4-Amino-5-substituted-3(2H)-pyridazinones as orally active antinociceptive agents: Synthesis and studies on the mechanism of action

Article abstract of DOI:10.1021/jm070161e

A number of 4-amino-5-vinylpyridazinones and 4-amino-5-heterocyclic- pyridazinones were synthesized and tested for their analgesic activity. Many of these compounds, tested at doses of 3-20 mg kg^-1 po, showed good antinociceptive activity, redu

Full text of DOI:10.1021/jm070161e

J. Med. Chem. 2007, 50, 3945-3953
3945
4-Amino-5-substituted-3(2H)-pyridazinones as Orally Active Antinociceptive Agents: Synthesis
and Studies on the Mechanism of Action
Maria Paola Giovannoni,*,† Nicoletta Cesari,† Claudia Vergelli,† Alessia Graziano,† Claudio Biancalani,† Pierfrancesco Biagini,†
Carla Ghelardini,‡ Elisa Vivoli,‡ and Vittorio Dal Piaz†
Dipartimento di Scienze Farmaceutiche, UniVersita` di Firenze, Via Ugo Schiff 6, Sesto Fiorentino, 50019 Firenze, Italy, and Dipartimento di
Farmacologia Clinica e Preclinica, UniVersita` di Firenze, Viale Pieraccini 6, 50139 Firenze, Italy
ReceiVed February 12, 2007
A number of 4-amino-5-vinylpyridazinones and 4-amino-5-heterocyclic-pyridazinones were synthesized and
tested for their analgesic activity. Many of these compounds, tested at doses of 3-20 mg kg-1 po, showed
good antinociceptive activity, reducing by more than 50% the number of writhes with respect to controls.
Compounds 16c, 19a, 20a, and 28 were the most potent of the series because they were able to induce a
potent antinociceptive effect at a dose of 3 mg kg-1 po. None of the active compounds at the analgesic dose
provoked any visible change in normal behavior, as demonstrated in the rotarod test. Studies on the mechanism
of action showed that the analgesia induced by the active compounds was completely prevented by
pretreatment with the R2-antagonist yohimbine, suggesting an involvement of R2-adrenoceptors. Further
investigation demonstrated an indirect activation of the noradrenergic system through an amplification of
noradrenaline release.
Introduction
Pain is widely accepted to be one of the most important
determinants of quality of life. A study reported by the World
Health Organization demonstrated that individuals who live with
persistent pain suffer 4-fold more from depression or anxiety
compared to healthy subjects.1 Moreover, the costs associated
with pain are very high, not only in medical terms but also with
regard to reduced work productivity.2 Advances in pharmaceuti-
Figure 1. 4-Amino-5-vinyl-3(2H)-pyridazinones with antinociceptive
activity.
cal research have greatly increased the options for analgesic
therapy, which fall under three chief categories: nonsteroidal
different functionalities produced inactive compounds. The only
anti-inflammatory drugs (NSAIDs), opioids, and “analgesic
modifications allowed are the homologation of the vinyl group
adjuvants”.3 As is well-known, this last category has primary
(propenyl) and the introduction of a bromine in position â of
the vinyl.
Considering these data, we decided to examine in more depth
the structure-activity relationships of the above compounds,
application for conditions other than pain but is also used for
specific forms of pain such as neuropathic, musculoskeletal, and
cancer pain.4 In spite of the large number of analgesic drugs
available on the market, the research in this field is very lively
performing modifications at position 6 while maintaining at the
with more potent molecules devoid of the secondary effects
other positions the groups that award the maximum activity.
typical of NSAIDs and opioids5,6 being sought. Since pain is a
At the same time, we replaced the vinyl group at position 5
very complex process in which many neuromodulators are
with nitrogen heterocyclic systems. In fact, the vinyl group is
involved7,8 (i.e., adrenaline, acetylcholine, adenosine, etc.),
potentially toxic,17 since it could generate the reactive epoxidic
researchers have also focused their efforts on molecules that
function in vivo. Finally, we investigated the mechanism of
work on these systems.9
action of all synthesized compounds.
Our research in the field of antinociceptive agents has led us
Design of the new molecules was carried out following a
to obtain pyridazine derivatives with very potent analgesic
modification of Lipinski’s rule of five,18 which is the most
activity.10-16
widely accepted technique for identifying druglike compounds.
In our previous papers11,13,16 we reported a number of
According to this method,19 our molecules fulfill the following
4-amino-5-vinyl-3(2H)-pyridazinones that exhibited interesting
requirements: (a) hydrogen bond donors (OH and NH), e5;
antinociceptive activity, with the most active compounds having
(b) hydrogen bond acceptors (O and N), e10; (c) molecular
an ED50 of <20 mg/kg sc in an abdominal constriction test
weight, e500; (d) log P e 5. Evaluation of lipophilicity was
(Figure 1). Structure-activity relationships revealed that modi-
performed by measuring log P with ACD/LogP Free LogP
fications at positions 2 and 6 of the pyridazine ring are well-
calculator. The value met the requirement, since our compounds
tolerated, while at position 4, the presence of an amino or low
alkylamino group is necessary for the activity. Furthermore,
showed log P in the range 0.22-2.24.
replacement of the vinyl group at position 5 with a variety of
Chemistry
All new compounds were synthesized as reported in Schemes
1-6.
Schemes 1 and 2 depict the synthetic procedures that afford
final products 5a-i (Table 2) and 12, which were modified at
* To whom correspondence should be addressed. Phone: +039-055-
4573682. Fax: +039-055-4573780. E-mail: mariapaola.giovannoni@unifi.it.
† Dipartimento di Scienze Farmaceutiche.
‡ Dipartimento di Farmacologia Clinica e Preclinica.
10.1021/jm070161e CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/13/2007

3946 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16
Table 1. Details for Compounds 1a-d
GioVannoni et al.
Scheme 2. Synthesis of Compound 12a
compd
R
1a
1b
1c
1d
4-pyridyl
2-thienyl
4-F-Ph
cyclopentyl
Table 2. Details for Compounds 2-5(a-i)
compds 2-5
R
R1
a
b
c
d
e
f
g
h
i
4-pyridyl
2-thienyl
4-F-Ph
4-F-Ph
cyclopentyl
4-pyridyl
2-thienyl
4-Br-Ph
CH3
nC4H9
nC4H9
nC4H9
CH3
CH3
CH3
CH3
CH3
nC4H9
Scheme 1. Synthesis of Final Compounds 5a-ia
a (a) NaOEt, anhydrous EtOH, 0 °C; (b) MeNHNH2, EtOH, room temp,
2 h; (c) 10% Pd/C, ammonium formate, EtOH, reflux, 1 h; (d) NaBH4,
MeOH, 1 h, room temp; (e) PPA, 60 °C, 2 h.
Scheme 3. Synthesis of Final Compounds 16a-da
a (a) Appropriate alkyl halide, anhydrous acetone, K2CO3, reflux, 2-3
h; (b) 10% Pd/C, ammonium formate, EtOH, reflux, 1 h; (c) NaBH4, MeOH,
40-60 min, room temp; (d) PPA, 60 °C, 2-6 h.
position 6 of the pyridazine ring. The isoxazolo[3,4-d]pyridazi-
nones 2a-i represent the starting material (Scheme 1). Com-
pounds 2a-e were prepared by alkylation under standard
conditions of the precursors 1a-d (1a,10 1b,20 1c,21 1d;22 Table
1), whereas compounds 2f-i were previously described (2f,10
2g,23 2h,24 2i22). Reductive cleavage with ammonium formate
and Pd/C gave the 4-amino-5-acetyl derivatives 3a-i in good
yield. Reduction of these intermediates with sodium borohydride
in methanol at room temperature afforded the corresponding
secondary alcohols, which were transformed into the final
4-amino-5-vinyl derivatives 5a-i by treatment with polyphos-
phoric acid (PPA). The same procedure was followed to obtain
compound 12 (Scheme 2), but in this case it was necessary to
synthesize the 4-cyclohexanecarbonyl-5-methylisoxazole 8 start-
a (a) Alkylamine, EtOH, room temp, 30-120 min; (b) NaBH4, MeOH,
40-60 min, room temp; (c) PPA, 60 °C, 2-6 h.
ing from 1-cyclohexylbutane-1,3-dione 6 and commercially
available chloro(hydroximino)acetate 7.
Modifications at position 4 of the pyridazine ring (Scheme
3) were performed by displacement of the nitro group in
compounds 13a-c (Table 3) with methyl or ethylamine at room
temperature (compounds 14a-d). These intermediates were
converted into the secondary alcohols 15a-d, which in turn
were transformed into the vinyl derivatives 16a-d (Table 4).

Pyridazinones as AntinociceptiVe Agents
Table 3. Details for Compounds 13a-c
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16 3947
Scheme 5. Synthesis of Compound 25a
compd
R
R1
Ph
CH3
Ph
13a
13b
13c
C4H9
CH3
CH3
Table 4. Details for Compounds 14-16(a-d)
compds 14-16
R
R1
R2
a
b
c
Ph
CH3
Ph
C4H9
CH3
CH3
CH3
CH3
CH3
C2H5
C2H5
d
CH3
Scheme 4. Synthesis of Final Compounds 19-22a
a (a) Br2, AcOH, HBr, 50 °C, 4 h; (b) thioacetamide, EtOH, 80 °C, 1 h.
Scheme 6. Synthesis of Compound 28a
a (a) SOCl2, 60 °C, 1 h, acetohydrazide, anhydrous dioxane, room temp,
3 h; (b) POCl3, 60 °C, 2 h.
The synthesis of 5-methylthiazolyl derivative 25 (Scheme 5)
was performed starting from the previously described compound
23. Treatment of 23 with bromine in acetic acid afforded product
24, which with thioacetamide in ethanol gave rise to final
product 25.
