IMAGING HERPES-VIRUS THYMIDINE KINASE GENE-TRANSFER AND EXPRESSION BYPOSITRON-EMISSION-TOMOGRAPHY

We report a series of studies that assess the feasibility and sensitiv ity of imaging of herpes virus type one thymidine kinase (HSV1-tk) gen e transfer and expression with -5-iodo-2'-fluoro-1-beta-D-arabinofuran osyl-uracil ([I-124]-FIAU) and positron emission tomography (PET) and the ability of [I-124]-FIAU-PET imaging to discriminate different leve ls of HSV1-tk gene expression. Studies were performed in rats bearing multiple s.c. tumors derived from W256 rat carcinoma and RG2 rat gliom a cells. In the first set, we tested the sensitivity of [I-124]-FIAU-P ET imaging to detect low levels of HSV1-tk gene expression after retro viral-mediated gene transfer, HSV1-tk gene transduction of one of pree stablished wild-type W256 tumor in each animal was accomplished by dir ect intratumoral injection of retroviral vector-producer cells (W256-- >256TK tumors), Tumors produced from W256 and W256TK+ cells served as the negative and positive control in each animal. Highly specific ima ges of [I-124]-FIAU-derived radioactivity mere obtained in W256TK tum ors (that were transduced in vivo) and in W256TK+ tumors but not in no ntransduced wild-type W256 tumors. The level of ''specific'' incorpora ted radioactivity in transduced portions of both W256TK and R256TK+ t umors was relatively constant between 4 and 50 h. In the second set, m e tested whether [I-124]-FIAU and PET imaging can measure and discrimi nate between different levels of HSV1-tk gene expression. Multiple s.c . tumors were produced from wild-type RG2 cells and stably transduced RG2TK cell lines that express different levels of HSV1-tk, A highly si gnificant relationship between the level of [I-124]-FIAU accumulation [% injected dose/g and incorporation constant (Ki)] and an independent measure of HSV1-tk expression (sensitivity of the transduced tumor ce lls to ganciclovir, IC50) was demonstrated, and the slope of this rela tionship was defined as a sensitivity index, We have demonstrated for the first time that highly specific noninvasive images of HSV1-tk expr ession in experimental animal tumors can be obtained using radiolabele d 2'-fluoro-nucleoside [I-124]-FIAU and a clinical PET system, The abi lity to image the location (distribution) of gene expression and the l evel of expression over time provides new and useful information for m onitoring clinical gene therapy protocols in the future.