The Concise Guide to PHARMACOLOGY 2023/24: Enzymes

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and about 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.16176. In addition to this overview, in which are identified 'Other protein targets' which fall outside of the subsequent categorisation, there are six areas of focus: G protein-coupled receptors, ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Uncovering the psychoactivity of a cannabinoid from liverworts associated with a legal high

Phytochemical studies on the liverwort Radula genus have previously identified the bibenzyl (-)-cis-perrottetinene (cis-PET), which structurally resembles (-)-Δ⁹-trans-tetrahydrocannabinol (Δ⁹-trans-THC) from Cannabis sativa L. Radula preparations are sold as cannabinoid-like legal high on the internet, even though pharmacological data are lacking. Herein, we describe a versatile total synthesis of (-)-cis-PET and its (-)-trans diastereoisomer and demonstrate that both molecules readily penetrate the brain and induce hypothermia, catalepsy, hypolocomotion, and analgesia in a CB1 receptor-dependent manner in mice. The natural product (-)-cis-PET was profiled on major brain receptors, showing a selective cannabinoid pharmacology. This study also uncovers pharmacological differences between Δ⁹-THC and PET diastereoisomers. Most notably, (-)-cis-PET and (-)-trans-PET significantly reduced basal brain prostaglandin levels associated with Δ⁹-trans-THC side effects in a CB1 receptor-dependent manner, thus mimicking the action of the endocannabinoid 2-arachidonoyl glycerol. Therefore, the natural product (-)-cis-PET is a psychoactive cannabinoid from bryophytes, illustrating the existence of convergent evolution of bioactive cannabinoids in the plant kingdom. Our findings may have implications for bioprospecting and drug discovery and provide a molecular rationale for the reported effects upon consumption of certain Radula preparations as moderately active legal highs.

Docking and QSAR Studies of Aryl-valproic Acid Derivatives to Identify Antiproliferative Agents Targeting the HDAC8

BACKGROUND

Histone deacetylase 8 (HDAC8) is a plausible target for the development of novel anticancer drugs using a metal-chelating group and hydrophobic moieties as pharmacophores. It is known that valproic acid (administered as its salt, sodium valproate; VPANa+) is an HDAC8 inhibitor characterized by its hydrophobic chains. Nevertheless, VPA is hepatotoxic and VPA analogues might be explored for less hepatotoxic antiproliferative compounds.

METHOD

In this work, docking and QSAR studies of 500 aryl-VPA derivatives as possible HDAC8 inhibitors were performed in order to explore and select potential anti-proliferative compounds. Docking results identified π-π, hydrogen bonds as the most important noncovalent interactions between HDAC8 (PDB: 3F07) and the ligands tested, whereas Belm4 was the best QSAR descriptor and classified as a 2D-BCUT descriptor.

RESULT

Based on theoretical studies, compound DAVP042 was synthesized and evaluated in vitro for its antiproliferative activities on several cancer cell lines (A549-lung, MCF-7-breast, HCT116-colon and U937- lymphoid tissue) in comparison to VPA, as well as for its inhibitory activity on HDAC8 using in vitro models. DAVP042 demonstrated to have antiproliferative activity on all cancer cell lines employed, not only suggesting that this compound should be further studied, but also demonstrating that the methodology herein employed is appropriated to identify new therapeutic candidates.

The cannabinoid CB₂ receptor-selective phytocannabinoid beta-caryophyllene exerts analgesic effects in mouse models of inflammatory and neuropathic pain

The widespread plant volatile beta-caryophyllene (BCP) was recently identified as a natural selective agonist of the peripherally expressed cannabinoid receptor 2 (CB₂). It is found in relatively high concentrations in many spices and food plants. A number of studies have shown that CB₂ is critically involved in the modulation of inflammatory and neuropathic pain responses. In this study, we have investigated the analgesic effects of BCP in animal models of inflammatory and neuropathic pain. We demonstrate that orally administered BCP reduced inflammatory (late phase) pain responses in the formalin test in a CB₂ receptor-dependent manner, while it had no effect on acute (early phase) responses. In a neuropathic pain model the chronic oral administration of BCP attenuated thermal hyperalgesia and mechanical allodynia, and reduced spinal neuroinflammation. Importantly, we found no signs of tolerance to the anti-hyperalgesic effects of BCP after prolonged treatment. Oral BCP was more effective than the subcutaneously injected synthetic CB₂ agonist JWH-133. Thus, the natural plant product BCP may be highly effective in the treatment of long lasting, debilitating pain states. Our results have important implications for the role of dietary factors in the development and modulation of chronic pain conditions.

