Design, synthesis, and preliminary evaluation of gabapentin-pregabalin mutual prodrugs in relieving neuropathic pain

Article abstract of DOI:10.1002/ardp.200400958

As a part of a program for the development of specific analgesics in relieving neuropathic pain, the purpose of the present study was to investigate a new concept that involves the conjugation of two drugs, gabapentin and pregabalin, as mutual prodrugs using a chemical modification approach. A series of gabapentin-pregabalin diester compounds were synthesized using linear or branch bis-hydroxyl linkers. Their pharmacological properties for treating neuropathic pain were investigated in a rat model of chronic sciatic nerve constriction injury (CCI). In-vivo evaluation demonstrated that 1a and 1b composed of two gabapentin molecules as well as 3a composed of gabapentin and pregabalin with the short linear linker, were effective in reversing tactile allodynia in CCI rats. Compounds with longer or side-branched linkers showed lower efficiencies and severe adverse effects.

Full text of DOI:10.1002/ardp.200400958

358
DOI 10.1002/ardp.200400958
Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
Design, Synthesis, and Preliminary Evaluation
of Gabapentin-Pregabalin Mutual Prodrugs in
Relieving Neuropathic Pain
Weiguo Shi, He Liu, Yanping Zhang, Bohua Zhong, Hongju Yang
As a part of a program for the development of specific analgesics in relieving neuropathic pain, the
purpose of the present study was to investigate a new concept that involves the conjugation of two
drugs, gabapentin and pregabalin, as mutual prodrugs using a chemical modification approach. A
series of gabapentin-pregabalin diester compounds were synthesized using linear or branch bis-hy-
droxyl linkers. Their pharmacological properties for treating neuropathic pain were investigated in a
rat model of chronic sciatic nerve constriction injury (CCI). In-vivo evaluation demonstrated that 1a
and 1b composed of two gabapentin molecules as well as 3a composed of gabapentin and pregabalin
with the short linear linker, were effective in reversing tactile allodynia in CCI rats. Compounds with
longer or side-branched linkers showed lower efficiencies and severe adverse effects.
Keywords: Neuropathic pain; Mutual prodrug; CCI; Gabapentin; Pregabalin
Received: November 29, 2004; Accepted: June 1, 2005
Introduction
convulsants are sedation and cerebella symptoms (nystag-
mus, tremor, and incoordination). In order to develop safe
and more effective treatments for neuropathic pain a new
strategy of drugs has to be considered. The direction for
new drug development is to use the combination of different
kinds of drugs based on mechanism classification in order
to block the pain symptom at different levels. Combination
is a potentially effective way to increase efficiency of drugs
and to reduce the adverse effect. On the other hand, the
conjugation of two drugs having same or different pharma-
cological activities, which is termed a mutual prodrug, has
been synthesized to improve the therapeutic index and re-
duce adverse effect [4].
Neuropathic pain results from disease or injury to the pe-
ripheral or central nervous system, including diabetic neuro-
pathy, trigeminal neuralgia, post-herpetic neuralgia, and
spinal cord injury. As a common symptom of a hetero-
geneous group of conditions, neuropathic pain frequently
runs a chronic course and will be severe and difficult to
treat [1]. The characteristic symptoms of neuropathic pain
are usually expressed as allodynia, hyperalgesia, and spon-
taneous pain. Different pain mechanisms may be involved
in the neuropathic pain, for one mechanism could be re-
sponsible for many different symptoms, and the same symp-
tom can be caused by different mechanisms. As a result,
more than one mechanism can operate in a single patient,
and these mechanisms may change with time [2]. Many
classical analgesics such as opioids and NSAIDs (non-
steroid anti-inflammation drugs) have a limited effect in
treatment of neuropathic pain, while opiates carry long-
term risks of habituation with chronic use [3].
