Kappa opioid antagonists: past successes and future prospects

Early role of the κ opioid receptor in ethanol-induced reinforcement

Effects of early ethanol exposure on later ethanol intake emphasize the importance of understanding the neurobiology of ethanol-induced reinforcement early in life. Infant rats exhibit ethanol-induced appetitive conditioning and ethanol-induced locomotor activation, which have been linked in theory and may have mechanisms in common. The appetitive effects of ethanol are significantly modulated by μ and δ opioid receptors, whereas μ but not δ receptors are involved in the motor stimulant effects of ethanol during early development. The involvement of the κ opioid receptor (KOR) system in the motivational effects of ethanol has been much less explored. The present study assessed, in preweanling (infant) rats, the modulatory role of the KOR system in several paradigms sensitive to ethanol-induced reinforcement. Kappa opioid activation and blockade were examined in second-order conditioned place preference with varied timing before conditioning and with varied ethanol doses. The role of KOR on ethanol-induced locomotion and ethanol-induced taste conditioning was also explored. The experiments were based on the assumption that ethanol concurrently induces appetitive and aversive effects and that the latter may be mediated by activation of kappa receptors. The main result was that blockade of kappa function facilitated the expression of appetitive ethanol reinforcement in terms of tactile and taste conditioning. The effects of kappa activation on ethanol conditioning seemed to be independent from ethanol's stimulant effects. Kappa opioid activation potentiated the motor depressing effects of ethanol but enhanced motor activity in control subjects. Overall, the results support the hypothesis that a reduced function of the KOR system in nondependent subjects should attenuate the aversive consequences of ethanol.

Early maternal separation affects ethanol-induced conditioning in a nor-BNI insensitive manner, but does not alter ethanol-induced locomotor activity

Early environmental stress significantly affects the development of offspring. This stress has been modeled in rats through the maternal separation (MS) paradigm, which alters the functioning of the HPA axis and can enhance ethanol intake at adulthood. Infant rats are sensitive to ethanol's reinforcing effects, which modulate ethanol seeking and intake. Little is known about the impact of MS on sensitivity to ethanol's appetitive and aversive effects during infancy. The present study assessed ethanol-induced conditioned place preference established through second-order conditioning (SOC), spontaneous or ethanol-induced locomotor activity and ethanol intake in preweanling rats that experienced normal animal facility rearing (AFR) or daily episodes of maternal separation (MS) during postnatal days 1-13 (PDs 1-13). Low-ethanol dose (0.5 g/kg) induced appetitive conditioned place preference (via SOC) in control rats given conventional rearing but not in rats given maternal separation in early infancy, whereas 2.0 g/kg ethanol induced aversive conditioned place preference in the former but not the latter. The administration of a kappa antagonist at PD 1 or immediately before testing did not alter ethanol-induced reinforcement. High (i.e., 2.5 and 2.0 g/kg) but not low (i.e., 0.5 g/kg) ethanol dose induced reliable motor stimulation, which was independent of early maternal separation. Ethanol intake and blood alcohol levels during conditioning were unaffected by rearing conditions. Pups given early maternal separation had lower body weights than controls and showed an altered pattern of exploration when placed in an open field. These results indicate that, when assessed in infant rats, earlier maternal separation alters the balance between the appetitive and aversive motivational effects of ethanol but has no effect on the motor activating effects of the drug.

YFa and analogs: Investigation of opioid receptors in smooth muscle contraction

AIM: To study the pharmacological profile and inhibition of smooth muscle contraction by YFa and its analogs in conjunction with their receptor selectivity.

METHODS: The effects of YFa and its analogs (D-Ala2) YFa, Y (D-Ala2) GFMKKKFMRF amide and Des-Phe-YGGFMKKKFMR amide in guinea pig ileum (GPI) and mouse vas deferens (MVD) motility were studied using an isolated tissue organ bath system, and morphine and DynA (1-13) served as controls. Acetylcholine was used for muscle stimulation. The observations were validated by specific antagonist pretreatment experiments using naloxonazine, naltrindole and norbinaltorphimine norBNI.

RESULTS: YFa did not demonstrate significant inhibition of GPI muscle contraction as compared with morphine (15% vs 62%, P = 0.0002), but moderate inhibition of MVD muscle contraction, indicating the role of κ opioid receptors in the contraction. A moderate inhibition of GPI muscles by (Des-Phe) YFa revealed the role of anti-opiate receptors in the smooth muscle contraction. (D-Ala-2) YFa showed significant inhibition of smooth muscle contraction, indicating the involvement of mainly δ receptors in MVD contraction. These results were supported by specific antagonist pretreatment assays.

