New 1,2,3,4-tetrahydropyrrolo[3,4-b]indole derivatives as selective CB2 receptor agonists

Article abstract of DOI:10.1016/j.bmcl.2007.09.019

The preparation and evaluation of a novel class of CB2 agonists based on a 1,2,3,4-tetrahydropyrrolo[3,4-b]indole moiety are reported. They showed binding affinities up to 4.2 nM toward CB2 with sub-nanomolar EC₅₀ values. They also showed moderate to good (>350-fold) selectivity over the CB1 receptor.

Full text of DOI:10.1016/j.bmcl.2007.09.019

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 17 (2007) 6183–6187
New 1,2,3,4-tetrahydropyrrolo[3,4-b]indole derivatives
as selective CB2 receptor agonists
a,
a
a
a
b
*
´
Daniel Page, Hua Yang, William Brown, Christopher Walpole, Manon Fleurent,
b
b
b
´
Meredith Fyfe, Franc¸ois Gaudreault and Stephane St-Onge
aDepartment of Medicinal Chemistry, AstraZeneca R&D Montreal, St-Laurent, Que., Canada H4S 1Z9
bDepartment of Molecular Pharmacology, AstraZeneca R&D Montreal, 7171 Frederick-Banting, St-Laurent, Que., Canada H4S 1Z9
Received 31 July 2007; revised 4 September 2007; accepted 5 September 2007
Available online 8 September 2007
Abstract—The preparation and evaluation of a novel class of CB2 agonists based on a 1,2,3,4-tetrahydropyrrolo[3,4-b]indole moiety
are reported. They showed binding affinities up to 4.2 nM toward CB2 with sub-nanomolar EC50 values. They also showed mod-
erate to good (>350-fold) selectivity over the CB1 receptor.
Ó 2007 Elsevier Ltd. All rights reserved.
The endocannabinoid system has been known to medi-
ate several important biological processes including met-
abolic regulation,1,2 pain,3–5 cellular proliferation,6
cancer,7 obesity,8 and different other CNS functions.9
These effects are mainly regulated via two primary
receptors, namely CB1 and CB2. The pharmacology of
these two cannabinoid receptors has been extensively
reviewed10–14 over the past few years, putting a much
larger emphasis on the more studied CB1 receptor,
resulting in huge commercial interests in CB1 receptor
ligands. CB1 antagonists are now being evaluated for
the treatment of obesity, metabolic syndrome, drug
abuse, and smoking cessation,2 while CB1 agonists are
currently in clinical trials for the treatment of acute
and chronic pain.3–5 However, because the CB1 receptor
is mainly expressed in the central nervous system, sev-
eral psychoactive side effects15–17 such as cognitive dis-
function, motor incoordination, or sedation have often
been associated with CB1 ligands.
inflammatory pain models,18–20 which would confer
them some potential therapeutic utility in chronic
inflammatory diseases such as atherosclerosis. Beneficial
effects of CB2 receptor activation in models of neurode-
generative disorders, such as Huntington and Alzhei-
mer’s diseases, were also reported, leading to a
possible neuroprotective role for CB2 ligands.21–23
Other preclinical studies have highlighted the potential
of CB2 ligands in the treatment of cancer by eliciting
apoptosis of brain and immune system tumor cells.24,25
Another potential role of the CB2 receptors could
include bone regeneration, as selective CB2 agonists
were shown to decrease bone loss in animal studies.26,27
The particular attractive feature about CB2-selective
ligands resides in the distribution of this receptor. CB2
is mainly expressed in immune tissues located in the
periphery, with only low levels found in neurons of the
central nervous system, minimizing therefore the side
effects observed with the CB1-selective ligands.
On the other hand, the exact physiological roles of the
CB2 receptor still remain to be fully defined even though
several reported experimental and clinical data show
promising applications in different disease areas. Previ-
ous studies have demonstrated that CB2 selective ago-
nists are analgesics in different neuropathic and
Several classes of CB2 ligands originating from natural
and synthetic sources have been reported in the litera-
ture.28–34 Making use of the ever increasing amount of
information reported in the literature, along with several
hit compounds obtained from a previous screening cam-
paign, a cannabinoid program was instigated at our
company a few years ago. This program enabled us to
develop several classes of CB1/CB2 ligands, like CB2
selective inverse agonists based on a benzimidazole scaf-
fold.35 More recently a different scaffold, based upon a
1,2,3,4-tetrahydropyrrolo[3,4-b]indole moiety, showed
good selectivity over CB1 and demonstrated a much dif-
Keywords: Cannabinoids; 1,2,3,4-Tetrahydropyrrolo[3,4-b]indole; CB2
receptor; Agonists.
*
Corresponding author. Tel.: +1 514 832 3200; fax: +1 514 832
3232; e-mail: daniel.page@astrazeneca.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.09.019