It has recently been reported that antidepressants, anticon-
vulsants, antiarrhythmic drugs, local anaesthetic, or NMDA
receptor antagonists become an alternative way for the
neuropathic pain treatment [5]. Gabapentin is an anticon-
vulsive drug that has been found to be effective for the treat-
ment of neuropathic pain. It is a structural analogue of the
inhibitory neurotransmitter gamma amino butyric acid
(GABA). S-(ϩ)-Pregabalin is another GABA analog that
inhibit the α2δ-subunit of the voltage-sensitive Ca2ϩ-chan-
nel, it is launched by Pfizer for the potential treatment of
central nervous system disorders, including neuropathic
pain. But gabapentin and pregabalin (Figure 1) exhibit some
drawbacks such as higher dosage (900 to 3600 mg/d and
300 mg/d for gabapentin and pregabalin, respectively) re-
sulting from the difficulty to diffuse across the blood-brain
barrier, which causes severe adverse effects (dizziness,
drowsiness, headache, and somnolence), and a shorter
Due to the intricate mechanisms of the neuropathic pain,
complete symptom reduction or complete functional resto-
ration is seldom achieved and the existing drugs focused on
one specific target could only alleviate pain symptoms to
some extent. Moreover, some of the drugs show severe ad-
verse effects, for the most common adverse effects of anti-
Correspondence: He Liu, No. 7 Department, Beijing Institute of
Pharmacology and Toxicology, Beijing 100850, P. R. China.
Phone: ϩ86 10 6687-4612, Fax: ϩ86 10 6821-1626, e-mail:
hliuamms@yahoo.com
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
Gabapentin-pregabalin mutual prodrugs
359
of 26 g was designated as the cut-off point. All sample com-
pounds screened were solved in 0.9% (w/v) saline. Two
weeks after surgery, the mechanical paw withdrawal thresh-
old was measured at 1 h after mutual prodrug adminis-
tration (60 mg/kg, i.p.). Broad screen found that compound
1a, 1b, and 3a exhibited anti-allodynic effects in CCI rats by
increasing the paw withdrawal threshold significantly when
compared with that of the preinjection (Table 1). The diester
compounds with longer or branch linker showed lower ef-
fectiveness and severe adverse effects (1c, 3d, 3g, and 3i,
symptoms such as ataxia, inertia, and logy), especially the
gabapentin-pregabalin 1,6-hexandiol diester 3e showed anti-
allodynic efficiency, but was more toxic than the references
(three rats died in this group).
Figure 1. Chemical structure of gabapentin and S-(+)-pre-
gabalin.
duration of action with the need to take the medication
orally three or four times daily [6Ϫ9].
In our work the objective is to get new potent anti-neuro-
pathic pain drugs with fewer side effects and better pharma-
cokinetic properties. Gabapentin and pregabalin were con-
jugated through their carboxyl groups to different bis-hy-
droxyl linkers. A series of diester compounds were designed
and synthesized as a mutual prodrug approach. In the pre-
sent study, the anti-allodynic effects of systemic adminis-
tration of mutual prodrugs were investigated in chronic sci-
atic nerve constriction injury (CCI) rats.
Discussion
In this report, we describe the synthesis of a series of diester
mutual prodrug of gabapentin and S-(ϩ)-pregabalin. Their
pharmacological properties to treat neuropathic pain were
investigated in a rat model of chronic sciatic nerve constric-
tion injury (CCI). Compounds 1a, 1b, and 3a were effective
in reversing tactile allodynia in vivo in CCI rats. From com-
paring the structures within the same system compounds
1aϪc, 2aϪc, and 3aϪI, respectively, the length and side-
branch of the linker are of vital influence for the biological
activity. The bis-gabapentin and gabapentin-pregabalin dies-
ter compounds with short linear linkers showed more effec-
tive. The bis-pregabalin and other diester compounds with
longer or branch linker showed low efficiencies, and the di-
ester compounds with side-branched linkers showed severe
adverse effects. There was only one exception: the gabapen-
tin-pregabalin diester 3e with the long linear 1,6-hexandiol
linker was highly potent but rather toxic.
