0022-3565/00/2942-0444$03.00/0
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 2000 by The American Society for Pharmacology and Experimental Therapeutics
JPET 294:444–450, 2000 /2639/839088
Vol. 294, No. 2
Printed in U.S.A.
Evaluation of Selective NK1 Receptor Antagonist CI-1021 in
Animal Models of Inflammatory and Neuropathic Pain
MARIA I. GONZALEZ, MARK J. FIELD, JOHN HUGHES, and LAKHBIR SINGH
Parke-Davis Neuroscience Research Centre, Cambridge University Forvie Site, Cambridge, United Kingdom
Accepted for publication April 24, 2000
This paper is available online at http://www.jpet.org
The tachykinin neuropeptides, substance P and neurokinin A, are colocalized in capsaicin-sensitive high threshold
nociceptive sensory afferents. They are released by a wide
variety of noxious stimuli from these neurones. Substance P
is the preferred endogenous ligand for the neurokinin-1
(NK1) receptor (Maggi et al., 1993). This receptor type is
widely distributed in specific regions of the central nervous
system (CNS; Kiyama et al., 1993). In the spinal cord, NK1
binding sites are distributed throughout the dorsal (particularly laminae I and II) and ventral horn (Yashpal et al.,
1990). For some time, the potential role of substance P as a
mediator of pain has received a major interest. The main
reason for this has been the discovery of nonpeptide, highly
selective NK1 receptor antagonists with the ability to penetrate the CNS. Several studies have reported that such compounds are active in animal models of inflammatory (Levine
et al., 1984; Yamamoto et al., 1993; Seguin et al., 1995) and
neuropathic (Coudoré-Civiale et al., 1998; Cumberbatch et
al., 1998) pain. Several distinct behavioral signs considered
Received for publication February 23, 2000.
CI-1021 (10 –100 mg/kg) and morphine (3 mg/kg) blocked all the
responses except dynamic allodynia. Carbamazepine (100 mg/kg)
was weakly effective against all the responses. Once daily administration of morphine (3 mg/kg, s.c.) in CCI rats led to the development of tolerance within 6 days. Similar administration of CI1021 (100 mg/kg, s.c.) for up to 10 days did not induce tolerance.
Moreover, the morphine tolerance failed to cross-generalize to
CI-1021. CI-1021 blocked the CCI-induced hypersensitivity in the
guinea pig, with a MED of 0.1 mg/kg, p.o. CI-1021 (10 –100 mg/kg,
s.c.) did not show sedative/ataxic action in the rat rota-rod test. It
is suggested that NK1 receptor antagonists possess a superior
side effect profile to carbamazepine and morphine and may have
a therapeutic use for the treatment of inflammatory and neuropathic pain.
to be representative of pain can be observed in animals after
tissue or nerve injury. However, most studies to date have
examined effects of NK1 receptor antagonists in these models
on only few of these signs. Therefore, the full profile of these
compounds in animal models of pain remains unclear.
CI-1021 (previously, PD 154075) is a selective NK1 receptor antagonist. It possesses nanomolar affinity for human
and guinea pig NK1 receptors. However, it has approximately
300 times lower affinity for the rodent NK1 receptor (Singh et
al., 1997). We have previously reported that CI-1021 is active
in rat models of surgical (Gonzalez et al., 1998) and neuropathic (Field et al., 1998) pain. This study further characterizes the role of NK1 receptors in inflammatory and neuropathic pain states by addressing several issues. First, we
evaluate and compare the profile of chemically different
classes of NK1 receptor antagonists in a model of inflammatory pain and also examine their site of action. Second, we
assess the activity of CI-1021 against a wide range of behavioral signs induced by nerve injury and compare it with other
compounds currently used in the clinic (carbamazepine, morphine). Third, our study addresses the possible development
of tolerance to the antiallodynic action of CI-1021.
ABBREVIATIONS: NK1, neurokinin-1; CCI, chronic constriction injury; MED, minimum effective dose; PEG200, polyethylene glycol 200; PHN,
postherpetic neuralgia; PWL, paw withdrawal latency; CI-1021, [(2-benzofuran)-CH2OCO]-(R)-␣-MeTrp-(S)-NHCH(CH3)Ph; CNS, central nervous
system.
