Chemistry of Cannabinoid Receptor Agonists

C H A P T E R

62 Chemistry of Cannabinoid Receptor Agonists M. Aghazadeh Tabrizi, P.G. Baraldi Department of Chemistry and Pharmaceutical Science, University of Ferrara, Ferrara, Italy

SUMMARY POINTS • Cannabinoid ligands are characterized by a wide chemical diversity. This diversity has different consequences. First, it has been fairly complicated to supply a common pharmacophore for all cannabinoid agonists. Second, the structure– activity relationships are not the same in the whole family. • Conspicuous antinociceptive effects of CB agonists in different models of inflammatory or neuropathic pain have been demonstrated. • The CB1 selective agonists include anandamide analogs (ACEA, ACPA) and O-1812, cyano analog of methandamide. • The CB2 selective agonists include nonendogenous cannabinoids, traditional cannabinoids such as L-759,633, JWH-133, HU308, aminoalkylindoles (AM-1241), and several different chemical structures are the purpose of the discussion in this chapter. • Several cannabinoid receptor agonists show distinct stereoselectivity in pharmacological assays, reflecting the presence of chiral centers in these molecules.

KEY FA C T S OF CANNAB INOID RECEPTO R S • Cannabinoid receptors are responsible for the mediation of the psychoactive effects of cannabis they are also implicated in a variety of physiological processes.

• The two types of cannabinoid receptors named CB1 and CB2 are G protein coupled receptors. • Activation of the cannabinoid receptors causes inhibition of adenylate cyclase, and a subsequent decrease in the concentration of cyclic adenosine monophosphate in the cell, resulting in the inhibition of neurotransmission. • ∆9-Tetrahydrocannabinol (∆9-THC), representing the psychoactive principle of Cannabis, was identified in 1964. • The CB1 receptor is expressed mainly in brain, lungs, liver, and kidneys; the CB2 receptor is expressed principally in the immune system. • Cannabinoid receptors are activated by phytocannabinoids (found in Cannabis), endocannabinoids (produced naturally in the body by humans and animals), and synthetic cannabinoids (produced chemically by humans).

LIST OF ABBREVIATIONS 2-AG 2 Arachidonoylglycerol ACEA Arachidonyl-2’-chloroethylamide ACPA Arachidonylcyclopropylamide AEA Anandamide cAMP Cyclic adenosine monophosphate CBD Cannabidiol CNS Central nervous system EC50 Median effective concentration FAAH Fatty acid amide hydrolase Ki Inhibition constant SAR Structure–activity relationship THC Tetrahydrocannabinol Handbook of Cannabis and Related Pathologies. http://dx.doi.org/10.1016/B978-0-12-800756-3.00072-7 Copyright © 2017 Elsevier Inc. All rights reserved.

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Endogenous cannabinoids and synthetic analogs

INTRODUCTION

593

Cannabinoids, the active components of Cannabis sativa and their derivatives, are known to mediate some of their actions through the cannabinoid receptors. Two distinct cannabinoid receptors, named CB1 and CB2, have been cloned and characterized from mammalian tissues. The CB1 receptor is abundantly expressed in the central nervous system (CNS) and is responsible for the psychotropic side effects. The CB2 receptor is mainly found in cells of the immune system, though it may be upregulated in the CNS under pathological conditions. The main signal transduction pathway triggered is through Gi proteins, resulting in an inhibition of adenylate cyclase activity, and a decrease in cyclic adenosine monophosphate (cAMP) levels (Pertwee et al., 2010). Several studies have demonstrated noticeable antinociceptive effects of CB agonists in different models of inflammatory or neuropathic pain (Guindon & Hohmann, 2008). These models could be useful for the preclinical evaluation and validation of the therapeutic efficacy of novel putative analgesics (Ashton & Milligan, 2008). However, it was mainly drugs interacting with CB1 receptors that showed marked central side effects that have disallowed their prevalent acceptance and therapeutic application (Kreitzer & Stella, 2009). The first part of this chapter describes cannabinoid ligands commonly classified into the following chemical groups: tricyclic and bicyclic analogs of tetrahydrocannabinol (THC); endogenous cannabinoids, and the aminoalkylindoles. In the second part, we describe the chemical structures of the recent entry of several different cores as cannabinoid ligands.

