Synthesis and opioid activity of new fentanyl analogs

Article abstract of DOI:10.1016/S0024-3205(02)01798-8

Three new fentanyl analogs (compounds 3-4-5) have been synthesized and evaluated for antinociceptive properties using the writhing test. The analgesic property of the active compound, N-[1-phenylpyrazol-3-yl]-N-[1-(2-phenethyl)-4-piperidyl)] propenamide (

Full text of DOI:10.1016/S0024-3205(02)01798-8

Life Sciences 71 (2002) 1023–1034
www.elsevier.com/locate/lifescie
Synthesis and opioid activity of new fentanyl analogs
a
a
a
a
a,
*
´
´
´
Rocıo Giron , Raquel Abalo , Carlos Goicoechea , M Isabel Martın
Luis F. Callado b, Carolina Cano c, Pilar Goya c, Nadine Jagerovic c
,
aUnidad de Farmacolog´ıa, Facultad de Ciencias de la Salud, Universidad Rey Juan Carlos, Avda. Atenas, s/n,
E-28922 Alcorco´n, Madrid, Spain
bDepartamento de Farmacolog´ıa, Universidad del Pa´ıs Vasco/Euskal Herriko Unibertsitatea, Leioa, Bizkaia, Spain
cInstituto de Qu´ımica Me´dica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain
Received 30 October 2001; accepted 16 January 2002
Abstract
Three new fentanyl analogs (compounds 3-4-5) have been synthesized and evaluated for antinociceptive
properties using the writhing test. The analgesic property of the active compound, N-[1-phenylpyrazol-3-yl]-N-[1-
(2-phenethyl)-4-piperidyl)] propenamide (compound 4), was tested using the hot plate test in mice. Its opioid
agonistic activity was characterized using three isolated tissues: guinea pig ileum, mouse vas deferens, and rabbit
vas deferens. Compound 4 was as effective as fentanyl or morphine and it showed less antinociceptive potency
than fentanyl but it was more potent than morphine. The duration of the antinociception was similar to that of
fentanyl. This compound inhibited the electrically evoked contractions of myenteric plexus-longitudinal muscle
strips of guinea pig ileum and of mouse vas deferens but not those of rabbit vas deferens. These effects could be
reversed by A selective antagonists (naloxone and/or CTOP) but not by the y selective antagonist naltrindole, thus
indicating that the compound acted as a A opioid agonist. Finally, the binding data confirmed that compound 4 had
high affinity and selectivity for the A-receptor. D 2002 Published by Elsevier Science Inc.
Keywords: Fentanyl analog; Nociception; In vitro assay; Opioid receptor; New analgesics
Introduction
Natural opioids and their synthetic derivatives belong to the most potent analgesics known so far and
are used in almost all fields of medicine. The main opioid receptor involved in pain perception is A
*
Corresponding author. Tel.: +34-91-4888855; fax: +34-91-4888831.
´
E-mail address: i.martin@cs.urjc.es (M.I. Martın).
0024-3205/02/$ - see front matter D 2002 Published by Elsevier Science Inc.
PII: S0024-3205(02)01798-8

