Synthesis and evaluation of a full-agonist orvinol for PET-imaging of opioid receptors: [11C]PEO

Diels–Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels–Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

Semi-synthesis of novel buprenorphine derivatives and their anti-nociceptive properties and dependency potential

Novel 1,2,3-triazole-tethered N-norbuprenorphine derivatives with an OMe or OH group at the C3 position were synthesized alongside with evaluation of their analgesic properties. The analgesic activities of the resulting library were investigated via tail flick test in mice. Our results indicated that 10b and 10e were as effective as the starting compounds 8 and 9 with ED50 equal to 16.59 and 19.44 mg/kg, respectively. To investigate the effect of a methyl group at C3 on biological properties, the most active compounds were O-demethylated and their anti-nociceptive effects were assessed. The new O-demethylated derivatives (11b and 11e) showed better analgesic properties than the parent compounds with ED50 of 14.73 and 15.80 mg/kg, respectively. Naloxone prevented the analgesic effect of the synthesized compounds, indicating that the opioid receptors are highly involved in the anti-nociceptive effects. The potential dependency effects of the most potent derivatives were studied by condition place preference test in mice and compared with morphine and buprenorphine. Interestingly, 10b, 10e, 11b, and 11e did not show any dependency effect, similar to buprenorphine.

Synthesis, Radiosynthesis and Biological Evaluation of Buprenorphine-Derived Phenylazocarboxamides as Novel μ-Opioid Receptor Ligands

Targeted structural modifications have led to a novel type of buprenorphine-derived opioid receptor ligand displaying an improved selectivity profile for the μ-OR subtype. On this basis, it is shown that phenylazocarboxamides may serve as useful bioisosteric replacements for the widely occurring cinnamide units, without loss of OR binding affinity or subtype selectivity. This study further includes functional experiments pointing to weak partial agonist properties of the novel μ-OR ligands, as well as docking and metabolism experiments. Finally, the unique bifunctional character of phenylazocarboxylates, herein serving as precursors for the azocarboxamide subunit, was exploited to demonstrate the accessibility of an 18 F-fluorinated analogue.

Synthesis, biochemical, pharmacological characterization and in silico profile modelling of highly potent opioid orvinol and thevinol derivatives

Morphine and its derivatives play inevitably important role in the μ-opioid receptor (MOR) targeted antinociception. A structure-activity relationship study is presented for novel and known orvinol and thevinol derivatives with varying 3-O, 6-O, 17-N and 20-alkyl substitutions starting from agonists, antagonists and partial agonists. In vitro competition binding experiments with [³H]DAMGO showed low subnanomolar affinity to MOR. Generally, 6-O-demethylation increased the affinity toward MOR and decreased the efficacy changing the pharmacological profile in some cases. In vivo tests in osteoarthritis inflammation model showed significant antiallodynic effects of thevinol derivatives while orvinol derivatives did not. The pharmacological character was modelled by computational docking to both active and inactive state models of MOR. Docking energy difference for the two states separates agonists and antagonists well while partial agonists overlapped with them. An interaction pattern of the ligands, involving the interacting receptor atoms, showed more efficient separation of the pharmacological profiles. In rats, thevinol derivatives showed antiallodynic effect in vivo. The orvinol derivatives, except for 6-O-desmethyl-dihydroetorfin (2c), did not show antiallodynic effect.

A Survey of Molecular Imaging of Opioid Receptors

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as μ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-¹¹C]morphine, -codeine and -heroin did not show significant binding in vivo. [¹¹C]Diprenorphine ([¹¹C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the μ-preferring agonist [¹¹C]carfentanil ([¹¹C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [¹¹C]DPN or [¹¹C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate μ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of μORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [¹¹C]GR103545 is validated for studies of κORs. Structures such as [¹¹C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

Synthesis and evaluation of three structurally related ¹⁸F-labeled orvinols of different intrinsic activities: 6-O-[¹⁸F]fluoroethyl-diprenorphine ([¹⁸F]FDPN), 6-O-[¹⁸F]fluoroethyl-buprenorphine ([¹⁸F]FBPN), and 6-O-[¹⁸F]fluoroethyl-phenethyl-orvinol ([¹⁸F]FPEO)