Finally, the 5-oxadiazolyl derivative 28 (Scheme 6) was
synthesized by treatment of precursor 26 with SOCl2. The crude
chloride was reacted with acetohydrazide affording compound
27, which in turn was cyclized with POCl3 to give compound
28.
a (a) N,N-Dimethylformammide dimethyl acetal, 100-110 °C, 3-4 h;
(b) hydrazine hydrate, EtOH, 80-90 °C, 1-2 h; (c) CH3I, anhydrous
acetone, K2CO3, reflux, 2-3 h; (d) for compound 20a, EtBr, NaH,
anhydrous DMSO, 80 °C, 90 min.
Modification of the vinyl group was performed as in Schemes
4-6 by inserting different heterocycles at position 5. Final
compounds 19-22 (Scheme 4) were obtained starting from
precursors 17a,b which when treated with N,N-dimethylform-
amide dimethylacetal, afforded the corresponding 3-dimethyl-
aminovinyl derivatives 18a,b. Treatment with the hydrazine
hydrate of 18a,b gave rise to 5-(1H)-pyrazolyl derivatives 19a,b,
which when alkylated under standard conditions, afforded
products 20a,b. The regioselectivity of alkylation was confirmed
by a positive NOE effect, detected between the methyl group
on the pyrazole ring and the proton at position 5′. Alkylation
performed on final compound 20a under drastic conditions
(EtBr, NaH, DMSO at 90 °C) permitted us to obtain compounds
21 and 22, depending on the amount of ethyl bromide.
Results and Discussion
In the present study, the antinociceptive activity of the inves-
tigated compounds was evaluated in the experimental model
of the abdominal constriction test in mice in which a painful
chemical stimulus was applied. All compounds were adminis-
tered po,a and their activity was compared with that of two
representative terms of the previous series (compounds A and
B, Figure 1), which were tested again to make the data uniform.
Compounds 5f, 12, 16a-d, 19a, 20a,b, 21, 25, and 28
a Abbreviations: po, per os; SGU, signal generation unit; LTQ, linear
trap quadruple.

3948 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16
GioVannoni et al.
Table 5. Antinociceptive Effect of Final Compounds in the Writhing
Test and Effect of Compounds 5a,h,i, 12, 16d, 19a, 20a,b, 21, and 25
on Noradrenaline Extracellular Levels from Rat Cerebral Cortex
Table 6. Effect of 5f, 12, 16a-d, 19a, 20a,b, 21, 25, and 28 in the
Mouse Rotarod Testa
no. of falls in 30 s
after treatment
30 min 45 min
no. of writhes no. of writhes
dose po
before
treatment
dose
in the absence in the presence
of yohimbine of yohimbine
% of NA
release
treatment (mg kg-1
)
15 min
treatmenta (mg kg-1
)
CMC
5f
12
4.5 ( 0.3 3.6 ( 0.4 3.0 ( 0.3 2.2 ( 0.4
4.8 ( 0.4 3.5 ( 0.3 2.8 ( 0.6 2.1 ( 0.2
5.0 ( 0.5 4.1 ( 0.5 3.4 ( 0.5 2.9 ( 0.4
4.7 ( 0.6 4.0 ( 0.5 3.3 ( 0.3 2.5 ( 0.3
4.9 ( 0.7 3.7 ( 0.3 2.5 ( 0.3 2.0 ( 0.2
4.4 ( 0.3 3.4 ( 0.5 2.3 ( 0.4 1.9 ( 0.3
5.1 ( 0.5 3.9 ( 0.5 3.2 ( 0.4 2.5 ( 0.4
4.9 ( 0.5 3.7 ( 0.4 2.8 ( 0.6 2.3 ( 0.5
4.8 ( 0.4 4.2 ( 0.6 3.1 ( 0.5 2.3 ( 0.4
5.0 ( 0.5 3.9 ( 0.3 2.6 ( 0.4 1.8 ( 0.5
4.5 ( 0.3 2.6 ( 0.4 1.8 ( 0.4 2.1 ( 0.3
4.9 ( 0.5 3.8 ( 0.5 2.7 ( 0.3 2.0 ( 0.4
4.3 ( 0.5 2.7 ( 0.4 2.3 ( 0.5 1.5 ( 0.4
CMC
yohimbine
5a
5b
5c
5d
5e
5f
5g
33.2 ( 2.1
31.7 ( 3.1
35.3 ( 4.4
25.2 ( 4.2
33.6 ( 4.1
27.2 ( 3.3
27.7 ( 3.9
100
20
10
20
10
3
10
3
3
30
20
10
3
3
20
20
20
20
10
20
20
20
20
10
20
10
3
10
3
3
30
20
20
10
3
97.5 ( 8.1
16a
16b
16c
16d
19a
20a
20b
21
18.7 ( 3.1b
30.2 ( 5.1
35.7 ( 3.8
36.7 ( 4.3
14.4 ( 4.2b
21.6 ( 4.5b
17.6 ( 2.8b
17.4 ( 3.3b
16.8 ( 4.9b
15.9 ( 3.1b
13.6 ( 3.0b
15.9 ( 4.3b
14.1 ( 3.7b
27.1 ( 4.2
14.7 ( 2.7b
19.5 ( 2.8b
19.7 ( 3.6b
20.4 ( 3.9b
34.1 ( 4.4c
5h
5i
12
107.0 ( 9.4
95.9 ( 8.8
32.5 ( 3.4c
31.9 ( 2.8c
32.0 ( 3.3c
28.1 ( 3.5c
29.0 ( 4.3c
26.5 ( 3.7c
31.1 ( 3.5c
31.7 ( 3.2c
29.6 ( 3.2c
182.1 ( 9.2b
25
28
16a
16b
16c
16d
19a
20a
20b
21
22
25
28
A
a Each value represents the mean at least of five mice.
176.2 ( 8.5b
168.4 ( 7.7b
190.1 ( 10.6b
188.5 ( 9.5b
173.7 ( 6.8b
In our experimental conditions, the R2-antagonist yohimbine
did not modify the pain threshold of mice in comparison with
control animals. The lack of effect of this antagonist agrees with
results of studies in which this compound did not modify the
nociceptive threshold against either thermal (hot-plate) or
chemical (writhing) noxious stimuli.27 We can therefore exclude
the possibility that the prevention of the antinociception induced
by assayed compounds was due to a hyperalgesic effect of the
R2-adrenoceptor antagonist used.
33.6 ( 3.0c
34.2 ( 4.1c
29.3 ( 3.1c
33.1 ( 2.6c
181.3 ( 9.4b
10
10
B
a All drugs were administered per os 30 min before test. b P < 0.01 versus
CMC treated mice. Each value represents the mean of at least two
experiments. c P < 0.05 in comparison with yohimbine-treated mice. Each
value represents the mean of at least two experiments.
None of the antinociceptive doses of the compounds provoked
any visible change in the normal behavior of the mice as
demonstrated in rotarod experiments in which no impairment
of mouse rotarod performance was observed (Table 6), as
revealed by the decrease of the number of falls in each session.
The R2-mediated analgesia can be induced by a direct R2-
adrenoceptor activation or by an indirect R2-adrenoceptor system
activation, such as an increase of noradrenaline release. To
elucidate whether the antinociceptive mechanism of action was
due to an indirect noradrenergic mechanism, noradrenaline
extracellular level from rat cerebral cortex treated with most
active compounds (12, 16d, 19a, 20a,b, 21, and 25) was
evaluated. Noradrenaline release was determined after admin-
istration of the above compounds at the doses able to induce a
reduction of writhes in the abdominal constriction test (Table
5). The increase of noradrenaline release exerted by 12
(+182%), 16d (+176%), 19a (+168%), 20a (+190%), 20b
(+188%), 21 (+173%), and 25 (+181%) indicates the existence
of a pharmacological correlation between the reduction of
writhes and the enhancement of noradrenaline levels. This
observation suggests that the antinociceptive action of the above
compounds could be due to an indirect activation of the
noradrenergic system through an amplification of noradrenergic
release. By contrast, 5a, 5h, and 5i, which were not endowed
with antinociceptive effect, were also unable to increase
noradrenaline release in the same experimental conditions.
Taking into account the fact that the tested compounds were
administered by oral route, structure-activity relationships can
be only partly determined. In fact the observed activity is a
consequence of both pharmacokinetic and pharmacodynamic
properties that could be affected differently by structural
modifications. Nevertheless, some SARs can be inferred. First
of all, we examined the role played by the substituent in position
6 in the subseries of 5-vinyl derivatives in comparison with the
lead A, keeping the methyl group at position 2 and amino group
at position 4. Thus, we observed that replacement of phenyl
with 4-fluorophenyl (5d), 4-bromophenyl (5h), 2-thienyl (5g),
were able to reduce the number of abdominal constrictions and
showed a potency comparable to that of reference compounds
A and B. In particular 12, 20a, 21, and 25, tested respectively
at 10, 3, 20, and 10 mg kg-1 po, were able to exhibit a good
antinociceptive activity reducing by more than 50% the number
of writhes with respect to controls (Table 5). Antinociceptive
effect induced by the above-mentioned compounds was therefore
exhibited at an interesting level since these compounds showed
potency and efficacy comparable to that shown by some
clinically employed analgesic drugs such as diphenhydramine,
baclofen, amitriptiline etc..25 Compounds 16c, 19a, 20a and 28
were the most potent of the series because they induce a
statistically significant antinociceptive effect at a dose of 3 mg
kg-1 po. Compounds 5f, 16a,b, 16d, and 20b reduced by less
than 50% the number of abdominal constrictions, and finally
compounds 5a-e,g-i and 22 were devoid of any effect in the
abdominal constriction test.
Previously we demonstrated that the analgesia induced by
structurally related compounds15 was completely prevented by
pretreatment with the R2-antagonist yohimbine. In order to
clarify whether the R2-adrenoceptor was involved in the
mechanism of action of this series, we pretreated animals with
yohimbine as antagonist.
The dose of yohimbine employed to prevent the analgesia
induced by the above-reported compounds is the minimal dose
able to antagonize antinociception induced by activation of R2-
adrenoceptor as demonstrated by the block exerted on amitrip-
tyline and imipramine antinociception.26 A complete reversal
of the antinociceptive effect of compounds 5f, 12, 16a-d, 19a,
20a,b, 21, 25, and 28 by the R2-antagonist yohimbine (3 mg
kg-1 ip) was evidenced in the mouse abdominal constriction
test, suggesting the involvement of R2-adrenoceptors in the
mechanism of analgesic action of the above-mentioned com-
pounds (Table 5).

