| Abstract
| - Human brain gliomas overexpress the receptor for epidermal growth factor (EGF), and radiolabeledEGF is a potential peptide radiopharmaceutical for imaging human brain tumors, should this peptidebe made transportable through the blood-brain barrier (BBB) in vivo. Peptide drug delivery to thebrain may be facilitated by conjugating peptide radiopharmaceuticals to BBB drug delivery vectorssuch as the OX26 monoclonal antibody (MAb), which undergoes receptor-mediated transcytosis throughthe 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[125I]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 boundto a conjugate of streptavidin (SA) and the OX26 MAb. An alternative linker strategy using poly(ethylene glycol) (PEG) of 3400 Da molecular mass (PEG3400) was evaluated, wherein EGF wasmonobiotinylated with NHS-PEG3400-biotin. Attachment of the [125I]EGF-PEG3400-biotin to the OX26/SA conjugate did not impair binding of the construct to the EGF receptor in C6 glioma cells. Thelength 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 MAbtransport vectors on binding of EGF to its cognate receptor on glioma cells. Attachment of EGF peptideradiopharmaceuticals to BBB drug delivery systems such as the OX26 MAb using extended PEGlinkers allows for retention of the bifunctionality of the conjugate with binding to both EGF andtransferrin receptors.
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