We report the synthesis and biological evaluation of a triplet of 6-O-(18)F-fluoroethylated derivatives of structurally related orvinols that span across the full range of intrinsic activities, the antagonist diprenorphine, the partial agonist buprenorphine, and the full agonist phenethyl-orvinol. [(18)F]fluoroethyl-diprenorphine, [(18)F]fluoroethyl-buprenorphine, and [(18)F]fluoroethyl-phenethyl-orvinol were prepared in high yields and quality from their 6-O-desmethyl-precursors. The results indicate suitable properties of the three 6-O-(18)F-fluoroethylated derivatives as functional analogues to the native carbon-11 labeled versions with similar pharmacological properties.

A fully automated radiosynthesis of [18F]fluoroethyl-diprenorphine on a single module by use of SPE cartridges for preparation of high quality 2-[18F]fluoroethyl tosylate

We have developed a new method for automated production of 2-[¹⁸F]fluoroethyl tosylate ([¹⁸F]FETos) that enables ¹⁸F-alkylation to provide PET tracers with high chemical purity. The method is based on the removal of excess ethylene glycol bistosylate precursor by precipitation and subsequent filtration and purification of the filtrate by means of solid phase extraction cartridges (SPE). The method is integrated to a single synthesis module and thereby provides the advantage over previous methods of not requiring HPLC purification, as demonstrated by the full radiosynthesis of the potent opioid receptor PET tracer [¹⁸F]fluoroethyldiprenorphine.

Design and synthesis of an ¹⁸F-labeled version of phenylethyl orvinol ([¹⁸F]FE-PEO) for PET-imaging of opioid receptors

The semisynthetic oripavine derivative phenethyl orvinol (PEO), a full agonist at opioid receptors (OR), is an attractive structural motif for developing ¹⁸F-labeled PET tracers with a high degree of sensitivity for competition between endogenous and exogenous OR-ligands. The target cold reference compound 6-O-(2-fluoroethyl)-6-O-desmethylphenylethyl orvinol (FE-PEO) was obtained via two separate reaction routes. A three-step synthesis was developed for the preparation of a tosyloxyethyl precursor (TE-TDPEO), the key precursor for a direct, nucleophilic radiofluorination to yield [¹⁸F]FE-PEO. The developed radiosynthesis provides the target compound in relevantly high yield and purity, and is adaptable to routine production.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Spinal long-term potentiation is associated with reduced opioid neurotransmission in the rat brain

INTRODUCTION

Neuronal events leading to development of long-term potentiation (LTP) in the nociceptive pathways may be a cellular mechanism underlying hyperalgesia. In the present study, we examine if induction of spinal LTP may be associated with functional changes in the supraspinal opioidergic system. The opioid receptors (ORs) play a key role in nociceptive processing and controlling the descending modulatory system to the spinal cord.

METHODS

Spinal LTP was induced by electrical high-frequency stimulation (HFS) conditioning applied to the sciatic nerve, and the excitability at spinal level was verified by spinal field potential recordings. To study supraspinal changes in opioid neurotransmission following the same HFS conditioning, we used small animal positron emission tomography (PET) and [(11)C]Phenethyl-Orvinol ([(11)C]PEO). All rats included in the PET study were scanned at baseline and 150 min after HFS, and specific binding was calculated with a reference tissue model.

RESULTS

A clear C-fibre LTP, i.e. increased C-fibre response and reduced C-fibre threshold, was observed 150 min after HFS conditioning (t-test, P<0.05, n = 6). Moreover, increased OR binding, relative to baseline, was observed after the same type of HFS conditioning ipsilaterally in the amygdala, hippocampus, somatosensory cortex and superior colliculus, and bilaterally in the nucleus accumbens, caudate putamen and hypothalamus (paired t-test, HFS>baseline, P<0.05, n = 8).

CONCLUSIONS

HFS conditioning of the sciatic nerve resulted in both spinal LTP and functional changes in supraspinal opioidergic signalling. Our findings suggest that induction of spinal LTP may be associated with reduced opioid neurotransmission in brain regions involved in pain modulation and affective-emotional responses.