chromen-1-ol 3 (HU-210) is the (+)-1,1-dimethylheptyl analog of 11-hydroxy-∆8-THC which is more potent than natural THC for both CB1 and CB2 receptors. The (−)enantiomer (HU-211) being inactive at cannabinoid receptors highlighted the preference for the 6aR, 10aR enantiomer. The 1-deoxy-∆8-THC derivatives were synthesized by replacing the phenolic hydroxyl function leading to 4 (JWH-051), that exhibited significantly enhanced affinity for both the CB2 and the CB1 receptors. Other 1-deoxy-∆8THC compounds such as 5 (JWH-056), and 6 (JWH-133) showed a total loss of CB1 affinity. In this series, JWH-133 has very high affinity and selectivity for the CB2 receptor (Ki 3.4 nM). Alternatively, methylation of the phenolic hydroxyl group also reduced affinity for the CB1 receptor such as that found in compound 7 (L759,633, Ki CB1 1043 nM) (Huffman, 2000). The bicyclic compounds fall into the nonclassical cannabinoids and derived from opening of the pyran ring of ∆9-THC. Cannabidiol (CBD) (8) is a bicyclic cannabinoid devoid of any psychoactive properties produced by ∆8-THC or ∆9-THC. Interestingly, a 3-phenylcyclohexanol derivative 9 (CP-47,497) which has been described as producing many of the pharmacological effects produced by ∆9-THC, acts as full receptor agonists for both the CB1 and CB2 cannabinoid receptors. Another related bicyclic analog 2-[(1R,2R,5R)-5-hydroxy2-(3-hydroxypropyl)cyclohexyl]-5-(2-methyloctan-2-yl) phenol 10 (CP-55,940) has been used to identify a cannabinoid binding site (Showalter, Compton, Martin, & Abood, 1996). The bicyclic ligand 11 (HU-308) which has the opposite absolute configuration from all other analogs, is a CB2 specific agonist. A slight modification of dimethylbicycloheptene leading to the 12 (­HU-910), enhanced the affinity of the molecule for CB2 receptor (Horvath et al., 2012).

TRICYCLIC AND BICYCLIC ANALOGS OF TETRAHYDROCANNABINOL

ENDOGENOUS CANNABINOIDS AND SYNTHETIC ANALOGS

Chemical structures are shown in Fig. 62.1; biological data are shown in Table 62.1. The tricyclic natural compounds fall into the classical cannabinoids. The prototypical (6aR,10aR)-∆9-THC (1) is the mainly well-known phytocannabinoid, and it is a nonselective agonist that binds to both CB1 and CB2 with a similar affinity in the order of 40 nM (Howlett et al., 2002). This natural product readily isomerizes to thermodynamically more stable isomer (2, ∆8-THC) under acidic conditions, which maintains a similar level of affinity. Classical and nonclassical cannabinoids include the tricyclic and bicyclic derivatives of ∆9-THC and, in particular, derived from more stable isomer ∆8-THC. For example, (6aR,10aR)-9-(hydroxymethyl)-6,6-dimethyl3-(2-methyloctan-2-yl)-6a,7,10,10a-tetrahydrobenzo[c]

Chemical structures are shown in Fig. 62.2; biological data are shown in Table 62.1. Generally, the endocannabinoids show better affinity for the CB1 than CB2 receptor. In 1992, the first endogenous ligand cannabinoid receptors was isolated and named anandamide (arachidonoylethanolamide, AEA, 13) (Howlett et al., 2002). Anandamide exerts its biological effects through both cannabinoid receptors as partial agonist. The endogenous agonist of the CB1 receptor, 2-arachidonoylglycerol (14, 2-AG), is the arachidonic acid ester of glycerol and has higher affinity and efficacy to CB1 and CB2 receptors than AEA. 2-Arachidonoylglyceryl ether (15, 2-AGE or noladin ether) is an ether-linked analog of 2-AG, binds to CB1 and very weakly to CB2 receptor.

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62.  Chemistry of Cannabinoid Receptor Agonists

FIGURE 62.1  Chemical structures of tricyclic and bicyclic analogs of THC.