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ABSTRACT
CI-1021 ([(2-benzofuran)-CH2OCO]-(R)-␣-MeTrp-(S)-NHCH(CH3)Ph)
is a selective and competitive neurokinin-1 (NK1) receptor antagonist. This study examines its activity in animal models of inflammatory and neuropathic pain. In mice, CI-1021 (1–30 mg/kg, s.c.)
dose dependently blocked the development of the late phase of
the formalin response with a minimum effective dose (MED) of 3
mg/kg. Two chemically unrelated NK1 receptor antagonists, CP99,994 (3–30 mg/kg) and SR 140333 (1–100 mg/kg), also dose
dependently blocked the late phase, with respective MEDs of 3
and 10 mg/kg. PD 156982, a NK1 receptor antagonist with poor
central nervous system penetration, failed to have any effect.
However, when administered i.c.v., it selectively blocked the late
phase of the formalin response. Chronic constrictive injury (CCI) to
a sciatic nerve in the rat induced spontaneous pain, thermal and
mechanical hyperalgesia, and cold, dynamic, and static allodynia.
2000
Effect of CI-1021 in Models of Pain
Materials and Methods
Animals
Male Sprague-Dawley rats (70 –90 g) and male BKTO mice (20 –25
g) were obtained from Bantin and Kingman, (Hull, UK) and used in
the rota-rod and formalin tests. Male Sprague-Dawley rats (180 –220
g) were also obtained from Charles River (Margate, UK) and used for
the chronic constriction injury (CCI) model. Male Dunkin Hartley
guinea pigs (200 –250 g) were obtained from Harlan Olac (Blackthorn, UK). Rats, guinea pigs, and mice were housed, respectively, in
groups of 6, 4 to 6, and 10 to 12 under a 12-h light/dark cycle (lights
on at 7:00 AM) with food and water ad libitum.
Surgery
Behavioral Tests
Formalin Test. Mice were habituated to perspex observation
chambers (24 cm ⫻ 24 cm ⫻ 24 cm) for at least 15 min before testing.
Formalin-induced hind paw licking and biting was initiated by a
20-␮l s.c. injection of a 5% formalin solution into the plantar surface
of the left hind paw. Immediately after the formalin injection, licking/biting of the injected hind paw was scored in 5-min bins for 60
min. The results are expressed as mean combined licking/biting time
for the early phase (0 –10 min) and late phase (10 – 45 min).
Spontaneous Pain. CCI rats were individually placed in perspex
observation chambers (24 cm ⫻ 24 cm ⫻ 24 cm). A mirror was placed
behind the box to aid observation, and animals were habituated for
5 min. Spontaneous pain was scored for 5 min as described previously by Choi et al. (1994). The test consisted of noting the cumulative duration that the rat holds its ipsilateral paw off the floor. The
paw lifts associated with locomotion or body repositioning were not
counted. It has been suggested that the foot lifts off a neutral temperature surface, in the absence of any overt external stimuli, are a
form of the paw withdrawal reflex that is associated with spontaneous pain, and are correlative of ongoing pain (Choi et al., 1994).
Thermal Hyperalgesia. Thermal hyperalgesia was assessed using the rat plantar test (Ugo Basile, Comerio, Italy) following a
modified method of Hargreaves et al. (1988). Rats were habituated to
the apparatus that consisted of three individual perspex boxes on a
glass table. A mobile radiant heat source was located under the table
and focused onto the desired paw; paw withdrawal latencies (PWLs)
were recorded. PWLs were taken three times for both hind paws of
each animal, the mean of which represented baselines for right and
left hind paws. The apparatus was calibrated to give a PWL of
approximately 10 s.
Static Allodynia. Rats were placed into wire mesh-bottomed
cages allowing access to the underside of their paws. They were
habituated to this environment before the start of the experiment.
Static allodynia was tested by touching the plantar surface of hind
paws with von Frey hairs (Stoelting, Wood Dale, IL) in ascending
order of force (0.7, 1.2, 1.5, 2, 3.6, 5.5, 8.5, 11.8, 15.1, and 29 g) for up
to 6 s. When a withdrawal response was established, the paw was
retested, starting with the next descending von Frey hair until no
response occurred. The highest force of 29 g lifted the paw as well as
elicited a response, and represented the cut off point. The lowest
amount of force required to elicit a response was recorded as the paw
withdrawal threshold in grams.
Mechanical Hyperalgesia. The plantar surface of the hind paws
was touched with the point of a safety pin. The duration of the
normal pinprick-evoked hindpaw withdrawal was too short to time
with a stopwatch and was arbitrarily assigned a duration of 0.5 s.
