Medication discovery for addiction: translating the dopamine D3 receptor hypothesis

Substance Addiction Rehabilitation Drugs

The relapse rate of substance abusers is high, and addiction rehabilitation adjunct drugs need to be developed urgently. There have been numerous reports on blocking the formation of substance addiction, but studies on drugs that can alleviate withdrawal symptoms are very limited. Both the dopamine transporter (DAT) hypothesis and D3 dopamine receptor (D3R) hypothesis are proposed. DAT activators reduce the extracellular dopamine level, and D3R antagonists reduce the neuron's sensitivity to dopamine, both of which may exacerbate the withdrawal symptoms subsequently. The D3R partial agonist SK608 has biased signaling properties via the G-protein-dependent pathway but did not induce D3R desensitization and, thus, may be a promising drug for the withdrawal symptoms. Drugs for serotoninergic neurons or GABAergic neurons and anti-inflammatory drugs may have auxiliary effects to addiction treatments. Drugs that promote structural synaptic plasticity are also discussed.

The Role of Estrogen Signaling and Exercise in Drug Abuse: A Review

BACKGROUND

Discovering how sex differences impact the efficacy of exercise regimens used for treating drug addiction is becoming increasingly important. Estrogen is a hormone believed to explain a large portion of sex differences observed during drug addiction, and why certain exercise regimens are not equally effective between sexes in treatment. Addiction is currently a global hindrance to millions, many of whom are suffering under the influence of their brain's intrinsic reward system coupled with external environmental factors. Substance abuse disorders in the U.S. alone cost billions of dollars annually.

REVIEW SUMMARY

Studies involving the manipulation of estrogen levels in female rodents, primarily via ovariectomy, highlight its impact regarding drug addiction. More specifically, female rodents with higher estrogen levels during the estrus phase increase cocaine consumption, whereas those in the non-estrus phase (low estrogen levels) decrease cocaine consumption. If estrogen is reintroduced, self-administration increases once again. Exercise has been proven to decrease relapse tendency, but its effect on estrogen levels is not fully understood.

CONCLUSIONS

Such findings and results discussed in this review suggest that estrogen influences the susceptibility of females to relapse. Therefore, to improve drug-abuse-related treatment, exercise regimens for females should be generated based on key sex differences with respect to males.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Association of dopamine D2-like and D3 receptor function with initial sensitivity to cocaine reinforcement in male rhesus monkeys

Identifying neurobiological characteristics that predict the development of cocaine use disorder would be of great value in prevention efforts. Because of their importance in mediating the abuse-related effects of cocaine, brain dopamine receptors are logical candidates for investigation. We analyzed data from two recently published studies that characterized availability of dopamine D2-like receptors (D2R) with [11C]raclopride PET imaging and dopamine D3 receptor (D3R) sensitivity with quinpirole-induced yawning in cocaine-naïve rhesus monkeys who subsequently acquired cocaine self-administration and completed a cocaine self-administration dose-effect curve. The present analysis compared D2R availability in several brain areas and characteristics of quinpirole-induced yawning, both acquired when monkeys were drug-naïve, with measures of initial sensitivity to cocaine. D2R availability in the caudate nucleus was negatively correlated with the ED50 of the cocaine self-administration curve, although the significance of this relationship was driven by an outlier and was not present after the outlier was removed. No other significant associations were observed between D2R availability in any examined brain region and measures of sensitivity to cocaine reinforcement. However, there was a significant negative correlation between D3R sensitivity, represented by the ED50 of the quinpirole-induced yawning curve, and the dose at which monkeys acquired cocaine self-administration. We also report no change from baseline D2R availability when a second PET scan was conducted after completion of the dose-effect curves. These data suggest the utility of D3R sensitivity, but not D2R availability, as a biomarker for vulnerability and resilience to cocaine. The well-established relationships between dopamine receptors and cocaine reinforcement in cocaine-experienced humans and animals may require extensive cocaine exposure.

Blockade of dopamine D3 receptor in ventral tegmental area attenuating contextual fear memory

The abnormal fear memory will lead to the onset of stress disorders, such as post-traumatic stress disorder (PTSD) and so on. Therefore, the intervention in the formation of abnormal fear memory will provide a new strategy for the prevention and treatment of PTSD. In our previous studies, we found that blockade of dopamine D3 receptor (DRD3) with highly selective antagonist YQA14 or knockout of DRD3 was able to attenuate the expression or retrieval of fear memory in PTSD animal models. However, the neurobiological mechanism of regulation of DRD3 in fear is unclear. In the present research, we clarified that DRD3 was expressed in the dopaminergic (DAergic) neurons in the ventral tegmental area (VTA). Then, we identified that microinjection of YQA14 (1 μg/0.2 μl/side) in VTA before the aversive stimuli in the training session or during days subsequent to the shock significantly meliorated the freezing behaviors in the inescapable electric foot-shock model. At last, using fiber photometry system, we found that microinjection of YQA14 in VTA promoted the dopamine neurotransmitter release in the basolateral amygdala (BLA), and pre-training YQA14 infusion in VTA lowered the increase of dopamine (DA) in BLA induced by shock during the training session or by context during the retrieval session. All above the results demonstrated that YQA14 attenuated the fear learning through the blockade of DRD3 in VTA decreasing the excitability of the projection to BLA. This study may provide new mechanisms and potential intervention targets for stress disorders with abnormal fear memory.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use disorders.

Design and Synthesis of Conformationally Flexible Scaffold as Bitopic Ligands for Potent D3-Selective Antagonists

Previous studies have confirmed that the binding of D₃ receptor antagonists is competitively inhibited by endogenous dopamine despite excellent binding affinity for D₃ receptors. This result urges the development of an alternative scaffold that is capable of competing with dopamine for binding to the D₃ receptor. Herein, an SAR study was conducted on metoclopramide that incorporated a flexible scaffold for interaction with the secondary binding site of the D₃ receptor. The alteration of benzamide substituents and secondary binding fragments with aryl carboxamides resulted in excellent D₃ receptor affinities (Ki = 0.8-13.2 nM) with subtype selectivity to the D₂ receptor ranging from 22- to 180-fold. The β-arrestin recruitment assay revealed that 21c with 4-(pyridine-4-yl)benzamide can compete well against dopamine with the highest potency (IC₅₀ = 1.3 nM). Computational studies demonstrated that the high potency of 21c and its analogs was the result of interactions with the secondary binding site of the D₃ receptor. These compounds also displayed minimal effects for other GPCRs except moderate affinity for 5-HT₃ receptors and TSPO. The results of this study revealed that a new class of selective D₃ receptor antagonists should be useful in behavioral pharmacology studies and as lead compounds for PET radiotracer development.

