A first synthesis of 18F-radiolabeled lapatinib: A potential tracer for positron emission tomographic imaging of ErbB1/ErbB2 tyrosine kinase activity

Article abstract of DOI:10.1002/jlcr.1898

Fluorine-18-labeled lapatinib has been successfully synthesized for the first time by the reaction of a dimethylformamide solution of meta-[ ¹⁸F]fluorobenzylbromide with a Boc-protected lapatinib precursor fragment. The reaction proceeded in th

Full text of DOI:10.1002/jlcr.1898

Note
Received 27 January 2011,
Revised 25 March 2011,
Accepted 7 April 2011
Published online 13 July 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1898
18
A first synthesis of F‐radiolabeled lapatinib:
a potential tracer for positron emission
tomographic imaging of ErbB1/ErbB2
tyrosine kinase activity
Falguni Basuli, Haitao Wu, Changhui Li, Zhen‐Dan Shi, Agnieszka Sulima,
*
and Gary L. Griffiths
Fluorine‐18‐labeled lapatinib has been successfully synthesized for the first time by the reaction of a dimethylformamide
solution of meta‐[18F]fluorobenzylbromide with a Boc‐protected lapatinib precursor fragment. The reaction proceeded in the
presence of K2CO3 at 110 °C for 10min in a microwave and was followed by Boc‐group deprotection with trifluoroacetic acid.
The overall radiochemical yield of the reaction starting from the radiofluorination of the iodonium salt was 8–12% (uncorrected,
n = 6) in a 140‐min synthesis time.
Supplementary information may be found in the online version of this article.
Keywords: Radiolabeling; Fluorine‐18; meta‐[18F]benzylbromide; [18F]Lapatinib
fluorination of a suitably activated precursor by [18F]fluoride ion or
radiofluorination of a benzyl intermediate prior to final assembly of
Inhibition of epidermal growth factor receptor tyrosine kinases,
the lapatinib drug.
Introduction
which are often overexpressed in tumors, is an attractive modern
Direct nucleophilic fluorination of an aromatic ring is known to
be difficult without an electron withdrawing group (NO2, CN,
CHO, RCO, COOR) in an ortho or para position to the leaving group
approach to cancer therapeutics1–5 and is an area of active inves-
tigations. Many drugs to inhibit tyrosine kinase activity have been
developed or are under development.3,6–9 Molecular probes and
radiolabeled analogs of epidermal growth factor receptor tyrosine
kinase inhibitor drugs engender multiple interests including stud-
ies of basic mechanisms of action, preclinical targeting studies, and
further drug development for new indications in oncology. In
addition, radiolabeled analogs may be useful in clinical applica-
tions to verify target presence, establish drug localization indices,
and monitor the effects of treatment protocols.10–19
(NO2, NMe3+, Cl, Br, I). The presence of such a group is posited to
reduce the electron density at the target carbon and/or act by
resonance stabilization of the putative sigma intermediate
complex. Because of the absence of any such electron with-
drawing group in lapatinib, the total synthesis of an [18F]lapatinib
analog becomes much more challenging. In addition, there are
very few examples of a direct replacement of a leaving group
(Cl, Br, I, NMe3+, NO2) by [18F]fluoride ion in compounds with an
electron withdrawing group (CN, NO2, Br, CHO) present in the
nonactivating meta ring position.29–32 Thus, indirect fluorination
was our only viable option for the preparation of [18F]lapatinib.
In this indirect approach,33–35 a first 18F‐labeled prosthetic group
or synthon is synthesized and then coupled with the remaining
fragment of the molecule. Specifically, the synthesis is based on
the ether bond formation between 3‐[18F]fluorobenzylbromide
and a Boc‐protected lapatinib fragment, tert‐butyl (5‐(4‐(3‐
chloro‐4‐hydroxyphenylamino)quinazolin‐6‐yl)furan‐2‐yl)methyl
Lapatinib ditosylate (Tykerb ) is a dual ErbB1/ErbB2 tyrosine
®
kinase inhibitor from GlaxoSmithKline and is clinically approved for
use in combination chemotherapy for the treatment of advanced
metastatic breast cancer.20–24 It is also under investigation in other
cancer indications.25,26 Further clinical and preclinical studies of
this drug would be greatly facilitated by noninvasive imaging
techniques that allow the measurement of drug concentrations in
tumor tissue and ErbB1/ErbB2 tyrosine kinase inhibition.
Ideally, [18F]lapatinib can be synthesized by exchanging the 19
F
(Figure 1) with [18F]F. Although the product from this reaction is of
low specific activity, it could still prove useful, given the high doses
of lapatinib used in both preclinical27 and clinical studies.28 This
Imaging Probe Development Center, National Heart, Lung, and Blood Institute,
direct substitution approach is reported infrequently in the National Institutes of Health, Rockville, MD 20850, USA
radiochemistry literature,29–32 mainly because of the unreactive
*Correspondence to: Gary L. Griffiths, Imaging Probe Development Center,
National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville,
MD 20850, USA.
nature of [18F]F under most nucleophilic conditions. Attempts to
take a similar approach for an [18F]lapatinib analog resulted
in failure.31 Alternative routes could involve direct nucleophilic E‐mail: griffithsgl@mail.nih.gov
J. Label Compd. Radiopharm 2011, 54 633–636
Published 2011. This article is a US Government work and is in the public domain in the USA.