CONCLUSION: YFa revealed its side-effect-free analgesic properties with regard to arrest of gastrointestinal transit. The study provides evidences for the involvement of κ and anti-opioid receptors in smooth muscle contraction.

Discovery of aminobenzyloxyarylamides as κ opioid receptor selective antagonists: application to preclinical development of a κ opioid receptor antagonist receptor occupancy tracer

Arylphenylpyrrolidinylmethylphenoxybenzamides were found to have high affinity and selectivity for κ opioid receptors. On the basis of receptor binding assays in Chinese hamster ovary (CHO) cells expressing cloned human opioid receptors, (S)-3-fluoro-4-(4-((2-(3-fluorophenyl)pyrrolidin-1-yl)methyl)phenoxy)benzamide (25) had a K(i) = 0.565 nM for κ opioid receptor binding while having a K(i) = 35.8 nM for μ opioid receptors and a K(i) = 211 nM for δ opioid receptor binding. Compound 25 was also a potent antagonist of κ opioid receptors when tested in vitro using a [(35)S]-guanosine 5'O-[3-thiotriphosphate] ([(35)S]GTP-γ-S) functional assay in CHO cells expressing cloned human opioid receptors. Compounds were also evaluated for potential use as receptor occupancy tracers. Tracer evaluation was done in vivo, using liquid chromatography-tandem mass spectrometry (LC/MS/MS) methods, precluding the need for radiolabeling. (S)-3-Chloro-4-(4-((2-(pyridine-3-yl)pyrrolidin-1-yl)methyl)phenoxy)benzamide (18) was found to have favorable properties for a tracer for receptor occupancy, including good specific versus nonspecific binding and good brain uptake.

Pharmacological characterization of 2-methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine (PF-04455242), a high-affinity antagonist selective for κ-opioid receptors

2-Methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine (PF-04455242) is a novel κ-opioid receptor (KOR) antagonist with high affinity for human (3 nM), rat (21 nM), and mouse (22 nM) KOR, a ∼ 20-fold reduced affinity for human μ-opioid receptors (MORs; K(i) = 64 nM), and negligible affinity for δ-opioid receptors (K(i) > 4 μM). PF-04455242 also showed selectivity for KORs in vivo. In rats, PF-04455242 blocked KOR and MOR agonist-induced analgesia with ID(50) values of 1.5 and 9.8 mg/kg, respectively, and inhibited ex vivo [(3)H](2-(benzofuran-4-yl)-N-methyl-N-((5S,7R,8R)-7-(pyrrolidin-1-yl)-1-oxaspiro[4.5]decan-8-yl)acetamide ([(3)H]CI977) and [(3)H](2S)-2-[[2-[[(2R)-2-[[(2S)-2-amino-3-(4-hydroxyphenyl) propanoyl]amino]propanoyl]amino]acetyl]-methylamino]-N-(2-hydroxyethyl)-3-phenylpropanamide ([(3)H]DAMGO) binding to KOR and MOR receptors with ID(50) values of 2.0 and 8.6 mg/kg, respectively. An in vivo binding assay was developed using (-)-4-[(3)H]methoxycarbonyl-2-[(1-pyrrolidinylmethyl]-1-[(3,4-dichlorophenyl)acetyl]-piperidine ([(3)H]PF-04767135), a tritiated version of the KOR positron emission tomography ligand (-)-4-[(11)C]methoxycarbonyl-2-[(1-pyrrolidinylmethyl]-1-[(3,4-dichlorophenyl)acetyl]-piperidine ([(11)C]GR103545) in which PF-04455242 had an ID(50) of 5.2 mg/kg. PF-04455242 demonstrated antidepressant-like efficacy (mouse forced-swim test), attenuated the behavioral effects of stress (mouse social defeat stress assay), and showed therapeutic potential in treating reinstatement of extinguished cocaine-seeking behavior (mouse conditioned place preference). KOR agonist-induced plasma prolactin was investigated as a translatable mechanism biomarker. Spiradoline (0.32 mg/kg) significantly increased rat plasma prolactin levels from 1.9 ± 0.4 to 41.9 ± 4.9 ng/ml. PF-04455242 dose-dependently reduced the elevation of spiradoline-induced plasma prolactin with an ID(50) of 2.3 ± 0.1 mg/kg, which aligned well with the ED(50) values obtained from the rat in vivo binding and efficacy assays. These data provide further evidence that KOR antagonists have potential for the treatment of depression and addiction disorders.