To improve metabolic stability and to increase the CB1 selectivity, the methanandamide series was synthesized. Compound 16 (R-(+)-methanandamide), named also AM356, is a stable chiral analog of anandamide and the CB1 selectivity of R-(+)-methanandamide derived from the introduction of a methyl group on the 1 carbon of AEA, a structural change that also confers better resistance to the hydrolytic inactivation by fatty acid amide hydrolase (FAAH). O-1812 (compound 17) is a cyano ­analog of methanandamide, behaves as potent CB1 receptor agonist, and acts as a potent and highly selective agonist for the CB1 receptor (Ki of 3.4 nM at CB1 and 3870 nM at CB2 receptors). Unlike most related compounds, O-1812 is ­metabolically stable against rapid breakdown by FAAH. Arachidonyl-2’-­chloroethylamide (18, ACEA) and arachidonylcyclopropylamide 19 (ACPA) were prepared by replacement of ethanolamide

chain with lipophilic moieties. These analogs are considered to be selective cannabinoid agonists, as they bind primarily to the CB1 receptor (Pertwee, 2005).

INDOLES Chemical structures are shown in Fig. 62.3; biological data are shown in Table 62.1. These ligands consisted of an indole core substituted by the lipophilic aroyl group at C3 and an aminoalkyl side chain at N1 positions (Huffman, 1999). The prototypical 20 (R-(+)-WIN-55,212), is one of the best ­characterized synthetic cannabinoids from the aminoalkylindole series that has been employed extensively in a number of investigations into the pharmacology of this group of compounds. The R-enantiomer is a potent, full

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595

Indoles

TABLE 62.1 CB1 and CB2 Binding Affinities of Traditional Cannabinoid Ligands and Indoles CB1 Ki(nM)a

Compound

CB2 Ki(nM)a

References

Agonists with similar CB1 and CB2 Affinities 1, ∆9-THC

53.3

75.5

Pertwee (2005)

2, ∆8-THC

47.6 (rat)

39.3 (mouse)

Pertwee (2005)

3, HU-210

0.06

0.52

Pertwee (2005)

8, CBD

4350 (rat)

2860 (rat)

Showalter et al. (1996)

10, CP-55,940

5.0

1.8

Howlett et al. (2002)

12, HU-910

1.37 b

6.0

Horvath et al. (2012) b

14, 2-AG

472

1400

Pertwee (2005)

20, R(+)-WIN55212

9.9 (rat)

16.2 (rat)

Pertwee (2005)

21, JWH-018

9.5

2.9

Pertwee (2005)

Weissman, Milne, and Melvin (1982)

Agonists with higher CB1 than CB2 affinity 9, CP-47,497

2.2

ND

13, AEA

61 (rat)

1930 (rat) b

Howlett et al. (2002)

15, 2-AGE

21.2 (rat)

>3000

Pertwee (2005)

16, AM-356

17.9 (mouse)

868 (rat)

Pertwee (2005)

17, O-1812

3.4 (rat)

3870 (mouse)

Pertwee (2005)

18, ACEA

1.4 (rat)

>2000 (rat)

Pertwee (2005)

19, ACPA

2.2 (rat)

715 (rat)

Pertwee (2005)

Agonists with higher CB2 than CB1 affinity 4, JWH-051

1.2 (rat)

0.03

Howlett et al. (2002)

5, JWH-056

>10000 (rat)

32

Han, Thatte, Buzard, and Jones (2013)

6, JWH-133

677 (rat)

3.4

Pertwee (2005)

7, L-759,633

1043

6.4

Huffman (2000)

11, HU-308

>10000 (rat)

22.7

Pertwee (2005)

22, JWH-015

383

14

Howlett et al. (2002)

23, AM-1241

580

7.1

Yao et al. (2009)

24, UR-12

245

11

Han et al. (2013)

25

16

1.0

Han et al. (2013)

26, A-796,260

945

4.6

Frost et al. (2010)

27, L-768242

2043

14

Gallant et al. (1996)

The structures of compounds listed in this table are shown in Figs. 72.1–72.3. a Human cannabinoid receptors, unless otherwise noted: nM, nanomolar. b Species unspecified.

agonist of both CB1 and CB2 receptors. In contrast, the S-(-)-WIN 55,212 was found to behave as a competitive neutral antagonist of the human CB2 receptor, and acts as a low potency partial inverse agonist at the human CB1 receptor (Savinainen et al., 2005).

A summary of SAR described by the research groups indicated that key structural features for potent cannabinoid binding activity of aminoalkylindoles are a bicyclic (particularly l-naphthyl) substituent at the 3-position of the indole, a small substituent, or better, no substituent, at

V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids

FIGURE 62.2  Chemical structures of endogenous cannabinoids and synthetic analogs.

