Efficacy of Naldemedine on Intestinal Hypomotility and Adhesions in Rodent Models of Postoperative Ileus.

Postoperative ileus (POI) often decreases patients' QOL because of prolonged hospitalization and readmission. Alvimopan, a peripheral µ-opioid receptor antagonist, is currently the only therapeutic drug for POI. The aim of this study was to examine the efficacy of naldemedine (a peripheral µ-opioid receptor antagonist with a non-competitive pharmacological profile different from that of alvimopan) on postoperative intestinal hypomotility and adhesion in rodent models, and compare it with the effects of alvimopan. Oral administration of naldemedine (0.3 mg/kg) and alvimopan (3 mg/kg) significantly inhibited the decrease in intestinal motility induced by mechanical irritation in mice (p < 0.01, for both). Naldemedine (1 mg/kg) significantly shortened the adhesion length in chemical-induced postoperative adhesion model rats (p < 0.05). Alvimopan (3 mg/kg) also significantly reduced the adhesion ratio (p < 0.01). These findings suggest that naldemedine is effective for postoperative intestinal hypomotility and adhesions in rodents (i.e., as for alvimopan). Thus, naldemedine may be a useful option for the treatment of POI.

Naldemedine: Peripherally Acting Opioid Receptor Antagonist for Treating Opioid-induced Adverse Effects.

Opioids are widely used for pain management in moderate-to-severe pain. However, opioids are associated with adverse events, such as constipation and emesis/vomiting. To reduce these undesired effects, a structure-activity relationship study of morphinan derivatives was conducted, and a promising lead compound with inhibitory effects on opioid receptors was obtained. Further improvement in the potency and pharmacokinetic profiles of the lead compound led to the discovery of naldemedine, which showed anti-constipation and anti-emetic effects against these adverse events that were induced by morphine without influencing morphine's analgesic effect. Naldemedine was launched in Japan and the USA in 2017 and in the EU in 2019, for treating opioid-induced constipation.

Preventive effects of naldemedine, peripherally acting µ-opioid receptor antagonist, on morphine-induced nausea and vomiting in ferrets.

AIMS: Naldemedine is a peripherally acting µ-opioid receptor antagonists (PAMORAs) indicated for the treatment of opioid-induced constipation (OIC). We investigated the preventive effect of naldemedine on morphine-induced nausea and vomiting in ferrets and conducted a pharmacokinetic/pharmacodynamic (PK/PD) analysis. MAIN METHODS: The antiemetic effect of naldemedine was evaluated as the frequency and time of retching (rhythmic abdominal contractile motion) and vomiting (throwing up vomit or similar reactions) caused by morphine in ferrets. After a single oral administration of naldemedine to ferrets, the plasma concentrations of naldemedine and morphine were measured by liquid chromatography-tandem mass spectrometry. KEY FINDINGS: Naldemedine showed a potent and dose-dependent anti-emetic effects against morphine-induced emetic responses, for up to 6 h. The dose of naldemedine that produced half the maximal effect (ED50) value for anti-emetic effect of naldemedine in the morphine-treated ferrets was 0.033 mg/kg. The PK/PD analysis revealed that the antiemetic effect was related to the plasma naldemedine concentration, with a half maximal effective concentration that produces half the maximal effect (EC50) of 3.51 ng/mL. The plasma concentration producing an antiemetic effect was almost 200-fold lower than that inducing an anti-analgesic effect in rats. SIGNIFICANCE: Naldemedine showed potent inhibition of morphine-induced vomiting for up to 6 h after dosing. These data suggest that naldemedine possesses antiemetic properties and could be effective against opioid-induced nausea and vomiting (OINV).

The antagonistic activity profile of naloxone in µ-opioid receptor agonist-induced psychological dependence.