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D. Page et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6183–6187
6184
ferent SAR than the one observed with the benzimid-
azole ligands. We report herein the synthesis and phar-
macological evaluation of this novel class of selective
CB2 agonists.
obtained from the cyclization of compound (6) with
cyclopentylamine under basic conditions. Deprotection
of the indole under basic conditions followed by either
N-alkylation, acylation, or sulfonylation afforded the fi-
nal compounds (11–20).
The synthesis of the 1,2,3,4-tetrahydropyrrolo[3,4-b]in-
dole core, outlined in Scheme 1, was adapted from a pre-
viously reported procedure36 in order to make necessary
modifications around it. Compound 8, an initial hit ob-
tained from our screening campaign, was used as the
model compound in this series. However, the highly
lipophilic nature of compound 8 made it difficult to be
used for biological tests. Therefore, modifications were
initially performed on the bottom part of the molecule
while keeping the left- and right-hand sides constant.
The synthesis started from the commercially available
2,3-dimethyl-1H-indole-5-carboxylic acid (1) that under-
went an amide coupling procedure, using the coupling
reagent N,N,N0,N0-tetramethyl-O-(7-azabenzotriazole-
1-yl)uranium hexafluorophosphate (HATU), followed
by N-sulfonylation to afford compound (4). This
intermediate was brominated under radical conditions
to give the corresponding bis-bromide product (6).
The 1,2,3,4-tetrahydropyrrolo[3,4-b]indole core was
Modifications on the left-hand side were done following
a similar synthetic route (Scheme 1), with the exception
that the starting acid (1) was initially esterified (com-
pound 3). After the core cyclization, the ester was
saponified under basic conditions to give the acid (10),
which underwent amide bond formation with different
amines. The N-phenylsulfonamide was then replaced
by an ethylsulfonamide group, which proved to enhance
the binding affinity and solubility properties, to give
products 21–27.
Finally, modifications on the right-hand side of the mol-
ecule (Scheme 2) made use of the previously prepared
bis-bromide intermediate (7), which was cyclized with
different amines under basic conditions (28–38). Sapon-
ification of the ester, followed by amide coupling with
4-methylpiperidine, afforded compounds 39–49. Once
again, the N-phenylsulfonamide was replaced with an
O
X
O
X
4 R = 4-Me-Piperidine, X = H
5 R = MeO, X = H
R
}
}
R
ii
iii
iv
N
6 R = 4-Me-Piperidine, X = Br
7 R = MeO, X = Br
N
H
O
S
O
Ph
1 R = OH
2 R = 4-Me-Pip
3 R = MeO
i
v
O
N
vi, vii
N
11 - 20
N
O
N
R
R
ix, vi, vii
N
O
S
O
O
Ph
N
R
8 R = 4-Me-Pip
9 R = MeO
21 - 27
viii
N
10 R = OH
O
S
O
Scheme 1. Reagents and conditions: (i) 4-methylpiperidine, HATU, DIPEA, DMF, rt; (ii) TMSCHN2, MeOH, 0 °C ! rt; (iii) benzenesulfonyl
chloride, NaH, DMF, 0 °C ! rt; (iv) NBS, benzoyl peroxide, CCl4, reflux; (v) cyclopentylamine, K2CO3, THF, reflux; (vi) 1.5 M KOH, 5:5:2 EtOH/
MeOH/H2O, 65 °C; (vii) RSO2Cl/R–X/RC(O)Cl, NaH, DMF, 0 °C ! rt; (viii) 1 M LiOH, dioxane, 65 °C; (ix) RNH2, HATU, DIPEA, DMF, rt.
O
O
R
O
R
Br
N
N
Br
N
O
R'
i
iii, iv
N
N
N
O
O
O
S
S
S
O
O
O
7
Ph
Ph
28 - 38 R' = MeO
39 - 49 R' = 4-Me-Pip
50-60
ii
Scheme 2. Reagents and conditions: (i) RNH2, K2CO3, THF, reflux; (ii) a—1 M LiOH, dioxane, reflux; b—4-methylpiperidine, HATU, DIPEA,
DMF, rt; (iii) 1.5 M KOH, 5:5:2 EtOH/MeOH/H2O, 65 °C; (iv) ethanesulfonyl chloride, NaH, DMF, 0 °C ! rt.

