Synthesis of hydroxyl silylated rhenium and (99mTc)technetium '3 + 1' mixed ligand complexes

Article abstract of DOI:10.1002/jlcr.567

The synthesis of hydroxyl silylated thiols as monodentate ligands is described. These monodentates were used to build with Re and ^99mTc trimethyl-, triethyl-and triphenyl-silylated '3 + 1' mixed ligand complexes, using 3thiapentane-1,5-dithiol

Full text of DOI:10.1002/jlcr.567

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2002; 45: 629–636.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.567
Research Article
Synthesis of hydroxyl silylated rhenium and
(99mTc)technetium ‘3+1’ mixed ligand
complexes
Torsten Kniess1, Hartmut Spies2, Isabel Santos1,*, and Alla Zablotska3
1 Departamento de Quimica, ITN, Estrada Nacional 10, 2686-953,
Sacavem Codex, Portugal
2 Institute of Bioinorganic & Radiopharmaceutical Chemistry, FZ-Rossendorf,
PF 510119, D-01314 Dresden, Germany
3 Latvian Institute of Organic Synthesis, LV-1006 Riga, Latvia
Summary
The synthesis of hydroxyl silylated thiols as monodentate ligands is described.
These monodentates were used to build with Re and 99mTc trimethyl-, triethyl-
and triphenyl-silylated ‘3+1’ mixed ligand complexes, using 3thiapentane-1,5-
1
dithiol as co-ligand. The Re complexes were characterized by H NMR and
elemental analysis, the 99mTc complexes were detected by radio HPLC. While
the trimethyl silylated derivatives hydrolysed in aqueous media, the triethyl-
and triphenyl silylated complexes have proved to be stable in neutral
solutions. Copyright # 2002 John Wiley & Sons, Ltd.
Key Words: hydroxyl silylated thiols; Re and 99mTc complexes
Introduction
99mTc and 186Re radiopharmaceuticals are widely used for diagnosis and
therapy in nuclear medicine. Their transportation and accumulation
in vivo is a crucial step but often unclear and many potential
applications suffer from insufficient concentration of the radioactive
drug in the target organ. Since membrane transport and accumulation
*Correspondence to: I. Santos, Departamento de Quimica, ITN, Estrada Nacional 10, 2686-953,
Sacavem Codex, Portugal. E-mail: isantos@itn1.itn.pt
Copyright # 2002 John Wiley & Sons, Ltd.
Received 14 November 2001
Revised 7 December 2001
Accepted 15 January 2002

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T. KNIESS ET AL.
of compounds in any organ system are governed by basic molecular
parameters such as molecular size, charge, lipophilicity, etc., the
availability of principles that allow one to control such parameters in
vivo are valuable tools in studying bio-distribution patterns.
A promising way to increase the lipophilicity of organic compounds is
the introduction of silyl groups, and several studies have shown that
silylation of biologically active molecules facilitates their transportation
in the organism.1–4 By applying this principle of silylation to Re and
99mTc complexes, their biological properties should be improved and
positive physiological effects are expected. Two major aims should be
pursued by introduction of silylated groups. First, silylation of hydroxyl
moieties usually increases the lipophilicity of the complex compared
with the non-silylated one. This should improve the bio-availability as
well, especially in terms of passing the blood brain barrier and the
uptake of the compound in brain tissues. Second, in case of silyl ethers,
which are hydrolyzable in vivo, the silylated radiopharmaceutical would
be able to act as a pro-drug, because after cleavage of the labile silyl
group the free hydroxyl compound should be trapped in the brain.
In previous work, the authors have described the synthesis of silylated
‘3+1’ rhenium(V)oxo-complexes by silylation of the free hydroxyl side
chain of hydroxylalkylthiolato(3thiapentane-1,5-dithiolato)oxo-rhe-
nium(V) compounds.5,6 The lipophilicity of the silylated Re complexes
was determined7 and their psychotropic activity and acute toxicity in
vivo have been investigated8. However, the above synthetic pathway is
not practicable for labelling with 99mTc and the silylated monodentate
ligands should be prepared beforehand.
This paper reports the synthesis of trimethyl-, triethyl- and triphenyl-
silylated alcohols bearing a free mercapto group and reports also the
reaction of these monodentate ligands with chloro(3thiapentane-1,5-
dithiolato)oxorhenium(V), an appropriate rhenium precursor in non-
aqueous medium. We also investigate whether the silylated mercapto
alcohols were stable against hydrolysis and suitable to form silylated
‘3+1’ mixed ligand 99mTc complexes in aqueous solution.