Unexpected opioid activity profiles of analogues of the novel peptide kappa opioid receptor ligand CJ-15,208

An alanine scan was performed on the novel κ opioid receptor (KOR) peptide ligand CJ-15,208 to determine which residues contribute to the potent in vivo agonist activity observed for the parent peptide. These cyclic tetrapeptides were synthesized by a combination of solid-phase peptide synthesis of the linear precursors, followed by cyclization in solution. Like the parent peptide, each of the analogues exhibited agonist activity and KOR antagonist activity in an antinociceptive assay in vivo. Unlike the parent peptide, the agonist activity of the potent analogues was mediated predominantly, if not exclusively, by μ opioid receptors (MOR). Thus analogues 2 and 4, in which one of the phenylalanine residues was replaced by alanine, exhibited both potent MOR agonist activity and KOR antagonist activity in vivo. These peptides represent novel lead compounds for the development of peptide-based opioid analgesics.

Effect of surfactants and pH on naltrexone (NTX) permeation across buccal mucosa

The objective of this pre-formulation study was to systematically investigate the effects of two surfactants (Brij 58(®) and Tween 80(®)) and change in solution pH on in vitro permeation of naltrexone HCl (NTX-HCl) across tissue engineered human buccal mucosa. For the study, 10mg/ml solutions of Tween 80(®) (0.1 and 1%, w/v) and Brij 58(®) (1%, w/v) were prepared in standard artificial saliva buffer solution (pH 6.8). For studying pH effects, solution pH was adjusted to either 7.5 or 8.2. As controls, three concentrations of NTX-HCl (2.5, 10 and 25mg/ml) were prepared. Using NTX standard solution (10mg/ml; pH 6.8), the permeation was observed between in vitro human and ex vivo porcine mucosa. It was observed that Brij 58(®) increased the permeation rates of NTX significantly. The flux of 10mg/ml solution (pH 6.8) increased from 1.9 ± 0.6 (× 10(2)) to 13.9 ± 2.2 (× 10(2))μg/(cm(2)h) (approximately 6-fold) in presence of 1% Brij 58(®). Increasing pH of NTX-HCl solution was found to increase the drug flux from 1.9 ± 0.6 (× 10(2)) (pH 6.8) to 3.0 ± 0.6 (× 10(2)) (pH 7.4) and 8.0 ± 3.5 (× 10(2)) (pH 8.2)μg/(cm(2)h), respectively. Histological analyses exhibited no tissue damage due to exposure of buccal tissue to Brij 58(®). The mean permeability coefficients (K(p)) for 2.5, 10 and 25mg/ml solutions of NTX-HCl (pH 6.8) were 5.0 (× 10(-2)), 1.8 (× 10(-2)) and 3.2 (× 10(-2))cm/h, respectively, consistent with data from published literature sources. Increase of NTX flux observed with 1% Brij 58(®) solution may be due to the effects of ATP. Increase in flux and the shortening of lag time observed by increasing in solution pH confirmed earlier finding that distribution coefficient (logD) of NTX is significantly affected by small increments in pH value and therefore plays an important role in NTX permeation by allowing faster diffusion across tissue engineered human buccal tissue.

[11C]GR103545: novel one-pot radiosynthesis with high specific activity

INTRODUCTION

GR103545 is a potent and selective kappa-opioid receptor agonist. Previous studies in non-human primates demonstrated favorable properties of [(11)C]GR103545 as a positron emission tomography tracer for in vivo imaging of cerebral kappa-opioid receptor. Nonetheless, advancement of [(11)C]GR103545 to imaging studies in humans was hampered by difficulties of its multiple-step radiosynthesis, which produces a final product with low specific activity (SA), which in turn could induce undesirable physiological side effects resulting from the mass associated with an injected amount of radioactivity. We report herein an alternative radiosynthesis of [(11)C]GR103545 with higher SA and radiochemical yields.

METHODS

The TRACERLab FXC automated synthesis module was used to carry out the two-step, one-pot procedure. In the first step, the desmethoxycarbonyl precursor was converted to the carbamic acid intermediate desmethyl-GR103545 via transcarboxylation with the zwitterionic carbamic complex, 1,8-diazabicyclo[5.4.0]undec-7-ene-carbon dioxide, in the presence and/or absence of cesium carbonate and tetrabutylammonium triflate. In the second step, the intermediate was radiolabeled at the carboxyl oxygen with [(11)C]methyl trifluoromethanesulfonate to give [(11)C]GR103545.

RESULTS

This novel synthesis produced [(11)C]GR103545 with ≥90% chemical and radiochemical purities and an SA of 290.45±99.9 MBq/nmol at the end of synthesis (n=26). Injectable radioactivity was 1961±814 GBq/μmol with 43 min of average synthesis time from the end of beam.

CONCLUSION

We have developed a practical one-pot method for the routine production of [(11)C]GR103545 with reliably high SA and radiochemical yield, thus allowing the advancement of this radiotracer to imaging applications in humans.

