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We synthesized derivatives of the δ opioid receptor (DOR) antagonists naltrindole (NTI) and compound 1 that were modified with small alkyl or fluorinated ethyl substituents on the 17-nitrogen. Although the derivatives showed decreased binding affinities for the opioid receptors, their selectivities for the DOR were higher than the parent compounds NTI and compound 1. Surprisingly, 17-fluoroethyl NTI derivatives exerted DOR inverse agonistic activities. The DOR inverse agonism of compounds 4c–e was less efficacious but significant, as compared with a standard DOR inverse agonist ICI-174864. On the other hand, compound 1 and its derivatives with small alkyl or monofluoroethyl substituents were partial agonists, but the derivatives having di- or trifluoroethyl group showed neither agonistic nor inverse agonistic activities.

We designed and synthesized the 1,3,5-trioxazatriquinane derivatives with m-hydroxyphenyl groups. These compounds include the phenethylamine structure within them, which is a common structure observed in morphinan derivatives like morphine. Among the synthesized compounds, (-)-8c with two m-hydroxyphenyl groups selectively bound and exerted full agonist activity toward the κ opioid receptor (KOR). Subcutaneously administered (-)-8c exhibited significant antinociceptive effects via the KOR in a dose-dependent manner. These results suggest the emergence of a novel class of KOR agonist.

We synthesized compounds 4a,c-f,h,i containing the oxazatricyclodecane structure from a novel rearrangement reaction product 2a. All the prepared compounds 4a,c-f,h,i exhibited full agonistic activities for the δ opioid receptor (DOR). Among them, the N-methyl derivative 4c was highly selective, and the most effective DOR agonist in functional assays. Subcutaneous administration of 4c produced dose-dependent and NTI (selective DOR antagonist)-reversible antinociception lacking any convulsive behaviors in the mice acetic acid writhing tests. The N-methyl derivative 4c is expected to be a promising lead compound for selective DOR agonists with a novel chemotype.

To clarify the essential structures of an opioid κ receptor selective agonist, nalfurafine, for binding to the κ receptor, we designed and synthesized the decahydro(iminoethano)phenanthrene derivatives with an oxygen functionality at the 3-position. The introduction of a hydroxy group to the derivatives increased the affinity and selectivity to the κ receptor regardless of the configuration at the 3-position. However, their affinities were lower than those of nalfurafine with the phenolic hydroxy group. The results suggested that the acidity of the hydroxy group would play an important role in the interaction with the opioid receptor. The low affinities of the 3-keto derivatives indicated that the 3-hydroxy group may participate in the hydrogen bonding with the receptor site not as a hydrogen acceptor but as a hydrogen donor. This is the first experimental evidence for a role as a hydrogen donor for the 3-hydroxy group in morphinans. Furthermore, the κ selectivities in these derivatives with the 6α-amide side chain were affected by the the 3-hydroxy group. The obtained structure-activity relationship information is expected to be useful for the design of more selective ligands for the κ receptor.

Previously reported propellane derivative KNT-42 preferred the κ receptor and functioned as a message part in the message-address concept, but its affinity for the κ receptor was not high. To improve affinity, we synthesized five pentacyclic propellane derivatives designed for the purpose of fixing the conformation of KNT-42. The etheno- and ethano-bridged derivatives SYK-347 and SYK-393 exhibited high affinity and selectivity for the κ receptor, whereas the other derivatives did not. These results would be due to the different ranges of movement of the basic nitrogens and less basicity of the nitrogens due to the electron withdrawing effect of the introduced hydroxy or keto group. SYK-347 and SYK-393 preferring the κ receptor were expected to be useful for designing selective ligands for opioid receptor types, especially the κ receptor.

