Pyridazinones as selective cyclooxygenase-2 inhibitors

Article abstract of DOI:10.1016/S0960-894X(02)01045-4

Pyridazinone was found to be an excellent core template for selective COX-2 inhibitors. Two potent, selective and orally active COX-2 inhibitors, which were highly efficacious in rat paw edema and rat pyresis models, have been obtained.

Full text of DOI:10.1016/S0960-894X(02)01045-4

Bioorganic & Medicinal Chemistry Letters 13 (2003) 597–600
Pyridazinones as Selective Cyclooxygenase-2 Inhibitors
Chun Sing Li,* Christine Brideau, Chi Chung Chan, Chantal Savoie, David Claveau,
Stella Charleson, Robert Gordon, Gillian Greig, Jacques Yves Gauthier, Cheuk K. Lau,
Denis Riendeau, Michel Therien, Elizabeth Wong and Petpiboon Prasit
Merck Frosst Centre for Therapeutic Research, PO Box 1005, Pointe-Claire-Dorval, Quebec, Canada H9R 4P8
Received 17 October 2002; revised 27 November 2002; accepted 7 December 2002
Abstract—Pyridazinone was found to be an excellent core template for selective COX-2 inhibitors. Two potent, selective and orally
active COX-2 inhibitors, which were highly efficacious in rat paw edema and rat pyresis models, have been obtained.
# 2003 Elsevier Science Ltd. All rights reserved.
Selective cyclooxygenase-2 (COX-2) inhibitors represent
a new generation of anti-inflammatory drugs. In clinical
trial, these compounds have demonstrated less gastro-
intestinal side effects than non-steroidal anti-inflamma-
tory drugs (NSAIDs), which also inhibit the
cytoprotective action of COX-1 in the GI tract.1 During
the last decade, several selective inhibitors based on
Dup-697 have been reported.2 A general modification of
leads based on Dup-697 is the replacement of the cen-
tral thiophene ring with other carbocylic or heterocyclic
ring system. Five membered rings were frequently stud-
ied systems and were the most produtive area. Three
compounds derived from five membered heterocycles
have reached the market, such as furanone of rofe-
coxib,3 pyrazole of celecoxib4 and isoxazole of valde-
coxib.5 For the six-member ring systems, only
benzene6,7 and pyridine7,8 were reported. The pyridine
analogue etoricoxib8 is now in advanced clinical trial.
Herein, we describe our SAR studies on a new class of
orally active COX-2 inhibitors based on the six-member
heterocyclic pyridazinone system.
The N-substituted pyridazinone analogue 3 could be
readily synthesized from furanone 1. Bromination of 1
followed by hydrolysis gave the 5-hydroxy intermediate
2, which was subsequently reacted with an appro-
priately substituted hydrazine, or with hydrazine to give
3. Alternatively, 3 can be obtained by reaction of 2 with
hydrazine following by an alkylation reaction with the
desired electrophile (Scheme 1). SAR at the 4-position
of the pyridazinone was most conveniently investigated
with the 4-bromo-substituted pyridazinone intermediate
8, which can be obtained from 4-bromo-furan-2-one 49
in 7 steps (Scheme 2). Suzuki coupling of 4 with
4-methylthiophenylboronic acid, followed by bromination
and oxidiation yielded the 3-bromo-furan-2-one inter-
mediate 5. This intermediate was then converted to the
pyridazinone intermediate 8 using chemistry as descri-
bed in Scheme 1. Suzuki reaction of 8 with a boronic
acid intermediate gave 3. Substitution reaction of 8 with
a nucleophile afforded 9.
The COX-2 inhibition was determined in stably trans-
fected chinese hamster ovary (CHO) cells expressing
human COX-210 as well as a human whole blood
(HWB) COX-2 assay.11 The COX-1 inhibition was
evaluated with a sensitive U-937 microsomal COX-1
*Corresponding author. Tel.: +1-5144-283-217; fax: +1-5144-284-
900; e-mail: chunsing_li@merck.com
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(02)01045-4

