In vivo and in vitro binding of iodinated monoclonal antibody A2B5 to RIN insulinoma cells.

Monoclonal antibody A2B5 reacts with the cell surface of a series of amine precursor uptake decarboxylation (APUD) cells and their tumors in many vertebrate species including chicken, rat, mouse, and man. We have studied the in vivo and in vitro binding of iodinated monoclonal antibody A2B5 to rat insulinoma cells. In vitro, radiolabeled A2B5 binds specifically to RINm5F insulinoma cells and the binding of 125I-A2B5 is inhibited by unlabeled A2B5 or a ganglioside extract of RINm5F cells. In vivo, scintigrams taken Day 0 to Day 5 after injection of 131I-labeled A2B5 showed a striking localization of 131I-A2B5 in transplanted RIN tumors grown in syngeneic rats. Other control radiolabeled monoclonal antibodies did not concentrate in the tumors. 131I-labeled A2B5 did not concentrate in other transplantable tumors (colon adenocarcinoma, osteosarcoma, renal cell carcinoma, and bladder transitional cell carcinoma) grown in nude mice. The tumor/blood ratio detected 5 days after antibody injection, was approximately two to 12 times higher in the insulinoma compared to other organs and only in the insulinoma did 131I-A2B5 show a higher concentration than control antibody 125I-P3X63.