Targeting the Cannabinoid Type-2 Receptor for Novel Anto-inflammatory Therapeutics

Type-2 cannabinoid receptor & endocannabinoid system

Type-2 cannabinoid receptor (CB₂R) is a G-protein-coupled receptor (GPCR) and an essential element of the endocannabinoid system (ECS) ^[1]. The ECS is defined as the ensemble of endogenous molecules, collectively termed ‘endocannabinoids’ (eCBs), their processing enzymes and molecular targets. The eCBs are endogenous activators or agonists of CB₂R and type-1 cannabinoid receptor (CB₁R). Anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) are the best-studied eCBs ^[2]. CB₂R and CB₁R are the most important molecular targets within the ECS, holding great therapeutic potential for a large number of indications.

Where do you find CB₂R, and how can you detect it?

In contrast to CB₁R, which is most abundant in the brain, CB₂R is under healthy conditions primarily expressed in all tissues and circulating cells of the immune system including macrophages, T and B cells, monocytes and polymorphonuclear neutrophils. Furthermore, moderate to low CB₂R levels have been reported in other peripheral districts (e.g. hepatic myofibroblasts, cardiomyocytes, endothelial, smooth muscle cells) ^[3] and in the CNS (e.g. microglial cells and retinal cells) ^[2].

The degree of CB₂R expression and activity depends on the type and activation of cells, as well as on the stimulus. Under various pathological conditions/disease states CB₂R expression can be markedly upregulated in affected tissues/cells ^[4]. This is often accompanied by elevated eCB levels as a part of the inflammatory response/tissue injury. The detection of CB₂R relies on assays of messenger RNA (mRNA) expression or methodologies which can detect the protein itself ^[5]. Due to a paucity of specific antibodies, the latter remained for a long time challenging. Only recently, highly selective labelled chemical probes such as radiotracers and fluorescent ligands have been developed that allow for a more reliable detection of CB₂R protein ^[6].

CB₂R activation has enormous therapeutic potential for treating inflammatory diseases

CB₂R is a Gi/o coupled GPCR. Increased local eCB levels and CB₂R expression upon inflammation/tissue injury activate the receptor and trigger distinct fast signalling responses in immune and other cells e.g. the modulation of cytokine release. The resulting downstream effects translate toward the modulation of disease pathogenesis of a mostly protective nature. Furthermore, eCBs and/or synthetic CB₂R activators have been reported to attenuate inflammation and associated tissue injury in a huge number of pathological conditions/diseases. These include heart disorders, gastrointestinal, liver, kidney, lung, neurodegenerative/neuroinflammatory diseases, pain, skin pathologies, rheumatoid arthritis, endometriosis and eye diseases ^{[4b, 7]} (Figure 2).

Molecules targeting CB₂R

Due to the enormous therapeutic potential of CB₂R, many academic and industry institutions have developed novel ligands. First publications and patents appeared in 1991 ^[8] and 1996 ^[9], respectively. Since then, more than 1000 CB₂R patent applications have been filed. The majority of them describe activators of the receptor. However, modulators, neutral antagonists, inverse gonists, and allosteric ligands are covered as well. While the majority of these ligands are classical small molecules, including also many labelled chemical probes, some of which are also of a peptidic nature ^[10]. Overall, a large variety of structurally diverse chemotypes is covered and CB₂R molecules can be subdivided into eCBs, plant-derived cannabinoids (phytocannabinoids), cannabinoid-like and synthetic CB₂R ligands. Early molecules were important chemical tools for deciphering CB₂R pharmacology and were continuously improved toward development candidates.

The importance of high-quality chemical tools for studying CB₂R pharmacology

Tool compounds are a prerequisite for target validation which is an essential element of drug discovery. Such chemical tools need to fulfill a multitude of criteria e.g. high potency on the target and excellent selectivity regarding off-targets. If applied in vivo, tool compounds need to be bioavailable, meaning they must be able to reach the target protein within the relevant diseased tissue compartment in sufficient concentration to trigger a pharmacological effect. For extrapolation toward the human situation, potency and selectivity profiles of chemical probes must be comparable in the species in which an in vivo efficacy study is conducted and in humans. Initial CB₂R probes were lacking one or more of the above-mentioned criteria leading to misinterpretations of in vivo efficacy data. To address this issue, an extensive molecular pharmacology characterization of the most widely used CB₂R ligands in a collaborative effort between multiple academic and industry laboratories was conducted, reaching a consensus on which CB₂R molecules to use for studying the role of CB₂R in biological and diseases processes [11].

Launched CB₂R ligands & molecules on the way to beside

In the search for CB₂R-based therapy, more than 20 CB₂R-activators have been or are currently being studied in humans ^[10b]. They cover a broad range of indications encompassing neurological and fibrotic diseases, pain, osteoarthritis and cancer. Phytocannabinoids dronabinol, which is synthetic Δ⁹-tetrahydrocannabinol (Δ⁹-THC), nabilone and cannabidiol (CBD) exerting their action partially through CB2R activation have been introduced to the market. Currently, more than seven CB₂R ligands are under active clinical development. In contrast to the first clinically evaluated CB₂R agonists, which often were also activating CB₁R and were designed for pain indications, the recent focus shifted toward peripheral indications with an inflammatory and/or fibrotic background. Latest development candidates are either highly selective against CB1R or possess limited blood-brain barrier (BBB) permeability to avoid the psychotropic effects resulting from CB1R activation in the brain ^[12], such as for the dual CB₁R/CB₂R agonist Δ⁹-THC, the main active ingredient of Cannabis sativa [13]. Furthermore, most recent CB2R development candidates possess excellent ADME properties translating into favourable pharmacokinetic profiles (Figure 3). Most advanced selective CB2R agonists are in phase 2 clinical trials.

Future perspectives on the understanding of CB2R

No CB₂R-related toxicity issues have been reported from clinical studies so far. However, the demonstration of target engagement and the identification of best-suited human disease condition(s) for the therapeutic use of CB₂R modulators still pose challenges for developing CB₂R-based therapies. The generation of translational animal models and a better understanding of CB₂R and the ECS, in general, will help unlock the receptor’s full therapeutic potential. Recently discovered high-quality labelled chemical probes have enabled a better understanding of CB₂R expression, mechanism of action and translatability of results toward the human situation. The in-depth understanding of signalling bias as well as CB₂R receptor homo- and heterodimers might translate into different functional properties and ultimately tailor-made CB₂R therapeutics.

Deeper insights into drug-target binding kinetics, their impact on receptor function, machine learning and artificial intelligence-based drug discovery applications will facilitate rational drug design. Furthermore, recent breakthroughs in the structural elucidation of CB₂R complexed with different ligands and its downstream G-proteins (Figure 1) ^[14] provide an important molecular basis for elucidating ligand binding modes, receptor activation and signaling mechanism, as well as knowledge on allosterism will aid the development of CB₂R-based therapies. Together with the huge chemical space available to generate tailor-made CB₂R modulators this will guide us to the discovery of potent, effective, and safe medicines for indications with a dire or even unmet need.

References

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