Evidence for a Stereoselective Mechanism of Action for Non‐competitive Antagonism of the D3 Dopamine Receptor by Extended‐Length Bitopic Ligands

Abstract

Dopamine receptors (DRs) are comprised of two subfamilies based on structure and pharmacology: D1‐like (D1R and D5R) and D2‐like (D2R, D3R, and D4R). The D3R expression is more restricted to the limbic system, as compared to the D2R, suggesting that modulation of the D3R may be useful for treating substance abuse or psychosis‐related disorders without the motor side effects associated with D2R blockade. Extensive structure‐activity relationship investigations indicate that D3R‐selective antagonists with extended‐length structures engage two distinct sites on the receptor. The extended‐length ligand's primary pharmacophore (PP) binds to the orthosteric site whereas the ligand's secondary pharmacophore (SP) interacts with a unique secondary binding pocket (SBP). Bivalent compounds that concurrently engage both the orthosteric site and a SBP that has allosteric properties, have been termed “bitopic”. We previously synthesized and characterized several extended‐length compounds with a conformationally restricted trans‐cyclopropyl linking chain based on the structure of SB269,652, a potent D3R‐preferring bitopic antagonist that acts in a noncompetitive/allosteric manner at both the D3R and the D2R. We found that relatively subtle changes in both the linker and the SP can affect the competitive/noncompetitive nature of the compounds, and identified two novel ligands that behave as noncompetitive antagonists at the D3R. We resolved the optical isomers of one of these compounds (VK04‐87) to gain a better understanding of the molecular basis of their allosteric activity. We found that the (+)‐isomer has higher affinity and is more D3R>D2R selective than the (−)‐isomer. Strikingly, using beta‐arrestin recruitment assays as a functional output, we found that (+)‐VK04‐87 inhibits the D3R in a non‐competitive manner, while (−)‐VK04‐87 behaves as a competitive antagonist. These data suggest that the bitopic interactions of VK04‐87 with the D3R are stereoselective in nature with the allosteric properties residing with the (+)‐isomer. To further investigate, we made a mutation (E90A) in the secondary binding pocket of the D3R that was previously found to decrease the affinity and negative cooperativity of SB269,652 for the D2R. We found that this mutation decreased the affinity of extended‐length ligands, including SB269,652, for the D3R, but had no effect on the affinities of smaller, competitive antagonists consisting of PPs. Further analyses of the interactions of the extended‐length ligands with the D3R E90A mutant are ongoing. These studies could help to determine how ligand interactions with the SBP can be utilized to engender allosterism, potentially producing compounds with unique pharmacological effects.Support or Funding InformationNINDS IRPThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.