FIGURE 62.3  Chemical structures of indoles.

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597

Monocyclic cores

the 2-position of the indole, and a morpholinoethyl or other cyclic aminoethyl substituent at the 1-position. Several studies have been established that the aminoalkyl segment of the molecule could be replaced by an alkyl group. In particular, 1-pentyl-2-methyl-3-(1-naphtoyl)indole 21 (JWH-018) demonstrates modest preference for CB2, it has high affinity for the brain receptor and exhibits typical cannabinoid pharmacology in vivo. The 1-propyl analog 22 (JWH-015) has selective affinity for the peripheral cannabinoid spleen receptor. In the family of aminoalkylindoles, the 3-benzoyl derivative 23 (AM-1241, 1-(methylpiperidin-2-ylmethyl)-3-(2-iodo-5-nitrobenzoyl)indole) was ­prepared as a hydrochloride salt, and showed high selectivity for the human CB2 receptor versus human CB1 in recombinant binding assays (Eissenstat et al., 1995). The C3 amido indole and its cyclized and conformationally constrained indolopyridone were found to bind selectively to the CB2 receptor. Representative compounds are N-(S)-­ fenchyl-1-(2 morpholinoethyl)-7-methoxyindole-3-carboxamide 24 (UR-12) and compound 25 (Han et al., 2013). Replacing the aromatic 3-benzoyl or 3-naphthoyl group of indole derivatives with the 3-tetramethylcyclopropylmethanone, enhances selectivity for CB2 receptors. Compound 26 (A-796,260) is an example which showed high affinity and selectivity for CB2 receptor having Ki values of

4.6 and 945 nM in a competition binding assay for human CB2 and CB1, respectively (Frost et al., 2010). CB2 selective cannabimimetic indoles include 1-(2,3-dichlorobenzoyl)2-methyl-3-(2-[1-morpholine]ethyl)-5-methoxyindole 27 (L-768242) obtained by introduction of benzoyl group on the N1 position (Ki values are 3.92 and 4772 nM for human CB2 and CB1 receptors, respectively). The morpholine moiety may be linked to the indole core through an ethylene or acetyl spacer with minimal change in potency on the human CB2 receptor. In this series, for compounds bearing a morpholine unit at C3, the 2,3-dichlorobenzoyl indole derivative was more active than the another chloro derivatives (30-fold) (Gallant et al., 1996). The second part of this chapter is focused on advances that have risen with the recent entry of several diverse chemical cores as cannabinoid ligands. Generally, these molecules are characterized by an aromatic heterocyclic core linked to a lipophilic segment, and commonly possess CB2 receptor affinity.

MONOCYCLIC CORES Chemical structures are shown in Figs. 62.4 and 62.5; biological data are shown in Table 62.2.

FIGURE 62.4  Chemical structures of monocyclic cores. V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids

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62.  Chemistry of Cannabinoid Receptor Agonists

FIGURE 62.5  Chemical structures of monocyclic cores.

Pyrimidines and Pyridines

Sulfamoyl Benzamides

Focused screen identified the pyrimidine ester as a partial agonist at the CB2 receptor with micromolar potency. Subsequent lead optimization identified 28 (GW842166X) as the optimal compound in the series. GW842166X is a potent and selective full agonist at the CB2 receptor. Replacement of the pyrimidine core of compound GW842166X with pyridine was investigated to improve the aqueous solubility over GW842166X (Mitchell et al., 2009). An extended SAR of the new pyridine-3-carboxamides template was evaluated, and was concluded in the identification of analog 6-(2,4-dichlorophenylamino)-4-cyclopropyl-N((tetrahydro-2H-pyran-4-yl)methyl)pyridine-3-carboxamide 29 which demonstrated efficacy in an in vivo model of inflammatory pain, despite its low aqueous solubility. Next studies reported a conformationally restricted morpholinyl motif (compound 30) which retained activity and selectivity (Zindell et al., 2009). A novel CB2 ligand based on the 3-carbamoyl-2-pyridone derivatives by adjusting the size of side chain at 1-, 5-, and 6-position was discovered. The structure–activity relationship around this template led to the identification of 31 (S-777469) as a selective CB2 receptor agonist, which exhibited moderate potency for CB2 receptor (hCB2 Ki 36 nM) and good selectivity (>120) (Odan et al., 2012).

