Variables affecting the tumor localization of 131I-antiferritin in experimental hepatoma

Ferritin is both a normal tissue- and tumor-associated protein. The in vivo localization of 131I-radiolabeled antitumor ferritin and normal IgG antibodies in the H-4-II-E rat hepatoma model was investigated in both tumor and normal tissues over a dose range of 0.67 micrograms to 5 mg of normal and antiferritin IgG and at labeling ratios (microCi 131I per micrograms IgG) of 15:1, 5:1, and 1:10. The total dose from nonpenetrating radiation in rads was calculated and demonstrated a maximum of 2.9 times greater dose deposition (rads) of antiferritin than normal IgG in hepatoma without specific increase in binding in normal tissues. The maximum tumor targeting achieved was dependent on the amount of injected IgG and not on the labeling ratio or procedure. The binding in tumor could be inhibited by unlabeled antiferritin but not by unlabeled normal rabbit IgG and demonstrated the requirement of specificity for tumor binding. Normal tissues did not target with antiferritin. Most normal tissues have a capacity to bind normal and antiferritin IgG nonspecifically that is linear in relationship to the amount of injected IgG. The results demonstrate that 131I-antiferritin selectively targets ferritin-secreting hepatoma over normal tissues and that the amount of targeting is dependent on the amount of antiferritin injected. The physiologic reasons for such selective localization is not known, but the term "biologic window" has been used to describe the differential availability of tumor ferritin for binding.