Probes for narcotic receptor mediated phenomena 49. N-substituted rac-cis-4a-arylalkyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols.

Racemic N-substituted -1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols containing cis-4a-aralkyl groups were explored as probes for opioid receptors. Specifically cis-4a-phenylpropyl, -phenylbutyl, and-phenylpentyl groups coupled with widely varied substituents on the nitrogen atom were synthesized and their pharmacological profiles at opioid receptors examined. The study yielded compounds with good affinity and moderate to potent antagonist activity at the µ- and δ-opioid receptors, and agonist activity at the κ-opioid receptor. An N-allyl substituent in the C4a phenylpropyl series induced 6-fold higher affinity at δ-than µ-receptors, while an N-CPM substituent in the C4a (CH2)3Ph series led to a compound with high δ-affinity and potent δ-antagonist activity.

Effect of Iboga alkaloids on µ-opioid receptor-coupled G protein activation.

OBJECTIVE: The iboga alkaloids are a class of small molecules defined structurally on the basis of a common ibogamine skeleton, some of which modify opioid withdrawal and drug self-administration in humans and preclinical models. These compounds may represent an innovative approach to neurobiological investigation and development of addiction pharmacotherapy. In particular, the use of the prototypic iboga alkaloid ibogaine for opioid detoxification in humans raises the question of whether its effect is mediated by an opioid agonist action, or if it represents alternative and possibly novel mechanism of action. The aim of this study was to independently replicate and extend evidence regarding the activation of µ-opioid receptor (MOR)-related G proteins by iboga alkaloids. METHODS: Ibogaine, its major metabolite noribogaine, and 18-methoxycoronaridine (18-MC), a synthetic congener, were evaluated by agonist-stimulated guanosine-5´-O-(γ-thio)-triphosphate ([(35)S]GTPγS) binding in cells overexpressing the recombinant MOR, in rat thalamic membranes, and autoradiography in rat brain slices. RESULTS AND SIGNIFICANCE: In rat thalamic membranes ibogaine, noribogaine and 18-MC were MOR antagonists with functional Ke values ranging from 3 uM (ibogaine) to 13 uM (noribogaine and 18MC). Noribogaine and 18-MC did not stimulate [(35)S]GTPγS binding in Chinese hamster ovary cells expressing human or rat MORs, and had only limited partial agonist effects in human embryonic kidney cells expressing mouse MORs. Ibogaine did not did not stimulate [(35)S]GTPγS binding in any MOR expressing cells. Noribogaine did not stimulate [(35)S]GTPγS binding in brain slices using autoradiography. An MOR agonist action does not appear to account for the effect of these iboga alkaloids on opioid withdrawal. Taken together with existing evidence that their mechanism of action also differs from that of other non-opioids with clinical effects on opioid tolerance and withdrawal, these findings suggest a novel mechanism of action, and further justify the search for alternative targets of iboga alkaloids.

Probes for narcotic receptor mediated phenomena. 48. C7- and C8-substituted 5-phenylmorphan opioids from diastereoselective alkylation.

The exploration of the effect of substituents at C7 and C8 of the 5-phenylmorphans on their affinity for opioid receptors was enabled by our recently introduced "one pot" diastereoselective synthesis that provided C7-oxo, hydroxy and alkyl substituents, C8-alkyl substituted 5-phenylmorphans, and compounds that had a new cyclohexane ring that includes the C7 and C8 carbon atoms of the 5-phenylmorphan. The affinity of the 5-phenylmorphans for opioid receptors is increased by a C8-methyl substituent, compared with its C7 analog. The affinity of the newly synthesized compounds is generally for the µ-opioid receptor, rather than the δ- or κ-receptors. Addition of a new cyclohexane ring to the C7 and C8 positions on the cyclohexane ring of the 5-phenylmorphans enhances µ-receptor affinity, bringing the Ki to the subnanomolar level. Unexpectedly, the N-methyl substituted compounds generally had higher affinity than comparable N-phenethyl-substituted relatives. The configurations of two compounds were determined by single-crystal X-ray crystallographic analyses.

Probes for narcotic receptor mediated phenomena. 47. Novel C4a- and N-substituted-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols.

