Long-acting fentanyl analogues: synthesis and pharmacology of N-(1-phenylpyrazolyl)-N-(1-phenylalkyl-4-piperidyl)propanamides.

Article abstract of DOI:10.1016/S0968-0896(01)00345-5

The synthesis of new fentanyl analogues in which the benzene ring of the propioanilido group has been replaced by phenylpyrazole is described. Antinociceptive activity was evaluated using the writhing and hot plate tests in mice. Two compounds, and, showe

Full text of DOI:10.1016/S0968-0896(01)00345-5

Bioorganic & Medicinal Chemistry 10 (2002) 817–827
Long-Acting Fentanyl Analogues: Synthesis and Pharmacology of
N-(1-Phenylpyrazolyl)-N-(1-phenylalkyl-4-piperidyl)propanamides
Nadine Jagerovic,a Carolina Cano,a Jose Elguero,a Pilar Goya,a,* Luis F. Callado,b
J. Javier Meana,b Rocıo Giron,c Raquel Abalo,c David Ruiz,c Carlos Goicoecheac
and M.a Isabel Martınc
a
´
Instituto de Quımica Medica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain
´
Departamento de Farmacologıa, Universidad del Pais Vasco/Euskal Herriko Unibertsitatea, Leioa, E-48940 Bizkaia, Spain
b
´
c
´
Facultad de Ciencias de la Salud. Area de Farmacologıa. Universidad Rey Juan Carlos.
Avda. de Atenas, s/n. E-28922 Madrid, Spain
Received 6 July 2001; accepted 27 September 2001
Abstract—The synthesis of new fentanyl analogues in which the benzene ring of the propioanilido group has been replaced by
phenylpyrazole is described. Antinociceptive activity was evaluated using the writhing and hot plate tests in mice. Two compounds,
3b and 3d, showed interesting analgesic properties, being more potent than morphine and less than fentanyl but with longer dura-
tion of action. These compounds inhibited the electrically evoked muscle contraction of guinea pig ileum and mouse vas deferens
but not that of rabbit vas deferens and the effects could be reversed by antagonists (naloxone and/or CTOP), thus indicating that
the compounds acted as m agonists. Finally, the binding data confirmed that the compounds had high affinity and selectivity for the
m receptor. # 2002 Elsevier Science Ltd. All rights reserved.
Introduction
important challenge in medicinal chemistry. Among the
new analgesic drugs on the market, gabapentin used for
Pain is the most common reason that patients seek
advice from pharmacists and other health professionals
and represents important medical and economic costs
for the community. Current analgesic therapy can be
divided in three large groups: NSAIDS (non-steroidal
antiinflammatory agents), opioids, and the so-called
analgesic adjuvants which include antidepressants and
local anesthetics. Despite their proven efficacy in alle-
viating symptoms and providing pain relief, all have
considerable side effects including gastrointestinal and
renal damage for the first, and respiratory depression,
emesis, and tolerance and/or addiction for opioids.1
This together with the fact that many pain sufferers are
not satisfied with their pain care, makes the search for
new analgesics, that can more effectively treat pain
either chronic, neuropathic or from any other origin an
neuropathic pain and tramadol, with a dual mode of
action (opioid and noradrenaline uptake inhibitor) are
worth mentioning.2
In what opioids are concerned,3 and despite the con-
siderable research effort of the past two decades, the
only new opioid analgesics either on the market, or in
clinical development are mainly alternative dosage
forms of the classic opioids, including controlled-
released morphine suppositories and supensions, and
transdermal fentanyl, a patch that allows 3-day dosing
and avoids the first-pass effect of the liver.4
Since the discovery of fentanyl by Janssen in 1962,5
many anilidopiperidines have been synthesized and
evaluated for SAR studies6 and for providing insight
into the key structural features required for high affinity
binding to the m receptor.7 However, only three fenta-
nyl-like compounds are commercially available (alfen-
tanil, remifentanil and sufentanil) (Chart 1) and due to
their high potency and short duration of action they are
mainly used for induction of general anesthesia.
*Corresponding author. Tel.: +34-91-562-2900; fax: +34-91-564-
4853; e-mail: iqmg310@iqm.csic.es
0968-0896/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(01)00345-5

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N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
Chart 1.
Therefore, fentanyl itself still represents the reference
drug in this kind of compounds and the increasing use
of its transdermal formulation for the treatment of
chronic and cancer pain suggests that the search of new
analogues with longer duration of action may represent
an interesting approach for novel analgesics.
structure, worth mentioning the recent description of
the potent 4-methylfentanyl derivative,9 the substitution
of the propioanilido benzene by five-membered rings
has not, in general, been explored. It has actually been
replaced by pyridine, six-membered heterocycles10 and
fused heteroaryl derivatives11 but with no substantial
improvement of the analgesic profile over fentanyl itself.
Therefore, in this paper we wish to report the synthesis
and pharmacological studies of new fentanyl analogues
bearing a phenylpyrazole moiety, some of which have
shown interesting analgesic properties with potency
higher than morphine, somewhat lower than fentanyl
but with considerably longer duration of action.
Within this context, we decided to modify the fentanyl
molecule, and taking into account our experience in
azole chemistry8 we decided to substitute the benzene of
the propioanilido group by a phenylpyrazole. It should
be commented that despite the many different structural
variations that have been performed on the fentanyl
Scheme 1. (i) 3-aminophenylpyrazole, NaBH4, PhCH3, p-TsOH or NaBH3CN, MeOH; (ii) 5-aminophenylpyrazole, NaBH4, PhCH3, p-TsOH or
NaBH3CN, MeOH; (iii) (Et2OC)2O; (iv) 1-ACE-Cl, CICH2CH2CI; (v) PhCH2CH2Br, DMA; (vi) CH3O2CCH¼CH2, CH3CN.

N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
819
Results
is also interesting to determine the receptor subtype.
Chemistry
The opioid activity profile of the new compounds was
functionally determined by in vitro bioassays using gui-
nea pig ileum for m and k receptors, mouse vas deferens
Compounds 1–7 described in this study were prepared
as outlined in Scheme 1.12 The benzyl derivatives of the
propanamides 3a, 3c and 4a were prepared starting
from 1-benzyl-4-piperidone. The phenethyl derivatives
were obtained either from 1-phenethyl-4-piperidone, as
was the case for the propanamide 3d, or from 1-benzyl-
4-piperidone, which was how the propanamide 4b was
prepared through the deprotected piperidine 6. In the
case of the phenethyl derivative of the propanamide 3b,
we describe both procedures: from 1-benzyl-4-piper-
idone 3b was obtained through 5a with an overall yield
of ꢀ 10% and from 1-phenethyl-4-piperidone the over-
all yield improved (24%). The first step of the reaction
sequences consists on a reductive amination of the
piperidones. This reaction was carried out in two steps:
formation of the corresponding imines by addition of
the corresponding aminopyrazole followed by reduction
with sodium borohydride (procedure A) or in a one pot
reaction using sodium cyanoborohydride as reductant13
(procedure B). Next, reflux of the resulting amines in
propionic anhydride afforded the desired amides 3a–d
and 4a. The debenzylation could not be achieved by
catalytic hydrogenation; however, treatment with 1-
chloroethylchloroformate14 followed by methanolysis
removed the benzyl group in good yield. Alkylation of
the N-deprotected derivatives 5a and 6 with 1-phenyl-2-
bromoethane gave the respective phenethyl derivatives
3b and 4b. Finally, the methoxycarbonylderivative 7,
which can be considered an analogue of remifentanil,
has been synthesized in good yield by alkylation of the
N-deprotected derivative 5a with methyl acrylate by
Michael reaction. This compound was prepared in order
to study the variation of the pharmacokinetic profile of
this series, since it is well known that modulation of the
duration of action can be achieved through alkyl ester
substitution.15
for
d receptors and rabbit vas deferens for k
receptors.18ꢁ21
Finally, compounds 3b and 3d showing m opioid profile
were evaluated for their binding affinities at the m, d and
k binding sites following reported procedures22 with
minor modifications. The inactive isomeric structure of
3b, 4b, and fentanyl were also tested for comparative
purposes. Mouse brain membranes (P2 fraction) were
used with [3H]DAMGO, [3H]DPDPE and [3H]U-69593
ligands for the m, d and k receptors respectively. Since
the binding data indicated that the compounds had an
extremely high affinity for the m receptor which did not
correlate linearly with the analgesic potencies and inhi-
bitory effects observed, further binding tests were per-
formed in a different system, in human recombinant
CHO-K1 cells following the methods of Wang23 and
Maguire.24
The synthesized compounds of this study have been
characterized by electrospray mass spectrometry and by
NMR spectroscopy: the signals have been attributed by
a combined used of 1H, 13C, HMQC and literature
references.16 The analgesic evaluation of compounds
3a–d, 4a, 4b and 7 has been carried on their oxalates.
