The opiate receptor: a model explaining structure-activity relationships of opiate agonists and antagonists

Neuroprotective effects of naltrexone in a mouse model of post-traumatic seizures

Traumatic Brain Injury (TBI) induces neuroinflammatory response that can initiate epileptogenesis, which develops into epilepsy. Recently, we identified anti-convulsive effects of naltrexone, a mu-opioid receptor (MOR) antagonist, used to treat drug addiction. While blocking opioid receptors can reduce inflammation, it is unclear if post-TBI seizures can be prevented by blocking MORs. Here, we tested if naltrexone prevents neuroinflammation and/or seizures post-TBI. TBI was induced by a modified Marmarou Weight-Drop (WD) method on 4-week-old C57BL/6J male mice. Mice were placed in two groups: non-telemetry assessing the acute effects or in telemetry monitoring for interictal events and spontaneous seizures both following TBI and naltrexone. Molecular, histological and neuroimaging techniques were used to evaluate neuroinflammation, neurodegeneration and fiber track integrity at 8 days and 3 months post-TBI. Peripheral immune responses were assessed through serum chemokine/cytokine measurements. Our results show an increase in MOR expression, nitro-oxidative stress, mRNA expression of inflammatory cytokines, microgliosis, neurodegeneration, and white matter damage in the neocortex of TBI mice. Video-EEG revealed increased interictal events in TBI mice, with 71% mice developing post-traumatic seizures (PTS). Naltrexone treatment ameliorated neuroinflammation, neurodegeneration, reduced interictal events and prevented seizures in all TBI mice, which makes naltrexone a promising candidate against PTS, TBI-associated neuroinflammation and epileptogenesis in a WD model of TBI.

Enantioselective de novo synthesis of 14-hydroxy-6-oxomorphinans

The enantioselective de novo synthesis of pharmacologically important 14-hydroxy-6-oxomorphinans is described. 4,5-Desoxynaltrexone and 4,5-desoxynaloxone were prepared using this route and their biological activities against the opioid receptors were measured.

Opioid ligands addressing unconventional binding sites and more than one opioid receptor subtype

Opioid receptors (ORs) represent one of the most significant groups of G-protein coupled receptor (GPCR) drug targets and also act as prototypical models for GPCR function. In a constant effort to develop drugs with less side effects, and tools to explore the ORs nature and function, various (poly)pharmacological ligand design approaches have been performed. That is, besides classical ligands, a great number of bivalent ligands (i.e. aiming on two distinct OR subtypes), univalent heteromer-selective ligands and bitopic and allosteric ligands have been synthesized for the ORs. The scope of our review is to present the most important of the aforementioned ligands, highlight their properties and exhibit the current state-of-the-art pallet of promising drug candidates or useful molecular tools for the ORs.

An organophotocatalytic late-stage N–CH3 oxidation of trialkylamines to N-formamides with O2 in continuous flow

We report an organophotocatalytic, N–CH₃-selective oxidation of trialkylamines in continuous flow. Based on the 9,10-dicyanoanthracene (DCA) core, a new catalyst (DCAS) was designed with solubilizing groups for flow processing. This allowed O₂ to be harnessed as a sustainable oxidant for late-stage photocatalytic N–CH₃ oxidations of complex natural products and active pharmaceutical ingredients bearing functional groups not tolerated by previous methods. The organophotocatalytic gas–liquid flow process affords cleaner reactions than in batch mode, in short residence times of 13.5 min and productivities of up to 0.65 g per day. Spectroscopic and computational mechanistic studies showed that catalyst derivatization not only enhanced solubility of the new catalyst compared to poorly-soluble DCA, but profoundly diverted the photocatalytic mechanism from singlet electron transfer (SET) reductive quenching with amines toward energy transfer (E_nT) with O₂.