A series of N-methyl rac-cis-4a-aralkyl- and alkyl-substituted-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols have been prepared (2a-l) using a simple previously designed synthetic route, in order to find a ligand that would interact with both µ- and δ-opioid receptors. A C4a-phenethyl derivative 2a, was found to have modest receptor affinity both at µ- (K(i)=60 nM) and δ-opioid receptors (K(i)=64 nM). The N-methyl substituent of 2a and that of other ligands in the series was then modified to obtain compounds with different N-substituents that might provide higher affinity at both receptors. A number of compounds differently substituted at C4a and N were synthesized and evaluated. Binding studies and functional assays revealed a moderately selective δ-antagonist (2l), selective µ-δ antagonists (3d, 3g), and a µ-κ antagonist (3f).

Probes for narcotic receptor mediated phenomena. 46. N-substituted-2,3,4,9,10,10a-hexahydro-1H-1,4a-(epiminoethano)phenanthren-6- and 8-ols - carbocyclic relatives of f-oxide-bridged phenylmorphans.

Oxide-bridged phenylmorphans were conceptualized as topologically distinct, structurally rigid ligands with 3-dimensional shapes that could not be appreciably modified on interaction with opioid receptors. An enantiomer of the N-phenethyl-substituted ortho-f isomer was found to have high affinity for the µ-receptor (K(i) = 7 nM) and was about four times more potent than naloxone as an antagonist. In order to examine the effect of introduction of a small amount of flexibility into these molecules, we have replaced the rigid 5-membered oxide ring with a more flexible 6-membered carbon ring. Synthesis of the new N-phenethyl-substituted tricyclic N-substituted-2,3,4,9,10,10a-hexahydro-1H-1,4a-(epiminoethano)phenanthren-6- and 8-ols resulted in a two carbon-bridged relative of the f-isomers, the dihydrofuran ring was replaced by a cyclohexene ring. The carbocyclic compounds had much higher affinity and greater selectivity for the µ-receptor than the f-oxide-bridged phenylmorphans. They were also much more potent µ-antagonists, with activities comparable to naltrexone in the [(35)S]GTP-γ-S assay.

14-Alkoxy- and 14-acyloxypyridomorphinans: µ agonist/δ antagonist opioid analgesics with diminished tolerance and dependence side effects.

In the search for opioid ligands with mixed functional activity, a series of 5'-(4-chlorophenyl)-4,5α-epoxypyridomorphinans possessing alkoxy or acyloxy groups at C-14 was synthesized and evaluated. In this series, the affinity and functional activity of the ligands were found to be influenced by the nature of the substituent at C-14 as well as by the substituent at N-17. Whereas the incorporation of a 3-phenylpropoxy group at C-14 on N-methylpyridomorhinan gave a dual MOR agonist/DOR agonist 17h, its incorporation on N-cyclopropylmethylpyridomorphinan gave a MOR agonist/DOR antagonist 17d. Interestingly, 17d, in contrast to 17h, did not produce tolerance or dependence effects upon prolonged treatment in cells expressing MOR and DOR. Moreover, 17d displayed greatly diminished analgesic tolerance as compared to morphine upon repeated administration, thus supporting the hypothesis that ligands with MOR agonist/DOR antagonist functional activity could emerge as novel analgesics devoid of tolerance, dependence, and related side effects.

An efficient synthesis of 3-OBn-6ß,14-epoxy-bridged opiates from naltrexone and identification of a related dual MOR inverse agonist/KOR agonist.

In an effort to better understand the conformational preferences that inform the biological activity of naltrexone and related naltrexol derivatives, a new synthesis of the restricted analog 3-OBn-6ß,14-epoxymorphinan 4 is described. 4 was synthesized starting from naltrexone in 50% overall yield, proceeding through the OBn-6α-triflate intermediate 8. Key steps to the synthesis include benzylation (96% yield), reduction (90% yield, α:ß:3:2), followed by a one-pot triflation/displacement sequence (96% yield) to yield the desired bridged epoxy derivative 4. X-ray crystallographic analysis of intermediate 3-OBn-6α-naltrexol 7a supports population of the key boat conformation required for the epoxy ring closure. We also report that the 6ß-mesylate 10-a high affinity opioid receptor ligand, the epimeric derivative of 11, and an analog of 12-functions as an inverse agonist at the mu opioid receptor using herkinorin pre-conditioned cells and an agonist at the kappa opioid receptor when evaluated in independent in vitro [(35)S]-GTP-γ-S assays.