Naloxone is a µ-opioid receptor antagonist that has been used to prevent overdose-related respiratory depression and deaths by the illicit use of opioids. Naloxone can also deter the abuse potential of opioids, but little has been reported regarding its antagonistic activity profile against opioid-induced psychological dependence. This study aimed to confirm the antagonistic activity profile of naloxone against several µ-opioid receptor agonists and investigate whether naloxone could affect the psychological dependence induced by widely used µ-opioid receptor agonist, oxycodone. In the Guanosine-5'-o-(3-thio) triphosphate (GTPγS) binding assay, naloxone (30-30,000 nM) inhibited the GTPγS binding induced by oxycodone, hydrocodone, morphine, and fentanyl. It elicited parallel rightward shifts in the concentration-response curves, indicating that naloxone possessed a competitive antagonistic activity profile against these µ-opioid receptor agonists. In the conditioned place preference test, oxycodone (0.01-1 mg/kg, i.v.) produced dose-dependent increases in place preference. The increased place preference induced by oxycodone (1 mg/kg) was significantly attenuated by co-administration of naloxone at a dose of 0.5 mg/kg but not 0.01 mg/kg. Naloxone (0.5 mg/kg, i.v.) also blocked oxycodone (1 mg/kg)-induced dopamine release in nucleus accumbens; however, at a lower dose (0.01 mg/kg), it did not affect the intrinsic dopamine release by oxycodone. These results indicate that the psychological dependence of oxycodone could be antagonized by naloxone, depending on the dose. This characterization might lead to a better understanding of the competitive antagonistic activity profile of naloxone for µ-opioid receptor in the brain.

Pharmacological Profile of Naldemedine, a Peripherally Acting µ-Opioid Receptor Antagonist: Comparison with Naloxone and Naloxegol.

Opioid-induced constipation (OIC), a typical side effect of opioids, is due to activation of the µ-opioid receptors in the enteric nervous system. Peripherally acting µ-opioid receptor antagonists (PAMORAs) can reverse OIC by inhibiting the peripheral action of opioids without affecting centrally mediated analgesia. Naldemedine is a PAMORA with potent antagonist activity against µ-, δ-, and κ-opioid receptors. In this study, the pharmacological profiles of naldemedine, compared with those of naloxone and naloxegol, were evaluated. In vitro, Schild plot analysis indicated that naldemedine was a noncompetitive antagonist of µ-opioid receptors, whereas other compounds were competitive antagonists. Also, naldemedine showed slower association and dissociation kinetics than the other compounds. In vivo, naldemedine dose-dependently ameliorated morphine-induced inhibition of small intestinal transit (SIT). The dose-response curve was not shifted at 1 and 3 mg/kg morphine. On the contrary, that of naloxegol was significantly shifted to the right from 1 to 3 mg/kg morphine. In morphine-dependent rats, naldemedine caused peripheral withdrawal symptoms (diarrhea) at doses higher than 1 mg/kg, whereas the dose that produced half the maximal preventive effect (ED50) against constipation was 0.03 mg/kg. Naldemedine showed slower onset and a lesser severity of diarrhea than the other compounds at close to the ED50 value in the SIT model. Our results reveal that naldemedine has different pharmacological profiles (type of antagonism and binding kinetics) to the other compounds. This might explain the differential inhibition of morphine-induced SIT and withdrawal symptoms among the three antagonist compounds. SIGNIFICANCE STATEMENT: Naldemedine is a novel peripherally acting µ-opioid receptor antagonist with potent antagonist activity against µ-, δ-, and κ-opioid receptors. Naldemedine showed a noncompetitive antagonism and slower association and dissociation kinetics against µ-opioid receptors than naloxone and naloxegol. Naldemedine showed insurmountable antagonism of morphine-induced inhibition and lower and slower peripheral withdrawal symptoms (diarrhea) than the other compounds. Therefore, naldemedine has a different pharmacological profile (the type of antagonism and binding kinetics) to the other compounds.

Pharmacologic effects of naldemedine, a peripherally acting µ-opioid receptor antagonist, in in vitro and in vivo models of opioid-induced constipation.

