Azidobupramine, an antidepressant-derived bifunctional neurotransmitter transporter ligand allowing covalent labeling and attachment of fluorophores

Article abstract of DOI:10.1371/journal.pone.0148608

The aim of this study was to design, synthesize and validate a multifunctional antidepressant probe that is modified at two distinct positions. The purpose of these modifications was to allow covalent linkage of the probe to interaction partners, and deco

Full text of DOI:10.1371/journal.pone.0148608

RESEARCH ARTICLE
Azidobupramine, an Antidepressant-Derived
Bifunctional Neurotransmitter Transporter
Ligand Allowing Covalent Labeling and
Attachment of Fluorophores
Thomas Kirmeier1☯¤a, Ranganath Gopalakrishnan2☯¤b, Vanessa Gormanns2☯, Anna
M. Werner2¤c, Serena Cuboni2¤d, Georg C. Rudolf3¤e, Georg Höfner4, Klaus T. Wanner4,
Stephan A. Sieber3, Ulrike Schmidt2, Florian Holsboer1¤a, Theo Rein2‡*, Felix Hausch2‡
*
1 Max Planck Institute of Psychiatry, Clinical Department, Munich, Germany, 2 Max Planck Institute of
Psychiatry, Department of Translational Research in Psychiatry, Munich, Germany, 3 Technical University
Munich, IAS, CIPSM, Department of Chemistry, Garching, Germany, 4 Department Pharmazie Zentrum für
Pharmaforschung, Ludwig-Maximilians-Universität München, Munich, Germany
☯ These authors contributed equally to this work.
¤a Current address: HMNC GmbH, Munich, Germany
¤b Current address: AstraZeneca MPI Satellite Unit, Max Planck Institute of Molecular Physiology,
Department of Chemical Biology, Dortmund, Germany
¤c Current address: Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany
¤d Current address: Prosteros Biostructures GmbH, Planegg-Martinsried, Germany
¤e Current address: BASF Chemicals India Pvt. Ltd., TTC Area, Turbhe, Navi Mumbai, India
‡ These authors also contributed equally to this work.
OPEN ACCESS
Citation: Kirmeier T, Gopalakrishnan R, Gormanns
V, Werner AM, Cuboni S, Rudolf GC, et al. (2016)
Azidobupramine, an Antidepressant-Derived
Bifunctional Neurotransmitter Transporter Ligand
Allowing Covalent Labeling and Attachment of
Fluorophores. PLoS ONE 11(2): e0148608.
doi:10.1371/journal.pone.0148608
* theorein@mpipsykl.mpg.de (TR); hausch@mpipsykl.mpg.de (F. Hausch)
Abstract
Editor: Kenji Hashimoto, Chiba University Center for
Forensic Mental Health, JAPAN
The aim of this study was to design, synthesize and validate a multifunctional antidepres-
sant probe that is modified at two distinct positions. The purpose of these modifications was
to allow covalent linkage of the probe to interaction partners, and decoration of probe-target
complexes with fluorescent reporter molecules. The strategy for the design of such a probe
(i.e., azidobupramine) was guided by the need for the introduction of additional functional
groups, conveying the required properties while keeping the additional moieties as small as
possible. This should minimize the risk of changing antidepressant-like properties of the
new probe azidobupramine. To control for this, we evaluated the binding parameters of azi-
dobupramine to known target sites such as the transporters for serotonin (SERT), norepi-
nephrine (NET), and dopamine (DAT). The binding affinities of azidobupramine to SERT,
NET, and DAT were in the range of structurally related and clinically active antidepressants.
Furthermore, we successfully visualized azidobupramine-SERT complexes not only in
SERT-enriched protein material but also in living cells stably overexpressing SERT. To our
knowledge, azidobupramine is the first structural analogue of a tricyclic antidepressant that
can be covalently linked to target structures and further attached to reporter molecules
while preserving antidepressant-like properties and avoiding radioactive isotopes.
Received: May 6, 2015
Accepted: January 20, 2016
Published: February 10, 2016
Copyright: © 2016 Kirmeier et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: The authors have no support or funding to
report.
Competing Interests: TK, RG, SC, GCR, and F.
Holsboer are employed by companies (as indicated
in the affiliations). This had no influence on the study.
This does not alter the authors' adherence to PLOS
ONE policies on sharing data and materials.
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Novel Probe to Identify Antidepressant Target Molecules
Introduction
Mapping monoamine transporters for relevant drug binding sites has been an integral part of
elucidating the molecular mechanisms of antidepressants regarding their effects on the mono-
aminergic system. To achieve this, various experimental approaches have been pursued,
including those employing chemically modified small molecules and genetic engineering. The
chemically modified molecules used in these mapping studies typically consist of a pharmaco-
logically active core structure enriched by a photo-inducible cross-linker and a radioactive iso-
tope. This design allows the formation of compound-target complexes that are detectable by
their radioactivity. In combination with genetic modifications of the target molecules, this
approach enables the identification of functionally relevant amino acids of known targets. This
strategy has successfully been used to characterize the binding sites of antidepressants to the
monoamine transporters NET, DAT, and SERT [1–4]. Intriguingly, similar chemically modi-
fied tricyclic compounds (i.e. tritium labelled photo-labile tricyclic antidepressants) pointed to
the existence of various binding partners in the cellular proteome that are most likely not iden-
tical to monoamine transporters [5–10]. However, not the least due to technical limitations at
that time, the molecular identity of these candidates has never been revealed. Moreover, after
the cloning of the monoamine transporters in the 1990s [11–13] the field focused mainly on
these transporter molecules and (in-)directly associated pathways while neglecting potential
alternative binding partners.
Today, several innovations in protein detection and chemical biology opened up hitherto
unknown possibilities in molecular pharmacology. This is exemplified not only by phenotypic
screening studies but also by the identification of direct interaction partners using multifunc-
tional small molecules [14,15]. In particular, technical innovations in organic chemistry
allowed the exchange of isotope labels by biologically inert chemical groups enabling for radio-
active-free labeling of small molecule-target complexes. Despite promising results in other dis-
ciplines, no equivalent multifunctional tool derived from clinically approved antidepressants
has been developed in the field of neuropsychopharmacology [16–21]. This may be due to the
fact that mental diseases are multifactorial disorders with several layers of complexity and that
antidepressant drugs are held to be promiscuous [22–25]. Moreover, like with other drug mod-
ifications, even small changes in chemical structure of psychoactive substances can result in
considerable changes in target binding or even complete loss of activity [26].