Semisynthetic neoclerodanes as kappa opioid receptor probes.

Modification of the furan ring of salvinorin A (1), the main active component of Salvia divinorum, has resulted in novel neoclerodane diterpenes with opioid receptor affinity and activity. Conversion of the furan ring to an aldehyde at the C-12 position (5) has allowed for the synthesis of analogues with new carbon-carbon bonds at that position. Previous methods for forming these bonds, such as Grignard and Stille conditions, have met with limited success. We report a palladium catalyzed Liebeskind-Srogl cross-coupling reaction of a thioester and a boronic acid that occurs at neutral pH and ambient temperature to produce ketone analogs at C-12. To the best of our knowledge, this is the first reported usage of the Liebeskind-Srogl reaction to diversify a natural product scaffold. We also describe a one-step protocol for the conversion of 1 to 12-epi-1 (3) through microwave irradiation. Previously, this synthetically challenging process has required multiple steps. Additionally, we report in this study that alkene 9 and aromatic analogues 12, 19, 23, 25, and 26 were discovered to retain affinity and selectivity at kappa opioid receptors (KOP). Finally, we report that the furan-2-yl analog of 1 (31) has similar affinity to 1. Collectively, these findings suggest that different aromatic groups appended directly to the decalin core may be well tolerated by KOP receptors, and may generate further ligands with affinity and activity at KOP receptors.

Probes for narcotic receptor mediated phenomena. 44. Synthesis of an N-substituted 4-hydroxy-5-(3-hydroxyphenyl)morphan with high affinity and selective µ-antagonist activity.

A simple three-step synthesis of 5-(3-hydroxyphenyl)-2-methyl-2-azabicyclo[3.3.1]nonan-4-ol (3a) was achieved using an osmium tetroxide mediated oxidation of the known intermediate 6. A pyrrolidine-ring variant of 3a (3-(7-(hydroxymethyl)-6-methyl-6-azabicyclo[3.2.1]octan-1-yl)phenol (5)) was isolated when other routes were used. The epimeric hydroxy analogue 4a was synthesized by simple inversion of the stereochemistry at C-4. Both N-methyl (3a and 4a) and N-phenethyl (3b and 4b) derivatives were synthesized. The compounds were examined for their opioid receptor affinity and the N-phenethyl analogue 3b was found to have relatively weak affinity for the µ-opioid receptor (K(i) = 74 nM). However, the N-phenethyl analogue of the C-4 epimer, 4b, had about 15 fold higher affinity than 3b and was selective for the µ-opioid receptor (K(i) = 4.6 nM). Compound 4b was a moderately potent µ-opioid antagonist (K(e) = 12 nM), as determined by [(35)S]GTP-γ-S assays. Compounds 3b and 4b were energy minimized at the level of B3LYP/6-31G*, and then overlaid onto the 5-phenylmorphan, the (1R,5R,9S)-(-)-enantiomer of 2b (Fig. 1) with the α or ß-OH group at the C-9 position. The spatial orientation of the hydroxyl moiety in 3b, 4b, 2a, and 2b is proposed to be the structural requirement for high µ-opioid receptor binding affinity and their agonist or antagonist activity. The modest change in spatial position of the hydroxyl moiety, and not the N-substituent, induced the change from potent agonist to an antagonist of moderate potency.

Probes for narcotic receptor mediated phenomena. 43. Synthesis of the ortho-a and para-a, and improved synthesis and optical resolution of the ortho-b and para-b oxide-bridged phenylmorphans: compounds with moderate to low opioid-receptor affinity.

