Radiosynthesis of [11C]docetaxel

Article abstract of DOI:10.1002/jlcr.861

Docetaxel (Taxotere) is an accepted chemotherapeutic agent for the treatment of breast cancer and non-small cell lung cancers. A potential means of predicting response is measuring tumor uptake of [¹¹C]docetaxel using Positron Emission Tomograp

Full text of DOI:10.1002/jlcr.861

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 763–777.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.861
Research Article
Radiosynthesis of [11C]docetaxel
E. W. van Tilburg1,*, E. J. F. Franssen1,2, J. J. M. van der Hoeven3,
M. van der Meij1, D. Elshove4, A. A. Lammertsma1 and A. D. Windhorst1
1 Department of Nuclear Medicine and PET Research, VU University Medical Center,
Amsterdam, The Netherlands
2 Department of Pharmacy, VU University Medical Center, Amsterdam,
The Netherlands
3 Department of Internal Medicine, Ziekenhuis van Amstelveen, Amstelveen,
The Netherlands
4 Aventis Pharma BV, Hoevelaken, The Netherlands
Summary
Docetaxel (Taxotere1) is an accepted chemotherapeutic agent for the treatment of
breast cancer and non-small cell lung cancers. A potential means of predicting
response is measuring tumor uptake of [11C]docetaxel using Positron Emission
Tomography (PET). The synthetic approach to introduce the 11C isotope in the 2-
benzoyl moiety of docetaxel unfortunately was unsuccessful. The radiosynthesis of
[11C]docetaxel (6b, Scheme 1), with the 11C isotope in the BOC moiety, was however,
successful using a second synthetic approach. It started with the reaction of [11C]tert-
butanol with 1,2,2,2-tetrachloroethyl chloroformate to give [11C]tert-butyl-l,2,2,2-
tetrachloroethyl carbonate in a good overall yield (62 ꢀ 9%). In the final step, the
[11C]tert-butoxycarbonylation of the free amine of docetaxel gave [11C]docetaxel 6b in
a satisfactory decay corrected yield of 10 ꢀ 1% (from [11C]CO2). Copyright # 2004
John Wiley & Sons, Ltd.
Key Words: docetaxel; [11C]tert-butanol ([2-11C]2-methylpropan-2-ol); tert-butoxy-
carbonylation; [11C]docetaxel; positron emission tomography (PET)
Introduction
Over the past several decades, the taxane class of antimicrotubule anticancer
agents is perhaps the most important class of new agents.1,2 The direct target
*Correspondence to: Dr. E. W. van Tilburg, Department of Nuclear Medicine & PET Research, VU
University Medical Center, Location Radionuclide Center, De Boelelaan 1083, 1081 HV Amsterdam, The
Netherlands. E-mail: etilburg@rnc.vu.nl
Copyright # 2004 John Wiley & Sons, Ltd.
Received 31 January 2004
Revised 26 March 2004
Accepted 3 April 2004

764
E. W. VAN TILBURG ET AL.
Figure 1. Docetaxel (Taxotere1), with trivial numbering of the molecule
(IUPACy)
of docetaxel (Taxotere1, Figure 1), one of the major taxanes, is the
heterodimeric protein tubulin3,4 that once polymerized ultimately leads to
mitotic arrest5,6 and thus inhibits cell proliferation. In the case of breast
cancer, reported response rates after first-line docetaxel therapy in meta-
static disease7–9 or after second-line therapy10,11 are in the order of 52–68%
and 50–58%, respectively. Obviously, methods that are able to predict the
response to docetaxel treatment would be very useful, as non-responders
could be offered alternative therapies at a much earlier stage without suffering
side effects from (ineffective) docetaxel administration. A potential means
of predicting response could be measuring tumor uptake of docetaxel,
labeled with a positron emitter, using Positron Emission Tomography
(PET).
The present study was undertaken to label docetaxel (chemical
nomenclaturey) with the positron emitter carbon-11 (t1/2=20.33 min). There
appeared to be two obvious synthetic strategies for the radiosynthesis
of [11C]docetaxel and both were attempted. The first synthetic approach
(Figure 2(I)) was to introduce the 11C isotope in the 2-benzoyl moiety of
docetaxel (6a)y with [11C]benzoyl chloride, which has been used for the
synthesis of paclitaxel (Taxol1) as well.12 The second approach was to
introduce the 11C isotope in the side chain (6b, Figure 2(II))y by a [11C]tert-
butoxycarbonylation of the free amine of docetaxel.
y (i) The chemical nomenclature for docetaxel (according to IUPAC recommendations) is: 15-[((1R,2S)-l-
hydroxy-2-((1,1,-dimethylethyloxy)carbonylamino)-2-phenylethyl)carbonyloxy]-(1S,2S,4S,7R,9S,10S,12R,
15S)-1,9,12-trihydroxy-10,14,17,17-tetramethyl-4-(methylcarbonyloxy)-11-oxo-2-(phenylcarbonyloxy)-6-
oxatetracyclo[11.3.1.03,10.04,7]heptadec-13-ene.
(ii) Compound 6a: 15-[((1R,2S)-l-hydroxy-2-((1,1-dimethylethyloxy)carbonylamino)-2-phenylethyl)carbo-
nyloxy]-(1S,2S,4S,7R,9S,10S,12R,15S)-l,9,12-trihydroxy-10,14,17,17-tetramethyl-4-(methylcarbonyloxy)-
11-oxo-2-(phenyl-[11C]carbonyloxy)-6-oxatetracyclo[11.3.1.03,10.04,7]heptadec-13-ene.