The goal of this study was to modify an established antidepressant in a way that enables for
covalent binding of the modified antidepressant to target structures and subsequent linkage of
reporter molecules. We created azidobupramine, a structural analogue of imipramine, featur-
ing two additional chemical groups, one for photoaffinity labelling (PAL) and the other for
copper(I)-catalyzed azide alkyne cycloaddition (CuAAC). The former group allows for cova-
lent linkage of azidobupramine to its target molecules and the latter to furnish the generated
drug-target complexes with reporter molecules like fluorophores. For the biological evaluation
of the functionality of azidobupramine, three canonical targets (i.e. SERT, NET and DAT)
were used. Primary endpoints of the study were the analysis of binding affinities of azidobupra-
mine to SERT, NET and DAT, and the functional evaluation of the added chemical moieties
for PAL and CuAAC employing SERT as model target.
Methods
Chemical synthesis
Chromatographic separations were performed either by manual flash chromatography or by
automated flash chromatography using an Interchim-Puriflash 430 with an UV detector.
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Novel Probe to Identify Antidepressant Target Molecules
Organic phases were dried over MgSO4, and the solvents were removed under reduced pres-
sure. Merck F-254 (thickness 0.25 mm) commercial plates were used for analytical TLC to fol-
low the progress of reactions. Silica gel 60 (Merck 70-230 mesh) was used for manual column
chromatography. Unless otherwise specified, 1H NMR spectra, 13C NMR spectra, 2D HSQC,
HMBC and COSY of all intermediates were analyzed on a Bruker AC 300, a Bruker XL 400, or
a Bruker AMX 600 at room temperature. Chemical shifts for 1H, 13C are given in ppm (δ) rela-
tive to tetramethylsilane (TMS) as internal standard. Mass spectra (m/z) were recorded on a
Thermo Finnigan LCQ DECA XP Plus mass spectrometer, while the high resolution mass
spectrometry was carried on Varian Mat711 mass spectrometer. The purity of the compounds
was verified by reversed phase HPLC (Jupiter 4 μ Proteo 90 A, 250Ã4.6 mm, Phenomenex, Tor-
rance, USA) using gradient A (acetonitrile: water gradient: 0.1% TFA of 0-100% in 45 min)
unless otherwise specified. Solvents were purchased from Roth, reagents were obtained from
Aldrich-Fluka unless otherwise noted. HPLC conditions for product analysis; Column: Jupiter
4 μm Proteo 90 A, 250 x 4.6 mm, Phenomenex, Torrance, USA, Wavelength: 224nm, 280nm
Flow rate: 1ml/min, Buffer A: 0.1% TFA in 5% MeCN/Water, Buffer B: 0.1% TFA in 95%
MeCN/water. Gradient A After 1min elution with 100% buffer A,: linear gradient of 0-100%
buffer B for 30 min.
Synthesis of 1-(3-azido-10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)ethanone 8. To a
stirred solution of 1-(3-amino-10,11-dihydro-5H-dibenzo [b,f]azepin-5-yl)ethanone 7 (Wako
Chemicals, 100 mg, 0.396 mmol) in 10% aqueous hydrochloric acid (2ml), a solution of sodium
nitrite (27.3 mg, 0.396 mmol) in water was added at 0–5°C with vigorous stirring. The mixture
was kept below 5°C for 30 min, and then a solution of sodium azide (28.3 mg, 0.436 mmol) in
water (5ml) was added dropwise while the reaction was kept at the same temperature. After
being stirred for 1h, the mixture was warmed to room temperature and extracted with EtOAc
and water. The organic layer was washed with brine, dried with MgSO4 and concentrated
under reduced pressure to give 110mg (0.396 mmol, 100%) of compound 8 as yellow oily liq-
uid. TLC (Hexane: EtOAc 11:9): Rf = 0.58. 1H NMR (400 MHz, CDCl3) δ = 2.03 (s, 3H, CH3),
2.77–2.86 (m, 2H, CH2CH2), 3.28–3.48 (m, 2H, CH2CH2), 6.85 (d, 1H, J = 8.4 Hz), 6.96 (s,
1H), 7.07 (s, 1H), 7.13 (d, 1H, J = 8.4 Hz), 7.29 (m, 4H). 13C NMR (100.5 MHz, CDCl3) δ =
22.64, 30.01, 30.59, 118.42, 119.19, 127.46, 127.60. 128.75, 129.88, 131.87, 137.41, 138.13,
140.94, 142.24, 143.69, 170.56. MS (ESI): m/z = 279.13 [M + H] +, 301.13 [M+ Na] +. Mass Cal-
culated: 278.31.
Synthesis of 3-azido-10,11-dihydro-5H-dibenzo[b,f]azepine 9. To 110 mg of compound
8 (0.395 mmol) was added potassium hydroxide (72mg, 1.38 mmol) dissolved in 10ml of meth-
anol and the reaction mixture was refluxed for 6 h under an argon atmosphere. Afterwards,
methanol was evaporated and the mixture was extracted with CH2Cl2. The oily liquid was dis-
solved in minimum amount of EtOAc and then recrystallized from hexane in the cold to yield
needle shaped crystals of 9 (85 mg, 0.359 mmol, 85%). TLC (Hexane: EtOAc 9:1): Rf = 0.6. 1H
NMR (300 MHz, CDCl3) δ = 3.07 (s, 4H), 6.39 (d, 1H, J = 2.1 Hz), 6.49 (dd, 1H, J = 2.1, 6 Hz),
6.75 (d, 1H, J = 0.9 Hz), 6.84 (dt, 1H, J = 1.2, 7.5 Hz), 7.03 (d, 1H, J = 8.1Hz), 7.06–7.15 (m,
2H). 13C NMR (75 MHz, CDCl3) δ = 34.55, 34.81, 108.06, 109.83, 118.15, 120.12, 125.39,
126.95, 129.02, 130.64, 132.06, 138.55, 141.87, 143.53. MS (ESI): m/z = 237.20 [M + H] +, Mass
Calculated: 237.27 [M + H] +.
