Discovery of 1-[4-(3-chlorophenylamino)-1-methyl-1H-pyrrolo[3,2-c]pyridin- 7-yl]-1-morpholin-4-ylmethanone (GSK554418A), a brain penetrant 5-azaindole CB2 agonist for the treatment of chronic pain

Article abstract of DOI:10.1021/jm9009857

We report the synthesis and SAR of a series of novel azaindole CB ₂ agonists. 6-Azaindole 18 showed activity in an acute pain model but was inactive in a chronic model. 18 is a Pgp substrate with low brain penetration. The template was redesign

Full text of DOI:10.1021/jm9009857

J. Med. Chem. 2009, 52, 5785–5788 5785
DOI: 10.1021/jm9009857
selective CB2 agonist, has been reported to have activity in
models of inflammatory10a and neuropathic pain.10b
We recently disclosed the discovery and pharmacological
characterization of our lead CB2 agonist 1 (GW842166X,
Figure 1),11 the first molecule of this class that has been tested
in human pain studies. Herein we describe a further series of
CB2 agonists that are more potent and have higher aqueous
solubility than 1.
Discovery of 1-[4-(3-Chlorophenylamino)-
1-methyl-1H-pyrrolo[3,2-c]pyridin-7-yl]-
1-morpholin-4-ylmethanone (GSK554418A),
a Brain Penetrant 5-Azaindole CB2 Agonist
for the Treatment of Chronic Pain
Recently a number of new chemotypes have been repor-
ted as selective CB2 agonists. In addition to 1, Abbott has
Gerard M. P. Giblin,* Andrew Billinton, Michael Briggs,
Andrew J. Brown, Iain P. Chessell, Nick M. Clayton,
Andrew J. Eatherton, Paul Goldsmith, Carl Haslam,
Matthew R. Johnson, William L. Mitchell, Alan Naylor,
Alcide Perboni,† Brian P. Slingsby, and Alex W. Wilson
reported indole 2 (A-796260),12 the Taisho group disclosed13
a
series of imino heterocycles exemplified by 3, and the Evotec
arylsulfonamide analogue 4 was recently reported.14
Earlier work highlighted the possibility of replacing the
pyrimidine ring of 1 by a pyridine,15 but this series of com-
pounds had unacceptably high metabolic clearance. We se-
lected 6-azaindole as an alternative template based on a
reported16 moderately basic pKa and designed a novel ring
synthesis to incorporate the substitution pattern previously
found to be required for CB2 activity (Figure 2). We hypothe-
sized that, in addition to the arylanilino and amide side chains
of 1, a methyl substituent would be required at the 3 position
of the azaindole to induce a twist into the amide side chain, a
role that we believe is performed by the trifluoromethyl group
in 1 and that is required for the bioactive conformation.17
We designed a route to the 6-azaindole template utilizing a
Bartoli reaction18 (Scheme 1). Iodination of commercially
available 2-hydroxy-3-nitropyridine 5 followed by chlorina-
tion gave 7. The Bartolicyclization wasperformed by addinga
solution of the iodonitropyridine to a solution of the Grignard
reagent 1-propenylmagnesium bromide (3.5 equiv) at 0 °C. By
use of these carefully optimized conditions, the Bartoli cycli-
zation routinely gave isolated yields of the 6-azaindole 8 of
35%. Compound 8 was protected with a BOC group, and 9
was then treated with isopropylmagnesium chloride at-40 °C
and quenched with CO2 gas to yield the key intermediate 10 in
77% yield. Standardamidecouplingfollowed displacement of
the chloride by the aniline using acid catalysis, and micro-
wave irradiation at 180 °C (with concomitant removal of the
BOC protecting group) furnished the desired amide pro-
ducts in good yield. By use of this synthetic route, a range of
6-azaindole analogues weresynthesized (Table1) and assessed
in recombinant assays in yeast expressing human CB1 and
CB2 receptor.19
We initially evaluated the CB2 SAR for the amide in
the presence of a 3-chloroanilino substituent, a group that
resulted in high CB2 efficacy in the pyrimidine series.11 The
4-tetrahydropyranylmethylamino analogue 11 was a full ago-
nist at the CB2 receptor with potency of 265 nM. Removing
the methylene, analogue 12, substantially reduced activity,
and replacing the tetrahydropyranyl by a 4-fluorophenyl
group, analogue 13, led to reduced efficacy. Analogues con-
taining a four- to seven-membered cyclic amine 14-17 gave
varying activity with the six-membered piperidine analogue16
showing optimal potency. Similarly the morpholino analogue
18 had high CB2 potency and we adopted the morpholino
amide as the basis for exploring the 7-amino substituent.
