Iodinated bleomycin: an unsatisfactory radiopharmaceutical for tumor localization

Cobalt 57 bleomycin is a clinically useful tumor localizing agent, but attempts to label bleomycin (BLEO) with other radionuclides have been made because of the long physical half life of ⁵⁷Co. As an alternative labeling approach, they iodinated BLEO both directly on the imidazole ring and indirectly by reaction with N succinimidyl 3 (4 hydroxy, 3 iodophenyl) propionate. Directly iodinated BLEO retained antibacterial activity, but in tumor bearing rats it showed a lower tumor to blood ratio (2.3) at 2 hr than did ⁵⁷Co BLEO (11.8). The antibacterial activity of the indirectly labeled BLEO was markedly reduced and this material showed a tumor to blood ratio of 0.55 at 2 hr. The radioiodinated bleomycins are not suitable substitutes for ⁵⁷Co BLEO as tumor imaging radiopharmaceuticals.