Antioxidative activities of novel diphenylalkyl piperazine derivatives with high affinities for the dopamine transporter

Article abstract of DOI:10.1016/j.bmcl.2004.05.091

A new series of diphenylalkyl piperazine derivatives with high affinities for the dopamine transporter (DAT), which were modified at both the diphenylalkyl moiety and the phenyl ring in the phenylamino moiety of 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-hydroxy-3-(phenylamino)propyl]piperazine 1, was evaluated for their inhibitory activities against auto-oxidative lipid peroxidation in canine brain homogenates. Some of these were approximately equivalent in activity to α-tocopherol as a potent antioxidant with IC₅₀ values of low micromolar order, and the 4-hydroxyphenyl derivative 11 showed the most potent antioxidative activity with an IC ₅₀ value of 0.32μM, exhibiting approximately 5-fold more potent activity than α-tocopherol. The structure-activity relationship (SAR) studies of the antioxidative activity of these derivatives are presented.

Full text of DOI:10.1016/j.bmcl.2004.05.091

Bioorganic & Medicinal Chemistry Letters 14 (2004) 4287–4290
Antioxidative activities of novel diphenylalkyl piperazine
derivatives with high affinities for the dopamine transporter
Makoto Kimura,a,* Tomoko Masuda,a Koji Yamada,a Nobuyuki Kawakatsu,a
Nobuo Kubota,a Masaki Mitani,a Kenichi Kishii,a Masato Inazu,b Yuji Kiuchi,c
Katsuji Oguchid and Takayuki Namikia,*
aPOLA Chemical Industries, Inc., Pharmaceutical R&D Laboratories, 560 Kashio-cho, Totsuka-ku,
Yokohama, Kanagawa 244-0812, Japan
bDepartment of Pharmacology, Tokyo Medical University, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160-8402, Japan
cDepartment of Pathophysiology, School of Pharmaceutical Sciences, Showa University, 1-5-8 Hatanodai,
Shinagawa-ku, Tokyo 142-8555, Japan
dDepartment of Pharmacology, School of Medicine, Showa University, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142-8555, Japan
Received 23 April 2004; accepted 31 May 2004
Available online
Abstract—A new series of diphenylalkyl piperazine derivatives with high affinities for the dopamine transporter (DAT), which were
modified at both the diphenylalkyl moiety and the phenyl ring in the phenylamino moiety of 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-
hydroxy-3-(phenylamino)propyl]piperazine 1, was evaluated for their inhibitory activities against auto-oxidative lipid peroxidation
in canine brain homogenates. Some of these were approximately equivalent in activity to a-tocopherol as a potent antioxidant with
IC50 values of low micromolar order, and the 4-hydroxyphenyl derivative 11 showed the most potent antioxidative activity with an
IC50 value of 0.32 lM, exhibiting approximately 5-fold more potent activity than a-tocopherol. The structure–activity relationship
(SAR) studies of the antioxidative activity of these derivatives are presented.
Ó 2004 Elsevier Ltd. All rights reserved.
Although much research has been focused on the etiol-
ogy and pathogenesis of the progressive neurodegener-
ative disorders1–4 such as Alzheimer’s disease,5
Huntington’s disease,6 Parkinson’s disease,7–10 and am-
yotrophic lateral sclerosis,11 these diseases have not been
elucidated clearly. In the last two decades, it has been
confirmed that oxidative stress in the central nervous
system (CNS) plays an important role in the pathogen-
esis and progression of these neurodegenerative disor-
ders, and also increases other risk factors for these
diseases.1–4 The brain is considered to be vulnerable to
oxidative stress, because a high concentration of poly-
unsaturated lipids in the brain can serve as substrates for
lipid peroxidation.3 Thus, the generation and sub-
sequent chain reaction of reactive oxygen species such as
the hydroxy radical, super oxide radical, and nitric
oxide, induce irreversible damage to neuronal cell
membranes through extensive lipid peroxidation, lead-
ing to the death of neuronal cells.1;2
Parkinson’s disease (PD), characterized by motor dys-
function with tremor, akinesia, rigidity, and postural
instability, is one of the best-known neurodegenerative
disorders. This disease is believed to be caused by the
depletion of dopamine (DA) due to neurodegeneration
in nigrostriatal dopaminergic neurons.12;13 Recently
there has been a focus of interest on the use of antiox-
idants such as a-tocopherol (Fig. 1) for the suppression
of neurodegeneration in nigrostriatal dopaminergic
neurons, and the clinical trials of a-tocopherol for the
treatment of PD have been performed.14 Moreover, the
antioxidative activities of conventional medications for
PD, such as bromocriptine15 and lazabemide16 (Fig. 1),
have been widely studied for an additional action, which
may prevent or delay the neuronal death in dopami-
nergic nerves.
Keywords: Diphenylalkyl piperazine derivative; Antioxidative activity;
Dopamine transporter affinity; Structure–activity relationship.
* Corresponding authors. Tel.: +81-45-826-7264; fax: +81-45-826-72-
49; e-mail addresses: m-kimura@pola.co.jp; t-namiki@pola.co.jp
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.05.091