Attempts to replace the phenyl ring with nonaromatic amines
resulted in analogues (20 and 21) of low potency. A small
range of aryl substituents (analogues 22-26) were evaluated
Neurosciences CEDD, GlaxoSmithKline, New Frontiers Science
Park, Third Avenue, Harlow, Essex, CM19 5AW, U.K. †Synthetic
Chemistry, GlaxoSmithKline, Verona, Italy.
Received July 3, 2009
Abstract: We report the synthesis and SAR of a series of novel
azaindole CB2 agonists. 6-Azaindole 18 showed activity in an acute
pain model but was inactive in a chronic model. 18 is a Pgp substrate
with low brain penetration. The template was redesigned, and the
resulting 5-azaindole 36 was a potent CB2 agonist with high CNS
penetration. This compound was efficacious in the acute model and
the chronic joint pain model.
Δ9-Tetrahydrocannabinol, the principalpharmacologically
active component of Cannabis sativa,1 acts on G-protein-
coupled receptors known as CB1,2 CB2,3 and GPR55.4 The
CB1 receptor is expressed in the central nervous system
(CNSa) and the periphery, whereas the CB2 receptor is
expressed principally on peripheral immune cells. Recently
expression of CB2 mRNA and protein in the CNS has been
confirmed5 leading to the association of CB2 with a range of
neurological diseases. It is clear that agonism of CB1 receptors
or CB2 receptors can produce analgesic effects in animal
models of pain,6 and consequently targeting one, or both,7
ofthesereceptors is an attractive strategyfor drugdiscoveryin
pain. Our approach has been to focus on selective CB2
agonists to minimize the CNS side effect and abuse potential
associated with agonism of the CB1 receptor.
Several reports confirm that selective agonism of the
CB2 receptor is sufficient for analgesia. For example, 1-(2,3-
dichlorobenzoyl)-5-methoxy-2-methyl-(2-(morpholin-4-yl)-
ethyl)-1H-indole (L-768,242 or GW405833),8a a moderately
selective partial CB2 receptor agonist, is active in the carra-
geenan model of inflammatory hyperalgesia and its anti-
hyperalgesic activity is blocked by a selective CB2 antagonist
but not by a selective CB1 antagonist.8b Furthermore (R)-(þ)-
1-[(N-methyl-2-piperidinyl)methyl]-3-(2-iodo-5-nitrobenzoyl)-
1H-indole (AM1241),9 a structurally distinct and moderately
*To whom correspondence should be addressed. Phone: þ44 1279
647539. Fax: þ44 1279 623210. E-mail: ged.m.giblin@gsk.com.
aAbbreviations: CFA, complete Freund’s adjuvant; Pgp, P-glycopro-
tein; CNS, central nervous system, BOC, tert-butyloxycarbonyl; SAR,
structure-activity relationship; THP, tetrahydropyranyl; CYP450,
cytochrome P450; cAMP, cyclic adenosine monophosphate; COX2,
cyclooxygenase-2.
r
2009 American Chemical Society
Published on Web 09/10/2009
pubs.acs.org/jmc

5786 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 19
Giblin et al.
