Discovery of an opioid kappa receptor selective pure antagonist from a library of N-substituted 4beta-methyl-5-(3-hydroxyphenyl)morphans

Nonpeptide small molecule agonist and antagonist original leads for neuropeptide FF1 and FF2 receptors

Neuropeptide FF1 and FF2 receptors (NPFF1-R and NPFF2-R), and their endogenous ligand NPFF, are one of only several systems responsible for mediating opioid-induced hyperalgesia, tolerance, and dependence. Currently, no small molecules displaying good affinity or selectivity for either subtype have been reported, to decipher the role of NPFF2-R as it relates to opioid-mediated analgesia, for further exploration of NPFF1-R, or for medication development for either subtype. We report the first nonpeptide small molecule scaffold for NPFF1,2-R, the guanidino-piperidines, and SAR studies resulting in the discovery of a NPFF1 agonist (7b, K(i) = 487 ± 117 nM), a NPFF1 antagonist (46, K(i) = 81 ± 17 nM), and a NPFF2 partial antagonist (53a, K(i) = 30 ± 5 nM), which serve as leads for the development of pharmacological probes and potential therapeutic agents. Testing of 46 alone was without effect in the mouse 48 °C warm-water tail-withdrawal test, but pretreatment with 46 prevented NPFF-induced hyperalgesia.

4β-Methyl-5-(3-hydroxyphenyl)morphan opioid agonist and partial agonist derived from a 4β-methyl-5-(3-hydroxyphenyl)morphan pure antagonist

In previous studies we reported that addition of 7α-acylamino groups to N-phenylpropyl-4β-methyl-5-(3-hydroxyphenyl)morphan (4) led to compounds that were pure opioid receptor antagonists. In contrast to these findings we report in this study that addition of a 7α-amino (5a), 7α-alkylamino (5b-e), or 7α-dialkylamino (5f-h) group to 4 leads to opioid receptor ligands with varying degrees of agonist/antagonist activity. The 7α-amino and 7α-methylamino analogues were full agonists at the μ and δ receptors and antagonists at the κ receptor. The 7α-cyclopropylmethylamino analogue 5h was a full agonist at the μ receptor with weaker agonist activity at the δ and κ receptors. Whereas the addition of a 7α-acylamino group to the pure nonselective opioid receptor antagonist N-phenylpropyl-4β-methyl-5-(3-hydroxyphenyl)morphan (4) led to κ selective pure opioid receptor antagonist, the addition of a 7α-amino, 7α-alkylamino, or 7α-dialkylamino group to 4 leads to opioid ligands that are largely μ or δ agonist with mixed agonist/antagonist properties.

Development of κ opioid receptor antagonists

κ opioid receptors (KORs) belong to the G-protein-coupled class of receptors (GPCRs). They are activated by the endogenous opioid peptide dynorphin (DYN) and expressed at particularly high levels within brain areas implicated in modulation of motivation, emotion, and cognitive function. Chronic activation of KORs in animal models has maladaptive effects including increases in behaviors that reflect depression, the propensity to engage in drug-seeking behavior, and drug craving. The fact that KOR activation has such a profound influence on behaviors often triggered by stress has led to interest in selective KOR antagonists as potential therapeutic agents. This Perspective provides a description of preclinical research conducted in the development of several different classes of selective KOR antagonists, a summary of the clinical studies conducted thus far, and recommendations for the type of work needed in the future to determine if these agents would be useful as pharmacotherapies for neuropsychiatric illness.

Diastereoselective one-pot synthesis of 7- and 8-substituted 5-phenylmorphans

Novel 7- and 8-alkyl and aryl substituted 5-phenylmorphans were synthesized from substituted allyl halides and N-benzyl-4-aryl-1,2,3,6-tetrahydropyridine by a highly efficient and diastereoselective reaction series, "one-pot" alkylation and ene-imine cyclization followed by sodium borohydride reduction. Mild cyclization conditions gave the desired substituted 5-phenylmorphans in good yield as a single diastereomer.

Natural products as tools for neuroscience: discovery and development of novel agents to treat drug abuse

Much of what we know about the neurosciences is the direct result of studying psychoactive natural products. Unfortunately, there are many gaps in our understanding of the basic biological processes that contribute to the etiology of many CNS disorders. The investigation of psychoactive natural products offers an excellent approach to identify novel agents to treat CNS disorders and to find new chemical tools to better elucidate their biological mechanisms. This review will detail recent progress in a program directed toward investigating psychoactive natural products with the goal of treating drug abuse by targeting kappa opioid receptors.

Flavonoids as opioid receptor ligands: identification and preliminary structure-activity relationships

Flavonoids have been recognized as the active ingredients of many medicinal plant extracts due to interactions with proteins via phenolic groups and low toxicity. Here, we report the investigation of the flavonoid core as a potential new scaffold for the development of opioid receptor ligands. Biological results suggest that stereochemistry of the C2 and C3 positions is important for antagonist activity and selectivity. Our results also suggest that the actions of Hypericum perforatum may be mediated in part by opioid receptors.