BACKGROUND: Naldemedine (S-297995) is a peripherally acting µ-opioid receptor antagonist developed as a once-daily oral drug for opioid-induced constipation (OIC) in adults with chronic noncancer or cancer pain. This study characterized the pharmacological effects of naldemedine in vitro and in vivo. METHODS: The binding affinity and antagonist activity of naldemedine against recombinant human µ-, δ-, and κ-opioid receptors were assayed in vitro. Pharmacologic effects of naldemedine were investigated using animal models of morphine-induced inhibition of small and large intestinal transit, castor oil-induced diarrhea, antinociception, and morphine withdrawal. KEY RESULTS: Naldemedine showed potent binding affinity and antagonist activities for recombinant human µ-, δ-, and κ-opioid receptors. Naldemedine significantly reduced opioid-induced inhibition of small intestinal transit (0.03-10 mg kg-1 ; P < 0.05) and large intestinal transit (0.3-1 µmol L-1 ; P < 0.05). Naldemedine (0.03-1 mg kg-1 ) pretreatment significantly reversed the inhibition of castor oil-induced diarrhea by subcutaneous morphine (P < 0.01). Naldemedine (1-30 mg kg-1 ) pretreatment (1 or 2 hours) did not alter the analgesic effects of morphine in a model measuring the latency of a rat to flick its tail following thermal stimulation. However, a significant delayed reduction of the analgesic effect of morphine was seen with higher doses of naldemedine (10-30 mg kg-1 ). Some centrally mediated and peripherally mediated withdrawal signs in morphine-dependent rats were seen with naldemedine doses ≥3 and ≥0.3 mg kg-1 , respectively. CONCLUSIONS & INFERENCES: Naldemedine displayed potent binding affinity to, and antagonistic activity against, µ-, δ-, and κ-opioid receptors. Naldemedine tempered OIC in vivo without compromising opioid analgesia.

Discovery of naldemedine: A potent and orally available opioid receptor antagonist for treatment of opioid-induced adverse effects.

Structure-activity relationship studies of several morphinan derivatives were conducted to obtain dual antagonists for µ- and δ-opioid receptors. We discovered peripherally restricted dual antagonists for µ/δ-opioid receptors as a new chemotype with a morphinan scaffold, which are orally available and do not easily pass the blood-brain barrier. As we expected, some of these compounds inhibit opioid-induced constipation and emesis/vomiting with limited potential to interfere the analgesic effects of morphine. Among them, naldemedine was selected as a potential drug candidate.

Sensitization of transient receptor potential vanilloid 4 and increasing its endogenous ligand 5,6-epoxyeicosatrienoic acid in rats with monoiodoacetate-induced osteoarthritis.

Transient receptor potential vanilloid 4 (TRPV4) receptor modulates pain, and this has been noted in several animal models. However, the involvement of TRPV4 in osteoarthritic (OA) pain remains poorly understood. This study assessed the functional changes in TRPV4 and the expression of its endogenous ligand 5,6-epoxyeicosatrienoic acid (5,6-EET) in a rat monoiodoacetate (MIA)-induced OA pain model (MIA rats). Monoiodoacetate-treated rats showed reduced grip strength as compared to sham-treated rats, and this loss in function could be recovered by the intraarticular administration of a TRPV4 antagonist (HC067047 or GSK2193874). By contrast, the intraarticular administration of the TRPV4 agonist, GSK1016790A, increased the pain-related behaviors in MIA rats but not in sham rats. TRPV4 expression was not increased in knee joints of MIA rats; however, the levels of phosphorylated TRPV4 at Ser824 were increased in dorsal root ganglion neurons. In addition, 5,6-EET was increased in lavage fluids from the knee joints of MIA rats and in meniscectomy-induced OA pain model rats. 5,6-EET and its metabolite were also detected in synovial fluids from patients with OA. In conclusion, TRPV4 was sensitized in the knee joints of MIA rats through phosphorylation in dorsal root ganglion neurons, along with an increase in the levels of its endogenous ligand 5,6-EET. The analgesic effects of the TRPV4 antagonist in the OA pain model rats suggest that TRPV4 may be a potent target for OA pain relief.

Pharmacological evaluation of novel (6-aminopyridin-3-yl)(4-(pyridin-2-yl)piperazin-1-yl) methanone derivatives as TRPV4 antagonists for the treatment of pain.

A novel series of (6-aminopyridin-3-yl)(4-(pyridin-2-yl)piperazin-1-yl) methanone derivatives were identified as selective transient receptor potential vanilloid 4 (TRPV4) channel antagonist and showed analgesic effect in Freund's Complete Adjuvant (FCA) induced mechanical hyperalgesia model in guinea pig and rat. Modification of right part based on the compound 16d which was disclosed in our previous communication led to the identification of compound 26i as a flagship compound. In this paper, we described the details about design, synthesis and structure-activity relationship (SAR) analysis at right and left part of these derivatives (Fig. 1).