We found that 18 has very low CNS penetration (mouse
brain/blood ratio of <0.05:1) possibly as a consequence of it
being a substrate for the P-glycoprotein (Pgp) efflux pump23
(Papp B-A/A-B efflux ratio 74). There is evidence that chronic
inflammatory pain has a significant component of central
sensitization,24 but there is no direct evidence of CB2 involve-
ment in central nociceptive pathways. However, we hypothe-
sized thatthe inactivityof 18 in the jointpainmodelwas due to
its low CNS penetration and we sought to identify a molecule
with reduced Pgp liability and improved CNS penetration to
evaluate this hypothesis.
Figure 1. Selective CB2 agonists.
We proposed that the Pgp interaction of 18 is associated
with one or more H-bond donors25 and consequently syn-
thesized the N1-methylated analogue of 18, compound27, but
found that its CB2 activity was significantly reduced. As an
alternative strategy, we envisaged that the isomeric 5-azain-
dole analogue, 36 would preserve the beneficial CB2 potency
and improved solubility of 18. Furthermore, by removal of an
NH bond, Pgp affinity may be reduced thus leading to greater
CNS penetration.
Figure 2. 6-Azaindole target.
Scheme 1a
A synthetic route based on the Bartoli reaction was devel-
oped,26 but acid 35 (Scheme 2) was isolated in very low overall
yield. Consequently an alternative synthesis was designed
based on a pyridine ring synthesis from a pyrrole enamine
precursor (Scheme 2). Readily available pyrrole 2827 was
esterified, then formylated with methyl formate to give enol
30. This was converted to the corresponding enamine 31, then
cyclized under basic catalysis to give the pyrrolopyridinone
32 which on treatment with phosphorus oxychloride gave the
4-choro-5-azaindole intermediate 33. Displacement of the
chloride with 3-chloroaniline gave ester 34 followed by sapo-
nification to furnish acid 35, the intermediate isolated in poor
yield from the Bartoli route. Finally the 5-azaindole target
36 was isolated using standard amide coupling.
a Conditions and reagents: (i) AcOH, H2O, H2SO4, H5IO6, I2, 90°C,
94% (ii) PhOP(O)Cl2, 180°C, 86%, (iii) 1-propenylmagnesium bromide,
0°C, tetrahydrofuran, 35% (iv) NaH, (BOC)2O, 82% (v) PrMgCl,
CO2, -40 °C, 81% (vi) HNR1R2, N-(3-dimethylaminopropyl)-3-ethyl-
carbodiimide, hydrochloride, 1-hydroxybenzotriazole, N-ethylmorpho-
line, dimethylformamide; (vii) ArNH2, MeSO3H, 1,4-dioxane, micro-
wave, 180 °C.
i
but showed no improvement in potency. All analogues
had >100-fold selectivity for CB2 relative to CB1 potency;
furthermore, many analogues were inactive at CB1 at
30 μM. From this series analogue 18 was selected for further
evaluation.
6-Azaindole 18 has low potency (EC50 = 10 000 nM,
efficacy 74%) at the human CB1 receptor with a selectivity
ratio of approximately a 1000-fold relative to its CB2 activity.
It has a low intrinsic clearance in both human (1.6 (mL/min)/g)
and rat (1.4 (mL/min)/g) liver microsomal clearance assays.20
It has an acceptable CYP450 inhibition profile (IC50: 1A2, 28
μM; 2C9, 4.7 μM; 2C19, 29 μM; 2D6, 14 μM; 3A4, 48 μM)
and, encouragingly, shows modest but improved solubility of
160 μg/mL in a simulated intestinal fluid assay (pH 5.0)
relative to 1 (solubility 10 μg/mL), reflecting its moderately
basic pKa of 5.8. In the acute complete Freund’s adjuvant
(CFA) model of inflammatory pain,21 18 is a highly potent
antihyperalgesic agent with ED50 of 0.12 mg/kg when dosed
orally. However, in a rat joint model of chronic pain that is
sensitive to COX2 inhibitors,22 18 had no significant analgesic
effect whendosedorally at1, 3, or 10mg/kg b.i.d. for5 days;in
contrast, rofecoxib gave full reversal of hyperalgesia at an oral
dose of 5 mg/kg b.i.d.
