Chloroxymorphamine, and opioid receptor site-directed alkylating agent having narcotic agonist activity.

Chloroxymorphamine, the 6beta-N,N-bis(2-chloroethyl) derivative of oxymorphone, is a potent nonequilibrium narcotic agonist in the longitudinal muscle preparation of guinea pig ileum. The corresponding naltrexone analog,chlornaltrexamine, is a potent nonequilibrium antagonist of morphine. These receptor sitedirected alkylating agents possess considerable potenial as pharmacologic and biochemical probes of apoid receptors.

Stereospecific accumulation of dihydromorphine and naltrexone by corpus striatal slices of morphine-dependent mice.

Stereospecific accumulation of [3H]dihydromorphine and [3H]naltrexone by striatal slices from morphine-dependent mice was examined in Krebs-Ringer bicarbonate medium. Striatal slices showed a saturable and stereospecific accumulation of both [3H]ligands. The accumulation constant of naltrexone, determined by Wilkinson's analysis, was significantly decreased in both morphine-dependent mice and dependent mice abruptly withdrawn for 6 hr. The maximal accumulation of naltrexone was not changed in withdrawn mice, but decreased in dependent mice. This could be due to the high concentration of residual morphine in the slices. There were no significant differences in the accumulation constant or maximal accumulation of dihydromorphine among the striatal slices from control, dependent and withdrawn mice. These data indicate that in morphine-dependent mice, there is an increased affinity of the opioid receptors for the narcotic antagonist, naltrexone but not for the agonist, dihydromorphine.

In vivo binding of naloxone to opioid receptors in morphine-dependent mice.

In vivo binding assay of opioid receptors for naloxone was tested in morphine-dependent mice. The content of naloxone in the brain (whole brain minus cerebellum) and cerebellum was determined as the total binding and non-specific binding, respectively, 20 min after the intravenous injection of [3H]-naloxone. Acute treatment with 8 mg/kg of morphine sulfate markedly enhanced the non-specific binding of naloxone. In contrast, withdrawal for 6 hr after 72 hr of implantation of morphine decreased the non-specific binding of naloxone. The Scatchard analysis of the specific binding revealed two binding sites. The apparent low affinity Kd values of both morphine-tolerant and morphine-withdrawn mice were significantly decreased when compared to their respective controls. The apparent high affinity Kd and the high and low Bmax values were not altered by the morphine dependent-tolerant state.

Selective opioid antagonist effects on opioid-induced inhibition of release of norepinephrine in guinea pig cortex.

Opioid agonists with selectivity for mu, delta and kappa-receptors have each been shown to inhibit the K+-stimulated release of [3H]norepinephrine (NE) from slices of guinea pig cortex maintained in vitro. In order to provide further evidence that each of these types of opioid receptor can regulate the release of NE in this tissue, experiments with receptor-type selective opioid antagonists have been conducted. In initial experiments, the selectivity of the antagonists for specific types of opioid receptors in the cortex of the guinea pig in an incubation medium of the same composition as that used for release studies was confirmed. The delta-receptor selective antagonist, ICI 174,864, prevented the inhibitory actions of the delta-selective agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), but had little effect on the inhibitory actions of the mu-selective agonist, Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), or the kappa-selective agonist, U-50,488H. In contrast, the kappa-selective antagonist, nor-binaltorphimine (nor-BNI) prevented the inhibitory actions of U-50,488H, but had little effect on the inhibitory actions of DPDPE or DAMGO. The greater potency of the partially mu-selective antagonist, naloxone, in reversing the effects of DAMGO relative to those of DPDPE or U-50,488H was confirmed. These results support the conclusion that mu- delta- and kappa-opioid receptors each exert a negative regulatory effect on the stimulated release of NE in the cortex of the guinea pig.

Design of peptidomimetic delta opioid receptor antagonists using the message-address concept.