Pharmacology
Target compounds 3a–d, 4a, 4b and 7 were tested in
order to determine their antinociceptive capacity. The
writhing test in mice was used because it is widely
employed at the first stages of the evaluation of anti-
nociceptive drugs.17 This test allows an easy and quick
discarding of the non-analgesic derivatives. Compounds
showing antinociceptive activity, were also studied using
the hot plate test, to corroborate the analgesia by this
more sensitive test. Naloxone is a selective opioid
antagonist that is generally used to verify any opioid
involvement in the effect of drugs. Data obtained from
the study of the antinociception and of the naloxone
antagonism permits to suggest that the activation of
opioid receptors plays a role in the effect of a drug but it
Figure 1. Lines show: A the % of inhibition ꢂ SEM of the number of
contractions induced by acetic acid administration, number of writhes
in saline solution treated animals (control group): 19ꢂ0.9 (n=12) and
B the % of Maximum Possible Effect (MPE) ꢂ SEM in the hot plate
test, induced by the administration of 3b (squares), 3d (triangles),
morphine (diamonds) and fentanyl (circles) in mice, 30 mm before the
test.

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N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
Discussion
Table 1 shows the EC50 values. The in vitro effect of all
the tested drugs was completely reversed by the in vitro
administration of naloxone (10ꢁ6 M). Since these tissues
mainly present m and k opioid receptors,25 it could be
suggested that m and/or k opioid receptors could be
involved in the effect of new compounds, although
effects on the d opioid receptor may not be discarded.
First of all, the antinociceptive activity of the new com-
pounds 3a–d, 4a–b and 7 was determined carrying out
the writhing test. Using this test, the phenethyl deriva-
tives 3b and 3d were able to induce a dose-dependent
antinociceptive effect, while the corresponding benzyl
derivatives 3a and 3c and the methoxycarbonyl deriva-
tive 7 did not demonstrate any significant analgesic
activity. Compounds 4a and 4b corresponding to the
positional isomers of 3a and 3b turned out to be inac-
tive. Then, a more selective test, the hot plate, was used
and the antinociception induced by morphine and by
fentanyl was also analysed in order to evaluate the effect
of the new compounds. Compounds 3b and 3d were
more potent than morphine but less potent than fenta-
nyl in both the writhing test and the hot plate test (Fig.
1). Naloxone was able to completely antagonize the
analgesia induced by the new compounds as well as that
induced by morphine or by fentanyl. From these find-
ings, it could be suggested that the antinociception
induced by both 3b and 3d is mediated through the
activation of opioid receptors.
To clarify these possibilities, we used mouse vas defe-
rens preparations that mainly contain d receptors
(although they have also m and k receptors), and rabbit
vas deferens, having k receptors only. The effect of 3b and
3d in these tissues was compared with that of two selective
agonists: [D-Pen2,D-Pen5]-enkephalin (DEPEN) (d)26
28
and U-50,488H (k).27 Naltrindole
was used as a d
selective antagonist and norbinaltorfimine29 as a k
antagonist. In comparison with the d agonist DEPEN,
3b and 3d were slightly less potent in inhibiting the
electrically induced contractions in the mouse vas defe-
rens (Table 1). However, the inhibition of the con-
tractile response induced by DEPEN (6ꢃ10ꢁ8 M) was
71.6ꢂ3.4% and in naltrindole (10ꢁ9 M) treated tissues
was 41.0ꢂ3.5%, whilst the inhibition induced by 3b
(2ꢃ10ꢁ7 M,% of inhibition: 70.5ꢂ8.1) and 3d (2ꢃ10ꢁ7
M,% of inhibition: 70.2ꢂ9.4) was not significantly
modified by the d antagonist (10ꢁ9 to 10ꢁ8 M). From
this result it could be suggested that the effect of 3b and
The duration of the analgesic effect of the new deriva-
tives was compared with that of fentanyl. In order to
perform the study, the analgesic effectiveness of several
doses of the compounds was tested 5 min after their
administration and equipotent (85–95% of inhibition of
the nociceptive response) doses of each compound were
selected. We found that whereas the fentanyl anti-
nociception ceased 120 min after the ip administration,
the effect of 3b and 3d persisted for at least 360 min
(Fig. 2). It is worth mentioning that no substantial dif-
ference in the duration of action was observed between
the parent compound 3b and 3d, in which on of the
probable metabolic degradation routes is blocked.
Table 1. Inhibitory effect (EC50) of morphine, fentanyl, 3b and 3d on
the electrically-induced contractions in guinea pig ileum and mouse
vas deferens
Compounds
EC50 (M)
Confidence intervals (95%)
Guinea pig ileum
Morphine
Fentanyl
3b
3d
Mouse vas deferens
5.2ꢃ10ꢁ7
5.2ꢃ10ꢁ9
1.3ꢃ10ꢁ7
9.4ꢃ10ꢁ8
2.1ꢃ10ꢁ7–1.3ꢃ10ꢁ6
4.1ꢃ10ꢁ9–6.5ꢃ10ꢁ9
8.2ꢃ10ꢁ8–2.0ꢃ10ꢁ7
5.4ꢃ10ꢁ8–1.6ꢃ10ꢁ7
D-Pen
3b
3d
4.0ꢃ10ꢁ9
6.0ꢃ10ꢁ8
4.7ꢃ10ꢁ8
2.5ꢃ10ꢁ9–6.6ꢃ10ꢁ9
3.9ꢃ10ꢁ8–9.2ꢃ10ꢁ8
3.0ꢃ10ꢁ8–7.5ꢃ10ꢁ8
To study the functional activity of the opioid agonists,
isolated tissues such as guinea-pig ileum,18 mouse vas
deferens19 and rabbit vas deferens21 were used. In gui-
nea pig ileum, 3b and 3d, as well as control opioids,
morphine and fentanyl, induced a dose-dependent inhi-
bition of the electrically induced contractions (Fig. 3).
Figure 3. Lines show the % of inhibition ꢂ SEM of the electrically-
induced contractions in the MP-LM strips of guinea pig ileum
incubated with 3b (squares), 3d (triangles), morphine (diamonds) and
fentanyl (circles).