As a result, there three compound 1a, 1b, and 3a were novel
potent anti-neuropathic pain compounds in the designed di-
ester mutual prodrug approach. The anti-allodynic effect,
combined with its partly non-toxic effect on nerve functions,
suggest that the prodrugs may be possible candidates for
further development. The pharmacokinetic profiles of these
compounds need further to be investigated.
Results
Chemistry
Synthesis of mutual prodrug diester compounds 1aϪc,
2aϪc, and 3aϪi is shown in Scheme 1. The amino groups
of gabapentin and pregabalin were protected by t-butoxy-
carbonyl(Boc) groups. The bis-gabapentin diester com-
pounds 1aϪc were synthesized by the protected gabapentin
with bis-hydroxyl compounds such as glycol or 1,3-propan-
diol using dicyclohexyl carbodiimide (DCC) and 4-(N,N-di-
methylamino)pyridine (DMAP) as carboxyl activator and
nucleophilic catalyst, respectively. The bis-pregabalin diester
compounds 2aϪc were synthesized by the protected pre-
gabalin in the same procedure. Synthesis of the gabapentin
monoester compounds was achieved by the protected gaba-
pentin with more excessive bis-hydroxyl compounds. The
unsymmetrically gabapentin-pregabalin diester compounds
3aϪi were synthesized by the gabapentin monoester with
the protected pregabalin accordingly. The amino-protecting
group was removed by the HCl-EtOAc solution, at the same
time the diester compounds hydrochloride were obtained.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (20302015).
Biological studies
The anti-allodynic effects studies were implemented in
chronic sciatic nerve constriction injury (CCI) rats. The CCI
surgery was conducted according to a procedure previously
reported by Bennett [10]. Paw withdrawal threshold was
measured by a series of von-Frey filaments, the strength of
a filament, which caused 4Ϫ6 responses stimulating 10
times, was designated as 50% response threshold. The force
Experimental
Chemistry
All reagents and solvent were of reagent grade and used without
further purification unless indicated otherwise. 1H-NMR spectra
were recorded using
a Bruker JNM-ECA-400C spectrometer
(Bruker BioSpin GmbH, Rheinstetten, Germany). Chemical shifts
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

360
Liu et al.
Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
Scheme 1. Synthesis routes to diester compounds.
are reported in parts per million (ppm) relative to TMS as internal
standard. All melting points are uncorrected and were determined
using a RY-1 type apparatus (Tianjin Analytical Instrument Fac-
tory, Tianjin, China). Column chromatography was performed on
Merck silica gel 60 (200Ϫ400mesh) (Merck, Darmstadt, Germany).
Elemental analyses were performed on a Perkin-Elmer model 240
instrument (Perkin Elmer, Norwalk, CT, USA). Mass spectra were
recorded on an API 3000 system instrument (Applied Biosystems,
Foster City, CA, USA). Gabapentin was obtained from commercial
products. S-(ϩ)-Pregabalin was synthesized as described in the
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
Gabapentin-pregabalin mutual prodrugs
361
Table 1. Effect of diester mutual prodrugs on allodynia response in CCI rats.