Highly selective nonpeptide ligands with potent delta opioid receptor antagonist activity have been developed using the message-address concept. This approach envisaged the delta opioid receptor to contain two major recognition subsites; a message subsite which recognizes the pharmacophore, and an address subsite that is unique for the delta receptor type and confers selectivity. The message and address components of the delta-selective enkephalins were postulated to be Tyr1 and Phe4, respectively, with Gly2-Gly3 functioning as a spacer. The message component of the target compounds in this study was derived from naltrexone and related structures. An indole system was fused to the C ring of naltrexone as a mimic of the address component. The benzene moiety of indole was viewed as the delta address component, mimicking the phenyl group of Phe4, and the pyrrole portion was used as a rigid spacer. Members of the series (1-23) were evaluated for opioid antagonist activity on the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations. Naltrindole (NTI, 1) was the most potent member of the series, with Ke values of approximately 0.1 nM at delta receptors. The antagonism by NTI was approximately 220- and 350-fold greater at delta than at mu and kappa opioid receptors. The binding of NTI and selected members of the series to guinea pig brain membranes was qualitatively consistent with their pharmacologic antagonist activity profiles in the MVD and GPI, but the Ki values were not in the same rank order. The selectivity of NTI arises mainly as a consequence of increased affinity at delta receptors. Thus, the Ke and Ki values of NTI were 1/530 and 1/90 that of the delta antagonist enkephalin analogue, ICI 174864. In contrast to NTI, ICI174864 derives its selectivity through greatly decreased recognition at mu and kappa receptors. The implications of the high affinity and selectivity of NTI as a consequence of its conformational rigidity are discussed. It is suggested that any attempt to model a receptor-bound conformation of an opioid peptide should consider affinity and potency at multiple receptor sites rather than selectivity alone.

Activities of morphinone and N-(cyclopropylmethyl)normorphinone at opioid receptors.

Morphinone (3) and N-(cyclopropylmethyl)normorphinone (4) were synthesized and tested on electrically stimulated smooth muscle preparations (guinea pig ileum and mouse vas deferens) and in mice. Compound 3 behaved as an agonist and 4 as an antagonist in vitro and in vivo. No pronounced nonequilibrium agonist or antagonist activity was observed with either compound.

Only one pharmacophore is required for the kappa opioid antagonist selectivity of norbinaltorphimine.

We have investigated whether one or two pharmacophores are required for the kappa opioid receptor selectivity of the bivalent opioid antagonist norbinaltorphimine, (-)-1 (nor-BNI), by the synthesis and testing of its meso isomer 2. In smooth muscle preparations 2 was more potent than 1 and about half as selective as a kappa antagonist. Since 2 contains only one antagonist pharmacophore but yet retains substantial kappa selectivity, it is concluded that kappa selectivity is not dependent on the presence of two (-)-naltrexone-derived pharmacophores of 1. It is suggested that the kappa selectivity of (-)-1 and 2 is derived from the portions of the second halves of these molecules in that they mimic key "address" components of dynorphin at kappa opioid receptors.

Narcotic antagonistic potency of bivalent ligands which contain beta-naltrexamine. Evidence for bridging between proximal recognition sites.

Two-bivalent ligands (P-X-P) containing the beta-naltrexamine pharmacophore (P) and a connecting oligoethylene glycol spanner (X) were synthesized and evaluated for narcotic antagonistic activity in the guinea pig ileum (GPI) and mouse vas deferens (MVD). The bivalent ligand 2 whose spanner contains three ethylene units possessed 10-fold greater antagonistic potency than its monovalent analogue (4) in antagonizing the effects of ethylketazocine (EK) on the GPI, while no differential antagonism of morphine was observed among the compounds. In the MVD, 2 was not substantially more potent than 4 as an antagonist against [D-Ala2,D-Leu5]enkephalin (DADLE). The bivalent ligand 3, whose spanner contains six ethylene units, exhibited 15 times greater potency in antagonizing the agonist effects of DADLE on the MVD than its monovalent ligand 4. No marked increase in the ability of 3 to antagonize the effects of morphine or EK on the GPI was observed. The data indicate that mu, kappa, and delta opioid receptors exhibit different selectivity toward bivalent ligands whose spanner lengths differ. The enhanced potency associated with different receptor interactions is consistent with simultaneous occupation of proximal recognition sites. Whether such proximal recognition sites are identical or different remains to be clarified. The distance between proximal sites appears to depend on the opioid receptor subtype involved.

