Identification of 2,3-diaryl-pyrazolo[1,5-b]pyridazines as potent and selective cyclooxygenase-2 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2004.07.089

GW406381 (8), currently undergoing clinical evaluation for the treatment of inflammatory pain is a member of a novel series of 2,3-diaryl-pyrazolo[1,5-b] pyridazine based cyclooxygenase-2 (COX-2) inhibitors, which have been shown to be highly potent and selective. Several examples of the series, in addition to possessing favourable pharmacokinetic profiles and analgesic activity in vivo, have also demonstrated relatively high brain penetration in the rat compared with the clinically available compounds, which may ultimately prove beneficial in the treatment of pain.

Full text of DOI:10.1016/j.bmcl.2004.07.089

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5445–5448
Identification of 2,3-diaryl-pyrazolo[1,5-b]pyridazines as potent
and selective cyclooxygenase-2 inhibitors
Paul Beswick, Sharon Bingham, Chas Bountra, Terry Brown, Kerry Browning,
Ian Campbell, Iain Chessell, Nick Clayton, Sue Collins, John Corfield, Stephen Guntrip,
Claudine Haslam, Paul Lambeth, Fiona Lucas, Neil Mathews, Graham Murkit,
Alan Naylor,* Neil Pegg, Elizabeth Pickup, Hazel Player, Helen Price, Alexander Stevens,
Sharon Stratton and Joanne Wiseman
Neurology and Gastrointestinal Diseases Centre of Excellence for Drug Discovery, GlaxoSmithKline,
New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
Received 2 April 2004; revised 26 July 2004; accepted 26 July 2004
Available online 17 September 2004
Abstract—GW406381 (8), currently undergoing clinical evaluation for the treatment of inflammatory pain is a member of a novel
series of 2,3-diaryl-pyrazolo[1,5-b]pyridazine based cyclooxygenase-2 (COX-2) inhibitors, which have been shown to be highly
potent and selective. Several examples of the series, in addition to possessing favourable pharmacokinetic profiles and analgesic
activity in vivo, have also demonstrated relatively high brain penetration in the rat compared with the clinically available com-
pounds, which may ultimately prove beneficial in the treatment of pain.
Ó 2004 Elsevier Ltd. All rights reserved.
O
S
O
S
1. Introduction
O
O
Me
Me
Interest in the identification of highly selective inhibitors
of cyclooxygenase-2 (COX-2) has been intense since the
demonstration that early inhibitors such as DuP697 (1)1
and SC 58125 (2)2 produced analgesic efficacy in preclin-
ical models that was not accompanied by the typical
gastrointestinal toxicity produced by nonsteroidal anti-
inflammatory drugs (NSAIDs) such as aspirin and indo-
methacin, and several groups have since reported potent
and selective COX-2 inhibitors. Currently marketed
inhibitors shown below (examples 3–6) have now dem-
onstrated clinical efficacy equivalent to NSAIDs in the
treatment of inflammatory and musculoskeletal pain,
but with a much improved gastric safety profile thus ful-
filling the promise of this class (Figs. 1 and 2).
N
N
Br
CF3
S
F
F
1, DuP697
2, SC58125
Figure 1.
is, a 1,2-diaryl-substituted monocyclic heterocyclic ring.
We were particularly interested in investigating struc-
ture–activity relationships of diaryl-substituted bicyclic
heterocycles to establish whether these agents would
demonstrate increased selectivity for the COX-2 isoform
by more efficient occupation of the larger active site
available in the COX-2 isoform relative to COX-1.
Many of the bicyclic templates studied failed to give
any significant activity. Two examples of active series
are the imidazo[1,2-a]pyridines (e.g., 7) and the pyraz-
olo[1,5-b]pyridazines (e.g., 8). We initially chose to
investigate a series of diaryl imidazo[1,2-a]pyridines
(e.g., 7),3 many of which demonstrated high potency
At the commencement of our work in this area all of the
reported potent, selective COX-2 inhibitors possessed
the typical core template associated with this class, that
*
Corresponding author. Tel.: +44 1279 87 5220; fax: +44 1279 62
2550; e-mail: alan.2.naylor@gsk.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.07.089