The hyperalgesic response seen ipsilateral to the nerve injury was
much longer and easy to time. We defined a withdrawal as being
abnormally prolonged if it lasted at least 2 s. A cut-off of 15 s was
applied to long withdrawals, many of which lasted for over 1 min.
Cold Allodynia. Rats were placed into wire mesh bottom cages
(allowing access to the underside of their paws). The cage was then
placed 10 mm deep into a water bath kept at 10°C. This allowed
immersion of the paws only, without struggle or evidence of stress.
The immersion lasted for a maximum of 15 s. The withdrawal latency was measured. This temperature is considered to be nonnoxious and normal rats do not exhibit paw withdrawal over a 15-s
period.
Dynamic Allodynia. Dynamic allodynia was assessed by lightly
stroking the plantar surface of both hind paws with a cotton bud
(Field et al., 1999a,b). Latency to paw withdrawal was noted. Two to
three measurements were taken at each time point. If no paw withdrawal was shown within 15 s, the procedure was terminated and
animals were assigned this withdrawal time. Thus, 15 s effectively
represents no withdrawal. A withdrawal response was often accompanied with repeated flinching or licking of the paw. Animals were
only selected for drug study if they exhibited a withdrawal time of 8 s
or under.
Weight Bearing. Guinea pigs with CCI were tested for hypersensitivity in the weight-bearing test, using an “Incapacitance
tester” (Linton Instruments, Norfolk, UK). Briefly, the animal was
placed in the apparatus, and the weight load exerted by the hind
paws was noted. The duration of the measurement was adjusted to
4 s. Three measurements were taken at 60-s intervals and the mean
was calculated.
Rota-Rod. Male Sprague-Dawley rats (70 –90 g) were trained to
stay on an accelerating rota-rod (Ugo Basile) for 2 min. On the next
day, animals were retested after s.c. administration of CI-1021 (10 –
100 mg/kg) or diazepam (10 mg/kg) 0.5 h before the test.
Chronic Administration of CI-1021. Two groups of CCI rats
were subjected to daily dosing of CI-1021 (100 mg/kg, s.c.) or vehicle
[polyethylene glycol 200 (PEG200), 1 ml/kg] over 12 days from postoperative day 10. Each animal was tested for static allodynia using
Von Frey hairs before and 30 min after drug administration to assess
the possible development of tolerance. Two separate groups of CCI
rats were subjected to daily dosing with morphine (3 mg/kg, s.c.) or
saline (1 ml/kg) over 6 days from postoperative day 10. They were
also tested every day before and 30 min after treatments for static
allodynia for development of tolerance. On day 7, the morphine
chronically treated animals received CI-1021 (100 mg/kg, s.c.) and
static allodynia was reassessed 30 min after to assess the possible
cross-generalization of opiate tolerance to CI-1021.
Drugs Used
CI-1021 ([(2-benzofuran)-CH2OCO]-(R)-␣-MeTrp-(S)-NHCH(CH3)Ph)
was synthesized at Parke-Davis Neuroscience Research Center (Cambridge, UK). CP-99,994 and SR 140333 were synthesized at Gödecke AG,
Freiburg. CI-1021 was dissolved in PEG200 (Sigma, UK) for s.c. administration, or in Gelucire 44/14 (Gateffossé, France) for oral administration.
CI-1021 was administered under brief isoflurane anesthesia in guinea
pigs. PD 156942 was synthesized at Parke-Davis Neuroscience Research
Center. It was dissolved in PEG200 for s.c. administration and in 20%
hydroxypropyl-␤-cyclodextrin for i.c.v. administration (Singh et al., 1990).
CP-99,994 and SR 140333 were dissolved, respectively, in saline and distilled water containing 1% Tween 80. All drugs were administered 30 min
before injection of formalin, except SR 140333, which was administered 1 h
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The CCI was induced in rats as previously described (Bennett and
Xie, 1988). Briefly, rats were anesthetized with 60 mg/kg i.p. sodium
pentobarbital (Sagatal; Rhone Merieux, distributed in the UK by
National Vet. Suppl. Ltd., Stoke-on-Trent, UK). Guinea pigs were
anesthetized with Hypnorm (1 ml/kg, i.m., containing 0.315 mg
fentanyl citrate and 10 mg fluanisone/ml; Janssen, distributed in the
UK by National Vet. Suppl. Ltd.) plus diazepam (2.5 mg/kg, i.p.). The
common left sciatic nerve was exposed at the level of the middle of
the thigh by blunt dissection through biceps femoris and proximal to
the sciatic trifurcation. Four ligatures (4.0 braided silk) were tied
loosely around it with about 1-mm spacing. The muscle was closed in
layers and two wound clips were applied to close the skin incision.