5-Azaindole 36 (GSK554418A) is a highly potent CB2
agonist with over 1200-fold selectivity for the human CB1
receptor. Furthermore, 36is a potentfullagonist atthe human
CB2 receptor expressed in Chinese hamster ovary cells using a
forskolin induced cAMP readout19b (EC50 = 8nM, efficacy
100%). It has high selectivity against a panel of 50 receptors,
ion channels, and enzymes28 and has low intrinsic clearance in
humanand ratliver microsomes (<0.6and<0.8 (mL/min)/g,
respectively) and a good CYP450 inhibition profile (IC50:
1A2, >100 μM; 2C9, 42 μM; 2C19, 41 μM; 2D6, 21 μM; 3A4,
24 μM). 5-Azaindole 36 has a pKa of 6.1 and has solubility
of 0.59 mg/mL in the simulated intestinal fluid preparation,
a 3-fold increase over the 6-azaindole 18. In a rat in vivo
pharmacokinetic study oral bioavailability was 82% and
blood levels of 36 were 1.1 μM, 0.8 h after an oral dose of

Letter
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 19 5787
Table 1. 6-Azaindole SARa
compd
X
Y
CB2 EC50 (nM)
CB2 E (%)
CB1 EC50 (nM)
CB2 E (%)
1
63
265
1705
145
504
153
14
95
95
79
58
66
68
73
76
82
78
84
59
72
83
82
73
86
>30000
>30000
>30000
>30000
>30000
>30000
4000
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
3-Cl-PhNH
3-Cl-PhNH
NHCH2(4-THP)
NH(4-THP)
3-Cl-PhNH
3-Cl-PhNH
NHCH2(4-fluorophenyl)
N-azetinyl
3-Cl-PhNH
3-Cl-PhNH
N-pyrrolidinyl
N-piperidinyl
N-azepinyl
32
3-Cl-PhNH
3-Cl-PhNH
3-Br-PhNH
107
11
>30000
10000
6300
N-morpholino
N-morpholino
N-morpholino
N-morpholino
N-morpholino
N-morpholino
N-morpholino
N-morpholino
N-morpholino
74
43
10
4-(THP)amino
3-pentylamino
2-MeO-PhNH
3-MeO-PhNH
2-Cl-PhNH
3-iPrO-PhNH
3-CF3-methyl PhNH
3302
4667
349
48
>30000
>30000
>10000
25100
69
56
123
188
15
15800
>30000
>30000
a THP = 4-tetrahydropyran. CB1 and CB2 assay data are the mean of at least three determinations. Assay reproducibility is monitored by the use of a
cannabinoid control agonist HU210 (CB1 EC50 = 40 ( 5 nM; CB2 EC50 = 3.0 ( 0.5 nM). Efficacy (E) is expressed as a percentage relative to the efficacy
of HU210.
Scheme 2a
a (i) p-Toluenesulphonic acid, MeOH, 78%; (ii) NaH, MeOCHO,
Figure 3. 5-Azaindole 36 is active in the chronic joint pain model
and at a dose of 10 mg/kg b.i.d. shows efficacy equivalent to that of
tetrahydrofuran, 94%; (iii) NH4OAc, MeOH 71%; (iv) NaOtBu, N,N-
dimethylformamide, 52%; (v) POCl3, 83%; (vi) 3-chloroaniline,
MeSO3H, 1,4-dioxane, 180°C, microwave, 43%; (vii) NaOH, MeOH,
84%; (viii) morpholine, N-(3-dimethylaminopropyl)-3-ethylcarbo-
diimide, hydrochloride, 1-hydroxybenzotriazole, N-ethylmorpholine,
dimethylformamide 30%.