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D. Page et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6183–6187
6185
ethylsulfonamide group to give products 50–60. All final
compounds were purified by reversed-phase chromatog-
raphy with a water/acetonitrile gradient containing
0.05% TFA v/v.
(19) or aromatic substituents (8, 14) decreased the bind-
ing affinity among the sulfonamides, these bottom
chains being probably too bulky to fit properly in the
receptor-binding pocket.
The CB1/CB2 binding results are summarized in Tables
37
The different substitutions made on the left-hand side
of the molecule seem to have less influence on the
CB2 affinity (Table 2). The removal of the methyl
group (21) or the introduction of polar atoms/groups
at the position 4 of the piperidine ring (compounds
22–24) reduced the CB2 affinity. Open-chain alkyl
derivatives (25–27) also showed decreased CB2 affinity
when compared to compound (16), which might be due
to the entropy cost introduced by the additional rotat-
able bonds.
1–3. The mixed CB1/CB2 agonists WIN55212-2 and
D9-THC were tested as standards for comparison pur-
poses (Table 1). The results of Table 1 showed that
the sulfonamide derivatives (8, 15–19) had higher CB2
binding affinities than the corresponding alkyl (12–14)
or amide (20) derivatives. Some bulkiness can be toler-
ated at this site as the isopropylsulfonamide derivative
(17) showed a CB2 binding affinity of 4.2 nM with a
selectivity factor of 36 over CB1. Longer alkyl chains
Table 1. CB2 binding results of 1,2,3,4-tetrahydropyrrolo[3,4-b]indoles with bottom part modifications
O
N
N
N
R
a
hCB2 EC50 (nM)
Compound
R
hCB1 Ki (nM)
hCB2 Ki (nM)
hCB1/hCB2
WIN55,212-2
—
—
40.9 1.7
20.1 3.4
259 27
>8000
2.9 0.3
140 42
14.1
0.14
14.5
—
1.5 0.1 (100%)
13.5 3.0 (64%)
15.6 2.4 (ꢀ62%)
nd
D9-THC
8
11
PhenylSO2
H
Methyl
17.9 4.5
4724 182
84.7 15.1
23.3 3.2
106 26
12
13
1811 59
695 41
658 11
279 56
66.3 14.6
150 12
137 44
195 41
2331 181
21.4
29.8
6.2
nd
5.0 0.6 (92%)
nd
Allyl
Benzyl
14
15
MethylSO2
EthylSO2
IsopropylSO2
CyclopropylSO2
n-PropylSO2
Acetyl
7.9 1.6
7.0 0.2
4.2 0.5
9.4 2.5
22.3 3.0
88.1 17.3
35.3
9.4
0.6 0.04 (105%)
0.8 0.1 (110%)
0.5 0.1 (105%)
1.5 0.1 (101%)
6.7 1.0 (90%)
nd
16
17
35.7
14.6
8.7
18
19
20
26.5
nd = not determined.
a Emax are reported in brackets.
Table 2. CB2 binding results of 1,2,3,4-tetrahydropyrrolo[3,4-b]indoles with left-hand side modifications
O
N
R
N
O
S
O
a
hCB2 EC50 (nM)
Compound
R
hCB1 Ki (nM)
hCB2 Ki (nM)
hCB1/hCB2
16
21
22
23
24
25
26
27
4-Me-piperidine
Piperidine
4-F-piperidine
66.3 14.6
818 92
7.0 0.2
17.5 2.8
44.2 5.1
308 11
27.6 7.3
107 44
22.3 5.1
38.1 9.3
9.5
0.8 0.1 (110%)
1.7 0.3 (107%)
46.7
65.9
>25
77.2
67.5
34.9
38.7
2914 251
>8000
nd
nd
Morpholine
4-MeO-piperidine
Isopropylmethylamine
n-Butylmethylamine
Isoamylmethylamine
2131 264
7222 251
779 164
1473 184
nd
nd
4.0 0.6 (111%)
nd
nd = not determined.
a Emax are reported in brackets.