Discovery of novel 2',4'-dimethyl-[4,5'-bithiazol]-2-yl amino derivatives as orally bioavailable TRPV4 antagonists for the treatment of pain: Part 2.

A series of 2',4'-dimethyl-[4,5'-bithiazol]-2-yl amino derivatives have been identified as selective TRPV4 antagonists that display inhibition potencies against 4α-phorbol 12,13-didecanoate (4αPDD), well known as a TRPV4 selective agonist and/or a hypotonicity. In particular, 9-(6-((2',4'-dimethyl-[4,5'-bithiazol]-2-yl)amino)nicotinoyl)-3-oxa-9-azabicyclo[3.3.1]nonan-7-one showed an analgesic effect in Freund's Complete Adjuvant (FCA) induced mechanical hyperalgesia model in guinea pig (reported in Part 1). However, there are some concerns such as species differences and the need for higher plasma exposure to achieve target efficacy for evaluation by an in vivo pain model. In this Letter, we report the resolution of some of the problems by further optimizing the chemical scaffold.

Discovery of novel 2',4'-dimethyl-[4,5'-bithiazol]-2-yl amino derivatives as orally bioavailable TRPV4 antagonists for the treatment of pain: Part 1.

A novel series of 2',4'-dimethyl-[4,5'-bithiazol]-2-yl amino derivatives were found by high throughput screening of the TRPV4 receptor, at which these compounds showed competitive antagonist potential against 4α-phorbol 12,13-didecanoate (4αPDD) as the selective TRPV4 agonist and showed excellent selectivity for TRPV1, N-type and L-type calcium ion channels, but poor ADME profile. In our SAR strategy, we found that the lead molecule 1 also having the unique 3-oxa-9-azabicyclo [3.3.1] nonan-7-one on the right part showed potent TRPV4 antagonist activity, good solubility at pH 6.8, good microsomal stability for human and better ADME profile including oral bioavailability. Moreover, compound 1 had an analgesic effect in Freund's Complete Adjuvant (FCA) induced mechanical hyperalgesia model in guinea pig. In this letter, we report a lead optimization process to identify the lead compound 1 (Fig. 1).

Effect of estrogen on morphine- and oxycodone-induced antinociception in a female femur bone cancer pain model.

Although estrous cycle has been reported to influence antiociceptive effect of morphine in several pain conditions, its effect on cancer pain is not well established. We investigated the effect of estrogen on morphine antinociception using a bone cancer pain model and compared its potency with that of oxycodone. Female mice were ovariectomized (OVX) for preparation of a femur bone cancer pain (FBC) model. ß-estradiol was subcutaneously (s.c.) administered and antinociceptive effects of opioids was assessed using the von Frey monofilament test. Although morphine (5-20mg/kg, s.c.) did have significant antinociceptive effects in the FBC-OVX group, its effects in the FBC-OVX+ß-estradiol (OVX+E) group was limited. Oxycodone (1-5mg/kg, s.c.) exhibited significant effects in both groups. Expression changes in opioid-related genes (µ-, κ-, δ-opioid receptors, prodynorphin, proenkephalin, proopiomelanocortin) in the spinal and supraspinal sites were examined among the sham-OVX, sham-OVX+E, FBC-OVX, and FBC-OVX+E groups by in situ hybridization. These studies detected a significant increase in prodynorphin in the spinal dorsal horn of the FBC-OVX+E group. Spinal injection of a dynorphin-A antibody to FBC-OVX+E mice restored antinociception of morphine. In conclusion, we detected a differential effect of estrogen on morphine- and oxycodone-induced antinociception in a female FBC model. The effect of morphine was limited with estrogen exposure, which may be due to estrogen- and pain-mediated spinal expression of dynorphin-A.

The contribution of Gi/o protein to opioid antinociception in an oxaliplatin-induced neuropathy rat model.