Sex differences in kappa opioid pharmacology

In recent years it has become apparent that sex is a major factor involved in modulating the pharmacological effects of exogenous opioids. The kappa opioid receptor (KOPR) system is a potential therapeutic target for pain, mood disorders and addiction. In humans mixed KOPR/MOPR ligands have been found to produce greater analgesia in women than men. In contrast, in animals, selective KOPR agonists have been found to produce greater antinociceptive effects in males than females. Collectively, the studies indicate that the direction and magnitude of sex differences of KOPR-mediated antinociception/analgesia are dependent on species, strain, ligand and pain model examined. Of interest, and less studied, is whether sex differences in other KOPR-mediated effects exist. In the studies conducted thus far, greater effects of KOPR agonists in males have been found in neuroprotection against stroke and suppression of food intake behavior. On the other hand, greater effects of KOPR agonists were found in females in mediation of prolactin release. In modulation of drugs of abuse, sex differences in KOPR effects were observed but appear to be dependent on the drug examined. The mechanism(s) underlying sex differences in KOPR-mediated effects may be mediated by sex chromosomes, gonadal hormonal influence on organization (circuitry) and/or acute hormonal influence on KOPR expression, distribution and localization. In light of the diverse pharmacology of KOPR we discuss the need for future studies characterizing the sexual dimorphism of KOPR neural circuitry and in examining other behaviors and processes that are modulated by the KOPR.

Synthesis of CJ-15,208, a novel κ-opioid receptor antagonist

The tryptophan isomers of the cyclic tetrapeptide CJ-15,208, reported to be a kappa opioid receptor (KOR) antagonist [Saito, T.; Hirai, H.; Kim, Y. J.; Kojima, Y.; Matsunaga, Y.; Nishida, H.; Sakakibara, T.; Suga, O.; Sujaku, T.; Kojima, N. J. Antibiot. (Tokyo)2002, 55, 847-854.], were synthesized to determine the tryptophan stereochemistry in the natural product. A strategy was developed to select linear precursor peptides that favor cyclization using molecular modeling, and optimized cyclization conditions are reported. The optical rotation of the l-Trp isomer is consistent with that of the natural product. Unexpectedly both isomers exhibit similar nanomolar affinity for KOR.

Analogues of (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic). Synthesis and in vitro and in vivo opioid receptor antagonist activity

The synthesis of compounds 6, 7a,b, 8a,b, 9a,b, and 10a,b where the amino -NH- group of JDTic (3) was replaced with an aromatic horizontal lineCH-, CH(2), O, S, or SO group was accomplished and used to further characterize the SAR of the compound 3 class of kappa opioid receptor antagonists. All of the compounds showed subnanomolar to low nanomolar K(e) values at the kappa opioid receptor. The most potent compound was 7a, where the amino -NH- group of 3 was replaced by a methylene (-CH(2)-) group. This compound had a K(e) = 0.18 nM and was 37- and 248-fold selective for the kappa relative to the mu and delta opioid receptors, respectively. Similar to compound 3, compound 7a antagonized selective kappa agonist U50,488-induced diuresis after sc administration in rats. In contrast to 3, where kappa antagonist activity lasted for three weeks, compound 7a did not show any kappa antagonist activity after one week.

N-Alkyl-octahydroisoquinolin-1-one-8-carboxamides: a Novel Class of Selective, Nonbasic, Nitrogen-Containing κ-Opioid Receptor Ligands

Herein we report that N-alkyl-octahydroisoquinolin-1-one-8-carboxamides are a novel class of readily-synthesized, selective ϰ-opioid receptor (KOR) ligands. A striking feature of this class of compounds is the absence of any basic nitrogen atoms. Many of these compounds have demonstrated exclusive affinity for the KOR over not only the δ-opioid receptor (DOR) and the μ-opioid receptor (MOR), but 38 other GPCR targets as well. The general binding affinity of this class of compounds for the KOR combined with a streamlined route for analog synthesis provide strong motivation for pursuing this interesting new scaffold as a basis toward new probes targeting the KOR.

Synthesis of 14-alkoxymorphinan derivatives and their pharmacological actions

Among opioids, morphinans play an important role as therapeutically valuable drugs. They include pain relieving agents such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol). Currently used opioid analgesics also share a number of severe side effects, limiting their clinical usefulness. The antagonist morphinans, naloxone and naltrexone are used to treat opioid overdose, opioid dependence, and alcoholism. All these opioid drugs produce their biological actions through three receptor types, mu, delta, and kappa, belonging to the G-protein-coupled receptor family. Considerable effort has been put forward to understand the appropriate use of opioid analgesics, while medicinal chemistry and opioid pharmacology have been continuously engaged in the search for safer, more efficacious and nonaddicting opioid compounds, with the final goal to reduce complications and to improve patient compliance. Toward this goal, recent advances in chemistry, ligand-based structure activity relationships and pharmacology of 14-alkoxymorphinans are reviewed in this chapter. Current developments of different structural patterns of 14-alkoxymorphinans as research tools and their potential therapeutic opportunities are also summarized.

