Radiolabeling of Epidermal Growth Factor with 99mTc and in Vivo Localization following Intracerebral Injection into Normal and Glioma-Bearing Rats

Abstract

High grade gliomas may have amplified expression of the epidermal growth factor receptor (EGFR) gene c-erb-B, which often is associated with increased expression of transmembrane EGFR. The purpose of the present study was to develop a method for labeling EGF with 99mTc and to determine whether the resulting radioligand would localize, following intracerebral injection, in rats bearing EGFR-positive gliomas. EGF has a relatively low molecular mass (approximately 6 kDa) compared to monoclonal antibodies, and this has allowed smaller bioconjugates, which should diffuse more rapidly within the brain and more effectively target disseminated glioma cells, to be constructed. In the present study, EGF has been labeled with either 131I or 99mTc, and in vitro uptake of the resulting radioligand has been investigated using C6EGFR rat glioma cells, which had been transfected with the EGFR gene. Cellular uptake of 131I radioactivity peaked after approximately 30 min of incubation with [131I]EGF, following which time it declined, while 99mTc radioactivity continued to increase over a 6 h incubation with [99mTc]-EGF. To determine if radiolabeled EGF had in vivo tumor-localizing properties, C6EGFR glioma cells were implanted stereotactically into the brains of Fischer rats. Four weeks later, either 99mTc- or 131I-labeled EGF was injected intracerebrally into normal or glioma-bearing animals using the same stereotactic coordinates. External gamma scintigraphy revealed that 131I radioactivity disappeared rapidly from the brain regions of tumor-bearing animals compared to 99mTc, approximately 50% of which remained in the tumor for up to 12 h. In contrast, only approximately 20% remained in the brains of non-tumor-bearing animals after 6 h. These studies are the first to describe a method for radiolabeling EGF with 99mTc and to detect it by external scintigraphy in the brains of tumor-bearing animals.