Experimental
Materials and methods
All chemicals were of reagent grade and used without further
purification. 2-Mercapto-ethanol, 3-mercapto-1-propanol, chloro-tri-
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SYNTHESIS OF HYDROXYL SILYLATED
631
methylsilane, chloro-triethylsilane, chloro-triphenylsilane and imidazole
were obtained from ALDRICH. The precursor chloro(3thiapentane-
1,5-dithiolato)-oxorhenium(V) was prepared according to the litera-
ture9. Stannous chloride and sodium gluconate were purchased from
ALDRICH. (99mTc)TcOÀ4 in saline solution was eluted from a
99Mo/99mTc generator from MDS Nordion S.A., Belgium.
Melting points were determined on a digital melting point apparatus
IA 9200, Electrothermal, UK. Elemental analysis was performed on a
1
Perkin-Elmer automatic analyser. The H NMR spectra were recorded
1
on a VARIAN Inova 300 MHz spectrometer. H chemical shifts were
referenced with the residual solvent resonance relative to tetramethyl-
silane. The NMR samples were prepared in CDCl3.
HPLC investigations were carried out with an RP 18 column (Nova-
Pack, 3.9 Â 150 mm, Waters, USA) using a gradient of methanol/water
as eluent with a flow rate of 1.0 ml/min. The products were determined
by UV absorbance at 254 nm with a UV-VIS detector LC 290 (Perkin-
Elmer) and by g-detection with a scintillation detector LB 507A
(Berthold).
O-alkylsiloxy-ethan-2thiols and -propan-3thiols 1a–e (general procedure)
2.0 mmol of mercaptoalcohol and 2.0 mmol imidazole (Imz) were stirred
in 3.0 ml of dry DMF under a nitrogen atmosphere. 2.0 mmol of the
chloro-trialkylsilane dissolved in 1.0 ml DMF was added and the
mixture stirred for 24 h at room temperature. The DMF was removed
by lyophilization under high vacuum and the residue was purified by
column chromatography using short columns with silicagel and
chloroform/hexane=1:1 as eluent. In case of 1a, 1c, and 1d the reaction
was carried out without DMF using the mercapto alcohol as solvent
and adding the pure chloro-trialkylsilanes.
1
1-Trimethylsiloxy-ethane-2thiol 1a; liquid, yield 68%: H NMR (d,
ppm): 0.12 (s, 9 H, (CH3)3Si), 1.49 (tr, 1 H, SH), 2,63 (q, 2 H, CH2–S);
3.68 (tr, 2 H, CH2–O).
1-Triphenylsiloxy-ethane-2thiol 1b; m.p. 82–848C, yield 65%: 1H
NMR (d, ppm): 1.56 (tr, 1 H, SH), 2.68 (q, 2 H, CH2–S), 3.91 (tr, 2 H,
CH2–O), 7.37–7.65 (m, 15 H, Ph). Elemental analysis: C20H20OSSi,
theory C 71.42, H 5.95, S 9.52, found C 71.89, H 6.40, S 9.26.
1
1-Trimethylsiloxy-propane-3thiol 1c; liquid, yield 49%: H NMR (d,
ppm): 0.10 (s, 9 H, (CH3)3Si), 1.33 (tr, 1 H, SH), 1.81 (m, 2 H, CH2), 2.61
(q, 2 H, CH2–S), 3.67 (tr, 2 H, CH2–O).
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T. KNIESS ET AL.
1
1-Triethylsiloxy-propane-3thiol 1d; liquid, yield 72%: H NMR (d,
ppm): 0.61 (q, 6 H, CH2–Si), 0.95 (tr, 9 H, CH3), 1.33 (tr, 1 H, SH), 1.81
(m, 2 H, CH2), 2.63 (q, 2 H, CH2–S), 3.70 (tr, 2 H, CH2–O).
1-Triphenylsiloxy-propane-3thiol 1e; m.p. 42–468c, yield 68%: 1H
NMR (d, ppm): 1.25 (tr, 1 H, SH), 1.86 (m, 2 H, CH2), 2.66 (q, 2 H,
CH2–S), 3.89 (tr, 2 H, CH2–O), 7.36–7.63 (m, 15 H, Ph). Elemental
analysis: C21H22OSSi, theory C 72.00, H 6.28, S 9.14, found C 72.62, H
6.71, S 8.65.
(O-alkylsiloxy-alkylthiolato)(3thiapentane-1,5-dithiolato)oxo-rhe-
nium(V) -complexes 2a–e:(general procedure):
To 100 mmol of chloro(3thiapentane-1,5-dithiolato)oxorhenium(V) dis-
solved in 5.0 ml acetonitrile was added under stirring 25 ml (350 mmol)
triethylamine followed by 200 mmol of the thiol 1 in 2.0 ml acetonitrile.
The stirring was continued for 4 h at room temperature and then the
solvent was evaporated off. The complexes were purified by column
chromatography.