The antinociceptive triterpene β-amyrin inhibits 2-arachidonoylglycerol (2-AG) hydrolysis without directly targeting cannabinoid receptors

BACKGROUND AND PURPOSE

Pharmacological activation of cannabinoid CB(1) and CB(2) receptors is a therapeutic strategy to treat chronic and inflammatory pain. It was recently reported that a mixture of natural triterpenes α- and β-amyrin bound selectively to CB(1) receptors with a subnanomolar K(i) value (133 pM). Orally administered α/β-amyrin inhibited inflammatory and persistent neuropathic pain in mice through both CB(1) and CB(2) receptors. Here, we investigated effects of amyrins on the major components of the endocannabinoid system.

EXPERIMENTAL APPROACH

We measured CB receptor binding interactions of α- and β-amyrin in validated binding assays using hCB(1) and hCB(2) transfected CHO-K1 cells. Effects on endocannabinoid transport in U937 cells and breakdown using homogenates of BV2 cells and pig brain, as well as purified enzymes, were also studied.

KEY RESULTS

There was no binding of either α- or β-amyrin to hCB receptors in our assays (K(i) > 10 µM). The triterpene β-amyrin potently inhibited 2-arachidonoyl glycerol (2-AG) hydrolysis in pig brain homogenates, but not that of anandamide. Although β-amyrin only weakly inhibited purified human monoacylglycerol lipase (MAGL), it also inhibited α,β-hydrolases and more potently inhibited 2-AG breakdown than α-amyrin and the MAGL inhibitor pristimerin in BV2 cell and pig brain homogenates.

CONCLUSIONS AND IMPLICATIONS

We propose that β-amyrin exerts its analgesic and anti-inflammatory pharmacological effects via indirect cannabimimetic mechanisms by inhibiting the degradation of the endocannabinoid 2-AG without interacting directly with CB receptors. Triterpenoids appear to offer a very broad and largely unexplored scaffold for inhibitors of the enzymic degradation of 2-AG.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

The cannabinoid CB1 receptor antagonists rimonabant (SR141716) and AM251 directly potentiate GABA(A) receptors

BACKGROUND AND PURPOSE

Rimonabant (SR141716) and the structurally related AM251 are widely used in pharmacological experiments as selective cannabinoid receptor CB(1) antagonists / inverse agonists. Concentrations of 0.5-10 µM are usually applied in in vitro experiments. We intended to show that these drugs did not act at GABA(A) receptors but found a significant positive allosteric modulation instead.

EXPERIMENTAL APPROACH

Recombinant GABA(A) receptors were expressed in Xenopus oocytes. Receptors were exposed to AM251 or rimonabant in the absence and presence of GABA. Standard electrophysiological techniques were used to monitor the elicited ionic currents.

KEY RESULTS

AM251 dose-dependently potentiated responses to 0.5 µM GABA at the recombinant α(1) β(2) γ(2) GABA(A) receptor with an EC(50) below 1 µM and a maximal potentiation of about eightfold. The Hill coefficient indicated that more than one binding site for AM251 was located in this receptor. Rimonabant had a lower affinity, but a fourfold higher efficacy. AM251 potentiated also currents mediated by α(1) β(2) , α(x) β(2) γ(2) (x = 2,3,5,6), α(1) β(3) γ(2) and α(4) β(2) δ GABA(A) receptors, but not those mediated by α(1) β(1) γ(2) . Interestingly, the CB(1) receptor antagonists LY320135 and O-2050 did not significantly affect α(1) β(2) γ(2) GABA(A) receptor-mediated currents at concentrations of 1 µM.

CONCLUSIONS AND IMPLICATIONS

This study identified rimonabant and AM251 as positive allosteric modulators of GABA(A) receptors. Thus, potential GABAergic effects of commonly used concentrations of these compounds should be considered in in vitro experiments, especially at extrasynaptic sites where GABA concentrations are low.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Effects of Echinaforce® treatment on ex vivo-stimulated blood cells

