Synthesis and biological evaluation of new N-substituted 4-(arylmethoxy)piperidines as dopamine transporter inhibitors

Article abstract of DOI:10.1016/j.mencom.2019.03.030

The library of new N-substituted 4-(arylmethoxy)piperidines as dopamine transporter inhibitors was designed and synthesized. H-Bond donors in piperidine ring were found to be important for reduced locomotor activity in mice. 4-[Bis(4-fluorophenyl)methoxy]piperidine has IC₅₀ 17.0 ± 1.0 nm for dopamine transporter and locomotor activity, which is lower than that for cocaine.

Full text of DOI:10.1016/j.mencom.2019.03.030

Mendeleev
Communications
Mendeleev Commun., 2019, 29, 203–205
Synthesis and biological evaluation of new N-substituted
4-(arylmethoxy)piperidines as dopamine transporter inhibitors
Gennady B. Lapa*a and Alla A. Lapab
a N. I. Pirogov Russian National Research Medical University (RNRMU), 117997 Moscow,
Russian Federation. E-mail: lapa.gb@yandex.ru
b Sandoz Pharmaceutical, 125315 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2019.03.030
F
cocaine
500
400
300
200
100
0
The library of new N-substituted 4-(arylmethoxy)piperidines
as dopamine transporter inhibitors was designed and syn-
thesized. H-Bond donors in piperidine ring were found to be
important for reduced locomotor activity in mice. 4-[Bis-
(4-fluorophenyl)methoxy]piperidine has IC50 17.0 1.0 nm
for dopamine transporter and locomotor activity, which is
lower than that for cocaine.
N
O
H
F
IC50 17.0 1.0 nM (DAT)
The dopamine transporter (DAT) is a transmembrane protein
responsible for reuptake of extraneuronal dopamine (DA) in
presynaptic terminals of DA neurons, thus being involved in
the regulation of dopaminergic transmission.1–3 DAT has been
considered as a primary target of cocaine action.1,2,4 Develop-
ment of new DAT inhibitors has been the focus of several
publications.1–4 Such compounds are interesting as molecular
tools for the study of DAT structure1,2 and potential therapeutic
agents for treatment of cocaine abuse.1,2,4 However, selective and
potent DAT inhibitors have demonstrated low efficacy for the
monotherapy of cocaine addiction.1,5 Moreover, DAT inhibitors
could be promising for treatment of some ‘dopamine diseases’,
i.e., attention deficit and hyperactivity disorders as well as
Parkinson’s disease.6 Recently DAT inhibitors attracted persistent
attention as components for triple reuptake inhibitors for anti-
depressant development.3 Enhanced studies of DAT inhibitors
with piperidine ring revealed its influence on H-1 and H-3 central
histamine receptors.7–10 This influence appears as an antagonism
to the behavioural and locomotor activity in rodent models of
behavioural disorders.7–10 Throughout the past decade, benzo-
tropine (BZT) analogues have been shown to be potent DAT
inhibitors.7 While the DAT binding affinities of BZT analogues
are superior to cocaine, their locomotor activities are less due
to affinity to the several M- and H-receptors.7 A recent study
examined the binding of BZT analogues to H-1, H-2, and H-3
histamine receptors and showed that BZT analogues have modest
central antihistamine effects.11 It serves only as a first step towards
understanding integral activity of BZT analogues on behavioural
reactions such as locomotor activity.11 Some of histamine anta-
gonists comprised piperidin-4-ol fragment, a popular scaffold for
development of H-1-12 and H-3-hystamine receptor antagonists.13
4-(Diphenylmethoxy)piperidine (DPP) 1 is a well-known H-1-
histamine antagonist with strong DAT inhibiting activity.14,15 It is
known that alpha-enantiomer of BZT is much more active than
beta-one in binding analysis.4 3D-structure of alpha-BZT does not
completely correspond to that of cocaine since there is agreement
only in tropane cycle but position of benzene moiety in the
space is different. Meanwhile, DPP 1 has conformationally mobile
piperidine cycle lacking two-carbon bridge. Thus, there is no
limitation in configuration at C-4 of the piperidine cycle. Piperidine
cycle, ether bond and one aromatic ring can coincide in space,
which is important for cocaine pharmacophore.2,4 The alignment
of molecules was made along the piperidine cycle and ether bond
(Figure 1).16 The root-mean-square deviation (RMSD) value for
this alignment was 3.28 that was acceptable for the proposals of
pharmacophore similarity and acceptance of qualitative specula-
tions about side radicals for this core.16,17
Previously, we discussed the comparison of pharmacological
profiles for cocaine and DPP and the benefits of 4-fluorophenyl
substituents in diarylmethoxy fragment.14,15 Also, we described
simple approach to the combinatorial library and more potent
DAT inhibitors with diverse locomotor activity in vivo.15 This
approach involved combinatorial modification of three points
of diversity in DPP structure and was limited by the nitrogen of
piperidine ring, methine group of diarylmethoxy fragment and
aromatic rings.15
CO2Me
O
N
Me
N
Me
O
N
O
BZT
cocaine
cocaine
O
N
O
Me
Arrows stand for
points of diversity
DPP
DPP
Figure 1 The alignment of DPP along the cocaine structure.