To clarify the essential structures of an opioid κ receptor selective agonist, nalfurafine, for binding to the κ receptor, we designed and synthesized some nalfurafine derivatives and the decahydro(iminoethano)phenanthrene derivatives with a cyclohexene moiety as a surrogate for the phenol ring. In addition to the 6-amide side chain and the 17-nitrogen substituted by a cyclopropylmethyl group, the 4,5-epoxy ring, phenolic hydroxy group, and angular hydroxy group played important roles in eliciting the binding properties of nalfurafine but these three moieties were not indispensable for binding to the κ receptor. Moreover, the phenol ring was also not essential for the binding to the κ receptor, and the cyclohexene moiety would play an important role in fixing the conformation of decahydro(iminoethano)phenanthrene derivatives to effectively raise the amide side chain, rendering a conformation that resembled the active one of nalfurafine.

A novel opioid ligand possessing a stable and cyclic imine 16 and its derivatives with an azabicyclo[2.2.2]octane skeleton were synthesized. The imine 16 showed higher affinity for the μ receptor than compound 21 with an oxabicyclo[2.2.2]octane skeleton. Azabicyclo[2.2.2]octane derivative 18d with a cyclopropylmethyl group on the 8-nitrogen showed the highest affinity for the μ receptor among the synthesized derivatives.

On the basis of the three-dimensional pharmacophore model of opioid κ agonists, we simplified the structure of nalfurafine (selective κ agonist) to find the essential structural moieties for binding the opioid receptors, especially κ receptor type. As a result, we found that the trans-fused decahydroisoquinoline derivatives without a phenol ring bound the opioid receptor in micromolar order and that both the amide side chain and the nitrogen substituted by the cyclopropylmethyl group were indispensable moieties for eliciting the κ selectivity. The simple decahydroisoquinoline without amide side chain also bound the opioid receptor without receptor type selectivity, suggesting that the message-address concept would be applicable to even these simple derivatives. These findings that the simple decahydroisoquinoline derivatives showed the affinities for the opioid receptors, especially some of the compounds showed κ selectivity, are the first example in the opioid field.

We previously reported that the δ opioid receptor (DOP) agonists SNC80 and TAN-67 produce potent antidepressant-like and antinociceptive effects in rodents. However, SNC80 produced convulsive effects. Recently, we succeeded in synthesizing a novel DOP agonist called KNT-127. The present study examined the convulsive, antidepressant-like, and antinociceptive effects of KNT-127 in mice. In contrast to SNC80, KNT-127 produced no convulsions at doses of up to 100mg/kg. In mice subjected to the forced swim test, a screening model for antidepressants, KNT-127 (1mg/kg, s.c.) significantly decreased the duration of immobility and increased the duration of swimming without influencing spontaneous locomotor activity. These behavioral changes were similar to that observed for the tricyclic antidepressant imipramine (6mg/kg). The antidepressant-like effect of KNT-127 in mice was antagonized by pretreatment with naltrindole (NTI), a selective DOP antagonist, or naltriben, a putative DOP(2) subtype antagonist. In addition, KNT-127 (3mg/kg, s.c.) significantly reduced the number of acetic acid-induced abdominal constrictions and the duration of licking time, respectively, in mice subjected to a writhing test and a formalin test. These antinociceptive effects were antagonized by pretreatment with either NTI or 7-benzylidenenaltrexone, a putative DOP(1) subtype antagonist. We propose that KNT-127 should be considered as a candidate compound for the development of DOP-based antidepressants and/or analgesics that lack convulsive effects.

An improved synthetic method for triplet drugs with the 1,3,5-trioxazatriquinane skeleton was developed that used p-toluenesulfonylmethyl isocyanide (TosMIC) instead of 1,3-dithiane. Using the improved method, we synthesized compounds with two identical pharmacophore units and an epoxymethano group, that is, capped homotriplets. Among the synthesized capped homotriplets, KNT-123 showed high selectivity for the μ receptor over the κ receptor, and the μ selectivity was the highest among the reported μ selective nonpeptide ligands. KNT-123 administered subcutaneously induced a dose-dependent analgesic effect in the acetic acid writhing assay, and its potency was 11-fold more potent than that of morphine. KNT-123 may serve as a useful tool for the study of the pharmacological actions mediated specifically via the μ receptor.

