Design and synthesis of fluorine-18 labeled matrix metalloproteinase inhibitors for cancer imaging

Article abstract of DOI:10.1002/jlcr.616

Matrix metalloproteinases (MMPs) are key enzymes involved in cancer invasion and metastasis. New ¹⁸F-labeled MMP inhibitors (1a-c) has been designed and synthesized for cancer imaging by positron emmision tomography. The precursors were synthesized in four steps starting from D-form of amino acids. Radiosynthesis of 1a-c were carried out by simple one-pot synthesis. The resulting radiofluorinated MMP inhibitors were obtained in overall radiochemical yields of 13-43% (EOB, decay corrected) within 60-70 min (including final preparative HPLC separation). Copyright

Full text of DOI:10.1002/jlcr.616

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2002; 45: 975–986.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.616
Research Article
Design and synthesis of fluorine-18 labeled
matrix metalloproteinase inhibitors for
cancer imaging
Shozo Furumoto1,2,*, Ren Iwata2, and Tatsuo Ido1,2
1 Graduate School of Pharmaceutical Science, Tohoku University,
Sendai 980-8578, Japan
2 Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578,
Japan
Summary
Matrix metalloproteinases (MMPs) are key enzymes involved in cancer
invasion and metastasis. New 18F-labeled MMP inhibitors (1a–c) has been
designed and synthesized for cancer imaging by positron emmision tomo-
graphy. The precursors were synthesized in four steps starting from d-form of
amino acids. Radiosynthesis of 1a–c were carried out by simple one-pot
synthesis. The resulting radiofluorinated MMP inhibitors were obtained in
overall radiochemical yields of 13–43% (EOB, decay corrected) within
60–70 min (including final preparative HPLC separation). Copyright # 2002
John Wiley & Sons, Ltd.
Key Words: Pet; matrix metalloproteinase; inhibitor; tumor imaging; fluorine-18
Introduction
Matrix metalloproteinases (MMPs) are a family of zinc-containing
endopeptidases involved in the extracellular matrix degradation. Under
strict control of their proteolytic activity in normal tissue, MMPs play a
*Correspondence to: S. Furumoto, Cyclotron and Radioisotope Center, Tohoku University,
Sendai 980-8578, Japan. E-mail: furumoto@cyric.tohoku.ac.jp
Copyright # 2002 John Wiley & Sons, Ltd.
Received 22 March 2002
Revised 27 May 2002
Accepted 3 June 2002

976
S. FURUMOTO ET AL.
significant physiological role in a variety of tissue remodeling processes.
However, the aberrant overexpression of MMPs has been implicated in
a number of pathological disease states such as rheumatoid arthritis,1
periodontal disease,2 multiple sclerosis,3 and malignant tumors.4
The mammalian MMP family is now known to include at least 20
enzymes and is categorized into several classes based on substrate
specificity and domain structure. Among the subfamilies of MMPs,
gelatinases (MMP-2 and MMP-9) have become attractive targets for
research on cancer and development of anticancer drugs. They have
been closely involved in the invasion, metastasis, and angiogenesis that
are crucial for the progression of malignant tumors.5,6 Moreover,
increased expressions and activities of gelatinases, especially MMP-2,
have been observed in human malignant tumor tissues of various organs
including the stomach, prostate, breast, colon, and lung.5 Thus, clinical
MMP measurements are considered to provide important data on the
metastatic potential of malignant tumors.7 For these reasons, we believe
that MMP inhibitors labeled with a positron emitter should become a
unique type of tracer that might be clinically useful for predicting cancer
metastasis through the use of positron emission tomography (PET).
Considerable effort has been devoted to the development of
potent MMP inhibitors and numerous inhibitors are reported in the
literature.8 Recent X-ray9 and NMR10 studies on the structures of
the MMP catalytic domain/inhibitor complexes indicate that the
inhibitor interactions with the active-site zinc (II) ion are essential for
defining the binding mode and the inhibitory potency. In addition, a
number of structure–activity relationship studies show that inhibitor
side chains, which may interact with specific enzyme subsites within the
active site, play a critical role in determining the inhibitory selectivity as
well as the potency.11 Current focus in the field of MMP inhibitor
development is directed towards the synthesis of selective inhibitors.
Shionogi12 and Bayer13 have reported gelatinase selective inhibitors
which contain a carboxylic acid group and a linear side chain (Figure 1).
The carboxylic acid group binds effectively to the catalytic zinc ion and
the linear side chain may specifically interact with the S0 1 subsite of the
gelatinase.