Figure 2. Lines show the duration ꢂ SEM of the analgesic effect of 3b
(2 mg/kg) (squares), 3d (1 mg/kg) (triangles) and fentanyl (0.4 mg/kg)
(inverted triangles) in the hot plate test in mice.

N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
821
Table 2. Affinity data [Ki (nM)] of Fentanyl, 3b, 3d and 4b for m, d
and k opioid receptorsa
different receptors whereas functional assays show the
intrinsic activity. For example, low activity in the GPI
assay despite strong binding to the k receptor has been
observed for a number of dynorphine analogues and
recently this has been reported to reflect an intrinsic
property of the ligand.33
[3H]DAMGO
(2 nM)
[3H]DPDPE
(4 nM)
[3H]U-69593
(2 nM)
Receptors:
m
d
k
Fentanyl
5.9ꢂ1.4
0.032ꢂ0.01
0.0025ꢂ0.0006
427ꢂ124
568ꢂ159
89ꢂ38
298ꢂ40
828ꢂ431
1383ꢂ227
4554ꢂ558
3b
3d
4b
In any case, the absolute Ki values should always be
considered with certain care since for example, in the
case of ohmefentanyl, a potent opioid already men-
tioned, in which eight optically active isomers are pos-
sible, there was controversy in the binding data of the
more potent isomers reported by two different
groups.32,34
86ꢂ23
7321ꢂ1656
aValues are expressed as meanꢂstandard error of the mean of three
experiments.
3d on the mouse vas deferens was not mediated by d
opioid receptors.
Nevertheless, a different m-binding study was performed
in human recombinant CHO-K1 cells. The Ki values in
this case were for 3b 1.21ꢂ0.11 nM and for 3d
1.04ꢂ0.05 nM. These values correlate well with all
other pharmacological assays and confirm that 3b and
3d have high affinity and selectivity for the m receptor.
When the m selective antagonist CTOP (10ꢁ6 M)30,31
was used to assess that the inhibitory effect was medi-
ated by m receptors a complete reversion of the effect of
3b and 3d was observed in guinea pig ileum and mouse
vas deferens. In rabbit vas deferens, as expected, U-
50,488H induced a dose dependent inhibition of the
electrically induced contractions and the selective k
antagonist norbinaltorfimine completely antagonized
this effect. On the contrary, neither 3b nor 3d sig-
nificantly modified the contractile response of the rabbit
vas deferens. This result discards any functional activity
of the new fentanyl derivatives on k receptors. The
possibility of these new compounds to selectively
antagonize the effect of U-50,488H on k receptors was
also tested in rabbit vas deferens: neither 3b (up to
5ꢃ10ꢁ7 M) nor 3d (up to 2.5ꢃ10ꢁ7 M) were able to
recover the contractile activity of the tissue after treat-
ment with the k agonist.
Conclusions
New piperidylpropanamides, structurally related to
fentanyl and incorporating a 1-phenylpyrazole rest have
been synthesized. Two derivatives 3b and 3d are able to
induce a dose dependent antinociceptive effect that may
be blocked by the opioid antagonist naloxone, and their
in vitro pharmacological profile suggests that they are
potent and selective m opioid agonists. Furthermore,
their in vitro and in vivo potency is greater than that of
morphine and, the duration of their analgesic effect is
longer than for fentanyl. Considering that the use of
fentanyl in chronic pain is mainly limited by the short
duration of its antinociceptive activity, it could be sug-
gested that 1-phenylpyrazole may represent an interest-
ing substitution for the benzene ring in fentanyl and
deserves further investigation. Studies on the tolerance
induced by these analgesics are currently underway.
These in vitro results indicated that the new compounds
are m opioid selective agonists. Their affinity for the m
opioid receptor was corroborated by radioligand bind-
ing assays.22 The binding data in mouse brain mem-
branes for the m, d and k receptors of 3b and 3d together
with those of 4b and fentanyl are provided in Table 2.
These data are in agreement with results obtained on
isolated tissues confirming that 3b and 3d are m agonists,
and corroborate the lack of affinity for the d and k
receptors. On the other hand, compound 4b, positional
isomer of 3b, which had shown no antinociceptive
activity in vivo, had no affinity for the m receptor sug-
gesting that the position of the 1-phenylpyrazole is cru-
cial for activity.
Experimental
Chemistry
The melting points were determined with a Reichert
Jung Thermovar apparatus and are uncorrected. Mass
spectra were recorded using Fast Atom Bombardment
in NBAmatrix or using electrospray. Flash column
chromatographies were run on silica gel 60 (230–400
mesh) or on medium pressure flash system with pre-
In terms of the potency of the binding affinity, the very
low values of Ki for the m receptors of 3b and 3d are
difficult to explain. These values, which are comparable
to those of some of the more potent isomers of ohme-
fentanyl, eg the oxalate salt of the 2R,3R,4S isomer has
a Ki=0.013ꢂ0.002 nM,32 do not correlate well with the
data obtained in functional assays and analgesic tests.
There is, however, a good correlationship between the
functional data obtained in isolated tissues and in the
antinociceptive tests. The considerable difference
between the binding data and the functional assays can
be accounted for by the fact that the binding study
provides information about affinity and selectivity upon
1
packed silica gel cartridge. H and 13C NMR spectra
were recorded on a 200, 300 and 400 unity spectro-
meters. Dry ethanol was obtained by distillation over
Mg(OH)2. Tetrahydrofuran and toluene were distilled
over sodium-benzophenone. The oxalate salts were pre-
cipitated from a solution of the corresponding free bases
in EtOAc to which was added oxalic acid in a light
excess. 1-Phenyl-3-amino-ꢀ2-pyrazoline, 1-(4-fluor-
ophenyl)-3-amino-ꢀ2-pyrazoline and 1-phenyl-5-ami-
nopyrazole were synthesized as reported in the

822
N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
literature.35 1-Phenyl- and 1-(4-fluorophenyl)-3-amino-
pyrazoles were prepared by oxidation of the corre-
sponding pyrazoline as follows:
d, J=11 Hz, H-2(Pip)), 2.13 (2H, q, J=7.5 Hz,
CH2CH3) 2.10 (2H, m, H-2(Pip)), 1.83 (2H, d, J=11
Hz, H-3(Pip)),1.56 (2H, m, H-3(Pip)), 1.07 (3H, t,
J=7.5 Hz, CH2CH3); 13C NMR (CDCl3) d (ppm) 174.0
(CO), 148.7 (C-3(Pz)), 139.6 (ipso-(C6H5Pz)), 138.1
(ipso-(C6H5CH2)), 129.4 (m-(C6H5CH2)), 129.0 (m-
(C6H5Pz)), 127.7 (C-5(Pz)), 127.6 (o-(C6H5CH2)), 126.8
(p-(C6H5CH2)), 126.7 (p-(C6H5Pz); 118.9 (o-(C6H5Pz)),
107.3 (C-4(Pz)), 62.9 (C6H5CH2), 52.9 (C-2(Pip)), 51.9
(C-4(Pip), 30.3(C-3(Pip)), 28.0 (CH2CH3), 9.4
(CH2CH3). Anal. (C24H28N4O): calcd C 74.20, H 7.26,
N 14.42; found C 73.58, H 7.06, N 14.22.
1 - Phenyl - 3 - aminopyrazole. 1-Phenyl-3-amino-ꢀ2-pyr-
azoline (3.7g, 23 mmol) was dissolved in dioxane (100
mL). DDQ (5.77 g, 25 mmol) was added to the solution.
