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)

Catalytic Asymmetric Benzylation of Azomethine Ylides Enabled by Synergistic Lewis Acid/Palladium Catalysis

The synergistic chiral Lewis acid/achiral Pd catalyst system was successfully applied in the enantioselective benzylation of various imine esters, giving a range of α-benzyl-substituted α-amino acid derivatives in satisfactory yield with excellent enantioselectivity. It is worth noting that this strategy exhibits good tolerance for bicyclic and monocyclic benzylic electrophiles. Furthermore, the utility of this synthetic protocol was demonstrated by the expedient preparation of enantioenriched antihypertensive drug α-methyl-l-dopa.

Synthesis and preliminary evaluation of 4-hydroxy-6-(3-[11C]methoxyphenethyl)pyridazin-3(2H)-one, a 11C-labeled d-amino acid oxidase (DAAO) inhibitor for PET imaging

Selective DAAO inhibitors have demonstrated promising therapeutic effects in clinical studies, including clinically alleviating symptoms of schizophrenic patients and ameliorating cognitive function in Alzheimer's patients with early phase. Herein we report the synthesis and preliminary evaluation of a ¹¹C-labeled positron emission tomography ligand based on a DAAO inhibitor, DAO-1903 (8). ¹¹C-Isotopologue of 8 was prepared in high radiochemical yield with high radiochemical purity (>99%) and high molar activity (>37 GBq/µmol). In vitro autoradiography studies indicated that the ligand possessed high in vitro specific binding to DAAO, while in vivo dynamic PET studies demonstrated that [¹¹C]8 failed to cross the blood-brain barrier possibly due to moderate brain efflux mechanism. Further chemical scaffold optimization is necessary to overcome limited brain permeability and improve specific binding.

Potent and Selective Tetrahydroisoquinoline Kappa Opioid Receptor Antagonists of Lead Compound (3 R)- N-[1 R)-1-(Cyclohexylmethyl)-2-methylpropyl]-7-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamide (CDTic)

Animal pharmacological studies suggest that potent and selective κ opioid receptor antagonists have potential as pharmacotherapies targeting depression, anxiety, and substance abuse (opiates, alcohol, nicotine, cocaine). We recently reported lead compound 1 as a new class of κ opioid receptor antagonists with only one basic amine group. Analogues were synthesized and evaluated for their in vitro opioid receptor antagonist properties using a [³⁵S]GTPγS binding assay. All analogues were pure opioid receptor antagonists with no agonist activity. Compounds 1, 8, 9, 13, and 14 ( K_e values 0.058-0.64 nM) are highly potent and highly selective for the κ relative to the μ and δ opioid receptors. Favorable calculated physiochemical properties were confirmed in rat PK studies, demonstrating brain penetration for selected compounds 1, 9, and 13. High κ opioid receptor potency and selectivity and highly favorable calculated physiochemical and PK properties for brain penetration suggest these compounds should be considered for further development.

Pharmacodynamic Relationships between Duration of Action of JDTic-like Kappa-Opioid Receptor Antagonists and Their Brain and Plasma Pharmacokinetics in Rats

JDTic is a potent and selective κ-opioid receptor (KOR) antagonist that reverses U50,488-induced diuresis in rats. It partitions into brain with a duration of action lasting for weeks. In a search for KOR antagonists that do not accumulate in the brain, we compared single doses of five methylated JDTic analogs (RTI-97, -194, -212, -240, and -241) for reversal of U50,488 diuresis and pharmacokinetic (PK) properties. All six compounds showed potent and selective KOR antagonism in a [³⁵S]GTPγS binding assay. Plasma half-lives ranged from 24 to 41 h and brain half-lives from 24 to 76 h. JDTic and RTI-194 showed increasing brain to plasma ratios over time, indicating increasing partitioning into brain and a longer duration of action for reversal of diuresis than did RTI-97. RTI-240 did not show significant brain accumulation. RTI-212 showed no substantive difference between brain and plasma levels and was inactive against diuresis. RTI-241, with a lower brain to plasma ratio than JDTic and RTI-194, formed JDTic as a metabolite, which still reduced diuresis after 9 weeks. The fact that the duration of action was correlated with the brain to blood plasma ratios and area under the concentration-time curves suggests that PK properties could help to predict safety and acceptable duration of action for KOR antagonists.

Diversity-oriented synthesis of medicinally important 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) derivatives and higher analogs

1,2,3,4-Tetrahydroisoquinoline-3-carboxylic acid (Tic) is a constrained analog of phenylalanine (Phe). The Tic unit has been identified as a core structural element present in several peptide-based drugs and forms an integral part of various biologically active compounds. This report covers the biological significance of the Tic core and provides a detailed account of various synthetic approaches available for the construction of Tic derivatives. Along with the traditional methods such as the Pictet-Spengler and Bischler-Nepieralski reactions, we cover various recent approaches such as enyne metathesis, [2 + 2 + 2] cycloaddition and the Diels-Alder reaction to generate Tic derivatives. In addition, syntheses of higher analogs of Tic are also discussed.

