Structural Insights from Recent CB1 X-Ray Crystal Structures

Over the past 2 years, X-ray crystal structures of the antagonist- and agonist-bound CB1 receptor have been reported. Such structures are expected to accelerate progress in the understanding of CB1 and should provide an exceptional starting point for structure-based drug discovery. This chapter examines the consistency of these X-ray structures with the CB1 experimental literature, including mutation, NMR and covalent labeling studies. These comparisons reveal discrepancies between this literature and the TMH1-2-3 region of each CB1 crystal structure. The chapter also examines crystal packing issues with each X-ray structure and shows that the discrepancies with the experimental literature can be attributed to crystal packing problems that force the N-terminus deep in the binding pocket of the two inactive state structures and force TMH2 to bend at G2.53/S2.54 and invade the binding pocket in the activated state structure. Revision is advisable before these structures are used for structure-based drug discovery

Up in smoke: Uncovering a lack of evidence for proton pump inhibitors as a source of tetrahydrocannabinol immunoassay false positives

Objective: It is recommended that positives in immunoassay drug screens be followed up with more specific confirmatory testing. The drug package insert for pantoprazole mentions reports of false-positive urine screening tests for tetrahydrocannabinol in patients receiving proton pump inhibitors, but no method details or data are given, referenced, or found in literature searches. Thus, we investigated this using our laboratory’s assay. Methods: A spiked sample and samples from 32 patients taking a proton pump inhibitor were analyzed using the EMIT II Plus Cannabinoid assay with a 20?ng/mL cutoff. Additionally, we examined urine samples from 50 patients with false-positive or low-positive screens for evidence of a proton pump inhibitor. To determine whether O-desmethyl pantoprazole sulfate, the major metabolite, shares any structural or electrostatic similarity to suggest a basis for cross-reactivity in the immunoassay, we used computational techniques for analyses. Molecular electrostatic potential energy (MEP) maps were calculated for the global minimum conformers, and the maximum common substructure Tanimoto similarity was calculated for the modeled compounds. Results: Neither the spiked sample nor the patient samples were found to screen positive. None of the false-positive or low-positive screens were found to contain a proton pump inhibitor. Computational studies showed very little similarity in shape or electrostatics between the two molecules. Conclusions: We find no supporting evidence of pantoprazole as the cause of false positives in the EMIT II Plus Cannabinoid assay and caution the use of proton pump inhibitors as an explanation for tetrahydrocannabinol immunoassay false positives

Towards a molecular understanding of the cannabinoid related orphan receptor gpr18: A focus on its constitutive activity

The orphan G-protein coupled receptor (GPCR), GPR18, has been recently proposed as a potential member of the cannabinoid family as it recognizes several endogenous, phytogenic, and synthetic cannabinoids. Potential therapeutic applications for GPR18 include intraocular pressure, metabolic disorders, and cancer. GPR18 has been reported to have high constitutive activity, i.e., activation/signaling occurs in the absence of an agonist. This activity can be reduced significantly by the A3.39N mutation. At the intracellular (IC) ends of (transmembrane helices) TMH3 and TMH6 in GPCRs, typically, a pair of oppositely charged amino acids form a salt bridge called the “ionic lock”. Breaking of this salt bridge creates an IC opening for coupling with G protein. The GPR18 “ionic lock” residues (R3.50/S6.33) can form only a hydrogen bond. In this paper, we test the hypothesis that the high constitutive activity of GPR18 is due to the weakness of its “ionic lock” and that the A3.39N mutation strengthens this lock. To this end, we report molecular dynamics simulations of wild-type (WT) GPR18 and the A3.39N mutant in fully hydrated (POPC) phophatidylcholine lipid bilayers. Results suggest that in the A3.39N mutant, TMH6 rotates and brings R3.50 and S6.33 closer together, thus strengthening the GPR18 “ionic lock”

Identification of CB1 Receptor Allosteric Sites Using Force-Biased MMC Simulated Annealing and Validation by Structure–Activity Relationship Studies

Positive allosteric modulation of the cannabinoid 1 receptor (CB1R) has demonstrated distinct therapeutic advantages that address several limitations associated with orthosteric agonism and has opened a promising therapeutic avenue for further drug development. To advance the development of CB1R positive allosteric modulators, it is important to understand the molecular architecture of CB1R allosteric site(s). The goal of this work was to use Force-Biased MMC Simulated Annealing to identify binding sites for GAT228 (R), a partial allosteric agonist, and GAT229 (S), a positive allosteric modulator (PAM) at the CB1R. Our studies suggest that GAT228 binds in an intracellular (IC) TMH1–2–4 exosite that would allow this compound to act as a CB1 allosteric agonist as well as a CB1 PAM. In contrast, GAT229 binds at the extracellular (EC) ends of TMH2/3, just beneath the EC1 loop. At this site, this compound can act as CB1 PAM only. Finally, these results were successfully validated through the synthesis and biochemical evaluation of a focused library of compounds