Synthesis of 3-dimethylamino-1-propylchloride 10. Sodium hydroxide and 3-dimethyla-
mino-1-propylchloride hydrochloride (TCI Europe) were dissolved separately in water (10
ml). These two solutions were mixed and the pH was adjusted to ~14. After extraction with
dichloromethane (3x30 ml), the extracts were dried over anhydrous sodium sulfate and the sol-
vent was removed to afford 50 mg (53%) of the free base. High vacuum was not used as the
amine obtained is volatile.
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Novel Probe to Identify Antidepressant Target Molecules
Synthesis of 3-(3-azido-10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N,N-dimethyl pro-
pan-1-amine 11 (azidopramine). A solution of compound 9 (50 mg, 0.212 mmol) was pre-
pared in dry toluene (sure seal, Fluka, 10ml) at 0°C under argon. To the solution was added a
suspension of NaH (6.09 mg, 0.254 mmol) in toluene (3ml) and the reaction was stirred for
30min. Freshly prepared solution of 3-dimethylamino-1-propylchloride 10 (33.5mg, 0.275
mmol) (generated from its hydrochloride salt) as described above was added dropwise and the
reaction was allowed to warm to room temperature. The reaction was heated to 60°C and stirred
overnight. TLC analysis showed complete disappearance of the starting educt 9. The reaction
mixture was poured into water and extracted with EtOAc. The organic layer was washed with
brine, dried over MgSO4 and concentrated by rotary evaporation. Column chromatography in
dichlorormethane: MeOH 95:05 of the crude reaction mixture was performed to give compound
11 (azidopramine, 45 mg, 0.14mmol, 66%). TLC (DCM: MeOH 9:1): Rf = 0.38. HPLC (gradient
A) Rt: 21.2min, Purity = 99%. 1H NMR (300 MHz, CDCl3) δ = 1.70–1.80 (m, 2H), 2.19 (s, 6H),
2.26–2.37 (m, 2H), 3.15 (s, 4H), 3.76 (t, 2H, J = 6.9 Hz) 6.61 (dd, 1H J = 2.4, 5.7 Hz), 6.73 (d, 1H,
J = 2.1 Hz), 6.984 (dt, 1H, J = 1.5, 5.4, 6.9 Hz), 7.06 (d, 1H, J = 8.1Hz), 7.11–7.20 (m, 3H). 13C
NMR (75 MHz, CDCl3) δ = 25.94, 31.65, 32.22, 45.42, 48.84, 57.49, 110.46, 112.45, 120.56,
123.22, 126.55, 129.43, 129.96, 131.33, 135.17, 137.87, 147.91, 149.20. MS (ESI): m/z = 322.27
[M + H] +. HRMS: 322.1861[M + H] +, Mass Calculated: 322.1853 [M + H] +.
Synthesis of 3-(tert-butoxycarbonyl(methyl)amino)propyl 4-methylbenzenesulfonate
13. To 3-(methylamino)propan-1-ol 12 (250 mg, 6.28 mmol) in acetonitrile was added BOC
anhydride (680 mg, 12.56 mmol) and a catalytic amount of DMAP. The reaction was stirred
for 2 hours until the disappearance of alcohol 12. The crude mixture was subjected to column
chromatography (Hexane: EtOAc 55:45) and dried under reduced pressure to obtain (460 mg,
2.43 mmol, 87%) of the desired product. TLC (Hexane: EtOAc 1:1): Rf = 0.46. 1H NMR (300
MHz, CDCl3) δ = 1.45 (s, 9H), 1.66–167 (m, 2H), 2.62 (s, 3H), 3.37 (s, 2H), 3.52 (s, 2H). 13
NMR (75 MHz, CDCl3) δ = 28.35, 29.63, 34.13, 44.21, 58.08, 79.96, 157.19.
C
To the above compound (90 mg, 0.475 mmol) in dichloromethane was added p-toluene sul-
fonylchloride (136 mg, 0.713 mmol) and triethylamine (96 mg, 0.951 mmol) and the mixture
was stirred at 0°C for 4 h. The reaction mixture was then quenched with water and extracted
using diethyl ether. The ethereal layer was washed with brine and dried over MgSO4 to yield the
crude product which was further subjected to column chromatography using Hexane: EtOAc
13: 7 to yield 135 mg (0.393 mmol, 84%) of 13 as white oily liquid. TLC (Hexane: EtOAc 1:1): Rf
= 0.46. 1H NMR (300 MHz, CDCl3) δ = 1.41 (s, 9H), 1.81–1.90 (m, 2H), 2.44 (s, 3H), 2.78 (s,
3H), 3.23 (t, 2H, J = 6.9 Hz), 4.02 (t, 2H, J = 6.3 Hz), 7.34 (d, 2H, J = 7.8 Hz), 7.77 (d, 2H, J = 8.4
Hz). 13C NMR (75 MHz, CDCl3) δ = 21.63, 27.53, 28.35, 34.59, 45.28, 68.2, 79.62, 125.94,
127.87, 129.10, 129.88, 132.86, 144.87, 155.60. MS (ESI): m/z = 244.13 [M—Boc] +, Mass Calcu-
lated: 244.08[M—Boc] +.
Synthesis of tert-butyl 3-(3-azido-10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)propyl
(methyl) caRbBAmate 14. To a solution of 3-azido-10,11-dihydro-5H-dibenzo[b,f]azepine 9
(130 mg, 0.550 mmol) in 5ml dry toluene was added 0.660ml of a 1M solution of sodium bis
(trimethylsilyl) amide in hexane (0.660 mmol) under an argon atmosphere at -78°C and the
mixture was stirred for 0.5h. Freshly prepared tosyl analog 13 (227 mg, 0.660 mmol) was
added dropwise to the above mixture and the reaction flask was allowed to warm to room tem-
perature. The reaction was further stirred at 70°C overnight and the completion of the reaction
was monitored using thin layer chromatography. The crude product was poured into water
and extracted with EtOAc. The organic layer was washed with brine and dried over MgSO4
and concentrated to dryness. Column chromatography of the crude reaction mixture was per-
formed in (Hexane: EtOAc 19:1) as eluent to give compound 14 (150 mg, 0.368 mmol, 67%).