(iii) Compound 6b: 15-[((1R,2S)-l-hydroxy-2-((1,l-dimethyl-l-[11C]ethyloxy)carbonylamino)-2-phenylethyl)-
carbonyloxy]-(1S,2S,4S,7R,9S,10S,12R,15S)-l,9,12-trihydroxy-10,14,17,17-tetramethyl-4-(methylcarbony-
loxy)-11-oxo-2-(phenylcarbonyloxy)-6-oxatetracyclo[11.3.1.03,10.04,7]heptadec-13-ene.
Copyright # 2004 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 763–777

RADIOSYNTHESIS OF [11C]DOCETAXEL
765
Figure 2. Different possible labeling positions of docetaxel
Results and discussion
Synthetic approach I: [11C]docetaxel (6a)y
The description of several methods for the selective hydrolysis of the 2-
benzoate group of paclitaxel (Taxol1) or docetaxel,13–18 along with the
expected easy access of [11C]benzoic acid or [11C]benzoylchloride for
reacylation prompted the selection of this synthetic approach for the synthesis
of [11C]docetaxel 6a (Scheme 1).
First, the most appropriate protecting groups for the 20-,7- and 10- hydroxyl
groups of docetaxel were selected. The tert-butyl carbamate moiety of
docetaxel (side chain) was anticipated to be unstable under acidic hydroly-
sis and deprotection reaction conditions, so a fast and quantitative removal
of the protection groups under basic conditions was a necessity. Using pub-
lished methods,14,16,18,19 the 20-hydroxyl was protected with a tert-butyldi-
methylsilyl protecting group and both the 7- and 10-hydroxyls with
triethylsilyl protecting groups in an average overall yield of 86 ꢀ 4%. The
bulkiness of the former protecting group was shown to be essential for the
prevention of cleavage of the entire C-13 side chain.13,16 Next, the hydrolysis
of the 2-benzoate ester of 1 was carried out. Compound 2 was obtained in a
yield of about 57%, using a selective C-2 deacylation method15,16 under
‘standard’ basic conditions (2 equiv. potassium tert-butoxide in THF,
ꢁ30–08C within ꢂ3 h).
After the deprotection of 1 (Scheme 1) was achieved in an unoptimized yield
of 65%, using HF/pyridine18–20 to reassure that 5 could be deprotected
similarly, a successful synthesis route for [11C]benzoylchloride (4) was set up
with some minor modifications compared to reported methods.12,21,22 The
radiochemical esterification of 4 with 2, involving a large excess of precursor
(2) compared to 4, however, was unsuccessful after numerous efforts, although
success of this type of reaction has been shown under standard (organic)
conditions.13,14,17 Analysis of the reaction mixtures indicated that the anion of
2 was not properly formed – recovery of unreacted [11C]benzoylchloride (4) –
with the use of small amounts of base (1.5 or 5 equiv of either Et3N, BuLi,
Copyright # 2004 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 763–777

766
E. W. VAN TILBURG ET AL.
Scheme 1. Synthetic approach I: [11C]docetaxel 6a
LDA, [Me3Si]2NLi, DBU, phosphazene base P2-t-Buz or phosphazene base
P4-t-Bu}) and in some cases also with larger amounts of base (10 to 100 equiv
of BuLi, P2-t-Bu or P4-t-Bu). The use of large amounts of the bases Et3N and
[Me3Si]2NLi lead to the decomposition of 4 (no conversion to the acid,
however). The amounts of the moisture-sensitive bases used were very small
(ml range) and although every attempt was made to exclude moisture from the
system (vials, tubing, syringes, chemicals and solvents), it was unclear how
much base remained intact during handling. And although the non-
nucleophilic strong bases P2-t-Bu and P4-t-Bu (with pKa values of 33 and
42, respectively) are not sensitive to moisture, they probably were too bulky to
reach the sterically hindered 2-hydroxyl group of 2.
Synthetic approach II: [11C]docetaxel (6b)
The side chain of docetaxel was explored as an alternative for the introduction
of the 11C isotope. This could be achieved at either the carbonyl or the t-butyl
function of the BOC moiety (Figure 2(II)).y The former position involves a
z C14H39N7P2, Mw 367.46. For the structure see Fluka catolog.
} C22H63N13P4, Mw 633.73. For the structure see Fluka catolog.
Copyright # 2004 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 763–777

RADIOSYNTHESIS OF [11C]DOCETAXEL
767
reaction of the primary amine of docetaxel (10) with, e.g. [11C]phosgene23,24
theoretically yielding the corresponding [11C]isocyanate, which in turn could
react with tert-butanol to form [11C]docetaxel. However, it is known that the
synthesis of [11C]phosgene is tedious and that it has been obtained in low
radiochemical yields.25 An alternative introduction of the 11C isotope in the
carbonyl position theoretically could be the synthesis of a [11C]carbamate
ester26 of 10, which, in principle, could be thermolyzed to yield the
corresponding [11C]isocyanate as well.27,28 The radiosynthesis of the preceding
[11C]haloalkyl- or aryl chloroformate (RCHCIO11CO2CI),29 with the 11C
isotope in the carbonyl function, would require the use of either [11C]phosgene
again or an alternative with the consequence that the 11C isotope has to be
introduced in an (too) early stage of the synthesis route (time limit).