The reaction mixture was stirred at room temperature
for 0.5 h. Then the solution was filtrated. The filtrate
was acidified with 1 N HCl and CH2Cl2 was added (150
mL). The organic layer was separated, extracted with
1 N HCl. The combined aqueous layers were then made
alkaline with 10% NaOH and extracted with CH2Cl2.
The organic layers were dried over MgSO4 and the sol-
vent was evaporated to yield 2.9 g (80ꢄ%) of a white
solid: mp 89–90 ꢄC [lit.35a (H2O) mp 101 C].
N-(1-Phenylpyrazol-3-yl)-N-(4-piperidyl)-propanamide
(5a). Propanamide 3a (270 mg, 0.70 mmol) was dis-
solved in dry 1,2-dichloroethane (6 mL). At 0 ꢄC was
added 1-chloroethylchloroformate (127 mL, 1.16
mmol). The reaction mixture was kept stirring at this
temperature for 15 min then was heated to reflux for 1
h. After evaporating the solvent, the residue was heated
in refluxing dry MeOH (15 mL) for 1.5 h. The solution
was then concentrated and the solid residue was dis-
solved in 0.5 N HCl (30 mL), this aqueous solution was
washed with Et2O. The aqueous layer was made alka-
line with 10% NaOH, then it was extracted with Et2O.
The ethereal layers were dried over MgSO4 and the sol-
vent was evaporated yielding 200 mg (95%) of a white
solid: mp 128 ꢄC; 1H NMR(CDCl3) d (ppm) 7.98 (1H, d,
J=2.4 Hz, H-5(Pz)), 7.69 (2H, d, J=7.5 Hz, o-
H(C6H5Pz)), 7.45 (2H, t, J=7.5 Hz, m-H(C6H5Pz)),
7.30 (1H, t, J=7.5 Hz, p-H(C6H5Pz)), 6.25 (1H, d,
J=2.4 Hz, H-4(Pz)), 4.66 (1H, m, H-4(Pip), 3.05 (2H, d,
J=11 Hz, H-2(Pip)), 2.69 (2H, t, H-2(Pip)), 2.15 (2H, q,
J=7.5 Hz, CH2CH3), 1.85 (2H, d, J=11 Hz, H-3(Pip)),
1.41 (2H, dd, H-3(Pip)), 1.10 (3H, t, J=7.5 Hz,
CH2CH3); 13C NMR (CDCl3) d (ppm) 173.6 (CO),
148.6 (C-3(Pz)), 139.4 (ipso-(C6H5Pz)), 129.2 (m-
(C6H5Pz)), 127.6 (C-5(Pz)), 126.6 (p-(C6H5Pz); 118.7 (o-
(C6H5Pz)), 107.1 (C-4(Pz)), 51.9 (C-4(Pip), 45.9 (C-
2(Pip)), 31.5 (C-3(Pip)), 27.8 (CH2CH3), 9.3 (CH2CH3).
MS (electrospray) [M+1] 299.2.
N-(1-Phenylpyrazol-3-yl)-N-(1-benzyl-4-piperidyl)-amine
(1a). Procedure A. To 1-benzyl-4-piperidone (0.7 mL,
3.78 mmol) in toluene (100 mL) was added 1-phenyl-3-
aminopyrazole (600 mg, 3.77 mmol) and few crystals of
p-TsOH. The reaction mixture was stirred and heated to
reflux for 18 h eliminating the H2O formed with a Dean-
Stark apparatus. The solvent was then evaporated and
the reddish oily residue was dissolved in MeOH (20
mL). Sodium tetrahydroborate (150 mg, 4.1 mmol) was
added portionwise and the solution was stirred for 3 h
at room temperature. The solvent was then evaporated,
the residue was dissolved in CH2Cl2, washed with H2O,
dried over Na2SO4. The solvent was evaporated and the
crude product was purified on silica gel [EtOAc/
TEA(200/1)] to yield 662 mg (53%) of a yellowish solid:
mp 52–54 ꢄC; H NMR (CDCl3) d (ppm) 7.61 (1H, d,
1
J=2.5 Hz, H-5(Pz)), 7.48 (2H, d, J=8 Hz, o-
H(C6H5Pz), 7.30 (2H, t, J=8 Hz, m-H(C6H5Pz)), 7.25
(5H, m, C6H5CH2), 7.07 (1H, t, J=8 Hz, p-H(C6H5Pz)),
5.70 (1H, d, J=2.5 Hz, H-4(Pz)), 3.45 (2H, s,
C6H5CH2), 3.29 (1H, m, H-4(Pip)), 2.77 (2H, d, J=13
Hz, H-2(Pip)), 2.10 (2H, t, J=9.5 Hz, H-2(Pip)), 2.01
(2H, d, J=13 Hz, H-3(Pip)), 1.47 (2H, m, H-3(Pip));
13C NMR (CDCl3) d (ppm) 157.0 (C-3(Pz)), 139.9 (ipso-
(C6H5Pz)), 138.1 (ipso- (C6H5CH2)), 128.8 (m-
(C6H5CH2)), 127.9 (C-5(Pz)), 126.9 (o-(C6H5CH2)),
126.7 (m-(C6H5Pz)), 124.3 (p-(C6H5Pz); p-(C6H5CH2)),
117.1 (o-(C6H5Pz)), 94.5 (C-4(Pz)), 63.7 (C6H5CH2),
52.9 (C-2(Pip)), 51.9 (C-4(Pip), 33.4 (C-3(Pip)). MS
(electrospray) [M+1]333.
N-(1-phenylpyrazol-3-yl)-N-(1-phenethyl-4-piperidyl)-
amine (1b) was prepared following the same procedure
as for the benzyl derivatives starting with the 1-phe-
nethylpiperidone.
N-(1-phenylpyrazol-3-yl)-N-(1-phenethyl-4-piperidyl)-
amine (1b). (Yield, 43% from procedure A): mp 107–
110 ꢄC; 1H NMR (CDCl3) d (ppm) 7.76 (1H, d, H-
5(Pz)), 7.63 (2H, d, o-H(C6H5Pz)), 7.40 (2H, t, o-
H(C6H5Pz)), 7.32 (6H, m, C6H5CH2, p-H(C6H5Pz)),
5.85 (1H, d, H-4(Pz)), 3.82 (1H, brm, NH), 3.48 (1H, m,
H-4(Pip)), 3.03 (2H, d, H-2(Pip)), 2.83 (2H, m,
C6H5CH2CH2), 2.62 (2H, m, C6H5CH2CH2), 2.24 (2H,
t, H-2(Pip)), 2.21 (2H, t, H-3(Pip)), 1.62 (2H, m, H-
3(Pip)); 13C NMR (CDCl3) d 156.8 (C-3(Pz)), 140.0 (ipso-
(C6H5Pz)), 139.8 (ipso-(C6H5CH2)), 128.8 (m-(C6H5CH2)),
128.2 (o-(C6H5CH2)), 127.9 (C-5(Pz)), 126.6 (m-
(C6H5Pz)), 125.6 (p-(C6H5CH2)), 124.2 (p-(C6H5Pz)),
117.1 (o-C6H5Pz)), 94.4 (C-4(Pz)), 60.2 (C6H5CH2CH2),
52.0 (C-2(Pip)), 50.8 (C-4(Pip), 33.4 (C6H5CH2CH2),
32.4 (C-3(Pip)). MS (electrospray) [M+1] 347.3.