TLC (Hexane: EtOAc 19:1): Rf = 0.23. 1H NMR (400 MHz, CDCl3) δ = 1.39 (s, 9H), 1.72–1.79
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Novel Probe to Identify Antidepressant Target Molecules
(m, 2H), 2.44 (s, 3H), 2.72 (s, 3H), 3.09–3.16 (m, 4H), 3.23 (t, 2H, J = 6.8 Hz), 3.69 (t, 2H,
J = 6.8 Hz), 6.59 (dd, 1H, J = 2.4, 5.6 Hz), 6.68 (d, 1H, J = 2 Hz), 6.96 (dt, 1H, J = 1.2, 7.2 Hz),
7.05 (t, 2H, J = 8 Hz), 7.10–7.16 (m, 2H). 13C NMR (100 MHz, CDCl3) δ = 26.26, 28.39, 31.58,
32.14, 34.23, 46.71, 48.09, 79.31, 110.27, 112.52, 120.36, 123.33, 126.54, 129.49, 129.91, 131.37,
135.17, 137.89, 147.67, 149.12, 155.69.
Synthesis of 3-(3-azido-10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)-N-methylpro-pan-
1-amine 15 (desazidopramine). Compound 14 (150mg, 0.368mmol) was deprotected in the
presence of 20% trifluoroacetic acid solution in DCM for 3.5h at room temperature to yield 15.
TFA was evaporated under reduced pressure and the crude mixture was subjected to a small
wash out silica gel column using hexane: EtOAc: TEA 3.8:6.0:0.2 to give pure desazidopramine
15 (85mg, 0.276mmol, 75%). TLC (Hexane: EtOAc: TEA 3.8:6.0:0.2): Rf = 0.29. HPLC (gradi-
ent A) Rt: 19.2 min, Purity = 98% 1H NMR (600 MHz, CDCl3) δ = 1.88–1.93 (m, 2H), 2.42 (s,
3H), 2.85 (t, 2H, J = 7.2), 3.07–3.12 (m, 4H), 3.75 (t, 2H, J = 6.6 Hz), 6.62 (dd, 1H, J = 2.4, 6
Hz), 6.64 (d, 1H, J = 1.8 Hz), 6.967(dt, 1H, J = 1.2, 6 Hz), 7.03 (q, 2H, J = 4.8, 7.2 Hz), 7.10–7.15
(m, 2H). 13C NMR (150 MHz, CDCl3) δ = 24.69, 31.48, 31.96, 33.26, 47.49, 47.62, 110.15,
112.95, 120.12, 123.71, 126.70, 129.68, 129.98, 131.50, 135.06, 138.08, 147.15, 148.68. MS (ESI)
m/z 308.12 [M + H] +. HRMS: 308.1685 [M + H] +, Mass Calculated: 308.1704 [M + H] +.
Synthesis of N-(3-(3-azido-10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)propyl)-N-
methylbut-3-yn-1-amine 17 (azidobupramine). To a solution of 15 (80 mg, 0.260 mmol) in
acetone (10ml) was added potassium carbonate (180mg, 1.30 mmol) and a catalytical amount
of potassium iodide. The mixture was stirred for 30 min and then further reacted with the
4-bromobut-1-yne 16 (41mg, 0.312 mmol) and refluxed at 60°C overnight. Acetone was evapo-
rated followed by an aqueous work up and extraction with CH2Cl2. The crude mixture was
subjected to column chromatography in DCM: MeOH mixture to give the desired product 17
(47 mg, 0.130 mmol, 50%). TLC (DCM: MeOH 9.2:0.8): Rf = 0.38. HPLC (gradient A) Rt: 19.8
min, Purity = 85% 1H NMR (300 MHz, CDCl3) δ = 1.69–1.79 (m, 2H), 1.94–1.96 (t, 1H, J = 2.4
Hz), 2.21 (s, 3H), 2.27–2.33 (m, 2H), 2.45 (t, 2H, J = 7.5 Hz), 2.56 (t, 2H, J = 7.2 Hz), 3.15 (s,
4H), 3.78 (t, 2H, J = 6.9 Hz), 6.62 (dd, 1H, J = 2.4 Hz), 6.74 (d, 1H, J = 2.4 Hz), 6.90–7.20 (m,
5H). 13C NMR (75 MHz, CDCl3) δ = 15.73, 24.45, 30.55, 31.33, 40.86, 47.62, 53.77, 54.90,
67.86, 81.68, 109.36, 111.29, 119.45, 122.09, 125.41, 128.30, 128.81, 130.21, 134.06, 136.75,
146.80, 148.10. MS (ESI): m/z = 360.16 [M + H] +. HRMS: 360.2057 [M + H] +, Mass Calcu-
lated: 360.2061[M + H] +.
Biological evaluation
Materials/Chemicals. The following materials were purchased: Dulbecco’s Modified
Eagle’s Medium (DMEM), FreeStyleTM 293 Expression medium, Fetal Bovine Serum (FBS),
antibiotic-antimycotic, streptomycine, sodium pyruvate, and tetracycline (Life Technologies,
CA, USA); blasticidin and zeocin (InvivoGen, CA, USA); citalopram, lofepramine, and paroxe-
tine (Kemprotec Limited, Middlesbrough, UK); desipramine, imipramine, polyethyleneimine,
ANTI-FLAG1 M2 Affinity Gel, SIGMAFASTTM Protease Inhibitor Cocktail EDTA-free (Sig-
maAldrich, MO, USA); [3H]-citalopram (84 Ci/mmol), clear 96-well Flexible PET Microplate
(PerkinElmer LAS, Boston, MA, USA); Rotiszint1 eco plus scintillant (Carl Roth, Karlsruhe,
Germany).
Cell culture. T-REx-SERT cells expressing rSERT-His10-FLAG were kindly provided by
C.G. Tate (MRC Laboratory of Molecular Biology, Cambridge, UK) [27]; cells were cultivated in
suspension (FreeStyle™ 293 Expression medium, 10% FBS, antibiotic-antimycotic, and pyru-
vate); for selection purpose, 5 μg/ml blasticidin and 200 μg/ml zeocin were used; expression
was induced by adding 1 μg/ml tetracycline for 5 days.
