The research shows that a diabody, an antibody surrogate just one third the size of native antibodies, can be used effectively as a targeting vehicle for radioimmunotherapy, said Gregory Adams, Ph.D., associate member of the Medical Science Division, Fox Chase Cancer Center, Philadelphia, Pa.
Diabodies are genetically engineered dimeric proteins produced in E. coli bacteria that contain the antigen-recognizing portion of antibodies formed by immune system cells to combat disease. The mini-antibody developed by Adams and colleagues, C6.5K-A, is a protein substitute for larger, naturally produced antibodies that specifically target the HER2/neu human tumor-associated antigen. When loaded with the beta-emitting radioisotope yttrium-90, C6.5K-A significantly inhibits the growth rate of human breast tumor xenografts in mice.
"The diabodies bound to the HER2 receptor produced by certain breast tumor cells." said Adams, the lead author on the paper. "Imaging indicated that the diabody was concentrated in the mammary tumors and in the kidney where it was excreted from the body."
Since diabodies are so much smaller than native antibodies, the genetically engineered protein is cleared much quicker from the body, Adams said. However, the affinity that the diabody has for its antigen target is so great that a significant amount of the cell-killing radioactive protein lodges in the mammary tumor cells.
"The dimeric C6.5K-A binds to its target antigen 40 times more tenaciously than its individual monomeric components, thus promoting prolonged retention in antigen-laden tumors. At the same time, its small size enables it to efficiently find and penetrate these tumors," Adams said.