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subtype, and A opioid agonists, such as morphine, methadone, codeine or fentanyl, are the most
commonly used opioids.
Ever since the potent synthetic opioid fentanyl was reported in the sixties [1] it has been the subject of
much interest. Over the years many analogs with variations of the basic 4-anilidopiperidine structure
have been synthesized, but despite the large number of derivatives reported in the literature fentanyl is
the most used drug of this group. It was first used as a component of general anaesthetic regimens
because of its high potency (75 to 100 times more than morphine [2] and short duration [3]). However its
low molecular weight and high lipid solubility make it suitable for transdermal administration; on this
basis, fentanyl has been incorporated into transdermal therapeutic systems designed to release the drug at
constant rate [4]. Despite of the undeniable relevance of its clinical use, fentanyl does not lack side
effects after intravenous or transdermal administration mainly: nausea, vomiting, constipation, somno-
lence and confusion among the most frequent problems ( > 10 %) and hipoventilation, syncope,
arrhythmia and paranoid reaction among the most severe although less frequent (1-2 %) adverse effects
[2]. In addition there are isolated cases of over-dosage that can take place by exposure to thermal heat
sources (heat pads, electric blankets, saunas. . .) and fever can also increase absortion of fentanyl [5].
Therefore, the synthesis of new fentanyl analogs with high potency but reduced side effects is still one
of the challenges of opioid research. Within this context, many anilidopiperidines have been synthesized
and evaluated for SAR studies [6]. The modifications carried out in the fentanyl molecule include the
replacement of the benzene ring by pyridine, six-membered heterocycles and fused heteroaryl
derivatives, the latter with reduced activity when compared to fentanyl itself [7]. More recently,
isothiocyanate derivatives of fentanyl have been reported to exhibit lower analgesic activity than the
corresponding parent compounds [8]. A considerable improvement seems to be the introduction of a
methyl group at position 4 since the preliminary pharmacological evaluation of 4-methyl-fentanyl
indicates that it is about four times more potent than fentanyl itself [9]. Within this context, we ourselves
have recently proved that substitution of the anilido phenyl ring by phenylpyrazole provides interesting
compounds (1, 2) with longer duration of action than fentanyl [10]. We now wish to report further
derivatives of this series, compound 3, a N-benzylpiperidine derivative, compound 4 with propenamide
replacing propanamide as the N-acyl substituent in the phenylpyrazole derivative structure (1) and
Fig. 1. Chemical structures of new fentanyl derivatives.

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compound 5 in which the fentanyl anilino ring has been ortho-substituted by a phenyl group. Fentanyl
derivative with a modified propanamido group as proposed in structure 4, has been reported for the
fentanyl itself with an interesting pharmacological profile [11] Fig. 1 shows the chemical structures of
the new fentanyl derivatives.
Material and methods
Synthesis
Compounds 3-4-5 were prepared as described in Fig. 2 [12]. Compound 3 was synthesized from 1-
benzyl-4-piperidone and compounds 4 and 5 from 1-phenethyl-4-piperidone. Reductive amination of the
piperidones with phenylpyrazolyl or biphenyl amines, in presence of cyanohydridoborate as reductive
agent [13], afforded the corresponding aminopiperidines. Acylation was carried out in refluxing
propionic anhydride yielding compounds 3 and 5 and was achieved at room temperature with 3-
chloropropionyl chloride in dichloromethane at basic pH giving compound 4. The pharmacological
activity of compounds 3-5 has been evaluated on their oxalate salts. These compounds have been fully
characterized by electrospray mass spectrometry and by NMR spectroscopy.
N-[1-(4-fluorophenyl)pyrazol-3-yl]-N-[1-benzyl-4-piperidyl) propanamide (3) [10]: overall yield =
50 %; mp (oxalate) = 192-193 jC; elemental analysis C24H27N4OF.C2H2O4 theorical (found): C: 62.90
(62.79); H: 5.84 (6.10); N: 11.29 (11.09).
N-[1-phenylpyrazol-3-yl]-N-[1-(2-phenethyl)-4-piperidyl)]propenamide (4): overall yield = 22 %; mp
(oxalate) = 190-192 jC; elemental analysis C25H28N4O.C2H2O4 theoretical (found): C: 66.12 (65.89);
H: 6.12 (6.30); N: 11.42 (11.29).
N-[2-biphenyl]-N-[1-(2-phenethyl)-4-piperidyl)]propanamide (5): overall yield = 10 %; mp (oxalate) =
205-210 jC; elemental analysis C28H32N2O.C2H2O4 theoretical (found): C: 71.71 (69.42); H: 6.77
(6.10); N: 5.57 (5.26).
Pharmacological assays
Writhing test
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-controlled room at 22 jC. Lighting was on a 12/12-h light/
dark cycle. The animals were housed for at least 1 day in the test-room before experimentation and then
Fig. 2. Synthesis of biologically evaluated compounds.