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Novel Probe to Identify Antidepressant Target Molecules
Membrane preparation. Tetracycline induced T-REx-SERT cells were mechanically dis-
rupted with a hypotonic Tris-buffer (50 mM Tris, pH 7.9, proteinase inhibitor) in a two step
procedure: dounce-homogenisation (Potter S, B.Braun, Melsungen, Germany) followed by an
ultrasound treatment (sonifier W-250, Branson, CT, USA). Homogenates were centrifuged first
at 800 rcf (10’, 4°C), followed by a second centrifugation step at 100,000 rcf (60’, 4°C). The
resulting pellet was re-suspended in reaction-buffer RB1 (50 mM Tris-HCl, 150 mM NaCl, 5
mM KCl, proteinase inhibitor, pH 7.9), and stored at -80°C after protein determination
(Pierce1 BCA Protein Assay, Thermo Scientific, IL, USA).
Analysis of substance 17 for its binding to SERT, NET, and DAT: Mass-Spectrometry
Based Assay (MSBA). Competitive MSBAs to characterize the affinity of test compounds
employing (1R,3S)-indatraline as a marker for hDAT, hNET and hSERT, respectively, were
performed exactly as described by Grimm et al. [28]. For this purpose, membrane fractions
obtained from HEK293 cell lines stably expressing the corresponding target protein were incu-
bated in presence of a fixed concentration of (1R,3S)-indatraline together with varying concen-
trations of test compounds in 96 well plates. After separation of the non-bound marker by
vacuum filtration from the binding samples, bound (1R,3S)-indatraline remaining on the filter
was eluted with acetonitrile (containing internal standard) and quantified by LC-ESI-MS/MS.
Subsequently, inhibition of (1R,3S)-indatraline binding caused by test compounds could be
analyzed essentially in the same way as described for radioligand binding assays.
Analysis of substances 17, 15, and 11 for their binding to rSERT: Radioligand Binding
Assays (RBA). Protocols for radioligand binding assays (RBA) were adapted from Basile (sat-
uration experiments) and Nakaki (competition experiments) [29,30]. Briefly, reactions were
carried out in reaction-buffer RB1 containing 1 μg protein of the membrane preparation over-
expressing rSERT, using various concentrations of [3H]-citalopram and, depending on the
assay, different competitors. Drugs were diluted in DMSO as solvent. Incubation was carried
out at 34°C for 60’, and terminated by rapid filtration over polyethyleneimine preincubated
GF/C filters using a Brandell MWXR-97TI cell harvester (Gaithersburg, MD, USA). Filters
were washed three times with 2 ml of ice cold reaction-buffer RB1 and then incubated with
scintillation liquid; radioactivity was measured by beta counting (MicroBeta, PerkinElmer LAS,
Boston, MA, USA). Maximal binding sites (Bmax) and equilibrium affinity constants (Kd) were
determined using [3H]-citalopram as radioligand in concentrations ranging from 0.09 nM to
60 nM; nonspecific binding was determined in the presence of 50 μM imipramine. For compe-
tition experiments, the concentration of [3H]-citalopram was set to 0.5 nM; each competitor
was titrated in ten dilution steps ranging from 1 fM to 30 μM; maximal radioligand binding
was determined in the absence of any competitor, and nonspecific binding in the presence of
50 μM imipramine.
Analysis of 17 for photoaffinity labeling (PAL), and Copper(I)-catalyzed azide alkyne
cycloaddition (CuAAC): fluorescence based assays. In addition to the determination of
binding affinities of azidobupramine (17) to hSERT, hNET and hDAT, also the functionality
of the added chemical groups of 17, namely photoaffinity labeling (PAL) and copper(I)-cata-
lyzed azide alkyne cycloaddition (CuAAC), were evaluated in different biological systems. For
all these experiments, rSERT was chosen as model target. The PAL-reaction was conducted
either using rSERT-enriched protein material or living cells. In both cases, the subsequent
CuAAC-reaction took place after the double-tagged rSERT-His10-FLAG was immobilized on
ANTI-FLAG1 M2 Affinity-gel (SigmaAldrich, MO, USA).
For the interaction analysis of azidobupramine (17) with rSERT-enriched material, 1 mg of
membrane preparation was solubilized in reaction-buffer RB3 (52.6 mM Tris/HCl, 126.4 mM
NaCl, 5.26 mM KCl, 1% Triton X-100, proteinase inhibitor, pH 7.9). Then rSERT was immobi-
lized on ANTI-FLAG1 M2 Affinity-gel (over night, 4°C, constant agitation), and unbound
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Novel Probe to Identify Antidepressant Target Molecules
protein was removed by three washing steps with reaction-buffer RB4 (25 mM HEPES/NaOH,
126.4 mM NaCl, 5.26 mM KCl, proteinase inhibitor, pH 7.9). Depending on the condition to
be tested, 17 (1 μM) alone or in the presence of 1000 fold molar excess (1 mM) of two different
competitors (i.e. paroxetine, mirtazapine) were added to the reaction (90’, RT, constant agita-
tion). This was followed by UV-light exposure (312 nm, 2x 45”, RT) using the Dual Transillu-
minator from Stratagene (5x8 Watt, La Jolla, CA, USA), and copper mediated click reaction
(60 μM Rhodamine-azide, 2.5 mM ascorbic acid, 250 μM CuSO4, and 500 μM bis[(tertbutyl-
triazoyl)methyl]-[(2-carboxymethyltriazoyl)methyl]amine (BTTAA), 60’, RT, constant agita-
tion) [31]. Finally, rSERT was eluted from affinity gel with Laemmli buffer (65°C, 15’), and
separated by SDS-PAGE (12%).
For the interaction analysis in living cells, 42 million suspended T-REx-SERT cells in 2ml
cultivation medium, overexpressing rSERT, were incubated with 17 (10 μM) alone or in the
presence of equimolar concentrations of paroxetine at constant agitation for 30’ (37°C, 5%
CO2). This was followed by UV-light induced covalent linkage (312 nm, 2x 60”, RT). The fur-
ther processing was as described above, i.e. membrane preparation, solubilization and immobi-
lization of rSERT to ANTI-FLAG1 M2 Affinity-gel, CuAAC-reaction followed by protein
elution, and SDS-PAGE.
Data analysis. Bmax and Kd values were determined by means of non-linear regression
analysis using SigmaPlot 11 (Systat Software Inc., IL, USA); applied algorithm: one side satura-
tion. pIC50 values were determined by means of non-linear regression analysis using SigmaPlot
11; applied algorithm: sigmoidal dose response; bottom (nonspecific binding) and top (no
competition) of the curves were set to 0 and 1, corresponding to 0% and 100% respectively.