A highly D3R-selective and efficacious partial agonist (S)-ABS01-113 compared to its D3R-selective antagonist enantiomer (R)-ABS01-113 as potential treatments for opioid use disorder

The non-medical use of opioids has become a national crisis in the USA. Developing non-opioid pharmacotherapies for controlling this opioid epidemic is urgent. Dopamine D₃ receptor (D₃R) antagonists and low efficacy partial agonists have shown promising profiles in animal models of opioid use disorders (OUD). However, to date, advancement to human studies has been limited. Here we report the effects of (S)- and (R)-enantiomers of (±)-ABS01-113, structural analogs of the D₃R partial agonist, (±)-VK4-40, in which the 3-OH in the linking chain is replaced by 3-F group. (S)- and (R)-ABS01-113 are identical in chemical structure but with opposite chirality. In vitro receptor binding and functional assays indicate that (S)-ABS01-113 is an efficacious (55%) and potent (EC₅₀ = 7.6 ± 3.9 nM) D₃R partial agonist, while the (R)-enantiomer is a potent D₃R antagonist (IC₅₀ = 11.4 nM). Both (S)- and (R)-ABS01-113 bind with high affinity to D₃R (K_i = 0.84 ± 0.16 and 0.37 ± 0.06 nM, respectively); however, the (S)-enantiomer is more D₃/D₂-selective (>1000-fold). Pharmacokinetic analyses indicate that both enantiomers display excellent oral bioavailability and high brain penetration. Systemic administration of (S)- or (R)-ABS01-113 alone failed to alter open-field locomotion in male rats and mice. Interestingly, pretreatment with (S)- or (R)-ABS01-113 attenuated heroin-enhanced hyperactivity, heroin self-administration, and (heroin + cue)-induced reinstatement of drug-seeking behavior. Together, these findings reveal that both enantiomers, particularly the highly selective and efficacious D₃R partial agonist (S)-ABS01-113, demonstrate promising translational potential for the treatment of OUD.

Effects of chronic cocaine and ethanol self‐administration on brain dopamine receptors in a rhesus monkey model of polysubstance abuse

Most individuals with cocaine use disorder also use alcohol; however, little is known about the behavioural and pharmacological mechanisms that promote co‐abuse. For example, although studies in humans and animals have documented that chronic use of either alcohol or cocaine alone decreases D2‐like receptor (D2R) availability, effects of co‐abuse of these substances on dopamine receptor function have not been characterized. These studies examined the effects of long‐term cocaine self‐administration in 12 male rhesus monkeys who also consumed either ethanol or an ethanol‐free solution each day (n = 6 per group). Specifically, all monkeys self‐administered cocaine (0.1 mg/kg per injection) 5 days per week in the morning. In the afternoon, six monkeys consumed 2.0 g/kg ethanol over 1 h to model binge drinking and six monkeys drank an ethanol‐free solution. Assessment of D2R availability using positron emission tomography (PET) and [¹¹C]raclopride occurred when monkeys were drug‐naïve and again when monkeys had self‐administered approximately 400‐mg/kg cocaine. D₃R function was assessed at the same time points by determining the potency of the D₃R‐preferring agonist quinpirole to elicit yawns. Chronic cocaine self‐administration decreased D2R availability in subregions of the basal ganglia in control monkeys, but not those that also drank ethanol. In contrast, D₃R sensitivity increased significantly after chronic cocaine self‐administration in ethanol‐drinking monkeys but not controls. These results suggest that co‐use of ethanol substantially changes the effects of chronic cocaine self‐administration on dopamine receptors, specifically implicating D₃R as a target for medications in these individuals.

Dopamine D3 receptor in the nucleus accumbens alleviates neuroinflammation in a mouse model of depressive-like behavior

We recently reported that dopamine D3 receptor (D3R) was involved in inflammation-related depression. Nucleus accumbens (NAc) inflammation is implicated in the development and progression of depression, but its regulatory mechanism remains largely unknown. In a mouse model of NAc neuroinflammation induced by bilateral NAc injection of lipopolysaccharide (LPS), we observed that NAc neuroinflammation triggered depressive-like behaviors, and D3R expression decline and microglial activation in the NAc. A selective knockdown of D3R in the NAc elicited depressive-like behaviors, while re-expression of D3R in the NAc of global D3RKO mice alleviated depressive-like behaviors induced by D3R deficiency. D3R downregulation in the NAc shifted microglia toward a proinflammatory state, which was validated with cultured mouse microglial cultures. Further in vitro results demonstrated that D3R inhibition induced microglia to enter a proinflammatory state primarily through the Akt signaling pathway. In conclusion, our results suggest that D3R expression in the NAc may inhibit microglial proinflammatory responses in the NAc, thus alleviating NAc neuroinflammation and subsequent depressive-like behaviors through the Akt signaling pathway.

Switching Clozapine to Cariprazine in Three Patients with Persistent Symptoms of Schizophrenia: A Case Series

Despite many available treatments for schizophrenia, several unmet needs persist in treating individuals with this disorder, and the response rate to first-line antipsychotics remains relatively low. Clozapine has shown efficacy in treating schizophrenia patients who failed to respond to previous antipsychotics. However, side effects and the need for routine blood tests have limited its use as a first-line treatment. Cariprazine is a D₂/D₃ partial agonist antipsychotic with a mechanism of action that differs from other antipsychotics due to its higher affinity for D₃ receptors. Several trials have demonstrated the efficacy of cariprazine on positive and negative symptoms of schizophrenia and have shown that it is a well-tolerated treatment. In this series, we present 3 cases of patients diagnosed with schizophrenia who were initially under treatment with clozapine. Despite some initial improvement, the patients showed persisting positive and negative symptoms or developed limiting side effects while in treatment with clozapine. Cariprazine treatment was titrated concurrently with clozapine tapering until its discontinuation. Significant improvement in both positive and negative symptoms was observed up to 14 months after starting cariprazine, and resolution of side effects was reported in all cases. Our case series supports cariprazine as an effective treatment for positive and negative symptoms in patients who failed to adequately respond or poorly tolerated treatment with clozapine, as well as a potential treatment in dual disorders, specifically psychotic disorders and cocaine use disorder.