We perceived, however, that these haloalkyl- or aryl chloroformates could
alternatively be used to introduce the 11C isotope at the t-butyl function of the
BOC moiety. The synthetic potential of haloalkyl- or aryl chloroformates via
their corresponding chloroalkyl tert-butyl carbonates (RCHC1O-
CO2C(CH3)3),26,30 has not been explored in radiochemistry yet, and thus a
method was developed here that used these carbonates for the synthesis of
[11C]docetaxel. In short, this method first involved the synthesis of [11C]tert-
butanol ([2-11C]2-methylpropan-2-ol, 7), which subsequently reacted with a
haloalkyl- or aryl chloroformate to yield its corresponding [11C]chloroalkyl
tert-butyl carbonate. Reaction of the [11C]chloroalkyl tert-butyl carbonate
with the primary amine of docetaxel (10) then yielded the desired
[11C]docetaxel (6b), as discussed below.
[11C]tert-Butanol (and also [1-11C]acetate) has been described as a side
product in the synthesis of [2-11C]acetone.31–33 The hypothesis that the
formation of tertiary alcohols (7) from [11C]CO2 is a kinetically controlled
reaction of still unhydrolyzed organolithium reagent with newly formed
ketone ([2-11C]acetone) during the hydrolysis step, was consistent with the
observation that the quantity of 7 could be controlled by changing the amount
of methyl lithium (MeLi).33 The results of the present study showed that the
formation of 7 compared to [2-11C]acetone indeed was increased by using
high(er) amounts of MeLi and omitting the destruction of its excess before
hydrolysis of lithium diolate. Consequently, reaction of [11C]CO2 with 400 ml
of MeLi (1.6 M solution in ether, 640 mmol) at 158C gave 7 in a decay corrected
overall yield of 85 ꢀ 3% (n=10, Scheme 2). Subsequent quenching of the
excess of MeLi, after the formation of 7, with 350 ml of 2 M HCl/ether
(700 mmol) gave the best results with the distillation of 7.
The subsequent selection of an appropriate haloalkyl- or aryl chloroformate
to react with 7 was mainly based on the stability and (expected) reactivity of its
corresponding [11C]chloroalkyl tert-butyl carbonate. Two candidates were
1-chloroethyl tert-butyl carbonate, which is a stable compound that can easily
Copyright # 2004 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 763–777

768
E. W. VAN TILBURG ET AL.
Scheme 2. Synthesis of [11C]tert-butyl-1,2,2,2-tetrachloroethyl carbonate (9)
be distilled without significant decomposition, and 1,2,2,2-tetrachloroethyl
tert-butyl carbonate, which is a crystalline compound that is stable for months
at room temperature.26,30 The increased electrophilicity of the carbonyl
(higher reactivity) of the latter compared to 1-chloroethyl tert-butyl carbonate
has been shown to lead to almost quantitative yields of carbamates starting
from several amines.26
While the cold reaction of 1,2,2,2-tetrachloroethyl chloroformate (8) with
tert-butanol proceeded quantitatively (98%), it was less straightforward to
obtain 9 in high yields radiochemically (Scheme 2). In initial experiments, 7
was distilled first and then the solution was neutralized with pyridine in the
second reaction vial. However, neutralization in the first reaction vial prior to
distillation appeared more convenient, and in this way it was also possible to
have 8 present in the second vial (in DCM, 0–58C) during the distillation.
Subsequently, pyridine (1 equiv) was added as a base in the second vial and the
solution was kept at 458C (3 min) to form 9 in a yield of 81 ꢀ 11% (n=10,
from 7). The use of an aqueous medium for this reaction in combination with
K2CO3 as a base, so as to prevent the possible formation of side products (e.g.
the corresponding aldehyde26), did not improve the yield of this reaction
further.
The reaction conditions for the next reaction (9+10, Scheme 3) were
explored with a model compound, instead of the primary amine of docetaxel
(10), in order to save on the expensive precursor. Benzylamine (11, Table 1)
was selected as the model compound to explore the optimal conditions and
minimize the amount of 11 needed for the reaction with 9. Compound 11 was
added to the crude mixture of 9, which still contained excess of 8. This excess
appeared to complicate the reaction of 11 with 9. It seems logical that 11 will
react with the excess of 8 first, and thus relatively large amounts of 11 had to
be used (up to 3 mmol) for the synthesis of 12. Entries 2 and 3 (Table 1) show
that use of only 2 equivalents of 11 reduced the yield of 12 from 97% to 25%.
Addition of pyridine improved the reaction significantly (entries 4 and 5), up
to a yield of about 90%. A further reduction of the amount of 11
(1 equivalent) deteriorated the yield, although this was somewhat compensated
by using a higher temperature. As the amount of 11 could not be reduced
Copyright # 2004 John Wiley & Sons, Ltd.
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RADIOSYNTHESIS OF [11C]DOCETAXEL
769
Scheme 3. Synthetic approach II: [11C]docetaxel 6b
Table 1. Model reaction of a primary amine with 9
Entry 8 (mmol) 9 SPEa 11 (mmol)
eqb
py ml Temp (8C) min Yield 12 (%c)
n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
170
170
170
170
170
170
170
130
100
170
170
180
180
180
180
180
180
No
No
No
No
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
2748
824
320
320
320
165
165
137
100
82
458
137
90
55
45
(16)
(5)
(2)
(2)
(2)
(1)
(1)
(1)
(1)
(0.5)
(2.7)
(0.75) 150
(0.5)
(0.3)
–
–
–
65
65
65
65
65
70
100
100
100
110
95
95
95
95
95
(5)
(5)
(5)
(4)
(4)
(4)
(4)
(5)
(4)
(4)
(3)
(3)
(3)
(3)
(3)
(3)
(3)
98 ꢀ 2
97 ꢀ 5
25 ꢀ 2
83 ꢀ 15
91
2
2
2
2
1
1
5
2
4
2
3
1
4
1
2
1
3
50
150
150
150
150
150
150
150
25
64 ꢀ 15d
4 ꢀ 5e
–
43 ꢀ 3
99 ꢀ 0
96
87 ꢀ 15
97
94 ꢀ 1
92
14 ꢀ 12
150
150
(0.25) 150
(0.15) 150
(0.10) 150
27
18
95
95
a Solid phase extraction (SPE) purification of 9 (Sep-Pak tC18 Plus).