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the writhing test was carried out: the mice were intraperitoneally (i.p.) injected with a 2% acetic acid
solution to produce the typical writhing reaction that is characterized by a wave of contraction of the
abdominal musculature followed by extension of the hind limbs. After the acetic acid administration,
mice were placed in individual transparent containers and, 5 min later, the number of writhes was
counted during a 10 min period.
To detect antinociceptive activity, several doses (0.05 to 10 mg/kg) of the three new compounds or
saline solution were i.p. administered to separated groups of mice (n z 10) before the acetic acid
administration. In order to compare the antinociceptive efficacy and potency, the effect of i.p. morphine
(2.5 to 10 mg/kg) or fentanyl (0.01 to 0.4 mg/kg) were also tested. Each mouse was used only once and
an observer who was unaware of the treatment performed the testing and data recording. After this test,
compounds showing no-antinociceptive effect at the tested doses were discarded.
To assess that the analgesia was mediated through opioid receptors, naloxone (1 mg/kg) was i.p.
injected 25 min before the administration of the larger dose of compound 4 or fentanyl.
In vitro assays
The functional activity of compound 4 was also tested on isolated tissues commonly used to study and
characterize opioid effects: guinea pig ileum, mouse vas deferens and rabbit vas deferens.
Male guinea-pigs weighing 300–450 g, male CD-1 mice weighing 25–30 g and male New Zealand
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 Ambache [14], the rabbit vas deferens was
prepared as described by Oka [15] and the mouse vas deferens as described by Hughes [16]. Tissues
were suspended in a 10 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 continuously 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 jC and were electrically stimulated through two platinum ring
electrodes. 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; 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 7A polygraph.
Cumulative concentration-response curves for selective A, y and n opioid receptor agonists or the new
compound were constructed in a step by step manner after the response to the previous concentration had
reached a plateau. The interval between application of increasing concentrations was 5 min. Curves were
constructed for:
1. The A agonist, morphine (5 ꢀ 10ꢁ8 to 8 ꢀ 10ꢁ7 M) and compound 4 (1.25 ꢀ 10ꢁ8 to 2 ꢀ 10ꢁ7
M) in guinea-pig MP-LM strips
2. The y agonist, [D-Pen2,D-Pen5]enkephalin (DEPEN) [15] (2 ꢀ 10ꢁ9 to 1.28 ꢀ 10ꢁ7 M) and
compound 4 (2.5 ꢀ 10ꢁ8 to 1.6 ꢀ 10ꢁ6 M) in mouse vas deferens
3. The n agonist, U-50,488H [16], (5 ꢀ 10ꢁ9 to 3.2 ꢀ 10ꢁ7 M) and compound 4 (2.5 ꢀ 10ꢁ8 to 1.6
ꢀ 10ꢁ6 M) in rabbit vas deferens.
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

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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 compound was
mediated through selective interaction with opioid receptors, one dose of selective antagonists was added
to the organ bath at the end of each experiment. Naloxone (5 ꢀ 10ꢁ8 M), nor-binaltorphimine [17] (10ꢁ8
M) and naltrindole [18] (10ꢁ9 and 10ꢁ8 M) were used as A, n and y antagonists, respectively. In mouse
vas deferens the effect of compound 4 was also antagonised by addition of the A selective antagonist
CTOP (10ꢁ6 M) [19].
In rabbit vas deferens the effect of U-50488H was also tested after de administration of the largest
concentration of compound 4 (1.6 ꢀ 10ꢁ6 M), to discard antagonistic activity.
Hot plate test
CD1 male mice housed as above described conditions were used. The test was carried out using a hot
plate at 55 jC as nociceptive stimulus. 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 5 min after
treatment with compound 4 or fentanyl. The cut-off time was 30 s and analgesia was quantified with the
formula of the Maximum Possible Effect (M.P.E.):
M:P:E: ¼ ½ðLatency after treatment ꢁ Control LatencyÞ
=ðCut ꢁ off time ꢁ Control latencyÞꢂ
ꢀ100
The control reaction latency of the animals was measured in each animal before the treatment, then
separated groups (n = 10) of mice were i.p. treated with: fentanyl (0.03 to 0.4 mg/kg), morphine (2.5 to
10 mg/kg) or compound 4 (0.1 to 2 mg/kg).
The opioid antagonists naloxone (1 mg/kg) and naltrindole (20 mg/kg) were i.p. injected in order to
assess the involvement of the opioid system. Separated groups of animals (n = 10) were treated with the
A or y antagonists and 25 min later with fentanyl (0.04 mg/kg) or compound 4 (0.75 mg/kg), and 5 min
later the hot plate test was carried out.
To test the duration of the analgesic effect one equipotent dose (90 F 5% inhibition of the nociceptive
response) of compound 4 (0.75 mg/kg), fentanyl (0.4 mg/kg) or morphine (10 mg/kg) was tested 5, 15,
30, 60, 90 and 120 min after their administration.
Binding assays
Preparation of membranes
Neural membranes (P2 fractions) were prepared from the whole brain of male Swiss Webster mice
[20]. 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 centrifuged at 1,100 g for 10 min, and the
supernatants were then recentrifuged at 40,000 g for 10 min. The resulting pellet was incubated at 25 jC
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 0.83 F 0.14 mg.mlꢁ1.