Compound
linker
PWT [g]
pre-injection
PWT [g]
post-injection
M.P.E
[%]
Gabapentin
2.23 0.36
3.43 0.39
3.12 0.51
3.40 0.77
4.80 0.44
4.00 0.57
2.90 0.49
6.00 0.00
4.50 0.78
3.33 0.42
3.60 0.36
4.17 0.91
2.80 0.73
3.33 0.47
13.50 4.35†
18.86 3.64‡
15.73 3.25
18.67 4.64†
19.40 2.45‡
5.60 0.89
47.41
68.36
55.11
67.56
68.86
7.72
Pregabalin
Gabapentin-
Pregabalin
combination
1a
1b
1c§
2a
2b
2c
-CH2CH2-
-CH2CH2CH2-
-CH2(CH2)2CH2-
-CH2CH2-
7.90 2.45
21.64
21.65
18.60
69.12
18.75
20.11
47.41
73.53
-CH2CH2CH2-
-CH2(CH2)2CH2-
-CH2CH2-
10.33 1.72
8.50 0.78
3a
3b
3c
19.00 3.40‡
7.80 4.92
-CH2CH2CH2-
-CH2(CH2)2CH2-
-CH2(CH2)3CH2-
-CH2(CH2)4CH2-
8.56 3.75
3d§
3e§
13.80 2.98‡
20.67 3.77‡
CH3
͉
ϪCH2CHϪ
3f
4.00 0.82
4.00 0.82
6.80 1.09
12.72
33.63
CH3
͉
3g§
11.40 3.76
ϪCH2CH2CHϪ
CH3
͉
3h
3i§
CH2
4.40 0.68
4.40 036
6.80 0.73
11.11
12.96
͉
ϪCH2CHϪ
CH3
͉
ϪCH2CC2HϪ
7.20 0.73
͉
CH3
†
‡
All compounds were administrated at (60 mg/kg, i.p.) 1 h before testing. Data were expressed as mean S.E.M, P < 0.05, P < 0.01 VS
§
pre-injection, Wilcoxon 2-Sample Test and Kruskal-Wallis Test. represents compounds with severe adverse effects.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Liu et al.
Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
literature [11]. All reactions were monitored by TLC on 15 ϫ 75
mm plastic sheets precoated with silica gel (GF-254) to a thickness
of 0.20 mm and viewed at 254 nm UV-light.
S-(ϩ)-Pregabalin 1,3-propandiol diester hydrochloride (2b)
Compound 2a was obtained as a white solid (yield 81%), mp.
132Ϫ134°C. 1H-NMR (CDCl3) δ: 0.91Ϫ0.94 (m, 12H, -CH3),
1.20Ϫ1.37 (m, 4H, -CHR1-CH2- CHR2-), 1.66 (m, 2H, -CH2-CHR-
CH2-), 2.01 (s, 2H, -(CH3)2-CH-CH2-), 2.39 (s, 2H, -COOCH2CH2-
CH2OOC-), 2.56 (m, 4H, -OOC-CH2-), 3.05 (d, 4H, -CH2-NH2),
4.23 (m, 4H, -COOCH2CH2CH2OOC-), 8.31 (s, 6H, -NH2HCl);
Anal. calcd. for C19H40N2O4Cl2: C, 52.90; H, 9.28; N, 6.50. Found:
C, 52.90; H, 9.54; N, 6.28.
General synthetic method for gabapentin and S-(ϩ)-pregabalin di-
esters hydrochlorides (1aϪc and 2aϪc)
2.5 equivalent gabapentin-Boc or S-(ϩ)-pregabalin-Boc and bis-hy-
droxyl compound were dissolved in anhydrous dichloromethane and
the mixture was stirred in an ice bath. DCC in anhydrous dichloro-
methane was dropped in slowly and maintained at 0°C for 1h,
DMAP was added. Then the mixture was reacted at room tempera-
ture. After the reaction was completed (4-8 h), the solution was
evaporated in vacuo. The residue (solid product) was dissolved in
EtOAc, washed with 5% NaHCO3 and water, respectively. The or-
ganic phase was dried over anhydrous MgSO4, filtered, and concen-
trated under reduced pressure. The colorless oil obtained was puri-
fied by silica column chromatography with petroleum ether:EtOAc:
MeOH (5:1.5:0.5). The diester compounds were dissolved in EtOAc,
4N HCl/EtOAc was slowly added dropwise. The mixture was stirred
at room temperature for 1h. The precipitation was filtered and
washed with cooled EtOAc and ether, respectively and dried under
vacuum. The gabapentin and S-(ϩ)-pregabalin diester hydrochlo-
ride was obtained as a white precipitate.