b Equivalents of 11 compared to 8.
c Calculated starting from 9 (vial 2).
d Also tried in 150 ml Et3N instead of pyridine or pyridine in combination with either 80 mmol
phenylpiperazine or 2,2,4,4 tetramethylpiperidine, without improved results.
e 9 was not (properly) formed.
further, the next optimization step was to use less 1,2,2,2-tetrachloroethyl
chloroformate (8) in the prior reaction. From entries 8 and 9 it is clear,
however, that at least 170 mmol of 8 was needed for a proper synthesis of 9
(and thus for the synthesis of 12) and that it was not possible to further reduce
the excess of 8. A solution to this problem was found in a solid-phase
Copyright # 2004 John Wiley & Sons, Ltd.
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E. W. VAN TILBURG ET AL.
Table 2. Optimizing reaction conditions for the synthesis of 6b (Scheme 3)
Entry
10a
(mmol)
Temp (8C)
min
Yield
Overall yield
eqb
6b (%c)
6b (%d)
n
Remarks
1
2
3
4
48
55
105
105
(0.27)
(0.30)
(0.59)
(0.59)
80 (3)
50 (3)
70 (3)
45 (6)
2
4
0.6
2
2 ꢀ 2
10 ꢀ 1
1
1
2
5
No reaction
No reaction
Decomposition
5 ꢀ 2
19 ꢀ 3
a Mixtures containing 9 were purified on Sep-Pak tC18 Plus prior to reaction with 10.
b Calculated starting from 9 (vial 2).
c Decay corrected from 9 (vial 2).
d Decay corrected from [11C]CO2.
extraction of the crude mixture containing 9 and 8, prior to the reaction with
11. The mixture was diluted with water (hydrolysis of the excess of 8) and
loaded onto a Sep-Pak tC18 Plus. Subsequently, 9 was eluted with THF and
collected in a vial for the reaction with 11. With this method 9 could be
synthesized in high yields, and compound 12 could be obtained in good yields
as well, with a reduced amount of 11 (0.15 equivalents, entries 7,11–17).
These optimized conditions (Table 1, entry 16) were then used for the
reaction with the primary amine of docetaxel (10) and 9. Neither 0.27 equiv
(48 mmol, 35 mg) nor 0.30 equiv (55 mmol, 40 mg) of 10, however, yielded any
product 6b (entries 1 and 2, Table 2) despite the proper formation of 9. The
required amount of 10 appeared to be 0.59 equiv (105 mmol, entry 4).
Furthermore, at high temperatures (708C, entry 3) decomposition of the
starting material (9) occurred. HPLC analysis showed that prolonged reaction
times (longer than 6 min) had no beneficial effect on the yield of 6b (due to
decay). Finally, using 0.59 equiv of 10 (105 mmol, 75 mg) at a temperature of
458C for 6 min (entry 4) gave optimal decay corrected overall yields of 6b
(10 ꢀ 1%).
Experimental
Materials and methods
Docetaxel was a gift from Aventis Pharma B.V. (Hoevelaken, the Nether-
lands). Unless otherwise stated, all other chemical reagents were purchased
from Aldrich (Aldrich Chemie, Sigma-Aldrich Chemie BV, Zwijndrecht, The
Netherlands). Solvents were purified and dried by standard procedures before
use. Thin-layer chromatography (TLC) was carried out using aluminum sheets
(20 ꢃ 20 cm2) with silica gel F254 from Merck. Spots were visualized under UV
(254 nm). For monitoring radiochemical reactions, TLC plates were scanned
using a Molecular Dynamics Storm 820 (Storm scanner control version 5.01)
phosphorimager and quantified using the software package ImageQuant
Copyright # 2004 John Wiley & Sons, Ltd.
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RADIOSYNTHESIS OF [11C]DOCETAXEL
771
(v5.2). Preparative column chromatography was performed on silica gel (230–
400 mesh ASTM). 1H- and 13C-NMR spectra were recorded on a Bruker AC-
200 or an MSL 400 spectrometer. Chemical shifts (d) were determined relative
to the solvent and converted to the tetramethylsilane scale using d=3.35 ppm
for CD3OD and 7.26 ppm for CDCl3. The assignment was confirmed using
1
1H-1H COSY and H-13C COSY NMR spectra (for numbering see Figure 1).
[11C]Carbon dioxide was produced by the 14N(p,a)11C nuclear reaction using
an IBA 18/9 cyclon.31 Abbreviations: tetrahydrofuran (THF), pyridine (py),
dichloromethane (DCM), ethyl acetate (EtOAc), N,N-dimethylformamide
(DMF), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), lithium diisopropylamide
(LDA) and methyl lithium (MeLi).