Highly Potent and Selective Phenylmorphan-Based Inverse Agonists of the Opioid δ Receptor

We recently reported the discovery of (+)-5-(3-hydroxyphenyl)-4-methyl-2-(3-phenylpropyl)-2-azabicyclo[3.3.1]non-7-yl-(1-phenyl-1-cyclopentane)carboxamide [(+)-KF4, (+)-5] as a novel chemotype possessing potent antagonist activity at the delta opioid receptor. Additional SAR studies involving changes to both the 2-amino and 7-amido N-substituents using this same (+)-morphan scaffold have revealed compounds with improved potency and selectivity for the delta opioid receptor. The highly potent and selective 2,2-dimethylphenylacetamide analogue (+)-N-[(1S,4R,5R,7S)-5-(3-hydroxyphenyl)-4-methyl-2-(3-phenylpropyl)-2-azabicyclo[3.3.1]non-7-yl]-2-methyl-2-phenylpropanamide (13d, delmorphan-A) showed picomolar inhibitory potency (Ke = 0.1 nM) in the [35S]GTPgammaS functional assay with delta opioid receptor selectivity ratios of 103- and 132-fold versus the mu and kappa opioid receptors, respectively. The compounds showed no agonist activity at any of the three opioid receptors; however, measurements of delta inverse agonist activity within this series illustrated a broad range of negative efficacy and IC50 values 650-fold more potent than the prototypical delta opioid receptor inverse agonist ICI 174,864 (22).

N-substituted 4beta-methyl-5-(3-hydroxyphenyl)-7alpha-amidomorphans are potent, selective kappa opioid receptor antagonists

In a previous study, we identified (-)-N-[(1R,4S,5S,7R)-5-(3-hydroxyphenyl)-4-methyl-2-(3-phenylpropyl)-2-azabicyclo[3.3.1]non-7-yl]-3-(1-piperidinyl)propanamide (5a, KAA-1) as the first potent and selective kappa opioid receptor antagonist from the 5-(3-hydroxyphenyl)morphan class of opioids. In this study we report an improved synthesis of this class of compounds. The new synthetic method was used to prepare analogues 5b-r where the morphan N-substituent and 7alpha-amido group were varied. Most of the analogues showed sub-nanomolar potency for the kappa opioid receptor and were highly selective relative to the mu and delta opioid receptors. (-)-3-(3,4-Dihydroisoquinolin-2(1H)-yl)-N-{(1R,4S,5S,7R)-5-(3-hydroxyphenyl)-4-methyl-2-[2-(2-methylphenyl)ethyl]-2-azabicyclo[3.3.1]non-7-yl}propanamide (5n, MTHQ) is at least as potent and selective as nor-BNI as a kappa opioid receptor antagonist in the [35S]GTP-gamma-S in vitro functional test.

Molecular modeling and 3D-QSAR studies of indolomorphinan derivatives as kappa opioid antagonists

Molecular modeling and 3D-QSAR studies were performed on 31 indolomorphinan derivatives to evaluate their antagonistic behaviors on kappa opioid receptor and provide information for further modification of this kind of compounds. Best predictions were obtained with CoMFA standard model (q2 = 0.693, N = 4, r2 = 0.900) and CoMSIA combined model (q2 = 0.617, N = 4, r2 = 0.904). Both models were further validated by an external test set of eight compounds with satisfactory predictions: r2 = 0.607 for CoMFA and r2 = 0.701 for CoMSIA. In addition, the 3D structure of human kappa opioid receptor was constructed based on the crystal structure of bovine rhodopsin, and the CoMSIA contour plots were then mapped into the structural model of kappa opioid receptor-GNTI complex to identify key residues, which might account for kappa antagonist potency and selectivity. The roles of nonconserved Glu297 and conserved Lys227 of human kappa opioid receptor were then discussed.

N-substituted cis-4a-(3-hydroxyphenyl)-8a-methyloctahydroisoquinolines are opioid receptor pure antagonists

N-Substituted cis-4a-(3-hydroxyphenyl)-8a-methyloctahydroisoquinolines (6a-g) were designed and synthesized as conformationally constrained analogues of the trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine (4) class of opioid receptor pure antagonists. The methyloctahydroisoquinolines 6a-g can exist in conformations where the 3-hydroxyphenyl substituent is either axial or equatorial, similar to the (3-hydroxyphenyl)piperidines 4. The 3-hydroxyphenyl equatorial conformation is responsible for the antagonist activity observed in the (3-hydroxyphenyl)piperidine antagonists. Single-crystal X-ray analysis of 6a shows that the 3-hydroxyphenyl equatorial conformation is favored in the solid state. Molecular modeling studies also suggest that the equatorial conformation has lower potential energy relative to that of the axial conformation. Evaluation of 6a-g in the [(35)S]GTP-gamma-S in vitro functional assay showed that they were opioid receptor pure antagonists. N-[4a-(3-Hydroxyphenyl)-8a-methyl-2-(3-phenylpropyl)octahydroisoquinoline-6-yl]-3-(piperidin-1-yl)propionamide (6d) with a K(e) of 0.27 nM at the kappa opioid receptor with 154- and 46-fold selectivity relative to those of the micro and delta receptors, respectively, possessed the best combination of kappa potency and selectivity.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

