Synthesis and preliminary biological evaluations of [18F]-1- deoxy-1-fluoro-scyllo-inositol

Article abstract of DOI:10.1039/b913317h

A novel PET radiotracer, [¹⁸F]-1-deoxy-1-fluoro-scyllo-inositol, was synthesized via a one-pot reaction in 16 ± 3% uncorrected radiochemical yield within 80 minutes; although this compound revealed low brain penetration it shows promise in rode

Full text of DOI:10.1039/b913317h

Chemical Communications
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Number 37 | 7 October 2009 | Pages 5465–5644
ISSN 1359-7345
COMMUNICATION
Neil Vasdev et al.
Synthesis and preliminary biological Min Shi et al.
FEATURE ARTICLE
18
evaluations of [ F]-1-deoxy-1-fluoro- Recent extensions of the Morita–
scyllo-inositol Baylis–Hillman reaction

COMMUNICATION
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Synthesis and preliminary biological evaluations of
[18F]-1-deoxy-1-fluoro-scyllo-inositolw
Neil Vasdev,*ab Jason Chio,c Erik M. van Oosten,ac Mark Nitz,c JoAnne McLaurin,d
Douglass C. Vines,ef Sylvain Houle,ab Raymond M. Reillyg and Alan A. Wilsonab
Received (in College Park, MD, USA) 6th July 2009, Accepted 22nd July 2009
First published as an Advance Article on the web 11th August 2009
DOI: 10.1039/b913317h
A novel PET radiotracer, [18F]-1-deoxy-1-fluoro-scyllo-inositol,
was synthesized via a one-pot reaction in 16 Æ 3% uncorrected
radiochemical yield within 80 minutes; although this compound
revealed low brain penetration it shows promise in rodent tumour
models for breast cancer imaging.
(4; Fig. 1b), was shown by our group to be effective at blocking
Ab1–42 aggregation in vitro.11 The objectives of the present
work were to radiolabel 4 with 18F and evaluate this new
PET radiopharmaceutical for potential use in imaging the
central nervous system and human breast cancer xenografts
in rodents.
Transformation of scyllo-inositol to the [18F]-labelled
derivative, [18F]4, required stereoselective substitution of one
hydroxyl group for a fluorine atom. A one-pot, three-step
reaction was developed to prepare [18F]4 (Scheme 1), using the
multi-functionalized precursor, 1,6:3,4-bis-[O-(2,3-dimethoxy-
butane-2,3-diyl)]-2-O-trifluoromethanesulfonyl-5-O-benzoyl-
myo-inositol11 (1). Compound 1 was reacted with azeotropically
dried potassium cryptand fluoride ([K222][18F]). Conversion to
[18F]2 yielded a high specific activity product (10.7 Ci mmolÀ1).
Fluorine-18 labelled 2 was reacted with 40% trifluoroacetic
acid (TFA) in acetonitrile to hydrolyze the diacetals to
produce [18F]3. The reaction mixture was evaporated to
dryness with nitrogen gas, followed by addition of 2N sodium
hydroxide to hydrolyze the benzoyl group. Each step in this
reaction sequence was carried out at 90 1C for 15 min and
monitored by radio-HPLC and radio-TLC. Solid-phase
purification and formulation of [18F]4 was achieved by passing
the crude mixture through anion exchange, C-18, and alumina
cartridges, connected in series, and filtering the solution into a
sterile dose vial containing sodium bicarbonate and concen-
trated saline. The radiosynthesis of [18F]4 was accomplished in
80 min, with an uncorrected radiochemical yield of 16 Æ 3%
(n = 6) and 498% radiochemical purity at the end of