598
C. S. Li et al. / Bioorg. Med. Chem. Lett. 13 (2003) 597–600
very clear that N-substitution was absolutely required
for good in vitro COX-2 inhibitory potency since the
unsubstituted analogue 3a was not potent. An increase
in size of the nitrogen substitutent such as 3b, 3c and 3d
improved COX-2 inhibitory potency, especially in the
human whole blood assay. However, a tertiary alkyl
susbstituent such as tert-butyl in 3e was not tolerated.
Cyclopropylmethyl substituent 3f and benzyl sub-
stitutent 3g, which exhibited excellent COX-2 potency
and selectivity, have the potential for further optimiza-
tion. A wide range of other large alkyl, alkenyl (3h),
cycloalkyl (3i), and substituted benzyl substituents have
also been investigated. Many of them retained COX-2
inhibitory potency and reasonable selectivity, but none
showed properties that were clearly superior to 3f and
3g. Heterocyclic substituents (3j, 3k and 3l), except for
the 2-thienylmethyl analogue 3l, were less potent. Com-
pound 3l, somehow suffered from the loss of selectivity
and poor pharmacokinetic. Substitution at the 6 posi-
tion of the pyridazinone template (3m), surprisingly
abolished the human whole blood COX-2 inhibitory
potency.
Scheme 1. (a) NBS; (b) HOAc, THF-H2O, ꢀ45% for 2 steps; (c)
RNHNH2 or (i) NH2NH2; (ii) R-X, NaOH, DMF, 30–75%.
Preliminary pharmacokinetic studies in rats indicated
that N-benzyl derivatives showed poorer pharmacoki-
netic than the corresponding N-cyclopropylmethyl deri-
vatives. For example, compound 3g has a Cmax of 1 mM
at 20 mg/kg oral dosing, while the corresponding cyclo-
propylmethyl analogue 3f has a Cmax of 2.2 mM. One of
the key objectives in the N-benzyl series was then to
improve the pharmacokinetic and maintain the COX-2
inhibitory potency and selectivity. This can be achieved
by varying substitutents at position 4. Several analogues
based on experiences with other templates were synthe-
sized (Table 2). Compound 10 with the celecoxib
4-methylphenyl substituent showed moderate improve-
ment in the human whole blood COX-2 inhibitory
potency as well as oral plasma level. Compound 10 has
good in vivo efficacy in the rat paw edema assay
(ED50=2.2 mg/kg), but the compound was not potent
Scheme 2. (a) 4-methylthiophenylboronic acid, PdCl2(PPh3)2,
Na2CO3, benzene, EtOH, quantitative; (b) Br2, CH2Cl2; (c) MMPP,
79% for 2 steps; (d) NBS; (e) HOAc, THF-H2O; 73% for 2 steps; (f)
NH2NH2, 84%; (g) base, R-X; (h) Ar-B(OH)2, Pd(0); (i) K2CO3 or
Cs2CO3, R1-YH, DMF, 8–71%.
assay12 at low arachidonic acid concentration. Selected
compounds were also tested in the rat paw edema13 and
rat pyresis13 assays for their in vivo efficacies.
Various N-substituted analogues were initially prepared
to evaluate the effect of N-substitution (Table 1). It was
Table 1. In vitro activity of N-substituted analogues
Compd
X
R
COX-2 IC50 (mM)
COX-1 IC50 (mM)
CHO
HWB
U-937
3a
3b
3c
3d
3e
3f
3g
3h
3i
H
H
H
H
H
H
H
F
F
H
F
F
H
H
Me
>5
0.3
0.5
0.08
>5
0.07
0.03
0.1
0.04
1.3
0.6
0.05
0.35
>33
21
11
>10
>10
>10
>10
>10
>10
ꢀ10
1–3
2,2,2-Trifluoroethyl
Phenyl
tert-Butyl
Cyclopropylmethyl
Benzyl
3-Methyl-2-butenyl
Cyclohexylmethyl
3-Pyridylmethyl
2-Thiazoylmethyl
2-Thienylmethyl
Benzyl (6-Me)
4.7
>33
1.3
0.9
0.3
0.41
7.1
0.3–1
ꢀ10
ꢀ10
ꢀ1
3j
3k
3l
5.9
<0.41
>33
3m
>10