Pyridazinones as AntinociceptiVe Agents
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16 3949
CH2CH3), 1.85 (m, 2H, CH2CH2CH2CH3), 2.70 (s, 3H, 3-CH3),
4.25 (t, 2H, CH2CH2CH2CH3), 7.65 (d, 2H, Ar), 8.85 (d, 2H, Ar).
6-n-Butyl-3-methyl-4-thiophen-2-ylisoxazolo[3,4-d]pyridazin-
and cyclopentyl (5e) at the same or at higher doses with respect
to the lead A was associated with complete loss of activity,
whereas the presence at position 6 of 4-pyridyl (5f) or
cyclohexyl (12) determined the appearance of good antino-
ciceptive effects (44% and 57% inhibition of the writhes
respectively).
In the same subseries, replacement of the amino group with
NHEt (16c) in the structure of A led to a still active compound
(48% inhibition at 3 mg/kg).
1
7(6H)-one 2b. Yield ) 73%; mp ) 92-95 °C (EtOH); H NMR
(CDCl3) δ 1.00 (t, 3H,(CH2)3CH3), 1.40-1.50 (m, 2H, (CH2)2CH2-
CH3), 1.80-1.90 (m, 2H, CH2CH2CH2CH3), 2.75 (s, 3H, 3-CH3),
4.20 (t, 2H, CH2CH2CH2CH3), 7.20 (m, 1H, Ar), 7.35 (m, 1H, Ar),
7.50 (m, 1H, Ar).
6-n-Butyl-4-(4-fluorophenyl)-3-methylisoxazolo[3,4-d]pyridazin-
7(6H)-one 2c. Yield ) 93%; mp ) 97-100 °C (EtOH); 1H NMR
(CDCl3) δ 1.00 (t, 3H, (CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3),
1.80-1.90 (m, 5H (3H, 3-CH3; 2H, CH2CH2CH2CH3)), 4.20 (t,
2H, CH2CH2CH2CH3), 7.15 (m, 2H, Ar), 7.45 (m, 2H, Ar).
4-(4-Fluorophenyl)-3,6-dimethylisoxazolo[3,4-d]pyridazin-
7(6H)-one 2d. Yield ) 71%; mp ) 223-225 °C (EtOH); 1H NMR
(CDCl3) δ 2.55 (s, 3H, 3-CH3), 3.85 (s, 3H, N-CH3), 7.25 (m,
2H, Ar), 7.55 (m, 2H, Ar).
4-Cyclopentyl-3,6-dimethylisoxazolo[3,4-d]pyridazin-7(6H)-
one 2e. Yield ) 20%; mp ) 103-104 °C (EtOH); 1H NMR
(CDCl3) δ 1.45-2.05 (m, 8H, cC5H9), 2.90 (s, 3H, 3-CH3), 3.25-
3.40 (m, 1H, cC5H9), 3.75 (s, 3H, N-CH3).
General Procedure for 3a-i. A mixture of appropriate 2-sub-
stituted isoxazolo[3,4-d]pyridazinones 2a-i (2f,10 2g,23 2h,24 2i22)
(0.6 mmol), 10% Pd/C (30-40 mg), and ammonium formate (2.0
mmol) in EtOH (5-6 mL) was refluxed for 1 h. After the mixture
was cooled, CH2Cl2 (6 mL) was added, the catalyst was filtered
off, and the solvent was evaporated in vacuo to afford 3a-i.
5-Acetyl-4-amino-2-n-butyl-6-pyridin-4-ylpyridazin-3(2H)-
one 3a. Yield ) 80%; mp ) 103 °C (EtOH); 1H NMR (CDCl3) δ
1.00 (t, 3H,(CH2)3CH3), 1.45 (m, 2H, (CH2)2CH2CH3), 1.80-1.90
(m, 5H (3H, COCH3; 2H, CH2CH2CH2CH3)), 4.20 (t, 2H, CH2-
CH2CH2CH3), 6.90 (exch br s, 2H, NH2), 7.50 (d, 2H, Ar), 8.75
(d, 2H, Ar).
5-Acetyl-4-amino-2-n-butyl-6-thiophen-2-ylpyridazin-3(2H)-
one 3b. Yield ) 82%; mp ) 109-112 °C (EtOH); 1H NMR
(CDCl3) δ 1.00 (t, 3H,(CH2)3CH3), 1.45 (m, 2H, (CH2)2CH2CH3),
1.85 (m, 2H, CH2CH2CH2CH3), 2.00 (s, 3H, COCH3), 4.20 (t, 2H,
CH2CH2CH2CH3), 7.10 (m, 2H, Ar), 7.45 (m, 1H, Ar), 7.55 (exch
br s, 2H, NH2).
Replacement of phenyl with a methyl group was well-
tolerated, as demonstrated by the activity of 16b,d in this series
and of 20b in the 5-heterocyclic series.
In this last subseries, good results were obtained with a variety
of five-membered systems at position 5. In fact, all the
synthesized compounds showed interesting levels of activity
(52-59% inhibition at doses ranging from 3 to 20 mg/kg) with
the exception of 22, which proved to be inactive. In this case,
the presence of the tertiary amino group NEt2 in position 4 is
probably responsible for the loss of activity. Taken together,
the obtained results in this subseries clearly demonstrate that
replacement of the metabolically suspect vinyl group with
several five-membered heterocycles did not affect the antino-
ciceptive activity. Moreover, these heterocyclic substituted
pyridazinones proved to have the same mechanism of action
with respect to the 5-vinyl series, as confirmed by results related
to antinociception evaluation after pretreatment with yohimbine
and percent increase of noradrenaline (Table 5).
Finally, for position 2, methyl groups proved to be better than
n-butyl substituents at least in the 5-vinyl series, as demonstrated
by the inactivity of 5a and 5i in comparison with 5f and 5g,
respectively.
In conclusion, with the present work we found that the best
groups around the pyridazinone core are a methyl group at
position 2, a primary or secondary amino group at position 4,
and a five-membered heterocycle, in particular a methylpyrazole,
at position 5. At position 6 a variety of groups like methyl,
cyclohexyl, phenyl, and 4-pyridyl are tolerated. Taking into
account the fact that all the new compounds were administered
by oral route, the obtained results may warrant further develop-
ment of some representative agents to investigate in more depth
the pharmacological profile of the present series.
5-Acetyl-4-amino-2-butyl-6-(4-fluorophenyl)pyridazin-3(2H)-
one 3c. Yield ) 95%; mp ) 143-145 °C (EtOH); 1H NMR
(CDCl3) δ 1.00 (m, 3H,(CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3),
1.75-1.85 (m, 2H, CH2CH2CH2CH3), 2.55 (s, 3H, COCH3), 4.20
(t, 2H, CH2CH2CH2CH3), 6.80 (exch br s, 2H, NH2), 7.25 (m, 2H,
Ar), 7.55 (m, 2H, Ar).
5-Acetyl-4-amino-6-(4-fluorophenyl)-2-methylpyridazin-3(2H)-
1
one 3d. Yield ) 79%; mp ) 215-216 °C, dec (EtOH); H NMR
(CDCl3) δ 1.80 (s, 3H, COCH3), 3.85 (s, 3H, N-CH3), 7.15 (m,
2H, Ar), 7.45 (m, 2H, Ar), 7.65 (exch br s, 2H, NH2).
5-Acetyl-4-amino-6-cyclopentyl-2-methylpyridazin-3(2H)-
one 3e. Yield ) 80%; mp ) 106-107 °C (EtOH); 1H NMR
(CDCl3) δ 1.50-2.00 (m, 8H, cC5H9), 2.60 (s, 3H, COCH3), 3.20-
3.35 (m, 1H, cC5H9), 3.75 (s, 3H, N-CH3), 6.95 (exch br s, 2H,
NH2).
Experimental Section
Chemistry. All melting points were determined on a Bu¨chi
apparatus and are uncorrected. IR spectra were measured as Nujol
mulls with a Perkin-Elmer spectrometer (FT-IR, Spectrum 1000).
1H NMR spectra were recorded with Avance 400 instruments
(Bruker Biospin, version 002 with SGU). Chemical shifts are
reported in ppm, using the solvent as internal standard. Mass spectra
(m/z) were recorded on a LTQ mass spectrometer (ThermoFisher,
San Jose, CA). Extracts were dried over Na2SO4, and the solvents
were removed under reduced pressure. Merck F-254 commercial
plates were used for analytical TLC to follow the course of reaction.
Silica gel 60 (Merck 70-230 mesh) was used for column
chromatography. Reagents and starting material 7 were com-
mercially available.
5-Acetyl-4-amino-2-methyl-6-pyridin-4-ylpyridazin-3(2H)-
one 3f. Yield ) 84%; mp ) 195-198 °C (EtOH); 1H NMR
(CDCl3) δ 1.90 (s, 3H, COCH3), 3.85 (s, 3H, N-CH3), 6.90 (exch
br s, 2H, NH2), 7.55 (d, 2H, Ar), 8.75 (d, 2H, Ar).
5-Acetyl-4-amino-2-methyl-6-thiophen-2-ylpyridazin-3(2H)-
one 3g. Yield ) 82%; mp ) 163-166 °C (EtOH); 1H NMR
(CDCl3) δ 2.00 (s, 3H, COCH3), 3.85 (s, 3H, N-CH3), 5.85 (exch
br s, 2H, NH2), 7.10 (m, 2H, Ar), 7.50 (m, 1H, Ar).
General Procedure for 2a-e. A mixture of isoxazolopyridazi-
nones 1a-d10,20-22 (1a,10 1b,20 1c,21 1d22) (1.2 mmol), K2CO3 (2.4-
3.6 mmol), and the appropriate alkyl halide (2.0-5.5 mmol) in
anhydrous acetone (5 mL) was refluxed under stirring for 2-3 h.
Then the mixture was concentrated in vacuo and diluted with cold
water, and the precipitate was recovered by suction.
