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Asymmetric synthesis of enantiomerically pure
18Ffluorinated amino acids for tumor imaging by
PET
R. N. Krasikova1, O. S. Fedorova1, O. F.
Kuznetsova1, V. I. Maleev2, Yu. N. Belokon2
1Institute of the Human Brain, RAS,
St.-Petersburg 2A.N. Nesmeyanov Institute of
Organo-Element Compounds RAS, Moscow
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Conventional PET radionuclides
15O
13N
11?
18F
2,04 min
9,96 min
20,4 min
109,8 min
Only 18F-radiotracers can be shipped away from
satellite cyclotron/radiochemistry facilities

• 96.9 ?
• Emax 0.635 MeV

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PET is the most expensive and complex nuclear
imaging technique
Radionuclide production in cyclotron
Transport to hot cell

• Radiotracer synthesis
• Formulation
• Filter sterilization

Dose dispensing
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PET in Russia
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Institute of the Human Brain (founded in 1991)

• Belongs to Russian Academy of Science
• 10 research laboratories and hospital
• First PET scanner in Russia (1991)

Research building
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18F-ratiotracers for oncology (excluding FDG)
AA transport
Amino acids
Cell proliferation
Nucleosides
Monitoring of gene therapy
Nitroimidazoles
Hypoxic tissues
Receptor status Apoptosis Angiogenesis
Peptides
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18F-labeled amino acids as metabolic tracers for
PET imaging of brain tumors

• malignat tumors represent a hypermetabolic state
  
• high uptake of 18F-labeled AA in tumor cells is
  due to increased transport via LAT- transport
  system
• 18F-labelled amino acids are not incorporated
  into proteins

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18F-labelled amino acids and FDG in PET imaging
of brain tumors

• FDG exhibits phisiologically high cortical
  metabolism (limitations of PET in detecting low
  grade gliomas)
• Due to low physiological uptake in a gray matter
  PET with labelled amino acids provides higher
  tumor/non-tumor contrast compared to FDG

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18F-labelled aminoacids in differentiation of
brain tumor and inflammation

• FDG shows high non-specific uptake in
  inflammatory cells and granulation tissues
• Labelled amino acids have lower uptake in
  inflammation tissues and may allow
  differentiation of tumor from inflammation

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The role of C-11 and F-18 labelled amino acids in
PET
So far L-11C-methyl-methionine remains a major
amino acid tracer for PET evaluation of brain
tumors
Routine clinical application of 18F-labelled
amino acids is still limited due to difficulties
in synthesis
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ISTC 2780 Development of novel methods for the
synthesis of enantiomerically pure fluorinated
amino acids labelled with fluorine-18,
radiotracers for positron emission tomography
(PET).

• INEOS RAS, Moscow, Russia
• IHB RAS, St.-Petersburg, Russia
• YSU, Yerevan, Republic of Armenia

• PET collaborators
• Karolinska Institute, Stockholm, Sweden
• ABX, Berlin, Germany
• IASON, Linz, Austria

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General demands to synthetic methodology for
18F-fluorinated amino acids

• Enantiomeric purity gt95
• Possibilty to produce both L- and D-enantiomers
• Easy automation
• Synthesis time not exceeding 2-3 hours

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New synthetic methods for 18F-labelled amino
acids within ISTC 2780 study
6-18F-fluoro-L-DOPA
2-18F-fluoro-L-tyrosine
O-(2-18Ffluoroethyl-L-tyrosine
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18F-fluorinated tyrosines label in the alkyl
chain
FET O-(2-18Ffluoroethyl-L-tyrosine
HJ Wester et al. J Nucl Med 1999 K Hamacher, HH
Coenen Appl Rad Isot 2002
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Diagnostic potential of FET in PET imaging of
brain tumours

• FET uptake in brain tumors is similar to that of
  L-11CMethionine (Weber et al. 2000)
• tumor-to-brain contrast for PET scans with FET
  is superior to that for 123I-IMT SPECT scans
  (Pauleit et al. 2004)
• in contrast to FDG, FET is not taken up by
  inflammatory tissues in patients with head and
  neck cancer (Pauleit et al. 2006)

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Two stage synthesis of O-(2-18Ffluoroethyl-L-ty
rosine via 18F-alkylation
HJ Wester et al. J Nucl Med 1999
Two semi-preparative HPLC purifications
difficult to automate
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Direct synthesis of O -(2-18Ffluoroethyl)tyrosi
ne
RCY 35 (EOS), 80 min synthesis
K Hamacher HH Coenen 2002
O-(2-tosyloxyethyl)-N-trityl-L-tyrosine
tert.butylester
95 Euro/15 mg ABX, Germany
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New type of precursor for 18FFET
ISTC 2780

• 18F incorporation rate
• OTs 60
• OMs 60
• OTf 5-10

Ni-BPB-Tyr-OCH2CH2OTos
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Synthesis of 18FFET using new precursor
advantages

• high stereospecificity and stability of
  precursor
• easy hydrolysis/deprotection under aqueous
  acidic conditions
• easy to automate
• radiochemical yield, not optimized, EOB, 25
• synthesis time (robotic) 55 min

