Radiosynthesis of 18F-labeled N-desmethyl-loperamide analogues for prospective molecular imaging radiotracers

Article abstract of DOI:10.1016/j.tetlet.2012.12.121

A simple procedure for preparing fluoroethyl-N-desmethyl-loperamide 4 and its analogue 5 was developed. Standard compound 4 was synthesized in useful yields for radiolabeling analysis. [N-Ethyl-18F]N-desmethyl-loperamide, 3, was rapidly and efficiently labeled with no-carrier added fluorine-18 (t _1/2 = 109.7 min) by treatment of readily prepared [ ¹⁸F]1-bromo-2-fluoro ethane with a N-desmethyl-loperamide precursor at a consistent 7% radiochemical yield. This procedure was also adapted to the radiosynthesis of 3 by [¹⁸F]ethylene tosylate, but at a lower 3% radiochemical yield. Copyright

Full text of DOI:10.1016/j.tetlet.2012.12.121

Tetrahedron Letters 54 (2013) 1412–1415
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Radiosynthesis of 18F-labeled N-desmethyl-loperamide analogues
for prospective molecular imaging radiotracers
⇑
Xiaofeng Bao , Duliang Liu
Department of Biochemical Engineering, Nanjing University of Science & Technology, Chemical Engineering Building, B302, 200 Xiaolinwei, Nanjing 210094, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple procedure for preparing fluoroethyl-N-desmethyl-loperamide 4 and its analogue 5 was
Received 12 November 2012
Revised 13 December 2012
Accepted 28 December 2012
Available online 7 January 2013
developed. Standard compound
4 was synthesized in useful yields for radiolabeling analysis.
[N-Ethyl-18F]N-desmethyl-loperamide, 3, was rapidly and efficiently labeled with no-carrier added
fluorine-18 (t1/2 = 109.7 min) by treatment of readily prepared [18F]1-bromo-2-fluoro ethane with a
N-desmethyl-loperamide precursor at a consistent 7% radiochemical yield. This procedure was also
adapted to the radiosynthesis of 3 by [18F]ethylene tosylate, but at a lower 3% radiochemical yield.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
F-18
Radiosynthesis
Loperamide
PET
Permeability-glycoprotein
Introduction
OH
R1
N
Progress in the synthesis of new fluorinated compounds to act
as drugs and 18F-labeled analogues as potential imaging agents
has grown dramatically.1,2 Many drugs, such as antibiotics, seda-
tives, antidepressants, and anti-tumor agents, are fluorinated com-
pounds.3–5 Radiosynthesis methods to introduce fluorine-18
(t1/2 = 109.7 min) into organic molecules have become increasingly
important for the development of radiotracers for position emis-
sion tomography (PET),6–8 a sensitive and powerful technique that
is valuable for both clinical research9,10 and drug development.11,12
Permeability-glycoprotein (P-gp) functions as a drug efflux
pump at the blood–brain barrier and at other tissues, including
some tumors.13,14 Radiotracers for imaging P-gp function in vivo
could be valuable to assess the role of P-gp in neuropsychiatric dis-
orders and in multi-drug resistance during cancer chemotherapy.15
Cl
N
R2
O
Loperamide,1, R1 = Me, R2 =Me
[
[
11C]dLop, 2,
R1 = H, R2 11CH3
=
18F]FEt-dLop, 3,R1 = H, R2
=
18FCH2CH2
FEt-dLop,4
FPr-dLop,5
R1 = H, R2 = FCH2CH2
R1 = H, R2 = FCH2CH2CH2
Figure 1. The structure of loperamide and its analogs.
greater promise because of its more favorable metabolic profile.21
In this Letter, we aimed to synthesize new fluoro derivatives of this
metabolite, such as 4 and 5. We also reported the radiosynthesis of
18F-labeled analogue of N-desmethyl-loperamide 3. We considered
that an 18F-labeled analog of [11C]dLop, 3, might also behave as a
prospective radiotracer for imaging P-gp function and potentially
offer the advantage of greater availability for a wider range of
applications.
Loperamide 1 (Fig. 1) is a potent l-receptor agonist that acts on the
gastrointestinal tract;16 this molecule is a safe antidiarrheal drug
with no undesirable central nervous system effects because it is
excluded from the brain by the efflux transporter-glycoprotein
(P-gp).17 Loperamide has been shown to be an avid substrate
for P-gp,18 and its radiolabeled [11C]loperamide has been proven
to be a promising radiotracer to study the function of P-gp at
the blood–brain barrier.19 In addition, its primary metabolite,
[N-methyl-11C]N-desmethyl-loperamide 2, has also been evalu-
ated as a radiotracer for imaging P-gp function20 and showed a
Result and discussion
Synthesis of compounds 4 and 5
To establish the reaction conditions for the preparation of 3, we
first tried a simple method to prepare the standard compounds 4
and 5 (Scheme 1). The intermediate compound b was prepared
⇑
Corresponding author. Tel./fax: +86 025 83415945.
E-mail address: baoxiaofeng@mail.njust.edu.cn (X. Bao).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.121

