DOE OSTI2015
Radiotracers based on the peptide A20FMDV2 selectively target the cell surface receptor integrin α v β 6 . This integrin has been identified as a prognostic indicator correlating with the severity of disease for several challenging malignancies. In previous studies of A20FMDV2 peptides labeled with 4- 18 F-fluorobenzoic acid ( 18 F-FBA), we have shown that the introduction of poly(ethylene glycol) (PEG) improves pharmacokinetics, including increased uptake in α v β 6 -expressing tumors. The present study evaluated the effect of site-specific C-terminal or dual (N- and C-terminal) PEGylation, yielding 18 F-FBA-A20FMDV2-PEG 28 (4) and 18 F-FBA-PEG 28 -A20FMDV2-PEG 28 (5), on α v β 6 -targeted tumor uptake and pharmacokinetics. The results are compared with 18 F-FBA–labeled A20FMDV2 radiotracers (1–3) bearing either no PEG or different PEG units at the N terminus. The radiotracers were prepared and radiolabeled on solid phase. Using 3 cell lines, DX3puroβ6 (α v β 6 +), DX3puro (α v β 6 –), and BxPC-3 (α v β 6 +), we evaluated the radiotracers in vitro (serum stability; cell binding and internalization) and in vivo in mouse models bearing paired DX3puroβ6–DX3puro and, for 5, BxPC-3 xenografts. Here, the size and location of the PEG units significantly affected α v β 6 targeting and pharmacokinetics. Although the C-terminally PEGylated 4 showed some improvements over the un-PEGylated 18 F-FBA-A20FMDV2 (1), it was the bi-terminally PEGylated 5 that displayed the more favorable combination of high α v β 6 affinity, selectivity, and pharmacokinetic profile. In vitro, 5 bound to α v β 6 -expressing DX3puroβ6 and BxPC-3 cells with 60.5% ± 3.3% and 48.8% ± 8.3%, respectively, with a significant fraction of internalization (37.2% ± 4.0% and 37.6% ± 4.1% of total radioactivity, respectively). By comparison, in the DX3puro control 5 showed only 3.0% ± 0.5% binding and 0.9% ± 0.2% internalization. In vivo, 5 maintained high, α v β 6 -directed binding in the paired DX3puroβ6–DX3puro model (1 h: DX3puroβ6, 2.3 ± 0.2 percentage injected dose per gram [%ID/g]; DX3puroβ6/DX3puro ratio, 6.5:1; 4 h: 10.7:1). In the pancreatic BxPC-3 model, uptake was 4.7 ± 0.9 %ID/g (1 h) despite small tumor sizes (20–80 mg). In 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 2 PEG units acted synergistically to result in an improved metabolic profile including high α v β 6 + tumor uptake and retention.
59 BASIC BIOLOGICAL SCIENCES↗