Cellular retention of radioactivity and increased radiation dose. Model experiments with EGF-dextran Åsa Liljegren Sundberg a , Erik Blomquist b , Jo ¨rgen Carlsson a, *, Ann-Charlott Steffen a , Lars Gedda a a Division of Biomedical Radiation Sciences, Rudbeck Laboratory, Uppsala University, S-751 85 Uppsala, Sweden b Division of Oncology, Rudbeck Laboratory, Uppsala University, S-751 85 Uppsala, Sweden Received 4 June 2002; received in revised form 3 October 2002; accepted 4 October 2002 Abstract Targeting of tumor cells with radiolabeled biomolecules is a possible approach to inactivate disseminated tumor cells. However, rapid degradation of the biomolecules after cellular internalization and subsequent excretion of the radioactivity is a problem. We studied the possibility of using dextran as a carrier of radionuclides to improve the intracellular retention. An EGF-dextran conjugate, aimed for targeting of tumor cells overexpressing the EGF-receptor, was used as model. Retention tests were performed with 125 I on different parts: [ 125 I]-EGF-dextran-[ 125 I], [ 125 I]-EGF-dextran and EGF-dextran-[ 125 I]. Comparisons were made with [ 125 I]-EGF. The radiolabeled com- pounds were incubated with cultured glioma cells for different times. The cellular retention of radioactivity was then measured for up to 24 h. Expected radiation doses at the cellular level were calculated assuming that 131 I, instead of 125 I, was coupled to EGF and EGF-dextran. The results indicated that the EGF-part of the conjugate was degraded and the EGF-attached radioactivity was rapidly excreted, whereas radioactivity on dextran was retained intracellularly to a high degree, i.e. 70 – 80% of the radioactivity bound to dextran was still cell-associated after 24 h. The retention after 24 h was significantly higher (p  0.001) when the radioactivity was on the dextran instead of the EGF-part. The radiolabeled EGF-dextran had a notably high specific radioactivity; up to 11 MBq/g. There was potential for at least hundred times increased radiation dose per receptor interaction when the radioactivity was on the dextran part. The advantage with radioactivity on the dextran part was the high cellular retention and the high specific radioactivity (higher than previously reported for other residualizing labels) without severe loss of receptor specific binding. Thus, dextran seems suitable as a carrier of radionuclides aimed for therapy and gives potential for a highly increased radiation dose. © 2003 Elsevier Science Inc. All rights reserved. Keywords: EGF; Dextran; Tumor targeting; Radiation dose; Retention; Residualizing label 1. Introduction A major problem in tumor therapy is to inactivate dis- seminated tumor cells that remain after surgery and/or ex- ternal radiotherapy. This problem is considerable when the tumor cells cannot be treated effectively by chemotherapy, as for example in patients with melanomas, highly malig- nant brain tumors and disseminated adenocarcinomas (e.g. breast and prostate carcinomas). An ideal therapy in these cases would be a targeting therapy where tumor cells are selectively killed without causing severe damage to normal tissue. One target structure considered for this kind of ther- apy is the epidermal growth factor receptor (EGFR). This receptor is frequently overexpressed in several malignan- cies, for example gliomas [7,30,34], bladder carcinomas [25,26], squamous carcinomas [9,24] and breast cancers [41]. In glioblastoma multiforme, the most malignant form of the gliomas, overexpression of EGFR is detected in up to 75% of cases [30]. The overexpression is due to gene amplification and/or increased transcription rates, and the number of 10 6 receptors per tumor cell has been reported [1,5,10]. Several targeting agents have been designed to deliver toxins or radioactive nuclides to tumor cells [8,17,20,23,31]. The primary choice of biomolecule for the delivery has been antibodies or fragments of antibodies, but these alternatives have disadvantages. First, the relatively large size of antibodies makes it difficult for them to pen- * Corresponding author: Tel.: +46-18-471-3841; fax: +46-18-471- 3432. E-mail address: Jorgen.Carlsson@bms.uu.se (J. Carlsson) Nuclear Medicine and Biology 30 (2003) 303–315 www.elsevier.com/locate/nucmedbio 0969-8051/03/$ – see front matter © 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0969-8051(02)00421-3