N-Phenethyl-substituted ortho-a and para-a oxide-bridged phenylmorphans have been obtained through an improved synthesis and their binding affinity examined at the various opioid receptors. Although the N-phenethyl substituent showed much greater affinity for µ- and κ-opioid receptors than their N-methyl relatives (e.g., K(i)=167 nM and 171 nM at µ- and κ-receptors vs >2800 and 7500 nM for the N-methyl ortho-a oxide-bridged phenylmorphan), the a-isomers were not examined further because of their relatively low affinity. The N-phenethyl substituted ortho-b and para-b oxide-bridged phenylmorphans were also synthesized and their enantiomers were obtained using supercritical fluid chromatography. Of the four enantiomers, only the (+)-ortho-b isomer had moderate affinity for µ- and κ-receptors (K(i)=49 and 42 nM, respectively, and it was found to also have moderate µ- and κ-opioid antagonist activity in the [(35)S]GTP-γ-S assay (K(e)=31 and 26 nM).

Probes for narcotic receptor mediated phenomena. Part 42: synthesis and in vitro pharmacological characterization of the N-methyl and N-phenethyl analogues of the racemic ortho-c and para-c oxide-bridged phenylmorphans.

A new synthesis of N-methyl and N-phenethyl substituted ortho-c and para-c oxide-bridged phenylmorphans, using N-benzyl- rather than N-methyl-substituted intermediates, was used and the pharmacological properties of these compounds were determined. The N-phenethyl substituted ortho-c oxide-bridged phenylmorphan(rac-(3R,6aS,11aS)-2-phenethyl-2,3,4,5,6,11a-hexahydro-1H-3,6a-methanobenzofuro[2,3-c]azocin-10-ol (12)) was found to have the highest µ-opioid receptor affinity (K(i)=1.1 nM) of all of the a- through f-oxide-bridged phenylmorphans. Functional data ([³5S]GTP-γ-S) showed that the racemate 12 was more than three times more potent than naloxone as an µ-opioid antagonist.

Altered gene expression in pulmonary tissue of tryptophan hydroxylase-1 knockout mice: implications for pulmonary arterial hypertension.

The use of fenfluramines can increase the risk of developing pulmonary arterial hypertension (PAH) in humans, but the mechanisms responsible are unresolved. A recent study reported that female mice lacking the gene for tryptophan hydroxylase-1 (Tph1(-/-) mice) were protected from PAH caused by chronic dexfenfluramine, suggesting a pivotal role for peripheral serotonin (5-HT) in the disease process. Here we tested two alternative hypotheses which might explain the lack of dexfenfluramine-induced PAH in Tph1(-/-) mice. We postulated that: 1) Tph1(-/-) mice express lower levels of pulmonary 5-HT transporter (SERT) when compared to wild-type controls, and 2) Tph1(-/-) mice display adaptive changes in the expression of non-serotonergic pulmonary genes which are implicated in PAH. SERT was measured using radioligand binding methods, whereas gene expression was measured using microarrays followed by quantitative real time PCR (qRT-PCR). Contrary to our first hypothesis, the number of pulmonary SERT sites was modestly up-regulated in female Tph1(-/-) mice. The expression of 51 distinct genes was significantly altered in the lungs of female Tph1(-/-) mice. Consistent with our second hypothesis, qRT-PCR confirmed that at least three genes implicated in the pathogenesis of PAH were markedly up-regulated: Has2, Hapln3 and Retlna. The finding that female Tph1(-/-) mice are protected from dexfenfluramine-induced PAH could be related to compensatory changes in pulmonary gene expression, in addition to reductions in peripheral 5-HT. These observations emphasize the intrinsic limitation of interpreting data from studies conducted in transgenic mice that are not fully characterized.

Opioid receptor probes derived from cycloaddition of the hallucinogen natural product salvinorin A.

As part of our continuing efforts toward more fully understanding the structure-activity relationships of the neoclerodane diterpene salvinorin A, we report the synthesis and biological characterization of unique cycloadducts through [4+2] Diels-Alder cycloaddition. Microwave-assisted methods were developed and successfully employed, aiding in functionalizing the chemically sensitive salvinorin A scaffold. This demonstrates the first reported results for both cycloaddition of the furan ring and functionalization via microwave-assisted methodology of the salvinorin A skeleton. The cycloadducts yielded herein introduce electron-withdrawing substituents and bulky aromatic groups into the C-12 position. Kappa opioid (KOP) receptor space was explored through aromatization of the bent oxanorbornadiene system possessed by the cycloadducts to a planar phenyl ring system. Although dimethyl- and diethylcarboxylate analogues 5 and 6 retain some affinity and selectivity for KOP receptors and are full agonists, their aromatized counterparts 13 and 14 have reduced affinity for KOP receptors. The methods developed herein signify a novel approach toward rapidly probing the structure-activity relationships of furan-containing natural products.