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D. Page et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6183–6187
6186
Table 3. CB2 binding results of 1,2,3,4-tetrahydropyrrolo[3,4-b]indoles with right-hand side modifications
O
R
N
N
N
O
S
O
Ra
hCB1 Ki (nM)
hCB2 Ki (nM)
hCB1/hCB2
b
hCB2 EC50 (nM)
Compound
16
50
51
52
53
54
55
56
57
58
59
60
Cyclopentyl
3-THF
4-THP
66.3 14.6
973 150
3021 505
6183 443
263 71
7.0 0.2
27.8 1.3
216 40
9.5
35
0.8 0.1 (110%)
nd
nd
14
2-THP
Allyl
17.6 2.8
6.4 0.4
8.9 0.7
351
41.1
47
1.7 0.3 (107%)
0.3 0.03 (104%)
0.9 0.1 (101%)
0.8 0.2 (104%)
2.7 0.3 (105%)
0.5 0.1
Isopropyl
n-Propyl
n-Butyl
418 21
604 49
11.1 1.1
16.8 1.7
6.3 1.2
54.4
20.3
27.9
0.4
341 47
176 32
–CH2-cyclopropyl
–CH2-cyclohexyl
–(CH2)2-cyclohexyl
Benzyl
339 73
2742 280
1273 132
935 149
1982 182
37.0 8.2
nd
nd
1.4
34.4
nd
nd = not determined.
a Abbreviations: THF = tetrahydrofuran; THP = tetrahydropyran.
b Emax are reported in brackets.
The modifications on the right-hand side of the molecule
yielded analogues with decent CB2 binding affinity (Ta-
ble 3). Polar atoms and their position in the ring of the
substituent had a great impact on the CB2 binding (50–
52). For example, compounds (50) and (52) showed
good CB2 affinity while compound (51) displayed poor
CB2 affinity. In addition, good binding affinities were
obtained with small alkyl substituents (53–57). How-
ever, the CB2 binding affinity was shown to decrease
with the length and/or bulkiness of the side chain (58–
59), demonstrating a restricted hydrophobic pocket in
the receptor. The addition of an aromatic substituent
(60) also resulted in a relatively lower CB2 binding affin-
ity (Ki = 37 nM).
The discrepancies observed between the binding and
functional assay data could be a result in the difference
of tissues used and the sensitivity of the assays, even
though such discrepancies have previously been re-
ported in the literature for other GPCR assays.38,39
To investigate the potential of the corresponding oxi-
dized class of compounds, bearing a 2,4-dihydropyrrol-
o[3,4-b]indole core, as CB2 ligands, compound (16) was
oxidized using DDQ (Scheme 3) to give the correspond-
ing 2,4-dihydropyrrolo[3,4-b]indole analogue (61). How-
ever, the CB2 binding affinity of this compound proved
to be about 23-fold lower than the corresponding
1,2,3,4-tetrahydropyrrolo[3,4-b]indole analogue (16),
with a CB2 Ki value of 146 61 nM.
Most of the compounds demonstrated moderate to
good CB1/CB2 selectivity, with compound (52) having
the best selectivity ratio of >350. Only the compounds
with a CB2 Ki < 25 nM were tested in a CB2 GTPc[35S]
assay. They all showed low to sub-nanomolar EC50 val-
ues with full agonist properties (relative to WIN55,212-
2) with the exception of compound (8), which proved to
be an inverse agonist. This is probably due to the pres-
ence of the aromatic sulfonamide bottom chain which
constraints the molecule in a different mode of binding.
In conclusion, these molecules, based on a 1,2,3,4-tetra-
hydropyrrolo[3,4-b]indole moiety, represent new scaf-
folds in the development of cannabinoid agonists.
These ligands demonstrated good binding affinities and
potencies toward the CB2 receptor, showing moderate
to good selectivity over the CB1 receptor. The biological
evaluation of this new class of ligands is currently under-
way in our laboratories in order to look at their poten-
tial biological application.
O
O
N
N
i
N
N
N
N
O
O
S
S
O
O
16
61
Scheme 3. Reagents and condition: (i) DDQ, toluene, rt.

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D. Page et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6183–6187
6187
N. M.; Wilson, A. W.; Chessell, I. P.; Wittington, A. R.;
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Supplementary data
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Selected syntheses and compound characterization
along with the description of the biological tests are in-
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can be found, in the online version, at doi:10.1016/
j.bmcl.2007.09.019.
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