CB2 receptor

Amyotropic lateral sclerosis (ALS) 3. AEA and 2-AG increase in the spinal cord of SOD1 transgenic mice, a model of ALS, to inhibit disease progress 3. CB2 receptor agonists or inhibitors of degradation... 

CBD has no activity at the CBl or CB2 receptor. It is well known that CBD influences the activity of THC if co-administered [140]. Another effect of CBD is the inhibition of cytochrome oxidase [141], which inversely to its antagonistic activity strongly potentiates THC effects above a certain threshhold. CBD is also active as a mild antipsychotic [142] and was proposed as a treatment for anxiety and panic attacks. The mechanism is not fully understood, but it might be caused by an interference with the endocannabi-... 

A series of palmitoylethanolamine-derived inhibitors has been described in the literature as FAAH inhibitors [77, 78]. This study explored the effect of shortening the chain length and replacement of the ethanolamine head group with primary, secondary and tertiary amide alternatives. Of the compounds synthesised and tested, two compounds gave reasonable affinities for FAAH inhibition, palmitoyl-isopropylamide (63) (IC50 = 13/rM) and palmitoyl-allylamide (64) (IC50 = 3.4/rM). Both these compounds had little affinity for either CBi or CB2 receptors. 

A -THC, the main psychoactive component of cannabis, is a moderately potent partial agonist of the CBi and CB2 receptors, while cannabidiol has little affinity for either receptor (Table 6.7). The term classical cannabinoids is used to describe cannabinoid receptor modulators structurally related to (67), which have a tricyclic dibenzopyran core. While several other structural types of cannabinoid receptor modulators have been discovered in recent years, the classical cannabinoids are still by far the most extensively studied group in terms of SAR and pharmacology. 

The dimethylheptyl side-chain analogue of 9-carboxy-A -THC, ajulemic acid, CT-3, (166) is currently in clinical development for treatment of pain and inflammation [121, 122]. Compound (166) does show some affinity for CBi and CB2 receptors, but may also exert anti-inflammatory and analgesic effects through other mechanisms. It does not appear to be psychoactive in humans [123]. 

In general, modification or deletion of the Cl phenolic hydroxyl group results in significantly reduced CBi receptor affinity [93]. A number of 1-deoxy and 1-alkoxy A -THC analogues have been shown to be selective ligands for the CB2 receptor (see below). 

Cannabidiol has low affinity for CBi and CB2 receptors and is not psychoactive, but has nevertheless shown a number of pharmacological activities of its own including anti-inflammatory and neuroprotective effects. Some side chain-modified cannabidiol derivatives have also been evaluated for can-nabinoid receptor affinity and these are shown in Table 6.15. 

A fourth important pharmacophoric element was established for the non-classical cannabinoid series in the form of a southern aliphatic hydroxyl group. Addition of this group to (192) resulted in the high-affinity CBi and CB2 receptor full agonist CP 55,940 (193) [129, 133], the tritiated form of which was used to first demonstrate specific cannabinoid binding sites in brain tissue [134]. Its enantiomer, CP 56,667 (194) has lower affinity for the CBi receptor (Table 6.17). 

As outlined earlier, anandamide was the first among the endogenous cannabinoid receptor agonists to be identified. It exhibits higher binding affinity for the CBi receptor Ki — 89 nM) than for the CB2 receptor (il = 371 nM) [81]. Anandamide has typical cannabinoid activities including decreased spontaneous motor activity, immobility and production of hypothermia and analgesia [147, 148]. However, this action in vivo is of shorter duration than... 

In a study looking at oxygenated metabolites of AEA and their interaction with the cannabinoid system, a series of hydroxylated alkyl chains was prepared using different lipoxygenases as biocatalysts [152-154]. Of the seven AEA derivatives prepared, only the 5R-hydroxy (208), 125 -hydroxy (209) and 155 -hydroxy (210) derivatives had any affinity for the CBi receptor. Interestingly, the IdV-hydroxy compound (211) that was inactive at the CBi receptor displayed some affinity for the CB2 receptor [152] (see Table 6.18). 