6-n-Butyl-3-methyl-4-pyridin-4-ylisoxazolo[3,4-d]pyridazin-
7(6H)-one 2a. Yield ) 69%; mp ) 120-121 °C (EtOH); 1H NMR
(CDCl3) δ 1.00 (t, 3H,(CH2)3CH3), 1.40-1.50 (m, 2H, (CH2)2-
5-Acetyl-4-amino-6-(4-bromophenyl)-2-methylpyridazin-3(2H)-
one 3h. Yield ) 87%; mp ) 203-206 °C (EtOH); 1H NMR
(CDCl3) δ 1.85 (s, 3H, COCH3), 3.85 (s, 3H, N-CH3), 5.90 (exch
br s, 2H, NH2), 7.30-7.50 (m, 4H, Ar).
5-Acetyl-4-amino-2-n-butyl-6-methylpyridazin-3(2H)-one 3i.
Yield ) 79%; mp ) 90-93 °C (EtOH); 1H NMR (CDCl3) δ 0.95
(t, 3H,(CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3), 1.80 (m, 2H,
CH2CH2CH2CH3), 2.55 (s, 3H, COCH3), 2.60 (s, 3H, 6-CH3), 4.10
(t, 2H, CH2CH2CH2CH3), 7.75 (exch br s, 2H, NH2).

3950 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16
GioVannoni et al.
General Procedure for 4a-i. To a stirred solution of the
appropriate 4-amino-5-acetyl derivative 3a-i (0.35 mmol) in MeOH
(3-6 mL), sodium borohydride (4.0-4.5 mmol) was added
portionwise. The reaction mixture was stirred for 40-60 min at
room temperature, concentrated in vacuo, and diluted with cold
water. The final products were recovered by suction.
5.60 (d, 1H, CHdCH2), 5.75 (d, 1H, CHdCH2), 6.30 (dd, 1H, CHd
CH2), 7.50 (d, 2H, Ar), 8.80 (d, 2H, Ar); MS (ESI) m/z 271.15
([M + H]+).
4-Amino-2-n-butyl-6-thiophen-2-yl-5-vinylpyridazin-3(2H)-
one 5b. Yield ) 80%; mp ) 80-83 °C (EtOH); IR (cm-1) 3375
1
and 3400 (NH2), 1635 (CO); H NMR (CDCl3) δ 1.00 (t, 3H,
4-Amino-2-n-butyl-5-(1-hydroxyethyl)-6-pyridin-4-ylpyridazin-
3(2H)-one 4a. Yield ) 87%; mp ) 150-153 °C (EtOH); 1H NMR
(CDCl3) δ 0.95 (t, 3H,(CH2)3CH3), 1.30-1.40 (m, 5H (3H,
CH(OH)CH3; 2H, (CH2)2CH2CH3)), 1.80 (m, 2H, CH2CH2CH2-
CH3), 2.80 (exch br s, 1H, OH), 4.20 (m, 2H, CH2CH2CH2CH3),
4.75 (m, 1H, CH(OH)CH3), 6.00 (exch br s, 2H, NH2), 7.40 (m,
2H, Ar), 8.80 (m, 2H, Ar).
(CH2)3CH3), 1.40-1.50 (m, 2H, (CH2)2CH2CH3), 1.80-1.90 (m,
2H, CH2CH2CH2CH3), 4.20 (m, 2H, CH2CH2CH2CH3), 5.40 (exch
br s, 1H, NH2), 5.65 (d, 1H, CHdCH2), 5.80 (d, 1H, CHdCH2),
6.60 (dd, 1H, CHdCH2), 7.05 (m, 1H, Ar), 7.35 (m, 2H, Ar); MS
(ESI) m/z 276.37 ([M + H]+).
4-Amino-2-n-butyl-6-(4-fluorophenyl)-5-vinylpyridazin-3(2H)-
one 5c. Yield ) 89%; mp ) 83-85 °C (EtOH); IR (cm-1) 3380
1
4-Amino-2-n-butyl-5-(1-hydroxyethyl)-6-thiophen-2-ylpyridazin-
and 3435 (NH2), 1645 (CO); H NMR (CDCl3) δ 1.00 (t, 3H,
1
3(2H)-one 4b. Yield ) 89%; mp ) 84-86 °C (EtOH); H NMR
(CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3), 1.85 (m, 2H, CH2CH2-
CH2CH3), 4.25 (t, 2H, CH2CH2CH2CH3), 5.45 (exch br s, 2H, NH2),
5.50 (d, 1H, CHdCH2), 5.70 (d, 1H, CHdCH2), 6.25 (dd, 1H, CHd
CH2), 7.15 (m, 2H, Ar), 7.45 (m, 2H, Ar); MS (ESI) m/z 288.33
([M + H]+).
(CDCl3) δ 0.95 (t, 3H,(CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3),
1.55 (d, 3H, CH(OH)CH3), 1.80 (m, 2H, CH2CH2CH2CH3), 4.20
(m, 2H, CH2CH2CH2CH3), 4.30 (exch br s, 1H, OH), 5.10 (q, 1H,
CH(OH)CH3), 7.10 (m, 2H, Ar), 7.40 (m, 1H, Ar), 7.65 (exch br
s, 2H, NH2).
4-Amino-6-(4-fluorophenyl)-2-methyl-5-vinylpyridazin-3(2H)-
4-Amino-2-n-butyl-6-(4-fluorophenyl)-5-(1-hydroxyethyl)pyrid-
azin-3(2H)-one 4c. Yield ) 86%; mp ) 119-120 °C (EtOH); 1H
NMR (CDCl3) δ 0.95 (t, 3H,(CH2)3CH3), 1.40-1.55 (m, 5H (3H,
CH3CH(OH); 2H, (CH2)2CH2CH3)), 1.80 (m, 2H, CH2CH2CH2-
CH3), 4.20 (m, 2H, CH2CH2CH2CH3), 4.30 (exch br s, 1H, OH),
4.80 (q, 1H, CH(OH)CH3), 5.95 (exch br s, 2H, NH2), 7.20 (m,
2H, Ar), 7.30 (m, 2H, Ar).
4-Amino-6-(4-fluorophenyl)-5-(1-hydroxyethyl)-2-methyl-
pyridazin-3(2H)-one 4d. Yield ) 66%; mp ) 208-210 °C
(EtOH); 1H NMR (CDCl3) δ 1.45 (d, 3H, CH(OH)CH3), 2.00 (exch
br s, 1H, OH), 3.80 (s, 3H, N-CH3), 4.80 (q, 1H, CH(OH)CH3),
5.95 (exch br s, 2H, NH2), 7.15 (m, 2H, Ar), 7.30 (m, 2H, Ar).
4-Amino-6-cyclopentyl-5-(1-hydroxyethyl)-2-methylpyridazin-
3(2H)-one 4e. Yield ) 70%; mp ) 171-173 °C (cyclohexane);
1H NMR (CDCl3) δ 1.55 (d, 3H, CH(OH)CH3), 1.70-2.00 (m,
8H, cC5H9), 2.90-3.00 (m, 1H, cC5H9), 3.75 (s, 3H, N-CH3), 4.50
(exch br s, 1H, OH), 5.20 (q, 1H, CH(OH)CH3), 5.75 (exch br s,
2H, NH2).
4-Amino-5-(1-hydroxyethyl)-2-methyl-6-pyridin-4-ylpyridazin-
3(2H)-one 4f. Yield ) 60%; mp ) 217-220 °C (EtOH); 1H NMR
(CDCl3) δ 1.50 (d, 3H, CH(OH)CH3), 3.80 (s, 3H, N-CH3), 4.10
(exch br s, 1H, OH), 4.75 (q, 1H, CH(OH)CH3), 6.00 (exch br s,
2H, NH2), 7.30 (d, 2H, Ar), 8.70 (d, 2H, Ar).
4-Amino-5-(1-hydroxyethyl)-2-methyl-6-thiophen-2-ylpyridazin-
3(2H)-one 4g. Yield ) 69%; mp ) 220-221 °C (EtOH); 1H NMR
(CDCl3) δ 1.55 (d, 3H, CH(OH)CH3), 3.85 (s, 3H, N-CH3), 4.85
(exch br s, 1H, OH), 5.10 (m, 1H, CH(OH)CH3), 6.20 (exch br s,
2H, NH2), 7.10 (m, 2H, Ar), 7.40 (m, 1H, Ar).
one 5d. Yield ) 72%; mp ) 74-76 °C (EtOH); IR (cm-1) 3385
1
and 3420 (NH2), 1640 (CO); H NMR (CDCl3) δ 3.85 (s, 3H,
N-CH3), 5.45 (exch br s, 2H, NH2), 5.55 (d, 1H, CHdCH2), 5.65
(d, 1H, CHdCH2), 6.25 (dd, 1H, CHdCH2), 7.05-7.15 (m, 2H,
Ar), 7.40-7.50 (m, 2H, Ar); MS (ESI) m/z 246.25 ([M + H]+).
4-Amino-6-cyclopentyl-2-methyl-5-vinylpyridazin-3(2H)-
one 5e. Yield ) 49%; mp ) 74-75 °C (EtOH); IR (cm-1) 3375
1
and 3450 (NH2), 1620 (CO); H NMR (CDCl3) δ 1.70-1.95 (m,
8H, cC5H9), 2.95-3.10 (m, 1H, cC5H9), 3.75 (s, 3H, N-CH3), 5.15
(exch br s, 2H, NH2), 5.55 (d, 1H, CHdCH2), 5.75 (d, 1H, CHd
CH2), 6.50-6.65 (m, 1H, CHdCH2); MS (ESI) m/z 220.18 ([M +
H]+).
4-Amino-2-methyl-6-pyridin-4-yl-5-vinylpyridazin-3(2H)-
one 5f. Yield ) 86%; mp ) 172-174 °C (EtOH); IR (cm-1) 3370
1
and 3450 (NH2), 1625 (CO); H NMR (CDCl3) δ 3.85 (s, 3H,
N-CH3), 5.60 (d, 1H, CHdCH2), 5.70 (d, 1H, CHdCH2), 5.80
(exch br s, 2H, NH2), 6.30 (dd, 1H, CHdCH2), 7.40 (d, 2H, Ar),
8.70 (d, 2H, Ar); MS (ESI) m/z 229.25 ([M + H]+).