2a: Column chromatography: Silicagel, ethyl acetate, m.p. 126–
1298C, yield 71%, 1H NMR (d, ppm): 0.15 (s, 9 H, (CH3)3Si), 1.97, 3.11,
3.90, 4.26 (m, 4 Â 2 H, S-(CH2)2–S–(CH2)2–S), 3.97 (m, 2 H, CH2–S),
4.01 (m, 2 H, CH2–O). Elemental analysis: C9H21O2S4SiRe, theory, C
21.47, H 4.17, S 25.44, found, C 22.14, H 3.34, S 26.00.
2b: Column chromatography: Silicagel, chloroform/hexane 9:1, m.p.
1
163–1678C, yield 75%, H NMR (d, ppm): 1.91, 3.04, 3.86, 4.23 (m,
4 Â 2 H, S–(CH2)2–S–(CH2)2–S), 4.08 (m, 2 H, CH2–S), 4.16 (m, 2 H,
CH2–O) 7.34–7.68 (m, 15 H, Ph). Elemental analysis: C24H27O2S4SiRe,
theory, C 41,79, H 5.92, S, 18.58, found, C 42.36, H 3.62, S 17.73.
2c: Column chromatography: silicagel, ethyl acetate, m.p. 92–968C,
1
yield 77%, H NMR (d, ppm): 0.13 (s, 9 H, (CH3)3Si), 1.94, 3.10, 3.78,
4.31 (m, 4 Â 2 H, S-(CH2)2–S–(CH2)2–S), 2.15 (m, 2 H, CH2), 3.88
(m, 2 H, CH2–S), 3.93 (m, 2 H, CH2–O). Elemental analysis:
C10H23O2S4SiRe, theory, C 23.12, H 4.44, S 24.75, found, C 22.05, H
3.35, S 26.01.
2d: Column chromatography: silicagel, chloroform, m.p. 76–808C,
1
yield 80%, H NMR (d, ppm): 0.62 (q, 6 H, CH2–Si), 0.96 (tr, 9 H,
CH3), 1.94, 3.10, 3.82, 4.29 (m, 4 Â 2 H, S–(CH2)2–S–(CH2)2–S), 2.12
(m, 2 H, CH2), 3.87 (m, 2 H, CH2–S), 3.91 (m, 2 H, CH2–O). Elemental
analysis: C13H29O2S4SiRe, theory, C 27.90, H 5.18, S 23.02, found, C
26.42, H 4.58, S 23.28.
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SYNTHESIS OF HYDROXYL SILYLATED
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2e: column chromatography: Silicagel, chloroform/hexane 95:5,
1
m.p.164–1668C, yield 74%, H NMR (d, ppm): 1.94, 3.10, 3.78, 4.31
(m, 4 Â 2 H, S–(CH2)2–S–(CH2)2–S), 2.15 (m, 2 H, CH2), 3.88 (m, 2 H,
CH2–S), 3.93 (m, 2 H, CH2–O), 7.34–7.66 (m, 15 H, Ph). Elemental
analysis: C25H29O2S4SiRe, theory, C 42.67, H 4.12, S 18,20, found, C
42.88, H 3.47, S 18.28
(
99mTc)O-alkylsiloxy-alkylthiolato)(3thiapentane-1,5-dithiolato)-oxo-
technetium(V) complexes 4 (general procedure)
To 500 ml of sodium gluconate (0.1 M) was added 500 ml of 99mTcO4À
(50–90 MBq) and the pertechnetate reduced using 10 ml of stannous
chloride in 0.1 M HCl (0.01 M). The complete reduction of the
pertechnetate to (99mTc)technetium gluconate was checked by thin
layer chromatography (silicagel, acetone). The pH of the solution was
adjusted to 7.1 by adding 500 ml of phosphate buffer (pH=7.54).
Afterwards, 1.5 ml of acetonitrile was added, followed by 2.0 mmol of
the monodentate ligand 1 dissolved in acetonitrile. The closed vial was
heated for 15 minutes at 508C, cooled and then 0.5 mmol of the
3thiapentane-1,5-dithiol in acetonitrile was added. The mixture was
heated for 15 minutes, at 508C, and analysed by HPLC, using an RP 18
column and a flow rate of 1.0 ml/min of methanol/water: Gradient:10 -
min 50% methanol ‘‘90% methanol, 5 min 90% methanol isokratic,
5 min 90% methanol’’ 50% methanol. The radiochemical yield of the
(
99mTc) complexes were determined from the radiochromatograms: 4b
yield 88%, 4d yield 80%, 4e yield 84%
Results and discussion
The synthetic pathway to obtain silylated ‘3+1’ mixed ligand complexes
of Re and 99mTc is outlined in Figure 1. The first step was the
preparation of silylated mercapto alcohols by silylation of the free
hydroxyl group and keeping the free mercapto functionality. These O-
silylated mercapto alcohols are not only required for following the
synthetic route, they are also a prerequisite for labelling with (99mTc)
technetium.
