NONINVASIVE IMAGING OF HERPES-VIRUS THYMIDINE KINASE GENE-TRANSFER AND EXPRESSION - A POTENTIAL METHOD FOR MONITORING CLINICAL GENE-THERAPY

Noninvasive imaging of herpes simplex virus type 1 thymidine kinase (H SV1-tk) gene expression is possible with a clinical gamma camera and b y single-photon emission tomography (SPECT) using I-131-labeled o-2'-d eoxy-1-beta-D-arabinofuranosyl-5-iodo-uracil (FIAU). Studies were perf ormed in rats bearing s.c. tumors, Tumors were produced by injection o f wild-type RG2 glioma or W256 mammary carcinoma cells into one flank and RG2TK+ glioma or W256TK+ mammary carcinoma cells (that had been tr ansduced in vitro with the HSV1-tk gene) into the opposite flank, In s ome animals, HSV1-tk gene transduction of the pre-established wild-typ e tumors was accomplished in vivo by direct intratumoral injection of retroviral vector-producer cells, Imaging studies were performed 2 wee ks after tumor transduction to allow time for production and spread of the retroviruses through the tumor and for sufficient growth and incr ease in size of the tumors to facilitate imaging, The gamma camera and SPECT images revealed highly specific localization of [I-131]FIAU-der ived radioactivity to areas of HSV1-tk gene expression at 24, 36, and 48 h after i.v. administration of 1.6-2.8 mCi of [I-131]FIAU. Comparat ive analysis of quantitative autoradiographic images obtained from the same tumors confirmed that the high levels of [I-131]FIAU-derived rad ioactivity (>1% dose) were localized to areas of HSV1-tk gene expressi on demonstrated by immunohistochemical staining for HSV1-tk protein, I n contrast, significantly lower levels of [I-131]FIAU-derived radioact ivity (<0.01%) were observed in the surrounding nontransduced tumor ti ssue, contralateral wild-type tumors, and other tissues that showed no immunohistochemical staining for the HSV1-tk protein, The magnitude o f FIAU accumulation in RG2TK+, W256TK+, and wild-type tumors correspon ded to the in vitro ganciclovir sensitivity of the cell lines used to produce these tumors, which indicates that the magnitude of FIAU accum ulation reflects the level of HSV1-tk gene expression, We suggest that ''clinically relevant'' levels of HSV1-tk gene expression in transfec ted tissue can be imaged with [I-131]FIAU and a gamma camera or SPECT, and that a significant improvement in imaging sensitivity and resolut ion is expected with [I-124]FIAU and PET.