N-(1-Phenylpyrazol-3-yl)-N-(1-benzyl-4-piperidyl)-propa-
namide (3a). Amine 1a (620 mg, 1.87 mmol) was heated
to 135 ꢄC in propionic anhydride for 4 h. Then the
reaction mixture was poured on ice–H2O, made alkaline
with concd NH4OH and extracted with CH2Cl2. The
combined organic layers were washed with H2O, dried
over MgSO4, and the solvent was evaporated. The crude
product was chromatographed on silica gel (EtOAc) to
yield 380 mg (52%) of a yellowish oil which solidified on
standing: mp 110–112 ꢄC; 1HNMR (CDCl3) d (ppm)
7.92 (1H, d, J=2.4 Hz, H-4(Pz)), 7.66 (2H, d, J=7.5
Hz, o-H(C6H5Pz)), 7.47 (2H, t, J=7.5 Hz, m-
H(C6H5Pz)), 7.30 (1H, t, J=7.5 Hz, p-H(C6H5Pz)), 7.26
(5H, m, C6H5CH2), 6.23 (1H, d, J=2.4 Hz, H-5(Pz)),
4.60 (1H, m, H-4(Pip), 3.46 (2H, s, C6H5CH2), 2.90 (2H,

N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
823
N-(1-Phenylpyrazol-3-yl)-N-[1-(2-phenethyl)-4-piperidyl)]
-propanamide (3b). From 1b: same procedure than for
the preparation of 3a, (yield=55%). From 5a: to pro-
panamide 5a (200 mg, 0.67 mmol) in DMA(5 mL) were
added 1-phenyl-2-bromoethane (92 mL, 0.67 mmol) and
TEA(110 mL, 0.79 mmol). This solution was stirred at
70 ꢄC for 18 h, then the solution was poured on H2O.
The white precipitate was extracted with CH2Cl2, the
combined organic layers were washed with H2O, dried
over MgSO4 and concentrated. The crude was purified
by chromatography on silica gel [EtOAc/cyclohexane
(1:1)] to yield 73 mg (36%) of a white solid: mp 75–
78 C; oxalate, mp 182 C; H NMR (CDCl3) d (ppm)
7.97 (1H, d, H-5(Pz)), 7.72 (2H, d, o-H(C6H5Pz)), 7.53
(2H, t, m-H(C6H5Pz)), 7.30 (6H, m, p-H(C6H5Pz),
C6H5CH2), 7.31 (1H, d, H-4(Pz)), 4.68 (1H, m, H-
4(Pip)), 3.08 (2H, d,H-2(Pip)), 2.80 (2H, m,
C6H5CH2CH2), 2.60 (2H, m, C6H5CH2CH2), 2.20 (2H,
m, H-2(Pip)), 2.21 (2H, q, J=7.5 Hz, CH2CH3), 1.91
(2H, d, H-3(Pip)), 1.65 (2H, m, H-3(Pip)), 1.13 (3H, t,
J=7.5 Hz, CH2CH3); 13C NMR (CDCl3) d (ppm) 174.1
(CO), 148.6 (C-3(Pz)), 140.1 (C-ipso-(C6H5Pz)), 139.9
(ipso-(C6H5CH2)), 129.4 (m-(C6H5Pz)), 128.6 (m-
(C6H5CH2)), 128.3 (o-(C6H5CH2)), 127.8 (C-5(Pz)),
126.8 (p-(C6H5Pz)), 125.9 (p-(C6H5CH2)), 119.0 (o-
(C6H5Pz)), 107.4 (C-4(Pz)), 60.5 (C6H5CH2CH2), 53.0
(C-2(Pip)), 51.8 (C-4(Pip), 33.7(C6H5CH2CH2), 30.3 (C-
3(Pip)), 28.0 (CH2CH3), 9.5 (CH2CH3). MS (electro-
spray) [M+1] 403.2. Anal. (C25H30N4O.C2H2O4): calcd
C 65.84, H 6.55, N 11.37; found C 65.59, H 6.55, N
11.26.
mixture was stirred at room temperature for 15 min
maintaining the pH at 6 with HCl–MeOH addition.
Molecular sieves were added, 10 min later the mixture
was filtrated over a Celite bed. The filtrate was evapo-
rated to dryness. The crude product was chromato-
graphed on a flash 40i cartridge [CH2Cl2/MeOH (24:1)]
to give 1.15 g (70%) of a yellowish solid: mp 58–62 ꢄC;
1H NMR (CDCl3) d (ppm) 7.55 (1H, d, J=2.5 Hz, H-
5(Pz)),7.46–6.94 (9H, p-F C6H4 and C6H5), 5.74 (1H, d,
J=2.5 Hz, H-4(Pz)), 3.81 (2H, s, C6H5CH2), 3.72 (1H,
m, H-4(Pip)), 3.10 (2H,m, H-2(Pip)), 2.50 (2H, m, H-
2(Pip)), 2.22 (2H, m, H-3(Pip)), 1.85 (2H, m, H-3(Pip));
13C NMR (CDCl3) d (ppm) 160.2 (d, JC-F=244 Hz, p-
(p-F-C6H4)), 156.9 (C-3(Pz)), 136.7 (d, JC-F=2.7 Hz,
ipso-(p-FC6H4)), 133.6 (ipso-(C6H5CH2)), 130.3, 128.8,
128.6-(C6H5), 127.6 (C-5(Pz)), 119.3 (d, JC-F=8 Hz, 0-
(p-F-C6H4)), 116.1 (d, JC-F=23 Hz, m-(p-F-C6H4)), 95.2
(C-4(Pz)), 62.1 (C6H5CH2), 51.6 (C-2(Pip)), 49.8 (C-
4(Pip), 31.0 (C-3(Pip)).
ꢄ
ꢄ
1
N-[1-(4-Fluorophenyl)pyrazol-3-yl]-N-(1-benzyl-4-piperi-
dyl) propanamide (3c) (oxalate salt). Mp 192–193 ꢄC,
ꢄ
1
free base, mp 37–40 C; H NMR (CD3OD) d (ppm)
8.45 (1H, d, J=2.3 Hz, H-5(Pz)), 7.94 (2H, m, (p-
FC6H4), 7.62 (5H, brs, C6H5), 7.43 (2H, m, (p-FC6H4),
6.40 (1H, d, J=2.3 Hz, H-4(Pz)), 4.94 (1H, m, H-4(Pip),
4.44 (2H, s, C6H5CH2), 3.69 (2H, m, H-2(Pip)), 3.33
(2H, H-2(Pip)), 2.39 (2H, q, J=7.4 Hz, CH2CH3), 2.06
(2H, m, H-3(Pip)), 2.01 (2H, m, H-3(Pip)), 1.20 (3H, t,
J=7.4 Hz,CH2CH3); 13C NMR (CD3OD) d (ppm)
177.0 (C2O4), 166.7 (CO), 163 (d, JC-F=245 Hz, p-C-(p-
F-C6H4)), 149.5 (C-3(Pz)), 136.2 (d, JC-F=3 Hz, ipso-(p-
FC6H4)), 132.6, 131.4, 130.8, 130.5-(C6H5 and C-5(Pz)),
122.7 (d, JC-F=8Hz, 0-(p-F-C6H4)), 117.5 (d, JC-F=24
Hz, m-(p-F-C6H4)), 108.8 (C-4(Pz)), 61.6 (C6H5CH2),
53.0 (C-2(Pip)), 51.4 (C-4(Pip), 29.2 (C-3(Pip)), 28.8
(CH2CH3), 10.1 (CH2CH3). MS (electrospray) [M+1]
407.3. Anal. (C24H27N4OF.C2H2O4): calcd C 62.90, H
5.84, N 11.29; found C 62.79, H 6.10, N 11.09.
