are underway to characterize the behavioral profile of these novel compounds.