rofecoxib.
achieve a target dose of 0.4 mg/kg/h). When dosed twice daily
for 5 days in the chronic joint pain model, 5-azaindole 36
showed full reversal of hypersensitivity at an oral dose of
10 mg/kg (blood concentration 6.1 μM) with efficacy equiva-
lent to that of an oral dose of rofecoxib (Figure 3) and a mini-
mal effective dose of 3 mg/kg (blood concentration 1.8 μM).
Finally, potential CB1 mediated effects were assessed in a
rat in vivo study.30 5-Azaindole 36 showed no significant
hypothermia after an oral dose of 130 mg/kg at 1 h, 3 h, and
5 h time points post dose (blood exposures in this study were,
respectively, 8.6, 10.0, and12.3μM, and brain/bloodratiowas
maintained at 1.1:1 throughout), demonstrating nosignificant
CB1 mediated effects at supraefficacious concentrations and a
significant therapeutic index relative to side effects.
2 mg/kg, suggesting rapid absorption of the drug from the
gastrointestinal tract. In the CFA model, 36 had unprece-
dented high potency for a CB2 agonist with an oral ED50 of
0.02 mg/kg and showed full reversal of hyperalgesia at a dose
of 0.1 mg/kg (blood concentration 97 nM) 1 h post dose. To
investigate the mechanism of action, 36 was dosed at 1 mg/kg
alone or in combination with the selective CB2 antagonist
{6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl}-
[4-(methyloxy)phenyl]methanone (AM630)29 (15 mg/kg i.p.).
Whereas 36 alone showed full reversal of hypersensitivity,
thetherapeuticeffectof36was completely reversed by the CB2
antagonist, confirming the mechanism of action to be CB2
receptor mediated.
In summary, we have discovered novel CB2 agonists31 with
high in vitro and in vivo potency and sufficient solubility to
achieve good oral absorption. 5-Azaindole 36 has high potency
in the CFA model of inflammatory pain and shows efficacy
equivalent to that of a COX2 inhibitor in the chronic rat joint
pain model probably as a result of good CNS penetration.
In contrast to 18, 5-azaindole 36 has a low affinity for Pgp
(Papp B-A/A-B efflux ratio 2.9) and, gratifyingly, has high
CNS penetration (rat brain/blood ratio of 1.04:1 after intra-
venous administration over 12 h via a femoral vein cannula, to

5788 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 19
Giblin et al.
5-Azaindole 36 is an excellent compound for further evalua-
tion of the utility of CB2 agonists for the treatment of pain. The
activity profile of this compound also provides compelling
evidence of a role for centrally located CB2 receptors in the
modulation of chronic inflammatory pain.
M.; Iida, I.; Tomishima, Y.; Toda, Y.; Saito, S. N-Alkylidenearylcarbox-
amides as new potent and selective CB2 cannabinoid receptor agonists
with good oral bioavailability. Bioorg. Med. Chem. Lett. 2007, 17,
6299–6304.
(14) Ermann, M.; Riether, D.; Walker, E. R.; Mushi, I. F.; Jenkins,
J. E.; Noya-Marino, B.; Brewer, M. L.; Taylor, M. G.; Amouzegh,
P.; East, S. P.; Dymock, B. W.; Gemkow, M. J.; Kahrs, A. F.;
Ebneth, A.; Loebbe, S.; O’Shea, K.; Shih, D.-T.; Thomson, D.
Arylsulfonamide CB2 receptor agonists: SAR and optimization of
CB2 selectivity. Bioorg. Med. Chem. Lett. 2008, 18, 1725–1729.
