Diabodies act as guided missiles targeted to mammary tumor growth

A mini-antibody bearing a payload of tumor-busting radiation thwarts the growth of human breast cancer in laboratory animals, according to research published in the September 1 issue of the journal Cancer Research.

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.

Using the HER2 receptor protein on tumor cell membranes as a portal into the cells, the radioactive diabody gained access inside tumors to induce cell death and restrict tumor growth.

Although radioactive C6.5K-A was effective against the human mammary tumors growing in mice, the biotherapeutic fell short of inhibiting human ovarian tumor cell growth in the lab animals. “The kinetics of the HER2 receptor on the mammary tumor cells are favorable to taking up the diabody,” Adams said. “Once the beta-emitter is delivered inside the mammary tumor cells, the radiation causes intracellular damage that probably triggers p53 driven apoptosis.”

The mammary tumor cells contained the normally functioning p53 tumor suppressor gene, while the p53-null ovarian tumor cells used in the studies lacked the protective action of that apoptosis-inducing gene. The mammary tumor cells also internalized the HER2-bound diabody fifteen times more rapidly than did the ovarian tumor cells.

Ongoing studies by Adams and his colleagues are aimed at determining refinements in the radioisotope-bearing diabody that will optimize tumor growth inhibition or reduce tumor mass while preserving the health and function of kidneys.