Unlabelled iododeoxyuridine increases the cytotoxicity and incorporation of [I-125]-iododeoxyuridine in two human glioblastoma cell lines

Iododeoxyuridine (IUdR), labelled with radioiodines emitting Auger, alpha o r beta (-) radiation, has been proposed as a therapeutic tool in the treatm ent of cancer. However, the low per cent incorporation in tumour cells and limited cytotoxicity are major obstacles for such an application. Using unl abelled IUdR as a modulator, we have studied the in vitro cytotoxicity of [ I-125]-IUdR in two human glioblastoma cell lines. Surprisingly, an enhanced cytotoxicity of[I-125]-IUdR was observed in the presence of 0.3-10 muM con centrations of unlabelled IUdR in U251 glioblastoma cells and to a lesser e xtent in LN229 cells. The presence of unlabelled IUdR unexpectedly increase d the incorporation of [I-125]-IUdR in both cell lines. Thymidine competiti vely blocked the cytotoxic effects of combined unlabelled and [I-125]-label led IUdR in these cells and DNA-incorporation of radiolabelled IUdR. ((C) 2 000 Lippincott Williams & Wilkins).