(15) Mitchell, W. L.; Giblin, G. M. P.; Naylor, A.; Eatherton, A. J.;
Slingsby, B. P.; Rawlings, A. D.; Jandu, K. S.; Haslam, C. P.;
Brown, A. J.; Goldsmith, P.; Clayton, N. M.; Wilson, A. W.;
Chessell, I. P.; Green, R. H.; Whittington, A. R.; Wall, I. D.
Pyridine-3-carboxamides as novel CB2 agonists for analgesia.
Bioorg. Med. Chem. Lett. 2009, 19, 259–263.
Supporting Information Available: Assay methods, synthetic
procedures, and compound characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Baker, D.; Pryce, G.; Giovannoni, G.; Thompson, A. J. The
therapeutic potential of Cannabis. Lancet Neurol. 2003, 2, 291–298.
(2) Matsuda, L. A.; Lolait, S. J.; Brownstein, M. J.; Yound, A. C.;
Bonner, T. I. Structure of a cannabinoid receptor and functional
expressionof the cloned cDNA. Nature (London) 1990, 346, 561–564.
(3) Munro, S.; Thomas, K. L.; Abu-Shaar, M. Molecular character-
ization of a peripheral receptor for cannabinoids. Nature (London)
1993, 365, 61–65.
(16) Unsubstituted 6-azaindole is reported to have pKa 8. See: Twine,
S. M.; Murphy, L.; Phillips, R. S.; Callis, P.; Cash, M. T.; Szabo,
A. G. The photophysical properties of 6-azaindole. J. Phys. Chem.
B 2003, 107, 637–645.
(17) Des-trifluoromethyl analogues of GW842166X have no significant
(4) Brown, A. J. Novel cannabinoid receptors. Br. J. Pharmacol. 2007,
152, 567–575.
activity at the CB2 receptor at 10 μM.
(18) Dalpozzo, R.; Bartoli, G. Bartoli indole synthesis. Curr. Org.
Chem. 2005, 9, 163–178.
(19) (a) Dowell, S. J.; Brown, A. J. Yeast assays for G-protein-coupled
receptors. Recept. Channels 2002, 8, 343–352.(b) Procedures for the
syntheses of compounds and details of the yeast CB2 reporter and
Chinese hamster ovary cAMP assays used to assess their potency at the
CB2 receptor are described in: Eatherton, A. J.; Giblin, G. M. P.;
Mitchell, W. L.; Naylor, A.; Page, L. W.; Swarbrick, M.; Sweeting, J. A.
PCT Int. Appl. WO 2007017237, 2007.
(5) Cabral, G. A.; Marciano-Cabral, F. Cannabinoid receptors in
microglia of the central nervous system: immune functional rele-
vance. J. Leukocyte Biol. 2005, 78, 1192–1197. Van Sickle, M. D.;
Duncan, M.; Kingsley, P. J.; Mouihate, A.; Urbani, P.; Mackie, K.;
Stella, N.; Makriyannis, A.; Piomelli, D.; Davison, J. S.; Marnett, L. J.;
Marzo, V. D.; Pittman, Q. J.; Patel, K. D.; Sharkey, K. A. Identification
and functional characterization of brainstem cannabinoid CB2 recep-
tors. Science 2005, 310, 329–332.
(6) Pertwee, R. G. Cannabinoid receptors and pain. Prog. Neurobiol.
(20) Clarke, S. E.; Jeffrey, P. Utility of metabolic stability screening:
comparison of in vitro and in vivo clearance. Xenobiotica 2001, 31,
591–598.
2001, 63, 569–611.
(7) For a recent example of dual CB1/CB2 agonists see the following:
Dziadulewicz, E. K.; Bevan, S. J.; Brain, C. T.; Coote, P. R.;
Culshaw, A. J.; Davis, A. J.; Edwards, L. J.; Fisher, A. J.; Fox, A.