Up to now the silylation of mercapto alcohols has not been referred
to in detail5,6 but it is known that in the reaction of chlorosilanes with
alcohols in dimethylformamide, imidazole acts as a nucleophilic
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T. KNIESS ET AL.
Figure 1. Synthetic pathway for silylated Re and 99mTc ‘3+1’ mixed ligand
complexes
catalyst10. By stirring 1,2-mercaptoethanol and 1,3-mercaptopropanol
with the corresponding chlorosilanes under the above conditions at
room temperature we found indeed a high rate of O-silylation and the
mercapto group has not been attacked. In case of compounds 1a, 1c and
1d the silylation was carried out in the absence of any solvent.
The silylated Re(V)oxo-complexes 2a–e were prepared by reacting the
free mercapto group of the silylated monodentates 1a–e with the
appropriate rhenium precursor chloro(3thiapentane-1,5-dithiolato)ox-
orhenium(V)9 (Figure 1). A slight excess of triethylamine is necessary to
avoid hydrolysis of the silyl group by the hydrochloric acid formed
during the reaction. The triethyl and triphenyl silylated complexes were
found to be stable against hydrolysis during the work up. In contrast, in
the trimethyl-silylated complexes 2a and 2c traces of the hydrolysed
by-products were still found, and the compounds hydrolysed completely
by treatment with protonic solvents. The hydrolysed complexes 3a and
3b, were synthesised as previously described, and used as a reference
(Figure 1)6.
The preparation of the silylated (99mTc)(3-thiapentane-1,5-dithiola-
to)oxo-technetium(V) complexes 4 was performed via 99mTc gluconate
in aqueous solution (Figure 1). After reduction of 99mTcO4À with
stannous chloride, the formation of 99mTc gluconate was checked by
HPLC (Figure 2). For preparing 4b, 4d and 4e the pH was adjusted to
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SYNTHESIS OF HYDROXYL SILYLATED
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Figure 2. (a) HPLC radio chromatogram of 99mTc gluconate; (b) HPLC radio
chromatogram of 99mTc complex 4b; (c) HPLC UV-VIS chromatogram of
rhenium complex 2b
about 7 with phosphate buffer to avoid hydrolysis. The labelling was a
two-step procedure: first the monodentate ligand 1 was added to the
99mTc gluconate solution and then the mixture was heated at 508C. In
the HPLC the 99mTc gluconate signal disappeared, but the intermediate
could not be detected because it was too lipophilic. Secondly, the
3thiapentane-1,5-dithiol (HSSSH) was added followed by additional
heating. The nature of the silylated ‘3+1’ 99mTc complexes 4 were
established by HPLC, by comparison with the analogous rhenium
compounds, fully characterized at the macroscopic level (Figure 2).
For the triphenyl- and triethyl-silylated (99mTc)technetium(V)oxo-
complexes 4b, 4d and 4e labelling yields between 80 and 90% were
found, together with some unreacted 99mTc gluconate. HPLC studies
(comparison of retention times with those of the rhenium complexes 3a
and 3b) showed that no hydrolysed 99mTc compounds were formed,
thereby confirming their stability under the experimental conditions.
In the case of the trimethyl-silylated ligands 1a and 1c the labelling
was unsuccessful. All the attempts made, led to a mixture of products
containing the hydrolysed complexes.
Concluding remarks
‘3+1’ mixed ligand complexes of Re and 99mTc with O-silylated
mercapto alcohols as monodentate ligands and 3-thia-pentane-1,5-
dithiol as tridentate ligand have been synthesized. The new ligands and
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1
the rhenium complexes were characterized by H NMR and elemental
analysis. The preparation of 99mTc complexes was carried out via 99mTc
gluconate and their purity evaluated by HPLC. The yields were found to
be 80–90% for the triethyl- and triphenyl-silylated complexes. These
compounds are stable in aqueous conditions (pH=7), in contrast to the
trimethyl-silylated complexes which are unstable and hydrolyse in protic
solvents. The silylated 99mTc complexes 4b, 4d and 4e are promising for
further biological investigation.
Acknowledgements
We would like to thank the Deutscher Akademischer Austauschdienst
*
(DAAD) and the Instituto de Cooperacao Cientıfica e Tecnologica
!
!
-
International (ICCTI) for a bilateral project (ICCTI/DAAD). T. Kniess
*
thanks the Fundacao para a Ciencia e Tecnologia (FCT) for a PRAXIS
#
-
postdoctoral grant.
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Copyright # 2002 John Wiley & Sons, Ltd.
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