125IUDR labelling efficiency and cytotoxicity for murine tumours in vivo and in vitro.

We have reviewed the literature on 125IUDR (5-iodo-2-deoxyuridine), the DNA label of choice for cell distribution studies. In previous studies, the cytotoxicity in relation to labeling efficiency has often been inadequately investigated or reported. We have studied four syngeneic mouse tumours and compared in vitro with in vivo labelling procedures. Ascites tumours could be effectively labelled in vivo by IP injections of 60-80 muCi per mouse during 24 h, without apparent cytotoxicity. Comparable in vitro labelling was also effective, but caused a dose-related cytotoxicity, as measured by growth rate and transplantability. Comparison with 'cold' IUDR disclosed that the toxicity was not chemical, but radiological. Our attempt to label a solid lymphoma in vivo was unsuccessful, obviously because the tumour grows too slowly. We conclude that 125IUDR-labelling is suitable for cell distribution studies, but in view of labelling efficiency versus cytotoxicity the procedure has to be adjusted to each new tumour individually.