A highly selective kappa-opioid receptor agonist with low addictive potential and dependence liability

[Challenges in the treatment of opioid dependence]

BACKGROUND

The number of persons using opioids has increased worldwide in the last decade, particularly the use of opioid analgesics in North America and Africa. In Germany, the prevalence of heroin addiction has remained relatively stable.

METHOD

Narrative review of the literature.

RESULTS

Opioid-assisted maintenance treatment (OMT) with the established substances methadone, levomethadone, slow-release morphine and buprenorphine is recommended as the first-line treatment for heroin dependence. The OMT reduces the use of heroin, mortality and individual suffering and improves the quality of life and physical health. A diamorphine and heroine-assisted treatment is an option for people who do not benefit from conventional OMT. An alternative to the use of diamorphine could be treatment with hydromorphone hydrochloride. The regulations on carrying out maintenance treatment in the Controlled Substances Prescription Act and the guidelines of the Federal Medical Association in Germany have been loosened based on the experiences of the COVID-19 pandemic, for example with respect to take-home prescriptions. There is an ongoing intensive discussion on how to deal with the decreasing number of outpatient clinics offering OMT.

CONCLUSION

The first-line treatment for opioid addiction is opioid-assisted substitution treatment, including diamorphine and heroin-assisted treatment. Long-acting depot medications and implants still play a subordinate role.

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.

Electrochemical N-Demethylation of 14-Hydroxy Morphinans: Sustainable Access to Opioid Antagonists

The most challenging step in the preparation of many opioid antagonists is the selective N-demethylation of a 14-hydroxymorphinan precursor. This process is carried out on a large scale using stoichiometric amounts of hazardous chemicals like cyanogen bromide or chloroformates. We have developed a mild reagent- and catalyst-free procedure for the N-demethylation step based on the anodic oxidation of the tertiary amine. The ensuing intermediates can be readily hydrolyzed to the target nor-opioids in very good yields.

Batch- and Continuous-Flow Aerobic Oxidation of 14-Hydroxy Opioids to 1,3-Oxazolidines-A Concise Synthesis of Noroxymorphone

14-Hydroxymorphinone is converted to noroxymorphone, the immediate precursor of important opioid antagonists, such as naltrexone and naloxone, in a three-step reaction sequence. The initial oxidation of the N-methyl group in 14-hydroxymorphinone with in situ generated colloidal palladium(0) as the catalyst and molecular oxygen as the terminal oxidant constitutes the key transformation in this new route. This oxidation results in the formation of an unexpected oxazolidine ring structure. Subsequent hydrolysis of the oxazolidine under reduced pressure followed by hydrogenation in a packed-bed flow reactor using palladium(0) as the catalyst provides noroxymorphone in high purity and good overall yield. To overcome challenges associated with gas-liquid reactions with molecular oxygen, the key oxidation reaction was translated to a continuous-flow process.

Pharmacological mechanisms underlying the antinociceptive and tolerance effects of the 6,14-bridged oripavine compound 030418

AIM

To investigate possible pharmacological mechanisms underlying the antinociceptive effect of and tolerance to N-methyl-7α-[(R)-1-hydroxy-1-methyl-3-(thien-3-yl)-propyl]-6,14-endo-ethanotetrahydronororipavine (030418), a derivative of thienorphine.

METHODS

The binding affinity and efficacy of 030418 were determined using receptor binding and guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) assays in CHO-μ, CHO-κ, CHO-δ, and CHO-ORL1 cell membranes. The analgesic activity of and tolerance to 030418 were evaluated in thermal nociceptive tests in mice. The effects of 030418 on opioid receptors were further investigated using in vivo pharmacological antagonist blockade and in vitro tissue preparations.

RESULTS

The compound 030418 displayed high binding affinity to all subtypes of opioid receptors with K(i) values in the nanomolar range. In [(35)S]GTPγS binding assay, the maximal stimulation of 030418 to μ-, κ-, δ-receptors and the ORL1 receptor was 89%, 86%, 67% and 91%, respectively. In hot-plate test, the antinociceptive effect of 030418 was more potent and longer than morphine. The nonselective opioid receptor antagonist naloxone could completely block 030418-induced antinociception, while both the μ-opioid receptor antagonist β-FNA and the κ-opioid receptor antagonist nor-BNI attenuated 030418-induced antinociception. In contrast, the ORL1 receptor antagonist J-113397 enhanced the antinociceptive effect of 030418. Additionally, chronic treatment with 030418 resulted in a dramatic development of tolerance that could not be effectively prevented by J-113397. In guinea pig ileum preparation, the existing action of 030418 could be removed with difficulty after prolonged washing.

CONCLUSION

The compound 030418 is a novel agonist of opioid receptors with high efficiency, long-lasting effect and liability to tolerance, which may be closely correlated with the methyl group at the N(17) position and the high hydrophobicity of the C(7)-thiophene group in its chemical structure.

Search for the "ideal analgesic" in pain treatment by engineering the mu-opioid receptor

The myriad of side effects that associate with morphine has been problematic in the clinical use to manage moderate to severe pain. It has been the holy grail of the pharmacologists to develop a compound, or treatment paradigm that could retain the analgesic effect of the drug as eliminating or reducing the side effects, mainly the tolerance and addiction development associates with chronic usage of the drug. In our earlier receptor structure/activities studies, we discovered an unique mutation of a conserved Ser in the fourth transmembrane domain of the opioid receptor that the alkaloid antagonist could activate the receptor. On the basis of this initial finding, we decide to explore the possibility of using virus to deliver the mutant mu-opioid receptor at the various sites of the nociceptive pathway and induce the antinociceptive responses with the systemic administration of opioid antagonists. In this article, we will summarize the progress of such approach and the probable advantages over the conventional approach of drug development in the treatment of chronic pain.