Within the fluorescence based interaction studies of 17 with rSERT, the fluorescence signal
was recorded with the ChemiDoc MP detection-system and analyzed using ImageLab (BioRad,
CA, USA); for quantification, only those fluorescent signals were taken that correspond to
Western blot verified rSERT (between 70-100 kDa). Recorded fluorescent signals were normal-
ized to the Western-blot signal of rSERT. Statistical evaluation was performed using either the
Student t-Test (normal distributed sample, two independent groups), the Wilcoxon-Mann-
Whitney-Test (non-normal distributed sample, two independent groups), or ANOVA (normal
distributed sample; more than two groups); for post-hoc analyses, most stringent tests were
used (indicated in each analysis). The levels of significance were Ãp < 0.05, ÃÃp < 0.01, and
ÃÃÃp < 0.001, respectively.
Results and Discussion
Design and synthesis of azidobupramine (17)
For the synthesis of azidobupramine (17), we began with the prototypic tricyclic antidepressant
imipramine (1) as a chemical starting point. Previous structure activity relationship studies
(SARS) on tricyclic antidepressants and recent cocrystal structures of antidepressant-trans-
porter complexes indicated that substituents at position three of the tricyclic ring system and at
the terminal amine of imipramine could be tolerated by classical monoamine transporters [32–
43]. Furthermore, clinically effective analogues like desipramine (2), clomipramine (3), cyano-
pramine (4) and lofepramine (5) suggested that substituents at the ring structure and at the ter-
minal amino group do not compromise antidepressant activity on the clinical level (Fig 1A).
We thus set out to introduce an azido group at position 3 of imipramine (1) to graft the known
photoreactivity of aromatic azides into the tricyclic ring system. The alkyne tag for CuAAC
was designed to be introduced at the terminal amino function.
For the chemical synthesis of the multifunctional tricyclic antidepressant analogues azido-
pra-mine (11), azidodesipramine (15), and azidobupramine (17), a common building block 9
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Novel Probe to Identify Antidepressant Target Molecules
Fig 1. Synthesis scheme of multifunctional antidepressants and reference compounds. (A) Reference substances: overview of clinically active
antidepressants (1, 2, 3, 4, 5); (B) Synthesis scheme of azidobupramine: a) NaN3, H2O, RT, 1h. b) NaNO2, 10% HCl c) KOH, MeOH, 60°C, 15h. d) NaH,
ClCH2CH2CH2N(CH3)2 (10), 0–60°C, toluene. e) (Boc)2O, DMAP, ACN, RT, 2h. f) TsCl, Et3N, DCM, 0°C. g) NaHMDS, toluene, -78°C to 70°C, 6h. h) 20%
TFA, DCM, RT, 4h. i) 1-bromo-3-butyne, K2CO3, KI, 60°C, 12h, acetone.
doi:10.1371/journal.pone.0148608.g001
was synthesized from the commercially available azepine analogue 1-(3-amino-10,11-dihydro-
5H-dibenzo[b,f]azepin-5-yl)ethanone (7). The primary amine was first converted to the corre-
sponding azide 8 followed by base-assisted deprotection of the amino group and subsequent
alkylation with 10 to yield the azidopramine (11) (Fig 1B). Azidobupramine (17) was synthe-
sized via the intermediate product azidodesipramine (15). The coupling of the necessary Boc-
protected building block 13 to azepine 9 turned out to be more demanding than that of the cor-
responding (3-chloropropyl) dimethylamine 10, which was used for the synthesis of 11.
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Novel Probe to Identify Antidepressant Target Molecules
Stronger activation with a tosyl group and optimization of the reaction conditions eventually
yielded compound 15; a further alkylation step finally yielded the desired fully equipped azido-
bupramine (17) (Fig 1B).
Analysis of binding properties to monoamine transporters (SERT, NET,
and DAT)
After the synthesis, it was necessary to check whether the two functional groups incorporated
into azidobupramine (17) may have changed known antidepressant-like pharmacological
properties inherent to the parent molecule. Several biochemical, cell- and animal-based test
systems have been developed to evaluate small molecules for their antidepressant-like pharma-
cological properties. While none of these tests alone or in combination can replace clinical tri-
als as ultimate test, analyzing binding properties of small molecules to monoamine
transporters is a frequently and successfully used approach to predict the likelihood of the
potential of these molecules to act as antidepressants.
To define binding parameters of azidobupramine (17) to the monoamine transporters two
complementary techniques were applied, the MS based Binding Assay (MSBA) and the classi-
cal Radioligand Binding Assay (RBA).
MSBA revealed that azidobupramine (17) is characterized by moderate to high affinities to
hSERT, hNET, and hDAT leading to a Ki-value-based ratio of 1:1:16 (Table 1). RBA with
rSERT as target structure indicated that binding affinities of azidobupramine (17) and two
related compounds (i.e. 15, 11) follow a well-known structure dependent pattern. This includes
the relation to five clinically active substances (Table 2; Fig 2A and 2B). While the substitution
of position 3 at the cyclic head-structure with an azido group consistently leads to higher affini-
ties (1 Ki = 17.5 nM, 11 Ki = 8.82 nM, 2 Ki = 217 nM and 15 Ki = 62.4 nM), variations at the ter-
minal amino group result in divergent effects. Secondary amino functions at the terminal
amino group go along with lower affinities (15: Ki = 62.4 nM, 2: Ki = 217 nM) and tertiary
amino functions with higher affinities (11: Ki = 8.82 nM, 1: Ki = 17.5 nM). Furthermore, it
appears evident that Ki values do not only depend on the type of the terminal amino group
(secondary versus tertiary) but also on the size and structure of the attached group—the more
sterically demanding the chemical group, the larger the influence on the binding parameters
[32,37]. This influence of the substitution at the terminal amino group on binding affinities to
rSERT can be illustrated by comparing the Ki-values of imipramine (1) (Ki = 17.5 nM) and
lofepramine (5) (Ki = 1.12Ã103 nM) and two newly synthetized ring substituted derivatives 11
(Ki = 8.82 nM) and 17 (Ki = 22.2 nM).