S-(ϩ)-Pregabalin 1,4-butandiol diester hydrochloride (2c)
Compound 2c was obtained as a white solid (yield 85%), mp.
114Ϫ116°C. 1H-NMR (CDCl3) δ: 0.91Ϫ0.94 (m, 12H, -CH3),
1.20Ϫ1.37 (m, 4H, -CHR1-CH2-CHR2-), 1.66 (m, 2H, -CH2-CHR-
CH2-), 2.05 (s, 2H, -(CH3)2-CH-CH2-), 2.36 (s, 4H, -CO-
OCH2CH2CH2CH2OOC-), 2.51Ϫ2.62 (m, 4H, -OOC-CH2-), 3.05
(m, 4H, -CH2-NH2), 4.20 (m, 4H, -COOCH2CH2CH2CH2OOC-),
8.31 (s, 6H, -NH2HCl). Anal. calcd. for C20H42N2O4Cl2: C, 53.93;
H, 9.44; N, 6.29. Found: C, 53.40; H, 9.48; N, 6.22.
General synthetic method for gabapentin-pregabalin diester hydro-
chloride (3aϪi)
Gabapentin-Boc (10mmol) and bis-hydroxyl compound (50mmol)
were dissolved in 20mL anhydrous dichloromethane. The mixture
was stirred in an ice bath. DCC (2.0g, 9.7mmol) in 5mL anhydrous
dichloromethane was slowly added droppwise. The mixture was
maintained in an ice bath for1h, 0.25g DMAP was added. Then the
mixture was stirredg at room temperature. After the reaction was
completed, the solution was evaporated under reduced pressure. The
solid product was dissolved in EtOAc, washed with 5% NaHCO3
and water, respectively. The organic phase was dried over anhydrous
MgSO4, then filtered and concentrated under reduced pressure. The
gabapentin-Boc monoester compounds were purified by silica col-
umn chromatography with petroleum ether:EtOAc:MeOH (5:2:0.5)
as a colorless liquid.
Gabapentin glycol diester hydrochloride (1a)
Compound 1a was obtained as a white precipitate (yield 81%), mp.
176Ϫ178°C. 1H-NMR (CD3OD) δ: 1.41Ϫ1.56 (m, 20H, -CH2-,
cyclohexyl), 2.57 (s, 4H, -OOC-CH2-), 3.07 (s, 4H, -CH2-NH2), 4.34
(s, 4H, -COOCH2CH2OOC-); MS, m/z: 368.9 (Mϩ1). Anal. calcd.
for C20H38N2O4Cl2: C, 54.42; H, 8.62; N, 6.35. Found: C, 54.21; H,
8.75; N, 6.30.
Gabapentin 1,3-propandiol diester hydrochloride (1b)
Gabapentin-pregabalin diester compounds were synthesized by the
gabapentin-Boc monoester with the pregabalin-Boc in the same pro-
cedure. The gabapentin-pregabalin diester hydrochloride was ob-
tained as a white precipitate from the protected gabapentin-pregaba-
lin diester by treatment with 4N HCl/EtOAc.
Compound 1b was obtained as a white solid (yield 73%), mp.
179Ϫ181°C. 1H-NMR (CDCl3) δ: 1.46Ϫ1.59 (m, 20H, -CH2-,
cyclohexyl), 2.00 (m, 2H, -COOCH2CH2CH2OOC-), 2.67 (s, 4H,
-OOC-CH2-), 3.09 (s, 4H, -CH2-NH2), 4.27 (m, 4H, -COOCH2CH2-
CH2OOC-), 8.33 (s, 6H, -NH2HCl); Anal. calcd. for
C
21H40N2O4Cl2: C, 55.38; H, 8.79; N, 6.15. Found: C, 55.25; H,
8.88; N, 6.06.
Gabapentin-Pregabalin glycol diester hydrochloride (3a)
Compound 3a was obtained as a white solid (yield 63%), mp.