The wound was then covered with topical antibiotics (rat) or iodine
and chloramine-T powder (guinea pig).
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Gonzalez et al.
Vol. 294
before formalin. Morphine sulfate was obtained from Savory and Moore
(Cambridge, UK) and dissolved in 0.9% w/v NaCl. Carbamazepine was
obtained from Sigma and suspended in 1% carboxymethyl-cellulose containing 0.1% Tween 80. Both carbamazepine and morphine were administered s.c. All compounds were administered in a dosing volume of 10
ml/kg for mice and 1 ml/kg for rats and guinea pigs except formalin, which
was administered in a volume of 20 ␮l. PD 156942 was administered i.c.v.
in a volume of 5 ␮l. The control group received in all cases the relevant
vehicle.
had no effect on the early phase of the formalin response,
consistent with CI-1021, (Table 1). However, CP 99,994 did
have a modest action on the early phase at the highest dose
(30 mg/kg) tested (Table 1). PD 156982 (1–30 mg/kg, s.c.)
failed to have any effect on either phase after systemic administration (Table 1). However, after i.c.v. administration
(30 ␮g) it selectively blocked the late phase of the formalin
test (Table 1).
Statistics
Results
Effect of NK1 Receptor Antagonists in the Mouse
Formalin Test. The administration of formalin into the
hind paw of mice induced a typical biphasic licking/biting
response with an early and a late phase. The s.c. administration of CI-1021 had no effect on the early phase of the
formalin response. However, it dose dependently (1–30 mg/
kg, s.c.) blocked the development of the late phase with a
minimum effective dose (MED) of 3 mg/kg (Table 1). Similar
administration of CP-99,994 (3–30 mg/kg) and SR 140333
(1–100 mg/kg) also dose dependently reduced the late phase,
with respective MEDs of 3 and 10 mg/kg (Table 1). SR 140333
TABLE 1
Mouse Formalin test
The data are shown as mean ⫾ S.E. (n ⫽ 7–10) time (s) spent licking/biting the
injected paw (20 ␮l of 5% formalin)
CI-1021 (mg/kg, s.c.)
0
1
3
10
30
CP 99,994 (mg/kg, s.c.)
0
3
10
30
SR 140333 (mg/kg, s.c.)
0
1
3
10
30
100
PD 156982 (mg/kg, s.c.)
0
1
3
10
30
PD 156982 (␮g/animal, i.c.v.)
0
30
Early Phase
(0–10 min)
Late Phase
(10–45 min)
129.0 ⫾ 6.2
111.8 ⫾ 6.1
144.0 ⫾ 18.0
94.0 ⫾ 12.2
105.9 ⫾ 11.3
206.0 ⫾ 8.2
162.7 ⫾ 15.1
105.3 ⫾ 20.2*
88.3 ⫾ 15.6**
83.3 ⫾ 16.4**
149.3 ⫾ 14.0
152.8 ⫾ 12.0
139.6 ⫾ 12.0
99.8 ⫾ 12.0*
264.5 ⫾ 22.0
186.3 ⫾ 33.7
159.7 ⫾ 25.0*
153.3 ⫾ 25.0*
125.8 ⫾ 13.0
134.2 ⫾ 13.0
123.2 ⫾ 11.0
125.3 ⫾ 14.0
110.0 ⫾ 9.0
116.0 ⫾ 17.0
252.2 ⫾ 19.0
253.0 ⫾ 26.0
162.4 ⫾ 23.0*
144.6 ⫾ 32.0**
118.6 ⫾ 24.0**
69.6 ⫾ 14.0**
122.0 ⫾ 10.0
141.0 ⫾ 14.0
122.0 ⫾ 14.0
114.0 ⫾ 5.0
147.3 ⫾ 17.0
149.4 ⫾ 20.0
178.0 ⫾ 31.0
176.9 ⫾ 25.0
151.0 ⫾ 28.0
171.0 ⫾ 30.0
63.0 ⫾ 9.7
45.5 ⫾ 8.0
203.4 ⫾ 20.4
58.0 ⫾ 20.0**
* P ⬍ .05, ** P ⬍ .01 significantly different from vehicle-treated (0). (ANOVA
followed by Dunnett’s t test; unpaired t test for i.c.v. data).