An N–CH₃-selective trialkylamine oxidation to N-formamides is reported in continuous flow using gaseous O₂. A novel, enhanced-solubility dicyanoanthracene organophotocatalyst switched the photochemical mechanism from electron to energy transfer.

Novel Dual-Target μ-Opioid Receptor and Dopamine D3 Receptor Ligands as Potential Nonaddictive Pharmacotherapeutics for Pain Management

The need for safer pain-management therapies with decreased abuse liability inspired a novel drug design that retains μ-opioid receptor (MOR)-mediated analgesia, while minimizing addictive liability. We recently demonstrated that targeting the dopamine D₃ receptor (D₃R) with highly selective antagonists/partial agonists can reduce opioid self-administration and reinstatement to drug seeking in rodent models without diminishing antinociceptive effects. The identification of the D₃R as a target for the treatment of opioid use disorders prompted the idea of generating a class of ligands presenting bitopic or bivalent structures, allowing the dual-target binding of the MOR and D₃R. Structure-activity relationship studies using computationally aided drug design and in vitro binding assays led to the identification of potent dual-target leads (23, 28, and 40), based on different structural templates and scaffolds, with moderate (sub-micromolar) to high (low nanomolar/sub-nanomolar) binding affinities. Bioluminescence resonance energy transfer-based functional studies revealed MOR agonist-D₃R antagonist/partial agonist efficacies that suggest potential for maintaining analgesia with reduced opioid-abuse liability.

In Vivo Antinociceptive, Muscle Relaxant, Sedative, and Molecular Docking Studies of Peshawaraquinone Isolated from Fernandoa adenophylla (Wall. ex G. Don) Steenis

Fernandoa adenophylla (Wall. ex G. Don) Steenis is traditionally used to cure various diseases and can be included as an ingredient in massage oils, which are supposed to comfort muscular tension and pain. This study was designed to assess the antinociceptive, muscle relaxant, and molecular docking properties of a novel compound, namely, (5aR,5a1R,6R,7aS,14bR,15R)15-hydroxy-7a-methyl-6-(2-methylprop-1-en-1-yl)-7,7a,14b,15-tetrahydro-5H-t-5a,15methanobenzo[g]benzo[5,6]azuleno[1,8-bc]chromene-5,9,14,16(5a1H,6H)- tetraone (peshawaraquinone), isolated from the methanolic extract of F. adenophylla in an animal model. The chemical structure of the isolated compound was elucidated using advanced spectroscopic techniques and further confirmed by XRD analysis. Compound 1 was tested against hot plate-induced noxious stimuli at various doses (2.5, 5, 10, and 15 mg/kg i.p.). The muscle relaxation potency of compound 1 was evaluated in the inclined and traction test, while the open-field test was used for the determination of sedative potential. The isolated compound was also subjected to acute toxicity analysis. The compound was then subjected to molecular docking analysis to determine the exact mechanism of action. Compound 1 demonstrated significant (p < 0.05) analgesic effect in a dose-dependent manner. A noticeable muscle relaxant effect was observed with the passage of time in both experimental models. The compound 1 showed a significant (p < 0.05) sedative effect, and in an acute toxicity study, the compound 1 was devoid of any noxious effects. The docking studies showed preferential affinity for μ-opioid and GABAA receptors. Hence, the prospective antinociceptive and muscle relaxant and sedative properties are probably mediated through these two target interactions.

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.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Validation and Cross-Reactivity Data for Fentanyl Analogs With the Immunalysis Fentanyl ELISA