1
101Ϫ103°C. H-NMR (CD3OD) δ: 0.92Ϫ0.95 (m, 6H, -CH3), 1.27
Gabapentin 1,4-butandiol diester hydrochloride (1c)
(m, 2H, -CHR1-CH2-CHR2-), 1.41Ϫ1.56 (m, 10H, -CH2-,cyclo-
hexyl), 1.66 (m, 1H, -CH2-CHR-CH2-), 2.24 (m, 1H, -(CH3)2-CH-
CH2-), 2.48 (m, 2H, -OOC-CH2-), 2.57 (s, 2H, -OOC-CH2-), 2.98
(m, 2H, -CH2-NH2), 3.07 (s, 2H, -CH2-NH2), 4.33 (s, 4H, -CO-
OCH2CH2 OOC-); MS m/z: 357.4(Mϩ1). Anal. calcd. for
Compound 1c was obtained as a white solid (yield 82%), mp.
171Ϫ173°C. 1H-NMR (CDCl3) δ: δ 1.44Ϫ1.56 (m, 20H, -CH2-,
cyclohexyl), 1.81 (m, 4H, -COOCH2CH2CH2CH2OOC-), 2.65 (s,
4H, -OOC-CH2-), 3.02 (s, 4H, -CH2-NH2), 4.15 (t, 4H, -CO-
OCH2CH2CH2CH2OOC-), 8.44 (s, 6H, -NH2HCl). Anal. calcd. for
C
19H38N2O4Cl2: C, 53.14; H, 8.86; N, 6.53. Found: C, 53.22; H,
8.83; N, 6.44.
C
22H42N2O4Cl2: C, 56.29; H, 8.95; N, 5.97. Found: C, 56.40; H,
8.83; N, 5.82.
Gabapentin-Pregabalin 1,3-propandiol diester hydrochloride (3b)
Compound 3b was obtained as a white solid (yield 60%), mp.
116Ϫ118°C. 1H-NMR (CDCl3) δ: 0.90Ϫ0.93 (m, 6H, -CH3), δ
1.22Ϫ1.65 (m, 13H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, and
-CH2-CHR-CH2-), 1.99 (m, 2H, -COOCH2CH2CH2OOC-), 2.56
(m, 1H, -(CH3)2-CH-CH2-), 2.56Ϫ2.65 (m, 4H, -OOC-CH2-),
3.12Ϫ3.13 (m, 4H, -CH2-NH2), 4.24 (t, 4H, -COOCH2CH2-
CH2OOC-), 8.30 (s, 6H, -NH2HCl); Anal. calcd. for
C20H40N2O4Cl2: C, 54.17; H, 9.03; N, 6.32. Found: C, 54.15; H,
9.03; N, 6.39.
S-(ϩ)-Pregabalin glycol diester hydrochloride (2a)
Compound 2a was obtained as a white solid (yield 77%), mp.
154Ϫ155°C. 1H-NMR (CD3OD) δ: 0.92Ϫ0.95 (m, 12H, -CH3), 1.27
(m, 4H, -CHR1-CH2-CHR2-), 1.66 (m, 2H, -CH2-CHR-CH2-), 2.24
(m, 2H, -(CH3)2-CH-CH2-), 2.48 (d, 4H, Jϭ6.6Hz, -OOC-CH2-),
2.98 (m, 4H, -CH2-NH2), 4.33 (s, 4H, -COOCH2CH2OOC-). MS
m/z 345.2 (Mϩ1). Anal. calcd. for C18H38N2O4Cl2: C, 51.80; H,
9.11; N, 6.71. Found: C, 51.65; H, 9.26; N, 6.65.
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Arch. Pharm. Chem. Life Sci. 2005, 338, 358−364
Gabapentin-pregabalin mutual prodrugs
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Gabapentin-Pregabalin 1,4-butandiol diester hydrochloride (3c)
calcd. for C21H42N2O4Cl2: C ,55.14; H, 9.19; N, 6.12. Found: C,
55.01; H, 9.15; N, 6.31.