Synthesis and in vitro opioid receptor functional antagonism of methyl-substituted analogues of (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl]-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic)

In previous structure-activity relationship (SAR) studies, (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl]-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic, 3) was identified as the first potent and selective kappa-opioid receptor antagonist from the trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine class of opioid antagonists. In the present study, we report the synthesis of analogues 8a-p of 3 and present their in vitro opioid receptor functional antagonism using a [(35)S]GTPgammaS binding assay. Compounds 8a-p are analogues of 3 containing one, two, or three methyl groups connected to the JDTic structure at five different positions. All the analogues with one and two added methyl groups with the exception of 8k had subnanomolar K(e) values at the kappa receptor. The three most potent analogues were the monomethylated (3R)-7-hydroxy-N-[(1S,2S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethylpiperidine-1-yl]methyl}-2-methylbutyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (8a) and (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethylpiperidin-1-yl]methyl}-(2-methylpropyl)]-3-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (8e) with K(e) values of 0.03 nM at the kappa receptor and (3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-methoxyphenyl)-3,4-dimethylpiperidin-1-yl]methyl}-2-methylpropyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (8d) with K(e) = 0.037 nM at the kappa receptor. All three compounds were selective for the kappa receptor relative to the micro and delta receptors. Overall, the results from this study highlight those areas that are tolerant to substitution on 3.

Differential role of mu, delta and kappa opioid receptors in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

The opioid system modulates ethanol intake and reinforcement in adult and preweanling rodents. While adult heterogeneous rats normally do not show ethanol-mediated locomotor stimulation, preweanling rats show it quite clearly. We recently observed that naloxone, a non-specific opioid antagonist, attenuated ethanol-induced locomotor activation in preweanling rats. In the present study we tested the role of specific opioid receptors (mu, delta and kappa) in ethanol-mediated locomotor stimulation and ethanol intake. In Experiment 1 13-day-old rats received naloxonazine (mu antagonist: 0, 7.5 or 15 mg/kg), naltrindole (delta antagonist: 0, 2 or 4 mg/kg) or nor-binaltorphimine (kappa antagonist: 0, 2, 4 or 8 mg/kg) before an intragastric administration of ethanol (0 or 2.5 g/kg), and subsequent locomotor activity assessment. In Experiment 2, the same opioid antagonists were administered on postnatal days 13 and 14 before consumption of ethanol (6%), saccharin (0.05%) or distilled water. In Experiment 1 only naloxonazine reduced ethanol-mediated locomotor stimulation. None of the opioid antagonists affected locomotor activity in water controls. In Experiment 2 naloxonazine and naltrindole suppressed ingestion of all the solutions tested. Similar to what has been reported in adult rodents, mu-opioid receptors seem to modulate ethanol-activating effects during early ontogeny. Hence, there seems to be a partial overlap of neurochemical mechanisms involved in the rewarding and stimulating effects of ethanol in preweanling rats. Mu-receptor antagonists reduced both ethanol-induced activity and ethanol intake, but it is unclear whether the latter effect is specific to ethanol or only a reflection of an effect on consummatory behavior generally, since mu and delta receptor antagonists also suppressed ingestion of water and saccharin.

Zyklophin, a systemically active selective kappa opioid receptor peptide antagonist with short duration of action

The cyclic peptide zyklophin {[N-benzylTyr(1),cyclo(D-Asp(5),Dap(8))-dynorphin A-(1-11)NH(2), Patkar KA, et al. (2005) J Med Chem 48: 4500-4503} is a selective peptide kappa opioid receptor (KOR) antagonist that shows activity following systemic administration. Systemic (1-3 mg/kg s.c.) as well as central (0.3-3 nmol intracerebroventricular, i.c.v.) administration of this peptide dose-dependently antagonizes the antinociception induced by the selective KOR agonist U50,488 in C57BL/6J mice tested in the 55 degrees C warm water tail withdrawal assay. Zyklophin administration had no effect on morphine- or SNC-80-mediated antinociception, suggesting that zyklophin selectively antagonizes KOR in vivo. Additionally, the antagonism of antinociception induced by centrally (i.c.v.) administered U50,488 following peripheral administration of zyklophin strongly suggests that the peptide crosses the blood-brain barrier to antagonize KOR in the CNS. Most importantly, the antagonist activity of zyklophin (3 mg/kg s.c.) lasts less than 12 h, which contrasts sharply with the exceptionally long duration of antagonism reported for the established small-molecule selective KOR antagonists such as nor-binaltorphimine (nor-BNI) that last weeks after a single administration. Systemically administered zyklophin (3 mg/kg s.c.) also prevented stress-induced reinstatement of cocaine-seeking behavior in a conditioned place preference assay. In conclusion, the peptide zyklophin is a KOR-selective antagonist that exhibits the desired shorter duration of action, and represents a significant advance in the development of KOR-selective antagonists.