Novel fluoroalkyl derivatives of selective kappa opioid receptor antagonist JDTic: Design, synthesis, pharmacology and molecular modeling studies

Novel N- and O-fluoroalkyl derivatives of the highly potent KOR antagonist JDTic were designed and synthesized. Their opioid receptor properties were compared in both in vitro binding assays and modeling approach. All compounds displayed nanomolar affinities for KOR. The fluoropropyl derivatives were more active than their fluoroethyl analogues. N-Fluoroalkylation was preferable to O-alkylation to keep a selective KOR binding. Compared to JDTic, the N-fluoropropyl derivative 2 bound to KOR with an only 4-fold lower affinity and a higher selectivity relative to MOR and DOR [Ki(κ) = 1.6 nM; Ki(μ)/Ki(κ) = 12; Ki(δ)/Ki(κ) = 159 for 2versus Ki(κ) = 0.42 nM; Ki(μ)/Ki(κ) = 9; Ki(δ)/Ki(κ) = 85 for JDTic]. Modeling studies based on the crystal structure of the JDTic/KOR complex revealed that fluorine atom in ligand 2 was involved in specific KOR binding. Ligand 2 was concluded to merit further development for KOR exploration.

The discovery and development of the N-substituted trans-3,4-dimethyl-4-(3'-hydroxyphenyl)piperidine class of pure opioid receptor antagonists

N-Substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines are a class of pure opioid receptor antagonists with a novel pharmacophore. This opioid receptor antagonist pharmacophore was used as a lead structure to design and develop several interesting and useful opioid receptor antagonists. In this review we describe: 1) early SAR studies that led to the discovery of LY255582 and analogues that are nonselective opioid receptor antagonists developed for the treatment of obesity; 2) the discovery and commercialization of LY246736 (alvimopan; ENTEREG®), a peripherally selective opioid receptor antagonist that accelerates the time to upper and lower GI recovery following surgeries that include partial bowel resection with primary anastomosis; and 3) the discovery and development of the potent and selective κ opioid receptor antagonist JDTic and analogues as potential pharmacotherapies for treating depression, anxiety, and substance abuse (nicotine, alcohol, and cocaine). In addition, the use of JDTic for obtaining the X-ray structure of the human κ opioid receptor is discussed.

Antagonists of the kappa opioid receptor

The research community has increasingly focused on the development of OPRK antagonists as pharmacotherapies for the treatment of depression, anxiety, addictive disorders and other psychiatric conditions produced or exacerbated by stress. Short-acting OPRK antagonists have been recently developed as a potential improvement over long-acting prototypic ligands including nor-BNI and JDTic. Remarkably the short-acting LY2456302 is undergoing phase II clinical trials for the augmentation of the antidepressant therapy in treatment-resistant depression. This Letter reviews relevant chemical and pharmacological advances in the identification and development of OPRK antagonists.

Characterization of BU09059: a novel potent selective κ-receptor antagonist

Kappa-opioid receptor (κ) antagonists are potential therapeutic agents for a range of psychiatric disorders. The feasibility of developing κ-antagonists has been limited by the pharmacodynamic properties of prototypic κ-selective antagonists; that is, they inhibit receptor signaling for weeks after a single administration. To address this issue, novel trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) piperidine derivatives, based on JDTic, were designed using soft-drug principles. The aim was to determine if the phenylpiperidine-based series of κ-antagonists was amenable to incorporation of a potentially metabolically labile group, while retaining good affinity and selectivity for the κ-receptor. Opioid receptor binding affinity and selectivity of three novel compounds (BU09057, BU09058, and BU09059) were tested. BU09059, which most closely resembles JDTic, had nanomolar affinity for the κ-receptor, with 15-fold and 616-fold selectivity over μ- and δ-receptors, respectively. In isolated tissues, BU09059 was a potent and selective κ-antagonist (pA2 8.62) compared with BU09057 (pA2 6.87) and BU09058 (pA2 6.76) which were not κ-selective. In vivo, BU09059 (3 and 10 mg/kg) significantly blocked U50,488-induced antinociception and was as potent as, but shorter acting than, the prototypic selective κ-antagonist norBNI. These data show that a new JDTic analogue, BU09059, retains high affinity and selectivity for the κ-receptor and has a shorter duration of κ-antagonist action in vivo.

Enantioselective Synthesis of Tetrahydroquinolines, Tetrahydroquinoxalines, and Tetrahydroisoquinolines via Pd-Catalyzed Alkene Carboamination Reactions

A catalyst composed of Pd₂(dba)₃ and (S)-Siphos-PE provides excellent results in Pd-catalyzed alkene carboamination reactions between aniline derivatives bearing pendant alkenes and aryl or alkenyl halides. These transformations generate tetrahydroquinolines and tetrahydroquinoxalines that contain quaternary carbon stereocenters with high levels of asymmetric induction. In addition this catalyst also effects the asymmetric synthesis of tetrahydroisoquinolines via related transformations of 2-allylbenzylamines. In contrast to most other approaches to the asymmetric synthesis of these compounds, which frequently involve functional group interconversion or a single C-C or C-N bond-forming event, the carboamination reactions generate both a C-N bond, a C-C bond, and a stereocenter.