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Binding assay
Total binding was measured in 0.55 ml-aliquots (50 mM Tris HCl, pH 7.5) of the neural membranes
which were incubated with [3H]DAMGO (2 nM), [3H]U-69593 (2 nM) or [3H]DEPEN (4 nM) for 60
min at 25 jC, or 37 jC in the [3H]DEPEN assays, in the absence or presence of the competing
compounds (10ꢁ16 M to 10ꢁ4 M, 14 concentrations). Total binding was determined and plotted as a
function of the compound concentration.
Incubations were terminated by diluting the samples with 5 ml of ice-cold Tris incubation buffer (4
jC). 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 OptiPhase ‘‘HiSafe’’ II cocktail and counted for radioactivity by liquid scintillation spectrometry.
Analysis of binding data
Analysis of competition experiments to obtain the inhibition constant (Ki) were performed by non-
linear regression using the EBDA-LIGAND program. All experiments were analysed assuming a one-
site model of radioligand binding.
Drugs
[3H]DAMGO (specific activity 50 Ci/mmol) was purchased from American Radiolabeled Chemicals
Inc., USA. [3H]U69593 (specific activity 41.4 Ci/mmol) and [3H]DEPEN (specific activity 45 Ci/mmol)
were obtained from NEN Life Science Products Inc., USA. Other reagents were obtained from Sigma
Chemical Co. (St. Louis, MO, USA).
Results
Writhing test
When the antinociceptive activity of the new compounds was determined carrying out the writhing
test, only compound 4 was able to significantly reduce the number of stretches. It was found that
compound 4 as well as the reference compounds, morphine and fentanyl, were able to induce a dose-
dependent antinociceptive effect. Compounds 3 and 5 did not induce any antinociceptive effect.
Compound 4 was more potent than morphine but less potent than fentanyl. The efficacy of the three
drugs was similar, they were able to completely abolish the writhes induced by the i.p. administration of
acetic acid (Fig. 3).
In vitro assays
In MP-LM strips of guinea pig ileum compound 4 as well as control opioid, morphine, induced a
dose-dependent inhibition of the electrically induced contractions (Fig. 4A). ID50 values were:
compound 4: 4.4 ꢀ 10ꢁ8 M (95% confidence interval 3.6 ꢀ 10ꢁ8 to 5.4 ꢀ 10ꢁ8 M); morphine: 1.8
ꢀ 10ꢁ7 (95% confidence interval: 1.4 ꢀ 10ꢁ7 to 2.3 ꢀ 10ꢁ7 M). The ‘‘in vitro’’ effect of all the tested
drugs was almost completely reversed by the ‘‘in vitro’’ administration of naloxone (5 ꢀ 10ꢁ8 M) or of
CTOP (10ꢁ6 M). The inhibition induced by the largest concentration of morphine (2 ꢀ 10ꢁ7 M) was in
control tissues 86.5 F 9.5 % (n = 10), after the administration of naloxone it was 5 F 2 % (n = 5) and

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Fig. 3. Each point represents the mean % F SEM (n = 12) of inhibition of the number of writhes induced by compound 4,
morphine or fentanyl. Mean number of writhes recorded in control animals 23 F 2.3 (n = 14).
after CTOP 16.4 F 12.3 % (n = 5). When compound 4 (8 ꢀ 10ꢁ7 M) was added to the organ bath the
mean inhibition was 86.4 F 6.4 % (n = 9), after naloxone 3 F 5.1 % (n = 5) and after CTOP 16 F 8.6 %
(n = 4).
Fig. 4. Each point shows the mean % of inhibition F SEM (n z 5) of the electrically induced contractile response by cumulative
administration of compound 4 and selective opioid agonists in the studied tissues.