Every year new fentanyl analog compounds, or fentanyls, appear on the drug scene. Development of immunoassays dedicated for screening individual molecules is challenging due to the short-lived presence of these compounds on the recreational drug market. Therefore, we investigated the detecting capabilities of the immunalysis fentanyl direct enzyme-linked immunosorbent assay (ELISA) kit against fentanyl in whole blood, and determined the cross-reactivity of nine fentanyl analogs (2-fluorofentanyl, acetylfentanyl, acrylfentanyl, carfentanil, cyclopropylfentanyl, tetrahydrofuranylfentanyl, furanylfentanyl, ocfentanil, valerylfentanyl) to confirm its validity for the general screening of fentanyls. Immunalysis ELISA assay was used to test whole blood samples fortified with fentanyl on a TECAN Freedom EVOlyzer platform, according to manufacturer specifications. The kit successfully was validated for fentanyl screening with a cutoff set at 0.5 ng/mL, and all tested analogs, with the exclusion of carfentanil, were detected. The lowest cross-reactivity with the kit was obtained with furanylfentanyl (20% ± 1, 95% confidence intervals (CI)) and 4-fluoroisobutyrfentanyl (25% ± 1, 95% CI), while the highest was recorded using acetylfentanyl (99% ± 11, 95% CI) and acrylfentanyl (94% ± 10, 95% CI). Post-mortem samples containing fentanyl, acrylfentanyl, cyclopropylfentanyl, THF-fentanyl and 4-fluoroisobutyrfentanyl were screened, and sensitivity and specificity of each analog were calculated. Positive screening results were generated by all post-mortem cases containing fentanyl (n = 14), acrylfentanyl (n = 11), cyclopropylfentanyl (n = 14), tetrahydrofuranylfentanyl (n = 13) and 4-fluoroisobutyrfentanyl (n = 10). Concentration of post-mortem fentanyl samples ranged from 0.5 ng/mL (cutoff) to 230 ng/mL, while the range for analogs was 3.4-36 ng/mL (cyclopentylfentanyl), 0.76-370 ng/mL (4-fluoroisobutyrfentanyl), 0.02-12 ng/mL (acrylfentanyl) and 2-26 ng/mL (tetrahydrofuranylfentanyl). The immunalysis fentanyl direct ELISA kit was successfully validated and showed significant cross-reactivity for all tested fentanyls, except carfentanil, making it a suitable technique for fentanyl and fentanyl analogs screening.

DARK Classics in Chemical Neuroscience: Morphine

As the major psychoactive agent in opium and direct precursor for heroin, morphine is a historically critical molecule in chemical neuroscience. A structurally complex phenanthrene alkaloid produced by Papaver somniferum, morphine has fascinated chemists seeking to disentangle pharmacologically beneficial analgesic effects from addiction, tolerance, and dependence liabilities. In this review, we will detail the history of morphine, from the first extraction and isolation by Sertürner in 1804 to the illicit use of morphine and proliferation of opioid use and abuse disorders currently ravaging the United States. Morphine is a molecule of great cultural relevance, as the agent that single-handedly transformed our understanding of pharmacognosy, receptor dynamics, and substance abuse and dependence disorders.

The "Cyclopropyl Fragment" is a Versatile Player that Frequently Appears in Preclinical/Clinical Drug Molecules

Recently, there has been an increasing use of the cyclopropyl ring in drug development to transition drug candidates from the preclinical to clinical stage. Important features of the cyclopropane ring are, the (1) coplanarity of the three carbon atoms, (2) relatively shorter (1.51 Å) C-C bonds, (3) enhanced π-character of C-C bonds, and (4) C-H bonds are shorter and stronger than those in alkanes. The present review will focus on the contributions that a cyclopropyl ring makes to the properties of drugs containing it. Consequently, the cyclopropyl ring addresses multiple roadblocks that can occur during drug discovery such as (a) enhancing potency, (b) reducing off-target effects,

μ Opioid receptor: novel antagonists and structural modeling

The μ opioid receptor (MOR) is a prominent member of the G protein-coupled receptor family and the molecular target of morphine and other opioid drugs. Despite the long tradition of MOR-targeting drugs, still little is known about the ligand-receptor interactions and structure-function relationships underlying the distinct biological effects upon receptor activation or inhibition. With the resolved crystal structure of the β-funaltrexamine-MOR complex, we aimed at the discovery of novel agonists and antagonists using virtual screening tools, i.e. docking, pharmacophore- and shape-based modeling. We suggest important molecular interactions, which active molecules share and distinguish agonists and antagonists. These results allowed for the generation of theoretically validated in silico workflows that were employed for prospective virtual screening. Out of 18 virtual hits evaluated in in vitro pharmacological assays, three displayed antagonist activity and the most active compound significantly inhibited morphine-induced antinociception. The new identified chemotypes hold promise for further development into neurochemical tools for studying the MOR or as potential therapeutic lead candidates.