Based on these structural features of the selective inhibitors, we began
our research to develop a novel PET tracer labeled with fluorine-18 for
evaluating gelatinase expression and/or activity in malignant tumors.
The design and radiosynthesis of the new MMP inhibitors selective for
gelatinases are reported in this publication.
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DESIGN AND SYNTHESIS OF [18F] LABELED MMP INHIBITORS
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side chain
side chain
Linear
Cl
Linear
ZBG
ZBG
O
S
O
H
N
HO
HO
S
O
O
O
Bayer
(BAY12-9566)
N
H
Shionogi
Figure 1. Selective MMP inhibitors especially for gelatinases. ZBG: zinc binding
group
n
18
side chai
F
Linear
ZBG
O
H
N
HO
S
O
O
R
1a . R = CH3SCH2CH2
1b . R = (3-Indolyl)CH2
1c . R = (CH3)2CH
Figure 2. New radiopharmaceuticals designed for gelatinase inhibition
Result and discussion
As shown in Figure 2, we designed carboxylic acid-based fluorine-18
labeled inhibitors containing the sulfonamide group. This type of
inhibitor can be easily constructed from the unnatural d-form of amino
acids and aryl sulfonyl chlorides. Moreover, the sulfonamide is suitable
sterically for carrying the sulfonyl substituent deeply into the gelatinase
S0 1 subsite. The latter forms a channel-like structure having a straight,
narrow, and deep pocket shape.14 The selectivity and potency of MMP
inhibitors increase with linear elongation of the substituent on the
sulfonyl group. Substituents were thus designed carrying para-sub-
stituted aryl alkynes attached to alkyl chains so as to form a long
straight structure. The alkyl terminal of the side chain was chosen for
the labeling position since this could be easily labeled by simple
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S. FURUMOTO ET AL.
nucleophilic displacement of a primary leaving group, such as the tosyl
group, with [18F]fluoride. On the basis of these design strategies, we
approached the development of fluorine-18 labeled gelatinase inhibitors.
The precursors (5a–c) were synthesized from the commercially
available d-form of methionine, tryptophan, and valine, respectively,
as shown in Scheme 1. Protection of the carboxyl group was carried out
in methanol under reflux with p-toluenesulfonic acid and p-toluene-
I
O
O
O
To
NH2
sOH
H
N
NH2
(i)
(ii)
S
H3CO
HO
H3CO
O
O
R
R
R
D-form of
aminoacids
2a
-c
3a-c
X
(iii)
O
H
N
4a-c X = OH
(iv)
S
H3CO
5a-c X = OTos
O
R
O
Scheme 1. Precursor synthesis of the newly designed MMP inhibitors from the
d-form of various amino acids. (i) TosCl, TosOH, MeOH, reflux; (ii) 4-
Iodobenzenesulfonyl chloride, NMM, CHCl3; (iii) 5-hexyne-1-ol, Pd(PPh3)2Cl2,
CuI, TEA, DMF; and (iv) TosCl, NMM, CHCl3
sulfonyl chloride.15 The obtained methyl esters of the amino acids (2a–c)
were coupled with 4-iodobenzenesulfonyl chlorides in the presence of N-
methylmoropholine (NMM). Utilizing the reaction conditions devel-
oped by Sonogashira,16 the obtained iodophenylsulfonamides (3a–c)
were coupled with 5-hexyn-1-ol in the presence of palladium(II), CuI(I),
and triethylamine (TEA) to yield the desired alkynyl phenylsulfona-
mides (4a–c) in good yield. Conversion of the hydroxyl group to the
tosylate by treating with p-toluenesulfonyl chloride and NMM
completed the precursor 5a–c synthesis.
The non-radioactive fluorinated inhibitors were synthesized from the
corresponding precursors by fluorination with tetrabutylammonium
fluoride in acetonitrile under reflux followed by removing the methyl
ester protective groups with aqueous sodium hydroxide in methanol.
The gelatinase inhibitors labeled with fluorine-18 (1a–c) were easily
prepared via a one-pot synthesis outlined in Scheme 2. The reactions
Copyright # 2002 John Wiley & Sons, Ltd.