Direct agonist activity of tricyclic antidepressants at distinct opioid receptor subtypes

Tricyclic antidepressants (TCAs) have been reported to interact with the opioid system, but their pharmacological activity at opioid receptors has not yet been elucidated. In the present study, we investigated the actions of amoxapine, amitriptyline, nortriptyline, desipramine, and imipramine at distinct cloned and native opioid receptors. In Chinese hamster ovary (CHO) cells expressing delta-opioid receptors (CHO/DOR), TCAs displaced [3H]naltrindole binding and stimulated guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding at micromolar concentrations with amoxapine displaying the highest potency and efficacy. Amoxapine and amitriptyline inhibited cyclic AMP formation and induced the phosphorylation of signaling molecules along the extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphatidylinositol-3 kinase pathways. Amoxapine also activated delta-opioid receptors in rat dorsal striatum and nucleus accumbens and human frontal cortex. In CHO cells expressing kappa-opioid receptors (CHO/KOR), TCAs, but not amoxapine, exhibited higher receptor affinity and more potent stimulation of [(35)S]GTPgammaS binding than in CHO/DOR and effectively inhibited cyclic AMP accumulation. Amitriptyline regulated ERK1/2 phosphorylation and activity in CHO/KOR and C6 glioma cells endogenously expressing kappa-opioid receptors, and this effect was attenuated by the kappa-opioid antagonist nor-binaltorphimine. In rat nucleus accumbens, amitriptyline slightly inhibited adenylyl cyclase activity and counteracted the inhibitory effect of the full kappa agonist trans-(-)-3,4dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50,488). At the cloned mu-opioid receptor, TCAs showed low affinity and no significant agonist activity. These results show that TCAs differentially regulate opioid receptors with a preferential agonist activity on either delta or kappa subtypes and suggest that this property may contribute to their therapeutic and/or side effects.

Kappa-opioid ligands in the study and treatment of mood disorders

The biological basis of mood is not understood. Most research on mood and affective states has focused on the roles of brain systems containing monoamines (e.g., dopamine, norepinephrine, serotonin). However, it is becoming clear that endogenous opioid systems in the brain may also be involved in the regulation of mood. In this review, we focus on the potential utility of kappa-opioid receptor (KOR) ligands in the study and treatment of psychiatric disorders. Research from our group and others suggests that KOR antagonists might be useful for depression, KOR agonists might be useful for mania, and KOR partial agonists might be useful for mood stabilization. Currently available KOR agents have some unfavorable properties that might be addressed through medicinal chemistry. The development of KOR-selective agents with improved drug-like characteristics would facilitate preclinical and clinical studies designed to evaluate the possibility that KORs are a feasible target for new medications.

Peptide kappa opioid receptor ligands: potential for drug development

While narcotic analgesics such as morphine, which act preferentially through mu opioid receptors, remain the gold standard in the treatment of severe pain, their use is limited by detrimental liabilities such as respiratory depression and drug dependence. Thus, there has been considerable interest in developing ligands for kappa opioid receptors (KOR) as potential analgesics and for the treatment of a variety of other disorders. These include effects mediated both by central receptors, such as antidepressant activity and a reduction in cocaine-seeking behavior, and activity resulting from the activation of peripheral receptors, such as analgesic and anti-inflammatory effects. While the vast majority of opioid receptor ligands that have progressed in preclinical development have been small molecules, significant advances have been made in recent years in identifying opioid peptide analogs that exhibit promising in vivo activity. This review will focus on possible therapeutic applications of ligands for KOR and specifically on the potential development of peptide ligands for these receptors.

Utilizing nature as a source of new probes for opioid pharmacology

Background: Traditional and current opioid pharmacology is fundamentally based on interactions between opioid receptors and compounds isolated from natural sources. Adverse effects associated with opioids have led to the search for compounds with diminished side effects. Discussion: Recent discoveries of non-nitrogenous and structurally diverse alkaloids as novel opioid ligands have led to renewed interest in the development of novel chemotypes for opioid receptors. Conclusion: The strong history of natural products as opioid receptor ligands suggests that nature is one of the most promising for the identification of novel opioids. This review highlights the vast potential of investigating natural products as novel probes of opioid receptors.