4-[(1-Oxopropyl)(1-phenylpyrazol-3-yl)amino]-N-piperi-
dinepropanoic acid methyl ester (7). Amixture of pro-
panamide 5a (200 mg, 0.67 mmol) in dry acetonitrile
and methylacrylate (120 mL, 1.32 mmol) was heated to
50 ꢄC for 2.2 h. The solution was then concentrated and
the residue was chromatographed on silica gel [EtOAc/
MeOH (8.5:0.5)] to yield 243 mg (96%) of a white solid:
mp 88–91 ꢄC; H NMR (CDCl3) d (ppm) 7.97 (1H, d,
1
H-5(Pz)), 7.66 (2H, d,o-H(C6H5Pz)), 7.43 (2H, t, m-
H(C6H5Pz)), 7.28 (2H, t, p-H(C6H5Pz)), 6.23 (1H, d, H-
4(Pz)), 4.56 (1H, m, H-4(Pip)), 3.60 (3H, s, CH3CO2),
2.87 (2H, d, H-2(Pip)), 2.64 (2H, t, CO2CH2CH2), 2.43
(2H, t, CO2CH2CH2), 2.12 (4H, m, CH2CH3, H-2(Pip)),
1.82 (2H, d, H-3(Pip)), 1.52 (2H, m, H-3(Pip)), 1.05
(3H, t, CH2CH3); 13CNMR (CDCl3) d (ppm) 172.4
(CO2), 173.7 (CO), 148.3 (C-3(Pz)), 139.3 (C-
ipso(C6H5Pz)), 129.1 (m-(C6H5Pz)), 127.6 (C-5(Pz)),
126.5 (p-(C6H5Pz)), 118.6 (o-(C6H5Pz)), 107.0 (C-4(Pz)),
53.0 (CO2CH2CH2), 52.5 (C-2(Pip)), 51.5 (C-4(Pip),
31.8 (CO2CH2CH2), 29.9 (C-3(Pip)), 27.7 (CH2CH3),
9.2 (CH2CH3). MS (electrospray) [M+1] 385.3. Anal.
(C21H28N4O3): calcd C 65.60, H 7.34, N 14.57; found C
65.28, H 7.05, N 14.40.
N-[1-(4-Fluorophenyl)pyrazol-3-yl]-N-[1-(2-phenethyl)-4-
ꢄ
1
piperidyl]-amine (1d). (Yield, 44%): mp 100–104 C; H
NMR (CDCl3) d (ppm) 7.76 (1H, d,H-5(Pz)), 7.63 (2H,
d, o-H(C6H5Pz)), 7.40 (2H, t, o-H(C6H5Pz)), 7.32 (6H,
m, C6H5CH2, p-H(C6H5Pz)), 5.85 (1H, d, H-4(Pz)), 3.82
(1H, brm, NH), 3.48 (1H, m, H-4(Pip)), 3.03 (2H, d, H-
2(Pip)), 2.83 (2H, m, C6H5CH2CH2), 2.62 (2H, m,
C6H5CH2CH2), 2.24 (2H, t, H-2(Pip)), 2.21 (2H, t,
H-3(Pip)), 1.62 (2H, m, H-3(Pip)); 13C NMR (CDCl3)
d (ppm) 160.1 (d, JC-F=244 Hz, p-(p-F-C6H4)), 157.1
(C-3(Pz)), 136.7 (ipso-(p-FC6H4)), 128.9 (ipso-
(C6H5CH2)), 128.7, 128.5, 126.2-(C6H5), 127.5 (C-
5(Pz)), 119.2 (d, JC-F=8 Hz, 0-(p-F-C6H4)), 115.9
(d, JC-F=23 Hz,m-(p-F-C6H4)), 94.9 (C-4(Pz)), 60.3
(C6H5CH2CH2), 52.3 (C-2(Pip)), 50.5(C-4(Pip), 33.3
(C6H5CH2CH2), 32.3 (C-3(Pip)). MS (electrospray)
[M+1] 365.2.
N-[1-(4-Fluorophenyl)pyrazol-3-yl]-N-(1-benzyl-4-piperi-
dyl) - amine (1c). Procedure B. 1-Benzyl-4-piperidone
(0.88 mL, 4.7 mmol) and NaBH3CN (170 mg, 2.8
mmol) were mixed in MeOH (50 mL). AHCl–MeOH
solution was added dropwise until pH 6. Then a solu-
tion of 1-(4-fluorophenyl)-3-aminopyrazole (1.23 g, 7
mmol) in MeOH (100 mL) was added. The reaction
N-[1-(4-Fluorophenyl)pyrazol-3-yl]-N-[1-(2-phenethyl)-4-
piperidyl)]propanamide (3d) (oxalate salt). Mp 195–
205 ꢄC; 1H NMR(CD3OD) d (ppm) 8.48 (1H, s, H-
5(Pz)), 7.98 (2H, m, m-H(p-FC6H5)), 7.47 (7H, m, o-
H(p-FC6H5), C6H5CH2), 6.67 (1H, d, H-4(Pz)), 5.0 (1H,

824
N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
m, H-4(Pip)), 3.88 (2H, d, H-2(Pip)), 3.43 (2H, m,
C6H5CH2CH2), 3.39 (2H, m, H-2(Pip)), 3.17 (2H, m,
C6H5CH2CH2), 2.39 (2H, q, CH2CH3), 2.14 (2H, d, H-
3(Pip)), 2.06 (2H, m, H-3(Pip)), 1.24 (3H, t, CH2CH3);
13C NMR (CD3OD) d (ppm) 176.7 (C2O4), 166.6 (CO),
162.9 (d, JC-F=245 Hz, p-(p-F-C6H4)), 149.2 (C-3(Pz)),
137.7 (ipso-(C6H5CH2)), 137.5 (d, ipso-(p-FC6H4)),
129.9, 129.8, 128.2-(C6H5), 131.0 (C-5(Pz)), 122.4 (d,
JC-F=9 Hz, 0-(p-F-C6H4)), 117.3 (d, JC-F=23 Hz, m-(p-
F-C6H4)), 108.6 (C-4(Pz)), 58.8 (C6H5CH2CH2), 53.1
(C-2(Pip)), 50.8 (C-4(Pip), 31.4 (C6H5CH2CH2), 29.0
(C-3(Pip)), 28.6 (CH2CH3), 9.8 (CH2CH3). MS (elec-
trospray) [M+1] 421.3. Anal. (C25H29N4OF.C2H2O4):
calcd C 63.52, H 6.07, N 10.98; found C 63.03, H 6.04,
N 10.52.