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DESIGN AND SYNTHESIS OF [18F] LABELED MMP INHIBITORS
979
X
18F
(ii)
O
O
H
N
H
N
(iii)
S
H3CO
S
HO
O O
O
O
R
R
1a-c
X=OTos 5a-c
X=18F
6a-c
(i)
Scheme 2. Radiosynthesis of 1a–c by a one-pot procedure: (i) [18F]KF, K2CO3,
Kryptofix 2.2.2, CH3CN, reflux; (ii) aqueous NaOH; and (iii) aqueous HCl
Table 1. Radio-TLC analysis of the radiosynthesis of 1a–c from 5a–c
Precursor
Ra
[18F]Fluorination (%)b
Deprotection (%)b
5a
5b
5c
CH2CH2SCH3
CH2(3-indolyl)
CH(CH3)2
61 (6 min)
29 (12 min)
78 (12 min)
99 (6 min)
95 (10 min)
57 (15 min)
a Side chains of 5a–c indicated in Scheme 2.
b Decay corrected yields.
were monitored by radio-thin-layer chromatography (radio-TLC) to
optimize the reaction time. The one-pot synthesis, radiofluorination and
deprotection, was done in the same vial with heating at 1108C using an
oil bath.
The initial fluorination of the precursor was performed by
nucleophilic displacement with [18F]fluoride in the presence of
potassium carbonate17 and Kryptofix 2.2.2. After fluorination, basic
hydrolysis of the methyl ester group of 6a–c was carried out by adding
2 N NaOH to the reaction solution. The results of radiofluorination and
deprotection profiled by radio-TLC are summarized in Table 1.
The radiofluorination of 5b did not proceed smoothly compared to 5a
or 5c and the yield was not improved by prolonged reaction time. The
reason for this could not be clarified in this study, but might be caused
by involvement of the indolyl side chain. On the other hand, the
deprotection of 6a and 6b proceeded easily and was almost complete
within a short time (6 and 12 min, respectively), while 6c was not
deprotected completely even after 15 min.The difficulty with the methyl
ester hydrolysis of 6c might be due to steric hindrance of the isopropyl
group at the carbonyl a-position.
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S. FURUMOTO ET AL.
The basic hydrolysis of the methyl ester was quenched by acidifying
the solution with aqueous HCl. The resulting acidic solution was diluted
with H2O followed by solid phase extraction using a Sep-Pak C18
cartridge. The final products (1a–c) were isolated from the extract by
semi-preparative reverse phase HPLC. The products (1a–c) had
retention times identical to those of the corresponding authentic non-
radioactive standards
From the results of the TLC analysis the optimum radiofluorination
time was set to 12 min (5a–c) and the deprotection time to 6 min (for 6a)
or 12 min (for 6b and 6c). Under these conditions the total radio-
synthesis times were about 60–70 min including the preparative HPLC
separation. The average of the overall radiosynthesis yields of 1a–c were
43, 13 and 33%, respectively (n=5–7, decay corrected).
After purification of 1a–c their radiochemical purities were evaluated
by analytical reverse phase HPLC. The purities of 1b and 1c stored in
preparative HPLC mobile phase remained 99% or more for at least 2 h.
On analysis of 1a, however, a slight impurity was seen 10 min after
purification by preparative HPLC (Figure 3A). The impurity seemed to
result from the decomposition of 1a because the radioactivity of the
impurity relative to 1a increased gradually with time (Figure 3B).The
chemical instability of 1a would make it unsuitable as a PET tracer even
though it was synthesized in the best radiochemical yield among the
three types of tracer.
(A)
(B)
1a
1a
Impurity
Impurity
0
5
10
15
0
5
10
15
Time (min)
Time (min)
Figure 3. Analytical HPLC profiles of 1a 10 min (A) and 180 min (B) after
preparative HPLC purification. Radiochemical purities of 1a after 10 and
180 min were 95 and 74%, respectively
Copyright # 2002 John Wiley & Sons, Ltd.
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DESIGN AND SYNTHESIS OF [18F] LABELED MMP INHIBITORS
981
Experimental section
General
All commercially available materials were of reagent grade and used
without further purification. Reactions were monitored by TLC using
Merck silica gel 60 F254 precoated aluminium sheets. Column
chromatography was performed using Silica Gel 60 (spherical,
1
40–50 mm) purchased from Kanto Chemical Co., Inc. H NMR spectra
were recorded on a JEOL GX-400 (400 MHz), reporting the chemical
shifts (d) in parts per million (ppm). Low-resolution mass spectra (MS)
and high-resolution mass spectra (HRMS) were recorded on a JEOL
JMS-DX303 or -DX500. Radio-TLCs were analyzed with a BAS 5000
system (Fuji film, Japan).