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

Linkers are emerging as a key component in regulating the pharmacology of bitopic ligands directed toward G-protein coupled receptors (GPCRs). In this study, the role of regio- and stereochemistry in cyclic aliphatic linkers tethering well-characterized primary and secondary pharmacophores targeting dopamine D2 and D3 receptor subtypes (D2R and D3R, respectively) is described. We introduce several potent and selective D2R (rel-trans-16b; D2R Ki = 4.58 nM) and D3R (rel-cis-14a; D3R Ki = 5.72 nM) agonists while modulating subtype selectivity in a stereospecific fashion, transferring D2R selectivity toward D3R via inversion of the stereochemistry around these cyclic aliphatic linkers [e.g., (-)-(1S,2R)-43 and (+)-(1R,2S)-42]. Pharmacological observations were supported with extensive molecular docking studies. Thus, not only is it an innovative approach to modulate the pharmacology of dopaminergic ligands described, but a new class of optically active cyclic linkers are also introduced, which can be used to expand the bitopic drug design approach toward other GPCRs.

Scaffold Hybridization Strategy Leads to the Discovery of Dopamine D3 Receptor-Selective or Multitarget Bitopic Ligands Potentially Useful for Central Nervous System Disorders

In the search for novel bitopic compounds targeting the dopamine D₃ receptor (D₃R), the N-(2,3-dichlorophenyl)piperazine nucleus (primary pharmacophore) has been linked to the 6,6- or 5,5-diphenyl-1,4-dioxane-2-carboxamide or the 1,4-benzodioxane-2-carboxamide scaffold (secondary pharmacophore) by an unsubstituted or 3-F-/3-OH-substituted butyl chain. This scaffold hybridization strategy led to the discovery of potent D₃R-selective or multitarget ligands potentially useful for central nervous system disorders. In particular, the 6,6-diphenyl-1,4-dioxane derivative 3 showed a D₃R-preferential profile, while an interesting multitarget behavior has been highlighted for the 5,5-diphenyl-1,4-dioxane and 1,4-benzodioxane derivatives 6 and 9, respectively, which displayed potent D₂R antagonism, 5-HT_1AR and D₄R agonism, as well as potent D₃R partial agonism. They also behaved as low-potency 5-HT_2AR antagonists and 5-HT_2CR partial agonists. Such a profile might be a promising starting point for the discovery of novel antipsychotic agents.

Can pharmacotherapy improve treatment outcomes in people with co-occurring major depressive and cocaine use disorders?

Introduction: Major depressive disorder (MDD) and cocaine use disorder (CUD) are prevalent and frequently co-occur. When co-occurring, the presence of one disorder typically negatively impacts the prognosis for the other. Given the clinical relevance, we sought to examine pharmacotherapies for co-occurring CUD and MDD. While multiple treatment options have been examined in the treatment of each condition individually, studies exploring pharmacological options for their comorbidity are fewer and not conclusive.Areas Covered: For this review, the authors searched the literature in PubMed using clinical query options for therapies and keywords relating to each condition. Then, they described potentially promising pharmacologic therapeutic options based on shared mechanisms between the two conditions and/or results from individual clinical trials conducted to date.Expert opinion: Medications like stimulants, dopamine (D3) receptors partial agonists or antagonists, antagonists of kappa opioid receptors, topiramate, and ketamine could be promising as there is significant overlap relating to reward deficiency models, antireward pathways, and altered glutamatergic systems. However, the available clinical literature on any one of these types of agents is mixed. Additionally, for some agents there is possible concern related to abuse potential (e.g. ketamine and stimulants).

Interaction of Ligands for PET with the Dopamine D3 Receptor: In Silico and In Vitro Methods

[¹⁸F]Fallypride and [¹⁸F]Fluortriopride (FTP) are two different PET radiotracers that bind with sub-nanomolar affinity to the dopamine D3 receptor (D₃R). In spite of their similar D₃ affinities, the two PET ligands display very different properties for labeling the D₃R in vivo: [¹⁸F]Fallypride is capable of binding to D₃R under "baseline" conditions, whereas [¹⁸F]FTP requires the depletion of synaptic dopamine in order to image the receptor in vivo. These data suggest that [¹⁸F]Fallypride is able to compete with synaptic dopamine for binding to the D₃R, whereas [¹⁸F]FTP is not. The goal of this study was to conduct a series of docking and molecular dynamic simulation studies to identify differences in the ability of each molecule to interact with the D₃R that could explain these differences with respect to competition with synaptic dopamine. Competition studies measuring the ability of each ligand to compete with dopamine in the β-arrestin assay were also conducted. The results of the in silico studies indicate that FTP has a weaker interaction with the orthosteric binding site of the D₃R versus that of Fallypride. The results of the in silico studies were also consistent with the IC50 values of each compound in the dopamine β-arrestin competition assays. The results of this study indicate that in silico methods may be able to predict the ability of a small molecule to compete with synaptic dopamine for binding to the D₃R.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Exploring the structural basis and atomistic binding mechanistic of the selective antagonist blockade at D3 dopamine receptor over D2 dopamine receptor

More recently, there has been a paradigm shift toward selective drug targeting in the treatment of neurological disorders, including drug addiction, schizophrenia, and Parkinson's disease mediated by the different dopamine receptor subtypes. Antagonists with higher selectivity for D₃ dopamine receptor (D3DR) over D₂ dopamine receptor (D2DR) have been shown to attenuate drug-seeking behavior and associated side effects compared to non-subtype selective antagonists. However, high conservations among constituent residues of both proteins, particularly at the ligand-binding pockets, remain a challenge to therapeutic drug design. Recent studies have reported the discovery of two small-molecules R-VK4-40 and Y-QA31 which substantially inhibited D3DR with >180-fold selectivity over D2DR. Therefore, in this study, we seek to provide molecular and structural insights into these differential binding mechanistic using meta-analytic computational simulation methods. Findings revealed that R-VK4-40 and Y-QA31 adopted shallow binding modes and were more surface-exposed at D3DR while on the contrary, they exhibited deep hydrophobic pocket binding at D2DR. Also, two non-conserved residues; Tyr36^1.39 and Ser182^45.51 were identified in D3DR, based on their crucial roles and contributions to the selective binding of R-VK4-40 and Y-QA31. Importantly, both antagonists exhibited high affinities in complex with D3DR compared to D2DR, while van der Waals energies contributed majorly to their binding and stability. Structural analyses also revealed the distinct stabilizing effects of both compounds on D3DR secondary architecture relative to D2DR. Therefore, findings herein pinpointed the origin and mechanistic of selectivity of the compounds, which may assist in the rational design of potential small molecules of the D₂ -like dopamine family receptor subtype with improved potency and selectivity.