N-(1-Phenylpyrazol-5-yl)-N-[1-(2-phenethyl)-4-piperidyl)
- propanamide (4b). (Yield 64%): oxalate, mp 204 ꢄC;
1H NMR(CDCl3) d (ppm) 7.69 (1H, d, H-3(Pz)), 7.30
(10H, C6H5Pz, C6H5CH2), 6.26 (1H, H-4(Pz), 4.40 (1H,
m, H-4(Pip)), 3.00 (2H, d, H-2(Pip)), 2.80 (2H, d, H-
3(Pip)), 2.70 (2H, m, (C6H5CH2CH2), 2.48 (2H, m,
(C6H5CH2CH2), 2.12 (2H, q, CH2CH3), 1.95 (2H, m,
H-2(Pip)), 1.60 (2H, m, H-3(Pip)), 1.07 (3H, t,
CH2CH3)); 13C NMR (CDCl3) d (ppm) 174.3 (CO),
140.0 (ipso-(C6H5CH2)), 139.9 (C-3(Pz)), 138.9 (C-
5(Pz)), 137.2 (ipso(C6H5Pz)), 129.1 (m-(C6H5Pz)), 128.4
(m-(C6H5CH2)), 128.2 (o-(C6H5CH2)), 127.4 (p-
(C6H5Pz)), 125.8 (p-(C6H5CH2)), 122.6 (o-(C6H5Pz)),
107.4 (C-4(Pz)), 60.1 (C6H5CH2CH2), 53.8 (C-4(Pip),
52.6 (C-2(Pip)), 52.5 (C-2(Pip)), 33.6 (C6H5CH2CH2),
30.0 (CH2CH3), 28.7 (C-3(Pip)), 28.3 (C-3(Pip)), 9.15
(CH2CH3). MS (FAB) [M+1] 403.2. Anal.
(C25H30N4O.C2H2O4): calcd C 65.84, H 6.55, N 11.37;
found C 65.02, H 6.51, N 10.97.
N-(1-Phenylpyrazol-5-yl)-N-(1-benzyl-4-piperidyl)-amine
(2a). (Yield, 84%): mp 94–96 ꢄC; H NMR(CDCl3) d
1
(ppm) 7.56–7.26 (10H, m, C6H5Pz, C6H5CH2), 7.52
(1H, brs, H-3(Pz)), 5.52 (1H, H-4(Pz)), 3.63 (1H, d,
NH), 3.52 (2H, s, C6H5CH2), 3.13 (1H, m, H-4(Pip)),
2.78 (2H, m, H-2(Pip)), 2.08 (4H, m, H-2(Pip), H-
3(Pip)), 1.47 (2H, m, H-3(Pip); 13C NMR (CDCl3) d
(ppm) 146.5 (C-5(Pz)), 140.4 (C-3(Pz)),138.0 (ipso-
C6H5), 137.5 (ipso-C6H5), 124.1, 127.0, 127.3, 128.2,
129.0, 129.5 (o, m, p-C6H5), 87.7 (C-4(Pz), 62.9
(C6H5CH2), 52.5 (C-4(Pip)), 51.9 (C-2(Pip)), 32.3 (C-
3(Pip)). MS (electrospray) [M+1] 333.3.
Pharmacological assays
In vivo assays. CD1 male mice weighing 25–30 g were
used. All the animals were supplied with food and water
‘ad libitum’ and were housed in a temperature-con-
trolled room at 23 ꢄC. Lighting was on a 12/12-h light/
dark cycle. The mice were housed for at least 1 day in
the test-room before experimentation.
N-(1-Phenylpyrazol-5-yl)-N-(1-benzyl-4-piperidyl)-propa-
namide (4a). (Yield 65%): mp 45–48 ꢄC; 1H NMR
(CDCl3) d (ppm) 7.63 (1H, d, H-3(Pz)), 7.28 (10H,
C6H5Pz, C6H5CH2), 6.18 (1H, d, H-4(Pz)), 4.30 (1H, m,
H-4(Pip)), 3.31 (2H, d, C6H5CH2), 2.80 (1H, brd, H-
2(Pip)), 2.60 (1H, brd, H-2(Pip)), 2.20–1.95 (5H, m,
CH2CH3, 1H-3(Pip), 2H-2(Pip)), 1.80 (2H, m, H-
3(Pip)), 1.49 (1H, m, H-3(Pip)), 0.99 (3H, t, CH2CH3);
13C NMR (CDCl3) d (ppm) 174.4 (CO), 139.9 (C-3(Pz)),
139.0 (C-5(Pz)), 138.0 (ipso(C6H5CH2)), 137.5 (ipso-
(C6H5Pz)), 129.2 (m-(C6H5Pz)), 128.9 (m-(C6H5CH2)),
128.1 (o-(C6H5CH2)), 127.4 (p-(C6H5Pz)), 126.9 (p-
(C6H5CH2)), 122.8 (o-(C6H5Pz)), 107.5 (C-4(Pz)),
62.7(C6H5CH2), 54.1 (C-4(Pip), 52.6 (C-2(Pip)), 30.1
(CH2CH3), 28.8 (C-3(Pip)), 28.4 (C-3(Pip), 9.2
(CH2CH3). MS (electrospray) [M+1] 389.2. Anal.
(C24H28N4O.C2H2O4): calcd C 65.26, H 6.32, N 11.71;
found C 64.00, H 6.21, N 11.49.
To detect antinociceptive activity, several doses (0.1–
20 mg/kg) of the tested compounds or saline solution
were intraperitoneally (ip) administered to separated
groups of mice (nꢅ10) 30 min before the analgesic effect
was tested. In order to evaluate the antinociception
level, the effect of ip morphine (2.5–10 mg/kg) was also
tested. Each mouse was used only once and an observer
who was unaware of the treatment performed the test-
ing and data recording.
Writhing test. The mice were ip injected with a 2% ace-
tic acid solution to produce the typical writhing reaction
which is characterized by a wave of contraction
of the abdominal musculature followed by exten-
sion of the hind limbs. After the acetic acid
administration, mice were placed in individual trans-
parent containers and, 5 min later, the number of
writhes was counted during a 10 min period. The
mean number of writhes in naıve animals was
19.8ꢂ0.7 (n=12). This value was not significantly
modified after the ip administration of saline slution
(19ꢂ0.9) (n=12). The effect of the studied compounds
was expressed as percentage of modification of this
control value.
N-(1-Phenylpyrazol-5-yl)-N-(4-piperidyl)-propanamide
(6). (Yield 66%): mp 198 ꢄC (dec. 145–150 ꢄC); 1H
NMR (CDCl3) d (ppm) 7.63 (1H, d, H-3(Pz)), 7.30 (5H,
C6H5Pz), 6.21 (1H, d, H-4(Pz)), 4.38 (1H, m, H-4(Pip)),
3.72 (2H, d, C6H5CH2), 3.05 (1H, brd, H-2(Pip)), 2.95
(1H, brd, H-2(Pip)), 2.80–2.40 (3H, m, H-(Pip)), 2.11
(1H, q, CH2CH3), 2.01 (1H, q, CH2CH3), 1.90–1.60
(2H, m, H-(Pip)), 1.44 (1H, qd, H-3(Pip)), 0.99 (3H, t,
CH2CH3); 13C NMR (CDCl3) d (ppm) 174.3 (CO),
140.0 (C-3(Pz)), 138.8 (C-5(Pz)), 137.2 (ipso-(C6H5Pz)),
129.2 (m-(C6H5Pz), 127.6 (p-(C6H5Pz)), 122.8 (o-
(C6H5Pz)), 107.3 (C-4(Pz)), 53.5 (C-2(Pip)), 50.9 (C-
4(Pip)), 28.4 (CH2CH3), 9.14 (CH2CH3), 30.1
(CH2CH3), 28.8 (C-3(Pip)), 28.4 (C-3(Pip)). MS (elec-
trospray) [M+1] 299.2.