C. S. Li et al. / Bioorg. Med. Chem. Lett. 13 (2003) 597–600
Table 2. In vitro and in vivo activity of N-benzyl analogues
599
Compd
R1
COX-2 IC50 (mM)
COX-1 IC50 (mM
)
max (CmM)
Rat paw ED50
mg/kg
,
Rat pyresis ED50,
mg/kg
CHO
HWB
U-937
20 mg/kg, PO
3g
10
11
12
13
14
15
16
Phenyl
0.03
0.02
1.2
0.02
0.26
0.02
0.003
0.37
0.9
0.35
1.1
0.09
0.9
1.7
>10
>10
>10
0.3–1
3–10
>10
ꢀ10
3–10
1.0
2.7
4.2
4-Methylphenyl
2-Methyl-5-pyridyl
4-Fluorophenoxy
5-Chloro-2-pyridyloxy
Isopropoxy
2.2
>10
>10
>10
0
2.8
0.8
<0.3
>10
<0.3
Cyclopropylmethoxy
Phenylthio
0.36
5.9
in the rat pyresis assay (ED50>10 mg/kg). Compound
11 with the etoricoxib 2-methyl 5-pyridyl substituent
was moderately potent in the CHO COX-2 assay and
the compound was not shift in the HWB COX-2 assay,
but this compound was not efficacious in the rat paw
edema assay (ED50>10 mg/kg). We have demonstrated
in the furanone series that an oxygen linked or sulfur-
linked substituents would provide COX-2 inhibitors
with excellent potency and selectivity. Indeed, all the
oxygen-linked derivatives 12, 13, 14 and 15 were potent
and selective COX-2 inhibitors. Unfortunately, the
4-fluorophenoxy compound 12 was not efficacious in vivo
(ED50>10 mg/kg in the rat paw edema assay) and the
5-chloro-2-pyridyloxy compound 13 showed very poor
pharmacokinetic in rats. On the other hand, the alkoxy
derivatives 14 and 15 showed moderate oral plasma
levels with Cmax of 2.8 mMfor 14, 0.8 mMfor 15. While
compound 14 was only moderately potent in the HWB
COX-2 assay, the in vitro activity of 14 translated
extremely well into excellent in vivo activity in the rat
paw edema (ED50<0.3 mg/kg) and rat pyresis
(ED50<0.3 mg/kg) assays. Surprisingly, compound 15
was not potent in the rat paw edema assay (ED50>10
mg/kg) and the poor oral plasma level might be partly
responsible for the lack of in vivo activity. The sulfur-
linked derivative 16 was not very potent in the human
whole blood COX-2 assay. Overall, the isopropoxy
analogue 14 represents the most promising compound
in the N-benzyl series of pyridazinone selective COX-2
inhibitors.
analogue 20, (2,2-dimethylcyclopropyl)methyl analogue
21, (2,3-dimethylcyclopropyl)methyl analogue 22 and
the (cis-2-methylcyclopropyl)methyl analogue 23,
though were potent and moderately selective, they
showed no oral plasma levels in rats. The a-methyl
substituted analogue 24 has excellent plasma levels in
Table 3. In vitro activity of N-cyclopropylmethyl analogues
Compd
R
COX-2 IC50 (mM)
COX-1 IC50 (mM)
U-937
CHO
0.2
HWB
1.7
17
18
19
20
>10
0.2
0.11
0.2
2.9
>33
0.7
>10
ꢀ10
3–10
21
22
23
24
25
26
0.03
0.05
0.05
0.2
0.8
0.7
2.4
0.7
1.7
0.3
1–3
1–3
The N-cyclopropylmethyl series was usually less potent
than the corresponding N-benzyl series in the COX-2
assays. Our initial observation suggested that increasing
the size of the nitrogen substituent might improve the
COX-2 potency. Several substituted N-cyclopropyl-
methyl analogues14 were subsequently prepared (Tables
3 and 4). Although most of these modifications
maintained or indeed slightly improved the COX-2
potency, pharmacokinetics of these derivatives seemed
to highly depend on the substitution patterns. Extension
or reduction of the linker between the cyclopropyl ring
and the nitrogen led to compounds with reduced COX-2
potency (18 and 19). The (1-methylcyclopropyl)methyl
3–10
3–10
3–10
ꢀ3
0.06
0.06