Investigation of the structural requirements for the kappa-selective opioid receptor antagonist, 6 beta,6 beta'-[ethylenebis(oxyethyleneimino)]bis[17-(cyclopropylmethyl)- 4,5 alpha-epoxymorphinan-3,14-diol] (TENA).

In an effort to determine whether or not the basic nitrogens in the spacer of the bivalent ligand 6 beta,6 beta'-[ethylenebis(oxyethyleneimino)]bis[17-(cyclopropylmethyl)4,5 alpha-epoxymorphinan-3,14-diol] (TENA, 1) is responsible for its selective kappa opioid antagonist activity, we have synthesized monovalent analogues 2-4 that contain a C-6 side chain with basic nitrogens. Analogue 2 behaved as a potent opioid agonist in the guinea pig ileum preparation (GPI) and possessed no significant kappa opioid antagonist activity (IC50 ratio = 1) relative to TENA (IC50 ratio = 20). The agonist activity of 3 and 4 interfered with the opioid antagonist assay and therefore did not permit evaluation of antagonist activity in a concentration range where TENA is effective. Although the results obtained with 2 are consistent with the requirement of a second opiate pharmacophore (rather than a second basic nitrogen in the spacer) for the kappa antagonist activity of TENA, the potent agonism associated with these monomers do not allow a firm conclusion in this regard.

Synthesis and opioid antagonist potencies of naltrexamine bivalent ligands with conformationally restricted spacers.

Bivalent ligands 1-4 with naltrexamine pharmacophores and spacers of different lengths containing a fumaryl moiety were synthesized and evaluated for mu and kappa opioid antagonist activity on the electrically stimulated guinea pig ileal longitudinal muscle (GPI). The fumaryl moiety was incorporated into the spacer in order to determine the effect of conformational restriction of the spacer on the relationship between spacer length and opioid antagonist potency. While it was found that the fumaryl and succinyl series (11) possessed a very similar structure-potency profile with respect to antagonism at mu opioid receptors, the interaction of these two series at kappa receptors differed substantially from one another. This difference was manifested by the longer spacer requirement for peak kappa antagonist potency in the fumaryl relative to the succinyl series. It is concluded that the conformational restriction imposed by the fumaryl group in a short spacer (n = 0) prevents effective interaction of both pharmacophores with vicinal recognition sites of the kappa receptor system; as the spacer is lengthened (n = 2) and becomes more flexible, the simultaneous occupation of vicinal recognition sites occurs with greater facility.

Synthesis and biological activity of analogues of beta-chlornaltrexamine and beta-funaltrexamine at opioid receptors.

beta-Chlornaltrexamine and beta-funaltrexamine analogues 4-7 with different length "arms" to which an electrophilic moiety is attached were synthesized in an effort to obtain affinity labels that would selectively and irreversibly block specific opioid receptor types and subtypes. One of the compounds, 4, was a potent, irreversible blocker of opioid receptors in the guinea pig ileum and mouse vas deferens preparations. The results of this study suggest that nucleophiles that are remote from the recognition locus are capable of alkylation by reactive electrophiles.

Irreversible blockage of opioid receptor types by ester homologues of beta-funaltrexamine.

A series of ester homologues 2-5 of the mu receptor nonequilibrium antagonist beta-funaltrexamine (1, beta-FNA) was synthesized. These ligands were of interest in our investigation of the relationship between the structure of the ester function and the ability to irreversibly block mu opioid receptors. While all of the ligands were potent reversible agonists in the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations, most appeared to behave as irreversible antagonists of morphine. The benzyl 5 and phenethyl 6 esters possessed irreversible mu antagonist potency that was of similar magnitude to that of beta-FNA in the GPI. In the MVD, all esters appeared to irreversibly block the agonist effect of morphine, but none of the compounds irreversibly antagonized [D-Ala2,D-Leu5]enkephalin to a significant degree. [3H]Dihydromorphine displacement studies revealed no relationship between the affinity of the esters 1-6 and the irreversible blockage of mu receptors in the GPI or MVD. Possible reasons for the observed structure-activity relationship are discussed.