Chemical structures are shown in Fig. 62.4; biological data are shown in Table 62.2. Several sufamoyl benzamides have been described (Worm et al., 2008), large lipophilic substituents such as S-fenchyl residue, led to improved receptor binding and selectivity for the CB2 receptor such as compound 32, with 120-fold functional selectivity for the CB2 receptor. Small changes in the sulfonamide part of the molecule produced a change from full agonist to inverse agonist. Replacement of the amide moiety with various heterocycles did not significantly improve affinity or selectivity for the CB2 receptor, in comparison to the lead compound 32; while reversal of the amide linkage led to the sulfamoyl benzamides with improved ­affinity and selectivity for the CB2 receptor. The best compound of this study, the tetramethylcyclopropy derivative 33, displayed poor metabolic stability in rat pain model (Goodman et al., 2009). Further SAR investigation in the sulfonamide series led to the identification of several compounds, showing potent affinity for the CB2 receptor (Ki  < 10 nM) and improved CB2 receptor selectivity (>200-fold). Compound 34, N-[3,4-dimethyl5-(morpholin-4-ylsulfonyl)phenyl]-2,2-dimethyl butanamide, displayed the best overall in vitro profile, and demonstrated robust efficacy in an animal model of postoperative pain (Sellitto et al., 2010).

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Monocyclic cores

TABLE 62.2 CB1 and CB2 Activities of Mono-, Bi-, and Tricyclic Cannabinoids Compound

CB1 (nM)a

CB2 (nM)a

References

28, GW842166X

EC50 > 30000

EC50 63

Mitchell et al. (2009)

29

EC50 > 30000

EC50 79

Zindell et al. (2009)

30

EC50 > 20000

EC50 10

Zindell et al. (2009)

31

Ki 4.6

Ki 36

Odan et al. (2012)

32

EC50 550000

EC50 4.6

Worm et al. (2008)

33

Ki 3400

Ki 23

Goodman et al. (2009)

34

Ki 2500

Ki 17

Sellitto et al. (2010)

35

Ki 93.6

Ki 1.0

Bhattacharjee, Gurley, and Moore (2009)

36

EC50 220

EC50 1.7

Cheng et al. (2008)

37

EC50 1430

EC50 7.0

DiMauro et al. (2008)

38

EC50 3500

EC50 13

Ohta et al. (2007)

39, A836339

Ki 270

Ki 0.64

Dart et al. (2007)

40, CBS0550

EC50 4000

EC50 2.9

Ohta et al. (2008)

41

EC50 > 2000

EC50 5.0

Marx et al. (2009)

42

EC50 > 2000

EC50 25

Marx et al. (2009)

43

Ki > 4600

Ki 38

Page et al. (2008)

44

ND

EC50 0.15

Ryckmans et al. (2009)

45

EC50 > 25000

EC50 30

Ando and Iwata (2010)

46

EC50 > 10000

EC50 2.7

Ryckmans et al. (2009)

47

EC50 1422

EC50 0.3

Verbist et al. (2008)

48

EC50 17000

EC50 33

Trotter et al. (2011)

49

EC50 2600

EC50 5.0

Trotter et al. (2011)

50

ND

EC50 17

Wu, Green, and Hartnett (2009)

51, GRC-10693

Ki 985

Ki 11.8

Muthuppalanippan, Balasubramanian, Gullapalli, Joshi, and Narayanan (2006)

52

Ki > 10000

Ki 0.6

Manera et al. (2007)

53

Ki 560

Ki 11

Pasquini et al. (2011)

54

Ki 4568

Ki 11.2

Aghazadeh Tabrizi et al. (2012)

55

Ki > 10000

Ki 2.5

Aghazadeh Tabrizi et al. (2013)

56

Ki 363 (mouse)

Ki 0.03 (mouse)

Murineddu et al. (2006)

57, Sch35966

Ki 2633

Ki 6.8 (rat)

Gonsiorek et al. (2007)

58

Ki 6183

Ki 17.6 (rat)

Page et al. (2007)

59

Ki 310

Ki 0.81

Baraldi et al. (2012)

60

Ki > 10000

Ki 8.12

Vincenzi et al. (2013)

a

Human cannabinoid receptors unless otherwise noted: nM, nanomolar; ND, no data.