ISTC 2780
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18F-fluorinated aromatic amino acids 18F in
benzene ring
6-18F-fluoro-L-DOPA
2-18F-fluoro-L-tyrosine
Garnett ES, Nature, 1983
HH Coenen et al J Nucl Med 1989 R Hustinx et al
J Nucl Med 2003
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1. 6-18FFluoro-L-DOPA application in PET
neurology
L-3,4-dihydroxyphenylalanine (L-DOPA) is a
precursor for the neurotransmitter dopamine
6-18FFluoro-L-DOPA (6-FDOPA), a tracer
analogue of L-DOPA, allows assessment of the
integrity of dopaminergic system implicated in
Parkinsons disease and other disorders
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2. 6-18F-fluoro-L-DOPA application in PET
oncology
Since recently 6-FDOPA has been applied as a
tumor seeking agent for PET

• A Becherer et al. Eur J Nucl Med Molec Imaging
  2003 brain tumors
• S Hoegerle et al. Eur J Nucl Med 2001
• medullary thyroid carcinoma

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Regiospecific fluoro-de-stannylation reactions in
the synthesis of 6-18F-fluoro-L-DOPA

• Namavari et al. Appl Rad Isot 1992
• E.F.J. De Vries et al, Appl Rad Isot, 1999

• commercially available stannyl precursor
• high stereospecificity
• automated production using standard module
• radiochemical yield 334

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2-18Ffluoro-L-tyrosine electrophilic synthesis
2-18Ffluoro-L-tyrosine, 21 RCY
Hess et al., Appl Rad Isot 2002
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Electrophilic fluorinations using 18FF2

Isotope production gas target in presence of
carrier (0.4 of F2)
18O(p,n)18F 20Ne(d,a)18F
Fluorinations 50 theoretical yield
Low specific activity
Crucial for toxic compounds or receptors
radioligands
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Why nucleophilic approach is attractive?
Electrophilic 18FF2
Nucleophilic 18Ffluoride

• some PET cyclotrons have no deuterons to run
  20Ne(d,a)18F reaction

18Ffluoride is available in high amounts from
18O(p,n)18F reaction in a small volume water
target Up to 12 Ci, no carrier added

• complex two-steps irradiation protocol is
  required for recovery of radionuclide produced
  via 18O(p,n)18F reaction in a gas target

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18F introduction into benzene ring of aromatic
amino acids via nucleophilic approach
Direct nucleophilic labeling is not possible
non-enantioselective
racemic L / D isomers
enantioselective (asymmetric)
Enantiomerically pure form
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Asymmetric synthesis of 18F-fluorinated aromatic
alpha amino acids
Phase transfer catalysis (PTC)
Stoichiometric
Diastereoselective alkylation of carbanions
derived from Gly-based chiral auxiliaries
Alkylaltion of achiral substrate using chiral
phase transfer catalyst

Yu.N. Belokon et al JACS 2003
ISTC 2780
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Stoichiometric asymmetric synthesis of
18F-labelled alpha amino acids
Lemaire C et al. 1993 Imidazolidinone chiral
inductor alkylation at -78oC
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Stoichimetric asymmetric synthesis of 6-FDOPA
using chiral Ni (II) complex of Schiff base of
(S)-O-(N-benzylprolyl)amino benzophenone and
glycine

• Enantiomeric purity
• of 6-FDOPA
• Ni-BPB-Gly 88
• Ni-CPB-Gly 95
• Ni-CBPB-Gly 92

Krasikova et al. Radiochem. 2006 in press
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The longest step is preparation of 18F-labelled
alkylating agent 55 min, radiochemical yield
45 (EOB)
6-18Ffluoro piperonil bromide
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Phase transfer catalytic approach in asymmetric
synthesis of 18F-labelled alpha amino acids
Achiral glycine complex
C. Lemaire et al. Eur J Org Chem 2004
6-18Ffluoro-L-dopa
Chiral catalyst (cinhonidium salt)
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Phase transfer catalytic alkylation in presence
of (S)-NOBIN
Chiral catalyst (S)-NOBIN
Achiral Ni (II) complex of Schiff base of
glycine and two-benzoylphenyl amide of
pyridine-two-carboxylic acid
Yu. N. Belokon et al Angew Chem Int Ed 2001
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PTC synthesis of 2-18F-fluoro-L-tyrosine 1.
Preparation of alkylating agent
Synthesis time - 55-60 min starting from
18F-fluoride
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PTC synthesis of 2-18F-fluoro-L-tyrosine 2.
Chiral alkylation and hydrolysis
R. Krasikova et al. Nucl Med Biol 2004
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PTC synthesis of 18F-labelled alpha aminoacids
using (S)-NOBIN

• Mild alkylation conditions Easy to automate
• (S)-NOBIN is cheap commercially available chiral
  catalyst
• Enantiomeric purity 96-98 at room temperature
  alkylation
• Radiochemical yield (EOB) 10-12 for FDOPA
  20-25 for 2-FTYR

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June 2006, ISTC 2780 project
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Acknowledgments

• PET radiochemistry studies at the IHB are
  supported
• by Swiss National Science Foundation,
• grant SUPJO62161.00/1
• by Scientific Center of St.-Petersburg, Russian
  Academy of Science
• by ISTC grant 2780