© 2019 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2019, 29, 203–205
Table 1 Biological evaluation and calculated logP data for compounds
1–15 and cocaine.
fragment can provide two symmetrical H-bond acceptors. N-Butyl
or N-isopropyl are bulky substituents which can increase activity
in comparison to compound 2. (p-Tolyl)methoxy fragment can
account for an importance of para-substituent with donor
properties. Additionally, 3,4-dichlorophenyl fragment could present
the series of meta-substituents. The influence of size of para-
substituents can explain small (fluoro-) or large (chloro-) side
groupings. N-Cyanomethyl fragment can provide knowledge on
the effect of donor–acceptor type of interaction.
We used 4-(benzhydryloxy)piperidins as the scaffold with
three points of diversity for N–H group, OCH fragment and
aromatic rings. Here we describe substitution at the N-atom
and in aromatic rings (Scheme 1). Recently,15 we reported on
route A. Obviously, route B is more economical and suitable for
combinatorial synthesis to introduce substituents at the nitrogen
atom of piperidine moiety at the last step and it was used to
prepare new compounds 10–15.†
Commercially available substituted benzophenones were
reduced (NaBH4, PriOH) to benzhydrols in nearly 100% yield.
The etherification of the latter with 5% molar excess of piperidin-
4-ols or N-methylpiperidin-4-ol was performed by reflux in toluene
with a Dean–Stark trap in the presence of TsOH,15,18 the yields
of the ethers ranged from 60 to 90%. N-Substituted piperidines
10, 11, 15 were prepared by alkylation of 4-[bis(4-fluorophenyl)-
methoxy]piperidine with alkyl halides in DMF in the presence of
anhydrous potassium carbonate in yields of approximately 90%.
Some compounds were converted into salts with oxalic acid in
acetone, which improved their solubility in water. Table 1 contains
the structures, data of binding assay and locomotor activity of
compounds.
Experimental Experimental Motion activity
Calc.
logP
Compound IC50/nm DAT, Ki/nm SERT, to basal level
123I-RTI-55
3H-citalopram 17.8 mmol kg–1 (%)
1 (DPP)
420 9.0a
22.1 5.7a
12.5 7.5a
44.0 10.9a
50.6 2.8a
155 10.0a
264 9.0a
277 10.0a
293 10.0a
32.0 1.4
61.6 7.2
5353 160
6000 150
17 2.0
13587 1920 311.0a
3.2
3.5
6.0
4.5
6.1
3.8
2.1
3.3
3.6
4.9
4.2
4.4
3.6
2.9
3.3
2.9
2
3
4
5
6
7
8
9
7518 1690
396 57
887.8a
98.7a
150.2a
41.4
>14000
406 100
2426 130
>14000
159.4a
28.9
3083 330
5534 1720
>14000
126.5
88.8
10
11
12
13
14
15
Cocaine
182.0
140.5
91.7
>14000
>14000
>14000
99.3
>14000
206.7
8.5
251 12.0
104 49.0a
>14000
297 13.0
426.0a
a See refs. 14 and 15.
Here we describe the further improvement and optimization
of the leading compound efficacy as well as the synthesis and
biological evaluation of novel 4-(4,4'-difluorobenzhydryloxy)-
piperidines and compare them with prototypical DAT inhibitors
such as early described compounds 1 and 2 (Table 1). Additionally
we provide new data on binding properties of earlier synthesized
compounds 1–9 and biological evaluation for the new ones
10–15.