Oxaliplatin is a chemotherapeutic agent that induces chronic refractory neuropathy. To determine whether opioids effectively relieve this chronic neuropathy, we investigated the efficacies of morphine, oxycodone, and fentanyl, and the mechanisms underlying opioid antinociception, in oxaliplatin-induced neuropathy in rats. Rats exhibited significant mechanical allodynia following 2 weeks of chronic oxaliplatin administration. Within the range of doses that did not induce sedation and/or muscle rigidity, morphine (3 mg/kg, subcutaneously, s.c.) and oxycodone (0.3-0.56 mg/kg, s.c.) completely reversed oxaliplatin-induced mechanical allodynia, whereas fentanyl (0.017-0.03 mg/kg, s.c.) showed partial antinociception. The antinociception of the optimal doses of morphine and oxycodone were completely inhibited by pertussis toxin (PTX; 0.5 µg/rat, i.c.v.), a Gi/o protein inhibitor, while the partial effect of fentanyl was not affected in the oxaliplatin model. In the [(35)S]-GTPγS binding assay, activation of µ-opioid receptor by fentanyl, but not by morphine or oxycodone, in the mediodorsal thalamus was significantly reduced in oxaliplatin-treated rats. These results indicate that the lower antinociceptive potency of fentanyl in the oxaliplatin model might in part result from the loss of PTX-sensitive Gi/o protein activation, and the degree of Gi/o protein activation might be related to the potency of antinociception by opioids in this model.

Establishment of opioid-induced rewarding effects under oxaliplatin- and Paclitaxel-induced neuropathy in rats.

The rewarding effects of µ-receptor agonists can be suppressed under several pain conditions. We recently showed that clinically used µ-receptor agonists possess efficacies for relieving the neuropathic pain induced by chemotherapeutic drug in rats; however, it is possible that the use of µ-receptor agonists may trigger the rewarding effects even under chemotherapeutic drug-induced neuropathic pain. Nevertheless, no information is available regarding whether µ-receptor agonists produce psychological dependence under chemotherapeutic drug-induced neuropathic pain. Therefore, we examined the effects of neuropathy induced by chemotherapeutic drugs on the rewarding effects of morphine, oxycodone, and fentanyl in rats. Repeated treatment with oxaliplatin or paclitaxel produced neuropathy as measured by the von Frey test. Rewarding effects produced by antinociceptive doses of µ-receptor agonists were not suppressed under oxaliplatin- or paclitaxel-induced neuropathy. Furthermore, the morphine-induced increase in the release of dopamine from the nucleus accumbens, which is a critical step in the rewarding effects of µ-receptor agonists, was not altered in paclitaxel-treated rats. These results suggest that the rewarding effects of µ-receptor agonists can still be established under oxaliplatin- or paclitaxel-induced neuropathic pain. Therefore, patients should be carefully monitored for psychological dependence on µ-receptor agonists when they are used to control chemotherapeutic drug-induced neuropathic pain.

Morphine and oxycodone, but not fentanyl, exhibit antinociceptive effects mediated by G-protein inwardly rectifying potassium (GIRK) channels in an oxaliplatin-induced neuropathy rat model.

It has begun to be understood that µ-opioid receptor (MOR) produces ligand-biased agonism, which contributes to differential physiological functions of MOR agonists. We previously demonstrated that in oxaliplatin-induced neuropathy in rats, morphine and oxycodone exhibited antinociceptive effects while antinociception of fentanyl was partial, and such different efficacies might result from the different level of Gi/o protein activation. Based on our background, to reveal further mechanism, we focused on the role of Gi/o protein-related downstream signaling, the G-protein inwardly rectifying K(+)1 (GIRK1) channel. The GIRK1 channel blocker tertiapin-Q (30pmol) was intracerebroventricularly (i.c.v.) or intrathecally (i.t.) administered to rats with oxaliplatin-induced neuropathy. The antinociception of systemic morphine (3mg/kg, subcutaneously (s.c.)) was suppressed only by pretreatment of i.t. tertiapin-Q, while supraspinal tertiapin-Q suppressed only the antinociception of systemic oxycodone (0.56mg/kg, s.c.). Partial antinocicpetion of fentanyl (0.017mg/kg, s.c.) was neither affected by i.c.v nor i.t. tertiapin-Q. These results demonstrated that GIRK1 channels differentially contribute to antinociceptive effects of MOR agonists, and that action site of GIRK1 channels is also different between morphine and oxycodone in oxaliplatin model. This study suggests the possibility that GIRK1 channels have a crucial role for antinociception of MOR agonists in oxaliplatin-induced neuropathy.