Probes for narcotic receptor mediated phenomena. 41. Unusual inverse µ-agonists and potent µ-opioid antagonists by modification of the N-substituent in enantiomeric 5-(3-hydroxyphenyl)morphans.

Conformational restraint in the N-substituent of enantiomeric 5-(3-hydroxyphenyl)morphans was conferred by the addition of a cyclopropane ring or a double bond. All of the possible enantiomers and isomers of the N-substituted compounds were synthesized. Opioid receptor binding assays indicated that some of them had about 20-fold higher µ-affinity than the compound with an N-phenylpropyl substituent (K(i) = 2-450 nM for the examined compounds with various N-substituents). Most of the compounds acted unusually as inverse agonists in the [(35)S]GTP-γ-S functional binding assay using nondependent cells that stably express the cloned human µ-opioid receptor. Two of the N-substituted compounds with a cyclopropane ring were very potent µ-opioid antagonists ((+)-29, K(e) = 0.17 and (-)-30, K(e) =0.3) in the [(35)S]GTP-γ-S functional binding assay. By comparison of the geometry-optimized structures of the newly synthesized compounds, an attempt was made to rationalize their µ-opioid receptor affinity in terms of the spatial position of N-substituents.

Potential Drug Abuse Therapeutics Derived from the Hallucinogenic Natural Product Salvinorin A.

Previous structure-activity relationship studies of salvinorin A have shown that modification of the acetate functionality off the C-2 position to a methoxy methyl or methoxy ethyl ether moiety leads to increased potency at KOP receptors. However, the reason for this increase remains unclear. Here we report our efforts towards the synthesis and evaluation of C-2 constrained analogs of salvinorin A. These analogs were evaluated at opioid receptors in radioligand binding experiments as well as in the GTP-γ-S functional assay. One compound, 5, was found to have affinity and potency at κ opioid (KOP) receptors comparable to salvinorin A. In further studies, 5 was found to attenuate cocaine-induced drug seeking behavior in rats comparably to salvinorin A. This finding represents the first example of a salvinorin A analog that has demonstrated anti-addictive capabilities.

In vitro and in vivo assessment of mu opioid receptor constitutive activity.

Constitutive (basal) signaling has been described and characterized for numerous G protein coupled receptors (GPCRs). The relevance of this activity to disease, drug discovery and development, and to clinical pharmacotherapy is just beginning to emerge. Opioid receptors were the first GPCR systems for which there was definitive evidence presented for constitutive activity, with numerous studies now published on the regulation of this activity (e.g., structure/activity of the receptor as it relates to basal activity, pharmacology of ligands that act as agonists, inverse agonists and "neutral antagonists," etc.). This chapter summarizes some of the methods used to characterize constitutive activity at the mu opioid receptor (MOR) in preclinical in vitro and in vivo model systems. This includes cell-based systems that are useful for higher throughput screening of novel ligands and for studying variables that can impact basal tone in a system. In vivo assays are also described in which constitutive activity is increased in response to acute or chronic opioid agonist exposure and where withdrawal is precipitated with antagonists that may function as inverse agonists or "neutral" antagonists. The methods described have inherent advantages and disadvantages that need to be considered in any drug discovery/development program. A brief discussion of progress toward understanding the clinical implications of MOR constitutive activity in the management of opioid addiction and chronic pain is also included in this chapter.

Synthesis and opioid activity of enantiomeric N-substituted 2,3,4,4a,5,6,7,7a-octahydro-1H-benzofuro[3,2-e]isoquinolines.