We have reported previously the novel δ-opioid agonist, SN-28, which was more potent in in vitro assays than the prototype δ-agonists, TAN-67 and SNC-80. However, when administered by subcutaneous injection, this compound showed no analgesic effect at dosages greater than 30mg/kg in the acetic acid writhing test. We speculated that SN-28 was not effective in the test because the presence of the charged ammonium groups prevented its penetration through the blood-brain barrier. On the basis of our proposal, we designed the novel δ-agonist, KNT-127, which was effective with systemic administration.

We synthesized pyrrolomorphinan derivatives 6, 7, and 9 to examine whether the pyrrole ring would be an accessory site in the kappa opioid receptor selective antagonist, nor-binaltorphimine. Derivative 6 had an alpha,beta-unsaturated ketone substituent that strongly bound to the kappa receptor. The compound with the highest kappa receptor selectivity, 6e, produced a dose-dependent antinociceptive effect in the mouse acetic acid writhing test. However, derivatives 7 and 9, which did not have alpha,beta-unsaturated ketone substituents, showed less kappa receptor selectivity than compound 6. Based on structure-activity relationships, we proposed that these compounds adopted active structures for kappa selective agonist activity. The pyrrole ring would not function as an accessory site, but the ability of the side chain on the pyrrole ring to localize above the C-ring appeared to confer kappa selective agonist activity. These results will promote the design of novel kappa agonists.

We synthesized novel 15-16 nornaltrexone derivatives 9, 11 and 22 to examine the importance of the cavity in the Beckett-Casy model, which was proposed to interact with the 15-16 ethylene moiety in the morphine structure. All the synthesized compounds showed lower affinities for the opioid receptor than did the naltrexone (10). The binding affinities of 14-OH derivatives 11, in which the rotation of the 9-17 bond would be restricted by an intramolecular hydrogen bond, was improved compared to the corresponding 14-H derivatives 9. Compound 22 whose 9-17 bond was strictly fixed by the ethylene bridge hardly bound to the opioid receptor. Compound 26 also showed very weak binding affinity in spite of the existence of the 15-16 ethylene unit. We proposed an important role for the orientation of the lone electron pair on the 17-nitrogen rather than the significance of the cavity in the Beckett-Casy model.

A dealkylation reaction of tertiary amines using chloroformate was a useful method for synthesizing morphinan derivatives without 17-substituents; however, the reaction has been applied to only the 14-hydromorphinans. In the course of the investigation of the 17-dealkylation reaction in 14-hydroxymorphinan, the novel cleavage reaction of the C16–N17 bond in the naltrexone derivative was found. A plausible reaction mechanism based on the stereoelectronic effect is presented. The examinations of dealkylation reactions in general tertiary amines ranked the tendency of cleavage as follows: benzyl > cyclopropylmethyl (CPM) ≈ allyl > methyl, ethyl. The preferable cleavage of the CPM group may be explained by the polarization of CPM–N17 bond due to a postulation of extreme stability of the cyclopropylcarbinyl cation.

Selective ring opening reaction of the N-cyclopropylmethyl group in naltrexone (1d) was effected in the presence of platinum (IV) oxide and hydrobromic acid under a hydrogen atmosphere at rt to selectively afford N-isobutyl derivative 10. The binding affinity of N-i-Bu derivative 10 for opioid receptors was 11–17 times less than that of the corresponding N-CPM compound, naltrexone (1d). However, compound 10 showed dose-dependent analgesic effects. Contrary to expectations based on previous structure–activity relationship studies for a series of N-substituted naltrexone derivatives that compound 10 would be an opioid antagonist, 10 showed dose-dependent analgesia in the mouse acetic acid writhing test (ED50: 5.05 mg/kg, sc), indicating it was an opioid agonist. This finding may have a great influence on the drug design of opioid agonists.

An analog of nor-binaltorphimine (nor-BNI) without the 4,5-epoxy bridge, 17,17'-bis(cyclopropylmethyl)-6,6',7,7'-tetrahydro-6,6'-imino-14beta,14'alpha-dihydroxy-3,3'-dimethoxy-7,7'-bimorphinan (4), which was the precursor of the designed compound 1 as a selective kappa(3) opioid receptor antagonist, was catalytically oxidized with oxygen in the presence of platinum to give the 5'-oxo derivative 3 with some other oxidized products. Morphinan derivatives without the 4,5-epoxy moiety were labile to oxygen, although the corresponding 4,5-epoxymorphinan derivatives resisted aerobic oxidation. One of the oxidized nor-BNI analogs without 4,5-epoxy bridge, compound 18, showed high affinity and selectivity for kappa opioid receptor.