Compound 3c was obtained as a white solid (yield 65%), mp.
105Ϫ107°C. 1H-NMR (CDCl3) δ: 0.91-0.93 (m, 6H, -CH3),
1.21Ϫ1.77 (m, 17H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-
CHR-CH2- and -COOCH2CH2CH2CH2OOC-), 2.35Ϫ2.67 (m,
5H, -(CH3)2-CH-CH2- and -OOC-CH2-), 3.06 (s, 4H, -CH2-NH2),
4.15 (s, 4H, -COOCH2CH2CH2CH2OOC-), 8.36 (m, 6H,
-NH2HCl). Anal. calcd. for C21H42N2O4Cl2: C, 55.14;, H 9.19; N,
6.12. Found: C, 54.99; H, 9.30; N, 6.18.
Gabapentin-Pregabalin 2,2-dimethyl-1,3-propandiol diester hydro-
chloride (3i)
Compound 3i was obtained as a white solid (yield 69%), mp.
148Ϫ149°C. 1H-NMR (CDCl3) δ: 0.90Ϫ0.98 (m, 12H, -CH3),
1.01Ϫ1.67 (m, 13H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-
CHR-CH2-), 2.50 (s, 1H, -(CH3)2-CH-CH2- ), 2.53Ϫ2.75 (m, 4H,-
OOC-CH2-), 3.02Ϫ3.16 (m, 4H, -CH2-NH2), 3.95 (s, 4H, -CO-
OCH2C(CH3)2CH2OOC-), 8.29 (s, 6H, -NH2HCl). Anal. calcd. for
C22H44N2O4Cl2: C, 56.06; H, 9.34; N, 5.94. Found: C, 56.04; H,
9.41; N, 5.98.
Gabapentin-Pregabalin 1,5-pentandiol diester hydrochloride (3d)
Compound 3d was obtained as a white solid (yield 68%), mp.
109Ϫ111°C. 1H-NMR (CDCl3) δ: 0.91Ϫ0.93 (m, 6H, -CH3),
1.19Ϫ1.78 (m, 19H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-
CHR-CH2- and -COOCH2CH2CH2CH2CH2OOC-), 2.33 (m, 1H,
-(CH3)2-CH-CH2- ), 2.55Ϫ2.61 (m, 4H, -OOC-CH2-), 3.05Ϫ3.12
(m, 4H, -CH2-NH2), 4.15 (s, 4H, -COOCH2CH2-CH2CH2-
CH2OOC-), 8.34Ϫ8.48 (m, 6H, -NH2HCl); Anal. calcd. for
C22H44N2O4Cl2: C, 56.06; H, 9.34; N, 5.94. Found: C, 56.09; H,
9.49; N, 6.04.
Preliminary biological evaluation
All procedures were conducted in accordance with the IASP guide-
lines on use of animals for investigation of experimental pain [12].
Surgical preparations and experimental protocols were approved by
the Animal Care and Use Committee of Beijing Institute of Phar-
macology and Toxicology (BIPT). Young adult male Sprague-Daw-
ley rats, weighting 160-180 g at entry were housed in clear plastic
cages at the Experimental Animal Center of BIPT. Rats housed in
groups of five per cage have been used to test the analgesic proper-
ties of one drug in a controlled 12-h light/12-h dark environment
and had free access to food and water. The mechanical allodynia
was determined by the paw withdrawal in response to a series of
filaments (Von Frey Hair, North Coast Company, Bellevue, WA,
USA) and was expressed in grams. A peripheral mono neuropathy
was produced in rats by placing loosely constrictive ligatures around
the common sciatic nerve according to the method described by
Bennett. Rats were anaesthetized with chloral hydrate. The right
common sciatic nerve was exposed at the level of the middle thigh
by blunt dissection through the biceps femoris. Proximal to the
nerve’s trifurcation, a 5-7 mm of nerve was freed of adhering tissue
and 4 ligature (4.0 chromic gut) were tied loosely around it with
about 1 mm spacing. Great care was taken to tie the ligatures such
that the diameter of the nerve was seen to be just barely constricted.