Recent advances in process development for opiate-derived pharmaceutical agents

This short review summarizes our work on the process development for the synthesis of buprenorphine, naltrexone, naloxone, and nalbuphine from naturally occurring opiates such as thebaine and oripavine. Several new methods for N-demethylation of morphinans have been developed during the pursuit of this research. The article traces the evolution of various approaches and provides a comparison for overall efficiency.

Heteroatom analogues of hydrocodone: synthesis and biological activity

Heteroatom analogues of hydrocodone, in which the N-methyl functionality was replaced with oxygen, sulfur, sulfoxide, and sulfone, were prepared by a short sequence from the ethylene glycol ketal of hydrocodone; a carbocyclic analogue of bisnorhydrocodone was also prepared. The compounds were tested for receptor binding and revealed moderate levels of activity for the sulfone analogue of hydrocodone.

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.

NOpiates: Novel Dual Action Neuronal Nitric Oxide Synthase Inhibitors with μ-Opioid Agonist Activity

A novel series of benzimidazole designed multiple ligands (DMLs) with activity at the neuronal nitric oxide synthase (nNOS) enzyme and the μ-opioid receptor was developed. Targeting of the structurally dissimilar heme-containing enzyme and the μ-opioid GPCR was predicated on the modulatory role of nitric oxide on μ-opioid receptor function. Structure-activity relationship studies yielded lead compound 24 with excellent nNOS inhibitory activity (IC50 = 0.44 μM), selectivity over both endothelial nitric oxide synthase (10-fold) and inducible nitric oxide synthase (125-fold), and potent μ-opioid binding affinity, K i = 5.4 nM. The functional activity as measured in the cyclic adenosine monosphospate secondary messenger assay resulted in full agonist activity (EC50 = 0.34 μM). This work represents a novel approach in the development of new analgesics for the treatment of pain.

Consensus 3D model of μ-opioid receptor ligand efficacy based on a quantitative Conformationally Sampled Pharmacophore

Despite being studied for over 30 years, a consensus structure-activity relationship (SAR) that encompasses the full range peptidic and nonpeptidic μ-opioid receptor ligands is still not available. To achieve a consensus SAR the Conformationally Sampled Pharmacophore (CSP) method was applied to develop a predictive model of the efficacy of μ-opioid receptor ligands. Emphasis was placed on predicting the efficacy of a wide range of agonists, partial agonists, and antagonists as well as understanding their mode of interaction with the receptor. Inclusion of all accessible conformations of each ligand, a central feature of the CSP method, enabled structural features between diverse μ-opioid receptor ligands that dictate efficacy to be identified. The models were validated against a diverse collection of peptidic and nonpeptidic ligands, including benzomorphans, fentanyl (4-anilinopiperidine), methadone (3,3-diphenylpropylamines), etonitazene (benzimidazole derivatives), funaltrexamine (C6-substituted 4,5-epoxymorphinan), and herkinorin. The model predicts (1) that interactions of ligands with the B site, as with the 19-alkyl substituents of oripavines, modulate the extent of agonism; (2) that agonists with long N-substituents, as with fentanyl and N-phenethylnormorphine, can bind in an orientation such that the N substitutent interacts with the B site that also allows the basic N-receptor Asp interaction essential for agonism; and (3) that the μ agonist herkinorin, that lacks a basic nitrogen, binds to the receptor in a manner similar to the traditional opioids via interactions mediated by water or a ion. Importantly, the proposed CSP model can be reconciled with previously published SAR models for the μ receptor.

