From the selective serotonin transporter inhibitor citalopram to the selective norepinephrine transporter inhibitor talopram: Synthesis and structure-activity relationship studies

Article abstract of DOI:10.1021/jm701602g

Citalopram and talopram are structurally closely related, but they have very distinct pharmacological profiles as selective inhibitors of the serotonin and norepinephrine transporters, respectively. A systematic structure-activity relationship study was p

Full text of DOI:10.1021/jm701602g

J. Med. Chem. 2008, 51, 3045–3048
3045
From the Selective Serotonin Transporter Inhibitor Citalopram to the Selective Norepinephrine
Transporter Inhibitor Talopram: Synthesis and Structure-Activity Relationship Studies
Jonas N. N. Eildal,†,‡ Jacob Andersen,†,‡ Anders S. Kristensen,† Anne Marie Jørgensen,‡ Benny Bang-Andersen,‡
Morten Jørgensen,‡ and Kristian Strømgaard*,†
Department of Medicinal Chemistry, The Faculty of Pharmaceutical Sciences, UniVersity of Copenhagen, UniVersitetsparken 2,
DK-2100 Copenhagen, Denmark, and Lundbeck Research Denmark, H. Lundbeck A/S, OtilliaVej 9, DK-2500 Valby, Denmark
ReceiVed December 20, 2007
Citalopram and talopram are structurally closely related, but they have very distinct pharmacological profiles
as selective inhibitors of the serotonin and norepinephrine transporters, respectively. A systematic
structure-activity relationship study was performed, in which each of the four positions distinguishing the
two compounds were varied. The inhibitory potencies of the resulting 16 compounds were tested at both
serotonin and norepinephrine transporters. This showed that particularly two of the four positions are
determinants for the biological activity.
Introduction
The serotonin transporter (SERT)a and the norepinephrine
transporter (NET) are integral membrane proteins that facilitate
the reuptake of the neurotransmitters 5-hydroxytryptamine (5-
HT, serotonin) and norepinephrine (NE), respectively, from the
extracellular space into neurons.1 SERT and NET are important
drug targets for treatment of psychiatric diseases such as
depression and anxiety.2 In particular, the development of
selective serotonin reuptake inhibitors (SSRIs) and combined
serotonin/norepinephrine reuptake inhibitors (SNRIs) has
resulted in important drugs used in the treatment of depression
such as the SSRIs sertraline, paroxetine, and fluoxetine in
addition to the SNRIs venlafaxine and duloxetine (Figure 1).3–5
In the search for novel antidepressants, the ring structure from
tricyclic antidepressants (TCAs) was replaced by bicyclic ring
structures (e.g., indanes, indenes, and phtalanes) at H. Lundbeck
A/S (Valby, Denmark) in the 1960s.6,7 The phenylsubstituted
phtalanes were found to be the most potent compounds, and a
series of approximately 60 phenylsubstituted phtalanes were
studied as inhibitors of SERT.8 Among these, citalopram (1,
Figure 1) was shown to be a highly selective and potent inhibitor
of SERT. Further studies demonstrated that the inhibitory
activity of citalopram almost exclusively resided in the (S)-
enantiomer (escitalopram).9 Following this discovery, it was
found that escitalopram has an improved clinical effect as
compared to SSRIs including citalopram.10–12 Talopram (2,
Figure 1)7 was synthesized in the same series of compounds,
and although the two compounds are structurally closely related,
they have very distinct pharmacological profiles. Citalopram (1)
is a potent, selective inhibitor of SERT,13 whereas talopram (2)
is a potent, selective inhibitor of NET.6,14 The two compounds
have the same phenylsubstituted phthalane skeleton, as well as
a propylamine moiety, and they differ in four positions only
(Figure 1). Citalopram (1) has two aromatic substituents, a
fluorine and a cyano group, whereas talopram (2) has no
aromatic substituents. On the other hand, talopram (2) has two
methyl substituents on the dihydro-isobenzenzofurane moiety,
Figure 1. Structures of citalopram (1), talopram (2), escitalopram
sertraline, paroxetine, fluoxetine, venlafaxine, and duloxetine.
while citalopram has no substituents in that position. Finally,
citalopram (1) contains an N,N-dimethyl propylamine group,
while talopram (2) has an N-methyl propylamine.
In a previous SAR study of citalopram analogues it was
demonstrated that the aromatic substituents were important for
inhibitory activity at SERT.8 However, a systematic SAR study
of citalopram and talopram analogues that could explain the
distinct differences in potency toward SERT and NET has so
far not been performed. Therefore, a SAR study was carried
out, where each of the four positions distinguishing citalopram
(1) and talopram (2) were modified independently leading to
16 analogues (Table 1). These compounds were synthesized,
and their inhibitory potencies at SERT and NET were deter-
mined. This enabled an analysis of the structural elements
* To whom correspondence should be addressed. Phone: +45 3533 6114.
Fax: +45 3533 6040. E-mail: krst@farma.ku.dk.
†
University of Copenhagen.
H. Lundbeck A/S.
‡
a Abbreviations: NET, norepinephrine transporter; SERT, serotonin
transporter; SSRI, selective serotonin reuptake inhibitor.
10.1021/jm701602g CCC: $40.75
2008 American Chemical Society
Published on Web 04/23/2008