Effects of CI-1021, Carbamazepine, and Morphine in the
CCI Rat
CI-1021. CI-1021 dose dependently (10 –100 mg/kg, s.c.)
blocked spontaneous pain 30 min after administration with a
MED of 30 mg/kg (5.60 ⫾ 1.80, 2.60* ⫾ 1.20, and 1.80* ⫾
0.70 s for 10, 30, and 100 mg/kg, respectively, versus 9.40 ⫾
2.40 s for vehicle group, *P ⬍ .05 Dunnett’s test, n ⫽ 6 –10).
It also dose dependently blocked thermal hyperalgesia (Fig.
1A), mechanical hyperalgesia (Fig. 2A), and static allodynia
(Fig. 3A) induced by CCI, with MEDs of 30, 30, and 10 mg/kg,
respectively. These actions of CI-1021 were maximum 30 min
after administration. The dose of 100 mg/kg completely
blocked thermal hyperalgesia and static allodynia at this
time point. The effects disappeared by 2 h. Cold allodynia
induced by the CCI was significantly reduced by 100 mg/kg
CI-1021 (Fig. 4A). This effect remained significant for up to
3 h. However, CI-1021 (30 –100 mg/kg, s.c.) failed to have any
effect on the maintenance of dynamic allodynia (3.30 ⫾ 0.36
and 3.22 ⫾ 0.49 s for 30 and 100 mg/kg, respectively, versus
2.88 ⫾ 0.39 s for vehicle group, n ⫽ 7–15).
Carbamazepine. Carbamazepine (50 –100 mg/kg, s.c.) at
the highest dose significantly reduced thermal hyperalgesia
induced by the CCI at 2.5 and 3.5 h after administration (Fig.
1B). However, unlike CI-1021, it did not produce a total
blockade of this hyperalgesic response. The 100-mg/kg dose
of carbamazepine also reduced mechanical hyperalgesia and
cold allodynia induced by CCI (Figs. 2B and 4B). Unlike
CI-1021, carbamazepine was weakly active against static
allodynia (Fig. 3B) and inactive against spontaneous pain
(8.60 ⫾ 1.70 s for 100 mg/kg versus 10.55 ⫾ 1.44 s for vehicle
group, n ⫽ 6 –10). Carbamazepine (50 –100 mg/kg, s.c.)
blocked dynamic allodynia only at the highest dose (100
mg/kg). The effect was small in magnitude and significant
only 30 min after administration (2.63 ⫾ 0.46 and 6.00* ⫾
1.49 s for 50 and 100 mg/kg, respectively, versus 2.38 ⫾ 0.41 s
for vehicle group, *P ⬍ .05, Dunnett’s test, n ⫽ 7–15).
Morphine. Morphine completely blocked thermal hyperalgesia at doses of 1 and 3 mg/kg at 30 min after administration (Fig. 1C). The effect of 1 mg/kg disappeared shortly
after 30 min, but that of 3 mg/kg remained significant for up
to 3.5 h (Fig. 1C). At these doses, morphine also blocked
mechanical hyperalgesia and static allodynia, with a MED of
1 mg/kg (Figs. 2C and 3C). The 3-mg/kg dose of morphine
blocked cold allodynia induced by CCI. The effect remained
significant for up to 3 h (Fig. 4C). This dose of morphine also
reduced spontaneous pain when administered 30 min before
the test (5.4 ⫾ 2.6 and 4.33* ⫾ 1.28 s for 1 and 3 mg/kg,
respectively, versus 10.55 ⫾ 1.44 s for vehicle group, *P ⬍
.05, Dunnett’s test, n ⫽ 6 –10). Morphine (1–3 mg/kg, s.c.)
failed to have a significant effect on the maintenance of
dynamic allodynia (3.70 ⫾ 0.72 and 6.50 ⫾ 1.83 s for 1 and 3
mg/kg, respectively, versus 4.25 ⫾ 0.92 s for vehicle group,
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Data obtained for the formalin test, spontaneous pain, thermal
hyperalgesia, mechanical hyperalgesia, cold allodynia, dynamic allodynia, and rota-rod were subjected to a one-way ANOVA followed
when significant by Dunnett’s test. Data obtained for static allodynia
were analyzed using a Kruskall-Wallis ANOVA for nonparametric
data, followed when significant by Mann-Whitney’s U test. All comparisons were analyzed separately for each time point. The difference in weight bearing between the ipsilateral and contralateral
paws was calculated and it was then subjected to a one-way ANOVA
followed by Dunnett’s test for each time point studied.
2000
n ⫽ 7–15). Data for static (Fig. 3C) and dynamic allodynia
(see above) have been taken from Field et al. (1999a).