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62.  Chemistry of Cannabinoid Receptor Agonists

Triaryl Derivatives Chemical structures are shown in Fig. 62.4; biological data are shown in Table 62.2. A series of cannabinoid ligands with a structurally triaryl core has been designed. This study was based on the previously observations, wherein an enhancement of CB2 receptor affinity was observed with a dichlorophenyl substituted ring in the biaryl series. The presence of a gem dimethyl group as a linker (compound 35) results in a CB2 selective compound with a Ki value of 1.07 nM at the CB2 receptor, compared to 93.6 nM at the CB1 receptor subtype (Bhattacharjee et al., 2009).

N-Arylamide Oxadiazoles Chemical structures are shown in Fig. 62.5; biological data are shown in Table 62.2. Targeted library screening and subsequent hit assessment have identified the five-membered oxadiazole core as a novel class of potent and selective CB2 agonists. SAR investigation into this group of compounds showed that the oxadiazole core was fundamental and substitutions at the 2- and 4-position of the phenyl ring joined to the oxadiazole were important for desired CB2 activity. The amino quinoline derivative 36 is a highly potent, selective CB2 agonist that displayed an excellent pharmacokinetic profile, with oral bioavailability in rats (Cheng et al., 2008). Structural modifications in the central portion of the N-arylamide oxadiazole scaffold led to the identification of N-arylpiperidine oxadiazoles as conformationally constrained analogs that offered improved stability, comparable potency, and selectivity (compound 37) (DiMauro et al., 2008).

Thiazolylidene, Pyrazolylidene, Amidosulfone Chemical structures are shown in Fig. 62.5; biological data are shown in Table 62.2. The five-membered heterocyclic cores such as thiazole ring are reported as CB2 selective agonists. The SAR study showed that the functional groups at 3- and 5-position in the thiazole ring and the aromatic group in the amide deeply influence the affinity and selectivity for the CB2 receptor. Thiazolylidene compounds with a bulky functional group like trifluoromethyl, exhibited much high CB2 affinities (compound 38, EC50 hCB2 13 nM, 270-fold selectivity) (Ohta et al., 2007). Another thiazolylidene, compound 39 (A-836339), bearing a bulky cycloalkylcarboxamide group, exhibited subnanomolar potency in competition binding assays for human and rat CB2, with good selectivity against the human and rat CB1 receptors (425- and 189-fold, respectively) (Dart et al., 2007).

Pyrazole-based CB2 agonists have also been explored, the SAR study highlighted that heterocycle nucleus operated as a linker, and that the functional groups at the 2-position in the pyrazole ring, and the aromatic group in the amide significantly affected affinity and selectivity for the CB2 receptor Pyrazolylidene 40 (CBS-0550) has good solubility in water, and in animal studies it was found to produce analgesic and antihyperalgesic effects. It acts as a potent and selective cannabinoid CB2 receptor agonist (EC50 human CB2 2.9 nM) (Ohta et al., 2008). The strictly correlated α-amidosulfones were found to be potent and selective agonists of CB2. The effect of the amide substituent on CB2 activity and selectivity was explored. 3,4-Disubstituted phenyl ring enhanced the potency and selectivity (compound 41), while disubstituted phenyl rings diminished agonist activity on both CB1 and CB2 receptors. For the five-membered heterocycles, it was found that tert-butyl group was important for activity, such as in compound 2-(4-chlorophenylsulfonyl)-N-(3-tert-butyl-1-methyl-1H-pyrazol-5-yl)-2-methylpropanamide 42. α-Amidosulfones behaved as selective full agonists of CB2 and showed high functional and cellular activity on CB2 receptors (Marx et al., 2009).

BICYCLIC CORES Chemical structures are shown in Fig. 62.6; biological data are shown in Table 62.2.

Benzimidazoles CB2 agonists based on benzimidazole template have been reported. One of the first was 2-(4-ethoxybenzyl)1-(2-(dimethylamino)ethyl)-N,N-diethyl-1H-benzo[d] imidazole-5-carboxamide 43. This molecule was studied as a starting template to improve the potency of benzoimidazole ligands. SAR studies of the N1 substituent exposed that various alkyl and aromatic groups were tolerated, with a preference for alkyl substituents, and that the binding interactions at the amide position appeared to be essentially hydrophobic (Page et al., 2008). Further investigations around this new class of ligands allowed the discovery of sulfone compounds with CB2 receptor agonistic activity. Incorporation of neopentyl chain into the 2-position, together with introduction of a sulfonyl group into the 5-position, led to compound 1-(1-(cyclopropylmethyl)-2-neopentyl-1H-benzo[d] imidazol-5-ylsulfonyl)cyclopropanecarboxamide (44) (Ryckmans et al., 2009), a selective and potent full agonist of CB2 receptor. In the sulfonyl family, it has been found out that a new class of N-substituted saturated heterocyclic sulfone compounds show CB2 agonistic activity with little affinity for CB1, such as 3-(1-(2-(dimethylamino)

