Design synthesis and structure–activity relationship of 5-substituted (tetrahydronaphthalen-2yl)methyl with N-phenyl-N-(piperidin-2-yl)propionamide derivatives as opioid ligands

Here, we report the design, synthesis and structure activity relationship of novel small molecule opioid ligands based on 5-amino substituted (tetrahydronaphthalen-2-yl)methyl moiety with N-phenyl-N-(piperidin-2-yl)propionamide derivatives. We synthesized various molecules including amino, amide and hydroxy substitution on the 5th position of the (tetrahydronaphthalen-2-yl)methyl moiety. In our further designs we replaced the (tetrahydronaphthalen-2-yl)methyl moiety with benzyl and phenethyl moiety. These N-phenyl-N-(piperidin-2-yl)propionamide analogues showed moderate to good binding affinities (850–4 nM) and were selective towards the μ opioid receptor over the δ opioid receptors. From the structure activity relationship studies, we found that a hydroxyl substitution at the 5th position of (tetrahydronapthalen-2yl)methyl group, ligands 19 and 20, showed excellent binding affinities 4 and 5 nM, respectively, and 1000 fold selectivity towards the μ opioid relative to the delta opioid receptor. The ligand 19 showed potent agonist activities 75 ± 21 nM, and 190 ± 42 nM in the GPI and MVD assays. Surprisingly the fluoro analogue 20 showed good agonist activities in MVD assays 170 ± 42 nM, in contrast to its binding affinity results.     

Synthesis of a novel universal opioid receptor agonist with the 1,3,5-trioxazatriquinane skeleton and its pharmacologies

We designed and synthesized of 1,3,5-trioxazatriquinanes with o- or p-hydroxyphenyl rings as analogs of the κ opioid receptor agonist SYK-146 with m-hydroxyphenyl groups. Although almost all tested compounds did not bind to the opioid receptors, only 17b (SYK-524) with two o-hydroxyphenyl rings showed moderate or potent binding affinities and exhibited agonistic activities for the three opioid receptor types. Because the basicity of the nitrogen atom in the 1,3,5-trioxazatriquinane structure was predicted to be very low due to the electron withdrawing effect of the three oxygen atoms, SYK-524 was a novel non-morphinan and nonpeptidic opioid universal agonist lacking a basic nitrogen atom.     

Design and synthesis of quinolinopropellane derivatives with selective δ opioid receptor agonism

Indolopropellane 2 was reported to show almost no binding affinity to the δ opioid receptor (DOR) in spite of the fact that 2 has both the propellane fundamental skeleton (message part) with binding ability to the opioid receptors and a possible DOR address structure (indole moiety). We developed the working hypothesis that almost no binding affinity of 2 to the DOR would be derived from its possibly stable bent conformer. To enable the propellane skeleton to adopt an extended conformation which would reasonably interact with the DOR, quinolinopropellanes 3a–d were designed which had an additional pharmacophore, quinoline nitrogen. The calculated binding free energies of ligand–DOR complexes strongly supported our working hypothesis. The synthesized quinolinopropellane 3a was a selective DOR full agonist, confirming our working hypothesis and the results of in silico investigation.     

Synthesis and biological evaluation of C-12 triazole and oxadiazole analogs of salvinorin A

Salvinorin A (1), the main active ingredient of Salvia divinorum, is a potent and selective κ-opioid receptor (KOPR) agonist. A series of C-12 triazole analogs and the oxadiazole (4) analog of 1 are synthesized and screened for binding affinity at κ, μ (MOPR), or δ (DOPR). Surprisingly, all triazole analogs have shown negligible binding affinity at opioid receptors and the oxadiazole 4, a reported MOPR and KOPR antagonist, exhibits very low affinities to opioid receptors and no antagonism in our binding assays. These results suggest that electronic factors that may affect either the electron density of hydrogen bond acceptor at C-12 or hydrophobic interactions between C-12 moiety and KOPR are critical to C-12 analog’s affinity for KOPR.     

Discovery of δ opioid receptor full agonists lacking a basic nitrogen atom and their antidepressant-like effects

We have recently reported that the elaboration of the N-substituent in the δ opioid receptor (DOR) antagonist naltrindole (NTI) enabled the regulation of the DOR activities from full inverse agonists to weak partial agonists. The investigations of amide-type NTI derivatives revealed that N-phenylacetyl and N-dihydrocinnamoyl derivatives 3a and 3b were DOR full agonists. The same transformations were applied to a DOR agonist KNT-127 to provide the more potent DOR agonists 6a and 6b. Among the tested compounds, the most efficacious compound 6a showed dose-dependent antidepressant-like effects in the mouse forced swim test. The antidepressant-like effects by 6a seemed to be more potent than those of KNT-127, which is a more potent DOR agonist in in vitro assays. The amide-type compound like 6a may more fully penetrate into the central nervous system.     