4-Amino-2-methyl-6-thiophen-2-yl-5-vinylpyridazin-3(2H)-
one 5g. Yield ) 65%; mp ) 95-98 °C (EtOH); IR (cm-1) 3390
1
and 3410 (NH2), 1645 (CO); H NMR (CDCl3) δ 3.85 (s, 3H,
N-CH3), 5.40 (exch br s, 2H, NH2), 5.65 (d, 1H, CHdCH2), 5.80
(d, 1H, CHdCH2), 6.60 (dd, 1H, CHdCH2), 7.10 (m, 1H, Ar),
7.30-7.40 (m, 2H, Ar); MS (ESI) m/z 234.29 ([M + H]+).
4-Amino-6-(4-bromophenyl)-2-methyl-5-vinylpyridazin-3(2H)-
one 5h. Yield ) 85%; mp ) 98-101 °C (EtOH); IR (cm-1) 3375
1
and 3470 (NH2), 1635 (CO); H NMR (CDCl3) δ 3.85 (s, 3H,
N-CH3), 5.45 (exch br s, 2H, NH2), 5.60 (m, 2H, CHdCH2), 6.30
(dd, 1H, CHdCH2), 7.30-7.60 (m, 4H, Ar); MS (ESI) m/z 307.16
([M + H]+).
4-Amino-6-(4-bromophenyl)-5-(1-hydroxyethyl)-2-methyl-
pyridazin-3(2H)-one 4h. Yield ) 60%; mp ) 223-226 °C
1
(EtOH); H NMR (CDCl3) δ 1.45 (d, 3H, CH(OH)CH3), 3.80 (s,
4-Amino-2-n-butyl-6-methyl-5-vinylpyridazin-3(2H)-one 5i.
Yield ) 81%; mp ) 40-43 °C (EtOH); IR (cm-1) 3330 and 3430
3H, N-CH3), 4.50 (exch br s, 1H, OH), 4.80 (q, 1H, CH(OH)-
CH3), 5.95 (exch br s, 2H, NH2), 7.35 (d, 2H, Ar), 7.45 (d, 2H,
Ar).
1
(NH2), 1630 (CO); H NMR (CDCl3) δ 0.95 (t, 3H,(CH2)3CH3),
1.35-1.45 (m, 2H, (CH2)2CH2CH3), 1.80 (m, 2H, CH2CH2CH2-
CH3), 2.25 (s, 3H, 6-CH3), 4.10 (t, 2H, CH2CH2CH2CH3), 5.20
(exch br s, 2H, NH2), 5.60 (d, 1H, CHdCH2), 5.70 (d, 1H, CHd
CH2), 6.50 (dd, 1H, CHdCH2); MS (ESI) m/z 208.14 ([M + H]+).
4-Cyclohexanecarbonyl-5-methylisoxazole-3-carboxylic Acid
Ethyl Ester 8. To a cooled (0 C°) and stirred solution of sodium
ethoxide, obtained from sodium (9.15 mmol) and anhydrous ethanol
(30 mL), a solution of 1-cyclohexylbutane-1,3-dione 628 (9.3 mmoL)
in the same solvent (13 mL) was slowly added. After the mixture
was further cooled to -5 °C, a solution of ethyl chloro(hydrox-
imino)acetate 7 (6.2 mmol) in anhydrous EtOH (10.5 mL) was
added dropwise. The mixture was neutralized with 6 N HCl, and
the solvent was evaporated in vacuo. The residue oil was washed
with cold 0.5 N NaOH and cold water, and finally it was extracted
with CH2Cl2 (3 × 20 mL). Evaporation of the solvent afforded
compound 8, which was purified by column chromatography using
4-Amino-2-n-butyl-5-(1-hydroxyethyl)-6-methylpyridazin-
1
3(2H)-one 4i. Yield ) 53%; mp ) 75-77 °C (cyclohexane); H
NMR (CDCl3) δ 0.95 (t, 3H,(CH2)3CH3), 1.35-1.50 (m, 5H (3H,
CH3CH(OH); 2H, (CH2)2CH2CH3)), 1.80 (m, 2H, CH2CH2CH2-
CH3), 2.20 (s, 3H, 6-CH3), 4.10 (t, 2H, CH2CH2CH2CH3), 4.30
(exch br s, 1H, OH), 5.05 (q, 1H, CH(OH)CH3), 6.80 (exch br s,
2H, NH2).
General Procedure for 5a-i. The appropriate derivative 4a-i
(0.35 mmol) was treated with PPA (35 mmol) at 60 °C for 2-6 h.
After treatment with ice-water (20 mL), the mixture was neutral-
ized with 3 N NaOH and compounds 5a-i were recovered by
suction.
4-Amino-2-n-butyl-6-pyridin-4-yl-5-vinylpyridazin-3(2H)-
one 5a. Yield ) 64%; mp ) 76-78 °C (EtOH); IR (cm-1) 3390
1
and 3420 (NH2), 1640 (CO); H NMR (CDCl3) δ 1.00 (t, 3H,
1
(CH2)3CH3), 1.40 (m, 2H, (CH2)2CH2CH3), 1.80 (m, 2H, CH2CH2-
CH2CH3), 4.20 (t, 2H, CH2CH2CH2CH3), 5.55 (exch br s, 2H, NH2),
cyclohexane/ethyl acetate, 9:1, as eluent. Yield ) 28%; oil; H
NMR (CDCl3) δ 1.20-1.90 (m, 10H, cC6H11), 1.45 (t, 3H,

Pyridazinones as AntinociceptiVe Agents
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16 3951
CH2CH3), 2.60 (s, 3H, 5-CH3), 2.85-2.95 (m, 1H, cC6H11), 4.50
(q, 2H, CH2CH3).
(OH)); 2H, (CH2)2CH2CH3), 1.80 (m, 2H, CH2CH2CH2CH3), 2.50
(exch br s, 1H, OH),3.40 (d, 3H, NH-CH3), 4.15 (t, 2H, CH2CH2-
CH2CH3), 4.80 (q, 1H, CH(OH)CH3), 6.20 (exch br s, 1H, NH-
CH3), 7.30-7.45 (m, 5H, Ar).
4-Cyclohexyl-3,6-dimethylisoxazolo[3,4-d]pyridazin-7(6H)-
one 9. To a solution of 8 (0.75 mmol) in EtOH (2 mL),
methylhydrazine (4.2 mmol) was added. The mixture was stirred
at room temperature for 2 h. After dilution with cold water (10-
15 mL), the suspension was extracted with ethyl acetate (3 × 20
mL) and the solvent was evaporated to afford 9, which was purified
by column chromatography using cyclohexane/ethyl acetate, 2:1,
5-(1-Hydroxyethyl)-2,6-dimethyl-4-methylaminopyridazin-
3(2H)-one 15b. Yield ) 84%; oil (purified by column chroma-
1
tography using CHCl3/CH3OH, 9:1, as eluent); H NMR (CDCl3)
δ 1.50 (d, 3H, CH(OH)CH3), 2.25 (s, 3H, 6-CH3), 3.25 (d, 3H,
NH-CH3), 3.65 (s, 3H, N-CH3), 4.60 (exch br s, 1H, OH), 5.10
(q, 1H, CH(OH)CH3), 6.10 (exch br s, 1H, NH-CH3).
4-Ethylamino-5-(1-hydroxyethyl)-2-methyl-6-phenylpyridazin-
3(2H)-one 15c. Yield ) 77%; mp ) 137-138 °C (cyclohexane);
1H NMR (CDCl3) δ 1.30 (t, 3H, CH2CH3), 1.45 (d, 3H, CH(OH)-
CH3), 3.75 (s, 3H, N-CH3), 3.75-3.85 (m, 2H, CH2CH3), 3.90
(exch br s, 1H, OH), 4.80 (q, 1H, CH(OH)CH3), 5.70 (exch br s,
1H, NH-CH2CH3),7.30-7.50 (m, 5H, Ar).
4-Ethylamino-5-(1-hydroxyethyl)-2,6-dimethylpyridazin-3(2H)-
one 15d. Yield ) 53%; mp ) 102-103 °C (cyclohexane); 1H NMR
(CDCl3) δ 1.25 (t, 3H, CH2CH3), 1.55 (d, 3H, CH(OH)CH3), 2.25
(s, 3H, 6-CH3), 3.65-3.75 (m, 5H (3H, N-CH3; 2H, CH2CH3)),
4.10 (exch br s, 1H, OH), 5.10 (q, 1H, CH(OH)CH3,), 5.70 (exch
br s, 1H, NH-CH2CH3).
General Procedure for 16a-d. Compounds 16a-d were
obtained starting from 15a-d following the general procedure
described for 5a-i. For compounds 16b-d, after dilution with
water the mixture was neutralized with 3 N NaOH and extracted
with CH2Cl2 (3 × 15 mL) and the solvent was evaporated in vacuo.
Compounds 16c,d were purified by column chromatography, using
cycloexane/ethyl acetate, 2:1, as eluent for compound 16c and using
cycloexane/ethyl acetate, 1:1, for compound 16d.
1
as eluent. Yield ) 24%; mp ) 166-167 °C (EtOH); H NMR
(CDCl3) δ 1.25-2.00 (m, 10H, cC6H11), 2.70-2.80 (m, 1H,
cC6H11), 2.85 (s, 3H, 3-CH3), 3.75 (s, 3H, N-CH3).
5-Acetyl-4-amino-6-cyclohexyl-2-methylpyridazin-3(2H)-
one 10. Compound 10 was obtained from compound 9 following
the general procedure described for 3a-i. Yield ) 54%; mp )
1
133-134 °C (EtOH); H NMR (CDCl3) δ 1.25-1.90 (m, 10H,
cC6H11), 2.55 (s, 3H, COCH3), 2.75-2.85 (m, 1H, cC6H11), 3.75
(s, 3H, N-CH3), 7.00 (exch br s, 2H, NH2).