5446
P. Beswick et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5445–5448
O
S
O
S
O
O
O
S
O
S
O
O
Me
H2N
Me
H2N
Me
Cl
N
N
O
O
N
CF3
N
Me
N
Me
O
3, Celecoxib
COX-2 IC50 = 68nM
COX-1 IC50 = 1689nM
4, Rofecoxib
COX-2 IC50 = 32nM
COX-1 IC50 = >100,000nM
6, Etoricoxib
COX-2 IC50 = 347nM
COX-1 IC50 = >100,000nM
5, Valdecoxib
COX-2 IC50 = 183nM
COX-1 IC50 = >100,000nM
Figure 2.
tion as shown in Scheme 1.4 Reaction of the acetylenic
esters (9)5 with the amino-pyridazinium salts (10)6,7 in
the presence of DBU generated the pyrazolo[1,5-
b]pyridazine ring system (11). Saponification of the
esters (11) furnished the corresponding acids (12).
Bromo-decarboxylation by treatment with N-bromosuc-
cinimide and sodium bicarbonate then provided the
intermediate bromides, suitable for a final Suzuki cou-
pling with the required boronic acid or boronate ester
to rapidly generate the final targets with ready access
to three points of diversity.
O
O
O
S
O
S
Me
Me
N
N
N
N
N
EtO
F
8
7
Figure 3.
against the COX-2 enzyme. However, several members
of the series surprisingly failed to show activity in a
rat model of inflammatory pain, regardless of possessing
adequate pharmacokinetic profiles. The lack of correla-
tion between in vitro and in vivo activity for some mem-
bers of the imidazo[1,2-a]pyridine template (7) could be
argued to be a consequence of the relatively high logD
values, and resulting unfavourable drug disposition for
this series, and we have thus investigated a number of
heterocyclic systems with reduced lipophilicity. This
communication describes our investigation of the novel
pyrazolo[1,5-b]pyridazine series (Fig. 3).
Alkoxy groups were introduced at R1 and R2 via deme-
thylation of the corresponding methoxy substituted pyr-
azolopyridazine analogues (using either boron
tribromide or pyridine hydrochloride) and subsequent
alkylation of the resulting phenols (Fig. 4).
A sulfone residue at R2 (e.g., 28) was formed by oxida-
tion of the corresponding methylsulfide (e.g., 27) using
OxoneÒ in methanol and water at 25°C (99% yield).
3. Results and discussion
The in vitro (recombinant human COX-1 and COX-2
enzymes8) and in vivo (FreundÕs Complete Adjuvant
(FCA)9-induced arthritis model) potencies of the pyraz-
olo[1,5-b]pyridazines 8 and 13–31 are shown in Table 1,
together with comparative data for known COX-2
2. Chemistry
The key strategy for formation of the pyrazolo[1,5-
b]pyridazine template was via a 1,3-dipolar cycloaddi-
CO2Me
MeO2C
R2
a
+
R2
N
N
N
N
N
+
I-
NH2
R1
R1
9
10
11
R3O2S
HO2C
c,d
b
R2
R2
N
N
N
N
N
N
R1
R1
12
8, 13- 28
Scheme 1. Reagents and conditions: (a) DBU, CH3CN, 25°C (25–84%); (b) NaOH, EtOH, H2O, reflux (72–99%); (c) NBS, NaHCO3, DMF, 25°C;
(d) 4-(methanesulfonyl)phenyl boronic acid or dipinacol-4-(aminosulfonyl) benzene borate ester,8 Pd(PPh3)4, Na2CO3, DME, H2O, reflux (32–62%
over steps c and d).