Synthesis and characterizations of novel quinoline derivatives having mixed ligand activities at the κ and μ receptors: Potential therapeutic efficacy against morphine dependence

Based on an established 3D pharmacophore, a series of quinoline derivatives were synthesized. The opioidergic properties of these compounds were determined by a competitive binding assay using ¹²⁵I-Dynorphine, ³H-DAMGO and ¹²⁵I-DADLE for κ, μ, and δ receptors, respectively. Results showed varying degree of activities of the compounds to κ and μ opioid receptors with negligible interactions at the δ receptor. The compound, S4 was the most successful in inhibiting the two most prominent quantitative features of naloxone precipitated withdrawal symptoms - stereotyped jumping and body weight loss. Determination of IC₅₀ of S4 revealed a greater affinity towards μ compared to κ receptor. In conclusion, quinoline derivatives of S4 like structure offer potential tool for treatment of narcotic addictions.     

Synthesis of quinolinomorphinan-4-ol derivatives as δ opioid receptor agonists

The previously reported morphinan derivative SN-28 showed high selectivity and agonist activity for the δ opioid receptor. In the course of examining the structure-activity relationship of SN-28 derivatives, the derivatives with the 4-hydroxy group (SN-24, 26, 27) showed higher selectivities for the δ receptor over the μ receptor than the corresponding SN-28 derivatives with the 3-hydroxy group (SN-11, 23, 28). Derivatives with the 4-hydroxy group showed potent agonist activities for the δ receptor in the [(35)S]GTPγS binding assay. Although the 17-cyclopropylmethyl derivative (SN-11) with a 3-hydroxy group showed the lowest selectivity for the δ receptor among the morphinan derivatives, the agonist activity toward the δ receptor was the most potent for candidates with the 3-hydroxy group.     

14-O-Heterocyclic-substituted naltrexone derivatives as non-peptide mu opioid receptor selective antagonists: Design,synthesis, and biological studies

Mu opioid receptor antagonists have clinical utility and are important research tools. To develop non-peptide and highly selective mu opioid receptor antagonist, a series of 14-O-heterocyclic-substituted naltrexone derivatives were designed, synthesized, and evaluated. These compounds showed subnanomolar-to-nanomolar binding affinity for the mu opioid receptor. Among them, compound 1 exhibited the highest selectivity for the mu opioid receptor over the delta and kappa receptors. These results implicated an alternative ‘address’ domain in the extracellular loops of the mu opioid receptor.     

Discovery of novel steroidal histamine H3 receptor antagonists/inverse agonists

Emerging from an HTS campaign, novel steroid-based histamine H₃ receptor antagonists were identified and characterized. Structural moieties of the hit compounds were combined to improve binding affinities which resulted in compound 4 as lead molecule. During the lead optimization due to the versatile modifications of diamino steroid derivatives, several in vitro potent compounds with subnanomolar binding affinities to histamine H₃ receptors were found. The unfavorable binding to rat muscarinic receptors was successfully reduced by tuning the basicity. Compound 20 showed significant in vivo activity in the rat dipsogenia model and could serve as a pharmacological tool in the future.     

Discovery of attenuation effect of orexin 1 receptor to aversion of nalfurafine: Synthesis and evaluation of D-nor-nalfurafine derivatives and analyses of the three active conformations of nalfurafine

The D-nor-nalfurafine derivatives, which were synthesized by contraction of the six-membered D-ring in nalfurafine (1), had no affinity for orexin 1 receptors (OX₁Rs). The 17N-lone electron pair in 1 oriented toward the axial direction, while that of D-nor-derivatives was directed in the equatorial configuration. The axial lone electron pair can form a hydrogen bond with the 14-hydroxy group, which could push the 6-amide side chain toward the downward direction with respect to the C-ring. The resulting conformation would be an active conformation for binding with OX₁R. The dual affinities of 1 for OX₁R and κ opioid receptor (KOR) led us to elucidate the mechanism by which only 1 showed no aversion but U-50488H. Actually, 1 selectively induced severe aversion in OX₁R knockout mice, but not in wild-type mice. These results well support that OX₁R suppresses the aversion of 1. This is the elucidation of long period puzzle which 1 showed no aversion in KOR.     

