Design and synthesis of (13S)-methyl-substituted arachidonic acid analogues: templates for novel endocannabinoids

Chiral Me-2-arachidonoyl Glycerols: The First Potent Endocannabinoid Glyceride Templates with Stability to COX-2

2-Arachidonoyl glycerol (2-AG) is the principal endogenously produced ligand for the cannabinoid CB1 and CB2 receptors (CBRs). The lack of potent and efficacious 2-AG ligands with resistance against metabolizing enzymes represents a significant void in the armamentarium of research tools available for studying eCB system molecular constituents and their function. Herein we report the first endocannabinoid glyceride templates with remarkably high potency and efficacy at CBRs. Two of our lead chiral 2-AG analogs, namely, (13S)- and (13R)-Me-2-AGs, potently inhibit excitatory neurotransmission via CB1 while they are endowed with excellent resistance to the oxidizing enzyme COX-2. Our SAR results are supported by docking studies of the key analog and 2-AG on the crystal structures of CB1.

( 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, (13 S,1' R)-dimethylanandamide (AMG315, 3a), a high affinity ligand for the CB1 receptor ( K_i of 7.8 ± 1.4 nM) that behaves as a potent CB1 agonist in vitro (EC₅₀ = 0.6 ± 0.2 nM). (13 S,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.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

Novel tail and head group prostamide probes

We report the design and synthesis of novel prostaglandin-ethanolamide (PGE2-EA) analogs containing head and tail group modifications to aid in the characterization of a putative prostamide receptor(s). Our synthetic approach utilizes Horner-Wadsworth-Emmons and Wittig reactions to construct the head and the tail moieties of the key PGE2 precursor, which leads to the final products through a peptide coupling, Swern oxidation and HF/pyridine assisted desilylation. The synthesized analogs were shown not to interact significantly with endocannabinoid proteins and recombinant EP1, EP3 and EP4 receptors and suggest a yet to be identified prostamide receptor as their site(s) of action.

Design and synthesis of novel prostaglandin E2 ethanolamide and glycerol ester probes for the putative prostamide receptor(s)

Novel prostaglandin-ethanolamide (PGE₂-EA) and glycerol ester (2-PGE₂-G) analogs were designed and synthesized to aid in the characterization of a putative prostamide receptor. Our design incorporates the electrophilic isothiocyanato and the photoactivatable azido groups at the terminal tail position of the prototype. Stereoselective Wittig and Horner-Wadsworth-Emmons reactions install the head and the tail moieties of the PGE₂ skeleton. The synthesis is completed using Mitsunobu azidation and peptide coupling as the key steps. A chemoenzymatic synthesis for the 2-PGE₂-G is described for first time.

Terpenes and lipids of the endocannabinoid and transient-receptor-potential-channel biosignaling systems

Endocananbnoid-system G-protein coupled receptors (GPCRs) and transient receptor potential (TRP) cation channels are critical components of cellular biosignaling networks. These plasma-membrane proteins are pleiotropic in their ability to interact with and engage structurally diverse ligands. The endocannabinoid and TRP signaling systems overlap in their recognition properties with respect to select naturally occurring plant-derived ligands that belong to the terpene and lipid chemical classes, the overlap establishing a physiological connectivity between these two ubiquitous cell-signaling systems. Identification and pharmacological profiling of phytochemicals engaged by cannabinoid GPCRs and/or TRP channels has inspired the synthesis of novel designer ligands that interact with cannabinoid receptors and/or TRP channels as xenobiotics. Functional interplay between the endocannabinoid and TRP-channel signaling systems is responsible for the antinocifensive action of some synthetic cananbinoids (WIN55,212-2 and AM1241), vasorelaxation by the endocannabinoid N-arachidonylethanolamide (anandamide), and the pain-relief afforded by the synthetic anandamide analogue N-arachidonoylaminophenol (AM404), the active metabolite of the widely used nonprescription analgesic and antipyretic acetaminophen (paracetamol). The biological actions of some plant-derived cannabinoid-receptor (e.g., Δ(9)-tetrahydrocannabinol) or TRP-channel (e.g,, menthol) ligands either carry abuse potential themselves or promote the use of other addictive substances, suggesting the therapeutic potential for modulating these signaling systems for abuse-related disorders. The pleiotropic nature of and therapeutically relevant interactions between cananbinergic and TRP-channel signaling suggest the possibility of dual-acting ligands as drugs.

C-ring cannabinoid lactones: a novel cannabinergic chemotype

As a part of our controlled-deactivation ligand development project, we recently disclosed a series of (-)-Δ(8)-tetrahydrocannabinols (THCs) with a metabolically labile ester group at the 2'-position of the side chain. Now, we have replaced the C-ring in the classical THC structure with a hydrolyzable seven-membered lactone. One of the synthesized analogues binds with high affinity to the CB1 receptor (K i = 4.6 nM) and exhibits much lower affinities for the mCB2 and the hCB2. Also, in vitro functional characterization found the compound to be an agonist at rCB1. Consistent with our rational design, the lead cannabinergic lactone identified here is susceptible to metabolic inactivation by plasma esterases, while the respective acid metabolite is inactive at CB receptors. These results are highlighted with molecular modeling of the two regiosomeric lactones.

Enantioselective synthesis of (10S)- and (10R)-methyl-anandamides

For the development of novel endocannabinoid templates with potential resistance to hydrolytic and oxidative metabolism, we are targeting the bis-allylic carbons of the arachidonoyl skeleton. Toward this end, we recently disclosed the synthesis and preliminary biological data for the (13S)-methyl-anandamide. We report now the total synthesis of the (10S)- and (10R)-methyl-counterparts. Our synthetic approach is stereospecific, efficient, and provides the analogs without the need for resolution. Peptide coupling, P-2 nickel partial hydrogenation, and cis-selective Wittig olefination are the key steps.

Application of Chemoenzymatic Hydrolysis in the Synthesis of 2-Monoacylglycerols

The selective biocatalyzed synthesis of 2-monoacylglycerols (2-MAGs) through the use of commercially available immobilized Candida antarctica (Novozym435) and Rhizomucor miehei is explored. Reactions at room temperature result in the formation of a 2-MAG and a corresponding ethyl ester of the fatty acid with immobilized Candida antarctica within 2h with yields ranging from 36%-83%. Similar reaction conditions with immobilized Rhizomucor miehei yielded exclusively the 2-MAG after 24h with yields ranging from 37% to 88%. Yields vary on the acyl group at the sn-2 position and choice of enzyme involved.