A. E. Takemori

Binaltorphimine and nor-binaltorphimine, potent and selective k-opioid receptor antagonists

The opioid antagonist activities of two bivalent ligands, BNI and nor-BNI, have been evaluated in smooth muscle preparations and in mice. Both ligands are highly potent and selective as kappa opioid receptor antagonists, with relatively feeble blocking activity at mu and delta opioid receptors. BNI and nor-BNI represent the first highly selective kappa opioid receptor antagonists and should be of great utility as molecular probes for identifying the interaction of agonist ligands with kappa opioid receptors in vitro and in vivo.

The irreversible narcotic antagonistic and reversible agonistic properties of the fumaramate methyl ester derivative of naltrexone

The fumaramate methyl ester derivatives of naltrexone (beta-FNA) and oxymorphone (beta-FOA) were both found to be reversible agonists on the guinea pig ileal longitudinal muscle preparation. In addition, beta-FNA possessed on irreversible antagonistic effect against morphine whereas beta-FOA had no such capacity. Analysis by pA2 values revealed that beta-FOA resembled pure agonists like morphine and enkephalin while beta-FNA resembled the mixed agonist-antagonists like nalorphine and pentazocine. The antagonism by beta-FNA was very selective in that it antagonized pure agonists but had little or no effect on the effects of either mixed agonist-antagonists, ethylketocyclazocine or other non-opiate-type agonists like norepinephrine.

A highly selective δ1-opioid receptor antagonist: 7-benzylidenenaltrexone

In guinea pig brain membranes 7-benzylideneraltrexone (BNTX) possesses 100-fold greater affinity (Ki = 0.1 nM) for [3H]DPDPE [3H][D-Pen2,D-Pen5]enkephalin) binding sites (δin1) relative to those of [3H]DSLET ([3H][D-Ser2, Leu5] enkephalin-Thr6) (δ2). The ED50 dose ratio (tail flick) in mice for the antagonism of DPDPE-induced antinociception of BNTX (6.3 pmol i.c.v.) was 7.2, whereas for DSLET, morphine and U69593 it was not significantly different from unity. The fact that there was no correlation of the binding or in vivo data for BNTX with antagonist potency in smooth muscle preparations suggest that the in vitro pharmacologic activity is mediated by δ-opioid subtypes that are different from those in the brain.

Pharmacological profiles of β-funaltrexamine (β-FNA) and β-chlornaltrexamine (β-CNA) on the mouse vas deferens preparation☆

The profiles of action of beta-funaltrexamine (beta-FNA) and beta-chlornaltrexamine (beta-CNA) have been assessed in the mouse vas deferens preparation. beta-FNA, but not beta-CNA, demonstrated a reversible agonist action that appeared to be mediated via kappa-receptor interaction. beta-CNA produced an irreversible antagonism of mu-, kappa- and delta-mediated agonist actions, whereas beta-FNA irreversibly antagonized mu-mediated agonist effects only. This selective action of beta-FNA could also be seen following administration in vivo. beta-CNA and particularly beta-FNA should prove valuable in the elucidation of multiple opioid receptors.

Effects of (−)-N6-(R-phenylisopropyl)-adenosine (PIA) and caffeine on nociception and morphine-induced analgesia, tolerance and dependence in mice☆

(-)-N6-(R-phenylisopropyl)-adenosine (PIA) was shown to possess analgesic activity in both the tail flick and acetic acid writhing assays. The analgesic actions of PIA were antagonized by caffeine in a dose-dependent manner. An apparent pA2 analysis in vivo suggested that the antagonism by caffeine was not competitive. Subanalgesic doses of PIA potentiated morphine-induced analgesia, tolerance and dependence. Caffeine antagonized these effects of morphine. PIA attenuated while caffeine exacerbated opiate withdrawal. While a low dose of caffeine antagonized PIA effects on withdrawal, a low dose of PIA did not antagonize the effects of caffeine. These results indicate that PIA can facilitate, and caffeine can antagonize the actions of morphine and that caffeine may be exerting some of its actions independent of adenosine receptor antagonism.

7-Benzylidenenaltrexone (BNTX): A selective δ1 opioid receptor antagonist in the mouse spinal cord

