Novel Dopamine D3/D2 Receptor Ligands

National Institute On Drug Abuse

Investigators

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Abstract

Our primary objective has been to develop novel and selective D3R antagonists and partial agonists with drug-like physicochemical properties. To this end we discovered very selective D3R antagonists and partial agonists with D3R/D2R-selectivites reaching >1000-fold. In addition, several of these analogues have been further screened for binding to receptors and ion channels and did not show significant binding affinities at any of these other (off) targets, highlighting that these agents are some of the most potent and selective D3R-antagonists and partial agonists reported to date. Moreover, the (+)- and (-)-enantiomers of several of our 3-OH analogues have been synthesized using enantioselective synthetic strategies or chiral chromatography. We combined small molecule SAR with the D3R crystal structure solved with the D2-like antagonist eticlopride to design our next generation of D3R-selective compounds. We hypothesized that the substituted-4-phenylpiperazine terminus, defined as the primary pharmacophore (PP), binds within the orthosteric binding site (OBS) of both the D2R and D3Rs, while the indole amide terminus termed as the secondary pharmacophore (SP), binds in a secondary binding pocket (SBP) at the interface of transmembrane domains (TMs) 1, 2, and 7 and the EL1, EL2, that significantly differ from the D2R. Site-directed mutagenesis studies have identified a single amino acid (Gly94) in the EL1 that differs between D2 and D3 receptors and is critically important for subtype selectivity. These studies have provided a structural basis for the contribution each component in these molecules to the binding and functional efficacy at D3R, and to the relative orientation of the primary and secondary pharmacophores for optimal D3R binding affinity, selectivity and efficacy. We have explored numerous substituted phenyl-piperazines as the PP as well as SP with different heteroaryl amides and have further investigated the 3-substitution on the butyl linking chain and separated enantiomers of both the 3-OH and 3-F analogues, identifying new lead molecules for investigation in vivo. We have identified two lead molecules: VK4-116 and VK4-40, which show promising behavioral results in rodent models of opioid misuse. Both compounds are metabolically stable and reduce acquisition to oxycodone self-administration suggesting that they might be useful as treatments for opioid dependence but also may be useful in preventing addiction to prescription opioids. We discovered that the R-enantiomer for both lead molecules was the eutomer and in both cases, an antagonist. Indeed, development of R-VK4-116 towards an Investigational New Drug (IND) application was cleared by the FDA for the treatment of opioid use disorder. R-VK4-40 was also effective in reducing cocaine self-administration and reinstatement to cocaine seeking in rats. More recently, the enantiomers of the 3-F analogue, ABS1-113, have been separated and evaluated in rodent models of opioid use disorder. The relatively efficacious partial agonist, S-ABS1-113 was surprisingly effective in these models suggesting that D3R partial agonists may have potential for development as treatments for SUD and possibly dual diagnoses with other affective disorders, which are notoriously difficult to treat. Further development of these compounds as well as other lead molecules is underway. Further, in nonhuman primates, the partial agonist VK4-40 was effective in reducing oxycodone self-administration while improving its analgesic potency. More recently, we have focused attention on D3R preferential partial agonists, based on cariprazine, that may be effective in treating psychostimulant use disorder, possibly comorbid with other neuropsychiatric disorders such as bipolar disorder or schizophrenia. These compounds are less selective over D2R than the VK analogues to test the hypothesis that D2R partial agonist activity may be required for effectiveness in this patient population. Indeed further evaluation of our lead molecules demonstrate superior effectiveness in numerous rodent models of cocaine use disorder as compared to cariprazine and highlight our lead molecule, ESG1-60, as a compound worthy of further development. In addition to bitopic ligands directed toward antagonists and partial agonists, we have also focused on D3R full agonists, using PD128,907 and PF592,379 as parent molecules. We have recently discovered one of the most D3R-selective ligands to date (FOB02-04A) which emphasized the critical role of chirality in both the PP and linker. This year we were successful in obtaining a cryoEM structure of FOB02-04A in D3R coupled to its G-protein, Go. This is the first bitopic D3R-selective ligand to provide the structural basis for D3R activation. We recently reported that FOB02-04A shows interesting effects in reducing impulsivity in highly impulsive rats. Finally, we have embarked on a dual target strategy to discover small molecules that bind both the mu opioid receptor (MOR) and D3R with partial agonist efficacy at MOR and a partial agonist/antagonist efficacy at D3R. We have synthesized a large series of loperamide analogues and have identified several lead molecules that have moderately high affinities at MOR, D2R and D3R, with a partial agonist profile at all three sites. The lead compounds in this series did not show consistent analgesia in the hot plate model in rats, likely due to low brain penetrance. We then embarked on a synthetic campaign using the opioid partial agonist and G-protein biased TRV130 as a scaffold and have discovered several novel bivalent ligands that bind to both D3R and MOR with moderate to high affinities. The lead molecules are all partial agonists at MOR and antagonists at D3R. Microsomal metabolic stability, pharmacokinetic analyses to determine brain penetration, and evaluation in rodent models of analgesia and reward.