Radiolabeled antitumor agents

Bleomycin serves as a useful prototype for a study of the various properties required in a radiopharmaceutical for tumor imaging. Both in vitro and in vivo results using bleomycin labeled with a large number of different radionuclides have now, been reported. In general, it has not been possible to predict in vivo biologic behavior solely from in vitro data. For example, 57Co-bleomycin and 111In-bleomycin both have identical patterns in several different chromatographic systems and do not break down with prolonged storage in saline, exposure to heat, or other cations. However, when the labeled bleomycin is exposed to serum transferrin (and to other as yet unidentified ligands in the body) a difference in stability becomes obvious. The stability and biologic activity of 57Co-bleomycin in humans demonstrate the validity of metal chelation as a labeling technique for this specific molecule, and also suggests that other stable chelates will have useful applications. Although 57Co-bleomycin has the most desirable biologic characteristics of all the chelates of bleomycin, the extremely long physical half-life of 57Co of 270 days creates a significant contamination problem in the hospital. Because of this, the use of 57Co-bleomycin is limited to a few specialized centers. All users have emphasized the need for a better radionuclide to produce a labeled bleomycin with the same or superior biologic characteristics to 57Co-bleomycin. Unfortunately, the elements forming the most stable chelates with bleomycin (copper, zinc, cobalt, and nickel) do not have radionuclides with suitable physical characteristics for scanning. Copper-67 may become available in the future from high-energy linear accelerators. However, even if it were available, copper will probably not have as good chemical properties as cobalt.