Lack of Development of Tolerance to the Antiallodynic Action of CI-1021. In the rat, morphine (3 mg/kg,
s.c.) blocked static allodynia on days 1 to 3 of chronic treatment (Fig. 5A). At these time points, morphine also significantly increased the withdrawal threshold on the contralateral paw, thus showing not only antihyperalgesic but also
antinociceptive action. On the 4th day of treatment, morphine still showed significant antiallodynic effect, although
smaller in magnitude and without producing antinociceptive
action. However, on the 6th day of administration, morphine
failed to show significant antiallodynic effect, indicating development of tolerance (Fig. 5A). In contrast, CI-1021 (100
mg/kg, s.c.) blocked static allodynia for up to 12 days without
447
Fig. 2. Effect of CI-1021 (A), carbamazepine (B), or morphine (C) on
mechanical hyperalgesia in the rat CCI model. Mechanical hyperalgesia
was assessed on postoperative days 29 to 36 by determining withdrawal
duration to the pinprick test. Baseline (BL) measurements were taken
before treatment. Animals then received a single s.c. injection of CI-1021
(10 –100 mg/kg), carbamazepine (50 –100 mg/kg), or morphine (1–3 mg/
kg), and withdrawal durations were reassessed at various times after
treatment. Results are expressed as mean ⫾ S.E. *P ⬍ .05, **P ⬍ .01
significantly different from ipsilateral paw of vehicle-treated animals
(ANOVA followed by Dunnett’s test; n ⫽ 7–15).
showing development of tolerance (Fig. 5B). CI-1021 also
produced total blockade of static allodynia in the animals
that received morphine for 6 days (Fig. 5A).
Effect of CI-1021 on CCI-Induced Hypersensitivity in
the Guinea Pig Measured Using Weight-Bearing Test.
The sciatic nerve ligation in the guinea pig led to the reduction in the weight load on the paw ipsilateral to the ligation,
whereas the weight load on the contralateral paw increased.
The difference in weight load between paws changed from 0
to 40 to 50 g. The difference in weight load was evident from
postoperative day 6 to 8 and was maintained for up to 3
weeks. Morphine (3 mg/kg, s.c.) blocked this hypersensitivity
induced by CCI. The effect was only significant 2 h after
administration (Fig. 6). Oral administration of CI-1021
blocked the CCI-induced hypersensitivity, with a MED of 0.1
mg/kg. The effect was significant for up to 2 h after administration (Fig. 6).
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Fig. 1. Effect of CI-1021 (A), carbamazepine (B), or morphine (C) on
thermal hyperalgesia in the rat CCI pain model. Thermal hyperalgesia
was assessed on postoperative days 26 to 28 by determining PWLs using
the plantar test. Baseline (BL) measurements were taken before treatment. Animals then received a single s.c. injection of CI-1021 (10 –100
mg/kg), carbamazepine (50 –100 mg/kg), or morphine (1–3 mg/kg), and
PWLs were reassessed at various times after treatment. Results are
expressed as mean ⫾ S.E. *P ⬍ .05, **P ⬍ .01 significantly different from
ipsilateral paw of vehicle-treated animals (ANOVA followed by Dunnett’s
test; n ⫽ 8 –12).
Effect of CI-1021 in Models of Pain
448
Gonzalez et al.
Effect of CI-1021 in the Rat Rota-Rod Test. Vehicle
(PEG200)-treated animals spent 119 ⫾ 1 s on the accelerating rota-rod. CI-1021 (10 –100 mg/kg), administered s.c. 30
min before the test, did not affect rota-rod performance and
thus failed to show a significant sedative/ataxic action [values expressed as mean (s) ⫾ S.E. are 102.1 ⫾ 12.7, 100.6 ⫾
12.3, and 108.6 ⫾ 7.46 for 10, 30, and 100 mg/kg, respectively; n ⫽ 8 per group).
