Imaging herpes viral thymidine kinase-1 reporter
gene expression with a new
18
F-labeled probe:
2V-fluoro-2V-deoxy-5-[
18
F]fluoroethyl-1-h-d-arabinofuranosyl uracil
Julius A. Balatoni
a,c
, Michael Doubrovin
b
, Ludmila Ageyeva
b
, Nagavarakishore Pillarsetty
b
,
Ronald D. Finn
a
, Juri G. Gelovani
c
, Ronald G. Blasberg
b,
T
a
Radiochemistry/Cyclotron Core Facility, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA
b
Department of Neurology, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA
c
Department of Experimental Diagnostic Imaging, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA
Received 25 May 2005; received in revised form 28 June 2005; accepted 7 July 2005
Abstract
The preparation and radiolabeling of 2V-fluoro-2V-deoxy-1-h-d-arabinofuranosyl-5-(2-fluoroethyl)-uracil (FFEAU) with
18
F and its
evaluation as a probe for imaging herpes simplex virus 1 thymidine kinase (HSV1-tk) gene expression are described. 2V-Fluoro-2V-deoxy-3V,
5V-di-O-benzoyl-1-h-d-arabinofuranosyl-3-N-benzoyl-5-(2-[
18
F]fluoroethyl) -uracil 12 was prepared by nucleophilic substitution of the
corresponding tosyl 8 or trifluoroethanesulfonyl 9 derivative with n-Bu
4
N[
18
F]F. Base hydrolysis was used to remove the benzoyl protecting
groups, followed by HPLC purification, to afford [
18
F]FFEAU 13. The trifluoroethanesulfonyl substrate 9 appears to be the better labeling
precursor. Carrier n-Bu
4
NF was added to the labeling reaction, which resulted in specific activities of 40–70 Ci/mmol (estimated).
Radiochemical purity averaged 94F4%. Although [
18
F]FFEAU was obtained in low radiochemical yield with 9 and further optimization of
the radiosynthesis will be required, sufficient product was available for a series of in vitro and in vivo studies. [
18
F]FFEAU was directly
compared with [
3
H]TdR in a series of in vitro accumulation studies involving a HSV1-tk stably transduced cell line, RG2TK+ and a
nontransduced, wild-type RG2 cells. The initial in vitro and in vivo imaging studies are promising; FFEAU has in vitro accumulation and
sensitivity characteristics similar to that previously reported for FIAU, but greater selectivity than FIAU due to lower uptake and retention in
nontransduced cells and tissues. The animal imaging experiment showed low levels of radioactivity in the lungs, with little or no radioactivity
seen in the heart, liver, spleen and intestines.
D 2005 Elsevier Inc. All rights reserved.
Keywords: 2V-Fluoro-2V-deoxy-1-h-d-arabinofuranosyl-5-(2-fluoroethyl)-uracil;
18
F; HSV1-tk; PET; Reporter gene
1. Introduction
Several different radiolabeled probes have been devel-
oped to image HSV1 thymidine kinase gene (HSV1-tk)
expression [1–12]. Pyrimidine nucleosides labeled with
radioiodine, such as FIAU, appear to have several advan-
tages over radiolabeled acycloguanosine probes such as
FHBG and FHPG for imaging stably transduced cells
[13,14], although FHBG may be a better probe for imaging
the sr39 HSV1-tk variant (HSV1-sr39tk) [8]. The main
advantage of using
18
F-labeled probes over radioiodine
(
123
I,
124
Ior
131
I)-based probes is that the short half-life of
18
F allows repetitive and sequential imaging of the subjects.
We recently demonstrated that good images of HSV1-tk
expression in transduced tissue can be obtained 1 to 2 h
after intravenous administration of [
124
I]FIAU [13]. There-
fore, we decided to synthesize an
18
F-labeled analogue
of FIAU that could be used for repetitive, sequential
imaging studies of HSV1-tk expression and retain the
imaging advantages of FIAU (greater sensitivity and
dynamic range).
This report describes the synthesis and initial evaluation
of
18
F-labeled 2V-fluoro-2V-deoxy-1-h-d-arabinofuranosyl-
5-(2-fluoroethyl)-uracil (FFEAU), a new probe for imaging
HSV1-tk expression. FFEAU has similar sensitivity char-
0969-8051/$ – see front matter D 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.nucmedbio.2005.07.007
T Corresponding author.
Nuclear Medicine and Biology 32 (2005) 811 –819
www.elsevier.com/locate/nucmedbio