Synthesis and Pharmacology of 1-Methoxy Analogs of CP-47,497

Three 1-methoxy analogs of CP-47,497 (7, 8 and 19) have been synthesized and their affinities for the cannabinoid CB1 and CB2 receptors have been determined. Although these compounds exhibit selectivity for the CB2 receptor none have significant affinity for either receptor. Modeling and receptor docking studies were carried out, which provide a rationalization for the weak affinities of these compounds for either receptor

(R)-N-(1-Methyl-2-hydroxyethyl)-13-(S)-methyl-arachidonamide (AMG315): A Novel Chiral Potent Endocannabinoid Ligand with Stability to Metabolizing Enzymes

The synthesis of potent metabolically stable endocannabinoids is challenging. Here we report a chiral arachidonoyl ethanolamide (AEA) analogue, namely, (13S,1'R)-dimethylanandamide (AMG315, 3a), a high affinity ligand for the CB1 receptor (Ki of 7.8 ± 1.4 nM) that behaves as a potent CB1 agonist in vitro (EC50 = 0.6 ± 0.2 nM). (13S,1'R)-dimethylanandamide is the first potent AEA analogue with significant stability for all endocannabinoid hydrolyzing enzymes as well as the oxidative enzymes COX-2. When tested in vivo using the CFA-induced inflammatory pain model, 3a behaved as a more potent analgesic when compared to endogenous AEA or its hydrolytically stable analogue AM356. This novel analogue will serve as a very useful endocannabinoid probe

Synthesis, Pharmacological Evaluation, and Docking Studies of Novel Pyridazinone-Based Cannabinoid Receptor Type 2 Ligands

In recent years, cannabinoid type 2 receptors (CB2R) have emerged as promising therapeutic targets in a wide variety of diseases. Selective ligands of CB2R are devoid of the psychoactive effects typically observed for CB1R ligands. Based on our recent studies on a class of pyridazinone 4-carboxamides, further structural modifications of the pyridazinone core were made to better investigate the structure–activity relationships for this promising scaffold with the aim to develop potent CB2R ligands. In binding assays, two of the new synthesized compounds [6-(3,4-dichlorophenyl)-2-(4-fluorobenzyl)-cis-N-(4-methylcyclohexyl)-3-oxo-2,3-dihydropyridazine-4-carboxamide (2) and 6-(4-chloro-3-methylphenyl)-cis-N-(4-methylcyclohexyl)-3-oxo-2-pentyl-2,3-dihydropyridazine-4-carboxamide (22)] showed high CB2R affinity, with Ki values of 2.1 and 1.6 nm, respectively. In addition, functional assays of these compounds and other new active related derivatives revealed their pharmacological profiles as CB2R inverse agonists. Compound 22 displayed the highest CB2R selectivity and potency, presenting a favorable in silico pharmacokinetic profile. Furthermore, a molecular modeling study revealed how 22 produces inverse agonism through blocking the movement of the toggle-switch residue, W6.48

Structural Mimicry in Class A G Protein-coupled Receptor Rotamer Toggle Switches

In this study, we tested the hypothesis that a CB1 TMH3-4-5-6 aromatic microdomain, which includes F3.25(190), F3.36(201), W5.43(280), and W6.48(357), is centrally involved in CB1 receptor activation, with the F3.36(201)/W6.48(357) interaction key to the maintenance of the CB1-inactive state. We have shown previously that when F3.36(201), W5.43(280), and W6.48(357) are individually mutated to alanine, a significant reduction in ligand binding affinity is observed in the presence of WIN 55,212-2 and SR141716A but not CP55,940 and anandamide. In the work presented here, we report a detailed functional analysis of the F3.36(201)A, F3.25(190)A, W5.43(280)A, and W6.48(357)A mutant receptors in stable cell lines created in HEK cells for agonist-stimulated guanosine 5′-3-O-(thio)triphosphate (GTPγS) binding and GIRK1/4 channel current effects in Xenopus oocytes where the mutant proteins were expressed transiently. The F3.36(201)A mutation showed statistically significant increases in ligand-independent stimulation of GTPγS binding versus wild type CB1, although basal levels for the W6.48(357)A mutant were not statistically different from wild type CB1. F3.36(201)A demonstrated a limited activation profile in the presence of multiple agonists. In contrast, enhanced agonist activation was produced by W6.48(357)A. These results suggest that a F3.36(201)/W6.48(357)-specific contact is an important constraint for the CB1-inactive state that may need to break during activation. Modeling studies suggest that the F3.36(201)/W6.48(357) contact can exist in the inactive state of CB1 and be broken in the activated state via a χ1 rotamer switch (F3.36(201) trans, W6.48(357) g+) → (F3.36(201) g+, W6.48(357) trans). The F3.36(201)/W6.48(357) interaction therefore may represent a “toggle switch” for activation of CB1