Opioid agonist and antagonist activities of morphindoles related to naltrindole.

A series of naltrindole-related ligands (4-10) with an N-methyl,N-phenethyl,N-cinnamyl, or an unsubstituted basic nitrogen were synthesized and tested for opioid agonist and antagonist activity in smooth muscle preparations and in mice. The nor compounds (4 and 6) and the phenethyl derivatives (5 and 8) displayed full agonist activity (IC50 = 85-179 nM) in the mouse vas deferens preparation (MVD) while the other members of the series exhibited partial agonist or weak antagonist activity. In the guinea pig ileum preparation (GPI), all compounds except 8 were partial agonists. The ligands that were evaluated in mice were found to produce antinociception that was not selectively mediated via delta opioid receptors. However, two of these ligands (4 and 5) appeared to be delta-selective opioid receptor antagonists at subthreshold doses for antinociception. The finding that all of the compounds bind selectively to delta opioid receptors in guinea pig brain membranes together with the in vitro pharmacology and in vivo antagonist studies suggests that the lack of delta agonist selectivity in vivo may be due to a number of factors, including a basic difference between the delta receptor system in the MVD and in the mouse brain. Further, it is suggested that the constellation of message and address components in the morphindole nucleus may tend to stabilize delta receptors in the brain in antagonist state.

Aspartic acid conjugates of 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1(3-aminophenyl)-2-(1-pyrrolidi nyl) ethyl]acetamide: kappa opioid receptor agonists with limited access to the central nervous system.

Aspartic acid conjugates of 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-aminophenyl)-2-(1-pyrrol idinyl) ethyl]acetamide (5) were synthesized and evaluated in mice for antinociceptive activity by intravenous and intracerebroventricular routes of administration. The intravenously-administered alpha-conjugate of L-Asp (2), its D-Asp diastereomer (3), and the beta -conjugate of L-Asp (4) were found to be 11-, 31-, and 40-fold, respectively, less effective than the parent ligand 1 (ICI 199,441) in producing central nervous system mediated antinociception in the mouse abdominal stretch assay. In addition, iv-administered 2 and 3 were found to also produce potent antinociception in the tonic phase of the mouse formalin assay, which is a model of tonic rather than acute pain. This study suggests that the attachment of a zwitterionic moiety to a position in the molecule that exhibits bulk tolerance is a viable strategy for the design of peripherally-selective and peripherally-active opioids.

7'-Substituted amino acid conjugates of naltrindole. Hydrophilic groups as determinants of selective antagonism of delta 1 opioid receptor-mediated antinociception in mice.

A series of amino acid conjugates (2-6) of naltrindole (1) were synthesized from 7'-carboxynaltrindole (7) in order to obtain delta antagonists that would have minimal access to the central nervous system (CNS) upon peripheral administration. All of the ligands (2-7) were tested in smooth muscle preparations and found to be potent and selective delta opioid antagonists. Receptor binding showed 2-7 to be highly delta-selective, with Ki ratios (mu/delta, kappa/delta) ranging from 127 to 38,000. Two of the more selective conjugates, the glycinate 2 and aspartate 3, were evaluated by the iv and icv routes in mice, and they afforded very high iv/icv dose ratios (112,766 and 46,667, respectively) consistent with poor CNS penetration. The in vivo testing revealed that 2 and 3 are delta 1-selective antagonists, in contrast to naltriben and related ligands which are delta 2-selective. The fact that the binding data are not consistent with the in vivo data suggests that the origin of the selectivity of naltrindole congeners may be related to selective access to tissue compartments in the CNS rather than to binding affinity differences between delta opioid receptor subtypes.

Synthesis of naltrexone-derived delta-opioid antagonists. Role of conformation of the delta address moiety.