The effects of C-terminal modifications on the opioid activity of [N-benzylTyr(1)]dynorphin A-(1-11) analogues

Structural modifications affecting the efficacy of analogues of the endogenous opioid peptide dynorphin (Dyn) A have focused on the N-terminal "message" sequence based on the "message-address" concept. To test the hypothesis that changes in the C-terminal "address" domain could affect efficacy, modified amino acids and cyclic constraints were incorporated into this region of the partial agonist [N-benzylTyr(1)]Dyn A-(1-11). Modifications in the C-terminal domain of [N-benzylTyr(1)]Dyn A-(1-11)NH(2) resulted in increased kappa opioid receptor (KOR) affinity for all of the linear analogues but did not affect the efficacy of these peptides at KOR. Cyclization between positions 5 and 8 yielded [N-benzylTyr(1),cyclo(d-Asp(5),Dap(8))]Dyn A-(1-11)NH(2) (zyklophin, 13) ( J. Med. Chem. 2005 , 48 , 4500 - 4503 ) with high selectivity for KOR. In contrast to the linear peptides, this peptide exhibits negligible efficacy in the adenylyl cyclase (AC) assay and is a KOR antagonist. These data are consistent with our hypothesis that appropriate modifications in the "address" domain of Dyn A analogues may affect efficacy.

Depletion of serotonin decreases the effects of the kappa-opioid receptor agonist U-69593 on cocaine-stimulated activity

Treatment with a kappa-opioid receptor agonist for 5 days decreases locomotor activity and reduces activity in response to a cocaine challenge 3 days later. In addition, chronic cocaine increases kappa-opioid receptor density, striatal dynorphin, and dynorphin gene expression in the striatum. The upregulation of kappa-opioid receptors after cocaine treatment occurs predominantly in brain regions that are highly innervated by serotonin. To determine if serotonin plays a role in the effects of kappa-opioid receptor agonists on cocaine-stimulated activity, parachloroamphetamine (PCA), which depleted serotonin by 53%-66%, or saline, was given prior to a five-day treatment with U-69593 or vehicle. Three days later each rat received a single injection of cocaine and locomotor activity was measured. Treatment with PCA had no effect on the ability of U-69593 alone to decrease locomotor activity. Thus, the behavioral effects of U-69593 alone were not dependent upon serotonin. In rats pretreated with saline, U-69593 treatment significantly blocked the locomotor-activating effects of cocaine. Following PCA pretreatment, however, there were no significant differences in locomotor activity in rats challenged with an injection of cocaine after treatment with U-69593 or vehicle. Thus, serotonin depletion prevented the long-lasting blockade of the locomotor-activating effects of cocaine subsequent to repeated administration of U-69593 but did not alter the effects of cocaine in rats that were treated with vehicle. Thus, the effects of PCA on U-69593 are not due to non-specific alterations in cocaine-induced locomotor activity. These findings suggest that serotonin plays an important role in mediating the effects of kappa-opioid receptor agonists on the behavioral response to cocaine.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Novel diastereomeric opioid tetrapeptides exhibit differing pharmacological activity profiles

A novel opioid peptide antagonist analogue, [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe, derived from the potent, delta-receptor selective TIPP tetrapeptide (Tyr-Tic-Phe-Phe) series was synthesized and radiolabeled by catalytic tritiation of its iodinated precursor peptide. The purified radioprobe exhibited a specific activity of 2.15 TBq/mmol (58 Ci/mmol). The novelty of this compound is that it contains structurally modified tyrosine residue (2',6'-dimethyltyrosine, Dmt1) replacing tyrosine (Tyr1) at the N-terminus, and beta-methyl substituted phenylalanine (betaMePhe3) at the third position. As the configuration of betaMePhe3 side-chain might be different due to diastereomerism, and accordingly can alter the biological activity, both unlabeled threo (2S,3R and 2R,3S) diastereomeric analogues were also prepared and included in this study. The affinity and selectivity (delta-opioid versus mu-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potencies were determined in [35S]GTPgammaS binding experiments using Chinese Hamster Ovary (CHO) cells selectively expressing delta- or mu-opioid receptors. The equilibrium binding of the radiolabeled peptide derivative [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe to rat brain membranes was saturable and the Scatchard analysis indicated a single binding site with a Kd of 0.3 nM and a Bmax of 127 fmol/mg protein. A study of [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe binding displacement by various receptor-type specific opioid ligands showed the rank order of competitor's potency delta > mu > kappa, suggesting selective labeling of opioid delta-sites. In the functional tests, the (2S,3R) and (2R,3S) peptides exhibited partial agonist behaviour by weakly stimulating regulatory G-proteins in CHO cell membranes transfected with different receptors. Both isomers were quite weak partial agonists at the delta-receptor and reasonable partial agonists at the mu-receptor, with a prevalence of (2S,3R) over (2R,3S) for the mu-receptor. Consistent with these observations both stereomers competitively inhibited the stimulation of [35S]GTPgammaS binding induced by the prototype delta-agonist peptide (pClPhe4)-DPDPE in delta(m) CHO cell membranes, and still the (2S,3R) compound exerted more potent delta-antagonist effect. [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe represents a high affinity new radioligand and also constitute further example of the influence of beta-methyl substitution on the potency and selectivity of TIPP analogues, thus becoming a valuable biochemical and pharmacological tool in opioid research.