Structure-activity relationship studies and discovery of a potent transient receptor potential vanilloid (TRPV1) antagonist 4-[3-chloro-5-[(1S)-1,2-dihydroxyethyl]-2-pyridyl]-N-[5-(trifluoromethyl)-2-pyridyl]-3,6-dihydro-2H-pyridine-1-carboxamide (V116517) as a clinical candidate for pain management.

A series of novel tetrahydropyridinecarboxamide TRPV1 antagonists were prepared and evaluated in an effort to optimize properties of previously described lead compounds from piperazinecarboxamide series. The compounds were evaluated for their ability to block capsaicin and acid-induced calcium influx in CHO cells expressing human TRPV1. The most potent of these TRPV1 antagonists were further characterized in pharmacokinetic, efficacy, and body temperature studies. On the basis of its pharmacokinetic, in vivo efficacy, safety, and toxicological properties, compound 37 was selected for further evaluation in human clinical trials.

Effect of the norepinephrine transporter (NET) inhibition on µ-opioid receptor (MOR)-induced anti-nociception in a bone cancer pain model.

Although norepinephrine transporter (NET) inhibition has an additional effect on µ-opioid receptor (MOR)-mediated anti-nociception in inflammatory and neuropathic pain, its effect on cancer pain is not well characterized. We investigated the additional effect of NET inhibition on MOR activation using a mouse femur bone cancer (FBC) pain model by comparing the anti-nociceptive effect of the dual-acting opioids tramadol and tapentadol and the clinically used MOR-targeted opioids oxycodone and morphine. The anti-nociceptive effects of subcutaneously administered opioids were assessed using the von-Frey filament test. Oxycodone (1 - 10 mg/kg) and morphine (5 - 50 mg/kg) dose-dependently exhibited potent anti-nociceptive effects, whereas tramadol (10 - 56 mg/kg) and tapentadol (10 - 30 mg/kg) exhibited partial effects. Rota-rod analyses of tapentadol at a higher dose (> 30 mg/kg) showed a significant decrease in motor coordination, which was partially recovered by pretreatment with MOR or α(1)-adrenoceptor antagonists. The partial anti-nociceptive effect of tapentadol (30 mg/kg) was completely suppressed by a MOR antagonist, but not by α(1)- or α(2)-adrenoceptor antagonists, suggesting that neither α(1)-adrenoceptor- nor α(2)-adrenoceptor-mediated pathways are involved in anti-nociception in the FBC model. We conclude that addition of NET inhibition does not contribute to MOR-mediated anti-nociception in bone cancer pain.

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.

Morphine, oxycodone, and fentanyl exhibit different analgesic profiles in mouse pain models.

Morphine, oxycodone, and fentanyl are clinically prescribed drugs for the management of severe pain. We investigated whether these opioids possess different efficacy profiles on several types of pain in mouse pain models. When the three opioids were tested in the femur bone cancer model, all of them significantly reversed guarding behavior, whereas the effects on limb-use abnormality and allodynia-like behavior differed among the opioids. Particularly, although oxycodone (5 - 20 mg/kg) and fentanyl (0.2 mg/kg) significantly reversed limb-use abnormality, not even a high dose of morphine (50 mg/kg) could reverse it. When the effects of these opioids were examined in a sciatic nerve ligation (SNL) model of neuropathic pain, oxycodone was the most effective, producing an antinociceptive effect without affecting the withdrawal threshold of sham-treated animals. When the effects of these opioids were examined with the tail-flick test using naive animals, oxycodone, morphine, and fentanyl exhibited antinociceptive effects on thermal nociception. These results show that the three opioids exhibit different efficacy outcomes in multiple pain models and that the efficacy profile of oxycodone does not overlap those of morphine and fentanyl.

Design and synthesis of novel delta opioid receptor agonists and their pharmacologies.

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.