The effect of dose loading and of double labelling with 57Co and 125I on the tissue distribution in animals.

Dose loading effects upon the performance of 57Co-bleomycin as a tumor localizing agent have been investigated in Rhabdomyosarcoma bearing Wag/Ry rats. The addition of non-radioactively labelled Co-bleomycin increased the relative uptake of 57Co-bleomycin in rapid growing tumors, but the addition of non-chelated bleomycin had no influence at all. In our experimental system, iodinated bleomycin generally labelled by reaction with ICl, was found to be an unsatisfactory tumor localizing agent. In order to combine the useful localizing properties of Co-bleomycin with the qualified detection properties of some iodine isotopes, we attempted to prepare bleomycin doubly labelled with Co and I. However, we were unable to prepare 57Co-125I-bleomycin by general labelling with ICl. This result indicates that both labels need the imidazole ring for the formation of a stable, labelled bleomycin.