Precursor synthesis
d-Valine methyl ester (2c). To a solution of d-valine (2.34 g, 20.0 mmol)
and p-toluenesulfonic acid monohydrate (4.18 g, 22.0 mmol) in metha-
nol (40 ml) was added p-toluenesulfonyl chloride (4.58 g, 24.0 mmol)
and thereafter reflux for 20 h. After being cooled to room temperature,
the mixture was concentrated under reduced pressure. The residue was
crystallized from methanol and ethyl ether to give 5.69 g (18.7 mmol,
93%) of 2c as a colorless crystal.
(2R)-2-[(4-iodophenyl)sulfonylamino]-3-methylbutyric acid methyl
ester (3c). To a solution of 2c (4.90 g, 16.1 mmol) and N-methylmor-
pholine (10 ml) in CHCl3 (50 ml) was added 4-idobenzenesulfonyl
chloride (5.37 g, 17.7 mmol). After stirring overnight at room tempera-
ture, CHCl3 was evaporated under vacuum and the residue was
dissolved in ethyl acetate followed by washing with H2O and brine. The
organic phase was dried over MgSO4 and evaporated under vacuum.
The oil residue was purified by column chromatography on silica gel
eluting with hexane/ethyl acetate (5:2) to give 5.52 g (13.9 mmol, 86%)
1
of 3c as a colorless crystal. H NMR (CDCl3/TMS): d 0.88 (3 H, d,
J=6.8 Hz), 0.95 (3 H, d, J=6.8 Hz), 2.00–2.09 (1 H, m), 3.49 (3 H, s),
3.74 (1 H, dd, J=10.4, 5.2 Hz), 5.28 (1 H, d, J=10.4 Hz), 7.55 (2 H, d,
J=6.0 Hz), 7.85 (2 H, d, J=6.0 Hz). MS (EI): 398 ([M+H]+).
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S. FURUMOTO ET AL.
(2R)-2-[[4-(6-hydroxyhex-1-ynyl)phenyl]sulfonylamino]-3-methyl-
butyric acid methyl ester (4c). To a solution of 3c (2.05 g, 5.16 mmol)
and triethylamine (3.0 ml) in DMF (9.0 ml) was added copper(I) iodide
(42.9 mg, 0.225 mmol), dichlorobis(triphenylphosphine)palladium(II)
(80.5 mg, 0.114 mmol), and 5-hexyn-1-ol (700 ml, 6.34 mmol) at room
temperature. The mixture was stirred under an argon atmosphere at
658C overnight. After being cooled to room temperature, the reaction
solution was diluted with 1 N HCl and extracted with ethyl ether. The
organic phase was dried over MgSO4 and evaporated under vacuum.
The oil residue was purified by column chromatography on silica gel
eluting with hexane/ethyl acetate (7:17) to give 1.62 g (4.41 mmol, 85%)
1
of 4c as a colorless solid. H NMR (CDCl3/TMS): d 0.870 (3 H, d,
J=6.8 Hz), 0.950 (3 H, d, J=6.8 Hz), 1.68-1.77 (4 H, m), 1.99-2.07 (1 H,
m), 2.48 (2 H, t, J=6.4 Hz), 3.46 (3 H, s), 3.70-3.75 (3 H, m), 5.18 (1 H,
d, J=10.4 Hz), 7.47 (2 H, d, J=8.4 Hz), 7.73 (2 H, d, J=8.8 Hz). MS
(EI): 367 (M+).
(2R)-3-methyl-2-[[4-[6-(toluene-4-sulfonyloxy)hex-1-ynyl]phenyl]
sulfonylamino]butyric acid methyl ester (5c). To a solution of 4c
(1.62 g, 4.41 mmol) and N-methylmorpholine (6.0 ml) in CHCl3 (10 ml)
was added p-toluenesulfonyl chloride (1.29 g, 6.76 mmol). After stirring
overnight at room temperature, CHCl3 was evaporated under vacuum.
Two normal HCl was added to the residue and the organic extracted
with ethyl acetate. The organic phase was dried over MgSO4 and
evaporated under vacuum. The oil residue was purified by column
chromatography on silica gel eluting with hexane/ethyl acetate (5:4) to
1
give 1.73 g (3.31 mmol, 75%) of 5c as colorless heavy oil. H NMR
(CDCl3/TMS): d 0.873 (3 H, d, J=6.8 Hz) 0.952 (3 H, d, J=6.8 Hz),
1.63-1.71 (2 H, m), 1.80–1.88 (2 H, m), 1.99–2.07 (1 H, m), 2.43 (2 H, t,
J=6.8 Hz), 2.45 (3 H, s), 3.47 (3 H, s), 3.74 (1 H, dd, J=10.0, 5.2 Hz),
5.07 (1 H, d, J=10.0 Hz), 7.35 (2 H, d, J=8.0 Hz), 7.59 (2 H, d,
J=8.8 Hz), 7.73 (2 H, d, J=8.8 Hz). 7.80 (2 H, d, J=8.4 Hz). MS (EI):
521 (M+).