P. Beswick et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5445–5448
5447
inhibitors. Full details of the profile of lead compounds
in models of chronic pain is to be reported elsewhere.8
pyrazolopyridazine template. As above replacement of
the sulfone group by a sulfonamide (e.g., 23 and 24) re-
sults in compounds which although still potent and
selective COX-2 inhibitors, are less selective than the
corresponding sulfones.
The compounds illustrated in Table 1 are highly potent
COX-2 inhibitors and demonstrate excellent selectivity.
Importantly, several of these compounds show a good
correlation between in vitro potency and efficacy in the
FCA model of inflammatory pain.
Following administration of GW406381 (8) at a dose of
2mg/kg to male rats as a 1h iv infusion, GW406381
demonstrated low clearance (9mL/min/kg) with a steady-
state volume of distribution indicative of tissue distribu-
tion of 3.0L/kg and a terminal half-life of 4.1h. When
administered by oral gavage at a dose of 5mg/kg, using
a clinically relevant formulation, oral bioavailability was
approximately 80%.
Substitution of the 2-phenyl ring (R1) of the pyrazo-
lopyridazine with fluorine, alkyl and alkoxy groups is
well tolerated. Although replacement of the sulfone moi-
ety with a sulfonamide group (e.g., 18–20) results in
compounds with increased potency against COX-2,
this is also accompanied by reduced selectivity against
COX-1.
There is an increasing body of evidence10,11 to suggest
that prostanoids play an important role in the central
nervous system (CNS). Thus we have also determined
the extent to which GW406381 and the current mar-
keted compounds celecoxib and rofecoxib cross the
blood–brain barrier in rats.8 In view of the relatively
high brain:blood concentration ratio of GW406381
(1.5:1) compared to rofecoxib (0.8:1) and celecoxib
(0.1:1) we are investigating the potential of this com-
pound in models of centrally mediated pain. The results
of these studies will be reported elsewhere.8
As is evident from Table 1, a range of substituents at the
6-position of the pyrazolo[1,5-b]pyridazine ring (R2) is
well tolerated with high potency and selectivity for the
COX-2 enzyme. This is particularly true when a phenyl
sulfone group is present at the 3-position of the central
R3O2S
R2
N
N
In summary, we have rapidly prepared a novel series of
pyrazolo[1,5-b]pyridazines as highly potent and selective
COX-2 inhibitors, from which several compounds dem-
onstrate good in vivo profiles.
N
R1
Figure 4.
Table 1. In vitro and in vivo properties of 2,3 diaryl-pyrazolopyridazines
Compound
R1
R2
R3
IC50 COX-2 (nM)a
IC50 COX-1 (nM)b
Selectivityc
FCA ED50 (mg/kg)d
8
13
4-OEt
4-F
3-F
H
Me
Me
Me
Me
Me
Me
NH2
NH2
NH2
Me
Me
Me
NH2
Me
Me
NH2
Me
NH2
Me
Me
—
3
39
>84,200
>100,000
58,300
>28,067
>2564
1715
1.9
0.3
H
14
15
H
34
78% @ 10
0.42
H
H
2
17
>100,000
>19,100
3880
>20,000
>1107
>388
16
17
4-Me
4-OMe
4-F
H
23% @ 10
NT
H
<10
<10
0.44
388
24
18
19
H
24,100
3828
>2410
8700
>258
0% @ 10
0.5
4-OEt
4-OCF3
4-F
H
20
21
H
>100,000
>100,000
>100,000
>35,100
2605
92% @ 10
5.7
Me
Et
>4167
>2083
>688
22
23
4-F
4-F
48
51
40% @ 10
10% @ 10
NT
OMe
OMe
Me
OMe
SMe
SO2Me
SO2Me
OEt
OiPr
—
24
25
4-F
26
100
4-H
4-Me
4-OEt
4-F
<31
50
43,900
>100,000
8630
>1416
>2000
664
1% @ 10
15% @ 10
NT
26
27
13
28
29
370
264
43
>100,000
>100,000
>100,000
>100,000
1689
>270
>379
5
4-OEt
4-F
4-F
0% @ 10
11% @ 10
3
30
31
>2326
>1754
24.8
57
68
Celecoxib
Rofecoxib
Valdecoxib
Etoricoxib
—
—
3.7
1.0
—
—
—
—
32
>100,000
>100,000
>100,000
>3125
>546
>288
—
—
183
347
3.2
1.0
—
—
a IC50 values for inhibition of PGE2 produced by arachidonic acid stimulated COS cells stably expressing human COX-2 as described in Ref. 8.
b IC50 values for inhibition of PGE2 produced by arachidonic acid stimulated COS cells stably expressing human COX-1 as described in Ref. 8.
c IC50 value for COX-1 divided by IC50 value against COX-2.
d Ref. 9, where ED50 values not determined % reversal at 10mg/kg quoted.

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Geer, S. M.; Detty, M. R. J. Org. Chem. 1987, 52,
3662–3668.
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