Discussion
The results of this study indicate that CI-1021 possesses
antihyperalgesic and antiallodynic actions in animal models
of inflammatory and neuropathic pain. These effects appear
Fig. 4. Effect of CI-1021 (A), carbamazepine (B), or morphine (C) on cold
allodynia in the rat CCI model. Cold allodynia was assessed on postoperative days 7 to 15 by measuring the withdrawal latency to paw immersion in water kept at 10°C. Baseline (BL) measurements were taken
before treatment. Animals then received a single s.c. injection of CI-1021
(10 –100 mg/kg), carbamazepine (50 –100 mg/kg), or morphine (1–3 mg/
kg), and withdrawal latencies were reassessed at various times after
treatment. Results are expressed as mean ⫾ S.E. *P ⬍ .05, **P ⬍ .01
significantly different from ipsilateral paw of vehicle-treated animals
(ANOVA followed by Dunnett’s test; n ⫽ 7–15).
to be mediated via an interaction at the NK1 receptor. Two
lines of evidence support this conclusion. First, the three
compounds examined in the formalin test are chemically
unrelated; the only property they are known to share is their
affinity for the NK1 receptor. Thus, their similar profile in
the formalin test implies that this action is likely to involve
the NK1 receptor. It is worth noting that unlike CI-1021 and
CP 99,994, SR 140333 does not show species differences but
possesses high affinity for the human, rat, and guinea pig
NK1 receptors (Emonds-Alt et al., 1993). However, in this
study it was no more potent than the other two compounds in
the formalin test. This may be related to its relatively poor
CNS penetration (Jung et al., 1994). Second, 2 orders of
magnitude lower doses in guinea pigs compared with rats
were required to block CCI-induced hypersensitivity. This is
consistent with the much higher affinity of CI-1021 for the
guinea pig NK1 receptor (Singh et al., 1997). Previous studies
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Fig. 3. Effect of CI-1021 (A), carbamazepine (B), or morphine (C) on static
allodynia in the rat CCI pain model. Static allodynia was assessed on
postoperative days 19 to 22 by measuring the force required in grams to
elicit paw withdrawal using von Frey hair filaments. Baseline (BL) measurements were taken before treatment. Animals then received a single
s.c. injection of CI-1021 (10 –100 mg/kg), carbamazepine (50 –100 mg/kg),
or morphine (1–3 mg/kg), and withdrawal thresholds were reassessed at
various times after treatment. Results are expressed median ⫾ first and
third quartiles. *P ⬍ .05, **P ⬍ .01, ***P ⬍ .001 significantly different
from ipsilateral or contralateral paw of vehicle-treated animals, respectively (Mann-Whitney U test; n ⫽ 8 –12).
Vol. 294
2000
Fig. 6. Effect of CI-1021 in the guinea pig CCI model. Hypersensitivity
was assessed using the weight-bearing test. Results are expressed as
mean difference in weight load between ipsilateral and contralateral
paws (g) ⫾ S.E. (n ⫽ 6 –9 per group). *P ⬍ .05, **P ⬍ .01, significantly
different from vehicle-treated animals (ANOVA followed by Dunnett’s
test).
have shown that PD 156982 is a functional NK1 receptor
antagonist in vivo but has poor CNS penetration (Field et al.,
1998). Its ability to block the late formalin response after
central but not systemic administration indicates the importance of the central NK1 receptor in inflammatory pain. This
is consistent with a previous report suggesting that the ac-
449
tions of CI-1021 in a neuropathic pain model are centrally
mediated (Field et al., 1998).
Neuropathic pain is often present as a combination of
several heterogeneous symptoms. A selective nerve damage
in animals also leads to induction of multiple signs that
appear to be indicative of pain. For example, rats with CCI
show hyperalgesia and allodynia provoked by thermal and
mechanical stimuli. These animals also show what has been
termed spontaneous pain. The results of this study show that
neither CI-1021, carbamazepine, nor morphine completely
blocked all of these responses. These data are consistent with
the suggestion that different mechanisms may be involved in
the mediation of these responses.
Mechanical allodynia is a prominent feature of inflammatory, neuropathic, and postoperative pain syndromes. The
results of this study support previous reports that two distinct types of mechanical allodynia can be detected in animal
models of neuropathic pain (Field et al., 1999a,b). They have
been termed static and dynamic allodynia after the type of
stimulus that leads to their induction. Both types of allodynia
are also present in patients suffering from neuropathic pain
(Ochoa and Yarnitsky, 1993). It has been suggested that
static allodynia is signaled by A␦ fibers, whereas dynamic
allodynia is signaled by A␤/capsaicin-insensitive A␦- primary
sensory neurones (Ochoa and Yarnitsky, 1993). The failure of
morphine to block dynamic allodynia supports our previous
studies showing that this is also the case in the diabetic and
Chung models of neuropathic pain (Field et al., 1999a,b).