Partial Agonists and Dual Disorders: Focus on Dual Schizophrenia

Dual disorder is a term applied to patients with an addictive disorder and other mental disorder. Epidemiological studies have established that dual disorders are an expectation rather than an exception. They are difficult to diagnose and treat and constitute a huge burden for both patients and their relatives and society. Current treatments are a combination of those needed to treat the addictive disorder with those focused on the co-occurring psychiatric disorder. Focusing specifically on schizophrenia, growing scientific evidence supports the existence of a shared vulnerability for substance use in these patients and those at risk. Various antipsychotics have been found to be useful in the treatment of psychotic symptoms and disorders; however, few effective treatments have been identified until now for substance use disorders in patients with dual schizophrenia. Partial agonism stands as a new pharmacological option available in recent years. Molecules with this kind of action may act as functional agonists or as antagonists, depending on the surrounding levels of the neurotransmitter. Studies have found their efficacy in schizophrenia, addiction, anxiety and depression. Certain partial agonist antipsychotics seem to have a role in the treatment of dual schizophrenia. That could be the case with cariprazine. Because of its higher affinity for dopaminergic D3 receptors compared to D2, a potential to prevent relapse to addiction, added to its antipsychotic efficacy, has been suggested. Here we briefly review current advances and future directions and introduce some personal insights into the role of partial agonists in co-occurring schizophrenia and substance use.

Evidence for a Stereoselective Mechanism for Bitopic Activity by Extended-Length Antagonists of the D3 Dopamine Receptor

The D3 dopamine receptor (D3R) has been suggested as a drug target for the treatment of a number of neuropsychiatric disorders, including substance use disorders (SUD). Many D3R-selective antagonists are bivalent in nature in that they engage two distinct sites on the receptor-a primary pharmacophore binds to the orthosteric site, where dopamine binds, whereas a secondary pharmacophore interacts with a unique secondary binding pocket (SBP). When engagement of the secondary pocket exerts allosteric activity, the compound is said to be bitopic. We recently reported the synthesis and characterization of two bitopic antagonists of the D3R, (±)-VK04-87 and (±)-VK05-95, which incorporated a racemic trans-cyclopropylmethyl linking chain. To gain a better understanding of the role of chirality in determining the pharmacology of such compounds, we resolved the enantiomers of (±)-VK04-87. We found that the (+)-isomer displays higher affinity for the D3R and exhibits greater selectivity versus the D2R than the (-)-isomer. Strikingly, using functional assays, we found that (+)-VK04-87 inhibits the D3R in a noncompetitive manner, while (-)-VK04-87 behaves as a purely competitive antagonist, indicating that the apparent allosteric activity of the racemate is due to the (+)-isomer. Molecular dynamic simulations of (+)-VK04-87 and (-)-VK04-87 binding to the D3R suggest that the (+)-isomer is able to interact with the SBP of the receptor whereas the (-)-isomer bends away from this pocket, thus potentially explaining their differing pharmacology. These results emphasize the importance of the linker, and its isomeric conformations, within extended-length molecules for their positioning and engagement within GPCR binding pockets.

Exception That Proves the Rule: Investigation of Privileged Stereochemistry in Designing Dopamine D3R Bitopic Agonists

In this study, starting from our selective D₃R agonist FOB02-04A (5), we investigated the chemical space around the linker portion of the molecule via insertion of a hydroxyl substituent and ring-expansion of the trans-cyclopropyl moiety into a trans-cyclohexyl scaffold. Moreover, to further elucidate the importance of the primary pharmacophore stereochemistry in the design of bitopic ligands, we investigated the chiral requirements of (+)-PD128907 ((+)-(4a R ,10b R )-2)) by synthesizing and resolving bitopic analogues in all the cis and trans combinations of its 9-methoxy-3,4,4a,10b-tetrahydro-2H,5H-chromeno[4,3-b][1,4] oxazine scaffold. Despite the lack of success in obtaining new analogues with improved biological profiles, in comparison to our current leads, a "negative" result due to a poor or simply not improved biological profile is fundamental toward better understanding chemical space and optimal stereochemistry for target recognition. Herein, we identified essential structural information to understand the differences between orthosteric and bitopic ligand-receptor binding interactions, discriminate D₃R active and inactive states, and assist multitarget receptor recognition. Exploring stereochemical complexity and developing extended D₃R SAR from this new library complements previously described SAR and inspires future structural and computational biology investigation. Moreover, the expansion of chemical space characterization for D₃R agonism may be utilized in machine learning and artificial intelligence (AI)-based drug design, in the future.

Effects of chronic treatment with bupropion on self-administration of nicotine + cocaine mixtures in nonhuman primates

Chronic health problems associated with long-term nicotine use are the leading cause of preventable death in the United States. The use of tobacco products is 3-4 times greater among individuals with cocaine use disorder than that observed in the general population. This may reflect the propensity of nicotine to augment the reinforcing effects of cocaine. However, the mechanism of action of nicotine differs from that of cocaine, which presents a significant challenge for the development of pharmacotherapeutic interventions for the management of nicotine + cocaine polydrug abuse. Bupropion, an FDA-approved smoking cessation aid, has pharmacological actions at both monoamine transporters and nicotinic receptors, suggesting that it may be effective at decreasing nicotine + cocaine coabuse. Here, rhesus monkeys (n = 4) responded for food pellets and, separately, intravenous injections of nicotine, cocaine, or nicotine + cocaine mixtures under a second-order FR2(VR16:S) schedule of reinforcement during 7- to 10-day continuous treatment with saline or bupropion (1.0 and 1.8 mg/kg/hr). Results show that bupropion treatment dose-dependently decreased self-administration of nicotine combined with a low dose of cocaine (0.0032 mg/kg/inj); however, when the dose of cocaine in the mixture was higher (i.e., 0.01 mg/kg/inj), bupropion attenuated self-administration in only a subset of subjects. The effective dosage of bupropion increased responding for cocaine alone, nicotine alone, and for saline injections and significantly increased measures of daily activity. The apparent stimulant-like effects of bupropion at the dosage required to decrease cocaine + nicotine self-administration does not support its clinical use for the management of nicotine + cocaine polydrug abuse. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