3D-QSAR Comparative Molecular Field Analysis on Opioid Receptor Antagonists: Pooling Data from Different Studies

Three-dimensional quantitative structure-activity relationship (3D-QSAR) models were constructed using comparative molecular field analysis (CoMFA) on a series of opioid receptor antagonists. To obtain statistically significant and robust CoMFA models, a sizable data set of naltrindole and naltrexone analogues was assembled by pooling biological and structural data from independent studies. A process of "leave one data set out", similar to the traditional "leave one out" cross-validation procedure employed in partial least squares (PLS) analysis, was utilized to study the feasibility of pooling data in the present case. These studies indicate that our approach yields statistically significant and highly predictive CoMFA models from the pooled data set of delta, mu, and kappa opioid receptor antagonists. All models showed excellent internal predictability and self-consistency: q(2) = 0.69/r(2) = 0.91 (delta), q(2) = 0.67/r(2) = 0.92 (mu), and q(2) = 0.60/r(2) = 0.96 (kappa). The CoMFA models were further validated using two separate test sets: one test set was selected randomly from the pooled data set, while the other test set was retrieved from other published sources. The overall excellent agreement between CoMFA-predicted and experimental binding affinities for a structurally diverse array of ligands across all three opioid receptor subtypes gives testimony to the superb predictive power of these models. CoMFA field analysis demonstrated that the variations in binding affinity of opioid antagonists are dominated by steric rather than electrostatic interactions with the three opioid receptor binding sites. The CoMFA steric-electrostatic contour maps corresponding to the delta, mu, and kappa opioid receptor subtypes reflected the characteristic similarities and differences in the familiar "message-address" concept of opioid receptor ligands. Structural modifications to increase selectivity for the delta over mu and kappa opioid receptors have been predicted on the basis of the CoMFA contour maps. The structure-activity relationships (SARs) together with the CoMFA models should find utility for the rational design of subtype-selective opioid receptor antagonists.

Discovery of the First N-Substituted 4β-Methyl-5-(3-hydroxyphenyl)morphan To Possess Highly Potent and Selective Opioid δ Receptor Antagonist Activity

A structurally novel opioid delta receptor selective antagonist has been identified. This compound, (+)-5-(3-hydroxyphenyl)-4-methyl-2-(3-phenylpropyl)-2-azabicyclo[3.3.1]non-7-yl-(1-phenyl-1-cyclopentane)carboxamide [(+)-KF4, (+)-4], showed a K(e) value of 0.15 nM in the [(35)S]GTPgammaS functional assay. (+)-KF4 is also a delta inverse agonist with an IC(50) value of 1.8 nM. To our knowledge, this is the first potent and selective delta opioid receptor antagonist from the 5-phenylmorphan class of opioids.

Kappa opioid receptor antagonism and prodynorphin gene disruption block stress-induced behavioral responses

Previous studies have demonstrated that stress may increase prodynorphin gene expression, and kappa opioid agonists suppress drug reward. Therefore, we tested the hypothesis that stress-induced release of endogenous dynorphin may mediate behavioral responses to stress and oppose the rewarding effects of cocaine. C57Bl/6 mice subjected to repeated forced swim testing (FST) using a modified Porsolt procedure at 30 degrees C showed a characteristic stress-induced immobility response and a stress-induced analgesia observed with a tail withdrawal latency assay. Pretreatment with the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI; 10 mg/kg, i.p.) blocked the stress-induced analgesia and significantly reduced the stress-induced immobility. The nor-BNI sensitivity of the behavioral responses suggests an activation of the kappa opioid receptor by a stress-induced release of dynorphin peptides. Supporting this hypothesis, transgenic mice possessing a disrupted prodynorphin gene showed no increase in immobility or stress-induced analgesia after exposure to repeated FST. Because both stress and the kappa opioid system can modulate the response to drugs of abuse, we tested the effects of forced swim stress on cocaine-conditioned place preference (CPP). FST-exposed mice conditioned with cocaine (15 mg/kg, s.c.) showed significant potentiation of place preference for the drug-paired chamber over the responses of unstressed mice. Surprisingly, nor-BNI pretreatment blocked stress-induced potentiation of cocaine CPP. Consistent with this result, mice lacking the prodynorphin gene did not show a stress-induced potentiation of cocaine CPP, whereas wild-type littermates did. The findings suggest that chronic swim stress may activate the kappa opioid system to produce analgesia, immobility, and potentiation of the acute rewarding properties of cocaine in C57Bl/6 mice.

Recent advances in selective opioid receptor agonists and antagonists

Opioid analgesics provide outstanding benefits for relief of severe pain. The mechanisms of the analgesia accompanied with some side effects have been investigated by many scientists to shed light on the complex biological processes at the molecular level. New opioid drugs and therapies with more desirable properties can be developed on the bases of accurate insight of the opioid ligand-receptor interaction and clear knowledge of the pharmacological behavior of opioid receptors and the associated proteins. Toward this goal, recent advances in selective opioid receptor agonists and antagonists including opioid ligand-receptor interactions are summarized in this review article.