Aerobic oxidation of indolomorphinan without the 4,5-epoxy bridge and subsequent rearrangement of the oxidation product to spiroindolinonyl-C-normorphinan derivative

Aerobic oxidation of indolomorphinan 1 without a 4,5-epoxy bridge proceeded in the presence of platinum catalyst to give indoleninomorphinan 2 or quinolono-C-normorphinan 5. The 4-hydroxy group would play an important role in deciding the course of the reaction. Treatment of 2a with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave spiroindolinonyl-C-normorphinan 3a whose structure resembles that of δ opioid receptor agonist spiroindanyloxymorphone (SIOM). Boron trichloride was effective for the reverse reaction from 3a to 2a without side reaction. This practical interconversion method between hydroxyindolenine and spiroindolinone would be useful for the design and construction of drug-like compound libraries. Although the compound 3b was expected to show the selectivity for δ opioid receptor because of the structural resemblance to SIOM, it was rather selective for μ opioid receptor (μ: K_i = 0.75 nM; δ: K_i = 2.90 nM; κ: K_i = 13.4 nM). The result suggests that the slight difference of the spatial location of the benzene rings in these compounds may definitively affect the binding affinity for δ opioid receptor.     

Design,synthesis, and characterization of 6β-naltrexol analogs,and their selectivity for in vitro opioid receptor subtypes

Since the mu opioid receptor (MOR) is known to be involved in the therapeutically relevant pathways leading to the manifestation of pain and addiction, we are currently studying the specific structural characteristics that promote antagonism at the MOR. The opiates 6β-naltrexol and 6β-naltrexamide function as neutral antagonists in in vitro and in vivo systems previously exposed to morphine, and are under investigation as improved treatments for narcotic dependence. In this research, we synthesized and characterized carbamate and sulfonate ester derivates of 6β-naltrexol that do not contain a protic group at C₆, and evaluated these compounds for opioid receptor affinity. In vitro receptor subtype (μ, κ, and δ opioid receptors) binding data of the carbamate and sulfonate derivatives is reported. All four compounds synthesized exhibited affinity for the MOR better than the standard 6β-naltrexol HCl. Based on K_i data, the order of MOR affinity is as follows: 9 > 13 > 14 > 10 > 6β-naltrexol HCl. Carbamate 9 and tosylate 13 displayed subnanomolar affinity for the MOR, while 10 was the most μ-selective compound synthesized. In conclusion, our data indicate that the absence of a hydrogen-bond donor on the C₆ oxygen enhances rather than impedes the in vitro affinity of naltrexol derivatives for the MOR. Additionally, data also suggest that increasing the bulk around C₆ may allow control of subtype selectivity within these compound series.     

Novel delta opioid receptor agonists with oxazatricyclodecane structure showing potent agonistic activities

We recently reported oxazatricyclodecane derivatives 1 as δ opioid receptor (DOR) agonists having a novel chemotype, but their DOR agonistic activities were relatively low. Based on the working hypothesis that the dioxamethylene moiety in 1 may be an accessory site and that it may interfere with the sufficient conformational change of the receptor required for exerting the full agonistic responses, we designed and synthesized new oxazatricyclodecane derivatives 2–4 lacking the dioxamethylene moiety. As we expected, the designed compounds 2–4 showed pronouncedly improved agonistic activities for the DOR. Compound 2a with the 17-cyclopropylmethyl substituent was a potent agonist with the highest selectivity for the DOR and was expected to be a lead compound for novel and selective DOR agonists.     

Redefining the structure–activity relationships of 2,6-methano-3-benzazocines. Part 9: Synthesis,characterization and molecular modeling of pyridinyl isosteres of N-BPE-8-CAC (1), a high affinity ligand for opioid receptors

Derivatives of the lead compound N-BPE-8-CAC (1) where each CH of the biphenyl group was individually replaced by N were prepared in hopes of identifying high affinity ligands with improved aqueous solubility. Compared to 1, binding affinities of the five possible pyridinyl derivatives for the μ opioid receptor were between threefold lower to fivefold higher with the K_i of the most potent compound being 0.064 nM. Docking of 8-CAC (2) into the unliganded binding site of the mouse μ opioid receptor (pdb: 4DKL) revealed that 8-CAC and β-FNA (from 4DKL) make nearly identical interactions with the receptor. However, for 1 and the new pyridinyl derivatives 4–8, binding is not tolerated in the 8-CAC binding mode due to the steric constraints of the large N-substituents. Either an alternative binding mode or rearrangement of the protein to accommodate these modifications may account for their high binding affinity.     

Synthesis and opioid receptor activity of indolopropellanes

A series of skeletal rearranged indolomorphinans 7a–d were obtained by N-demethylation of 3-methoxy-N-methyl-14-hydroxymorphinan-6-one 12 followed by N-realkylation, reduction and Fischer indole cyclization. The structure of the novel skeleton was confirmed by X-ray analysis. These new indoles displayed moderate binding affinity and selectivity at the μ receptor, with compound 7b showing the highest affinity at this receptor with a K_i value of 40 nM, and 6- and 25-fold selectivity against δ and κ receptors, respectively. Function assays showed that indolopropellanes 7b and 7c possessed full agonistic activity at all the opioid receptors indicating a different interaction model existed.     