600
C. S. Li et al. / Bioorg. Med. Chem. Lett. 13 (2003) 597–600
Table 4. Plasma levels and in vivo activity of N-cyclopropylmethyl
analogues
Wong, E.; Xu, L.-J.; Young, R. N.; Zamboni, R.; Boyce, S.;
Rupniak, N.; Forrest, M.; Visco, D.; Patrick, D. Bioorg. Med.
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Compd
Cmax (mM)
Rat paw
Rat pyresis
20 mg/kg, PO
ED50, mg/kg
ED50, mg/kg
17
24
25
26
8.5
13
4.8
16
3–10
1.8
40% @ 10
0.7
>10
1.1
5. Talley, J. J.; Brown, D. L.; Carter, J. S.; Graneto, M. J.;
Koboldt, C. M.; Masferrer, J. L.; Perkins, W. E.; Rogers,
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J. L.; Perkins, W. E.; Seibert, K.; Zhang, Y.; Zweifel, B. S.;
Reitz, D. B. J. Med. Chem. 1996, 39, 1846.
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Orwat, M. J.; Wang, S.; Jetter, J. W.; Sherk, S. R.; Houghton,
G. C.; Copeland, R. A.; Covington, M. B.; Trzaskos, J. M.;
Magolda, R. L. Bioorg. Med. Chem. Lett. 1999, 9, 919.
8. Friesen, R. W.; Brideau, C.; Chan, C.-C.; Charleson, S.;
Deschenes, D.; Dube, D.; Ethier, D.; Fortin, T.; Gauthier,
J. Y.; Girard, Y.; Gordon, R.; Greig, G.; Riendeau, D.;
Savoie, C.; Wang, Z.; Wong, E.; Visco, D.; Xu, L.-J.; Young,
R. N. Bioorg. Med. Chem. Lett. 1998, 8, 2777.
rats and showed a Cmax of 13 mM. Compound 24 has
good in vivo activity in the rat paw edema assay
(ED50=1.8 mg/kg), but it was not potent in the rat
pyresis assay (ED50>10 mg/kg). Interestingly, the two
optically pure (trans-2-methylcyclopropyl)methyl ana-
logues (R,R)-25 and (S,S)-26 showed 6-fold difference in
the HWB COX-2 potency although both compounds
have similar CHO COX-2 activity. While the racemic cis
isomer 23 has poor oral plasma level in rats, the trans-
(S,S)-25 and the trans-(R,R)-26 both showed decent oral
plasma levels15 with a Cmax 4.8 mMfor 25 and 16 mM
for 26. Compound 25 was not potent in the rat paw
edema assay (40% @ 10 mg/kg). Compound 26 on the
other hand showed excellent in vivo activity in the rat
paw edema assay (ED50=0.7 mg/kg) and the rat pyresis
assay (ED50=1.1 mg/kg). Compound 26 is so far the
optimal compound in the N-cyclopropylmethyl series of
pyridazinone selective COX-2 inhibitors.
9. Jas, G. Synthesis 1991, 11, 965.
10. Kargman, S.; Wong, E.; Greig, G.; Falgueyret, J.-P.;
Cromlish, W.; Ethier, D.; Yergey, J.; Riendeau, D.; Evans, J.;
Kennedy, B.; Tagari, P.; Francis, D.; O’Neill, G. P. Biochem.
Pharmacol. 1996, 52, 1113.
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E. W.; Rodger, I. W.; Chan, C.-C. Inflamm. Res. 1996, 45, 68.
12. Riendeau, D.; Charleson, S.; Cromlish, W.; Mancini,
J. A.; Wong, E.; Guay, J. Can. J. Phys. Pharmacol. 1997, 75,
1088.
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Hutchinson, A. W.; Gordon, R.; Guay, D.; Hill, R.; Li, C.-S.;
Mancini, J.; Penneton, M.; Prasit, P.; Rasori, R.; Riendeau,
D.; Roy, P.; Targari, P.; Vickers, P.; Wong, E.; Rodger, I. W.
J. Pharmacol. Exp. Ther. 1995, 274, 1531.
In conclusion, two potent and selective COX-2 inhibi-
tors 14 and 26 have been identified from the pyr-
idazinone template. These two compounds also showed
excellent efficacy in animal models of anti-inflammation,
the rat paw edema and rat pyresis assays.
References and Notes
14. The chiral cyclopropylmethyl alcohols for 25 and 26 were
prepared according to Charette’s condition: Charette, A. B.;
Prescott, S.; Brochu, C. J. Org. Chem. 1995, 60, 1081. Other
substitutents were eithier commercially available or prepared
from the corresponding olefins.
15. The oral samples were prepared by the addition of 0.5 mL
of a PEG 200 solution of compounds to 9.5 mL of 1% meth-
ocel under vigorously stirring. Otherwise, plasma levels would
be expected much lower (e.g., racemic 26 has a Cmax=2.4
mM).
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