Retention of biologic activity of human epidermal growth factor following conjugation to a blood-brain barrier drug delivery vector via an extended poly(ethylene glycol) linker

Human brain gliomas overexpress the receptor for epidermal growth factor (EGF), and radiolabeled EGF is a potential peptide radiopharmaceutical for imaging human brain tumors, should this peptide be made transportable through the blood-brain barrier (BBB) in vivo. Peptide drug delivery to the brain may be facilitated by conjugating peptide radiopharmaceuticals to BBB drug delivery vectors such as the OX26 monoclonal antibody (MAb), which undergoes receptor-mediated transcytosis through the BBB via the brain capillary endothelial transferrin receptor. EGF was biotinylated with NHS-XX-biotin, where NHS N-hydroxysuccinimide and -XX- = bis (aminohexanoyl) spacer arm. The [I-125]EGF-XX-biotin rapidly bound to C6 rat glioma cells transfected with the human EGF receptor. However, no binding to the C6 EGF receptor was detected when the [125I]EGF-XX-biotin was bound to a conjugate of streptavidin (SA) and the OX26 MAb. An alternative linker strategy using poly(ethylene glycol) (PEG) of 3400 Da molecular mass (PEG(3400)) was evaluated, wherein EGF was monobiotinylated with NHS-PEG(3400)-biotin. Attachment of the [I-125]EGF-PEG(3400)-biotin to the OX26/SA conjugate did not impair binding of the construct to the EGF receptor in C6 glioma cells. The length of the -PEG- spacer arm and the -XX- spacer arm was >200 atoms and 14 atoms, respectively. These studies demonstrate that the use of the extended PEG linker releases steric hindrance of MAb transport vectors on binding of EGF to its cognate receptor on glioma cells. Attachment of EGF peptide radiopharmaceuticals to BBB drug delivery systems such as the OX26 MAb using extended PEG linkers allows for retention of the bifunctionality of the conjugate with binding to both EGF and transferrin receptors.