Chirality of Novel Bitopic Agonists Determines Unique Pharmacology at the Dopamine D3 Receptor

Abstract

The dopamine D2/D3 receptor (D2R/D3R) agonists are used as therapeutics for Parkinson’s disease (PD) and other motor disorders. Selective targeting of D3R over D2R is attractive because of D3R’s restricted tissue distribution with potentially fewer side-effects and its putative neuroprotective effect. However, the high sequence homology between the D2R and D3R poses a challenge in the development of D3R selective agonists. To address the ligand selectivity, bitopic ligands were designed and synthesized previously based on a potent D3R-preferential agonist PF592,379 as the primary pharmacophore (PP). This PP was attached to various secondary pharmacophores (SPs) using chemically different linkers. Here, we characterize some of these novel bitopic ligands at both D3R and D2R using BRET-based functional assays. The bitopic ligands showed varying differences in potencies and efficacies. In addition, the chirality of the PP was key to conferring improved D3R potency, selectivity, and G protein signaling bias. In particular, compound AB04-88 exhibited significant D3R over D2R selectivity, and G protein bias at D3R. This bias was consistently observed at various time-points ranging from 8 to 46 min. Together, the structure-activity relationships derived from these functional studies reveal unique pharmacology at D3R and support further evaluation of functionally biased D3R agonists for their therapeutic potential.

Chiral Cyclic Aliphatic Linkers as Building Blocks for Selective Dopamine D2 or D3 Receptor Agonists

“…To generate SAR, the new library of compounds was tested in radioligand binding assays at both human-cloned D 2 R (hD 2 R) and D 3 R (hD 3 R) receptor subtypes using agonist [ 3 H]-( R )-(+)-7-OH-DPAT and/or antagonist [ 3 H]- N -methylspiperone (Table ). We have found that the D 2 R active state agonist binding is significantly more sensitive ,,, to the radioligand used with respect to D 3 R. As a consequence, the use of [ 3 H]-( R )-(+)-7-OH-DPAT allows an accurate determination of apparent affinities for both targets, and selectivity, valuable to better correlate measured binding values with predicted and/or experimentally determined functional profiles, ,,, which would be otherwise dramatically overestimated in favor of the D 3 R when the antagonist [ 3 H]- N -methylspiperone is used [Table , comparison between D 3 R and D 2 R selectivity ratios between K i s obtained using [ 3 H]- N -methylspiperone or [ 3 H]-( R )-(+)-7-OH-DPAT].…”

“…Methyl-2-(3-cyanocyclopentylidene)acetate (10). In a roundbottom flask, methyl 2-(dimethoxyphosphoryl)acetate (2.98 g, 16.4 mmol) was dissolved in 100 mL of MeOH, and the solution cooled to 0 °C in an ice bath.…”

“…(+)-3 is a D 3 R selective noncompetitive antagonist. 7a , and rel - trans -8 are known D 3 R and D 2 R selective agonists, respectively.…”

“…Moreover, the low expression level of D 3 R, relative to D 2 R, in brain regions controlling reward and locomotor processes − further increases the challenge, underscoring the need for highly D 3 R-selective agonists in order to discern the roles of D 3 R from D 2 R activation pathways. In this pursuit, we have recently started developing structure–activity relationships (SARs), resulting in agonists with significant D 3 R ( 7a in Figure D) or D 2 R ( rel - trans -8 in Figure D) subtype selectivity. − …”

“…(+)-3 8 is a D 3 R selective noncompetitive antagonist. 7a 9,10 and rel-trans-8 11 are known D 3 R and D 2 R selective agonists, respectively. D 3 R-selective agonists in order to discern the roles of D 3 R from D 2 R activation pathways.…”