The herb Echinacea purpurea, also called purple coneflower, is regarded as an immune modulator. This study examined changes in cytokine production in blood samples from 30 volunteers before and during 8-day oral administration with an ethanolic extract of fresh Echinacea purpurea (Echinaforce(®)). Daily blood samples were ex vivo stimulated by LPS/SEB or Zymosan and analysed for a series of cytokines and haematological and metabolic parameters. Treatment reduced the proinflammatory mediators TNF-α and IL-1β by up to 24% (p<0.05) and increased anti-inflammatory IL-10 levels by 13% (p<0.05) in comparison to baseline. This demonstrated a substantial overall anti-inflammatory effect of Echinaforce(®) for the whole group (n=28). Chemokines MCP-1 and IL-8 were upregulated by 15% in samples from subjects treated with Echinaforce(®) (p<0.05). An analysis of a subgroup of volunteers who showed low pre-treatment levels of the cytokines MCP-1, IL-8, IL-10 or IFN-γ (n=8) showed significant stimulation of these factors upon Echinaforce(®) treatment (30-49% increases; p<0.05), whereas the levels in subjects with higher pre-treatment levels remained unaffected. We chose the term "adapted immune-modulation" to describe this observation. Volunteers who reported high stress levels (n=7) and more than 2 colds per year experienced a significant transient increase in IFN-γ upon Echinaforce(®) treatment (>50%). Subjects with low cortisol levels (n=11) showed significant down-regulation of the acute-phase proteins IL1-β, IL-6, IL-12 and TNF-α by Echinaforce(®) (range, 13-25%), while subjects with higher cortisol levels showed no such down-regulation. This is the first ex vivo study to demonstrate adapted immune-modulation by an Echinacea preparation. While Echinaforce(®) did not affect leukocyte counts, we speculate that the underlying therapeutic mechanism is based on differential multi-level modulation of the responses of the different types of leukocytes. Echinaforce(®) thus regulates the production of chemokines and cytokines according to current immune status, such as responsiveness to exogenous stimuli, susceptibility to viral infection and exposure to stress.

Mutagenesis and computer modeling studies of a GPCR conserved residue W5.43(194) in ligand recognition and signal transduction for CB2 receptor

W5.43(194), a conserved tryptophan residue among G-protein coupled receptors (GPCRs) and cannabinoid receptors (CB), was examined in the present report for its significance in CB2 receptor ligand binding and adenylyl cyclase (AC) activity. Computer modeling postulates that this site in CB2 may be involved in the affinity of WIN55212-2 and SR144528 through aromatic contacts. In the present study, we reported that a CB2 receptor mutant, W5.43(194)Y, which had a tyrosine (Y) substitution for tryptophan (W), retained the binding affinity for CB agonist CP55940, but reduced binding affinity for CB2 agonist WIN55212-2 and inverse agonist SR144528 by 8-fold and 5-fold, respectively; the CB2 W5.43(194)F and W5.43(194)A mutations significantly affect the binding activities of CP55940, WIN55212-2 and SR144528. Furthermore, we found that agonist-mediated inhibition of the forskolin-induced cAMP production was dramatically diminished in the CB2 mutant W5.43(194)Y, whereas W5.43(194)F and W5.43(194)A mutants resulted in complete elimination of downstream signaling, suggesting that W5.43(194) was essential for the full activation of CB2. These results indicate that both aromatic interaction and hydrogen bonding are involved in ligand binding for the residue W5.43(194), and the mutations of this tryptophan site may affect the conformation of the ligand binding pocket and therefore control the active conformation of the wild type CB2 receptor. W5.43(194)Y/F/A mutations also displayed noticeable enhancement of the constitutive activation probably attributed to the receptor conformational changes resulted from the mutations.

Cytochrome P450 inhibitory action of Echinacea preparations differs widely and co-varies with alkylamide content

Echinacea preparations are one of the best selling herbal medicinal products with a well established therapeutic use in the prophylaxis of upper respiratory tract infections. Their consumption is increasing, but information about their ability to inhibit cytochrome P450 enzymes (CYP) is fragmentary. The picture is further complicated by a lack of phytochemical characterization of previously tested preparations. Due to its well characterized immunomodulatory activity, the standardized Swiss registered Echinacea purpurea (L.) Moench Echinaforce extract was selected for detailed study. With the single baculovirus-expressed CYP isoforms 1A2, 2C19, 2D9 and 3A4, inhibitory actions were measured by monitoring fluorescent metabolites derived from enzyme substrates (supersome assay). The Echinaforce extract induced mild inhibition of all these isoforms, with CYP 3A4 being the most, and CYP 2D6 the least sensitive enzyme. To assess whether CYP inhibition might be a general feature of Echinacea preparations, an additional nine commercially available preparations were screened using CYP 3A4. All tested preparations were able to inhibit CYP 3A4, but inhibitory potencies (expressed as median inhibitory concentration, IC50) varied by a factor of 150. The alkylamides are thought to be responsible for the immunomodulatory activity of Echinacea, and so the concentration of 2E,4E,8Z,10E/Z-tetranoic acid isobutylamide (1) and total alkylamide content were determined in all preparations, and the latter was found to be associated with their CYP 3A4 inhibitory potency. The chemically pure alkylamides dodeca-2E,4E,8Z,10E/Z-tetranoic acid isobutylamide (1) and dodeca-2E,4E-dieonoic acid isobutylamide (2) showed inhibitory activity on CYP 2C19, 2D6 and 3A4. However, unlike the Echinaforce extract, the alkylamides did not induce CYP 1A2 inhibition. Thus, other, as yet unidentified constituents also contribute to the overall weak inhibitory effects seen with Echinacea preparations in-vitro.