J.; Gentry, C.; Groarke, A.; Hart, T. W.; Huber, W.; James, I. F.;
Kesingland, A.; La Vecchia, L.; Loong, Y.; Lyothier, I.; McNair,
K.; O’Farrell, C.; Peacock, M.; Portmann, R.; Schopfer, U.;
Yaqoob, M.; Zadrobilek, J. Naphthalen-1-yl-(4-pentyloxynaph-
thalen-1-yl)methanone: a potent, orally bioavailable human CB1/
CB2 dual agonist with antihyperalgesic properties and restricted
central nervous system penetration. J. Med. Chem. 2007, 50,
3851–3856.
(8) (a) Gallant, M.; Dufresne, C.; Gareau, Y.; Guay, D.; Leblanc, Y.;
Prasit, P.; Rochette, C.; Sawyer, N.; Slipetz, D. M. New class of
potent ligands for the human peripheral cannabinoid receptor.
Bioorg. Med. Chem. Lett. 1996, 6, 2263–2268. (b) Clayton, N.;
Marshall, F. H.; Bountra, C.; O'Shaughnessy, C. T. CB1 and CB2
cannabinoid receptors are implicated in inflammatory pain. Pain
2002, 96, 253–260.
(9) Malan, T. P., Jr.; Ibrahim, M. M.; Deng, H.; Liu, Q.; Mata, H. P.;
Vanderah, T.; Porreca, F.; Makriyannis, A. CB2 cannabinoid recep-
tor-mediated peripheral antinociception. Pain 2001, 93, 239–245.
(10) (a) Quartilho, A.; Mata, H. P.; Ibrahim, M. M.; Vanderah, T. W.;
Porreca, F.; Makriyannis, A.; Malan, T. P., Jr. Inflammatory
hyperalgesia by activation of peripheral CB2 cannabinoid recep-
tors. Anesthesiology 2003, 99, 955–960. (b) Ibrahim, M. M.; Deng, H.;
Zvonok, A.; Cockayne, D. A.; Kwan, J.; Mata, H. P.; Vanderah, T. W.;
Lai, J.; Porreca, F.; Makriyannis, A.; Malan, T. P., Jr. Activation of CB2
cannabinoid receptors by AM1241 inhibits experimental neuropathic
pain: pain inhibition by receptors not present in the CNS. Proc. Natl.
Acad. Sci.U.S.A. 2003, 100, 10529–10533.
(11) Giblin, G. M. P.; O’Shaughnessy, C. T.; Naylor, A.; Mitchell, W.
L.; Eatherton, A. J.; Slingsby, B. P.; Rawlings, D. A.; Goldsmith,
P.; Brown, A. J.; Haslam, C. P.; Clayton, N. M.; Wilson, A. W.;
Chessell, I. P.; Wittington, A. R.; Green, R. Discovery of 2-[(2,4-
dichlorophenyl)amino]-N-[(tetrahydro-2H-pyran-4-yl)methyl]-
4-(trifluoromethyl)-5-pyrimidinecarboxamide, a selective CB2 re-
ceptor agonist for the treatment of inflammatory pain. J. Med.
Chem. 2007, 50, 2597–2600.
(21) Iadarola, M. J.; Douglass, J.; Civelli, O.; Naranjo, J. R. Differential
activation of spinal cord dynorphin and enkephalin neurons during
hyperalgesia: evidence using cDNA hybridization. Brain Res. 1988,
455, 205–212. Hay, C. H.; Trevethick, M. A.; Wheeldon, A.; Browers,
J. S.; De Belleroche, J. S. The potential role of spinal cord cycloox-
ygenase-2 in the development of Freund's complete adjuvant-induced
changes in hyperalgesia and allodynia. Neuroscience 1997, 78, 843–
850. See Supporting Information for protocol .
(22) Wilson, A. W.; Medhurst, S. J.; Dixon, C. I.; Bontoft, N. C.;
Winyard, L. A.; Brackenborough, K. T.; De Alba, J.; Clarke, C. J.;
Gunthorpe, M. J.; Hicks, G. A.; Bountra, C.; McQueen, D. S.;
Chessell, I. P. An animal model of chronic inflammatory pain:
pharmacological and temporal differentiation from acute models.