3046 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Table 1. Structures of Compounds 1-16
Brief Articles
Scheme 1a
a Reagents and conditions: (a) MeMgCl, THF, 0 °C f rt, 1 h.
Scheme 2a
a Reagents and conditions: (a) NiBr2, NaCN, NMP, microwave, 200 °C,
0.5 h. (b) 23 or 24, THF, 0 °C, 0.5 h. (c) (i) 25, THF, reflux, 1 h; (ii)
EtOH/HCl (1:1). (d) Chloroethyl chloroformate, dichloroethane, microwave,
180 °C, 0.5 h.
required for selective inhibition of SERT and NET, respectively,
by the phenylsubstituted phthalanes.
Results and Discussion
and subsequently reduced providing III. Treatment of III with
strong base affords anion IV, which is alkylated with a N,N-
dimethylaminopropyl halide to give the desired compound V.
Alternatively, in route B V is obtained directly from II by
treatment with the Grignard reagent derived from (3-chloro-
propyl)-dimethyl-amine followed by acid promoted ring-closure.
For the synthesis of compounds 4-9 and 12 both strategies
were initially attempted, but it was quickly realized that the
double Grignard strategy (route B) was superior in this case.
Initially, 5-bromoisobenzofuran-1,3-dione (17) was treated with
methyl magnesium chloride to provide bromo-phtalide 18. The
reaction gave a mixture of two isomers 18 and 1915 (Scheme
1) in a 2:1 ratio. Gratifyingly, compound 18 was readily isolated
by crystallization from ethanol in an overall yield of 33%.
Bromo-phtalide 18 was reacted with aryl magnesium bromide
(23 or 24) generating a 2-(hydroxymethyl)-benzophenone in situ,
which was treated with Grignard reagent 25, and the desired
intermediates, compounds 26 and 28, were obtained after acid
promoted ring-closure (Scheme 2). In a similar manner, phtalides
20 and 2116 were reacted with aryl magnesium bromides 23 or
24, followed by reaction with 25 providing intermediate 27 and
final compound 12.
Chemistry. Of the 16 analogues included in this study (Table
1), citalopram (1), talopram (2), and compounds 3, and 10-16
had previously been synthesized7,8 and were available in our
laboratories. Compounds 4-9 have not previously been de-
scribed. Thus, these compounds were prepared as described
below; this method was also used for a revised synthesis of
compound 12.7
Phenylsubstituted phthalanes have generally been synthesized
by two different routes (Figure 2): routes A (the anion strategy)
and B (the double Grignard strategy), respectively.7,8 In both
cases, the starting point is a phthalide (I), which is reacted with
an aryl magnesium halide giving a 2-hydroxymethyl benzophe-
none (II). In the anion strategy, II is ring closed by use of acid
Inspired by the commercial production of citalopram (1),
where a double Grignard strategy is applied, we evaluated
compound 22 as a starting material for cyanated analogues 4-9.
Despite the fact that this route works very well for the
preparation of 1 it was found that the acid-promoted ring-closure
after the double Grignard reaction could not be performed with
compound 22. The only structural difference compared to the
Figure 2. Two synthetic routes generally used for the synthesis of
phenylsubstituted phthalanes: routes A (the anion strategy) and B (the
double Grignard strategy).

