Tritiation of SR141716 by metallation-iodination-reduction: tritium-proton nOe study

Efficacy in diet-induced obese mice of the hepatotropic, peripheral cannabinoid 1 receptor inverse agonist TM38837

BACKGROUND AND PURPOSE

The cannabinoid CB1 receptor has a well-established role in appetite regulation. Drugs antagonizing central CB1 receptors, most notably rimonabant, induced weight loss and improved the metabolic profile in obese individuals but were discontinued due to psychiatric side effects. However, metabolic benefits were only partially attributable to weight loss, implying a role for peripheral receptors, and peripherally restricted CB1 receptor antagonists have since been of interest. Herein, we describe the evaluation of the peripherally restricted potent CB1 receptor inverse agonists TM38837 and TM39875, with acidic functionality, which were administered daily to diet-induced obese (DIO) mice for 5 weeks at doses for which CNS-mediated effects were minimal.

EXPERIMENTAL APPROACH

Compounds were tested in dose-response in acute studies to compare efficacy (gastric transport) and extent of CNS exposure (hypothermia and satiety sequence) to demonstrate peripheral restriction and select doses for the subsequent chronic DIO study.

KEY RESULTS

TM38837 but not TM39875 produced considerable (26%) weight loss, linked to a sustained reduction in food intake, together with improvements in plasma markers of inflammation and glucose homeostasis. Pharmacokinetic analysis indicated high plasma and low brain levels for both compounds with high liver levels for TM38837 (but not TM39875) due to hepatic uptake.

CONCLUSION AND IMPLICATIONS

Weight loss and metabolic benefits of TM38837 are likely not CNS-mediated but could be linked to enhanced liver exposure, which implicates intracellular CB1 receptors in hepatocytes as a possible driver of obesity and co-morbidities.

Bivalent ligands that target μ opioid (MOP) and cannabinoid1 (CB1) receptors are potent analgesics devoid of tolerance

Given that μ opioid (MOP) and canabinoid (CB1) receptors are colocalized in various regions of the central nervous system and have been reported to associate as heteromer (MOP-CB1) in cultured cells, the possibility of functional, endogenous MOP-CB1 in nociception and other pharmacologic effects has been raised. As a first step in investigating this possibility, we have synthesized a series of bivalent ligands 1-5 that contain both μ agonist and CB1 antagonist pharmacophores for use as tools to study the functional interaction between MOP and CB1 receptors in vivo. Immunofluorescent studies on HEK293 cells coexpressing both receptors suggested 5 (20-atom spacer) to be the only member of the series that bridges the protomers of the heteromer. Antinociceptive testing in mice revealed 5 to be the most potent member of the series. As neither a mixture of monovalent ligands 9 + 10 nor bivalents 2-5 produced tolerance in mice, MOR-CB1 apparently is not an important target for reducing tolerance.

Design and synthesis of cannabinoid receptor 1 antagonists for peripheral selectivity

Antagonists of cannabinoid receptor 1 (CB1) have potential for the treatment of several diseases such as obesity, liver disease, and diabetes. Recently, development of several CB1 antagonists was halted because of adverse central nervous system (CNS) related side effects observed with rimonabant, the first clinically approved CB1 inverse agonist. However, recent studies indicate that regulation of peripherally expressed CB1 with CNS-sparing compounds is a viable strategy to treat several important disorders. Our efforts aimed at rationally designing peripherally restricted CB1 antagonists have resulted in compounds that have limited blood-brain barrier (BBB) permeability and CNS exposure in preclinical in vitro and in vivo models. Typically, compounds with high topological polar surface areas (TPSAs) do not cross the BBB passively. Compounds with TPSAs higher than that for rimonabant (rimonabant TPSA = 50) and excellent functional activity with limited CNS penetration were identified. These compounds will serve as templates for further optimization.

Towards rational design of cannabinoid receptor 1 (CB1) antagonists for peripheral selectivity

CB1 receptor antagonists that are peripherally restricted were targeted. Compounds with permanent charge as well as compounds that have increased polar surface area were made and tested against CB1 for binding and activity. Sulfonamide and sulfamide with high polar surface area and good activity at CB1 were rationally designed and pharmacologically tested. Further optimization of these compounds and testing could lead to the development of a new class of therapeutics to treat disorders where the CB1 receptor system has been implicated.

Synthesis and biological evaluation of bivalent ligands for the cannabinoid 1 receptor

Dimerization or oligomerization of many G-protein-coupled receptors (GPCRs), including the cannabinoid 1 (CB1) receptor, is now widely accepted and may have significant implications for medications development targeting these receptor complexes. A library of bivalent ligands composed of two identical CB1 antagonist pharmacophores derived from SR141716 linked by spacers of various lengths were developed. The affinities of these bivalent ligands at CB1 and CB2 receptors were determined using radiolabeled binding assays. Their functional activities were measured using GTP-γ-S accumulation and intracellular calcium mobilization assays. The results suggest that the nature of the linker and its length are crucial factors for optimum interactions of these ligands at CB1 receptor binding sites. Finally, selected bivalent ligands (5d and 7b) were able to attenuate the antinociceptive effects of the cannabinoid agonist CP55,940 (21) in a rodent tail-flick assay. These novel compounds may serve as probes that will enable further characterization of CB1 receptor dimerization and oligomerization and its functional significance and may prove useful in the development of new therapeutic approaches to G-protein-coupled receptor mediated disorders.

