Binding, internalization and excretion of TGF alpha-dextran associated radioactivity in cultured human glioma cells

Abstract

Conjugates based on transforming growth factor alpha, TGF alpha, or epidermal growth factor, EGF, are candidates for targeted radiotherapy against EGF-receptor rich tumours such as gliomas or squamous carcinomas. In this study, binding, internalization and excretion of radiolabelled TGF alpha and TGF alpha-dextran conjugates was analysed in an EGF-receptor rich human glioma cell line. The binding of 125I-TGF alpha was EGF-receptor specific and the binding pattern was similar to that of 125I-EGF. The TGF alpha-dextran conjugate also bound specifically but gave maximum binding for a longer time during continuous incubation compared to when only TGF alpha was used. The excretion pattern of internalized radioactivity was somewhat slower for 125I-TGF alpha-dextran, with 125I-labelling on the TGF alpha part, as compared to 125I-TGF alpha although most of the radioactivity in both cases was excreted within 4 hours. The fate of the dextran part of the conjugate, as followed by means of 125I-labelling of the dextran, was different since all radioactivity in that case remained cell-associated for at least up to 22 hours. Furthermore, by comparison with previously published results, it was seen that the radioactivity delivered through the TGF alpha part of TGF alpha-dextran was retained for a shorter period of time by the cells than when delivered by EGF in EGF-dextran conjugates. However, when the radioactivity was delivered by the dextran part of the conjugates, the radioactivity seemed to be retained equally well or even better when TGF alpha-dextran was applied. It is concluded that TGF alpha-dextran, as well as EGF-dextran, have interesting properties for targeting against EGF-receptors and that the dextran part is well retained in the cells and therefore might be a suitable carrier for toxic agents such as radionuclides. It is of high interest to continue with toxicological and pharmacological in vivo studies of the conjugates.