Precursors 5a and 5b were prepared from d-methionine and
d-tryptophan, respectively, using the same procedures as for the
synthesis of 5c.
(2R)-4-methylsulfanyl-2-[[4-[6-(toluene-4-sulfonyloxy)-hex-1-ynyl]-
1
phenyl]sulfonylamino]butyric acid methyl ester (5a). H NMR (CDCl3/
TMS): d 1.62–1.71 (2 H, m), 1.79–1.87 (2 H, m), 1.87–1.95 (1 H, m),
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DESIGN AND SYNTHESIS OF [18F] LABELED MMP INHIBITORS
983
1.98–2.07 (1 H, m), 2.05 (3 H, s), 2.43 (2 H, t, J=7.2 Hz), 2.44 (3 H, s),
2.47–2.61 (2 H, m), 3.54 (3 H, s), 4.05–4.12 (3 H, m), 5.53 (1 H, d,
J=9.2 Hz), 7.35 (2 H, d, J=8.0 Hz), 7.46 (2 H, d, J=8.8 Hz), 7.74-7.82
(4 H, m). MS (EI): 494 ([MꢀCH3COꢁ+H]+), 554 ([M+H]+).
(2R)-3-(1H-indol-3-yl)-2-[[4-[6-(toluene-4-sulfonyloxy)-hex-1-ynyl]-
1
phenyl]sulfonylamino]propionic acid methyl ester (5b). H NMR (CDCl3/
TMS): d 1.63–1.71 (2H, m), 1.80-1.89 (2H, m), 2.43 (2H, t, J=6.8 Hz),
2.44 (3H, s), 3.21 (1H, dd, J=14.8, 5.6 Hz), 3.46 (1H, dd, J=14.8, 6.0 Hz),
3.47 (3H, s), 4.10 (2H, t, J=6.4 Hz), 4.23-4.29 (1H, m), 5.14 (1H, d,
J=8.8 Hz), 6.99 (1H, s), 7.08 (1H, t, J=7.2 Hz), 7.17 (1H, t, J=7.2 Hz),
7.31–7.46 (6H, m), 7.58 (2H, d, J=8.4 Hz), 7.79 (2H, d, J=8.4 Hz), 8.10
(1H, brs). MS (EI): 608 (M+).
Synthesis of non-radiolabeled standard
Non-radiolabeled standards of 1a–c were synthesized by following the
general procedures. To a solution of the corresponding precursor in
acetonitrile was added 2–4 equivalents of tetrabutylammonium fluoride
(1 M/THF, Aldrich) and the solution was stirred under reflux. After
checking the disappearance of the precursor by TLC, acetonitrile was
evaporated off under vacuum. To the residue was added H2O and the
organic extracted with ethyl acetate followed by drying over MgSO4.
The fluorinated compound was purified by column chromatography on
silica gel. It was then dissolved in methanol, after which 2 N NaOH was
added to the solution at room temperature. The mixture was heated at
608C for several hours, monitoring the hydrolysis of the methyl ester by
TLC. After completion of the reaction, the reaction solution was
acidified with aqueous HCl followed by evaporation under vacuum. To
the residue was added H2O and the organic extracted with ethyl acetate.
The extract was dried over MgSO4 and evaporated under vacuum. The
residue was purified by column chromatography on silica gel using
hexane/ethyl acetate with acetic acid (1%). The analytical data for the
non-labeled standard are given below.
(2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-4-methyl-
1
sulfanylbutyric acid (non-radiolabeled standard of 1a). H NMR
(DMSO-d6/TMS): d 1.72–1.99 (5H, m), 2.03–2.14 (1H, m), 2.05 (3H,
s), 2.47–2.63 (4H, m), 4.12–4.18 (1H, m), 4.51 (2H, dt, J=47.2, 5.8 Hz),
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5.44 (1H, d, J=8.8 Hz), 6.50–7.20 (1H, br), 7.49 (2H, d, J=8.8 Hz), 7.77
(2H, d, J=8.4 Hz).