4-Amino-6-cyclohexyl-5-(1-hydroxyethyl)-2-methylpyridazin-
3(2H)-one 11. Compound 11 was obtained starting from 10
following the general procedure described for 4a-i. Yield ) 34%;
mp ) 203-204 °C (cyclohexane); 1H NMR (CDCl3) δ 1.25-1.90
(m, 13H (10H, cC6H11; 3H, CH(OH)CH3)), 2.40-2.50 (m, 1H,
cC6H11), 3.75 (s, 3H, N-CH3), 5.15 (q, 1H, CH(OH)CH3), 5.80
(exch br s, 2H, NH2).
4-Amino-6-cyclohexyl-2-methyl-5-vinylpyridazin-3(2H)-one 12.
Compound 12 was obtained starting from 11 following the general
procedure described for 5a-i. Yield ) 65%; mp ) 118-119 °C
1
(EtOH); IR (cm-1) 3385 and 3430 (NH2), 1640 (CO); H NMR
(CDCl3) δ 1.20-1.90 (m, 10H, cC6H11), 2.45-2.60 (m, 1H,
cC6H11), 3.75 (s, 3H, N-CH3), 5.15 (exch br s, 2H, NH2), 5.55 (d,
1H, CHdCH2), 5.75 (d, 1H, CHdCH2), 6.50 (dd, 1H, CHdCH2);
MS (ESI) m/z 234.18 ([M + H]+).
2-Butyl-4-methylamino-6-phenyl-5-vinylpyridazin-3(2H)-
one 16a. Yield ) 67%; mp ) 65-67 °C (EtOH); IR (cm-1) 3290
1
(NH), 1640 (CO); H NMR (CDCl3) δ 1.00 (t, 3H,(CH2)3CH3),
General Procedure for Compounds 14a-d. A mixture of the
required 5-acetyl-2-alkyl-4-nitropyridazinone 13a-c29,30 (0.3 mmol)
and the appropriate amine (1.5-1.8 mmol) in EtOH (3-4 mL) was
stirred at room temperature for 30-120 min. Then the reaction
mixture was concentrated in vacuo and addition of cold water (10
mL) furnished compounds 14a,b, which were recovered by suction.
For compounds 14c,d the suspension was extracted with CH2Cl2
(3 × 15 mL) and evaporation of the solvent afforded a crude
precipitate for 14c and an oil for 14d. This last one was purified
by column chromatography using cyclohexane/ethyl acetate/
methanol, 1:2:0.1, as eluent.
1.40 (m, 2H, (CH2)2CH2CH3), 1.85 (m, 2H, CH2CH2CH2CH3), 2.95
(d, 3H, NH-CH3), 4.20 (t, 2H, CH2CH2CH2CH3), 5.00 (d, 1H,
CHdCH2), 5.45 (d, 1H, CHdCH2), 6.10 (exch br s, 1H, NH-
CH3), 6.50 (dd, 1H, CHdCH2), 7.40 (s, 5H, Ar); MS (ESI) m/z
284.17 ([M + H]+).
2,6-Dimethyl-4-methylamino-5-vinylpyridazin-3(2H)-one 16b.
Yield ) 88%; mp ) 88-91 °C (EtOH); IR (cm-1) 3310 (NH),
1620 (CO); 1H NMR (CDCl3) δ 2.20 (s, 3H, 6-CH3), 2.95 (d, 3H,
NH-CH3), 3.75 (s, 3H, N-CH3), 5.20 (d, 1H, CHdCH2), 5.65 (d,
1H, CHdCH2), 5.95 (exch br s, 1H, NH-CH3), 6.65 (dd, 1H, CHd
CH2); MS (ESI) m/z 179.22 ([M + H]+).
5-Acetyl-2-n-butyl-4-methylamino-6-phenylpyridazin-3(2H)-
4-Ethylamino-2-methyl-6-phenyl-5-vinylpyridazin-3(2H)-
one 16c. Yield ) 11%; mp ) 74-76 °C (EtOH); IR (cm-1) 3300
(NH), 1630 (CO); 1H NMR (CDCl3) δ 1.25 (t, 3H,CH2CH3), 3.30
(m, 2H, CH2CH3), 3.85 (s, 3H, N-CH3), 5.00 (d, 1H, CHdCH2),
5.45 (d, 1H, CHdCH2), 6.20 (exch br s, 1H, NH-CH3), 6.50 (dd,
1H, CHdCH2), 7.40 (s, 5H, Ar); MS (ESI) m/z 256.18 ([M + H]+).
4-Ethylamino-2,6-dimethyl-5-vinylpyridazin-3(2H)-one 16d.
Yield ) 40%; mp ) 65-66 °C (cyclohexane); IR (cm-1) 3295
(NH), 1635 (CO); 1H NMR (CDCl3) δ 1.20 (t, 3H,CH2CH3), 2.20
(s, 3H, 6-CH3), 3.30 (m, 2H, CH2CH3), 3.75 (s, 3H, N-CH3), 5.25
(d, 1H, CHdCH2), 5.62 (d, 1H, CHdCH2), 5.80 (exch br s, 1H,
NH-CH3), 6.60 (dd, 1H, CHdCH2); MS (ESI) m/z 194.12 ([M +
H]+).
1
one 14a. Yield ) 45%; mp ) 105 °C (EtOH); H NMR (CDCl3)
δ 0.95 (t, 3H,(CH2)3CH3), 1.45 (m, 2H, (CH2)2CH2CH3), 1.75-
1.90 (m, 5H (3H, COCH3; 2H, CH2CH2CH2CH3)), 2.95 (d, 3H,
NH-CH3), 4.15 (t, 2H, CH2CH2CH2CH3), 6.25 (exch br m, 1H,
NH-CH3), 7.50 (m, 5H, Ar).
5-Acetyl-2,6-dimethyl-4-methylaminopyridazin-3(2H)-one 14b.
Yield ) 57%; mp ) 99-102 °C (EtOH); 1H NMR (CDCl3) δ 2.20
(s, 3H, COCH3), 2.50 (s, 3H, 6-CH3), 2.90 (d, 3H, NH-CH3), 3.75
(s, 3H, N-CH3), 5.50 (exch br m, 1H, NH-CH3).
5-Acetyl-4-ethylamino-2-methyl-6-phenylpyridazin-3(2H)-
1
one 14c. Yield ) 71%; mp ) 136-138 °C (EtOH); H NMR
(CDCl3) δ 1.30 (t, 3H, CH2CH3), 1.90 (s, 3H, COCH3), 3.35 (m,
2H, CH2CH3), 3.80 (s, 3H, N-CH3), 4.85 (exch br m, 1H, NH-
CH2CH3), 7.45 (s, 5H, Ar).
General Procedure for 18a,b. A suspension of 17a,b29 (1.1
mmol) in N,N-dimethylformammide dimethyl acetal (3 mL, 22
mmol) was stirred at 100-110 °C for 3-4 h. After the mixture
was cooled, the precipitate was recovered by suction.
5-Acetyl-4-ethylamino-2,6-dimethylpyridazin-3(2H)-one 14d.
1
Yield ) 35%; mp ) 85-86 °C (cyclohexane); H NMR (CDCl3)
δ 1.30 (t, 3H, CH2CH3), 2.20 (s, 3H, COCH3), 2.50 (s, 3H, 6-CH3),
3.15 (m, 2H, CH2CH3), 3.75 (s, 3H, N-CH3), 6.15 (exch br s, 1H,
NH-CH2CH3).
3-(2-Dimethylaminovinyl)-6-methyl-4-phenyisoxazolo[3,4-d]-
pyridazin-7(6H)-one 18a. Yield ) 73%; mp ) 203-204 °C
1
(EtOH); H NMR (CDCl3) δ 2.80 (s, 6H, N(CH3)2, 3.80 (s, 3H,
General Procedure for 15a-d. Compounds 15a-d were
obtained starting from 14a-d following the same general procedure
described for 4a-i. 15c was obtained after extraction with ethyl
acetate (3 × 20 mL) and evaporation in vacuo.
N-CH3), 4.75 (d, 1H, CHdCH-N), 7.40-7.60 (m, 6H (5H, Ar;
1H, CHdCH-N)).
3-(2-Dimethylaminovinyl)-4,6-dimethylisoxazolo[3,4-d]pyrid-
azin-7(6H)-one 18b. Yield ) 85%; mp ) 197-198 °C (EtOH);
1H NMR (CDCl3) δ 2.40 (s, 3H, CH3), 3.05 (s, 6H, N(CH3)2), 3.65
(s, 3H, N-CH3), 5.15 (d, 1H, CHdCH-N), 7.50 (d, 1H, CHd
CH-N).
2-n-Butyl-5-(1-hydroxyethyl)-4-methylamino-6-phenylpyrid-
azin-3(2H)-one 15a. Yield ) 79%; mp ) 102 °C (EtOH); 1H NMR
(CDCl3) δ 0.95 (t, 3H,(CH2)3CH3), 1.30-1.50 (m, 5H (3H, CH3CH-

3952 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16
GioVannoni et al.
General Procedure for 19a,b. A suspension of compounds
18a,b (0.9 mmol) in ethanol (2-3 mL) and hydrazine hydrate (15-
20 mmol) was stirred at 80-90 °C for 1-2 h. Then the mixture
was concentrated in vacuo and cooled for 3-4 h and the precipitate
was recovered by suction.
4-Amino-2-methyl-6-phenyl-5-(1H-pyrazol-3-yl)pyridazin-
3(2H)-one 19a. Yield ) 54%; mp ) 267-268 °C (cyclohexane);
IR (cm-1) 3385 and 3430 (NH2), 3290 (NH), 1640 (CO); 1H NMR
(CDCl3) δ 3.70 (s, 3H, N-CH3), 5.15 (d, 1H, Ar), 6.40 (exch br s,
2H, NH2), 7.10-7.40 (m, 5H, Ar), 7.60 (d, 1H, Ar), 13.05 (exch
br s, 1H, NH); MS (ESI) m/z 268.18 ([M + H]+).