N-Alkyl dien- and trienamides from the roots of Otanthus maritimus with binding affinity for opioid and cannabinoid receptors

Two new thienylheptatrienamides (1, 5) and one new neo-lignan (12), together with thirteen known compounds (2, 3, 4, 6–11, 13–16) were isolated from the roots of Otanthus maritimus. The structures of the new compounds were elucidated on the basis of extensive 1D and 2D NMR experiments as well as high resolution mass spectrometry. All the isolated amides (1–10), the known pontica epoxide (11) and the new neo-lignan (12) were evaluated for their binding affinity to the CB1 and CB2 as well as to the μ and δ opioid receptors. Some alkylamides showed moderately high binding affinity for CB2 receptors and 1-[(2E,4E,8Z)-tetradecatrienoyl]piperidine (10) resulted the most active one with a K_i value of 160 nM. As far as we know, this is the first example of a tertiary alkylamide that binds CB2 receptors with significant potency. Compounds that showed the highest affinity for cannabinoid receptors (6–8, 10) were much less potent against opioid receptors. Primary structure–activity relationship is discussed. Docking experiments were carried out with the aim to understand the key interactions of the most active compounds with CB2 receptor.     

Investigation of Beckett–Casy model 3: Synthesis of novel naltrexone derivatives with contracted and expanded D-rings and their pharmacology

Novel naltrexone derivatives 7 and 8 with contracted and expanded D-rings were synthesized to investigate the importance of orientation of lone electron pair on the nitrogen for binding abilities to the opioid receptor. Compound 7 showed almost no binding affinity, whereas compound 8 was comparable to naltrexone (6) in binding affinity. Conformational analyses and NOE experiments in D₂O of compounds 6–8 suggested that the lone electron pairs of compounds 6 and 8 with respective six- and seven-membered D-rings would project in the pseudo-axial orientation, whereas compound 7 with five-membered D-ring would have the lone electron pair directing in pseudo-equatorial position. These results strongly supported the proposal that the axial orientation of the lone electron pair on nitrogen would provide sufficient binding abilities to the opioid receptor and that the 15–16 ethylene moiety in the morphine structure would play a role in fixation of the lone electron pair in the axial direction rather than interaction with the putative cavity in the Beckett–Casy model.     

Synthesis and evaluation of 4-substituted piperidines and piperazines as balanced affinity μ opioid receptor (MOR) agonist/δ opioid receptor (DOR) antagonist ligands

In this letter, we describe a series of 4-substituted piperidine and piperazine compounds based on tetrahydroquinoline 1, a compound that shows balanced, low nanomolar binding affinity for the mu opioid receptor (MOR) and the delta opioid receptor (DOR). We have shown that by changing the length and flexibility profile of the side chain in this position, binding affinity is improved at both receptors by a significant degree. Furthermore, several of the compounds described herein display good efficacy at MOR, while simultaneously displaying DOR antagonism. The MOR agonist/DOR antagonist has shown promise in the reduction of negative side effects displayed by selective MOR agonists, namely the development of dependence and tolerance.     

The most effective influence of 17-(3-ethoxypropyl) substituent on the binding affinity and the agonistic activity in KNT-127 derivatives, δ opioid receptor agonists

We investigated the structure–activity relationship of KNT-127 (opioid δ agonist) derivatives with various 17-substituents which are different in length and size. The 17-substituent in KNT-127 derivatives exerted a great influence on the affinity and agonistic activity for the δ receptor. While the compounds with electron-donating 17-substituents showed higher affinities for the δ receptor than those with electron-withdrawing groups, KNT-127 derivatives with 17-fluoroalkyl groups (the high electron-withdrawing groups) showed high selectivities for the δ receptor among evaluated compounds. In addition, the basicity of nitrogen as well as the structure of the 17-N substituent such as the length and configuration at an asymmetric carbon atom contributed to agonist properties for the δ receptor. Thus, the analog with a 17-(3-ethoxypropyl) group showed the best selectively and potent agonistic activity for the δ receptor among KNT-127 derivatives. These findings should be useful for designing novel δ selective agonists.     

Effect of ring-constrained phenylpropyloxyethylamines on sigma receptors

A series of ring-constrained phenylpropyloxyethylamines, partial opioid structure analogs and derivatives of a previously studied sigma (σ) receptor ligand, was synthesized and evaluated at σ and opioid receptors for receptor selectivity. The results of this study identified several compounds with nanomolar affinity at both σ receptor subtypes. Compounds 6 and 9 had the highest selectivity for both σ receptor subtypes, compared to μ opioid receptors. In addition, compounds 6 and 9 significantly reduced the convulsive effects of cocaine in mice, which would be consistent with antagonism of σ receptors.