Brief Articles
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10 3047
corresponding starting material for 1 is the two methyl groups
(R2, Scheme 2) in compound 22.
Next, conditions for cyanation of compounds 26-28 were
investigated using bromo-phtalide (18) as a model substrate.
Initially, cyanation was attempted by treatment of 18 with either
CuCN and K2CO3 in N-methylpyrrolidone (NMP)17 or using
Buchwald’s aromatic Finkelstein cyanation procedure18 under
microwave irradiation. However, in both cases only very low
degree of conversion was observed (as determined by LC-MS).
Fortunately, applying Leadbeater’s protocol using NaCN and
NiBr2 in NMP under microwave irradiation19 provided the
cyanated analogue 22 in high yield (Scheme 2). These conditions
were then applied in the cyanation of 26-28 to provide the
desired compounds 4, 6, and 8, respectively.
The N-monomethylated compounds 5, 7, and 9 were obtained
by demethylation of 4, 6, and 8, respectively, by treatment with
chloroethyl chloroformate under microwave irradiation and
subsequent treatment with warm (70 °C) ethanol.20
Biology. The potency of citalopram (1), talopram (2), and
their analogues for inhibition of SERT and NET function has
over the course of time been determined in widely different
model systems such as human blood platelets,8 rat brain
synaptosomes,14 or mammalian cell lines transiently expressing
recombinant SERT or NET.13,21 The inherent biological dif-
ferences of these model systems complicate direct comparison
of pharmacological profiles of many of these compounds. In
the present study, the % inhibition at 100 nM of compounds
1-16 was determined at human SERT (hSERT) and NET
(hNET) expressed in the COS-7 kidney cell line through stable
transfection of hSERT and hNET cDNA, respectively. A
reuptake inhibition assay was used,22 where COS-7 cells stably
expressing either hSERT or hNET were incubated with [3H]5-
HT (hSERT assay) or [3H] dopamine (hNET assay) and 100
nM inhibitor or no inhibitor (control). Values for % inhibition
are given as mean ( SEM determined in six independent
experiments each performed in triplicate.
The results of the biological testing at hSERT and hNET show
a distinct relationship between the substitution pattern of the
phenylsubstituted phthalanes and their biological activity (Figure
3 and Table 1 in Supporting Information). Citalopram (1)
together with compounds 3 and 7 are the most potent analogues
at hSERT with 52-58% inhibition when tested at 100 nM,
whereas compounds 6, 10, and 11 were relatively potent with
20-27% inhibition. Compound 3 is a metabolite of citalopram
(1), and it has previously been demonstrated to be equipotent
to the parent compound.8 All other analogues were much less
potent and had <10% inhibition when tested at 100 nM.
Characteristic for the six most potent compounds is that none
of them have methyl substituents in the R2-position (Table 1),
which thus can be identified as the primary factor in reducing
hSERT inhibitory activity. The aromatic CN substitutent (R1,
Table 1), which is present in compounds 1, 3, 6, and 7, is
important for enhancing SERT inhibitory activity. Conversely,
the aromatic fluorine atom (R3, Table 1) and particularly the
amino substitution (R4, Table 1) are less important factors for
hSERT inhibition.
Figure 3. Biological activity of compounds 1-16 at SERT and NET,
respectively. % inhibition of [3H]5-HT (SERT, A) and [3H]DA (NET,
B) uptake by 100 nM inhibitor. Bars represent mean ( SEM obtained
from six independent experiments each performed in triplicate.
structural feature of the most potent compounds is the absence
of a CN substitutent (R1, Table 1), while the presence of methyl
substituents in the R2-position (Table 1) increases potency.
Similarly to hSERT activity, the aromatic fluorine atom (R3,
Table 1) and the amino substitution (R4, Table 1) are less
important for hNET activity.
Conclusions
Citalopram (1) and talopram (2) are structurally related but
show very different pharmacological profiles, being highly
selective and potent inhibitors of SERT and NET, respectively.
The two compounds share the phenyl-substituted phthalane
scaffold, and the differences in chemical structure are limited
to substitutions in four positions. A systematic variation of these
substitutions leads to 16 analogues, which have been investigated
in the present study. Nine of these compounds had been prepared
before and were available in-house, one compound was
synthesized according to a revised procedure, and six com-
pounds were successfully synthesized according to a double
Grignard strategy. Pharmacological characterization of the series
of compounds at cloned hSERT and hNET show that two of
the four positions, the cyano and dimethyl substituents, are the
primary determinants for potency at either hSERT or hNET.
The data demonstrates that the parent compounds citalopram
For hNET activity, talopram (2) is the most potent compound
with a 63% inhibition when tested at 100 nM, while compounds
13 and 15 are relatively potent with 37% and 36% inhibition,
respectively. Interestingly, compound 11 is among the more
potent hNET inhibitors with a 19% inhibition, and this
compound also showed significant potency (20% inhibition) at
hSERT, thus being a SNRI. The remaining analogues generally
produced less than 10% inhibition at 100 nM. The common