V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids



Bicyclic Cores

ethyl)-2-neopentyl-1H-benzo[d]imidazol-5-ylsulfonyl) azetidine-carboxamide 45 (Ando & Iwata, 2010). In this context, 2-tert-butyl-substituent benzoimidazole was found to influence both potency and efficacy. Among them, 2-tert-butyl-1-(cyclopropylmethyl)-5(ethylsulfonyl)-1H-benzo[d]imidazole 46 was described as a potent and selective CB2 agonist (>38,000-fold

601

against CB1) (Ryckmans et al., 2009; Watson et al., 2011). A strictly related class of benzimidazole CB2 receptor agonists was synthesized, and the SAR was explored. The size of the substituent on the 2-position determined the level of agonism, ranging from inverse to partial or full agonism, which was more pronounced for the rat CB2 receptor. The benzoimidazole cannabinoid agonists

FIGURE 62.6  Chemical structures of bicyclic cores.

V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids

602

62.  Chemistry of Cannabinoid Receptor Agonists

bearing a substituted aryl group, in particular, the pyridyl sulfone such as compound 2-tert-(butyl-5-(2-ethoxypyridin-4-sulfonyl)-1-tetrahydropyran-4-ylmethyl)-1Hbenzimidazole. 47 had excellent binding affinity and selectivity for CB2 (Verbist et al., 2008).

Imidazopyridines Imidazopyridine-based CB2 agonists have been described. Exploration of SAR in this chemical series by variation of substituents on both carbon positions of the imidazo ring has produced CB2 agonists. Compounds N-((1-(hydroxymethyl)cyclopentyl)methyl)3-morpholinoH-imidazo[1,5-a]pyridine-1-carboxamide 48 and (2,3-dichlorophenyl)(1-(morpholinomethyl)Himidazo[1,5-a]pyridin-3-yl)methanone 49, which were completely selective for CB2 versus CB1 are representatives of this series. In vivo evaluation of these compounds indicates a significant impact of the degree of ­selectivity for CB2 on analgesic effects (Trotter et al., 2011). A related series of 3-arylimidazopyridine analogs was reported, wherein 3-(3-(trifluoromethyl)phenyl)-1-((4,4difluoropiperidin-1-yl)methyl)H-imidazo[1,5-a]pyridine 50 was the most potent CB2 agonist disclosed (Wu et al., 2009).

Bicyclic Pyrazoles The bridged bicyclic pyrazole compounds are reported to be CB2 agonists. Within this family, (4S,7R)-1-(2,4difluorophenyl)-N-(1,1-dimethylethyl)-4,5,6,7-tetrahydro-4,7-methano-1Hindazole-3-carboxamide (51, GRC-10693) has >4700-fold ­ functional ­ selectivity for CB2 over CB1, making it one of the most selective CB2 agonists reported in development (­Muthuppalanippan et al., 2006).

Quinolones and Naphthyridines The CB2 receptor agonists also include 4-oxo-1,8naphthyridine-3-carboxamide and 4-oxo-1,4dihydroquinoline-3-carboxamide derivatives, endowed with high affinity and selectivity toward CB2 (Manera et al., 2007). Some of these analogs were demonstrated to act as agonists or inverse agonists in functional activity assays, depending on the nature of the substituents on the different positions of the heterocyclic scaffold. In the series of 8-substituted quinolones, the best results in terms of both CB2 affinity and selectivity were obtained with the introduction of electron-donating groups, such as methyl or methoxy group (compound 52, Ki human CB1 > 10,000, human CB2 0.6 nM). In contrast, the double substitution at positions 6 and 8 of the quinolone nucleus was not favorable. The quinolone derivative 53 is reported to be potent and selective for CB2 receptor

(Ki human CB1  > 10,000 nM, human CB2 0.6 nM), and possessed antinociceptive effects (Pasquini et al., 2011).