F. Basuli et al.
microwave. The formation of the product was monitored by HPLC
using a semi‐preparative column. The complete consumption
of meta‐[18F]fluorobenzylbromide was observed within 10min.
Deprotection of the Boc‐protected amine group was achieved with
TFA at 100 °C for 10 min. Radio‐HPLC analysis showed that the
coupling reaction proceeded cleanly with only two minor peaks
(Figure 2). The reaction mixture was partially neutralized with 6 N
NaOH, and the final product was purified by semi‐preparative
HPLC. The collected fraction was diluted with 10 mL of water and
passed through a Sep‐Pak light C18 cartridge (preconditioned
®
Figure 1. Lapatinib structure.
with 5 mL of ethanol, 10 mL of water, 10 mL of air). The trapped [18F]
lapatinib was eluted with 1 mL of ethanol. The compound was
(2‐(methylsulfonyl)ethyl)carbamate, which contains a phenolic formulated for further use by evaporating the ethanol at 70 °C
group (2), followed by deprotection of the N‐Boc secondary under nitrogen, and the [18F]lapatinib was dissolved in 2 mL of 10%
amino protecting group with trifluoroacetic acid (TFA) to give ethanol in 1× PBS. The solution was then passed through a 0.25‐µm
[18F]lapatinib.
sterile filter (Millipore). The final pH of the product solution was
between 6 and 7.
Results and discussion
The overall radiochemical yield starting from radiofluorination of
the iodonium salt was 8–12% (uncorrected, n = 6) in a 140‐min
synthesis time, with a radiochemical purity of >98% by analytical
HPLC (Figure 3a). The specific activity, determined by injection of
an aliquot of known activity onto an analytical HPLC and
comparison of the integrated sample UV signal area with a
calibrated µmol/UV area curve for the nonradioactive lapatinib, was
35–430 Ci/mmol (n = 6). The specific activity remained low because
of the low amount37 of starting [18F]fluoride ion (10–50 mCi) used
in this manual reduction‐to‐practice synthesis. The identity of the
[18F]lapatinib was further confirmed by comparing its HPLC
retention time with that of co‐injected authentic nonradioactive
lapatinib (Figure 3b), using different analytical HPLC conditions
(Supplementary Information).
>tert‐Butyl (5‐(4‐(4‐(benzyloxy)‐3‐chlorophenylamino)quinazo-
lin‐6‐yl)furan‐2‐yl)methyl(2‐(methylsulfonyl)ethyl)carbamate (com-
pound 1) was synthesized according to a modified literature
method (see Supplementary Information for detailed procedure
and references). Hydrogenolysis of the benzyl ether of 1 afforded
an N‐Boc‐protected lapatinib precursor (compound 2; Scheme 1)
with 76% yield. Formation of compound 2 was confirmed by both
1H and 13C NMR and high‐resolution mass spectrometry (MS).
Fluorine‐18 labeling of lapatinib involved two steps: first, the
preparation of meta‐[18F]fluorobenzylbromide; second, the
coupling of this prosthetic group with the lapatinib precursor
compound 2 (Scheme 2).
meta‐[18F]Fluorobenzylbromide was synthesized according to
the recent procedure36 reported by our group. meta‐[18F]
Fluorobenzylbromide was purified using a Sep‐Pak plus silica
Materials and methods
®
cartridge. The coupling reaction of meta‐[18F]fluorobenzylbromide 4‐Chloro‐6‐bromoquinazoline was purchased from Ark Pharm, Inc.
with a Boc‐protected lapatinib precursor fragment (2) was carried (Libertyville, IL), and used as received. All other commercially
out at 110°C in dimethylformamide in the presence of K2CO3 in a available organic precursors and dry solvents were obtained from
Scheme 1. Synthesis of tert‐butyl (5‐(4‐(3‐chloro‐4‐hydroxyphenylamino)quinazolin‐6‐yl)furan‐2‐yl)methyl(2‐(methylsulfonyl)ethyl)carbamate. rt, room temperature.
Br
18
18F
2
o
3
o
Scheme 2. Synthesis of [18F]lapatinib.
www.jlcr.org
Published 2011. This article is a US Government work
and is in the public domain in the USA.
J. Label Compd. Radiopharm 2011, 54 633–636