Moreover, these observations are consistent with electrophysiological observations that morphine can block small (Cand A␦-) but not large diameter (A␤-) fiber-evoked responses
into the dorsal horn (Le Bars et al., 1979; Dickenson and
Sullivan, 1986). The A-fibers that appear to signal dynamic
allodynia (Koltzenburg et al., 1992; Gracely et al., 1993) do
not contain substance P. This may explain the failure of
CI-1021 to block this type of allodynia. It has been shown
that chronic inflammation can induce synthesis of substance
P in A␤-fibers (Neumann et al., 1996). The failure of CI-1021
to block dynamic allodynia in this study suggests that this is
unlikely to be the case after nerve injury.
The results presented here indicate that unlike morphine,
repeated administration does not lead to the development of
tolerance to the antiallodynic action of CI-1021. However, it
remains to be seen whether administration for a longer period leads to tolerance. This study further shows that opiate
tolerance does not cross-generalize to the NK1 receptor antagonist. It remains to be seen whether NK1 receptor antagonists can block development of opiate tolerance and also
reduce abuse liability. The other difference between CI-1021
and morphine is that the NK1 receptor antagonist did not
block the first phase of the formalin response or affect the
contralateral paw in the CCI model. This would suggest that
unlike opiate analgesic agents, NK1 receptor antagonists do
not block transient or physiological pain. The failure of CI1021 to show deficits in the rota-rod suggests that it has an
improved side effect profile; this further distinguishes NK1
receptor antagonists from morphine.
To date, clinical studies have failed to show efficacy of NK1
receptor antagonists against painful neuropathy. Thus, CP99,994 was found to be ineffective in peripheral painful neuropathy trials (Suarez et al., 1994). However, it has been
suggested that the doses used may have been too low to
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 31, 2017
Fig. 5. Lack of tolerance to the effect of CI-1021 on static allodynia in the
rat CCI model. Morphine (3 mg/kg, s.c.) was administered over 6 days to
CCI animals, beginning on postoperative day 10 (A). The next day, the
animals received CI-1021 (100 mg/kg, s.c.) (A). CI-1021 (100 mg/kg, s.c.)
was administered over 12 days, also beginning on postoperative day 10
(B). Withdrawal thresholds were reassessed 30 min after treatment.
Results are expressed as median ⫾ first and third quartiles. *P ⬍ .05,
**P ⬍ .01, ***P ⬍ .001 significantly different from ipsilateral paw of
vehicle-treated animals (Mann-Whitney U test; n ⫽ 7).
Effect of CI-1021 in Models of Pain
450
Gonzalez et al.
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Send reprint requests to: Dr. Lakhbir Singh, Parke-Davis Neuroscience
Research Centre, Cambridge University Forvie Site, Robinson Way, Cambridge, CB2 2QB, UK. E-mail: [email protected]
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 31, 2017
adequately block NK1 receptors. More recently, it has been
reported that L-754,030 failed to block pain induced by
postherpetic neuralgia (PHN; Block et al., 1998). It is known
that PHN can lead to destruction of peptidergic C-fiber function. A lack of effect of an NK1 receptor antagonist in PHN
patients with loss of substance P function is not surprising.
The results of this study show that CI-1021, morphine, and
carbamazepine have differential effects on various behavioral responses induced by CCI. The potency of CI-1021 in the
experimental pain model reflects its receptor affinity in that
it is 100 times more potent in the guinea pig than in the rat.
The limited antiallodynic action of morphine and carbamazepine may explain the inconsistent efficacy of these compounds observed against neuropathic pain (Arner and Meyerson, 1988; Leijon and Boivie, 1989; McQuay et al., 1995).
The failure of CI-1021 to block dynamic allodynia suggests
that NK1 receptor antagonists may not be effective against
A␤-fiber-signaled responses. These results taken together
with clinical studies indicate that a mechanistic approach
may represent a better way for designing future clinical
studies with NK1 receptor antagonists. Thus, in addition to
using visual analog scale as a measure of overall pain relief,
effects on individual (e.g., spontaneous pain, hyperalgesia,
and allodynia) components of pain will provide invaluable
information for discovering novel treatments. Chronic pain
syndromes consist of a wide range of symptoms likely to be
mediated by multiple mechanisms. Therefore, blockade of
just one of these mechanisms may not provide full pain relief
in every patient. A better approach may be a combination
therapy involving two compounds with independent mechanisms of action. Mechanical hypersensitivity can be the most
debilitating symptom in some patients suffering from neuropathic pain. It remains to be seen whether a combination of
CI-1021 with a compound that blocks dynamic allodynia,
which is insensitive to the NK1 receptor antagonist, proves to
be a superior treatment of neuropathic pain.
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