The Long-Acting D3 Partial Agonist MC-25-41 Attenuates Motivation for Cocaine in Sprague-Dawley Rats

The dopamine D3 receptor is a prime target for developing treatments for cocaine use disorders (CUDs). In this study, we conducted a pre-clinical investigation of the therapeutic potential of a long-acting, D3 receptor partial agonist, MC-25-41. Male rats were pre-treated with MC-25-41 (vehicle, 1.0, 3.0, 5.6, or 10 mg/kg, intraperitoneal (IP)) five minutes prior to tests of cocaine or sucrose intake on either a progressive ratio schedule of reinforcement or a variable interval 60 s multiple schedule consisting of 4, 15-min components with sucrose or cocaine available in alternating components. A separate cohort of rats was tested on a within-session, dose-reduction procedure to determine the effects of MC-25-41 on demand for cocaine using a behavioral economics analysis. Finally, rats were tested for effects of MC-25-41 on spontaneous and cocaine-induced locomotion. MC-25-41 failed to alter locomotion, but reduced reinforcement rates for both cocaine and sucrose on the low-effort, multiple schedule. However, on the higher-effort, progressive ratio schedule of cocaine reinforcement, MC-25-41 reduced infusions, and active lever presses at doses that did not alter sucrose intake. The behavioral economics analysis showed that MC-25-41 also increased cocaine demand elasticity compared to vehicle, indicating a reduction in consumption as price increases. Together, these results suggest that similar to other D3-selective antagonists and partial agonists, MC-25-41 reduces motivation for cocaine under conditions of high cost but has the added advantage of a long half-life (>10 h). These findings suggest that MC-25-41 may be a suitable pre-clinical lead compound for development of medications to treat CUDs.

Dopamine D3 receptor-based medication development for the treatment of opioid use disorder: Rationale, progress, and challenges

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the current national opioid crisis. Although several opioid-based pharmacotherapies are available (e.g., methadone, buprenorphine, naloxone), they show limited effectiveness in long-term relapse prevention. In response to the opioid crisis, the National Institute on Drug Abuse proposed a list of pharmacological targets of highest priority for medication development for the treatment of opioid use disorders (OUD). Among these are antagonists of dopamine D3 receptors (D3R). In this review, we first review recent progress in research of the dopamine hypothesis of opioid reward and abuse and then describe the rationale and recent development of D3R ligands for the treatment of OUD. Herein, an emphasis is placed on the effectiveness of newly developed D3R antagonists in the animal models of OUD. These new drug candidates may also potentiate the analgesic effects of clinically used opioids, making them attractive as adjunctive medications for pain management and treatment of OUD.

Discovery, Optimization, and Characterization of ML417: A Novel and Highly Selective D3 Dopamine Receptor Agonist

To identify novel D₃ dopamine receptor (D3R) agonists, we conducted a high-throughput screen using a β-arrestin recruitment assay. Counterscreening of the hit compounds provided an assessment of their selectivity, efficacy, and potency. The most promising scaffold was optimized through medicinal chemistry resulting in enhanced potency and selectivity. The optimized compound, ML417 (20), potently promotes D3R-mediated β-arrestin translocation, G protein activation, and ERK1/2 phosphorylation (pERK) while lacking activity at other dopamine receptors. Screening of ML417 against multiple G protein-coupled receptors revealed exceptional global selectivity. Molecular modeling suggests that ML417 interacts with the D3R in a unique manner, possibly explaining its remarkable selectivity. ML417 was also found to protect against neurodegeneration of dopaminergic neurons derived from iPSCs. Together with promising pharmacokinetics and toxicology profiles, these results suggest that ML417 is a novel and uniquely selective D3R agonist that may serve as both a research tool and a therapeutic lead for the treatment of neuropsychiatric disorders.

Cannabidiol attenuates the rewarding effects of cocaine in rats by CB2, 5-HT1A and TRPV1 receptor mechanisms

Cocaine abuse continues to be a serious health problem worldwide. Despite intense research there is currently no FDA-approved medication to treat cocaine use disorder. The recent search has been focused on agents targeting primarily the dopamine system, while limited success has been achieved at the clinical level. Cannabidiol (CBD) is a U.S. FDA-approved cannabinoid for the treatment of epilepsy and recently was reported to have therapeutic potential for other disorders. Here we systemically evaluated its potential utility for the treatment of cocaine use disorder and explored the underlying receptor mechanisms in experimental animals. Systemic administration (10-40 mg/kg) of CBD dose-dependently inhibited cocaine self-administration, shifted a cocaine dose-response curve downward, and lowered break-points for cocaine self-administration under a progressive-ratio schedule of reinforcement. CBD inhibited cocaine self-administration maintained by low, but not high, doses of cocaine. In addition, CBD (3-20 mg/kg) dose-dependently attenuated cocaine-enhanced brain-stimulation reward (BSR) in rats. Strikingly, this reduction in both cocaine self-administration and BSR was blocked by AM630 (a cannabinoid CB2 receptor antagonist), WAY100135 (a 5-HT1A receptor antagonist), or capsazepine (a TRPV1 channel blocker), but not by AM251 (a CB1 receptor antagonist), CID16020046 (a GPR55 antagonist), or naloxone (an opioid receptor antagonist), suggesting the involvement of CB2, 5-HT1A, and TRPV1 receptors in CBD action. In vivo microdialysis indicated that pretreatment with CBD (10-20 mg/kg) attenuated cocaine-induced increases in extracellular dopamine (DA) in the nucleus accumbens, while CBD alone failed to alter extracellular DA. These findings suggest that CBD may have certain therapeutic utility by blunting the acute rewarding effects of cocaine via a DA-dependent mechanism.