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.

Duration of action of a broad range of selective κ-opioid receptor antagonists is positively correlated with c-Jun N-terminal kinase-1 activation

The κ-opioid receptor is a widely expressed G-protein-coupled receptor that has been implicated in biological responses to pain, stress, anxiety, and depression, and its potential as a therapeutic target in these syndromes is becoming increasingly apparent. However, the prototypical selective κ-opioid antagonists have very long durations of action that have been attributed to c-Jun N-terminal kinase (JNK) 1 activation in vivo. To test generality of this proposed noncompetitive mechanism, we used C57BL/6 wild type mice to determine the durations of antagonist action of novel κ-opioid receptor ligands and examined their efficacies for JNK1 activation compared with conventional competitive antagonists. Of the 12 compounds tested, 5 had long durations of action that positively correlated with JNK activation: RTI-5989-97 [(3S)-7-hydroxy-N-[(1S)-1-[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-(2-methylpropyl]-2-methyl-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide], RTI-5989-194 [(3R)-7-hydroxy-N-[(1S)-1-[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-(2-methylbutyl]-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide], RTI-5989-241 [(3R)-7-hydroxy-N-[(1S)-1-{[(3R,4R)-4-(3-methoxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl]-1,2,3,4-tetrahydroisoquinoline-3-carboxamide)], nor-binaltorphimine (nor-BNI); and (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). Seven had short durations of action and did not increase phospho-JNK-ir: RTI-5989-212[(3R)-N-[(1S)-1-[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-(2-methylpropyl]-7-methoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamide], RTI-5989-240 [(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], JSPA0658 [(S)-3-fluoro-4-(4-((2-(3,5-dimethylphenyl)pyrrolidin-1-yl)methyl)phenoxy)benzamide], JSPA071B [(S)-3-fluoro-4-(4-((2-(3,5-bis(trifluoromethyl)phenyl)pyrrolidin-1-yl)methyl)phenoxy)benzamide]. PF-4455242 [2-methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine], PF-4455242 [2-methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine], FP3FBZ [(S)-3-fluoro-4-(4-((2-(3-fluorophenyl)pyrrolidin-1-yl)methyl)phenoxy)benzamide], and naloxone. After long-acting antagonist treatment, pJNK-ir did not increase in mice lacking the κ-opioid receptor; increased pJNK-ir returned to baseline by 48 h after treatment; and a second challenge with nor-BNI 72 h after the first did not increase pJNK-ir. Long-lasting antagonism and increased phospho-JNK-ir were not seen in animals lacking the JNK1 isoform. These results support the hypothesis that the duration of action of small molecule κ-opioid receptor antagonists in vivo is determined by their efficacy in activating JNK1 and that persistent inactivation of the κ-receptor does not require sustained JNK activation.

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.

Effectiveness of analogs of the 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) to reduce U50,488-induced diuresis and stress-induced cocaine reinstatement in rats

RATIONALE

The kappa opioid receptor (KOR) antagonist, JDTic, was reported to prevent stress-induced reinstatement of cocaine-maintained responding and to have antidepressant-like effects.

OBJECTIVES

Our objectives were to determine whether analogs of JDTic retained KOR antagonist activity and whether an orally effective analog prevented footshock-induced cocaine reinstatement.

METHODS

RTI-194 (i.g. 1-30 mg/kg, s.c. 0.3-10 mg/kg, and i.p. 30 mg/kg), RTI-212 (s.c. 0.3-10 mg/kg and i.p. 30 mg/kg), and RTI-230 (i.g. 3-30 mg/kg and i.p. 1-30 mg/kg) were evaluated for their ability to block diuresis induced by 10-mg/kg U50,488 in rats. RTI-194 was additionally evaluated i.g. (3-100 mg/kg) for its ability to prevent footshock-induced reinstatement of responding previously reinforced with 0.5-mg/kg/inf cocaine.

RESULTS

RTI-194 significantly (p < 0.05) attenuated U50,488-induced diuresis when given i.g., s.c., and i.p. RTI-194s effectiveness increased 1 week following administration. RTI-212 was ineffective. RTI-230 was ineffective when given i.g., but blocked diuresis at 24 h and 8 days (1, 10, and 30 mg/kg), 15 days (10 and 30 mg/kg), 22 and 29 days (30 mg/kg) following i.p. administration. Footshock reinstated responding in vehicle-but not RTI-194 (30 and 100 mg/kg)-treated rats.

CONCLUSIONS

RTI-194 and RTI-230 are effective KOR antagonists, and RTI-194 is now included with JDTic as the only reported compounds capable of antagonizing the KOR following oral administration. The failure of stress to reinstate cocaine seeking in rats treated with RTI-194 is consistent with results reported with JDTic, although it had less efficacy in lowering response levels than JDTic, suggesting a diminished overall effectiveness relative to it.