Abstract Recent reports provided evidence that at least two δ opioid receptors may mediate antinociception in mice. In this study, we studied further the involvement of δ opioid receptor subtypes in mediating antinociception at spinal sites in mice using subtype selective agonists and antagonists. The antinociceptive ED 50 values (95 % C.I.) of i.t. administered DPDPE [(D-Pen 2 , D-Pen 5 )enkephalin] ( δ 1 receptor agonist) and DELT II [(D-Ala 2 )deltorphin II] ( δ 2 receptor agonist) were 6.3 (5.2–7.6) and 6.4 (5.4–7.7) nmol/mouse, respectively. Administration of BNTX, s.c. increased the antinociceptive ED 50 value of DPDPE 5.9-fold whereas that of DELT II was not changed significantly. On the other hand administration of maltriben (NTB, the benzofuran derivative of naltrindole), s.c. increased the antinociceptive ED 50 value of DELT II 12.5-fold but did not alter that of DPDPE. Similarly administration of BNTX, i.t. increased the antinociceptive ED 50 value of DPDPE 4-fold without altering significantly that of DELT II. NTB given i.t. enlarged the antinociceptive ED 50 of DELT II 11-fold without affecting significantly that of DPDPE. BNTX, s.c. did not alter the antinociceptive ED 50 values of the μ-agonists, DAMGO [(D-Ala 2 , MePhe 4 , Gly-ol 5 ) enkephalin] and morphine or that of the κ-agonist, U50,488H [trans(±)−3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinylcyclohexyl]benzeneacetamide] These results demonstrate that BNTX is highly selective for δ 1 opioid receptors at spinal sites. Also, the present data provide for the involvement of both δ 1 and δ 2 opioid receptors in mediating antinociception at spinal sites in mice.

Agonist and antagonist activities of ligands derived from naltrexone and oxymorphone

The pharmacological profile of naltrindole (NTI) and three of its analogues, N-methyl-NTI (N-Me-NTI), oxymorphindole (OMI) and naltriben (NTB) were studied in antinociceptive assays. The compounds were found to have agonist activities that appear to be mediated mainly by kappa opioid receptors because norbinaltorphimine (nor-BNI), the selective kappa opioid receptor antagonist inhibited their effects significantly. All of the compounds, behaved as antagonists at doses that were lower than those that produced agonist effects and they possessed a profile that was very selective for inhibiting the antinociceptive activities of delta opioid receptor agonists. Differential antagonism by NTB of the activities of DSLET and DPDPE was demonstrated.

Opioid agonist and antagonist bivalent ligands as receptor probes

Bivalent ligands are molecules which contain two pharmacophores linked by a connecting chain (spanner). The present report describes the use of oxymorphamine (Oxy) and naltrexamine (Nal) as the opioid agonist and antagonist pharmacophores separated by a variable length spanner composed of succinyl-bis-oligoglycine. The agonist series, [CH2CO(Gly)nOxy]2, and antagonist series, [CH2CO(Gly)nNal]2, were synthesized (n = 0-4) and tested on the electrically stimulated GPI. All of the antagonist bivalent ligands (Nal) antagonized the effects of morphine, with the greatest potency enhancement (60 x) residing with the succinyl (n = 0) congener. A dramatically different SAR profile was observed in the agonist (Oxy) series where the greatest potency enhancement (17 x) occurs when n = 2. By contrast with the antagonist series the agonist bivalent ligand with n = 0 is equipotent to its monovalent agonist analogue. The significance of these results with respect to the possibility of discrete opioid agonist and antagonist recognition sites are discussed.

Improved assays for the assessment of ϰ- and δ-properties of opioid ligands

Abstract The highly selective non-equilibrium μ-antagonist β-funaltrexamine (β-FNA) produced a maximal 20-fold shift in the IC 50 for the μ-agonist morphine on the guinea-pig ileum preparation, whilst producing no significant change in the IC 50 for the ϰ-agonist ethylketazocine. On preparations pretreated with β-FNA, the pA 2 values for the interaction of morphine and ethylketazocine with naloxone were similar. These values were similar to the pA 2 value for the interaction of ethylketazocine and naloxone determined on control tissues, but significantly different from the pA 2 value for morphine-naloxone on control tissues, indicating that the agonist actions of morphine on preparations pretreated with high concentrationsof β-FNA are mediated by ϰ-, rather than μ-receptor interaction. On the mouse was deferens preparation, co-incubation with the highly selective δ-agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) and the non-selective non-equilibrium opiate antagonist β-chlornaltrexamine (β-CNA) resulted in marked inhibition of the agonist actions of morphine but had no effect upon the agonist actions of the δ-agonist leucine-enkephalin. The pA 2 values for the interactions of naloxone with leucine-enkephalin and etorphine were unaltered by pretreatment with β-CNA and DSLET. In similarly pretreated tissues, the agonist actions of ethylketazocine were markedly inhibited. It is concluded that manipulation of the guinea-pig ileum and mouse vas deferens preparations in the described manner results in assay systems that possess a largely homogeneous receptor population, and as such are valuable tools with which to evaluate opioid activity.

Comparative antagonism by naltrexone and naloxone of μ, κ, and δ agonists☆

In the guinea pig ileum preparation, naltrexone was 3.5 to 5 times more potent than naloxone in antagonizing morphine but the antagonists were equipotent in antagonizing ethylketazocine. In the mouse vas deferens preparation, naltrexone and naloxone were equipotent in antagonizing both morphine and [D-Ala2-D-Leu5]enkephalin. The data provide evidence that the naltrexone-reversible μ population of receptors in the guinea pig ileum are different from those in the mouse vas deferens. These findings also point out the uniqueness of the μ receptor system in the guinea pig ileum which is reflected by the potency differences between naltrexone and naloxone.