Microglial activation contributes to depressive-like behavior in dopamine D3 receptor knockout mice

We previously demonstrated that the dopamine D3 receptor (D3R) inhibitor, NGB2904, increases susceptibility to depressive-like symptoms, elevates pro-inflammatory cytokine expression, and alters brain-derived neurotrophic factor (BDNF) levels in mesolimbic dopaminergic regions, including the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and ventral tegmental area (VTA) in mice. The mechanisms by which D3R inhibition affects neuroinflammation and onset of depression remain unclear. Here, using D3R-knockout (D3RKO) and congenic wild-type C56BL/6 (WT) mice, we demonstrated that D3RKO mice displayed depressive-like behaviors, increased tumornecrosisfactor-α (TNF-α), interleukin-1β (IL-1β), and IL-6 levels, and altered BDNF expression in selected mesolimbic dopaminergic regions. D3R expression was localized to astrocytes or microglia in the mPFC, NAc, and VTA in WT mice. D3RKO mice exhibited a large number of Iba1-labelled microglia in the absence of glial fibrillary acidic protein (GFAP)-labelled astrocytes in mesolimbic dopaminergic brain areas. Inhibition or ablation of microglia by minocycline (25 mg/kg and 50 mg/kg) or PLX3397 (40 mg/kg) treatment ameliorated depressive-like symptoms, alterations in pro-inflammatory cytokine levels, and BDNF expression in the indicated brain regions in D3RKO mice. Minocycline therapy alleviated the increase in synaptic density in the NAc in D3RKO mice. These findings suggest that microglial activation in selected mesolimbic reward regions affects depressive-like behaviors induced by D3R deficiency.

2016 Philip S. Portoghese Medicinal Chemistry Lectureship: Designing Bivalent or Bitopic Molecules for G-Protein Coupled Receptors. The Whole Is Greater Than the Sum of Its Parts

The genesis of designing bivalent or bitopic molecules that engender unique pharmacological properties began with Portoghese's work directed toward opioid receptors, in the early 1980s. This strategy has evolved as an attractive way to engineer highly selective compounds for targeted G-protein coupled receptors (GPCRs) with optimized efficacies and/or signaling bias. The emergence of X-ray crystal structures of many GPCRs and the identification of both orthosteric and allosteric binding sites have provided further guidance to ligand drug design that includes a primary pharmacophore (PP), a secondary pharmacophore (SP), and a linker between them. It is critical to note the synergistic relationship among all three of these components as they contribute to the overall interaction of these molecules with their receptor proteins and that strategically designed combinations have and will continue to provide the GPCR molecular tools of the future.

Design, synthesis, and evaluation of N-(4-(4-phenyl piperazin-1-yl)butyl)-4-(thiophen-3-yl)benzamides as selective dopamine D3 receptor ligands

As part of our on-going effort to explore the role of dopamine receptors in drug addiction and identify potential novel therapies for this condition, we have a identified a series of N-(4-(4-phenyl piperazin-1-yl)butyl)-4-(thiophen-3-yl)benzamide D₃ ligands. Members of this class are highly selective for D₃ versus D₂, and we have identified two compounds (13g and 13r) whose rat in vivo IV pharmacokinetic properties that indicate that they are suitable for assessment in in vivo efficacy models of substance use disorders.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Drug addiction: a curable mental disorder?

Drug addiction is a chronic, relapsing brain disorder. Multiple neural networks in the brain including the reward system (e.g., the mesocorticolimbic system), the anti-reward/stress system (e.g., the extended amygdala), and the central immune system, are involved in the development of drug addiction and relapse after withdrawal from drugs of abuse. Preclinical and clinical studies have demonstrated that it is promising to control drug addiction by pharmacologically targeting the addiction-related systems in the brain. Here we review the pharmacological targets within the dopamine system, glutamate system, trace amine system, anti-reward system, and central immune system, which are of clinical interests. Furthermore, we discuss other potential therapies, e.g., brain stimulation, behavioral treatments, and therapeutic gene modulation, which could be effective to treat drug addiction. We conclude that, although drug addiction is a complex disorder that involves complicated neural mechanisms and psychological processes, this mental disorder is treatable and may be curable by therapies such as gene modulation in the future.

[O-methyl-11C]N-(4-(4-(3-Chloro-2-methoxyphenyl)-piperazin-1-yl)butyl)-1H-indole-2-carboxamide ([11C]BAK4-51) Is an Efflux Transporter Substrate and Ineffective for PET Imaging of Brain D₃ Receptors in Rodents and Monkey

Selective high-affinity antagonists for the dopamine D₃ receptor (D₃R) are sought for treating substance use disorders. Positron emission tomography (PET) with an effective D₃R radioligand could be a useful tool for the development of such therapeutics by elucidating pharmacological specificity and target engagement in vivo. Currently, a D₃R-selective radioligand does not exist. The D₃R ligand, N-(4-(4-(3-chloro-2-methoxyphenyl)piperazin-1-yl)butyl)-1H-indole-2-carboxamide (BAK4-51, 1), has attractive properties for PET radioligand development, including full antagonist activity, very high D₃R affinity, D₃R selectivity, and moderate lipophilicity. We labeled 1 with the positron-emitter carbon-11 (t_1/2 = 20.4 min) in the methoxy group for evaluation as a radioligand in animals with PET. However, [¹¹C]1 was found to be an avid substrate for brain efflux transporters and lacked D₃R-specific signal in rodent and monkey brain in vivo.

Validation of gallbladder absorbed radiation dose reduction simulation: human dosimetry of [18F]fluortriopride

BACKGROUND

[¹⁸F]Fluortriopride (FTP) was developed as a dopamine D3-selective radiotracer, thought to be important to neurobiological reward pathways and implicated in drug addiction, Parkinson's disease, and schizophrenia. Preclinical radiation dosimetry studies found the gallbladder wall received the highest dose. A gallbladder dose reduction intervention was simulated using a novel reduction model for healthy adults following fatty-meal consumption. The goals of this study were to assess whole body FTP human dosimetry and determine the feasibility of reducing absorbed dose to the gallbladder wall.

RESULTS

Effective dose without a fatty meal was 0.022 ± 0.002 mSv/MBq (± standard deviation) with highest organ dose of 0.436 ± 0.178 mSv/MBq to the gallbladder wall (n = 10). Predicted gallbladder dose reduction with fatty meal consumed was 67.4% (n = 10). Meal consumption by four repeat volunteers decreased average gallbladder dose by 71.3% (n = 4) compared to the original ten volunteers.

CONCLUSIONS

Observed effective doses were adequately low to continue studying FTP uptake in humans. Validated dosimetry simulations indicate up to a 71% reduction in gallbladder dose can be achieved by employing intrinsic physiology to contract the gallbladder via fatty meal ingestion. This methodology for predicting gallbladder absorbed dose reduction from fatty meal consumption can be applied to other radiopharmaceuticals and radiotherapies.