N-Methyl-4-(4,4'-difluorobenzhydryloxy)piperidine 2 was
chosen as a leading compound to increase DAT affinity because it
showed high affinity and selectivity to DAT. This compound has
shown remarkably increased locomotor activity which depends
on the penetration through blood–brain barrier.15 We used big–
small and donor–acceptor pairs of substituents for the new
compounds to expand previously described combinatorial library
since our alignment showed some similarity to the cocaine
structure and some similarity to the BZT analogues. From this
point of view, new six compounds 10–15 (see Table 1) were
chosen for the synthesis. p-Fluoro substituents in diarylmethoxy
Binding assay was performed as described previously.19
Determination of doses for in vivo tests (17.8 mmol kg–1) and
locomotor activity test was reported in previous publications.14,15
The results of locomotor activity test were expressed as percents
to the basal level of motion activity when mice were injected with
saline (Figure 2). Synthesized compounds significantly changed
locomotor activity levels (P < 0.05). Statistical one-way analysis
of variance (ANOVA) was carried out.
Our IC50 results have revealed the same trend of SAR known
from published series of BZT analogues.2–4,7 The use of our
scaffold (DPP or 3) with lower molecular weight as compared to
BZT allowed us to avoid some limits of ‘Lipinsky’s rule of five’.
Using known method20 we calculated logP, which helped us to
obtain compounds with better balance, lower molecular weight
and lipophilicity by increasing size of substituents at the N atom
of piperidine ring.20 Binding assay showed that all compounds
have excellent selectivity to DAT in comparison to SERT (see
Table 1). Motion activity tests revealed less predictable results
than IC50 because it reflects total effects of DAT inhibitors in
living animal. This trend depends on factors such as binding
i
ii
R4
HN
OH
route A
route B
R3
N
HN
R4
OH
O
R1
R2
R3
ii
i
500
400
300
200
100
0
cocaine
N
O
R1
R2
1–15
R3 R4
14
R1
R2
Ph
4-FC6H4
4-ClC6H4 Cl
4-ClC6H4 Cl
R1
R2
R3 R4
1
2
3
4
5
6
7
8
Me
Me
Bu
H
F
H
H
H
H
H
H
H
H
9 Me
10 Bu
11 Pri
12 Me
13 Me
14 H
Ph
H
F
F
Me
H
H
4-FC6H4
4-FC6H4
Ph
Ph
4-FC6H4
Me
Cl Cl
Me H
IC50 17.0 1.0 nM (DAT)
4-FC6H4CH2 4-ClC6H4 Cl
Me
Me
Me
Ph
Me
Ph
Cl
F
F
F
F
H
H
Figure 2 Comparison of locomotor activity in mice for cocaine and com-
pound 14 in doses 17.8 mmol kg–1 given as a percentage to basal level
(injection of saline).
15 CH2CN 4-FC6H4
Scheme 1 Reagents and conditions: i, R1Hal, K2CO3, DMF, 40 °C;
†
ii, 3-R4-4-R3-C6H3CH(R2)OH, TsOH, toluene–DMF (25:1), reflux.
For the synthetic details, see Online Supplementary Materials.
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Mendeleev Commun., 2019, 29, 203–205
affinity, penetration through blood–brain barrier and distribution
among different types of receptors.
We are grateful to Professors S. R. Jones and S. R. Childers
(Wake Forest University Health Science, NC) and Dr. Jill J. Harp
(Department of Life Sciences, Winston-Salem State University,
NC) for comprehensive consultation on binding experiments, lab
routines of radioactive safety and in vivo experiments. We are
thankful to Dr. V. Grinevich (Asinex, NC) for friendly advice
and comments.
Almost all compounds had IC50 better than that of DPP. As
we predicted and found earlier,15 the introduction of lipophilic
moieties at nitrogen atom in compounds 2, 5, 10 led to an increase
in IC50, however increase in N-alkyl size was not sufficient.
Although, the structure of compound 7 has the best alignment
with the cocaine one (see Scheme 1) because carbonyl group was
changed with methyl one, this did not lead to an increase in IC50.
The most active compound 3 had the strongest IC50 and biggest
logP but it is too lipophilic to penetrate through the blood brain
barrier (see Table 1).15 Both H-bond donors at piperidine nitrogen
(compound 14) and alkyl substituents (compounds 2, 10 and
11) caused an increase in IC50. However, the in vivo tests have
shown a lot of restrictions for this direction in piperidine scaffold
modification.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2019.03.030.
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In conclusion, we revealed that DPP analogues with lipophilic
N-substituents as well as ones para-substituted in benzhydroxy
fragment showed the trend of increasing the IC50 value. Both
aliphatic and halogen meta-substituents in benzhydryloxy moiety
cause decrease in IC50. The most promising candidates for
further studies are new compounds 11, 14 and 15. Perhaps, the
introduction of H-bond acceptors into N-piperidine substituent
can be a good tool for further improving activity within this
combinatorial library.
Received: 29th June 2018; Com. 18/5623
– 205 –