4-Amino-2,6-dimethyl-5-(1H-pyrazol-3-yl)pyridazin-3(2H)-
one 19b. Yield ) 73%; mp ) 250-253 °C, dec (EtOH); 1H NMR
(CDCl3) δ 2.40 (s, 3H, 6-CH3), 3.75 (s, 3H, N-CH3), 6.55 (d, 1H,
Ar), 7.35 (exch br s, 2H, NH2), 7.85 (d, 1H, Ar), 11.05 (exch br s,
1H, NH).
(0.34 mmol) in EtOH (3 mL) was heated at 80 °C for 1 h. After
the mixture was cooled, water (10-15 mL) was added and the final
product was recovered by suction. Yield ) 21%; mp ) 129-130
°C (cyclohexane); IR (cm-1) 3385 and 3430 (NH2), 1640 (CO);
1H NMR (CDCl3) 2.75 (s, 3H, 2′-C-CH3), 3.85 (s, 3H, N-CH3),
6.20 (s, 1H, Ar), 6.70 (exch br s, 2H, NH2), 7.25-7.40 (m, 5H,
Ar); MS (ESI) m/z 299.18 ([M + H]+).
5-Amino-1-methyl-6-oxo-3-phenyl-1,6-dihydropyridazine-4-
carboxylic Acid N′-Acetylhydrazide 27. A mixture of compound
2611 (0.4 mmol) in SOCl2 (1.5 mL) was refluxed for 1 h. The excess
of SOCl2 was removed in vacuo, and then acetohydrazide (0.5
mmol) was added to the residue oil dissolved in cold anhydrous
dioxane. The mixture was stirred at room temperature for 3 h and
diluted with cold water, and the precipitate was recovered by
suction. Yield ) 40%; mp ) 264-265 °C (EtOH); 1H NMR
(CDCl3) 1.90 (s, 3H, COCH3), 3.70 (s, 3H, N-CH3), 7.15 (exch
br s, 2H, NH2), 7.35-7.50 (m, 5H, Ar), 10.25 (exch br s, 1H, NH),
10.35 (exch br s, 1H, NH).
General Procedure for 20a,b. A mixture of compounds 19a,b
(0.24 mmol), K2CO3 (0.48 mmol), and CH3I (0.5-1 mmol) in
anhydrous DMF (1.5-2.5 mL) was stirred at 90-100 °C for 3 h.
After cooling, the suspension was diluted with cold water (10-15
mL) and extracted with ethyl acetate (3 × 15 mL) for compound
20a and with CH2Cl2 (3 × 15 mL) for compound 20b. Evaporation
of the solvent afforded compounds 20a,b, which were purified by
column chromatography using CHCl3/MeOH, 9:1, as eluent.
4-Amino-2-methyl-5-(1-methyl-1H-pyrazol-3-yl)-6-phenylpyrid-
azin-3(2H)-one 20a. Yield ) 60%; mp ) 151-153 °C (EtOH);
4-Amino-2-methyl-5-(5-methyl-[1,3,4]oxadiazol-2-yl)-6-phenyl-
pyridazin-3(2H)-one 28. A suspension of 27 (0.23 mol) in POCl3
(2 mL) was stirred at 60 °C for 2 h. After cooling, the mixture was
slowly poured into ice-water and then extracted with CH2Cl2 (3
× 20 mL). Evaporation of the solvent afforded 28. Yield ) 31%;
mp ) 183-184 °C (EtOH); IR (cm-1) 3385 and 3430 (NH2), 1640
1
(CO); H NMR (CDCl3) 2.25 (s, 3H, 5′-C-CH3), 3.90 (s, 3H,
N-CH3), 7.20 (exch br s, 2H, NH2), 7.35-7.50 (m, 5H, Ar); MS
(ESI) m/z 284.29 ([M + H]+).
1
IR (cm-1) 3380 and 3410 (NH2), 1630 (CO); H NMR (CDCl3)
Biological Assays. Animals. Male Swiss albino mice (23-30
g) from the Morini breeding farm (Italy) and male Sprague-Dawley
(200-250) rats from Harlan Laboratories (Italy) were used. Fifteen
mice and four rats were housed per cage. The cages were placed
in the experimental room 24 h before the test for acclimatization.
The animals were kept at 23 ( 1 °C with a 12 h light/dark cycle,
light at 7 a.m., with food and water ad libitum. All experiments
were carried out according to the guidelines of the European
Community Council.
3.85 (s, 3H, N-CH3), 3.95 (s, 3H, 2′-N-CH3), 5.20 (d, 1H, Ar),
7.10 (d, 1H, Ar), 7.35 (s, 5H, Ar), 7.55 (exch br s, 2H, NH2); MS
(ESI) m/z 282.18 ([M + H]+).
4-Amino-2,6-dimethyl-5-(1-methyl-1H-pyrazol-3-yl)pyridazin-
3(2H)-one 20b. Yield ) 30%; mp ) 167-169 °C (EtOH); IR
1
(cm-1) 3370 and 3435 (NH2), 1645 (CO); H NMR (CDCl3) 2.35
(s, 3H, 6-CH3), 3.75 (s, 3H, N-CH3), 3.95 (s, 3H, 2′-N-CH3),
6.40 (d, 1H, Ar), 7.45 (d, 1H, Ar), 7.80 (exch br s, 2H, NH2); MS
(ESI) m/z 220.11 ([M + H]+).
Abdominal Constriction Test. Mice were injected ip with a
0.6% solution of acetic acid (10 mL kg-1), according to the
procedure of Koster et al.32 The number of stretching movements
was counted for 10 min, starting 5 min after acetic acid injection.
Statistical Analysis. All experimental results are given as the
mean ( SEM. Analysis of variance (ANOVA), followed by Fisher’s
protected least significant difference (PLSD) procedure for post
hoc comparison, was used to verify the significance between two
means. Data were analyzed with the StatView software for the
Macintosh (1992). P values of less than 0.05 were considered
significant.
Drugs. The following drug was used: yohimbine hydrochloride
(Tocris). Other chemicals were of the highest quality commercially
available. Yohimbine was dissolved in isotonic (NaCl, 0.9%) saline
solution, and tested compounds were dispersed in sodium car-
boxymethylcellulose, 1%.
4-Ethylamino-2-methyl-5-(1-methyl-1H-pyrazol-3-yl)-6-phenyl-
pyridazin-3(2H)-one 21. A mixture of 20a (0.35 mmol), bromo-
ethane (0.7 mmol), and NaH (60% dispersion in mineral oil) (3.3
mmol) in anhydrous DMSO (1.5 mL) was stirred at 80 °C for 90
min. After dilution with cold water (10-15 mL), the suspension
was extracted with CH2Cl2 (3 × 20 mL) and the solvent was
evaporated in vacuo to afford 21, which was purified by column
chromatography using cyclohexane/ethyl acetate, 1:2, as eluent.
1
Yield ) 38%; oil; IR (cm-1) 3300 (NH), 1640 (CO); H NMR
(CDCl3) 1.00 (t, 3H, CH3CH2-N), 2.95 (q, 2H, CH3CH2-N), 3.90
(m, 6H (3H, N-CH3; 3H, 2′-N-CH3)), 5.85 (d, 1H, Ar), 7.10-
7.35 (m, 5H, Ar), 7.40 (d, 1H, Ar); MS (ESI) m/z 310.36 ([M +
H]+).
4-Diethylamino-2-methyl-5-(1-methyl-1H-pyrazol-3-yl)-6-phenyl-
pyridazin-3(2H)-one 22. Compound 22 was obtained from com-
pound 20a following the general procedure described for 21. The
reaction was carried out using a 1:10 molar ratio of substrate 20a
to bromoethane. Compound 22 was purified by column chroma-
tography (cyclohexane/ethyl acetate, 1:3). Yield ) 63%; mp ) 79-
81 °C (cyclohexane); IR (cm-1) 1630 (CO); 1H NMR (CDCl3) 1.00
(m, 6H, (CH3CH2)2-N), 3.10 (m, 4H, (CH3CH2)2-N), 3.80 (s, 3H,
N-CH3), 3.90 (s, 3H, 2′-N-CH3), 5.90 (d, 1H, Ar), 7.10-7.25
(m, 5H, Ar), 7.30 (d, 1H, Ar); MS (ESI) m/z 338.44 ([M + H]+).
4-Amino-5-(2-bromoacetyl)-2-methyl-6-phenylpyridazin-3(2H)-
one 24. To a stirred and heated (40 °C) suspension of 2331 (1.05
mmol) and a catalytic amount of HBr in AcOH (3.5 mL), a solution
of Br2 (1.05 mmol) in acetic acid (1.9 mL) was slowly added. The
mixture was heated for 4 h at 50 °C. After dilution with cold water
(20-30 mL) the precipitate was recovered by suction and purified
by column chromatography using cyclohexane/ethyl acetate, 1:1,
Noradrenaline Release from Rat Cerebral Cortex Evaluated
by Microdialysis Technique. Experiments were performed fol-
lowing the methods described by Oropeza et al.33
Rats were anesthetized with 8% chloral hydrate (400 mg/kg) and
placed in a stereotaxic apparatus with the skull flat. A small burr
hole was made in the skull centered at 3.2 mm anterior, and ( 0.9
mM MgCl2 and 4 mM KCl were continuously perfused through
the probe by a microliter infusion pump. Approximately 18 h
following surgery, dialysate samples were collected every 20 min.
Dialisate samples were stored at -80 °C for subsequent analysis
by HPLC-ED. Vertical concentric microdialysis probes were used.
The amount of NE in the dialysate samples was determined with
HPLC-ED. Dialysate samples were injected into the HPLC system.
The detection system consisted of an ESA Coulochem II electro-
chemical detector. The baseline value against which drug admin-
istration was compared to was derived from the average of three
samples just prior to manipulation. The neurochemical data were
expressed as the mean ( SEM. All statistics were performed using
StatView software.
1
as eluent. Yield ) 44%; mp ) 191-193 °C (EtOH); H NMR
(CDCl3) 3.50 (s, 2H, CH2Br), 3.85 (s, 3H, N-CH3), 6.80 (exch br
s, 2H, NH2), 7.50 (s, 5H, Ar).