X. Bao, D. Liu / Tetrahedron Letters 54 (2013) 1412–1415
Cl
1413
transferred to a mixture of the amide b (0.36 mmol) and NaH
(0.39 mmol) in DMF (5 mL). This mixture was then stirred for
12 h at 80 °C. Chromatography (silica gel; hexane/EtOAc, 1:3 v:v;
then EtOAc) of the crude mixture, followed by HPLC on a Luna
C18 column (250 Â 10 mm) eluted at 8 mL/min with 0.025% aq
NH4OH (A)-MeCN (B), with B increased from 30% to 100% over
30 min, gave 4 (tR = 16.8 min) at a 25% yield with 99% purity. Other
attempts to achieve the alkylation of amide b, either with
1-bromo-2-fluoroethane, fluoroethyl tosylate, or with 1-fluoro-2-
iodoethane, achieved lower yields. The synthesis of compound 5
was analogous to that of compound 4 through the activation of a
hydroxyl group on 3-fluoropropan-1-ol with triflic anhydride, fol-
lowed by coupling with the amide precursor b to obtain 5 at a
30% yield with 99% purity. The successful synthesis of 4 and 5 con-
firmed the susceptibility of amide alkylation on N-desmethyl-lop-
eramide. Although 4 was only synthesized at a 25% yield, the
amount was adequate to serve as a chromatographic reference
material.
HO
Cl
Cl
i
HO
Br
CN
N
HO
N
+
NC
ii H2N
O
N
H
a
b
Cl
HO
H
N
N
iv
iii
F
OTf
F
F
OH
n
n
n
O
5, n=2
6, n=3
7, n=2
8, n=3
4, n=2, 25%
5, n=3, 30%
Scheme 1. Synthesis of standard compounds 4 and 5. Reagents and conditions: (i)
DIPEA, CH3CN, 80 °C, 31 h, 60%; (ii) KOH, tBuOH, 3 d, reflux, 87%; (iii) (CF3SO2)2O,
Et3N CH2Cl2, rt, 1 h; (iv) b, NaH, DMF, 80 °C, 24 h.
from commercially available 4-(4-chlorophenyl)-4-hydroxyl piper-
idine and 4-bromo-2, 2-diphenylbutane nitrile as described previ-
ously.20 Compound 7 was prepared without purification by slowly
adding triflic anhydride (10 mmol) to a solution of 2-fluoroethanol
(10 mmol) and Et3N (10 mmol) in CH2Cl2 (2 mL). The reaction mix-
ture was stirred for 1 h at room temperature, concentrated, and
Synthesis of radiolabeling precursors
An aliphatic nucleophilic substitution reaction with [18F]fluo-
ride ions can be highly efficient if the leaving groups are sulfonates
(tosylate, mesylate, or triflate, etc.) or other halides (Cl, Br, or I) and
the reaction is performed in a polar aprotic solvent, such as DMF,
THF, DMSO, CH3CN, etc.22 The aliphatic bromide and tosylate pre-
cursors used for the radiolabeling of [18F]4 and [18F]5 were de-
signed and tried via a number of reaction conditions, as shown in
Table 1. The desired precursors 9, 10, and 11 were not successfully
obtained by a reaction of amide b with ethylene ditosylate, 1,
2-dibromoethane, 1-bromoethyl tosylate, or 1-bromopropyl tri-
flate under various reaction conditions (Table 1). The unexpected
cyclic byproducts 12 and 13 (Scheme 2) were isolated, and their
structures were determined using 1H NMR and HRMS. The failure
to prepare the desired precursors probably due to the affection of
the hydroxyl group on b. This group is also a strong nucleophile
under basic conditions and is able to activate product decomposi-
tion through cyclization (Scheme 2), as the byproducts 12 and 13
have been detected by MS (Fig. 2) at the mass of the proposed
cyclic. Because this approach to prepare the aliphatic bromide
and tosylate precursors for aliphatic nucleophilic substitution with
Table 1
Synthesis of R(CH2)n-N-desmethyl-loperamide from the amide b
Cl
Cl
HO
HO
H
N
O
N
N
H2N
R
n
O
b
9
R=OTs, n=2
10 R=Br, n=2
11 R=Br, n=3
Reagents
Reaction conditions
Product
Yield
TsO
NaH, DMF, MW, 110 °C, 30 min
NaH, DMF, MW, 110 °C, 15 min
NaH, DMF, 80 °C, 12 h
9
—
—
—
—
OTs
OTs
Br
Br
Br
10
10
10
[
18F]fluoride ions was not feasible, alternate strategies were
OTs
Br
adopted to achieve radiosynthesis through the use of other condi-
tions and labeling agents, as shown in Scheme 3.
NaH, DMF, 80 °C, 24 h
Br
Br
NaH, DMSO, 80 °C, 24 h
KOH, DMF, 80 °C, 24 h
10
10
—
—
Br
Radiosynthesis of [18F]FEt-dLop
Br
A cyclotron-produced [18F]fluoride ion solution (100À120 mCi)
was mixed with kryptofix 2.2.2 (5 mg) and K2CO3 (0.5 mg) in
MeCN–H2O (95:5 v:v; 0.1 mL) and then dried by two addition–
evaporation cycles of MeCN (2 mL). 2-Bromoethyl tosylate
Br
Br
Br
NaH, DMF, 80 °C, 12 h
K2CO3, tBuOH, 80 °C, 12 h
NaH, DMF, 80 °C, 24 h
11
11
11
—
—
—
Br
OTf
OTf
(30 lL) in t-butanol plus 1,2-dichlorobenzene (1 mL; 1:9 v:v) was
Base
H
R
Cl
O
n
N
HN
O
Base
NH
O
O
n
Cl
N
9
R=OTs, n=2
12 n=2
13 n=3
10 R=Br, n=2
11 R=Br, n=3
Scheme 2. Hypothesized decomposition of R(CH2)n-N-desmethyl-loperamide by cyclization.