Synthesis and pharmacological activities of 6-glycine substituted 14-phenylpropoxymorphinans, a novel class of opioids with high opioid receptor affinities and antinociceptive potencies

The synthesis and the effect of a combination of 6-glycine and 14-phenylpropoxy substitutions in N-methyl- and N-cycloproplymethylmorphinans on biological activities are described. Binding studies revealed that all new 14-phenylpropoxymorphinans (11−18) displayed high affinity to opioid receptors. Replacement of the 14-methoxy group with a phenylpropoxy group led to an enhancement in affinity to all three opioid receptor types, with most pronounced increases in δ and κ activities, hence resulting in a loss of μ receptor selectivity. All compounds (11−18) showed potent and long-lasting antinociceptive effects in the tail-flick test in rats after subcutaneous administration. For the N-methyl derivatives 13 and 14, analgesic potencies were in the range of their 14-methoxy analogues 9 and 10, respectively. Even derivatives 15−18 with an N-cyclopropylmethyl substituent acted as potent antinociceptive agents, being several fold more potent than morphine. Subcutaneous administration of compounds 13 and 14 produced significant and prolonged antinociceptive effects mediated through peripheral opioid mechanisms in carrageenan-induced inflammatory hyperalgesia in rats.

Influence of intramolecular hydrogen bonding on the electronic structure of oxymorphone

Approximate ab initio molecular orbital methods are used to examine the structural and electronic properties of oxymorphone. The most stable conformation of the molecule is found to include an intramolecular hydrogen bond between the C-14 hydroxyl group and the nitrogen atom in agreement with available experimental data. The total molecular electron density is transformed to a set of localized molecular orbitals, one of which corresponds to the lone electron pair on nitrogen. The hydrogen bond is shown to produce substantial bending and stretching of the lone pair when compared to its shape when such hydrogen bonding is precluded.

Synthesis of 14-alkoxymorphinan derivatives and their pharmacological actions

Among opioids, morphinans play an important role as therapeutically valuable drugs. They include pain relieving agents such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol). Currently used opioid analgesics also share a number of severe side effects, limiting their clinical usefulness. The antagonist morphinans, naloxone and naltrexone are used to treat opioid overdose, opioid dependence, and alcoholism. All these opioid drugs produce their biological actions through three receptor types, mu, delta, and kappa, belonging to the G-protein-coupled receptor family. Considerable effort has been put forward to understand the appropriate use of opioid analgesics, while medicinal chemistry and opioid pharmacology have been continuously engaged in the search for safer, more efficacious and nonaddicting opioid compounds, with the final goal to reduce complications and to improve patient compliance. Toward this goal, recent advances in chemistry, ligand-based structure activity relationships and pharmacology of 14-alkoxymorphinans are reviewed in this chapter. Current developments of different structural patterns of 14-alkoxymorphinans as research tools and their potential therapeutic opportunities are also summarized.

Probable activation of the opioid receptor-nitric oxide-cyclic GMP-K+ channels pathway by codeine

There is evidence that local peripheral administration of morphine produces antinociception through the activation of the nitric oxide (NO)-cyclic GMP-K(+) channels pathway. Therefore we evaluated the possible participation of this pathway in the antinociceptive action produced by codeine in the rat 5% formalin test. Local peripheral injection of codeine produced a dose-dependent antinociception during the first and second phases of the test. Local pretreatment of the paws with the NO synthase inhibitor N(G)-L-nitro-arginine methyl ester (L-NAME), the soluble guanylyl cyclase inhibitor methylene blue, the ATP-sensitive K(+) channel inhibitors glibenclamide and tolbutamide, the non-selective voltage-gated K(+) channel inhibitors 4-aminopyridine (4-AP) and tetraethylammonium (TEA) and the opioid receptor blocker naloxone prevented codeine-induced antinociception in both phases of the test. L-NAME, methylene blue, K(+) channel blockers and naloxone by themselves did not modify formalin-induced nociceptive behavior. Our data suggest that codeine could activate the opioid receptor-NO-cyclic GMP-K(+) channels pathway in order to produce its peripheral antinociceptive effect in the formalin test.