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.

Agonist activity of N-desmethylclozapine at delta-opioid receptors of human frontal cortex

The clozapine metabolite N-desmethylclozapine (NDMC) has been recently shown to act at different neurotransmitter receptors and to display both antagonist and agonist activities. We have previously reported that in cells over-expressing the recombinant delta-opioid receptor NDMC behaved as partial agonist with high intrinsic activity, but its action at the receptors naturally expressed in human brain remained to be investigated. In the present study, we examined whether NDMC was able to bind to and activate delta-opioid receptors in membranes of post-mortem human frontal cortex. In radioligand binding assays, NDMC competition curves displayed high- (K(i)=26 nM) and low-affinity (K(i)=3 microM) components, whose proportion was regulated by guanine nucleotides in an agonist-like fashion. In functional assays, NDMC stimulated [(35)S]GTPgammaS binding (EC(50)=905 nM) and inhibited cyclic AMP formation (EC(50)=590 nM) as effectively as delta-opioid agonists, whereas clozapine was much less potent and efficacious and clozapine N-oxide was completely inactive. The NDMC agonist activity was potently antagonized by the delta-opioid antagonist naltrindole, but not by the micro-opioid receptor antagonist CTAP (D-phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) or the kappa-opioid antagonist nor-binaltorphimine. Moreover, blockade of either acetylcholine muscarinic, dopamine D(2) or serotonin 5HT(1A) receptors failed to affect NDMC agonist activity. These data demonstrate that at clinically relevant concentrations NDMC behaves as an efficacious agonist at delta-opioid receptors of human frontal cortex.

Synthesis and in vitro opioid receptor functional antagonism of analogues of the selective kappa opioid receptor antagonist (3R)-7-hydroxy-N-((1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic)

In previous structure-activity relationship (SAR) studies, we identified (3 R)-7-hydroxy- N-((1 S)-1-{[(3 R,4 R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic, 1) as the first potent and selective kappa opioid receptor antagonist from the trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine class of opioid antagonist. In the present study, we report the synthesis and in vitro opioid receptor functional antagonism of a number of analogues of 1 using a [ (35) S]GTPgammaS binding assay. The results from the studies better define the pharmacophore for this class of kappa opioid receptor antagonist and has identified new potent and selective kappa antagonist. (3 R)-7-Hydroxy- N-[(1 S,2 S)-1-{[(3 R,4 R)-4-(3-hydroxyphenyl)-3,4-dimethylpiperidin-1-yl]methyl}-2-methylbutyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxamide ( 3) with a K e value of 0.03 nM at the kappa receptor and 100- and 793-fold selectivity relative to the mu and delta receptors was the most potent and selective kappa opioid receptor antagonist identified.

Depersonalization disorder: pharmacological approaches

Depersonalization disorder (DPD) is a chronic and distressing condition with a prevalence in the general population between 0.8 and 2%. Several neurobiological studies in the last decade have shown that patients have suppressed limbic activation to emotional stimuli. Such findings are in line with a model which suggests that the condition is generated by an anxiety-triggered, 'hard-wired' inhibitory response to threat. Such a mechanism would ensure the preservation of adaptive behavior, during situations normally associated with overwhelming and potentially disorganizing anxiety. In DPD, such a response would become chronic and dysfunctional. Depersonalization remains a condition for which no definitive treatment exists, and for which conventional medications, such as antidepressants or antipsychotics, have been found to be of little value. Fortunately, a few promising lines of pharmacological treatment have emerged in recent years, although more rigorous studies are needed. For example, a number of studies suggest that opioid receptor antagonists such as naltrexone and naloxone are useful in at least a subgroup of patients. In spite of initial expectations, the use of lamotrigine as a sole medication has not been found useful. However, open-label trials suggest that its use as an add-on treatment with selective serotonin reuptake inhibitors (SSRIs) is beneficial in a substantial number of patients. Similarly, the use of clonazepam, particularly in conjunction with SSRI antidepressants, appears to be beneficial in patients with high levels of background anxiety. In line with the stress-related model of depersonalization, those neurotransmitter systems of relevance to depersonalization are known to play important inhibitory roles in the regulation of the stress response.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Long-acting kappa opioid antagonists disrupt receptor signaling and produce noncompetitive effects by activating c-Jun N-terminal kinase