3048 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 10
Brief Articles
procedures for the biological testing. This material is available free
of charge via the Internet at http://pubs.acs.org.
(1) and talopram (2) are the most potent and most selective for
hSERT and hNET, respectively, within this series compounds.
Studies aiming at understanding the molecular basis for this
selectivity are under way in our laboratories.
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General Procedure A. Synthesis of Compounds 4, 6, and 8.
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{3-[1-(4-Fluoro-phenyl)-3,3-dimethyl-1,3-dihydro-isobenzo-
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cooled to 0 °C on an ice bath. Grignard reagent 23 (37 mL, 37
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reaction mixture to stir at rt for 0.5 h. Grignard reagent 25 (69 mL,
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solution) was added dropwise over 0.5 h and the mixture refluxed
for 1 h. The reaction mixture was quenched with 10% aqueous
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mL). The organic phases were concentrated in vacuo giving a brown
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37% aqueous HCl (1:1) and then concentrated in vacuo giving the
crude product as a brown oil. The product was dissolved in EtOAc,
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and concentrated in vacuo giving a brown oil, which was dried for
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treated with oxalic acid (1 equiv) in acetone to precipitate the
oxalate of 12 as a white solid (3.04 g, 48%). Anal. (C21H26FNO·
C2H2O4 ·0.59H2O) C, H, N.
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Acknowledgment. We thank the Drug Research Academy,
The Faculty of Pharmaceutical Sciences, University of Copen-
hagen, for support (Ph.D. scholarship to J.A.), and H. Lundbeck
A/S and the Danish Ministry of Science, Technology and
Innovation for support (Industrial Ph.D. scholarship to A.M.J.).
Note Added after ASAP Publication. This paper was published
on April 23, 2008, with errors in Figure 1 and Table 1. The correct
version was published on May 1, 2008.
Supporting Information Available: Experimental details for
the synthesis of compounds 4-9, 12, 18, and 25-28, spectroscopic
data, and elemental analysis results, as well as experimental
JM701602G