Synthethic approach I: [11C]docetaxel (6a)
20-(tert-butyldimethylsilyl)-docetaxel. Docetaxel (55 mg, 68.2 mmol) was dis-
solved in DMF (600 ml). To the mixture were added imidazole (23 mg,
0.34 mmol, 5 eq.) and tert-butyldimethylsilylchloride (51 mg, 0.34 mmol, 5 eq.)
and the solution was stirred at 608C for 2 h. After cooling down, again 5
equivalents of both imidazole and tert-butyldiethylsilylchloride were added.
The mixture was stirred at room temperature overnight. The mixture was
diluted with ethyl acetate and extracted with water (2 ꢃ 30 ml) and brine
(1 ꢃ 30 ml). The organic layer was dried (MgSO4), concentrated and purified
by column chromatography ðEtOAc=hexane ¼ 1=2Þ: Yield 62 mg (66.8 mmol,
l
98%), Rf 0.87 (EtOAc); H NMR (200 MHz, CDCI3) d 8.05 (d, 2 H,
J=7.02 Hz, benzoylortho), 7.53 (t, 2 H, J=7.1 l Hz, benzoylpara), 7.43 (q, 2 H,
J=7.6 l Hz, benzoylmeta), 7.38–7.19 (m, 5 H, phenyl), 6.26 (t, 1 H, J=8.6 Hz,
H-13), 5.63 (d, lH, J=7.08 Hz, H-2), 5.47 (d, 1 H, J=9.51 Hz, H-30), 5.20
(s, 1 H, H-10), 4.92 (d, 1 H, J=7.88 Hz, H-5), 4.46 (m, 1 H, H-20), 4.30–4.18
(m, 3 H, H-7, H-20), 3.88 (d, 1 H, J=7.07 Hz, H-3), 2.49 (s, 3 H, COCH3),
2.48–2.10 (m, 3 H, 1 ꢃ H-6, H-14), 1.85 (s, 4 H, 1 ꢃ H-6, H-18), 1.70 (s, 3 H, H-
19), 1.24 (s, 9 H, OC(CH3)3), 1.19 (s, 3 H, H-l6), 1.15 (s, 3 H, H-l7), 0.68 (s, 9 H,
SiC(CH3)3), ꢁ0.19 (s, 3 H, SiCH3), ꢁ0.37 (s, 3 H, SiCH3) ppm.
20-(tert-butyldimethylsilyl)-7,10-di-(triethylsilyl)-docetaxel (1). 20-(tert-butyl-
dimethylsilyl)-docetaxel (62 mg, 66.8 mmol) was dissolved in DMF (500 ml). To
the mixture were added imidazole (50 mg, 0.74 mmol) and triethylsilylchloride
(105 m1, 0.63 mmol) and the solution was stirred overnight at room
temperature. The mixture was diluted with EtOAc and extracted with water
(2 ꢃ 30 ml) and brine (1 ꢃ 30 ml). The organic layer was dried (MgSO4),
concentrated and purified by column chromatography (EtOAc/hexane= 1/4).
Yield 65 mg (56.5 mmol, 83%), Rf 0.42 (EtOAc/hexane= 1/4); 1H NMR
(400 MHz, CDC13) d 8.04 (d, 2 H, J=7.00 Hz, benzoylortho), 7.52 (t, 2 H,
J=7.10 Hz, benzoylpara), 7.38 (q, 2 H, J=6.89 Hz, benzoylmeta), 7.31–7.20 (m,
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E. W. VAN TILBURG ET AL.
5 H, phenyl), 6.18 (t, 1 H, J=8.6 Hz, H-13), 5.61 (d, 1 H, J=6.99 Hz, H-2),
5.35–5.26 (m, 1 H, H-30), 5.10 (s, 1 H, H-10), 4.87 (d, 1 H, J=8.21 Hz, H-5),
4.46 (m, 1 H, H-20), 4.33 (m, 1 H, H-7), 4.18 (q, 2 H, J=16.34 Hz, H-20), 3.81
(d, 1 H, J=6.97 Hz, H-3), 2.49 (s, 3 H, COCH3), 2.48–1.85 (m, 3 H, 1 ꢃ H-6,
H-14), 1.78 (s, 4 H, 1 ꢃ H-6, H-18), 1.61 (s, 3 H, H-19), 1.26 (s, 9 H,
OC(CH3)3), 1.16 (s, 3 H, H-16), 0.90 (t, 12 H, J=8.05 Hz, H-17, SiCH2CH3),
0.68 (s, 9 H, SiC(CH3)3), 0.52 (q, 12 H, =8.01 Hz, SiCH2CH3), ꢁ0.19 (s, 3 H,
SiCH3), ꢁ0.38 (s, 3 H, SiCH3) ppm; 13C NMR (100-MHz, CDC13) d 200.87
(C-9), 171.38 (C-10), 170.20 (COCH3), 167.09 (COC6H5), 155.13 (NHCO),
139.41 (phenyl), 138.53 (C-12), 135.83 (C-11), 133.52 (benzoylpara), 130.18
(benzoylortho), 129.29 (benzoyl), 128.66 (benzoylmeta), 128.53 (phenylortho),
127.64 (phenylpara), 126.42 (phenylmeta), 84.28 (C-5), 81.00 (C-4), 79.89
(OC(CH3)3), 79.13 (C-1), 76.58 (C-20), 75.70 (C-20), 75.11 (C-2), 74.01 (C-
10), 72.79 (C-7), 71.34 (C-13), 57.73 (C-8), 56.21 (C-30), 46.45 (C-3), 43.26 (C-
15), 37.23 (C-6), 35.75 (C-14), 28.13 (OC(CH3)3), 26.64 (C-16), 25.45
(SiC(CH3)3), 22.97 (COCH3), 21.08 (C-17), 18.16 (SiC(CH3)3), 14.20 (C-18),
10.14 (C-19), 6.73, 6.47 (SiCH2CH3), 5.17, 4.27 (SiCH2CH3), ꢁ5.39, ꢁ5.97
(SiCH3) ppm.