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M. Kimura et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4287–4290
O
N
O
N
O
H
HO
N
O
OH
HN
Br
H
O
N
H
Bromocriptine
α −Tocopherol
HO HN
N
N
Cl
H
N
F
N
NH2
O
F
1
Lazabemide
Figure 1.
In our previous study of novel diphenyl piperazine
derivatives, we have found that a series of compounds
represented by 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-hy-
droxy-3-(phenylamino)propyl]piperazine 1 shows potent
DAT binding affinities on rat striatal membranes.17;18
Moreover, from a pharmacological study using in vivo
brain microdialysis, compound 1 has been found to
display significant and dose-dependent increases of rat
striatal extracellular DA levels.17 These findings have
suggested that these novel diphenyl piperazine deriva-
tives are potent DA uptake inhibitors in the CNS, and
may prove effective for the treatment of CNS disorders
such as PD, which are caused by the depletion of DA or
related to the DAT. On the other hand, compound 1
and its analogues, which were modified at the b-amino-
2-propanol moiety in 1 have also been found to possess
antioxidative activities against auto-oxidative lipid per-
oxidation in canine brain homogenates.19 Therefore, to
study further antioxidative activity of these novel diph-
enylalkyl piperazine derivatives as an additional action
for more effective treatment of PD, we evaluated them
modified at the diphenylalkyl moiety and the phenyl
ring in the phenylamino moiety of 1 and examined their
SARs.
As described in our previous studies of these novel di-
phenyl piperazine derivatives, we have synthesized a
series of diphenylalkyl piperazine derivatives modified at
the diphenylalkyl moiety and the phenyl ring in the
phenylamino moiety of 1. Herein, we outline their syn-
thetic routes. The synthetic method of these novel
diphenylalkyl piperazine derivatives is divided into two
routes, as shown in Schemes 1 and 2. Alkylation of the
N-protected aniline derivatives 2a and 2b with epi-
bromohydrin using sodium hydride in N,N-dimethyl-
formamide gave the epoxides 3a and 3b. The ring
opening of the epoxides 3a and 3b with the diph-
enylalkyl piperazines 4a–e in ethanol, followed by de-
protection of the N-protective group in the resultant
intermediates, gave the diphenylalkyl piperazine deriv-
atives 1, 5a–d, and 6 (Scheme 1). Alkylation of the
diphenylbutyl piperazine 4d with the chlorohydrin
derivatives 8a–h prepared from the corresponding ani-
line derivatives 7a–h and epichlorohydrin gave the
R1
HO HN
b,c
R1
a
R1
(CH2)n
F
N
N
HN
R2
N
O
F
R2
1, 5a-d, 6
3a,b
2a,b
a: R1=H, R2=COOEt
b: R1=3,5-di-tert-Bu-4-OH, R2=Boc
1: R1=H, n=3; 5a: R1=H, n=0; 5b: R1=H, n=1; 5c: R1=H, n=2; 5d: R1=H, n=4;
6: R1=3,5-di-tert-Bu-4-OH, n=3
(CH2)n
F
N
NH
F
4a : n=0
4b : n=1
4c : n=2
4d : n=3
4e : n=4
Scheme 1. Reagents and conditions: (a) NaH, epibromohydrin, DMF, room temperature, 81% for 3a,3b was used for the next reaction without
purification; (b) diphenylalkyl piperazine (4a–e), EtOH, room temperature, 28–100%; (c) NaOH(aq), EtOH, reflux, for 1 and 5a–d, 53–95%, and HCl,
AcOEt, room temperature, for 6, 6 was converted to its trimaleate without purification.