A series of enantiomeric N-substituted 2,3,4,4a,5,6,7,7a-octahydro-1H-benzofuro[3,2-e]isoquinolines was synthesized. The (-)-enantiomers had much greater kappa-, mu-, and delta-opioid receptor binding affinity than the corresponding (+)-enantiomers. Compounds (-)-1a, (-)-1b, and (-)-1c displayed subnanomolar binding affinity for the mu-receptor, and (-)-1b had a high affinity for the kappa-receptor. Compound (-)-1a was a mu-partial agonist and kappa-antagonist. Compound (-)-1b was a potent neutral mu-antagonist (K(d) = 0.22 nM) and a kappa-partial agonist.

Probes for narcotic receptor mediated phenomena. 40. N-substituted cis-4a-ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-8-ols.

A series of N-substituted rac-cis-4a-ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-8-ols have been prepared using a simple synthetic route previously designed for synthesis of related cis-2-methyl-4a-alkyl-1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ols. The new phenolic compounds, where the aromatic hydroxy moiety is situated ortho to the oxygen atom in the oxide-bridged ring, do not interact as well as the pyridin-6-ols with opioid receptors. The N-para-fluorophenethyl derivative had the highest mu-opioid receptor affinity of the examined compounds (K(i)=0.35 microM).

Identification of a novel "almost neutral" micro-opioid receptor antagonist in CHO cells expressing the cloned human mu-opioid receptor.

The basal (constitutive) activity of G protein-coupled receptors allows for the measurement of inverse agonist activity. Some competitive antagonists turn into inverse agonists under conditions where receptors are constitutively active. In contrast, neutral antagonists have no inverse agonist activity, and they block both agonist and inverse agonist activity. The mu-opioid receptor (MOR) demonstrates detectable constitutive activity only after a state of dependence is produced by chronic treatment with a MOR agonist. We therefore sought to identify novel MOR inverse agonists and novel neutral MOR antagonists in both untreated and agonist-treated MOR cells. CHO cells expressing the cloned human mu receptor (hMOR-CHO cells) were incubated for 20 h with medium (control) or 10 microM (2S,4aR,6aR,7R,9S,10aS,10bR)-9-(benzoyloxy)-2-(3-furanyl)dodecahydro-6a,10b-dimethyl-4,10-dioxo-2H-naphtho-[2,1-c]pyran-7-carboxylic acid methyl ester (herkinorin, HERK). HERK treatment generates a high degree of basal signaling and enhances the ability to detect inverse agonists. [(35)S]-GTP-gamma-S assays were conducted using established methods. We screened 21 MOR "antagonists" using membranes prepared from HERK-treated hMOR-CHO cells. All antagonists, including CTAP and 6beta-naltrexol, were inverse agonists. However, LTC-274 ((-)-3-cyclopropylmethyl-2,3,4,4alpha,5,6,7,7alpha-octahydro-1H-benzofuro[3,2-e]isoquinolin-9-ol)) showed the lowest efficacy as an inverse agonist, and, at concentrations less than 5 nM, had minimal effects on basal [(35)S]-GTP-gamma-S binding. Other efforts in this study identified KC-2-009 ((+)-3-((1R,5S)-2-((Z)-3-phenylallyl)-2-azabicyclo[3.3.1]nonan-5-yl)phenol hydrochloride) as an inverse agonist at untreated MOR cells. In HERK-treated cells, KC-2-009 had the highest efficacy as an inverse agonist. In summary, we identified a novel and selective MOR inverse agonist (KC-2-009) and a novel MOR antagonist (LTC-274) that shows the least inverse agonist activity among 21 MOR antagonists. LTC-274 is a promising lead compound for developing a true MOR neutral antagonist.

Synthetic studies of neoclerodane diterpenes from Salvia divinorum: role of the furan in affinity for opioid receptors.

Further synthetic modification of the furan ring of salvinorin A (1), the major active component of Salvia divinorum, has resulted in novel neoclerodane diterpenes with opioid receptor affinity and activity. A computational study has predicted 1 to be a reproductive toxicant in mammals and is suggestive that use of 1 may be associated with adverse effects. We report in this study that piperidine 21 and thiomorpholine 23 have been identified as selective partial agonists at kappa opioid receptors. This indicates that additional structural modifications of 1 may provide ligands with good selectivity for opioid receptors but with reduced potential for toxicity.