1414
X. Bao, D. Liu / Tetrahedron Letters 54 (2013) 1412–1415
Figure 2. MS spectra of the cyclic compounds 12 and 13.
TsO(CH2)2Br
18F
OH
O
i
N
HN
Cl
18F(CH2)2Br
OH
ii
O
3
H2N
N
Cl
TsO(CH2)2OTs
iii
18F(CH2)2OTs
iv
18F
OH
O
b
N
HN
Cl
3
Scheme 3. Radiosynthesis of 3. Reagents and conditions: (i) [18F]fluoride ion, K2CO3, K2.2.2, t-butanol and 1,2-dichlorobenzene, 90 °C, 10 min; (ii) NaH, DMF, 110 °C, 10 min,
RCY ꢀ7%; (iii) [18F]fluoride ion, K2CO3, K 2.2.2, CH3CN, 110 °C, 10 min, 84%; (iv) NaH, DMF, 110 °C, 10 min, RCY ꢀ3%.
added and then heated at 90 °C for 10 min. (Scheme 3). [18F]
2-Fluoroethyl bromide (20À25 mCi) was distilled out, passed
through a silica Sep-Pak cartridge, and trapped in a sealed V-vial
containing amide b (2 mg) and NaH (0.5 mg) in DMF (250 lL).
The reaction mixture was heated to 110 °C for 10 min, cooled,
and diluted with MeCN–H2O (1:1 v:v). A sample was injected onto