F. Basuli et al.
radio 254 nm
(a)
0.3
0.2
0.1
0.0
3.0
2.0
1.0
0
0
1
2
3
4
5
6
7
Time (min)
0
2
4
6
8
10
12
14
16
Time (min)
(b)
Figure 2. HPLC analysis of the crude reaction mixture of [18F]lapatinib. HPLC
conditions: Agilent Eclipse XDB C18 column, 5 µm, 9.4 × 250 mm; eluent, 40%
acetonitrile, 60% water with 0.1% TFA; flow rate, 4 mL/min. Solid line, in‐line
radiodetection; dotted line, UV detection at 254 nm. (This figure is available in color
online at http://www.interscience.wiley.com/journal/jlcr).
Sigma‐Aldrich (St. Louis, MO) and usedas receivedunlessotherwise
noted. Compound 1 was synthesized by an improved modification
of the literature‐described method (see Supplementary Informa-
tion for detailed procedure and references). meta‐[18F]Fluoroben-
zylbromide was prepared using our recently described method.36
Thin‐layer chromatography visualization was achieved using both
254‐nm or 360‐nm UV detection. Flash column chromatography
was performed on an Ana‐Logix IntelliFlash 280 system, using
0
2
4
6
8
10
Time (min)
Figure 3. HPLC analysis of (a) [18F]lapatinib after formulation and (b) [18F]
lapatinib co‐injected with the nonradioactive standard. HPLC conditions: Agilent
Eclipse XDB C18 column, 5 µm, 4.6 × 150 mm; 20–90% acetonitrile in water (0.1%
TFA) in 10 min; flow rate, 1 ml/min. Solid line, in‐line radiodetection; dotted line, UV
detection at 254 nm. (This figure is available in color online at http://www.
interscience.wiley.com/journal/jlcr).
Biotage SNAP Cartridges. Atmospheric pressure chemical ioniza-
®
tion MS was performed on a 6130 Quadrupole LC/MS Agilent
Technologies instrument equipped with a diode array detector. 1H
and 13C NMR spectra were recorded on a Varian spectrometer
(400 MHz). Chemical shifts (ppm) are reported relative to the solvent
residual peak of dimethyl sulfoxide (δ 1H, 2.50; 13C, 40.45 ppm). High‐
resolution MS was performed by the Scripps Research Institute
Center for Metabolomics and Mass Spectrometry (La Jolla, CA).
Fluorine‐18 was purchased from PETNET Solutions (North Wales, PA).
Radiosynthesis was performed manually. The product was purified
by HPLC on a Beckman Coulter System Gold instrument equipped
with a multiwavelength detector using an Agilent Eclipse C18
column (5 µm, 9.4 × 250 mm). Analytical HPLC analyses for radio-
chemical work were performed on an Agilent 1200 Series instrument
equipped with multiwavelength detectors using an Agilent Eclipse
XDB C18 column (4.6 × 150 mm, 5 µm) with a flow rate of 1.0 mL/min.
All the microwave reactions for fluorine‐18 labeling were done in a
Biotage Initiator 2.5 using Biotage 10‐mL V‐shaped vials at constant
yellow solid. LC‐MS (Method II, positive mode atmospheric
pressure chemical ionization): retention time = 4.51 min, m/z 573
[M + 1]; high‐resolution ESI‐TOF MS calculated for C27H30ClN4O6S,
1
573.1569, found 573.1557 [M + 1]. H NMR (400 MHz, DMSO‐d6):
δ 10.07 (s, 1H), 9.77 (s, 1H), 8.73 (d, J = 1.3, 1H), 8.53 (s, 1H), 8.10
(dd, J = 1.7, 8.8 Hz, 1H), 7.87 (d, J = 2.5 Hz, 1H), 7.79 (d, J = 8.7 Hz,
1H), 7.57 (dd, J = 2.5, 8.8 Hz, 1H), 7.06–7.02 (m, 2H), 6.52 (d,
J = 3.1 Hz, 1H), 4.55 (s, 2H), 3.70 (br, 2H), 3.40 (br, 2H, mixed
with water peak), 2.30 (s, 3H), 1.44 (s, 9H). 13C NMR (100 MHz,
DMSO‐d6): δ 157.6, 154.4, 154.2 (br), 152.0, 149.6, 148.8, 131.1,