20-(tert-butyldimethylsilyl)-2-debenzoyl-7,10-di-(triethylsilyl)-docetaxel (2). 1
(65 mg, 57.4 mmol) was dissolved in THF (1 ml). The mixture was cooled (ice/
salt bath) and K-t-OBu (13 mg, 115 mmol, 2 eq.) and H2O (1.2 ml, 1.2 eq.) were
added. The mixture was stirred at ꢁ308C (cold room) for 1 h. Then it was
allowed to reach 08C in 3.5 h, and was stirred at 08C for 0.5 h. The mixture was
quenched with a saturated NH4Cl solution. The aqueous layer was extracted
with EtOAc (2 ꢃ 50 ml) and the combined organic layers were washed with
H2O (2 ꢃ 50 ml) and brine (1 ꢃ 75 ml), dried (MgSO4) and concentrated. The
crude mixture was purified by column chromatography (EtOAc/hexane= 1/2).
1
Yield 34.2 mg (32.7 mmol, 57%), Rf 0.21 (EtOAc/hexane= 1/2); H NMR
(400 MHz, CDCl3) d 7.36–7.17 (m, 5 H, phenyl), 6.17 (t, 1 H, J=10 Hz, H-13),
5.46 (d, 1 H, J=9.14 Hz, NH), 5.21 (d, 1 H, J=8.63 Hz, H-30), 5.09 (s, 1 H,
H-10), 4.94 (d, 1 H, J=9.39 Hz, H-5), 4.62 (q, 2 H, J=12.69 Hz, H-20), 4.44
(bs, 1 H, H-20), 4.34 (dd, 1 H, J=10.15 Hz, H-7), 3.93–3.88 (m, 1 H, H-2),
3.50–3.47 (m, 1 H, H-3), 2.56–2.47 (m, 1 H, H-6), 2.39 (s, 3 H, COCH3),
2.20–2.06 (m, 1 H, H-14), 1.98–1.88 (m, 1 H, H-6), 1.79 (s, 4 H, H-14, H-18),
1.64 (s, 3 H, H-19), 1.42 (s, 9 H, OC(CH3)3), 1.21 (s, 3 H, H-16), 1.11 (s, 3 H,
H-17), 1.01–0.94 (m, 18 H, SiCH2CH3), 0.74 (s, 9 H, SiC(CH3)3), 0.68–0.52 (m,
12 H, SiCH2CH3), ꢁ0.11 (s, 3 H, SiCH3), ꢁ0.34 (s, 3 H, SiCH3) ppm; 13C
NMR (100.1 MHz, CDC13) d 205.87 (C-9), 171.64 (C-l0), 169.56 (COCH3),
155.27 (NHCO), 138.82 (phenyl), 137.94 (C-11), 134.08 (C-12), 128.49, 127.67,
126.47 (phenyl), 83.65 (C-5), 82.57 (C-4), 80.01 (OC(CH3)3), 78.28 (C-1), 78.12
(C-20), 76.47 (C-10, C-20), 74.82 (C-2), 72.79 (C-7), 72.15 (C-13), 58.34 (C-8),
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RADIOSYNTHESIS OF [11C]DOCETAXEL
773
56.79 (C-30), 46.63 (C-3), 42.80 (C-15), 37.45 (C-6), 35.68 (C-14), 28.24
(OC(CH3)3), 26.04 (C-16), 25.48 (SiC(CH3)3), 23.07 (COCH3), 20.73 (C-17),
18.12 (SiC(CH3)3), 13.64 (C-18), 10.70 (C-19), 6.94, 6.87 (SiCH2CH3), 6.08,
5.28 (SiCH2CH3), ꢁ5.35, ꢁ5.97 (SiCH3) ppm.
Docetaxel. 1 (14 mg, 12.2 mmol) was dissolved in dry cold THF (500 ml, 58C) in
a Teflon flask. HF/pyridine (420 ml) was added and the solution was stirred at
room temperature for 2 h, and subsequently at 408C for 30 min. The solution
was diluted with EtOAc (30 ml). The organic layer was washed successively
with 5% NaHCO3 (2 ꢃ 10 ml), 5% HC1 (2 ꢃ 10 ml), water (10 ml) and brine
(15 ml). The organic phase was dried (MgSO4) and concentrated. The crude
mixture was purified by column chromatography (EtOAc/hexane= 1/1).