4-Amino-2-methyl-5-(2-methylthiazol-4-yl)-6-phenylpyridazin-
3(2H)-one 25. A suspension of 24 (0.34 mmol) and thioacetamide

Pyridazinones as AntinociceptiVe Agents
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 16 3953
(19) Mozziconacci, J. C.; Arnoult, E.; Baurin, N.; Marot, C.; Morin-Allory,
L. Preparation of a Molecular Database from a Set of 2 Million
Compounds for Virtual Screening Applications: Gathering, Structural
Analysis and Filtering. Presented at the 9th Electronic Computa-
tional Chemistry Conference (ECCC9), March 2003; http://
www.univ-orleans.fr/icoa/eposter/eccc9/ECCC9i.htm.
(20) Dal Piaz, V.; Graziano, N.; Haider, N.; Holzer, W. Synthesis and
Complete 1H, 13C and 15N NMR Assignment of Substituted Isox-
azolo[3,4-d]pyridazin-7(6H)-ones. Magn. Reson. Chem. 2005, 43,
240-245.
(21) Dal Piaz, V.; Giovannoni, M. P.; Vergelli, C.; Aguilar, I. N.
Preparation of Pyridazin-3(2H)-ones as Phosphodiesterase 4 (PDE4)
Inhibitors. PTC Int. Appl. WO 2003097613, 2003; 145 pp.
(22) Giovannoni, M. P.; Vergelli, C.; Biancalani, C.; Cesari, N.; Graziano,
A.; Biagini, P.; Gracia, J.; Gavalda`, A.; Dal Piaz, V. Novel
Pyrazolopyrimidopyridazinones with Potent and Selective Phos-
phodiesterase 5 (PDE5) Inhibitory Activity as Potential Agents for
Treatment of Erectile Dysfunction. J. Med. Chem. 2006, 49, 5363-
5371.
(23) Dal Piaz, V.; Rascon, A.; Dubra, M. E.; Giovannoni, M. P.; Vergelli,
C.; Castellana, C. Isoxazolo[3,4-d]pyridazinones and Analogues as
Leishmania Mexicana PDE Inhibitors. Farmaco 2002, 57,
89-96.
(24) Dal Piaz, V.; Ciciani, G.; Giovannoni, M. P. Synthesis of 4,5-
Functionalized-2-methyl-6-(substituted aryl)-3(2H)-pyridazinones: A
New Group of Potent Platelet Aggregation Inhibitors. Farmaco 1997,
52, 173-178.
(25) Galeotti, N.; Ghelardini, C.; Vinci, M. C.; Bartolini, A. Role of
Potassium Channels in the Antinociception Induced by Agonists of
R2-Adrenoceptors. Br. J. Pharmacol. 1999, 126, 1214-1220.
(26) Ghelardini, C.; Galeotti, N.; Bartolini, A. Antinociception Induced
by Amitriptyline and Imipramine Is Mediated by R2-Adrenoceptors.
Jpn. J. Pharmacol. 2000, 82, 130-137.
(27) Millan, M. J.; Bervoets, K.; Rivet, J. M.; Widdowson, P.; Renouard,
A.; Le Marouille-Girardon, S.; Gobert, A. Multiple alpha-2 Adren-
ergic Receptor Subtypes. Evidence for a Role of Rat Ralpha-2A
Adrenergic Receptors in the Control of Nociception, Motor Behaviour
and Hippocampal Synthesis of Noradrenaline. J. Pharmacol. Exp.
Ther. 1994, 270, 958-972.
(28) Fargeas, V.; Baalouch, M.; Metay, E.; Baffreau, J.; Menard, D.;
Gosselin, P.; Berge, J. P.; Barthomeuf, C.; Lebreton, J. New Access
to 1,3-Diketones from Aldehydes. Tetrahedron 2004, 60, 10359-
10364.
Supporting Information Available: Elemental analysis results
for all target compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Gureje, O.; Von Korff, M.; Simon, G. E.; Gater, R. Persistent Pain
and Well-Being: A World Health Organization Study in Primary
care. JAMA, J. Am. Med. Assoc. 1998, 280, 147-151.
(2) Katz, N. The Impact of Pain Management on Quality of Life. J. Pain
Symptom Manage. 2002, 24, 38-47.
(3) Gordon, D. B.; Love, G. Pharmacologic Management of Neuropathic
Pain. Pain Manag. Nursing 2004, 5, 19-33.
(4) Russell, K.; Portenoy, M. D. Current Pharmacotherapy of Chronic
Pain. J. Pain Symptom Manage. 2000, 19, S16-S20.
(5) Allison, M. C.; Howatson, A. G.; Torrance, C. J.; Lee, F. D.; Russel,
R. I. Gastrointestinal Damage Associated with the Use of Non-
steroidal Antiinflammatory Drugs. N. Engl. J. Med. 1992, 327, 749-
754.
(6) Walsh, T. D. Prevention of Opioid Side Effects. J. Pain Symptom
Manage. 1990, 5, 362-367.
(7) Dickenson, A. Mechanisms of Central Hypersensitivity: Excitatory
Amino Acid Mechanisms and Their Control. In The Pharmacology
of Pain. Handbook Experimental Pharmacology; Dickenson, A. H.,
Besson, J. M., Eds.; Springer-Verlag: Berlin, 1997; pp 167-210.
(8) Ghelardini, C.; Galeotti, N.; Gualtieri, F.; Bellucci, C.; Manetti, D.;
Borea, P. A; Bartolini, A. Antinociceptive Property of the Nicotinic
Agonist AG-4 in Rodens. Drug DeV. Res. 1997, 41, 1-9.
(9) Collins, S. D.; Chessell, I. P. Emerging Therapies for Neuropathic
Pain. Expert Opin. Emerging Drugs 2005, 10, 95-108.
(10) Dal Piaz, V.; Ciciani, G.; Turco, G.; Giovannoni, M. P.; Miceli, M.;
Pirisino, R.; Perretti, M. 5-Acyl-6-aryl-4-nitro-3(2H)-pyridazinones
and Related 4-Amino Compounds: Synthesis and Pharmacological
Evaluation. J. Pharm. Sci. 1991, 80, 341-348.
(11) Dal Piaz, V.; Giovannoni, M. P.; Ciciani, G.; Barlocco, D.; Giardina,
G.; Petrone, G.; Clarke, G. D. 4,5-Functionalized 6-Phenyl-3(2H)-
pyridazinones: Synthesis and Evaluation of Antinociceptive Activity.
Eur. J. Med. Chem. 1996, 31, 65-70.
(12) Pieretti, S.; Dal Piaz, V.; Matucci, R.; Giovannoni, M. P.; Galli, A.
Antinociceptive Activity of 3(2H)-pyridazinone Derivative in Mice.
Life Sci. 1999, 65, 1381-1394.
(13) Dal Piaz, V.; Pieretti, S.; Vergelli, C.; Castellana, M. C.; Giovannoni,
M. P. 4-Amino-5-vinyl-3-(2H)-pyridazinones and Related Com-
pounds: Synthesis and Evaluation of Antinociceptive Activity. J.
Heterocycl. Chem. 2002, 39, 523-533.
(14) Dal Piaz, V.; Vergelli, C.; Giovannoni, M. P.; Scheideler, M. A.;
Petrone, G.; Zaratin, P. 4-Amino-5-vinyl-3(2H)-pyridazinones Bear-
ing Arylpiperazinylalkyl Groups and Related Compounds: Synthesis
and Antinociceptive Activity. Farmaco 2003, 58, 1063-1071.
(15) Giovannoni, M. P.; Vergelli, C.; Ghelardini, C.; Galeotti, N.; Bartolini,
A.; Dal Piaz, V. [(3-Chlorophenyl)piperazinylpropyl]pyridazinones
and Analogues as Potent Antinociceptive Agents. J. Med. Chem.
2003, 46, 1055-1059.
(16) Cesari, N.; Biancalani, C.; Vergelli, C.; Dal Piaz, V.; Graziano, A.;
Biagini, P.; Ghelardini, C.; Galeotti, N.; Giovannoni, M. P. Aryl-
piperazinylalkylpyridazinones and Analogues as Potent and Orally
Active Antinociceptive Agents: Synthesis and Studies on Mechanism
of Action. J. Med. Chem. 2006, 49, 7826-7835.
(17) Hoyer, P. B.; Devine, P. J.; Hu, X.; Thompson, K. E.; Sipes, I. G.
Ovarian Toxicity of 4-Vinylcyclohexene Diepoxide: A Mechanistic
Model. Toxicol. Pathol. 2001, 29, 91-99.
(29) Dal Piaz, V.; Ciciani, G.; Turco, G. Oxidative Cleavage of 7-Oxo-
6,7-dihydroisoxazolo[3,4-d]pyridazines by Cerium(IV) Diammonium
Nitrate. A Synthetic Approach to New 5-Acyl-4-nitro-3-oxo-2,3-
dihydropyridazines. Synthesis 1989, 213-214.
(30) Dal Piaz, V.; Ciciani, G.; Giovannoni, M. P.; Franconi, F. Synthesis
and Evaluation as Platelet Aggregation Inhibitors of 6-Phenyl-2,4-
substituted-3(2H)-pyridazinones and Their Rigid Analogues Benzo-
[h]cinnolin-3,5-diones. Drug Des. DiscoVery 1996, 14, 53-75.
(31) Dal Piaz, V.; Ciciani, G.; Chimichi, S. Pyrazolo[1′,5′:1,6]pyrimido-
[4,5-d]pyridazin-7(8H)-one: A New Heterocyclic Ring System from
Isoxazolopyridazinones. Heterocycles 1986, 24, 3143-3148.
(32) Koster, R.; Anderson, M.; De Beer, E. J. Acetic Acid for Analgesic
Screening. Fed. Proc. 1959, 18, 412.
(33) Oropeza, V. C.; Page, M. E.; Van, Bockstaele, E. J. Systemic
Administration of WIN 55,212-2 Increases Norepinephrine Release
in the Rat Frontal Cortex. Brain Res. 2005, 1046, 45-54.
(18) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J.
Experimental and Computational Approaches To Estimate Solubility
and Permeability in Drug Discovery and Development Settings. AdV.
Drug DeliVery ReV. 2001, 46, 3-26.
JM070161E