Effects of lorcaserin and buspirone, administered alone and as a mixture, on cocaine self-administration in male and female rhesus monkeys

Cocaine use disorder is a serious public health issue for which there is no effective pharmacotherapy. One strategy to speed development of medications for cocaine use disorder is to repurpose drugs already approved for use in humans based on their ability to interact with targets known to be important for addiction. Two such drugs, lorcaserin (Belviq; a drug with serotonin [5-HT]2C receptor agonist properties) and buspirone (Buspar; a drug with 5-HT1A receptor partial agonist and dopamine D3/D4 receptor antagonist properties) can produce modest decreases in cocaine self-administration in rhesus monkeys. The current study evaluated the effectiveness of mixtures of lorcaserin and buspirone (at fixed dose ratios of 3:1, 1:1, and 1:3 relative to each drug's ID50) to reduce responding for 0.032 mg/kg/inf cocaine under a progressive ratio schedule of reinforcement in 2 male and 2 female rhesus monkeys. Dose addition analyses were used to determine if the effects of the drug mixtures differed from those predicted for an additive interaction between lorcaserin and buspirone. Dose-dependent reductions of cocaine self-administration were observed when lorcaserin and buspirone were administered alone, as well as when they were administered as 3:1, 1:1, and 1:3 fixed ratio mixtures of lorcaserin + buspirone. The effects of the 1:1 mixture of lorcaserin + buspirone on cocaine self-administration were supraadditive, whereas the effects of 3:1 and 1:3 mixtures were additive. Together, these results indicate that a combination therapy containing a mixture of lorcaserin and buspirone might be more effective than either drug alone at treating cocaine use disorder. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Dopamine D1-Like Receptor Agonist and D2-Like Receptor Antagonist (-)-Stepholidine Reduces Reinstatement of Drug-Seeking Behavior for 3,4-Methylenedioxypyrovalerone (MDPV) in Rats

Psychostimulant reinforcement is mediated by stimulation of both dopamine (DA) D1-like and D2-like receptors, suggesting that pharmacotherapy agents with a dual DA receptor mechanism may be useful for managing psychostimulant abuse. (-)-Stepholidine (L-SPD) is a Chinese herbal extract that functions as a D1-like receptor agonist and D2-like receptor antagonist. L-SPD has been shown to attenuate the reinforcing effects of heroin; however, its effects on the synthetic cathinone 3,4-methylenedioxypyrovalerone (MDPV) have not been examined. The current study determined the effects of L-SPD on reinstatement of MDPV-seeking behavior in the drug intravenous self-administration (IVSA) and conditioned place preference (CPP) paradigms. To determine whether the effects of L-SPD were specific to psychostimulant reinforcement, we also examined its effects on sucrose-seeking behavior. Using a locomotor activity assay, we tested the locomotor effects of L-SPD, as well as its effects on MDPV-induced hyperactivity. The results of a battery of in vitro binding and functional assays confirmed that L-SPD functioned as a D1-like receptor agonist and D2-like receptor antagonist. In behavioral experiments, L-SPD dose-dependently attenuated cue plus MDPV-primed reinstatement of MDPV-seeking behavior in the IVSA model. The highest dose of L-SPD also attenuated MDPV-primed reinstatement of MDPV CPP, as well as cue-induced reinstatement of sucrose-seeking. L-SPD had no significant locomotor effects, and did not modulate the robust hyperactivity induced by MDPV. The current findings show for the first time a robust reinstatement effect with MDPV, which can be reduced by L-SPD. These results establish a role for DA receptors in drug-seeking behavior for MDPV.

Synthesis of 3-(3-hydroxyphenyl)pyrrolidine dopamine D3 receptor ligands with extended functionality for probing the secondary binding pocket

A series of 3-(3-hydroxyphenyl)pyrrolidine analogues which incorporate N-alkyl groups and N-butylamide-linked benzamide functionality have been synthesized and their in vitro binding affinities at human dopamine receptors have been evaluated. Our ligand design strategy was to take the 3-(3-hydroxyphenyl)pyrrolidine scaffold and extend functionality from the orthosteric binding site to the secondary binding pocket for enhancing affinity and selectivity for the D₃ receptor. The N-alkyl analogues constitute a homologous series from N-pentyl to N-decyl to probe the length/bulk tolerance of the secondary binding pocket of the D₃ receptor. Enantiomeric 3-(3-hydroxyphenyl)pyrrolidine analogues were also prepared in order to test the chirality preference of the orthosteric binding site for this scaffold. Benzamide analogues were prepared to enhance affinity and/or selectivity based upon the results of the homologous series.

Design, synthesis, and biological evaluation of structurally constrained hybrid analogues containing ropinirole moiety as a novel class of potent and selective dopamine D3 receptor ligands

Two series of hybrid analogues were designed, synthesized, and evaluated as a novel class of selective ligands for the dopamine D3 receptor. Binding affinities of target compounds were determined (using the method of radioligand binding assay). Compared to comparator agent BP897, compounds 2a and 2c were found to demonstrate a considerable binding affinity and selectivity for D3 receptor, and especially compound 2h was similarly potent and more selective D3R ligand than BP897, a positive reference. Thus, they may provide valuable information for the discovery and development of highly potent dopamine D3 receptor ligands with outstanding selectivity.

Highly Selective Dopamine D3 Receptor Antagonists with Arylated Diazaspiro Alkane Cores

A series of potent and selective D3 receptor (D3R) analogues with diazaspiro alkane cores were synthesized. Radioligand binding of compounds 11, 14, 15a, and 15c revealed favorable D3R affinity (Ki = 12-25.6 nM) and were highly selective for D3R vs D3R (ranging from 264- to 905-fold). Variation of these novel ligand architectures can be achieved using our previously reported 10-20 min benchtop C-N cross-coupling methodology, affording a broad range of arylated diazaspiro precursors.

The effects of buspirone on occupancy of dopamine receptors and the rat gambling task

BACKGROUND

The dopamine D3 receptor (DRD₃) has been proposed as a target for drug development for the treatment of addictive disorders. Recently, the anxiolytic buspirone has been shown to have affinity for DRD₃ and DRD₄, and interest in repurposing it for addictive disorders has grown.

METHODS

Binding of [³H]-(+)-PHNO in the rat cerebellum and striatum was used to measure occupancy by buspirone of DRD₃ or DRD₂, respectively. Effects of buspirone in the rat gambling task (rGT) and the five-choice serial reaction time task (5-CSRTT) were examined.