X. Bao, D. Liu / Tetrahedron Letters 54 (2013) 1412–1415
1415
Figure 3. Chromatograms from the HPLC analysis of crude [18F]-FEt-dLop.
a Prodigy column (250 Â 4.6 mm) and eluted at 1 mL/min with
References and notes
mobile phase A–B (3:7 v:v). The identity of [18F]FEt-dLop 3
(tR = 11.5 min) was confirmed using LC–MS and the observation
of co-elution with reference compound 4 in radio-HPLC analysis.
The decay-corrected radiochemical yield (RCY) of 3 (tR = 11.5 min)
from the labeling agent was estimated from the radio-chromato-
gram (Fig. 3). The RCY of 3 was consistently observed to be
7% 2 (n = 6). Another approach to prepare 3 from ethylene ditosy-
late to form [18F]2-fluoroethyl tosylate, followed by a coupling
with the amide precursor b to afford 3 was also attempted using
a similar procedure (Scheme 3) with a 3% 1 RCY (n = 3).
1. Maisonial, A.; Kuhnast, B.; Papon, J.; Boisgard, R.; Bayle, M.; Vidal, A.; Auzeloux,
P.; Rbah, L.; Bonnet-Duquennoy, M.; Miot-Noirault, E.; Galmier, M. J.; Borel, M.;
Askienazy, S.; Dollé, F.; Tavitian, B.; Madelmont, J. C.; Moins, N.; Chezal, J. M. J.
Med. Chem. 2011, 54, 2745–2766.
2. Bourdier, T.; Shepherd, R.; Berghofer, P.; Jackson, T.; Fookes, C. J.; Denoyer, D.;
Dorow, D. S.; Greguric, I.; Gregoire, M. C.; Hicks, R. J.; Katsifis, A. J. Med. Chem.
2011, 54, 1860–1870.
3. Yildiz-Oren, I.; Yalcin, I.; Aki-Sener, E.; Ucarturk, N. Eur. J. Med. Chem. 2004, 39,
291–298.
4. Wegner, F.; Deuther-Conrad, W.; Scheunemann, M.; Brust, P.; Fischer, S.; Hiller,
A.; Diekers, M.; Strecker, K.; Wohlfarth, K.; Allgaier, C.; Steinbach, J.; Hoepping,
A. Eur. J. Pharmacol. 2008, 580, 1–11.
5. Brantley, E.; Antony, S.; Kohlhagen, G.; Meng, L.; Agama, K.; Stinson, S. F.;
Sausville, E. A.; Pommier, Y. Cancer Chemother. Pharmacol. 2006, 58, 62–72.
6. Milicevic Sephton, S.; Mu, L.; Schweizer, W. B.; Schibli, R.; Krämer, S. D.;
Ametamey, S. M. J. Med. Chem. 2012, 55, 7154–7162.
Conclusion
7. Bao, X.; Lu, S.; Liow, J. S.; Zoghbi, S. S.; Jenko, K. J.; Clark, D. T.; Gladding, R. L.;
Innis, R. B.; Pike, V. W. J. Med. Chem. 2012, 55, 2406–2415.
8. Pathuri, G.; Hedrick, A. F.; Awasthi, V.; Gali, H. Nucl. Med. Biol. 2012, 39, 1195–
1201.
9. Heiss, W. D.; Zimmermann-Meinzingen, S. J. Neurol. Sci. 2012, 322, 268–273.
10. Diehl, M.; Risse, J. H.; Brandt-Mainz, K.; Dietlein, M.; Bohuslavizki, K. H.;
Matheja, P.; Lange, H.; Bredow, J.; Körber, C.; Grünwald, F. Eur. J. Nucl. Med.
2001, 28, 1671–1676.
11. Cunningham, V. J.; Parker, C. A.; Rabiner, E. A.; Gee, A. D.; Gunn, R. N. Drug
Discovery Today: Technol. 2005, 2, 311–315.
In summary, we have developed a simple procedure for prepar-
ing 4 and 5 in useful yields as standard compounds for radiolabel-
ing analysis. We also developed a mild and operational procedure
for preparing [18F]FEt-dLop, 3, at a consistent 7% radiochemical
yield by the alkylation of the radiolabeling agent [18F]1-bromo-2-
fluoro ethane with the amide precursor b. This procedure was also
adapted to the radiosynthesis of 3 using [18F]ethylene tosylate, but
this approach resulted in a lower radiochemical yield (3% RCY). The
new fluoro compounds are expected to resist defluorination
in vivo. The new [18F]FEt-dLop may be proven to be a useful radi-
otracer for imaging P-gp function.
12. Seo, Y. Curr. Radiopharm. 2008, 1, 17–21.
13. Schinkel, A. H.; Wagenaar, E.; Mol, C. A.; van Deemter, L. J. Clin. Invest. 1996, 97,
2517–2524.
14. Szaka´cs, G.; Paterson, J. K.; Ludwig, J. A.; Booth-Genthe, C.; Gottesman, M. M.
Nat. Rev. Drug Disc. 2006, 5, 219–234.
15. Fromm, M. F. Trends Pharmacol. Sci. 2004, 25, 423–429.
16. Awouters, F.; Megens, A.; Verlinden, M.; Schuurkes, J.; Niemegeers, C.; Janssen,
P. A. J. Dig. Dis. Sci. 1993, 38, 977–995.
Acknowledgments
17. Gottesman, M. M.; Fojo, T.; Bates, S. E. Nat. Rev. Cancer 2002, 2, 48–58.
18. Zoghbi, S. S.; Liow, J. S.; Yasuno, F.; Hong, J.; Tuan, E.; Lazarova, N.; Gladding, R.
L.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2008, 49, 649–656.
19. Wilson, A. A.; Passchier, J.; Garcia, A.; Vasdev, N.; Stableford, W.; Lawrie, K.;
Fellows, I.; Gee, A. D. J. Labelled Compd. Radiopharm. 2005, 48, S142.
20. Lazarova, N.; Zoghbi, S. S.; Hong, J.; Seneca, N.; Tuan, E.; Gladding, R. L.; Liow, J.
S.; Taku, A.; Innis, R. B.; Pike, V. W. J. Med. Chem. 2008, 51, 6034–6043.
21. Seneca, N.; Zoghbi, S. S.; Liow, J. S.; Kreisl, W.; Herscovitch, P.; Jenko, K.;
Gladding, R. L.; Taku, A.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2009, 50, 807–813.
22. Cai, L.; Lu, S.; Pike, V. W. Eur. J. Org. Chem. 2008, 2008, 2853–2873.
This research was supported by the Jiangsu Science Foundation
BK2011704 and NJUST Research Funding (No. 2011ZDJH08).We
thank Dr. Jiang Wei for helpful suggestions.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
12.121.