Modeling and simulation of the human delta opioid receptor

A model for the human delta opioid receptor has been generated via sequence alignment, structure building using the crystal structure of bovine rhodopsin as a template, and refinement by molecular dynamics simulation. The model building suggested that, in addition to the previously postulated interaction between D128 and Y308, an internal salt bridge also exists between residues D128 and R192, both of which are conserved in all the opioid receptors. The model and salt bridge were then shown to be stable during a 20-nsec simulation in a lipid bilayer. It is therefore proposed that both of these interactions play a role in stabilizing the inactive state of the receptor. The model is also used in an effort to rationalize many of the mutational studies performed on delta opioid receptors, and to suggest a plausible explanation for the differences between known delta opioid agonists and antagonists.

Synthesis and biological evaluation of 14-alkoxymorphinans. 18. N-substituted 14-phenylpropyloxymorphinan-6-ones with unanticipated agonist properties: extending the scope of common structure-activity relationships

The synthesis, biological, and pharmacological evaluations of 14beta-O-phenylpropyl-substituted morphinan-6-ones are described. The most striking finding of this study was that all of the compounds from the novel series of differently N-substituted 14beta-O-phenylpropylmorphinans acted as powerful opioid agonists. Even with N-substituents such as cyclopropylmethyl and allyl, which are usually associated with distinct antagonist properties, only agonists were obtained. Compared to morphine, the N-cyclopropylmethyl derivative 15 showed considerably increased potency in the in vivo assays in mice (600-fold in the tail-flick assay, 60-fold in the paraphenylquinone writhing test, and 400-fold in the hot-plate assay). Remarkably, most of the new ligands were nonselective and exhibited binding affinities in the subnanomolar range at opioid receptors (mu, kappa, delta), with the N-propyl derivative 19 displaying the highest affinity for the mu-receptor (K(i) = 0.09 nM).

3-Carboxamido analogues of morphine and naltrexone. synthesis and opioid receptor binding properties

In response to the unexpectedly high affinity for opioid receptors observed in a novel series of cyclazocine analogues where the prototypic 8-OH was replaced by a carboxamido group, we have prepared the corresponding 3-CONH(2) analogues of morphine and naltrexone. High affinity (K(i)=34 and 1.7nM) for mu opioid receptors was seen, however, the new targets were 39- and 11-fold less potent than morphine and naltrexone, respectively.

Apparent thermodynamic parameters of ligand binding to the cloned rat mu-opioid receptor

The apparent thermodynamic parameters of binding of ten ligands to the cloned rat mu-opioid receptor stably expressed in Chinese hamster ovary (CHO) cells were investigated. For every ligand, the Kd or Ki values at 0 degrees C, 12 degrees C, 25 degrees C and 37 degrees C were determined, a van't Hoff plot was generated and deltaH degrees' , deltaS degrees' and -TdeltaS degrees' and deltaG degrees' were calculated. Changes in free energy (deltaG degrees') ranged from -10.35 to -15.65 kcal/mol. The binding of sufentanil, ohmefentanyl, diprenorphine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-penicillamineThr-NH2 (CTAP) was endothermic (deltaH degrees' > 0) and driven by an increase in entropy (-TdeltaS degrees' = -13.08 to -18.57 kcal/mol). The binding of naltrexone was exothermic (deltaH degrees' = -12.56 kcal/mol) and essentially enthalpy-driven. The binding of morphine, methadone, pentazocine, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) was exothermic (deltaH degrees' = -3.53 to -9.95 kcal/mol) and occurred with an increase in entropy (-TdeltaS degrees' = -2.48 to -7.92 kcal/mol). Plots of enthalpy versus entropy and enthalpy versus free energy were linear, although enthalpy-entropy compensation was not evident. The entropy changes were not correlated with apparent lipophilicity of the compounds. These results suggest that: (1) opioid ligands bind to the mu receptor by specific mechanisms, unrelated to lipid solubility; (2) the mechanism of binding is not universally different for peptide and non-peptide ligands; (3) the nature of binding does not a priori determine intrinsic activity. The results reveal a novel differentiation of opioid ligands into two groups (group 1: ohmefentanyl, sufentanil, diprenorphine, CTAP and PL017; group 2: naltrexone, morphine, methadone, DAMGO, pentazocine), based on two distinct relationships between enthalpy versus free energy of binding, the details of which are yet to be elucidated.