synthesis (Fig. 2). It is noteworthy that our group and others
have recently published proceedings describing the synthesis of
[18F]4.12–14 Our procedure gave the highest radiochemical
yield of [18F]4, and is attributed to the stability of the
precursor, 1. The measured logD of [18F]4 was À2.8 Æ 0.3,
as determined using an established procedure.15
scyllo-Inositol (Fig. 1a) is a naturally occurring stereoisomer
of inositol. Recent advances in our laboratories have revealed
that scyllo-inositol interacts with Ab1–42 peptides,1–3
considered to be a neurotoxic component of senile plaques
deposited in patients suffering from Alzheimer’s disease (AD).
This compound is emerging as a promising therapeutic for AD
and is presently in phase II of clinical trials. scyllo-Inositol
may also be a new marker for breast cancer, as it has been
detected in human breast tumour extracts.4 Given the high
socioeconomic burden of AD and breast cancer there is a
critical need for new methods to enable early diagnosis, to
monitor disease progression and to evaluate drug therapies for
these illnesses.
Extensive efforts are underway to develop radiopharma-
ceuticals for medical imaging with positron emission tomography
(PET)5–7 for both AD8,9 and breast cancer,10 however, the
existing radiopharmaceuticals for these targets require
significant improvements to be reliable for early and accurate
detection. scyllo-Inositol derivatives would represent a new
class of PET radiopharmaceuticals for these illnesses.
Despite the extensive efforts, there are still no ideal PET
radiopharmaceuticals for imaging AD labelled with the
desirable isotope fluorine-18 (18F; t1/2 = 109.7 min, b+
97%). A fluorinated analog, 1-deoxy-1-fluoro-scyllo-inositol
a PET Centre, Centre for Addiction and Mental Health,
250 College St., Toronto, ON, Canada M5T 1R8.
E-mail: neil.vasdev@camhpet.ca; Fax: +1-416-979-4656;
Tel: +1-416-535-8501 ext. 4680
b Department of Psychiatry, University of Toronto, Toronto, ON,
Canada M5T 1R8
The non-radioactive intermediates were prepared as
analytical standards using literature procedures.11 Although
3 was not previously isolated, it was readily synthesized by
hydrolysis of the diacetals on 2-O-benzoyl-1,6:3,4-bis-[O-(2,3-
dimethoxybutane-2,3-diyl)]-5-fluoro-scyllo-inositol (2)11 with
95% TFA. Further characterization of [18F]4 was achieved
c Department of Chemistry, University of Toronto, Toronto, ON,
Canada M5S 1A1
d Centre for Research in Neurodegenerative Diseases,
University of Toronto, Toronto, ON, Canada M5S 3H2
e STTARR Innovation Centre, Radiation Medicine Program.,
Princess Margaret Hospital, Toronto, ON, Canada M5G 1L7
f Department of Radiation Oncology, University of Toronto,
Toronto, ON, Canada M5S 3E2
g Leslie Dan Faculty of Pharmacy, University of Toronto,
Toronto, ON, Canada M5S 3M2
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and characterization of 3, 19F NMR spectrum of
4, radiosynthesis and characterization of [18F]4, and biological studies.
See DOI: 10.1039/b913317h
Fig. 1 Chemical structures of (a) scyllo-inositol and (b) 1-deoxy-1-
fluoro-scyllo-inositol (4).
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Chem. Commun., 2009, 5527–5529 | 5527