Conformationally constrained analogues of N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4- dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716): design, synthesis, computational analysis, and biological evaluations

Structure-activity relationships (SARs) of 1 (SR141716) have been extensively documented, however, the conformational properties of this class have received less attention. In an attempt to better understand ligand conformations optimal for receptor recognition, we have designed and synthesized a number of derivatives of 1, including a four-carbon-bridged molecule (11), to constrain rotation of the diaryl rings. Computational analysis of 11 indicates approximately 20 kcal/mol energy barrier for rotation of the two aryl rings. NMR studies have determined the energy barrier to be approximately 18 kcal/mol and suggested atropisomers could exist. Receptor binding and functional studies with these compounds displayed reduced affinity and potency when compared to 1. This indicates that our structural modifications either constrain the ring systems in a suboptimal orientation for receptor interaction or the introduction of steric bulk leads to disfavored steric interactions with the receptor, and/or the relatively modest alterations in the molecular electrostatic potentials results in disfavored Coulombic interactions.

Synthesis and Cannabinoid Activity of a Variety of 2,3-Substituted 1-Benzo[b]thiophen Derivatives and 2,3-Substituted Benzofuran: Novel Agonists for the CB1 Receptor

An exploratory chemical effort has been undertaken to develop a novel series of compounds as selective CB1 agonists. It is hoped that compounds of this type will have clinical utility in pain control and cerebral ischaemia following stroke or traumatic head injury. We report here medicinal chemistry studies directed towards the investigation of several classes of 1-benzo[b]thiophen and benzofuran derivatives as novel CB1 agonists. We have discovered a novel series of compounds, which contain a 1-benzo[b]thiophen or a benzofuran group as the central aromatic group. Our investigation of this series of compounds has enhanced our understanding of the importance of binding sites within the CB1 receptor for favourable CB1 potency. Our understanding of these factors allowed us to modify the structure of a 1-benzothiophen derivative and improve its potency at the CB1 receptor.

Biarylpyrazole inverse agonists at the cannabinoid CB1 receptor: importance of the C-3 carboxamide oxygen/lysine3.28(192) interaction

The biarylpyrazole, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716; 1) has been shown to act as an inverse agonist/antagonist at the cannabinoid CB1 receptor. Our previous mutant cycle study suggested that K3.28(192) is involved in a direct interaction with the C-3 substituent of 1 in wild-type (WT) CB1.(1) However, these results did not establish what part of the C-3 substituent of 1 is involved in the K3.28(192) hydrogen bond, the carboxamide oxygen or the piperidine nitrogen. Furthermore, our previous calcium channel assay results for 5-(4- chlorophenyl)-3-[(E)-2-cyclohexylethenyl]-1-(2,4-dichlorophenyl)-4- methyl-1H-pyrazole (VCHSR; 2) (an analogue of 1 that lacks hydrogen-bonding capability in its C-3 substituent) showed that this compound acts as a neutral antagonist, a result that is in contrast to 1, which acts as an inverse agonist in this same assay.(1) These results suggested a relationship between biarylpyrazole interaction with K3.28(192) at CB1 and inverse agonism, but these results were for a single pair of compounds (1 and 2). The work presented here was designed to test two hypotheses derived from our modeling and mutant cycle results. The hypotheses are as follows: (1) it is the carboxamide oxygen of the C-3 substituent of 1 that interacts directly with K3.28(192) and (2) the interaction with K3.28(192) is crucial for the production of inverse agonism for biarylpyrazoles such as 1. To determine whether the carboxamide oxygen or the piperidine nitrogen of the C-3 substituent may be the interaction site for K3.28(192), we designed, synthesized, and evaluated a new set of analogues of 1 (3-6, Chart 1) in which modifications only to the C-3 substituent of 1 have been made. In each case, the modifications that were made preserved the geometry of this substituent in 1. The absence of the piperidine nitrogen was not found to affect affinity, whereas the absence of the carboxamide oxygen resulted in a reduction in affinity. CB1 docking studies in an inactive state model of CB1 resulted in the trend, 3,1<5,4<2<6 for ligand/CB1 interaction energies. This trend was consistent with the trend in WT CB1 Ki values versus [3H]CP55,940 reported here. In calcium channel assays, all analogues with carboxamide oxygens (1, 3, and 4) were found to be inverse agonists, whereas those that lacked this group (2, 5, and 6) were found to be neutral antagonists. Taken together, these results support the hypothesis that it is the carboxamide oxygen of the C-3 substituent of 1 that engages in a hydrogen bond with K3.28(192) in WT CB1. Furthermore, functional results for 1-6 support the hypothesis that the interaction of 1 with K3.28(192) may be key to its inverse agonism.