The 528 murine
monoclonal antibody (MAb) to the human
epidermal growth factor receptor (EGFR) was sequentially cationized with
hexamethylenediamine and conjugated with diethylenetriaminepentaacetic
acid (
DTPA) as a potential antibody
radiopharmaceutical for imaging EGFR-expressing
cancer. The cationized 528 MAb was characterized with isoelectric focusing and electrophoresis, and an immunoradiometric assay, which showed the affinity of the 528 MAb for the human EGFR was retained following cationization. The native or cationized 528 MAb, labeled with (111)In, was injected intravenously in scid mice bearing human U87 flank
tumors, which express the EGFR, and
tumor imaging was performed with both external detection in live animals and with whole body autoradiography. However, the
tumor signal was not increased with the cationized MAb, relative to the native MAb, and this was due to a serum inhibition phenomenon that was confirmed by a pharmacokinetics analysis in control mice. In an attempt to block the serum inhibition, the cationized 528 MAb was pegylated with 2000 Da poly(
ethylene glycol), and the cationized/pegylated MAb was conjugated with
DTPA and labeled with (111)In. However, a pharmacokinetics analysis showed the pegylation did not reverse the serum inhibition of the cationic charge on the MAb. These studies describe methods for reformulating
monoclonal antibodies to develop improved
radiopharmaceuticals, but show that radiolabeling a cationized MAb with
DTPA produces a serum neutralization of the initial cationization modification.