Conformational and electrostatic properties of naphthazarin, juglone, and naphthoquinone: an ab initio theoretical study

Conformational features of naphthazarin, juglone, and naphthoquinone have been examined via ab initio (Hartree-Fock) SCF calculations at 3-21G level. The results suggest a planar structure for all the three molecules and internally hydrogen-bonded structure for naphthazarin and juglone to be their preferred conformation. The optimized structural features are essentially the same as their crystal geometries. Molecular electrostatic potential (MEP) calculations using ab initio SCF methods ranging from 3-21G to 6-31G levels have been performed to visualize their three-dimensional pharmacophoric patterns and topography. The results indicate that two factors--(i) the depth, extent, and relative location of negative potential around hydroxyl and quinonoid oxygens, and (ii) a gradual loss of negative potential over the molecular plane due to the presence and orientation of the hydroxyl groups in the phenolic part of the molecules--are crucial for recognition interaction of the compounds with their receptors. Aqueous solvation seems to have significant influence on the MEP profiles of the molecules. Although intrinsic nucleophilicity increases for all the compounds, including the different conformers, due to aqueous solvation, the intrinsic electrophilicity shows remarkable decrease for all. It appears that the acidic nature of the hydrogens in these compounds and conformers decreases sharply along with shifts of positions while going from the gas phase to the aqueous phase. These observations may help to explain the mechanism of action(s) of the anthracyclin family of cytotoxic antibiotics.

Phenyl-substituted normethadones: synthesis and pharmacology

Phenyl-substituted normethadone derivatives were synthesized and their affinity (IC50) for opioid receptors was determined by displacement of the specific binding sites of [3H]sufentanyl on rat brain preparations. Substitution resulted in a decrease of affinity in-vitro. These results suggest that normethadone-like compounds may interact with the P subsite of the mu-opioid receptor and that the P subsite has a well-defined cavity shape of stringent dimensions.

Frog skin opioid peptides: a case for environmental mimicry

Naturally occurring environmental substances often mimic endogenous substances found in mammals and are capable of interacting with specific proteins, such as receptors, with a high degree of fidelity and selectivity. Narcotic alkaloids and amphibian skin secretions, introduced into human society through close association with plants and animals through folk medicine and religious divination practices, were incorporated into the armamentarium of the early pharmacopoeia. These skin secretions contain a myriad of potent bioactive substances, including alkaloids, biogenic amines, peptides, enzymes, mucus, and toxins (noxious compounds notwithstanding); each class exhibits a broad range of characteristic properties. One specific group of peptides, the opioids, containing the dermorphins (dermal morphinelike substances) and the deltorphins (delta-selective opioids), display remarkable analgesic properties and include an amino acid with the rare (in a mammalian context) D-enantiomer in lieu of the normal L-isomer. Synthesis of numerous stereospecific analogues and conformational analyses of these peptides provided essential insights into the tertiary composition and microenvironment of the receptor "pocket" and the optimal interactions between receptor and ligand that trigger a biological response; new advances in the synthesis and receptor-binding properties of the deltorphins are discussed in detail. These receptor-specific opioid peptides act as more than mimics of endogenous opioids: their high selectivity for either the mu or delta receptor makes them formidable environmentally derived agents in the search for new antagonists for treating opiate addiction and in the treatment of a wide variety of human disorders.