Hot plate test. This test was carried out with a hot plate
at 55 ꢄC as a nociceptive stimulus. The control reaction
latency of the animals was measured before the treat-
ment. The time of licking of the front paw was taken as
an index of nociception. The latency was measured
before drug or saline administration (control) and 30
min after treatment. The cut-off time was 30 s and
analgesia was quantified with the formula of the Max-
imum Possible Effect (MPE):

N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
825
MPE=(Latency after treatment ꢁ Control Latency)/
(Cut-off timeꢁControl latency). Control latency was
evaluated in each individual experiment, the main value
being 10.7ꢂ0.3 (n=12).
concentration had reached a plateau. The interval
between application of increasing concentrations was 5
min. Curves were constructed for:
1. The m agonist, morphine (10ꢁ7–1.6ꢃ10ꢁ6 M) and for
3b and 3d (3b: 5ꢃ10ꢁ8–8ꢃ10ꢁ7 mM; 3d: 2.5ꢃ10ꢁ8 to
4ꢃ10ꢁ7 M) in guinea-pig MP-LM strips
The opioid antagonist naloxone was used in order to
assess the involvement of the opioid system. Separated
groups of animals (n ꢅ10) were ip treated with nalox-
one (1 mg/kg ip) and one of the active compounds (0.1–
2 mg/kg), and 30 min later either the writhing test or the
hot plate test were carried out. The inhibition of mor-
phine (5 mg/kg) antinociception induced by naloxone
was used as a control.
2. The d agonist, DEPEN ([D-Pen2,D-Pen5]enkephalin)
(2ꢃ10ꢁ9–1.6ꢃ10ꢁ8 M) and for 3b and 3d (3b: 5ꢃ10ꢁ8
–
8ꢃ10ꢁ7 M; 3d: 2.5ꢃ10ꢁ8–4ꢃ10ꢁ7 M) in mouse vas
deferens
3. The k agonist, U-50,488H, (10ꢁ8 M–1.6ꢃ10ꢁ7 M)
and for 3b and 3d (3b: 5ꢃ10ꢁ8–8ꢃ10ꢁ7 M; 3d:
2.5ꢃ10ꢁ8–4ꢃ10ꢁ7 M) in rabbit vas deferens.
Finally, the duration of the analgesic effect of the new
compounds was compared with that of fentanyl. The
analgesic effect of fentanyl (0.03–0.4 mg/kg), 3b (0.4–
2 mg/kg) and 3d (0.3–1 mg/kg) was tested 5 min
after their ip administration, from these dose–response
curves equipotent (85–95% of inhibition of the noci-
ceptive response) doses were selected and the
analgesic effect of fentanyl (0.4 mg/kg) was tested
5, 15, 30, 60 and 120 min after its ip administra-
tion and the effect of 3b (2 mg/kg) and of 3d (1 mg/kg)
were tested 5, 30, 60, 120, 240 and 360 min after its ip
administration.
The effect of the drugs was evaluated 5 min after the
addition of each dose, as % of inhibition, taking the
amplitude of the last contraction before the first
addition of agonist as 100%. The opioid agonists
were added to the organ bath 15 min after the
beginning of electrical stimulation. Each tissue was
employed to construct only one concentration–response
curve.
To corroborate that the inhibitory effect of the
selective opioid agonists or of the new compounds
was mediated through interaction with their respec-
tive opioid receptors, one dose of selective antago-
nists was added to the organ bath at the end of each
experiment. Naloxone (5ꢃ10ꢁ8 M), nor-binaltorphi-
mine (10ꢁ8 M) and naltrindole (10ꢁ9 and 10ꢁ8 M)
were used as m, k and d antagonists, respectively. In
mouse vas deferens the effect of active compounds,
3b (2ꢃ10ꢁ7 M) and 3d (4ꢃ10ꢁ7 M), was also antag-
onized by addition of the m selective antagonist CTOP
(10ꢁ6 M).30,31
The compounds showing antinociceptive activity were
tested for their functional activity on isolated tissues
commonly used to study and characterize opioid effects
(guinea pig ileum, mouse vas deferens and rabbit vas
deferens).
In vitro assays
Isolated tissues. Male guinea-pigs weighing 300–450 g,
male CD-1 mice weighing 25–30 g and male New Zeal-
and rabbits weighing 3–3.5 kg were used for this study.
Myenteric plexus-longitudinal muscle strips (MP-LM)
were isolated from guinea-pig ileum as described by
Binding assays
Ambache,36 the rabbit vas deferens was prepared as
Neural membranes from whole mice brain. Preparation
of membranes: Neural membranes (P2 fractions) were
prepared from the whole brain of male Swiss Webster
mice. Briefly, the tissue samples were homogenized in 5
mL of ice-cold Tris sucrose buffer (5 mM Tris–HCl, 250
mM sucrose, pH 7.4). The homogenates were cen-
trifuged at 1100g for 10 min, and the supernatants were
then recentrifuged at 40,000g for 10 min. The resulting
pellet was incubated at 25 ꢄC for 30 min to remove
endogenous opioids. After that, the pellet was washed
twice and resuspended in 50 mM Tris–HCl buffer
(pH 7.5) to a final protein content of 0.83ꢂ0.14 mg
37
described by Oka
and the mouse vas deferens as
described by Hughes.38 Tissues were suspended in a 20
mL organ bath containing Krebs solution (NaCl 118,
KCl4.75; CaCl2 2.54; KH2PO4 1.19; MgSO4 1.2;
NaHCO3 25; glucose 11 mM). This solution was con-
tinuously gassed with 95% O2 and 5% CO2. Tissues
were kept under 1 g (guinea pig ileum) or 0.5 g (rabbit
or mouse vas deferens) of resting tension at 32 ꢄC and
were electrically stimulated through two platinum ring
electrodes. Guinea-pig MP-LM strips were stimulated
with rectangular pulses of 70 V, 0.1 ms duration and 0.3
Hz frequency. Rabbit vas deferens were stimulated with
rectangular pulses of 70 V, 0.1 ms duration and 0.1 Hz
frequency, and mouse vas deferens with trains of 15
rectangular pulses of 70 V, 15 Hz and 2 ms duration
each min. The isometric force was recorded on a Grass
model 7Apolygraph.
mLꢁ1
.
Binding assay: Total binding was measured in 0.55-mL-
aliquots (50 mM Tris–HCl, pH 7.5) of the neural mem-
branes which were incubated with [3H]DAMGO (2
nM), [3H]U-69593 (ꢄ2 nM) or [3H]DPDPE (4 nM) for 60
min at 25 ꢄC, or 37 C in the [3H]DPDPE assays, in the
absence or presence of the competing compounds
(10ꢁ16–10ꢁ4 M, 14 concentrations). Total binding was
determined and plotted as a function of the compound
concentration.
Cumulative concentration–response curves for selective
m, d or k opioid receptor agonists or the new anti-
nociceptive compounds (3b and 3d) were constructed in
a step by step manner after the response to the previous

826
N. Jagerovic et al. / Bioorg. Med. Chem. 10 (2002) 817–827
Incubations were terminated by diluting the samples
with 5 mL of ice-cold Tris incubation buffer (4 ꢄC).
Membrane bound was separated by vacuum filtration
through Whatman GF/C glass fibre filters. Then, the
filters were rinsed twice with 5 mL of incubation buffer
and transferred to minivials containing 3 mL of Opti-
Phase ‘HiSafe’ II cocktail and counted for radioactivity
by liquid scintillation spectrometry.
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Acknowledgements
This work was supported by Spanish grants SAF 97-
0044-CO2 and SAF 00-0114-C02. L.F.C. is recipient of
a postdoctoral fellowship from the Basque Government.
C.C. is grateful to Dr Esteve S.A. for financial support.
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