RESULTS

Buspirone occupied both the DRD₂ and DRD₃ at high doses and the DRD₃, but not the DRD₂, in the narrow dose range of 3 mg/kg. At 10 mg/kg, a disruption of performance on rGT was observed. All measures of performance on the rGT, except for perseverations, were affected at 3 mg/kg. On the 5-CSRTT, omissions were increased. Impairments in the rGT were not mimicked by the effects induced by satiation. Further, buspirone did not impair food-maintained responding under a progressive ratio schedule of reinforcement at any dose, suggesting that the effects of buspirone on the rGT cannot be explained by non-selective actions.

CONCLUSIONS

Although buspirone had effects on the rGT at the dose that selectively occupied the DRD₃, the effects found do not parallel those found in previous studies of the effects of selective DRD₃ antagonists on the rGT. Thus, buspirone may impair performance on the rGT through actions at multiple receptor sites.

Advances and challenges in the search for D2 and D3 dopamine receptor-selective compounds

Compounds that target D2-like dopamine receptors (DRs) are currently used as therapeutics for several neuropsychiatric disorders including schizophrenia (antagonists) and Parkinson's disease (agonists). However, as the D₂R and D₃R subtypes are highly homologous, creating compounds with sufficient subtype-selectivity as well as drug-like properties for therapeutic use has proved challenging. This review summarizes the progress that has been made in developing D₂R- or D₃R-selective antagonists and agonists, and also describes the experimental conditions that need to be considered when determining the selectivity of a given compound, as apparent selectivity can vary widely depending on assay conditions. Future advances in this field may take advantage of currently available structural data to target alternative secondary binding sites through creating bivalent or bitopic chemical structures. Alternatively, the use of high-throughput screening techniques to identify novel scaffolds that might bind to the D₂R or D₃R in areas other than the highly conserved orthosteric site, such as allosteric sites, followed by iterative medicinal chemistry will likely lead to exceptionally selective compounds in the future. More selective compounds will provide a better understanding of the normal and pathological functioning of each receptor subtype, as well as offer the potential for improved therapeutics.

Asymmetric total synthesis of tetrahydroprotoberberine derivatives and evaluation of their binding affinities at dopamine receptors

Cocaine addiction remains a serious challenge for clinical and medical research because there is no effective pharmacological treatment. l-THP, a natural product isolated from Corydalis yanhusuo W.T. Wang, is one of the most frequently used traditional herbs to treat drug addiction in China. Our laboratory first reported that its demethylated metabolites l-ICP, l-CD, and l-CP had high affinity at dopamine D1, D2, and D5 receptors. Here we report the chemical synthesis of these metabolites and other derivatives and their binding affinities at dopamine receptors. The synthesis of these bioactive metabolites will allow further in vivo study of their potential in treating cocaine addiction.

Synthesis and Pharmacological Characterization of Novel trans-Cyclopropylmethyl-Linked Bivalent Ligands That Exhibit Selectivity and Allosteric Pharmacology at the Dopamine D3 Receptor (D3R)

The development of bitopic ligands directed toward D₂-like receptors has proven to be of particular interest to improve the selectivity and/or affinity of these ligands and as an approach to modulate and bias their efficacies. The structural similarities between dopamine D₃ receptor (D₃R)-selective molecules that display bitopic or allosteric pharmacology and those that are simply competitive antagonists are subtle and intriguing. Herein we synthesized a series of molecules in which the primary and secondary pharmacophores were derived from the D₃R-selective antagonists SB269,652 (1) and SB277011A (2) whose structural similarity and pharmacological disparity provided the perfect templates for SAR investigation. Incorporating a trans-cyclopropylmethyl linker between pharmacophores and manipulating linker length resulted in the identification of two bivalent noncompetitive D₃R-selective antagonists, 18a and 25a, which further delineates SAR associated with allosterism at D₃R and provides leads toward novel drug development.

Insights from Preclinical Choice Models on Treating Drug Addiction

Substance-use disorders are a global public health problem that arises from behavioral misallocation between drug use and more adaptive behaviors maintained by nondrug alternatives (e.g., food or money). Preclinical drug self-administration procedures that incorporate a concurrently available nondrug reinforcer (e.g., food) provide translationally relevant and distinct dependent measures of behavioral allocation (i.e., to assess the relative reinforcing efficacy of the drug) and behavioral rate (i.e., to assess motor competence). In particular, preclinical drug versus food 'choice' procedures have produced increasingly concordant results with both human laboratory drug self-administration studies and double-blind placebo-controlled clinical trials. Accordingly, here we provide a heuristic framework of substance-use disorders based on a behavioral-centric perspective and recent insights from these preclinical choice procedures.

The neuroimmune transcriptome and alcohol dependence: potential for targeted therapies

Transcriptome profiling enables discovery of gene networks that are altered in alcoholic brains. This technique has revealed involvement of the brain's neuroimmune system in regulating alcohol abuse and dependence, and has provided potential therapeutic targets. In this review, we discuss Toll-like-receptor pathways, hypothesized to be key players in many stages of the alcohol addiction cycle. The growing appreciation of the neuroimmune system's involvement in alcoholism has also led to consideration of crucial roles for glial cells, including astrocytes and microglia, in the brain's response to alcohol abuse. We discuss current knowledge and hypotheses on the roles that specific neuroimmune cell types may play in addiction. Current strategies for repurposing US FDA-approved drugs for the treatment of alcohol use disorders are also discussed.

Highly Selective Dopamine D3 Receptor (D3R) Antagonists and Partial Agonists Based on Eticlopride and the D3R Crystal Structure: New Leads for Opioid Dependence Treatment

The recent and precipitous increase in opioid analgesic abuse and overdose has inspired investigation of the dopamine D3 receptor (D3R) as a target for therapeutic intervention. Metabolic instability or predicted toxicity has precluded successful translation of previously reported D3R-selective antagonists to clinical use for cocaine abuse. Herein, we report a series of novel and D3R crystal structure-guided 4-phenylpiperazines with exceptionally high D3R affinities and/or selectivities with varying efficacies. Lead compound 19 was selected based on its in vitro profile: D3R Ki = 6.84 nM, 1700-fold D3R versus D2R binding selectivity, and its metabolic stability in mouse microsomes. Compound 19 inhibited oxycodone-induced hyperlocomotion in mice and reduced oxycodone-induced locomotor sensitization. In addition, pretreatment with 19 also dose-dependently inhibited the acquisition of oxycodone-induced conditioned place preference (CPP) in rats. These findings support the D3R as a target for opioid dependence treatment and compound 19 as a new lead molecule for development.