Eur. J. Pain 2006, 10, 537–549.
(23) Polli, J. W.; Wring, S. A.; Humphreys, J. E.; Huang, L.; Morgan,
J. B.; Webster, L. O.; Serabjit-Singh, C. S. Rational use of in vitro
P-glycoprotein assays in drug discovery. J. Pharmacol. Exp.Ther.
2001, 299, 620–628.
(24) Aurilio, C.; Pota, V.; Pace, M. C.; Passavanti, M. B. e; Barbarisi,
M. Ionic channels and neuropathic pain: physiopathology and
applications. J. Cell. Physiol. 2008, 215, 8–14.
(25) Raub, T. J. P-Glycoprotein recognition of substrates and circum-
vention through rational drug design. Mol. Pharmaceutics 2006, 3,
3–25.
(26) For original Bartoli synthetic route see Supporting Information.
(27) Bottaccio, G.; Campolmi, S.; Carletti, V.; Marchi, M. 3-Carboxy-
1-methylpyrrole-2-acetic Acid and Its Alkali Metal Salts. Eur. Pat.
Appl. EP 105664, 1984.
(28) 36 has <10% activity (binding) across a CEREP panel of targets
at 1 μM with the exception of R2 (25%), κ opioid (11%), 5HT1A
(24%), 5HT2A (11%), 5HT5A (15%), NE transporter (15%).
Further details are in Supporting Information.
(29) Ross, R. A.; Brockie, H. C.; Stevenson, L. A.; Murphy, V. L.;
Templeton, F.; Makriyannis, A.; Pertwee, R. G. Agonist-inverse
agonist characterization at CB1 and CB2 cannabinoid receptors of
L759633, L759656 and AM630. Br. J. Pharmacol. 1999, 126, 665–
672.
(12) Frost, J. M.; Dart, M. J.; Tietje, K. R.; Garrison, T. R.; Grayson,
G. K.; Daza, A. V.; El-Kouhen, O. F.; Miller, L. N.; Li, L.; Yao,
B. B.; Hsieh, G. C.; Pai, M.; Zhu, C. Z.; Chandran, P.; Meyer,
M. D. Indol-3-yl-tetramethylcyclopropyl ketones: effects of indole
ring substitution on CB2 cannabinoid receptor activity. J. Med.
Chem. 2008, 51, 1904–1912.
(13) Ohta, H.; Ishizaka, T.; Tatsuzuki, M.; Yoshinaga, M.; Iida, I.;
Yamaguchi, T.; Tomishima, Y.; Futaki, N.; Toda, Y.; Saito, S.
Imine derivatives as new potent and selective CB2 cannabinoid
receptor agonists with an analgesic action. Bioorg. Med. Chem.
2008, 16, 1111–1124. Ohta, H.; Ishizaka, T.; Tatsuzuki, M.; Yoshinaga,
(30) For technical reasons, a CB1 or CB2 antagonist challenge is not
possible in the joint pain protocol. For a more detailed discussion
see Supporting Information. For hypothermia methods, see the
following: Fox, A.; Kesingland, A.; Gentry, C.; McNair, K.; Patel,
S.; Urban, L.; James, I. The role of central and peripheral canna-
binoid 1 receptors in the anti-hyperalgesic activity of cannabinoids
in a model of neuropathic pain. Pain 2001, 92, 91–100.
(31) Eatherton, A. J.; Giblin, G. M. P.; Johnson, M. R.; Mitchell, W. L.;
Perboni, A.; Slingsby, B. P. Preparation of Pyrrolopyridine Deri-
vatives as Cannabinoid Receptor Modulators. PCT Int. Appl. WO
2005121140, 2005.