In Vitro and Initial In Vivo Evaluation of 68Ga-Labeled Transferrin Receptor (TfR) Binding Peptides as Potential Carriers for Enhanced Drug Transport into TfR Expressing Cells

The transferrin receptor (TfR) is one of the most attractive targets to overcome the blood-brain barrier (BBB). It has recently been shown that THRPPMWSPVWP binds to the TfR and is subsequently internalized into TfR-expressing cells. Here, we evaluated the ability of THRPPMWSPVWP to become internalized into human TfR-expressing cells via endocytosis to determine its potential to act as a carrier system for the transport of small molecules across the BBB. To validate the underlying concept of a conjugate consisting of a small brain imaging tracer and a large peptidic carrier molecule, a conjugate of the high affinity D2 receptor ligand fallypride and the TfR targeting peptide THRPPMWSPVWP has been synthesized. Furthermore, two derivatives of THRPPMWSPVWP were labeled with Ga-68 in high radiochemical yields (> 96%) and a radiochemical purity of 96-98% and evaluated in vitro and in vivo. The fallypride-THRPPMWSPVWP conjugate still displayed a K (i) of 27 nM. The uptake of the Ga-68-labeled peptides into TfR-bearing cells was investigated using U87MG and HT-29 cells to assess the capability of the peptide to act as a carrier molecule targeting the TfR. The in vitro binding studies revealed negligible uptake of the tested Ga-68-labeled conjugates ranging from 0.08% to 0.66% after 60 min incubation at 37A degrees C. Initial in vivo experiments with Ga-68-DOTA-S-maleimido-THRPPMWSPVWP in two healthy rats showed a mean brain uptake of 0.037% injected dose per gram, confirming the results obtained in vitro. These results suggest that the accumulation of the Ga-68-radiolabeled conjugates of the TfR-binding peptide THRPPMWSPVWP into TFR expressing human cell lines is nonspecific and too low to render this peptide suitable as a possible carrier molecule for a receptor-mediated transport of compounds across the BBB.