Naltrindole (1) (NTI) is a highly potent and selective delta-opioid receptor antagonist. In an effort to understand the origin of the high potency, affinity, and selectivity of NTI, we have examined the conformational role of its indolic benzene moiety through the synthesis of related naltrexone derivatives 3-8, which contain the benzene moiety in different orientations and at different attachments in the molecule. One of these naltrexone derivatives, 5, whose 7-indanyl benzene moiety is orthogonal to ring C of the morphinan system, is a potent delta-opioid receptor antagonist in vitro and in vivo. Computer-assisted molecular overlay studies of the minimized structures (2-8) revealed the importance of the position of the benzene moiety for effective interaction with delta-opioid receptors. In compounds 2, 4, and 5, the aromatic ring falls in the same region of space as that of the indolic benzene moiety of NTI, and all of these ligands possessed significant activity at delta-opioid receptors. Analogues (3 and 6-8) which were shown to have relatively weak delta-opioid receptor antagonist potency have their aromatic groups located in a space that is different from that of the more potent analogues.

N-benzylnaltrindoles as long-acting delta-opioid receptor antagonists.

The indolic nitrogen of the delta-opioid receptor antagonist, naltrindole (1), was derivatized with benzyl or substituted benzyl to afford a series (2-9) that retained delta-opioid receptor antagonist activity and selectivity in vitro. The two most potent members (2 and 8) of the series were evaluated in mice and were found to produce delta-selective antagonism of [D-Ser2,Leu5]enkephalin-Thr6 which lasted 5 days. N-Benzylnaltrindole (2) should be useful as a delta 2-selective antagonist for in vivo studies where prolonged action is desired.

Synthesis and delta-opioid receptor antagonist activity of a naltrindole analogue with a regioisomeric indole moiety.

Indolomorphinans 2 and 3, in which the indole moiety is fused to the 7,8-position of the morphinan system, have been synthesized from dihydropseudocodeinone 4 and evaluated for antagonist activity on the mouse vas deferens (MVD) and guinea pig ileum (GPI) preparations. Indolomorphinan 2 was found to be approximately 1/60th as potent as naltrindole 1 in the MVD and an agonist in the GPI preparation. A comparable difference in affinity between 1 and 2 was observed. The methyl analogue 3 was inactive in both preparations. The results of this study support the idea that the regio orientation of the indolic benzene moiety of 1 is optimal for delta-opioid receptor antagonist activity. It is proposed that the proper alignment of the benzene moiety with an address subsite on the delta receptor is critical for potent delta antagonist activity.

Synthesis and kappa-opioid antagonist selectivity of a norbinaltorphimine congener. Identification of the address moiety required for kappa-antagonist activity.

Compound 2, which represents a structurally simplified congener of norbinaltorphimine 1a, was synthesized in order to evaluate the role of its second basic nitrogen in conferring kappa-opioid receptor antagonist selectivity. Congener 2 was found to be at least twice as selective as 1a as a kappa antagonist, while its N-carbobenzoxy derivative (3) was inactive at kappa-receptors. This study establishes the importance of the second basic nitrogen of 1a for kappa-receptor recognition. It is proposed that this basic group mimics the guanidinium moiety of Arg7, which may be the key kappa-address component of dynorphin.

Electrophilic N-benzylnaltrindoles as delta opioid receptor-selective antagonists.

The N-benzyl group of N-benzylnaltrindole (1, BNTI), a potent and selective delta 2 opioid receptor antagonist, was employed as a scaffold to hold electrophilic moieties (isothiocyanate and haloacetamide) in an effort to obtain selective affinity labels (2-4 and 8-11). The corresponding acetamide derivatives (5-7) also were synthesized to serve as nonelectrophilic controls. The o- and p-isothiocyanates (2 and 4) and the haloamides (8-11) were selective delta opioid receptor antagonists in the mouse vas deferens (MVD) preparations, while the meta isomer 3 was a delta-selective full agonist (IC50 = 5 nM). The fact that the effect of 2 and 4 was found to increase as a function of time in MVD suggests a covalent mechanism for the wash resistant component. The m-isothiocyanate 3 was found to be a delta-selective and irreversible agonist in the MVD, and it is suggested that it may be covalently binding to an agonist recognition site. In the mouse abdominal stretch antinociceptive assay, compounds 2-4 and 9 were delta-selective antagonists but exhibited delta 2/delta 1 selectivity ratios than that of BNTI.