The binding affinities of azidobupramine (17) to SERT determined by RBA and MSBA dif-
fer to some degree. While the radioactivity-based assay revealed a Ki-value of 22.2 nM, there
was a 4.6-fold drop in affinity using the mass-spectrometry-based assay (103 nM). This differ-
ence might be due to the origin of the protein source (rat for RBA, human for MSBA), varia-
tions in the harvesting procedure and buffer conditions, and different reporter ligands used
(citalopram for RBA, indatraline for MSBA). Although the reason for these differences remains
to be investigated, we would like to point out that the magnitude of the difference between
these two methods is not unusual [28]. In addition, the Ki-values determined here with each
method are well in the range of data reported in the literature [45,46].
Analysis of 17 for photoaffinity labelling (PAL), and copper(I)-catalyzed
azide alkyne cycloaddition (CuAAC)
Having proven that azidobupramine (17) is characterized by high affinity to the monoamine
transporters, we next tested the functionality of the two additional groups of azidobupramine
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Novel Probe to Identify Antidepressant Target Molecules
Table 1. Equilibrium affinity constants pKi SEM for MSBA and pIC50 SEM for RBA and corresponding binding affinities (Ki) of 17 to SERT, NET,
and DAT; affinity constants were measured using two different technical approaches based on mass spectrometry (MSBA), and based on classi-
cal radioligand binding assays (RBA); each data point represents the average value of three independent experiments, each performed in
triplicates.
Method
SERT
NET
DAT
pKi/pIC50
Ki [nM]
pKi/pIC50
Ki [nM]
pKi/pIC50
Ki [nM]
MSBA
RBA
6.99 0.03
7.52 0.03
103
6.59 0.06
----
115
----
5.80 0.07
----
1.64*103
22.2
----
doi:10.1371/journal.pone.0148608.t001
(17): UV-light induced photoaffinity labelling (PAL) by the aryl-azide group and copper(I)-
catalyzed azide alkyne cycloaddition (CuAAC) by the terminal alkyne group. Specifically, we
evaluated whether azidobupramine (17) can be covalently linked to rSERT by UV-light (PAL)
and subsequently be furnished with fluorochromes by CuAAC for visualization.
In a first step, it was necessary to demonstrate that azidobupramine (17) forms covalent
complexes with rSERT dependent on the expression status of rSERT, the PAL-reaction, and
the CuAAC-reaction. To this end, membrane preparations from rSERT overexpressing and
control cells were solubilized and loaded onto ANTI-FLAG1 M2 Affinity gel matrix. The
experiments were designed in such a way that visualization of rSERT by fluorescence is only
possible if three conditions are fulfilled: presence of rSERT, functionality of the incorporated
chemical moieties into (17) for PAL and CuAAC. In fact, the specific fluorescent signal became
apparent only when rSERT was expressed, azidobupramine (17) was added to the reaction,
and the material was exposed to UV-light (Fig 3, lane 5). We conclude that all functional fea-
tures of azidobupramine (17) are operative and that 17 interacts predominantly with rSERT
after the transporter was enriched onto ANTI-FLAG1 M2 Affinity gel.
To further elaborate the interaction of azidobupramine (17) and rSERT, competition exper-
iments were designed in the presence of either paroxetine, which is characterized by high bind-
ing affinity to SERT (Fig 4 lane 3) or mirtazapine, which shows no relevant binding to the
transporter (Fig 4 lane 2). As in the experiments before, the binding reactions were performed
after rSERT was immobilized on ANTI-FLAG1 M2 Affinity gel. It was apparent, that only par-
oxetine significantly displaced azidobupramine (17) from rSERT while mirtazapine does not.
Table 2. Equilibrium affinity constants (pIC50 SEM) and corresponding binding affinities (Ki) of 17, 15, 11, and the antidepressants paroxetine,
imipramine (1), desipramine (2), clomipramine (3) and lofepramine (5) at rSERT; the conversion of IC50 to Ki values was carried out according to
Cheng-Prusoff while using the actual [3H]-citalopram concentration for each calculation [44]; each data point represents the average value of at
least eight independent experiments.
Compound
Acronym
pIC50 SEM
Ki [nM]
Non-Tricyclic Antidepressants
Paroxetine
PAR
10.02 0.07
0.07
Tricyclic Antidepressants (unmodified)
Clomipramine
CMI
IMI
8.88 0.07
7.63 0.04
6.53 0.06
5.81 0.08
0.97
Imipramine
17.5
Desipramine
DMI
LOF
217
1.12*103
Lofepramine
Analogues of Tricyclic Antidepressants
Azidopramine
11
17
15
7.92 0.04
7.52 0.03
7.07 0.03
8.82
22.2
62.4
Azidobupramine
Desazidobupramine
doi:10.1371/journal.pone.0148608.t002
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Novel Probe to Identify Antidepressant Target Molecules
Fig 2. Binding parameters of 17 and reference compounds for rSERT. (A) Saturation analysis using 1 μg
membrane homogenate per reaction from recombinant rSERT expressing cells with [3H]-citalopram as radio-
ligand; the maximal number of binding sites (Bmax SEM) and equilibrium dissociation constants for the
radioligand (Kd SEM) were calculated by means of non-linear regression analysis (Bmax 77.2 1.8 pmol/mg;
Kd 1.8 0.2 nM) and verified after Scatchard transformation (inset: Bmax 80.3 1.5 pmol/mg; Kd 2.1 0.4 nM);
each data point represents the average value ( SEM) of 12 independent experiments. (B) 0.5 nM [3H]-
citalopram was used for competition experiments; dilutions were prepared from 17, 15, 11, and paroxetine,
clomipramine, imipramine, desipramine and lofepramine; each data point represents the average value of at
least eight independent experiments.