Computer-aided structure-affinity relationships in a set of piperazine and 3,8-diazabicyclo[3.2.1]octane derivatives binding to the mu-opioid receptor

Molecular modeling studies were carried out on a set of piperazine and 3,8-diazabicyclo[3.2.1]octane derivatives with the aim to highlight the main factors modulating their affinity for the mu-opioid receptor. Structure-affinity relationships were developed with the aid of molecular mechanics and semiempirical quantum-mechanics methods. According to our proposed pharmacodynamic model, the binding to the mu-receptor is promoted by the following physico-chemical features: the presence of hydrocarbon fragments on the nitrogen ring frame capable of interacting with one of two hypothesized hydrophobic receptor pockets; a 'correct' orientation of an N-propionyl side chain so as to avoid a sterically hindered region of the receptor; the possibility of accepting a hydrogen bond from a receptor site complementary to the morphine phenol oxygen.

Is the analgesic activity of epibatidine caused by a chemical reaction with the morphine opioid receptor?

Using the molecular modeling program SYBYL, a conformational analysis of epibatidine has been performed. Two pairs of stable conformations due to the rotational degree of freedom for the pyridine ring have been found. These conformations were compared with morphine regarding spatial arrangements as well as electronic aspects. A very close agreement between the essential receptor positions occurring in morphine and epibatidine could be demonstrated. The protonable nitrogen atom in epibatidine is in exactly the same spatial position as in morphine, if the pyridine ring and the phenolic ring of morphine were matched to each other. Interestingly, it is also apparent that the pyridine nitrogen atom is in a close position to the bridging oxygen atom of morphine. Furthermore, the chlorine substituent fits very well with the hydroxyl group of morphine. A chemical reaction is postulated to permit epibatidine to function as an analgesic. The carbon-chlorine bond should be activated by the neighbourhood of the nitrogen atom in the pyridine ring and therefore undergo a chemical reaction resulting in formation of a covalent bond, perhaps of an oxygen bridge between the opioid receptor and epibatidine.

Development of a conformational search strategy for flexible ligands: a study of the potent mu-selective opioid analgesic fentanyl

An extensive conformational search of the potent opioid analgesic, fentanyl, was performed using the semiempirical quantum mechanical method AM1 and the CHARMm potential energy function. A combination of two procedures was used to search the conformational space for fentanyl, which included nested dihedral scans, geometry optimization and molecular dynamics simulation at different temperatures. In addition, the effect of a continuum solvent environment was taken into account by use of appropriate values for the dielectric constant in the CHARMm computations. The results of the conformational search allowed the determination of the probable conformation of fentanyl in polar and nonpolar solvents and of three candidate conformers for its bioactive form.

Bioactive conformation of linear peptides in solution: an elusive goal?

Bioactive peptides of natural origin have, in general, short linear sequences, and are characterized by a large conformational flexibility. It is very difficult to study their conformation in solution since they exist, almost invariably, as a complex mixture of numerous conformers, most of which are extended. The so-called bioactive conformation may be one of them, although the solvents used in solution studies often have properties drastically different from those of the biological system in which the peptide acts. There is, however, no simple way of identifying the bioactive conformation amid the many existing conformers. It is possible to approach a solution to this problem using two distinct strategies: (a) Limiting the conformational freedom of the peptide, e.g., by increasing the viscosity of the solution and decreasing the temperature, in the assumption that the bioactive conformation is, even slightly, more stable than the others. (b) Trying to mimic in solution the physicochemical features of the more reliable receptor models. These two approaches will be illustrated with examples taken mainly from opioid peptides.