Replacement of the ethanolamine head group is also well tolerated. Substitution with a cyclopropyl (243) [37], allyl (244) or propargyl group (245) [164] all led to an increase in binding affinity compared to AEA. Replacement of the head group with aromatics is also allowed. The phenyl derivative (246) retains affinity at the CBi receptor [37], whereas the 2-substituted A-methyl pyrrole (247) has a 2-fold improved affinity compared to AEA [167]. Interestingly, the 3-substituted furan derivative (23) that has micromolar affinity for the AEA transporter (see above) does not bind to the CBi receptor, but has good affinity for the CB2 receptor [167]. These results are summarised in Table 6.20. 

Noladin ether (3) was recently isolated from porcine brain [16] and found to bind to the CBi receptor (/fj = 21.2 nM), to bind weakly to the CB2 receptor K, > 3 /iM) and it causes typical cannabinoid-like effects such as sedation, hypothermia, intestinal immobility and mild antinociception in mice [16]. This endocannabinoid had previously been synthesised independently by both Mechoulam and co-workers [176] and Sugiura et al. [173]. SAR studies of this endocannabinoid are lacking in the literature, however, a recent publication highlighted the importance of the tetra-unsaturated C20 chain... 

More recently, the utility of the indole group as a scaffold for cannabinoid agonists has been demonstrated by a number of new patent applications appearing in the literature (286)-(290) [187-190]. Of particular note is compound (286) that is reported to have 18-fold selectivity for the CBi receptor (CBp Ki — 0.08 nM CB2 Ki — 1.44nM). In addition to the indole scaffold, a number of patent applications by AstraZeneca claim indole-like scaffolds such as benzimidazoles (289) [191-193] and azaindoles (290) [194]. Although these compounds bind to both CBi and CB2 receptors, the inventors claim that they may be useful in treating diseases without the associated CNS side effects. 

In addition, Novartis filed a patent application on a series of quinazolines as cannabinoid agonists [209]. Compound (320) is one of the two compounds specifically claimed and exhibited CBi and CB2 binding with if values of 34 and 11 nM, respectively. It was shown to be a full agonist at the CBi receptor with an EC50 of 132nM (no functional data for the CB2 receptor). Compound (320) was also active in the neuropathic pain model described above with an ED50 of 0.5mg/kg after oral dosing. 

AstraZeneca has filed a patent application on novel bis-aryl compounds as CB1/CB2 agonists that lack CNS penetration and thus avoid the unwanted side effects associated with activation of central CBi receptors [210]. Over 100 compounds are specifically claimed e.g. (321). Compounds were tested in receptor binding assays using human CBi and CB2 receptor preparations. Respective K values were in the ranges 50-5,000 and 15-2,800 nM, although no specific data were presented. 

The CB2 receptor has been shown to tolerate shorter C3 side chains than the CBi receptor and the Huffman group has exploited this in combination with Cl modifications to develop highly selective CB2 receptor ligands (Tables 6.31 and 6.32) [214, 215]. 

The 11-hydroxy analogues of the IT -dimethyl compounds in Tables 6.31 and 6.32 were also prepared by the Huffman group. In general, these showed higher affinity for both CBi and CB2 receptors than the simple methyl analogues, but reduced CB2 selectivity (data not shown) [215]. 

Several groups have published on structural analogues of (382), one of the earliest being the disclosure of CP 272871 (383) from Pfizer, which displays lower affinity for the CBi receptor than (382), in addition to reduced selectivity over the CB2 receptor subtype. Both (382) and (383) have been shown to act as inverse agonists rather than neutral antagonists in vitro [265]. A recently published patent application from Sanofi-Aventis claims a series of 4-cyanopyrazole analogues of (382), with 42 specific examples [266]. 

Several studies have been reported on the application of conformational restraint to the 1,5-diaryl-pyrazole series in an attempt to provide compounds with modified properties. In one approach, a Sanofi-Synthelabo patent application claimed a series of conformationally restrained compounds, exemplified by compound (391). Compounds of the invention were stated to be CBi receptor antagonists with K[ values below 5 x 10 M and selectivity over CB2 receptors of at least 10-fold [274]. 

Thomas and co-workers [277] reported that irradiation of (382) with a 450 W high-pressure mercury lamp brought about photocyclisation to a constrained analogue (394). The structure of the product was elucidated through NMR and X-ray diffraction analysis. The compound retained high affinity for the CBi receptor (Xj = 48 nM) and good selectivity over the CB2 receptor (K[ — 3,340 nM). 

The initial 4-Cl analogue (516) was found to exhibit only modest affinity, however derivative (517) showed CBi inverse agonist activity comparable with (382) while also demonstrating good selectivity over the CB2 receptor. 

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