doi:10.1371/journal.pone.0148608.g002
Reasons for the incomplete displacement of azidobupramine (17) from rSERT (i.e. 60%) could
be attributed to the hypothesized additional low affinity binding sites of tricyclic antidepres-
sants at the monoamine transporters [5,47] that could hardly be monitored by conventional
binding assays such as RBA or MSBA. Alternatively, we cannot exclude that detergents alter
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Novel Probe to Identify Antidepressant Target Molecules
kDa
1
2
3
4
5
6
130
100
70
55
35
SERT(WB)
TETi (1µg/ml)
17 (1µM)
UV (312nm)
CuAAC
Fig 3. Analysis of 17 for PAL and CuAAC. Membrane fractions were prepared from tetracycline induced
(TETi) and non-induced T-REx-SERT cells; differential expression of rSERT was verified by Western blot
analysis (lower panel); while the CuAAC-mediated rhodamine-labelling reaction was carried out in all
conditions, 17-rSERT complexes are only visualized if rSERT is expressed, 17 is present and the reaction
was exposed to UV-light (upper panel, lane5); the blot displayed is a representative example of five
independent experiments. The full, uncropped original images of the fluorescence and Western blot analyses
are provided in S1 and S2 Figs.
doi:10.1371/journal.pone.0148608.g003
membrane proteins in their secondary and tertiary structure leading to the presentation of
unspecific binding sites [48]. Binding of azidobupramine (17) to such sites would be less likely
outcompeted by paroxetine. Based on this, we hypothesize that the 60% decrease of signal
intensity corresponds to the canonical and intact binding sites of antidepressants to the trans-
porter molecules, while the remaining 40% represents the non-specific interaction of azidobu-
pramine (17) with rSERT, be it the proposed additional binding sites for tricyclic
antidepressants or hydrophobic surfaces exposed by partial denaturation.
To control for possibly detrimental detergent mediated effects on protein structures, the
interaction of azidobupramine (17) with rSERT was also examined in living cells (Fig 5). Azi-
dobupramine (17) was allowed to form complexes with rSERT under cell culture conditions
(including PAL), while the indicator CuAAC reaction took place after rSERT was enriched on
ANTI-FLAG1 M2 Affinity gel. We could demonstrate that azidobupramine (17) also forms
covalent bonds with rSERT after UV-light exposure in living cells. In competition experiments
using an equimolar amount of paroxetine (10μM), a decrease of fluorescent signal of 60–70%
was observed (Fig 5, compare lanes 1 and 2 in panel B, quantification in panel A). Considering
the higher affinity of the competitor paroxetine in comparison to azidobupramine (17)
(Table 2), one might expect higher displacement of 17 by paroxetine. In the case of living cells,
detergent cannot serve as explanation. It is not unusual that overexpression systems produce
partly misfolded protein; thus, part of rSERT may feature non-specific binding sites. In addi-
tion, the hypothesized low affinity binding sites may contribute to some degree to the competi-
tion resistant signal.
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Novel Probe to Identify Antidepressant Target Molecules
Fig 4. Competition experiments of azidobupramine (17) binding at enriched rSERT by paroxetine
(PAR) and mirtazapine (MRT). Membrane preparations from rSERT expressing cells were incubated with
17 in the absence or presence of PAR or MRT. (A) After PAL and CuAAC-mediated fluorescent labelling,
proteins were separated by SDS-PAGE and fluorescence was monitored; expression of rSERT was verified
by Western blot analysis (lower panel). (B) PAR leads to a 60% decrease in binding signal (***p < 0.001)
while MRT has no effect; for data analysis one way ANOVA followed by Bonferroni adjustment was applied;
each box-plot represents the average of 12 independent experiments, each performed in triplicates.
doi:10.1371/journal.pone.0148608.g004
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Novel Probe to Identify Antidepressant Target Molecules
Fig 5. Analysis of azidobupramine (17) binding to rSERT in living cells. Living rSERT expressing cells
were exposed to 17, in the presence or absence of PAR followed by PAL-reaction, still in living cells;
immediately afterwards we performed cell disruption, membrane purification, affinity enrichment and CuAAC-
mediated rhodamin labelling. (A) Relative change of 17-specific binding to rSERT depending on the
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Novel Probe to Identify Antidepressant Target Molecules
presence of the competitor PAR; Wilcoxon-Mann-Whitney test was applied, ***p <0.001; each box-plot
represents the average of 20 independent experiments, each performed in triplicates. (B) Representative
fluorescence scans (lanes 1–3) and corresponding Coomassie-stained gels (lanes 1’-3’); lanes 3 and 3’ show
control eluates from affinity gels that were not loaded with protein material.
doi:10.1371/journal.pone.0148608.g005
Taken together, these data indicate that all functional features of azidobupramine (17) are
operative, namely antidepressant-like binding to canonical substrates, UV-induced cross-lining
to a known target and furnishing this drug-target complex with a fluorophore as an example
for a reporter molecule. Furthermore, it was possible to demonstrate that azidobupramine (17)
forms covalent complexes with rSERT not only in rSERT-enriched material but also in living
cells stably overexpressing rSERT. Finally, azidobupramine (17) could be competed out of
rSERT using paroxetine as competitor.
Conclusions
To our knowledge, azidobupramine (17) is the first non-radioactive structural analogue of tri-
cyclic antidepressants that can be covalently linked to target structures and furnished with
reporter molecules while preserving certain antidepressant-like properties. These characteris-
tics are necessary preconditions to conduct extended target identification approaches employ-
ing the latest technologies for protein identification. Accordingly, we propose that future
studies using azidobupramine (17) as model substance have the potential to contribute sub-
stantially to a better understanding of the diversity of direct interaction partners of antidepres-
sants. Nevertheless, considerable tasks are ahead, such as the adaptation of the procedures to
conditions of endogenous levels of unknown interaction partners. In addition, target identifica-
tion strategies may benefit from using CuACC to add tags suitable for purification of drug-tar-
get complexes, instead of visualization by fluorophores.
Supporting Information
S1 Fig. Uncropped image of the fluorescence analysis of SERT.
(PDF)
S2 Fig. Uncropped image of the Western analysis of SERT.
(PDF)
Acknowledgments
Special thanks are due to Ms. E. Weyher and Dr. S. Uebel (MPI of Biochemistry) and Ms. C.
Dubler (Ludwig-Maximilians-Universität Munich) for HRMS and NMR measurements.
Author Contributions
Conceived and designed the experiments: TK RG VG GH KTW F. Holsboer TR F. Hausch.
Performed the experiments: TK RG VG AMW SC GH. Analyzed the data: TK RG VG AMW
SAS US TR F. Hausch. Contributed reagents/materials/analysis tools: RG GCR SAS F. Hausch.
Wrote the paper: TK RG VG US F. Holsboer TR F. Hausch.
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