The α-iminoamide derivative, 4b was designed and synthesized as a novel agonist selective for the opioid κ receptor. The amide was constrained to an orientation horizontal to the F-ring of the azabicyclo[2.2.2]octane skeleton, which remarkably improved its affinity, selectivity, and agonistic activity for the κ receptor. This finding was newly established by chemical modification of the nitrogen atom at the 8-position in the azabicyclo[2.2.2]octane skeleton. This modification would never have been found with KNT-63, a derivation of oxabicyclo[2.2.2]octane. These results may provide valuable information for the future development of novel κ selective 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.

Several derivatives with an azabicyclo[2.2.2]octane skeleton having a 7-amide side chain were synthesized. Compounds that had an electron-donating group exhibited high affinity for the μ opioid receptor while those with a bulky substituent at the 8-nitrogen atom had low affinities for all receptor types. High affinities and selectivities for the κ receptor resulted from the introduction of the longer amide side chain at the 7α-position. Our studies indicate that the orientation of the amide side chain at the 7-position within the azabicyclo[2.2.2]octane skeleton is related to selectivity for the κ receptor.

trans-Fused decahydroisoquinoline derivatives without a phenol ring (I) are found to bound the opioid receptor in the micromolar range.

We have previously reported the essential structure of the opioid κ receptor agonist nalfurafine hydrochloride (TRK-820) for binding to the κ receptor. In the course of this study, we focused on the effect of the substituent at 17-N in nalfurafine on the binding affinity for the κ receptor. The exchange of the 17-N substituent in nalfurafine from cyclopropylmethyl to fluoro-substituted alkyl groups, which are strong electron withdrawing substituents, almost completely diminished the binding affinities for the μ and δ opioid receptors, but the binding affinity for the κ receptor was still maintained. As a result, nalfurafine derivatives with 17-fluoro-substituted alkyl groups showed higher selectivities for the κ receptor than did nalfurafine itself. With regard to the κ agonistic activities, the conversion of the 17-N substituent in nalfurafine from cyclopropylmethyl to fluoro-substituted alkyl groups led to the gradual decrease of the agonistic activities in the order corresponding to their binding affinities for the κ receptor. In contrast, the derivative with the bulky 17-isobutyl group showed lower affinity and agonistic activity for the κ receptor than the derivatives with the smaller functional groups. This research suggested that both the electronic property and the steric characteristics of the 17-N substituent would have a great influence on the binding property for the κ receptor.

Novel double-capped triplet drugs, which have one pharmacophore unit and two epoxymethano or dimethylepoxymethano structures (termed cap or diMe-cap structures, respectively) were synthesized. Key intermediate oxazoline 16 derived from acetone enabled the effective synthesis of double-capped triplets. SYK-134 (7a) and SYK-135 (8a) with N-cyclopropylmethyl substituent and cap structures showed selectivities for the κ opioid receptor. On the other hand, the N-Me series exhibited selectivities for the μ opioid receptor. The double-capped triplet drugs with diMe-cap structures preferred the μ receptor independently of their N-substituents. SYK-385 (19b), one of the μ-selective double-capped triplet drugs, showed the highest selectivity for the μ receptor among the reported μ-selective nonpeptide ligands.

We designed and synthesized propellane derivatives with a 6- or 7-amide side chain on the basis of the active conformation of the κ selective agonist nalfurafine. The 6-amides showed high affinities for the κ receptor, and one of the 6β-amides showed higher κ selectivity than nalfurafine. On the other hand, although the affinities of the 7-amides decreased compared to the 6-amides, some 7α-amides showed the highest selectivities for the κ receptor among the tested compounds. The affinities of 7β-isomers were extremely low, which was postulated to result from the shielding effect of the 7β-amide side chain against the lone electron pair on the 17-nitrogen. This is the first conformational information about the 7-amide side chain in propellane derivatives.

