Epidermal growth factor radiopharmaceuticals:: 111In chelation, conjugation to a blood-brain barrier delivery vector via a biotin-polyethylene linker, pharacokinetics, and in vivo imaging of experimental brain tumors

Epidermal growth factor (EGF) is a potential peptide radiopharmaceutical for detection of brain tumors, because many human gliomas overexpress the EGF receptor (EGFR). The transport of EGF to the brain, however, is restricted by the blood-brain barrier (BBB). The purpose of the present study was to develop a vector-mediated brain delivery system for radiolabeled EGF. Human EGF was monobiotinylated with NHS-PEG(3400)-biotin, where NHS is N-hydroxysuccinimide and PEG(3400) is poly(ethylene glycol) of 3400 Da molecular mass. EGF-PEG(3400)-biotin was radiolabeled with either I-125 or In-111 through the metal chelator, diethylenetriaminepentaacetic acid (DTPA). The radiolabeled EGF was then conjugated to a BBB delivery vector comprised of a complex of the OX26 monoclonal antibody (MAb) to the rat transferrin receptor, which was coupled to streptavidin (SA). Following intravenous injection in rats, the I-125 conjugate was rapidly degraded in vivo, while the In-111 conjugate was metabolically stable. The brain delivery of [In-111]DTPA-EGF-PEG(3400)-biotin was enabled by conjugation with OX26/SA and was optimized by co-injection of unlabeled EGF to saturate EGF receptors in the liver. The specific binding of the [In-111]DTPA-EGF-PEG(3400)-biotin conjugated to OX26/SA to the EGF receptor was confirmed in C6 rat glioma cells, which had been transfected with a gene encoding for the human EGF receptor under the regulation of a dexamethasone-inducible promoter. In vivo studies of CG-EGFR experimental tumors in Fischer 344 rats demonstrated successful brain imaging only when the peptide radiopharmaceutical was conjugated to the BBB delivery system, although the C6-EGFR tumors did not express EGFR in vivo. In conclusion, these studies describe the molecular formulation of a peptide radiopharmaceutical that can be used for imaging brain tumors behind the BBB.