Pyrazolopyridines Recently, a related series of heteroaryl-4-oxopyridine derivatives has been described (Aghazadeh Tabrizi et al., 2012). In this family, the core structure of molecules defines the type of activity to be seen, while the substituents around the core structure can be used to modulate the potency of that activity. Compound 54 was among the most potent analogs (Ki values of 4.6 µM for human CB1 and 11.4 nM for human CB2 receptor). This compound was found to act as partial agonists while the isoxazolopyridine parent compound behaved as full agonist. Further efforts led to discovery of 7-oxopyrazolo[1,5-a] pyrimidine-6-carboxamides, structural isomers of the previously reported pyrazolo[3,4-b]pyridines. The novel series exemplified by structure such as compound 55 shows stimulatory effects on forskolin-induced cAMP production acting as inverse agonists (Aghazadeh ­Tabrizi et al., 2013).

TRICYCLIC CORES Chemical structures are shown in Fig. 62.7; biological data are shown in Table 62.2. Rigidifying the CB1 antagonist N-(piperidin-1-yl)5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1Hpyrazole-3-carboxamide (SR141716A) resulted in 1,4-dihydroindeno[1,2-c]pyrazol-based CB2 agonist ligands. Compound 56 exerted affinity and selectivity for CB2 receptor (mouse KiCB2 0.03 nM) (Murineddu et al., 2006). Tricyclic benzoquinolizinone compound 57 (Sch35966) was found to be a potent CB2 agonist in four different species; it binds with low nanomolar potency to CB2 in both primates and rodents. It has >450-fold selectivity for CB2 over central cannabinoid receptor CB1 in primates (humans and cynomolgus monkeys) and rodents (rats and mice). Sch35966 is an agonist as it effectively inhibited forskolin-stimulated cAMP synthesis in CHO-human CB2 cells (Gonsiorek et al., 2007). Several molecules, based on a 1,2,3,4-tetrahydropyrrolo [3,4-b]indole moiety, are described as cannabinoid agonists. These ligands demonstrated good binding affinities and potencies toward the CB2 receptor, showing moderate to good selectivity over the CB1 receptor. SAR studies showed that the sulfonamide moiety produced higher CB2 binding affinities (compound 58) than the corresponding amide or N-alkyl analogs, and the piperidine nucleus enhanced CB2 binding affinity. C ­ ompound 58 is described as a full agonist (EC50 human CB2 17.6 nM), and possessed the best binding selectivity in the series (>350-fold over CB1) (Page et al., 2007).

V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids



REFERENCES

603

FIGURE 62.7  Chemical structures of tricyclic cores.

Very recently, 7-oxo-[1,4]oxazino[2,3,4-ij]quinoline6-carboxamide chemotype is reported as a novel cannabinoid ligand possessing high CB2 receptor affinity. Structural modifications led to the identification of several compounds as potent and selective cannabinoid receptor agonists (59, Ki human CB2 0.81, human CB1 310 nM). The effect of a chiral center on the biological activity was also investigated, and it was found that the R-enantiomers exhibited greater affinity at the CB2 receptor than the S-enantiomers. The novel series behaved as full agonists, exhibiting functional activity at the human CB2 receptor (Baraldi et al., 2012). Oxazinoqiunolone 60 (MT178) produced a robust analgesia in different pain models via CB2 receptors, providing an interesting approach to analgesic therapy in inflammatory and chronic pain, without CB1-mediated central side effects (Vincenzi et al., 2013).

MINI-DICTIONARY Affinity  The affinity of a drug for a receptor is the power with which it binds to the receptor. Enantiomers  Enantiomers are optical isomers that have the same chemical and physical properties, except for their capacity to rotate the plane of polarized light by identical amounts in opposing sense. Ligand  Ligand is a molecule capable to make a complex with a biomolecule to induce a biological function. Structure–activity relationship (SAR)  The relationship between the chemical structure of a molecule and its biological activity is SAR, and allows it to modify the activity of a compound by changing its chemical structure. The half maximal effective concentration (EC50)  The EC50 is a measure of the potency of a substance required to produce 50% of the maximal effect.

The inhibitory or affinity constant (Ki) The Ki expresses the binding affinity, and is the direct indicator of affinity between a ligand and a receptor; it could be defined as the concentration of competing ligand that is mandatory to decrease the maximal effect of the reactions by half.

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V.  Pharmacology and cellular activities of cannabinoids and endocannabinoids