In every animal, an identical dissection was performed on the op-
posite side except that the sciatic nerve was not ligated. The left paw
was untouched. Data were expressed as mean S.E.M and analyzed
by SAS data process soft. The raw data of mechanical threshold
were converted to a percentage of the maximal possible effect
(% M.P.E) for the ED50 calculation.% M.P.E was defined according
to the following formula in the CCI model: M.P.E% ϭ postdrug
thresholdϪpredrug threshold)/(Max thresholdϪpredrug thres-
hold)%. The max thresholds are designed as 26 g.
Gabapentin-Pregabalin 1,6-hexandiol diester hydrochloride (3e)
Compound 3e was obtained as a white solid (yield 66%), mp.
88Ϫ90°C. 1H-NMR (CDCl3) δ: 0.91Ϫ0.93 (m, 6H, -CH3),
1.18Ϫ1.68 (m, 21H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-
CHR-CH2- and -COOCH2CH2CH2CH2CH2CH2OOC-), 2.35 (m,
1H, -(CH3)2-CH-CH2- ), 2.53Ϫ2.58 (m, 4H, -OOC-CH2-),
3.05Ϫ3.12 (d, 4H, -CH2-NH2), 4.12 (s, 4H, -COOCH2CH2CH2-
CH2 CH2CH2OOC-), 8.34Ϫ8.50 (d, 6H, -NH2HCl); Anal. calcd.
for C23H46N2O4Cl2: C, 56.91; H, 9.48; N, 5.77. Found: C, 56.95; H,
9.63; N, 5.87.
Gabapentin-Pregabalin 1,2-propandiol diester hydrochloride (3f)
Compound 3f was obtained as a white solid (yield 63%), mp.
87Ϫ88°C. 1H-NMR (CDCl3) δ: 0.91-0.93 (m, 6H, -CH3), 1.19Ϫ1.65
(m, 16H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-CHR-
CH2-, and ϪCH3), 2.38Ϫ3.19 (m, 9H, -(CH3)2-CH-CH2-, -OOC-
CH2-, -CH2-NH2), 3.96Ϫ4.40 (m, 2H, -COOCH2CH(CH3)OOC-),
5.18Ϫ5.23 (m, 1H, -COOCH2CH(CH3)OOC-), 8.30 (s, 6H,
-NH2HCl); Anal. calcd. for C20H40N2O4Cl2: C, 54.17; H, 9.03; N,
6.32. Found: C, 53.76; H, 9.02; N, 6.27.
Gabapentin-Pregabalin 1,3-butandiol diester hydrochloride (3g)
Compound 3g was obtained as a white solid (yield 61%), mp.
86Ϫ88°C. 1H-NMR (CDCl3) δ: 0.86Ϫ0.93 (s, 6H, -CH3), 1.17Ϫ1.68
(m, 18H, -CHR1-CH2-CHR2-, -CH2-, cyclohexyl, -CH2-CHR-CH2-,
and ϪCOOCH2CH2CH(CH3)OOC-), 1.90Ϫ1.95 (m, 1H, -(CH3)2-
CH-CH2-), 2.39Ϫ2.68 (m, 4H, -OOC-CH2-), 3.02Ϫ3.17 (m, 4H,
-CH2-NH2), 4.11Ϫ4.39 (m, 2H, -COOCH2CH2- CH(CH3)OOC-),
5.04Ϫ5.07 (m, 1H, -COOCH2CH(CH2CH3)OOC-), 8.26Ϫ8.38 (m,
6H, -NH2HCl). Anal. calcd. for C21H42N2O4Cl2: C, 55.14; H, 9.19;
N, 6.12. Found: C, 55.21; H, 9.06; N, 6.26.
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