128.4, 128.2, 127.9, 124.3, 122.8, 118.9, 116.7, 116.1, 115.2, 110.4
(br), 110.1 (br), 107.7, 79.84, 51.75, 44.01, 43.04, 40.59, 27.85.
Radiosynthesis of meta‐[18F]fluorobenzylbromide
Phenyl(3‐formylphenyl)iodonium bromide (1–2 mg), TEMPO
(0.5–1 mg), and dried Cs[18F]F (10–50 mCi) were heated at
110 °C in a microwave for 5 min.36 meta‐[18F]Fluorobenzaldehyde
was then converted to corresponding alcohol by treat-
ment with aqueous NaBH4 and finally converted to meta‐[18F]
fluorobenzylbromide by reaction with triphenylphosphine dibro-
mide. The meta‐[18F]Fluorobenzylbromide thus obtained was
temperature mode. All the Sep‐Pak cartridges used in this synthesis
®
were obtained from Waters (Billerica, MA).
tert‐Butyl (5‐(4‐(3‐chloro‐4‐hydroxyphenylamino)
quinazolin‐6‐yl)furan‐2‐yl)methyl(2‐(methylsulfonyl)ethyl)
carbamate (2)
loaded onto a silica plus Sep‐Pak cartridge and eluted with 2 mL
of dry dichloromethane.
®
A solution of tert‐butyl (5‐(4‐(4‐(benzyloxy)‐3‐chlorophenylamino)
quinazolin‐6‐yl)furan‐2‐yl)methyl(2‐(methylsulfonyl)ethyl)carbamate
(1; 1.00 g, 1.5mmol) in EtOH (1.4L) was hydrogenated for 2 days
at room temperature in the presence of 10% Pd/C (0.5g,
0.5mmol) and then filtered. The solvent was evaporated to
dryness, and the residue was purified by chromatography (30%
ethyl acetate in hexane) to afford the title compound tert‐butyl
(5‐(4‐(3‐chloro‐4‐hydroxyphenylamino)quinazolin‐6‐yl)furan‐2‐yl)
methyl(2‐(methylsulfonyl)ethyl)carbamate (2; 0.66 g, 76%) as a
Radiosynthesis of [18F]lapatinib
A dichloromethane solution of meta‐[18F]fluorobenzylbromide
was evaporated under a gentle flow of nitrogen at 60 °C to
obtain dry meta‐[18F]fluorobenzylbromide. To the residue was
added 25 mg of activated (dried overnight at 100 °C in a oven)
K2CO3 and 2–3 mg of 2 in dimethylformamide (0.5 mL). The
J. Label Compd. Radiopharm 2011, 54 633–636
Published 2011. This article is a US Government work
and is in the public domain in the USA.
www.jlcr.org

F. Basuli et al.
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solution was heated in a microwave oven at 110 °C for 10 min. To
the solution was added 1 mL of TFA, and the solution was
heated in a microwave oven at 100 °C for a further 10 min. The
resulting solution was partially neutralized with 6 N NaOH (2 mL)
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In conclusion, we have successfully developed the first reliable
and reproducible strategy for the synthesis of [18F]lapatinib
using meta‐[18F]fluorobenzylbromide as the fluorination synthon.
The radiosynthesis is suited for the preparation of the com-
pound in quantities and times practicable when considering its
use as a positron emission tomography radiopharmaceutical or
investigational radiochemical. Further development and work
toward an automated radiolabeling protocol are under way.
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Supplementary Information
Supplementary information may be found in the online version
of this article: full characterization of all compounds and HPLC
data of radiolabeling experiments.
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Published 2011. This article is a US Government work
and is in the public domain in the USA.
J. Label Compd. Radiopharm 2011, 54 633–636