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Fig. 5. Hot plate test. Each point shows the % of MPE F SEM (n z 12) induced by compound 4, morphine and fentanyl.
The effect of the new compound was also tested on mouse vas deferens and on rabbit vas deferens and
its effect was compared with that of two selective agonists: DEPEN (y) and U-50,488H (n). Naltrindole
was used as y selective antagonist and norbinaltorfimine as n antagonist.
In comparison with the y agonist DEPEN, compound 4 was 10 folds less potent, being their ID50:
compound 4: 7.4 ꢀ 10ꢁ7 M (95 % confidence interval: 6.3 ꢀ 10ꢁ7 to 8.6 ꢀ 10ꢁ7 M); DEPEN 6.2 ꢀ
10ꢁ8 M (95% confidence interval: 4.1 ꢀ 10ꢁ7 to 9.6 ꢀ 10ꢁ7 M) in inhibiting the electrically induced
contractions in the mouse vas deferens (Fig. 4B). The inhibition of the contractile response induced by
the new derivative was not significantly reduced by naltrindole (10ꢁ9 or 10ꢁ8 M), as expected the effect
of DEPEN (6 ꢀ 10ꢁ8 M) was antagonized by naltrindole (10ꢁ9 M), the inhibition induced by the y
agonist was 55.3 F 10.2 % (n = 6) in control tissues and after naltrindole administration it was 36 F 7.9
% (n = 5).
Fig. 6. Each point shows the % of MPE F SEM (n z 12) induced by administration of equipotent doses of compound 4 (0.75
mg/kg), morphine (10 mg/kg) and fentanyl (0.4 mg/kg).

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Table 1
Binding values of fentanyl and compound 4 to A, y and n opioid receptors
Receptors
[3H]DAMGO (2 nM)
[3H]DEPEN (4 nM)
[3H]U-69593 (2 nM)
A
y
n
Fentanyl
Compound 4
6 F 1.5
44 F 3
663 F 184
>10.000
298 F 40
4617 F 1851
Values are expressed as mean F Standard Error of the Mean of three experiments.
In rabbit vas deferens, U-50,488H induced a dose dependent inhibition of the electrically induced
contractions, ID50: 1.4 ꢀ 10ꢁ7 M (confidence interval: 1.2 ꢀ 10ꢁ7 to 1.7 ꢀ 10ꢁ7 M). The selective n
antagonist norbinaltorfimine completely antagonized this effect, the inhibition induced by the largest
concentration of U-50,488H was: 74.4 F 13.3 % M and after norbinaltorfimine (10ꢁ8 M) addition to the
organ bath the inhibition was: 7.8 F 3 % (n = 5). Compound 4 (2.5 ꢀ 10ꢁ8 to 1.6 ꢀ 10ꢁ6 M (n=5)) did
not significantly modify the contractile response of the rabbit vas deferens (Fig. 4C) and the largest
tested dose (1.6 ꢀ 10ꢁ6 M) was not able to recover the contractile activity of the tissue after treatment
with the n agonist.
Hot plate test
Compound 4 showed more potent antinociceptive effect than morphine but less than fentanyl in the
hot plate test (Fig. 5).
When the duration of the analgesic effect of equipotent doses (85 to 95 % of inhibition of the
nociceptive response) of new compound was compared with those of the reference compounds, fentanyl
and morphine, it was find to be quite similar to that of fentanyl and shorter than that of morphine (Fig. 6).
The selective involvement of A opioid receptor in the analgesic effect of compound 4 was
corroborated because naloxone (A antagonist) but not naltrindole (y antagonist) was able to antagonize
the antinociception induced by compound 4 and fentanyl.
Binding
The binding data for the A, y and n receptors of compound 4 and fentanyl are provided in Table 1. In
terms of binding affinity, compound 4 showed greater affinity for the A site than for y and n sites.
Fentanyl, tested for comparative purposes, showed, as expected, high affinity for the A receptor.
Discussion
The writhing test in mice was used because it is widely employed at the first stages of the
evaluation of antinociceptive Drugs [21] and it was useful to discard compounds lacking analgesic
activity. This first step of the evaluation of compounds 3, 4 and 5 showed that only one (4) of the new
compounds exerted antinociceptive activity, being even more potent than morphine. Moreover it could
be suggested that the opioid system could be involved in this effect because naloxone, an opioid