Scheme 1 Synthesis of [18F]4.
Fig. 3 Coronal PET-CT fused images of representative athymic mice
implanted subcutaneously in the right shoulder with MDA-MB-231
human breast cancer xenografts 14 days post-inoculation and
following tail vein injection of [18F]4. (a) Image through the plane of
the brain at 3 min reveals no brain penetration and only high uptake in
the kidneys; (b) image through the plane of tumour at 45 min (peak)
reveals 4.5% i.d. gÀ1 in the tumour. Image intensities were adjusted for
optimal delineation of volumes-of-interest and not for equivalent
visualization.
Fig. 2 Radio-HPLC chromatogram of [18F]4 after purification.
Radio-HPLC conditions: mobile phase = 80 : 20 CH3CN to H2O;
flow rate = 2.0 mL minÀ1; column = Phenomenex Analytical Luna
NH2 250 Â 4.6 mm (5 mm), l = 254 nm.
at all time points). Successful radiotracers for imaging the
central nervous system typically express Z 0.5% i.d. gÀ1 of
tissue in rodent brain.17 The data confirmed that [18F]4 does
not readily pass the blood brain barrier and is unlikely to be
useful for imaging the central nervous system.
by
a ‘‘carrier-added’’ reaction. The radiosynthesis was
repeated as above with the addition of 100 mg KF (5 : 1 mole
equivalent of 1 : KF) prior to the azeotropic drying step.
This reaction produced [18F]4 in 11% radiochemical yield
(uncorrected). The final product was co-spotted with
authentic 4 and analyzed by radio-TLC in conjunction with
a phosphomolybdic acid stain (Rf 0.6). The process was
repeated with KF as the only source of fluorine and the 19F
NMR spectrum of the combined purified product conformed
to that of authentic 4.
A preliminary small animal imaging study was carried out
to investigate the potential use of [18F]4 for detection of
human breast cancer xenografts (MDA-MB-231 cell line) in
athymic mice (Fig. 3b). Dynamic scans revealed that tumour
uptake peaked at 45 min post tail vein injection of [18F]4, with
4.5% i.d. gÀ1. The uptake of [18F]4 is similar to that of
[18F]2-fluoro-2-deoxy-D-glucose (FDG), considered the
‘‘gold standard’’ for cancer imaging with PET, in the same
xenograft tumour model (5.5 Æ 1.7% i.d. gÀ1 at 60 min
post-injection) under similar experimental conditions.18
Limitations of FDG PET in the follow-up of breast cancer
patients include the relatively high rate of false-positive results
in patients with known or suspected malignancy, as FDG
uptake in non-malignant inflammatory conditions has been
observed. The rather low specificity of FDG PET may be
improved by the use of [18F]4 and other inositol-based imaging
agents. The development of several new [18F]-labelled inositol
derivatives, optimization and automation of the radio-
chemistry, and continued oncology imaging studies are
currently under way in our laboratories.
Small animal imaging studies were conducted continuously
over 2 hours (dynamic scans) in female CD1 nude mice
following tail vein injection of [18F]4. Mice were fasted for
12 h prior to imaging under isoflurane anaesthesia. Dynamic
imaging revealed that [18F]4 had very low brain penetration
and accumulated primarily in the kidneys (representative
image shown in Fig. 3a). In order to confirm this finding, we
carried out a preliminary cerebral ex vivo biodistribution study
in conjunction with an in vivo SIC (Sonde IntraCerebrale;
´ ´
a b-sensitive intracerebral probe) study following tail vein
injection of [18F]4 in rats.16
Ex vivo biodistribution studies in conscious male Sprague–
Dawley rats further confirmed that [18F]4 has negligible brain
penetration (o0.1% injected dose per gram of wet tissue
(% i.d. gÀ1)) in cortex, cerebellum and rest of brain at 3 and
30 min post-injection of the radiotracer, without correction for
the vascular compartment. Whole blood levels were 0.8% and
0.5% i.d. gÀ1 at the same time points, respectively. The SIC
probe study was conducted from 0 to 40 min to measure
radioactivity in two localized areas of the rat brain (cerebellum
and frontal cortex) and showed similar results (o0.2% i.d. gÀ1
This study was conducted with the support of the Natural
Sciences and Engineering Research Council and Canadian
Institutes for Health Research (CHRPJ 365537-2009), a
Young Investigators Grant from the Alzheimer’s Society of
Canada (M.N.) as well as the Ontario Institute for Cancer
Research through funding provided by the Government of
Ontario. All animal experiments were carried out under
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5528 | Chem. Commun., 2009, 5527–5529

humane conditions, with approval from the Animal Care
Committee at the Centre for Addiction and Mental Health
or the University Health Network and in accordance with the
guidelines set forth by the Canadian Council on Animal Care.
The authors thank Armando Garcia and Winston Stableford
for fluorine-18 production as well as Dr David Green, Dr Jun
Parkes, Patrick McCormick, and Kristin McLarty for helpful
discussions and their expertise with the biological studies.
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