(2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-(1H-indol-
1
3-yl)propionic acid (non-radiolabeled standard of 1b). H NMR
(CDCl3/TMS): d 1.78–1.80 (2H, m), 1.80–1.94 (2H, m), 2.49 (2H, t,
J=7.0 Hz), 3.18 (1H, dd, J=14.4, 6.6 Hz), 3.28 (1H, dd, J=14.4,
5.2 Hz), 4.22–4.24 (1H, m), 4.51 (2H, dt, J=47.2, 5.8 Hz), 5.39 (1H, d,
J=8.4 Hz), 6.94 (1H, s), 7.06 (1H, t, J=7.6 Hz), 7.17 (1H, t, J=7.6 Hz)
7.25–7.48 (4H, m), 7.52 (2H, d, J=8.4 Hz), 7.56–7.90 (1H, br), 8.10 (1H,
brs). HRSM (EI): calculated for C23H23FN2O4S 442.1357 (M+), found
442.1337.
(2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric
1
acid (non-radiolabeled standard of 1c). H NMR (CDCl3/TMS): d 0.87
(3H, d, J=6.7 Hz), 0.98 (3H, d, J=6.7 Hz), 1.71–1.96 (4H, m), 2.51 (2H,
t, J=6.7 Hz), 3.79–3.84 (1H, m), 4.52 (2H, dt, J=47.2, 5.8 Hz), 5.15
(1H, d, J=9.4 Hz), 7.47 (2H, d, J=8.1 Hz), 9.75 (2H, d, J=8.3 Hz).
HRSM (EI): calculated for C17H22FNO4S 355.1248 (M+), found
355.1224.
Radiosynthesis (general procedures)
[18F]Fluoride was produced by the 18O(p, n)18F nuclear reaction on a
[18O]water target using an HM12 cyclotron (Sumitomo Heavy
Industries, Japan) and the fluoride was converted to potassium
[18F]fluoride with potassium carbonate. To the [18F]fluoride solution
including potassium carbonate (8-10 mg) in a vial (5 ml volume) was
added Kryptofix 2.2.2 (16 mg). Azeotropic removal of water with
acetonitrile (3 ꢂ 3 ml) was carried out by heating at 1108C under a
stream of nitrogen. To the activated Kryptofix 2.2.2/potassium
[18F]fluoride was added the precursor (10 mmol) dissolved in acetonitrile
(1 ml). The mixture was heated at 1108C for 12 min before adding 2 N
NaOH (300 mL). After 6–12 min, 2 N HCl (450 ml) and H2O (10 ml) were
added to the reaction solution. The resulting acidic aqueous solution
was passed through a Sep-Pak C18 column followed by eluting with
acetonitrile (10 ml). The acetonitrile solution was evaporated in vacuo
to dryness. The residue was purified by semi-preparative reversed phase
HPLC using a YMC-Pack ODS-A column (300 ꢂ 10 mm, s-5 mm) eluted
with acetonitrile/methanol/water/acetic acid (200:400:400:10 for 1a and
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DESIGN AND SYNTHESIS OF [18F] LABELED MMP INHIBITORS
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1b, and 200:400:380:10 for 1c) at a flow rate of 5.0 ml/min. The retention
times of 1a, 1b, and 1c were 19.3, 24.7, and 16.3 min, respectively.
Analytical HPLC was performed using a LiChrosorb1 RP-8
(250 ꢂ 4 mm, s-7 mm, Cica-MERCK) column eluted with the same
mobile phase of semi-preparative HPLC.
Conclusion
We have designed and synthesized a new type of PET tracer for cancer
imaging, carboxylic acid-based MMP inhibitors labeled with fluorine-
18. The precursors were easily prepared from the d-form of various
amino acids in four steps. Fluorine-18 labeled compounds were
synthesized from the precursors by a simple one-pot preparation and
purified by HPLC of the three types of labeled compounds. 1a was
prepared in the best total radiochemical yield, but turned out to be
unstable chemically and thus unsuitable for use to biological studies. 1b
and 1c are undergoing biological evaluation as prospective candidates
for novel and potent PET tracers for cancer imaging. This evaluation
includes in vitro enzymatic inhibition and biodistribution studies using
several tumor models. The results of these studies will be reported
elsewhere in the near future.
Acknowledgements
The authors thank Dr F. Brady (Imaging Research Solutions Limited,
Cyclotron Building, Hammersmith Hospital, London) for helpful and
critical discussion.
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