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 [35S]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.

We synthesized symmetrical and nonsymmetrical triplet drugs with 1,3,5-trioxazatriquinane skeletons. The isolation of key intermediates, oxazoline dimers, made it possible to effectively produce nonsymmetrical triplets. Among the synthesized triplets, KNT-93, composed of three identical opioid μ receptor agonists, showed dose-dependent antinociception via the μ receptor. The effect was 56-fold more potent than that of morphine, a representative μ agonist. The profound analgesic effect induced by KNT-93 might result from simultaneous occupation of three μ opioid receptors.

Acetylcholinesterase inhibitor (−)-homogalanthamine 3 was synthesized from μ opioid antagonist naltrexone (2) in 16% total yield. The synthesis features Grob fragmentation as a key reaction, which was especially accelerated in the presence of 15-crown-5.

A novel 6,14-epoxymorphinan benzamide derivative (NS22) that was previously reported showed opioid κ receptor agonistic activity and analgesic activity. The unsatisfactory κ selectivity of NS22 led us to synthesize its derivatives to improve the opioid κ receptor selectivity and the agonist activity. In the course of SAR of the various derivatives, 17-benzyl-6,14-epoxymorphinan derivatives (KNT-33, 53, 55, 80, 90, 133) were found to show high selectivities and affinities for the opioid κ receptor. In addition, KNT-33, 53, 55 showed dose-dependent analgesic effects in acetic acid writhing tests. Therefore, 17-benzyl substituents may play an important role for developing κ selectivity.

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.

A conformational analysis of kappa opioid receptor agonists, TRK-820 and U-50,488H indicated an active conformation of TRK-820 in which the C-ring was in the boat form with the 14-OH interacting with the amide nitrogen. Based on the obtained active conformation of TRK-820, we designed and synthesized a novel kappa agonist KNT-63 with oxabicyclo[2.2.2]octane skeleton. KNT-63 showed profound antinociceptive effects via the kappa receptor which were as potent as that of TRK-820.

We re-examined the accessory site of the 4,5-epoxymorphinan skeleton by camdas conformational analysis in an effort to deign novel delta opioid receptor antagonists. We synthesized three novel compounds (SN-11, 23 and 28) with a 10-methylene bridge and without a 4,5-epoxy ring. Among them, compounds SN-23 (17-isobutyl derivative) and SN-28 (17-methyl derivative) showed very strong agonist activity (over 10 times more than TAN-67). SN-28 also showed high delta selectivity. The delta agonist activity of SN-23 was weaker than that of SN-28, but in terms of the delta selectivity, SN-23 was higher than that of SN-28. These unexpected results indicated that the 4,5-epoxy ring, but not the 10-methylene bridge, was an accessory site in delta opioid receptor agonists.

Selective ring opening reaction of the N-cyclopropylmethyl group in naltrexone (1d) was effected in the presence of platinum (IV) oxide and hydrobromic acid under a hydrogen atmosphere at rt to selectively afford N-isobutyl derivative 10. The binding affinity of N-i-Bu derivative 10 for opioid receptors was 11–17 times less than that of the corresponding N-CPM compound, naltrexone (1d). However, compound 10 showed dose-dependent analgesic effects. Contrary to expectations based on previous structure–activity relationship studies for a series of N-substituted naltrexone derivatives that compound 10 would be an opioid antagonist, 10 showed dose-dependent analgesia in the mouse acetic acid writhing test (ED50: 5.05 mg/kg, sc), indicating it was an opioid agonist. This finding may have a great influence on the drug design of opioid agonists.

A novel 6,14-epoxymorphinan benzamide derivative (NS22) was synthesized, which showed opioid kappa receptor agonistic activity in the [(35)S]GTPgammaS binding assay. The antinociceptive effect of NS22 was evaluated in the tail-flick and the hot-plate test. This compound showed a potent antinociceptive activity in mice by s.c. administration, which was attenuated with nor-BNI (selective opioid kappa receptor antagonist).