Abstract
Radiotracers based on the peptide A20FMDV2 selectively target the cell surface receptor integrin αvβ6. This integrin has been identified as a prognostic indicator correlating with the severity of disease for several challenging malignancies. In previous mouse imaging studies of A20FMDV2-peptides labeled with 4-[18F]fluorobenzoic acid ([18F]FBA), we have shown that introduction of a poly(ethylene glycol) (PEG) unit improves pharmacokinetics, including increased uptake in αvβ6-expressing tumors. The present study evaluated the effect of site-specific C-terminal or dual (N- and C-terminal) PEGylation, yielding [18F]FBA-A20FMDV2-PEG28 (4) and [18F]FBA-PEG28-A20FMDV2-PEG28 (5), on αvβ6-targeted tumor-uptake and pharmacokinetics of the radiotracers. The results are compared to [18F]FBA-labeled A20FMDV2 radiotracers (1-3) bearing either no PEG, or different PEG units at the N-terminus. Methods: The radiotracers were prepared and radiolabeled on solid phase. Using three cell lines, DX3puroβ6 [αvβ6(+)], DX3puro [αvβ6(-)], and BxPC-3 [αvβ6(+)], the radiotracers were evaluated in vitro (serum stability; cell binding and internalization) and in vivo in mouse models bearing paired DX3puroβ6/DX3puro and, for 5, BxPC-3 xenografts. Results: Size and location of the PEG units significantly affected αvβ6-targeting and pharmacokinetics. While the C-terminally PEGylated 4 showed some improvements over the unPEGylated [18F]FBA-A20FMDV2 (1), it was the bi-terminally PEGylated 5 that displayed the more favorable combination of high αvβ6-affinity, -selectivity and pharmacokinetic profile. In vitro, 5 bound to the αvβ6-expressing cell lines DX3puroβ6 and BxPC-3 with 60.5±3.3% and 48.8±8.3%, respectively, with a significant fraction of the radioactivity internalized (37.2±4.0% and 37.6±4.1% of total radioactivity, respectively). By comparison, the DX3puro control showed only 3.0±0.5% binding and 0.9±0.2% internalization. In vivo, 5 maintained high, αvβ6-directed binding in the paired DX3puroβ6/DX3puro model (1 h: DX3puroβ6: 2.3±0.2% ID/g; DX3puroβ6/DX3puro ratio: 6.5/1; 4 h: 10.7/1). In the pancreatic BxPC-3 model the uptake was 4.7±0.9% ID/g (1 h) despite small tumor sizes (20-80 mg). Conclusion: The bi-PEGylated radiotracer 5 showed a greatly improved pharmacokinetic profile, beyond what was predicted from individual N- or C-terminal PEGylation. It appears that the two PEG units acted synergistically to result in an improved metabolic profile including high αvβ6(+)-tumor uptake and retention. PEGylation is a simple and straight-forward way of achieving these goals.
- Animal Imaging
- Molecular Imaging
- Peptides
- PET
- PEGylation
- integrin αvβ6
- metabolism
- peptide
- positron emission tomography
- Copyright © 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.