M. Kimura et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4287–4290
4289
R1
HO HN
R1
R1
a
HO HN
Cl
b
H2N
N
N
F
9a-h
F
c
7a-h
8a-h
10
11
9f
d
9h
a:R1=3,4-diCl, b:R1=4-Cl, c:R1=4-Me, d:R1=4-OMe, e:R1=4-N(CH3)2, f:R1=4-NO2, g:R1=3,4,5-triOMe, h:R1=4-OBn, 10:R1=4-NH2, 11:R1=4-OH
Scheme 2. Reagents and conditions: (a) epichlorohydrin, EtOH, reflux, 42–72%; (b) 4d, K2CO3, KI, EtOH, reflux, 31–78%; (c) SnCl2, EtOH, heat,
81%; (d) 10% Pd-black, HCOOH, MeOH, room temperature, 95%.
compounds 9a–h with various substituents on the phe-
nyl ring in the phenylamino moiety of 1 (Scheme 2).
Reduction of the 4-nitro derivative 9f with tin(II) chlo-
ride gave a 4-amino derivative 10 (Scheme 2). Depro-
tection of the benzyl group in the 4-benzyloxy derivative
9h using palladium catalyst gave a 4-hydroxy derivative
11 (Scheme 2).
apparent antioxidative activities with IC50 values of
approximately micromolar order, showing that the most
potent compound 1 among them displayed slightly less
inhibitory activity than a-tocopherol. The difference in
the antioxidative activity between these five compounds
was less than 2-fold, and these results suggest that the
modification at the connective between the diphenyl-
methyl and piperazine moieties has little influence on the
antioxidative activity.
All of the final compounds synthesized as described
above were used as their corresponding salts listed in
Table 1 for their inhibitory activities against auto-oxi-
dative lipid peroxidation in canine brain homogenates.20
The results are shown in Table 1.
Next, the compounds modified at the phenyl ring in the
phenylamino moiety of 1 were evaluated for their an-
tioxidative activities. Since the Topliss method22 has
often been applied to modification of the substituent of
the phenyl ring in optimization strategies, we attempted
to use this method in order to investigate the phenyl ring
substituent effects. According to the Topliss method,
four compounds, 3,4-dichloro 9a, 4-chloro 9b, 4-methyl
9c, and 4-methoxy 9d derivatives, excluding the un-
substituted compound 1 described above, were selected
and evaluated. These four derivatives showed potent
antioxidative activities. As compared with the un-
substituted compound 1, the introduction of the meth-
oxy group 9d led to a 4-fold increased activity, whereas
the introduction of dichlorine atoms 9a exhibited a 3.5-
fold less activity. Of the above compounds, the
4-methoxy derivative 9d showed the most potent an-
tioxidative activity, with an IC50 value of 1.6 lM, which
was approximately equipotent in activity to a-tocoph-
erol. The ranked order in the antioxidative activity of
these five compounds including 1, was : 9d > 9c =
1 = 9b > 9a. When these results are applied to the To-
pliss method, it is suggested that a contribution of the
Àr parameter, which is closely related to the electron-
donating property of a substituent on the phenyl ring, is
dominant in the antioxidative activity of these diph-
enylalkyl piperazine derivatives.
At first, the compounds 5a–d, which were modified at
the connective between the diphenylmethyl and pipera-
zine moieties in compound 1, were evaluated for their
antioxidative activities, as well as 1. All of them showed
Table 1. Antioxidative activities of diphenylalkyl piperazine deriva-
tives
F
R1
HO HN
(CH2)n
N
N
F
Compounds
n
R1
Salta IC50 (lM)b
5a
5b
5c
1
0
1
2
3
4
3
3
3
3
3
3
3
3
3
3
H
H
H
H
H
A
A
A
A
A
C
B
A
A
B
C
B
B
B
B
6.9
11.0
11.0
6.5
5d
9a
9b
9c
9d
9e
9f
9g
10
11
6
8.3
3,4-diCl
4-C1
4-Me
23.0
8.5
6.1
1.6
4-OMe
4-N(CH3)2
4-NO2
2.2
These results encouraged us to make further investiga-
tion of the phenyl ring substituent effects according to
the Topliss method, and the compounds possessing a
substituent with a higher Àr effect at the para-position
of the phenyl ring were expected to increase the an-
tioxidative activity. Three compounds possessing the
4-dimethylamino group 9e, 4-amino group 10, and
4-hydroxy group 11 on the phenyl ring were evalu-
ated. As expected, they all exhibited notably potent
182.0
1.4
2.7
3,4,5-triOMe
4-NH2
4-OH
0.32
2.8
3,5-di-tert-Bu-4-OH
a-Tocopherol
1.5
a A: trihydrochloride; B: trimaleate; C: dihydrochloride.
b See Ref. 21.

4290
M. Kimura et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4287–4290
antioxidative activities. Surprisingly, the 4-hydroxy
derivative 11 displayed the most potent activity, show-
ing a 4.7-fold more potent activity than a-tocopherol,
with an IC50 value of 0.32 lM. This may be due to its
higher capacity as a hydrogen donor to form a stable
radical, as well as the phenols known as antioxi-
dants,23;24 in addition to the Àr effect. In contrast with
the results described above, the nitro derivative 9f hav-
ing a high r effect as an electron-withdrawing sub-
dual actions of dopamine uptake inhibition and an-
tioxidative activity.
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antioxidative
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Subsequently, as another approach to the enhancement
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derivatives with high affinities for the DAT, which were
modified at both of the diphenylalkyl moiety and the
phenyl ring in the phenylamino moiety of compound 1,
was evaluated for antioxidative activity in canine brain
homogenates. Most of them showed apparent antioxi-
dative activities with IC50 values of approximately
micromolar order, including some compounds with
activity equal to or higher than a-tocopherol. Among
them, the 4-hydroxyphenyl derivative 11 exhibited the
most potent activity with an IC50 value of 0.32 lM,
which was approximately 5-fold more potent than a-
tocopherol. The SAR studies suggested that the modi-
fication at the site between the diphenylmethyl and
piperazine moieties has no great influence on the anti-
oxidative activity, and that the introduction of a sub-
stituent having a Àr effect as an electronic property into
the phenyl ring of the phenylamino moiety is more likely
to increase the activity. Moreover, compound 11 was
suggested to display the extraordinary antioxidative
activity, probably due to the formation of a stable
radical in addition to the Àr effect. These results also
suggest that these novel diphenylalkyl piperazine deriv-
atives may serve as effective treatments for PD, with
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