Preclinical evaluation of the penciclovir analog 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine for in vivo measurement of suicide gene expression with PET.
The gene for herpes simplex virus thymidine kinase (HSV-tk) is widely used as a suicide gene in experimental gene therapy of cancer. 9-(4-Fluoro-3-hydroxymethylbutyl)guanine (FHBG) is an antiviral nucleoside analog that is rapidly phosphorylated by viral thymidine kinase but is a poor substrate for mammalian thymidine kinase. Recently, FHBG labeled in the 4-fluoro position with (18)F has shown promise relative to other similar compounds for imaging in vivo expression of HSV-tk using PET. In this study, we evaluated the uptake of [(18)F]FHBG in vitro and in vivo using transduced and wild-type human colon cancer cells (HT-29). We also imaged [(18)F]FHBG and measured the radioactivity concentrations of circulating [(18)F]FHBG and its metabolites in monkeys.


METHODS
Sterile, pyrogen-free [(18)F]FHBG was produced routinely in good yields. Cells were transduced with the retroviral vector G1Tk1SvNa containing HSV-tk gene. In vitro uptake studies were performed by incubating cells with [(18)F]FHBG at 37 degrees C for 1 and 5 h. Biodistribution studies were performed at 2 and 5 h after injection in nude mice bearing tumors grown from wild-type or transduced cells. Sequential, whole-body PET scans of cynomolgus monkeys were obtained over a period of >2 h after intravenous injection of [(18)F]FHBG. Arterial plasma samples obtained from monkeys 15-120 min after intravenous injection were subjected to acid extraction, and the acid-soluble fractions were analyzed by high-performance liquid chromatography.


RESULTS
In vitro studies showed 31 and 71 (P < 0.001) times higher uptake of the probe at 1 and 5 h, respectively, in transduced cells compared with nontransduced cells. In vivo studies in mice showed that tumor uptake of the radiotracer was 4-fold (P < 0.05) and 13-fold (P < 0.001) higher at 2 and 5 h, respectively, in tumors grown from transduced cells compared with control cells. Transduced tumor-to-normal tissue ratios ranged from 2 to 25 at 2 h and from 2 to 22 at 5 h. Recirculating labeled metabolites had only a minor effect on the biodistribution of radiolabel from [(18)F]FHBG in monkeys.


CONCLUSIONS
These results indicate that [(18)F]FHBG may yield high-contrast PET images of HSV-tk expression in tumors and, therefore, it is a very promising radiotracer for monitoring of gene therapy of cancer with PET.
Alauddin
M M
MM
Department of Radiology, PET Imaging Science Center, University of Southern California, Los Angeles 90033-4609, USA.
Shahinian
A
A
Gordon
E M
EM
Bading
J R
JR
Conti
P S
PS
D000212:Acyclovir / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D000818:Animals; D000998:Antiviral Agents / Q000378:metabolism / Q000493:pharmacokinetics; D002851:Chromatography, High Pressure Liquid; D015971:Gene Expression Regulation, Enzymologic / Q000235:genetics*; D015316:Genetic Therapy; D006147:Guanine / Q000031:analogs & derivatives*; D019073:HT29 Cells; D006801:Humans; D008252:Macaca fascicularis; D008297:Male; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D019275:Radiopharmaceuticals; D018139:Simplexvirus / Q000201:enzymology*; D013937:Thymidine Kinase / Q000096:biosynthesis / Q000235:genetics*; D014018:Tissue Distribution
D000998:Antiviral Agents; D019275:Radiopharmaceuticals; C051755:9-(4-hydroxy-2-(hydroxymethyl)butyl)guanine; C053539:penciclovir; D006147:Guanine; D013937:Thymidine Kinase; D000212:Acyclovir