Characteristics of the calcium-triggered mitochondrial permeability transition in nonsynaptic brain mitochondria: effect of cyclosporin A and ubiquinone O

The objective of the present study was to assess the capacity of nonsynaptic brain mitochondria to accumulate Ca2+ when subjected to repeated Ca2+ loads, and to explore under what conditions a mitochondrial permeability transition (MPT) pore is assembled. The effects of cyclosporin A (CsA) on Ca2+ accumulation and MPT pore assembly were compared with those obtained with ubiquinone 0 (Ubo), a quinone that is a stronger MPT blocker than CsA, when tested on muscle and liver mitochondria. When suspended in a solution containing phosphate (2 mM) and Mg2+ (1 mM), but no ATP or ADP, the brain mitochondria had a limited capacity to accumulate Ca2+ (210 nmol/mg of mitochondrial protein). Furthermore, when repeated Ca2+ pulses (40 nmol/mg of protein each) saturated the uptake system, the mitochondria failed to release the Ca2+ accumulated. However, in each instance, the first Ca2+ pulse was accompanied by a moderate release of Ca2+, a release that was not observed during the subsequent pulses. The initial release was accompanied by a relatively marked depolarization, and by swelling, as assessed by light-scattering measurements. However, as the swelling was <50% of that observed following addition of alamethicin, it is concluded that the first Ca2+ pulse gives rise to an MPT in a subfraction of the mitochondrial population. CsA, an avid blocker of the MPT pore, only marginally increased the Ca(2+)-sequestrating capacity of the mitochondria. However, CsA eliminated the Ca2+ release accompanying the first Ca2+ pulse. The effects of CsA were shared by Ubo, but when the concentration of Ubo exceeded 20 microM, it proved toxic. The results thus suggest that brain mitochondria are different from those derived from a variety of other sources. The major difference is that a fraction of the brain mitochondria, studied presently, depolarized and showed signs of an MPT. This fraction, but not the remaining ones, contributed to the chemically and electron microscopically verified mitochondrial swelling.

Mechanism of inhibition of a poxvirus topoisomerase by the marine natural product sansalvamide A

At present no antiviral agents are available for treatment of infection by the pathogenic poxvirus molluscum contagiosum virus (MCV). Here we report the identification and characterization of an inhibitor active against the virus-encoded type-1 topoisomerase, an enzyme likely to be required for MCV replication. We screened a library of marine extracts and natural products from microorganisms using MCV topoisomerase assays in vitro. The cyclic depsipeptide sansalvamide A was found to inhibit topoisomerase-catalyzed DNA relaxation. Sansalvamide A was inactive against two other DNA-modifying enzymes tested as a counterscreen. Assays of discrete steps in the topoisomerase reaction cycle revealed that sansalvamide A inhibited DNA binding and thereby covalent complex formation, but not resealing of a DNA nick in a preformed covalent complex. Sansalvamide A also inhibits DNA binding by the isolated catalytic domain, thereby specifying the part of the protein sensitive to sansalvamide A. These data specify the mechanism by which sansalvamide A inhibits MCV topoisomerase. Cyclic depsipeptides related to sansalvamide A represent a potentially promising chemical family for development of anti-MCV agents.

Mechanism of inhibition of a poxvirus topoisomerase by the marine natural product sansalvamide A

At present no antiviral agents are available for treatment of infection by the pathogenic poxvirus molluscum contagiosum virus (MCV). Here we report the identification and characterization of an inhibitor active against the virus-encoded type-1 topoisomerase, an enzyme likely to be required for MCV replication. We screened a library of marine extracts and natural products from microorganisms using MCV topoisomerase assays in vitro. The cyclic depsipeptide sansalvamide A was found to inhibit topoisomerase-catalyzed DNA relaxation. Sansalvamide A was inactive against two other DNA-modifying enzymes tested as a counterscreen. Assays of discrete steps in the topoisomerase reaction cycle revealed that sansalvamide A inhibited DNA binding and thereby covalent complex formation, but not resealing of a DNA nick in a preformed covalent complex. Sansalvamide A also inhibits DNA binding by the isolated catalytic domain, thereby specifying the part of the protein sensitive to sansalvamide A. These data specify the mechanism by which sansalvamide A inhibits MCV topoisomerase. Cyclic depsipeptides related to sansalvamide A represent a potentially promising chemical family for development of anti-MCV agents.