Norbinaltorphimine (NorBNI), guanidinonaltrindole, and atrans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) piperidine (JDTic) are selective kappa opioid receptor (KOR) antagonists having very long durations of action in vivo despite binding non-covalently in vitro and having only moderately high affinities. Consistent with this, we found that antagonist treatment significantly reduced the subsequent analgesic response of mice to the KOR agonist U50,488 in the tail-withdrawal assay for 14-21 days. Receptor protection assays were designed to distinguish between possible explanations for this anomalous effect, and we found that mice pretreated with the readily reversible opioid antagonists naloxone or buprenorphine before norBNI responded strongly in the tail-flick analgesia assay to a subsequent challenge with U50,488 1 week later. Protection by a rapidly cleared reagent indicates that norBNI did not persist at the site of action. In vitro binding of [(3)H]U69,593 to KOR showed that K(d) and Bmax values were not significantly affected by prior in vivo norBNI exposure, indicating that the agonist binding site was intact. Consistent with the concept that the long-lasting effects might be caused by a functional disruption of KOR signaling, both norBNI and JDTic were found to stimulate c-Jun N-terminal kinase (JNK) phosphorylation in HEK293 cells expressing KOR-GFP but not in untransfected cells. Similarly, norBNI increased phospho-JNK in both the striatum and spinal cord in wild type mice but not in KOR knock-out mice. Pretreatment of mice with the JNK inhibitor SP600125 before norBNI attenuated the long acting antagonism. Together, these results suggest that the long duration KOR antagonists disrupt KOR signaling by activating JNK.

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.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Effect of GNTI, a kappa opioid receptor antagonist, on MK-801-induced hyperlocomotion and stereotypy in mice

AIM

To examine the effect of GNTI [5'-guanidinyl-17-(cyclopropylmethyl)-6,7- dehydro-4,5alpha-epoxy-3,14-dihydroxy-6,7-2',3'-indolomorphinan], a selective antagonist for the kappa opioid receptor, in the MK-801 (dizocilpine maleate)-induced behavioral model of psychosis in schizophrenia as a way to explore the involvement of the kappa opioid receptor in modulating psychotic symptoms of schizophrenia.

METHODS

Two doses of MK-801 (0.3 mg/kg and 0.6 mg/kg) were administered by systemic injection in mice to induce psychosis-like behavior as a rodent schizophrenia model, preceded by an injection of different doses of GNTI. Both locomotion and stereotypy were measured as the behavioral endpoints for quantitative analysis.

RESULTS

GNTI inhibited MK-801-induced hyperlocomotion and stereotypy. In particular, GNTI showed differential modulation of stereotypy induced by 0.3 mg/kg vs 0.6 mg/kg MK-801.

CONCLUSION

Antagonism of kappa opioid receptors attenuates MK-801-induced behavior, suggesting a potential involvement of the kappa opioid receptor in psychosis-like symptoms of schizophrenia. GNTI appears to be a useful pharmacological tool to explore the kappa opioid receptor function in vivo.

A combined ligand-based and target-based drug design approach for G-protein coupled receptors: application to salvinorin A, a selective kappa opioid receptor agonist

Combined ligand-based and target-based drug design approaches provide a synergistic advantage over either method individually. Therefore, we set out to develop a powerful virtual screening model to identify novel molecular scaffolds as potential leads for the human KOP (hKOP) receptor employing a combined approach. Utilizing a set of recently reported derivatives of salvinorin A, a structurally unique KOP receptor agonist, a pharmacophore model was developed that consisted of two hydrogen bond acceptor and three hydrophobic features. The model was cross-validated by randomizing the data using the CatScramble technique. Further validation was carried out using a test set that performed well in classifying active and inactive molecules correctly. Simultaneously, a bovine rhodopsin based "agonist-bound" hKOP receptor model was also generated. The model provided more accurate information about the putative binding site of salvinorin A based ligands. Several protein structure-checking programs were used to validate the model. In addition, this model was in agreement with the mutation experiments carried out on KOP receptor. The predictive ability of the model was evaluated by docking a set of known KOP receptor agonists into the active site of this model. The docked scores correlated reasonably well with experimental pK (i) values. It is hypothesized that the integration of these two independently generated models would enable a swift and reliable identification of new lead compounds that could reduce time and cost of hit finding within the drug discovery and development process, particularly in the case of GPCRs.

3-(4-Piperidinyl)indoles and 3-(4-piperidinyl)pyrrolo-[2,3-b]pyridines as ligands for the ORL-1 receptor

A novel series of indoles and 1H-pyrrolo[2,3-b]pyridines having a piperidine ring at the 3-position were synthesized and found to bind with high affinity to the ORL-1 receptor. Structure-activity relationships at the piperidine nitrogen were investigated in each series. Substitution on the phenyl ring and nitrogen atom of the indole and 1H-pyrrolo[2,3-b]pyridine cores generated several selective high-affinity ligands that were agonists of the ORL-1 receptor.