Yield 6.4 mg (7.9 mmol, 65%), Rf 0.08 (EtOAc/hexane= 1/2); 1H NMR
(400 MHz, CDCl3) d 8.09(d, 2 H, J=6.99 Hz, benzoylortho), 7.61 (t, 2 H,
J=7.10 Hz, benzoylpara), 7.50 (t, 2 H, J=6.89 Hz, benzoylmeta), 7.33–7.23 (m,
5 H, phenyl), 6.21 (t, 1 H, J=8.40 Hz, H-13), 5.65 (d, 1 H, J=6.99 Hz, H-2),
5.28–5.24 (m, 1 H, H-30), 5.18 (s, 1 H, H-10), 4.94 (d, 1 H, J=8.21 Hz, H-5),
4.61 (d, 1 H, H-20), 4.33–4.14 (m, 1 H, H-7), 4.33–4.14 (m, 2 H, H-20), 3.92 (d,
1 H, J=6.89 Hz, H-3), 2.64–2.53 (m, 1 H, H-6), 2.36 (s, 3 H, COCH3), 2.27 (m,
2 H, H-14), 1.88–1.79 (m, 1 H, H-6), 1.84 (s, 3 H, H-18), 1.76 (s, 3 H, H-19),
1.34 (s, 9 H, OC(CH3)3), 1.25 (s, 3 H, H-16), 1.14 (s, 3 H, H-17) ppm; 13C
NMR (100 MHz, CDCl3) d 211.36 (C-9), 170.26 (C-10), 169.20 (COCH3),
167.02 (COC6H5), 141.21 (phenyl), 138.40 (C-12), 135.95 (C-11), 133.69
(benzoylpara), 130.16 (benzoylortho), 129.15 (benzoyl), 128.83 (benzoylmeta),
128.70 (phenylortho), 127.07 (phenylpara), 126.75 (phenylmeta), 84.08 (C-5),
81.07 (C-4), 80.22 (OC(CH3)3), 78.82 (C-1), 76.70 (C-20), 74.79 (C-2), 74.54
(C-10), 73.65 (C-20), 72.46 (C-13), 72.02 (C-7), 57.64 (C-8), 57.10 (C-30), 46.46
(C-3), 43.07 (C-15), 37.06 (C-6), 35.74 (C-14), 28.19 (OC(CH3)3), 26.44 (C-16),
22.70 (COCH3), 22.56 (C-17), 14.36 (C-18), 9.84 (C-19) ppm.
Radiosynthesis of [a-11C]benzoylchloride (4). [a-11C]benzoylchloride was
prepared according to published methods with minor modifications.21,22
Briefly, at the end-of-bombardment (EOB), the [11C]CO2 was transferred from
the target with a helium flow of 1000 ml/min and trapped in a stainless-steel
coil cooled in liquid nitrogen. The cooling bath was removed and the [11C]CO2
was transferred to the silicone septum sealed reaction vial via a helium sweep
gas (10–15 ml/min). The reaction vial was purged with helium and charged
with 75 ml of 2 M phenylmagnesiumchloride (Aldrich), diluted with 225 ml of
dry THF. The amount of [11C]CO2 trapped usually was >95%. After
obtaining maximum radioactivity in the vial (ꢂ4 min), the sweep gas was
switched off and the vial was heated at 558C for 2 min and then cooled to 218C.
The mixture was treated with 150 ml of 1 M HCl/ether and SOC12 (0.3 mmol in
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774
E. W. VAN TILBURG ET AL.
THF) and heated at 658C for 3 min. The excess of chemicals and solvent was
removed by keeping the vial at 688C and purging with helium (50 ml/min
above the solution). During this process some of the activity was lost (trapped
on the ascarite column, ꢂ30%). THF was added to the reaction vial (500 ml)
and subsequently [11C]benzoylchloride (4) was distilled for 5 min using helium
sweep gas (ꢂ20 ml/min) from the vial (908C) to a vacuumized second vial
containing solvent at ꢁ708C. Finally, the vacuum and the sweep gas were
removed.
20-(tert-butyldimethylsilyl)-7,10-di-(triethylsilyl)-[11C]docetaxel (5). [a-11C]
benzoylchloride (4) was prepared and distilled as described above. In general,
the second vial contained precursor 20-(tert-butyldimethylsilyl)-2-debenzoyl-
7,10-di-(triethylsilyl)-docetaxel (2, 4 mg, 3.8 mmol) in 400 ml dioxane that was
treated with a base 1–3 min (r.t) prior to distillation of 4 into this vial. After
distillation, the mixture was heated at 45–508C for 3–6 min. The base used was
either Et3N (10 or 100 equiv), BuLi (1, 5, 10 or 50 equiv), LDA (1.5 or 5
equiv), [Me3Si]2NLi (1.5, 5 or 10 equiv), DBU (1.5 or 5 equiv), phosphazene
base P2-t-Buz (1.5, 5,10 or 15 equiv) or phosphazene base P4-t-Bu} (10 equiv).