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antagonist, was able to fully antagonize the effect of compound 4 as well as the effect of the reference
compounds morphine and fentanyl.
To functionally determine the subtype of opioid receptor that mediated the analgesia, isolated tissues
were used: guinea pig ileum for A and n receptors [22], mouse vas deferens for y receptors and rabbit vas
deferens for n receptors [19,22,23].
Compound 4 inhibited, in a concentration-dependent manner, the electrically induced contractions in
the guinea pig ileum and this effect was antagonized by naloxone and by the A selective antagonist CTOP
[19,24]. From this data it could be accepted that A receptors play and important role in the effect of
compound 4. However, from these data, the involvement of n and y receptors could not be disregarded.
To clarify these possibilities, mouse vas deferens preparations that mainly contain y receptors
(although they have also y and n receptors), and rabbit vas deferens, having only n opioid receptors,
were used.
Compound 4 inhibited the mouse vas deferens contractile responses but this inhibition was not
antagonised by the y selective antagonist naltrindole. These data discard the involvement of y opioid
receptors on the compound 4 actions.
The absence of effectiveness of compound 4 in rabbit vas deferens discards any functional activity of
the new fentanyl derivative upon n receptors. The possibility of this new compound to antagonize the
effect of U-50,488H acting on n receptors was also tested in rabbit vas deferens and could be also
disregarded because it did not modify the inhibition induced by the n agonist. Taken together, these ‘‘in
vitro’’ results indicate that the new compound is a A opioid agonist.
The hot plate test was used to corroborate the analgesia because it is a more sensitive test and is
largely employed to evaluate opioids [25]. Compound 4 induced a dose-dependent antinociceptive effect
that was antagonized by naloxone; from these data it could be suggested that the activation of opioid
receptors play a role in the effect of this drug.
The functional activity of this new compound on A opioid receptors as well as the absence of effect on
y and n receptors is in agreement with binding data that provided evidence of the high affinity of
compound 4 for A receptors with very low affinity for y or n receptors.
The present results along with our previous studies on phenylpyrazole fentanyl derivatives [10]
provide the three following observations concerning variations in fentanyl structure related to activity:
(i) Hot plate test of fentanyl bearing propenamide instead of propanamide moiety have been reported
[11]. The author suggested a mode of interaction with A receptors different from morphine.
(ii) In our previous studies we reported compound 2 (Fig. 1) as a very potent and selective opioid
agonist with an in vivo potency greater than morphine and a duration of analgesic effect longer than
fentanyl. Substitution of the phenethyl group of 2 by a benzyl group (compound 3) led to significant
losses in activity.
(iii) The results obtained for compound 5 together with those presented in our previous study about 1-
phenyl-5-yl propanamide derivative, an isomeric structure of 2, clearly indicate that a phenyl group in
ortho position with respect to the propanamide group produces loss of activity.
Conclusion
Three new fentanyl analogs have been synthesized and evaluated for antinociceptive activity. Only
one compound (4) has shown interesting analgesic properties with a potency superior to morphine and

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1033
less than fentanyl and a duration of action similar to that of fentanyl. Its pharmacological profile suggests
that it is a very potent and selective A opioid agonist. This, together with previous results from our group
[10], suggests that substitution of the phenyl ring of fentanyl by a 1-phenylpyrazole rest is a good
alternative for preparing new fentanyl analogs with selective interaction with A receptors and potent
antinociceptive effect. Evaluation of side effects of these new compounds is currently under study.
Acknowledgements
This work was supported by Spanish government grants SAF 97-0044-CO2 and SAF 00-0114-C02,
and Laboratorios del Dr. Esteve, S.A.
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