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Am J Nucl Med Mol Imaging 2013;3(1):71-84

Original Article
Synthesis and preclinical evaluation of a new C-6
alkylated pyrimidine derivative as a PET imaging
agent for HSV1-tk gene expression

Ursina Müller, Tobias L Ross, Charlene Ranadheera, Roger Slavik, Adrienne Müller, Mariana Born,
Evelyn Trauffer, Selena Milicevic Sephton, Leonardo Scapozza, Stefanie D Krämer, Simon M Ametamey

Institute of Pharmaceutical Sciences, ETH Zurich, Wolfgang-Pauli Strasse 10, 8093 Zurich, Switzerland; University
of Geneva, School of Pharmaceutical Sciences, Quai Ernest-Ansermet 30, 1211 Geneva 4, Switzerland

Received September 27, 2012; Accepted November 2, 2012; Epub January 5, 2013; Published January 15, 2013

Abstract: [18F]FHOMP (6-((1-[18F]-fluoro-3-hydroxypropan-2-yloxy)methyl)-5-methylpyrimidine-2,4(1H,3H)-dione), a
C-6 substituted pyrimidine derivative, has been synthesized and evaluated as a potential PET agent for imaging herpes
simplex virus type 1 thymidine kinase (HSV1-tk) gene expression. [18F]FHOMP was prepared by the reaction of
the tosylated precursor with tetrabutylammonium [18F]-fluoride followed by acidic cleavage of the protecting groups.
In vitro cell accumulation of [18F]FHOMP and [18F]FHBG (reference) was studied with HSV1-tk transfected HEK293
(HEK293TK+) cells. Small animal PET and biodistribution studies were performed with HEK293TK+ xenograftbearing
nude mice. The role of equilibrative nucleoside transporter 1 (ENT1) in the transport and uptake of [18F]
FHOMP was also examined in nude mice after treatment with ENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside
phosphate (NBMPR-P). [18F]FHOMP was obtained in a radiochemical yield of ~25% (decay corrected) and the
radiochemical purity was greater than 95%. The uptake of [18F]FHOMP in HSV1-TK containing HEK293TK+ cells was
52 times (at 30 min) and 244 times (at 180 min) higher than in control HEK293 cells. The uptake ratios between
HEK293TK+ and HEK293 control cells for [18F]FHBG were significantly lower i.e. 5 (at 30 min) and 81 (240 min).
In vivo, [18F]FHOMP accumulated to a similar extend in HEK293TK+ xenografts as [18F]FHBG but with a higher general
background. Blocking of ENT1 reduced [18F]FHOMP uptake into brain from a standardized uptake value (SUV)
of 0.10±0.01 to 0.06±0.02, but did not reduce the general background signal in PET. Although [18F]FHOMP does
not outperform [18F]FHBG in its in vivo performance, this novel C-6 pyrimidine derivative may be a useful probe for
monitoring HSV1-tk gene expression in vivo. (ajnmmi1209003).

Keywords: HSV1-TK, reporter gene, gene expression monitoring, PET, [18F]FHOMP, [18F]FHBG

Address all correspondence to:
Dr. Simon M Ametamey
Center for Radiopharmaceutical Sciences of ETH
PSI and USZ, Institute of Pharmaceutical Sciences
ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093
Zurich, Switzerland.
Phone: +41 44 633 74 63
Fax: +41 44 633 13 67