Synthetic approach II: [11C]docetaxel (6b)
The primary amine of docetaxel (10). Docetaxel (290 mg, 0.36 mmol) was
dissolved in 25 ml of concentrated formic acid, and the solution was stirred for
4 h at room temperature. The mixture was concentrated and coevaporated
with toluene (4 ꢃ ). The residue obtained was extracted with a NaHCO3
solution (5% w/v, 2 ꢃ 40 ml) and EtOAc (3 ꢃ 25 ml). The organic layer was
dried and concentrated, and a white powder was obtained. This was purified
by column chromatography (EtOAc/MeOH = 95/5). Yield 230 mg
1
(0.33 mmol, 92%). H NMR (400 MHz, MeOD) d 8.04 (d, 2 H, J=8.80 Hz,
benzoylortho), 7.70 (t, 1 H, J=7.20 Hz, benzoylpara), 7.59 (t, 2 H, J=7.20 Hz,
benzoylmeta), 7.55–7.48 (m, 4 H, phenylortho+meta), 7.43–7.38 (m, 1 H,
phenylpara), 6.08 (t, 1 H, J=7.20 Hz, H-13), 5.60 (d, 1 H, J=5.60 Hz, H-2),
5.23 (s, 1 H, H-10), 4.96 (m, 1 H, H-5), 4.59 (s, 2 H, H-20, H-30), 4.19–4.22 (m,
1 H, H-7), 4.14 (s, 2 H, H-20), 3.80 (d, 1 H, J=7.20 Hz, H-3), 2.48–2.39 (m,
1 H, H-6), 2.20 (s, 3 H, H-21), 2.01–1.90 (m, 1 H, H-14), 1.88 (s, 3 H, H-18),
1.87–1.78 (m, 1 H, H-6), 1.77–1.69 (m, 1 H, H-14), 1.68 (s, 3 H, H-19), 1.12 (s,
3 H, H-16), 1.09 (s, 3 H, H-17) ppm; 13C NMR (100 MHz, MeOD) d 210.18
(C-9), 171.57 (C-10), 170.71 (COCH3), 166.58 (COC2H5), 137.81 (C-12), 137.13
(C-11), 133.71 (benzoylpara), 133.59 (phenyl), 130.42 (benzoyl), 130.07
(benzoylortho, phenylpara), 129.51 (phenylortho), 128.70 (benzoylmeta), 128.13
(phenylmeta), 84.90 (C-5), 81.30 (C-4), 78.15 (C-1), 76.56 (C-20), 75.35 (C-2),
74.59 (C-10), 73.47 (C-20), 71.66 (C-7), 71.53 (C-13), 57.98 (C-30), 57.87 (C-8),
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RADIOSYNTHESIS OF [11C]DOCETAXEL
775
46.87 (C-3), 43.43 (C-15), 36.47 (C-6), 35.75 (C-14), 25.97 (C-16), 22.22 (C-21),
20.58 (C-17), 13.46 (C-18), 9.51 (C-19) ppm.
Radiosynthesis of [11C]tert-butanol ([2-11C]2-methylpropan-2-ol, 7). A reac-
tion vial was purged with helium and charged with 400 ml MeLi (1.6 M in
ether). The [11C]CO2 was transferred to the vial (158C). After obtaining
maximum radioactivity in the vial (ꢂ3 min), the sweep gas was switched off
and the mixture was treated with 350 ml of 2 M HCl/ether and 65 ml of
pyridine. The product was analyzed by HPLC (Kromasil 100 RP C-18 column
250 ꢃ 4.6 mm; CH3CN/H2O 20/80, 1 ml/min, 254 nm). The retention time was
5.8 min and the overall decay corrected radiochemical yield was 85 ꢀ 3%
(n=10).
Radiosynthesis of [11C]tert-butyl-1,2,2,2-tetrachloroethyl carbonate (9). [11C]tert-
butanol (7) was distilled (He-flow 15 ml/min) from the reaction vial (1008C) to a
second vial (108C) containing 0.20 mmol (30 ml) of 1,2,2,2-tetrachloroethyl
chloroformate (8) in 200ml of DCM. After completion of the distillation (5–
6 min) the sweep gas was removed, and 0.27 mmol (22 ml) of pyridine (in 100 ml
THF) was added. The mixture was heated at 608C (3 min) and then cooled to
208C. The product was analyzed by HPLC (Partisil 10 mm Silica column
250 ꢃ 4.6 mm; hexane/EtOAc 80/20; 0.7 ml/min, 254 nm). The retention time was
9.3 min and the decay corrected radiochemical yield was 81 ꢀ 11% (n=10, from
7) and (62ꢀ 9% (from [11C]CO2).
Radiosynthesis of [11C]docetaxel (6b). The mixture containing [11C]tert-
butyl-1,2,2,2-tetrachloroethyl carbonate (9) was diluted with water (18 ml),
and loaded onto a Sep-Pak tC18 Plus. Compound 9 was eluted with THF
(700 ml) and collected in another reaction vial and the solution was cooled
(108C). Subsequently, 75 mg (0.11 mmol) of the primary amine of docetaxel
(10, in 225 ml THF) and pyridine (150 ml) were added. The mixture was heated
at 458C (6 min) and then cooled to ambient temperature. The product was
analyzed by HPLC, using either a Kromasil 100 RP C-18 column
(250 ꢃ 4.6 mm; CH3CN/H2O 20/80, 1 ml/min, 221 nm with a retention time
of 12.6 min) or using a LiChrosphere 100 RP C-8 column (150 ꢃ 4.6 mm;
MeOH/H2O 65/35, 1 ml/min, 221 nm with a retention time of 12.3 min). The
decay corrected radiochemical yield was 19 ꢀ 3% (n=5, from 9); and
10 ꢀ 1% (n=5, from [11C]CO2).
Conclusions
Methods that are able to predict response to docetaxel treatment would be
very useful to select the non-responders from the responders. Then, alternative
therapies could be offered at a much earlier stage. The radiosynthesis of
[11C]docetaxel 6b was developed using a two-pot procedure. The synthesis
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E. W. VAN TILBURG ET AL.
started with [11C]tert-butanol, that in turn was reacted with 1,2,2,2-
tetrachloroethyl chloroformate to give [11C]tert-butyl-1,2,2,2-tetrachloroethyl
carbonate. Finally, the [11C]tert-butoxycarbonylation of the free amine of
docetaxel gave [11C]docetaxel 6b. Research concerning the purification of 6b
via a semipreparative HPLC system, as well as its formulation is in progress.
The pharmacokinetics of a tracer dose of docetaxel may be studied afterwards.
The main objective will then be to assess whether uptake of [11C]docetaxel by
breast cancer tumor deposits has prognostic value for response to therapeutic
doses.
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