The Effects of an Albumin Binding Moiety on the Targeting and Pharmacokinetics of an Integrin αvβ6-Selective Peptide Labeled with Aluminum [18F]Fluoride.

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The αvβ6-BP peptide selectively targets the integrin αvβ6, a cell surface receptor recognized as a prognostic indicator for several challenging malignancies. Given that the 4-[18F]fluorobenzoyl (FBA)-labeled peptide is a promising PET imaging agent, radiolabeling via aluminum [18F]fluoride chelation and introduction of an albumin binding moiety (ABM) have the potential to considerably simplify radiochemistry and improve the pharmacokinetics by increasing biological half-life. The peptides NOTA-αvβ6-BP (1) and NOTA-K(ABM)-αvβ6-BP (2) were synthesized on solid phase, radiolabeled with aluminum [18F]fluoride, and evaluated in vitro (integrin ELISA, albumin binding, cell studies) and in vivo in mouse models bearing paired DX3puroβ6 [αvβ6(+)]/DX3puro [αvβ6(-)], and for [18F]AlF 2, BxPC-3 [αvβ6(+)] cell xenografts (PET imaging, biodistribution). The peptides were radiolabeled in 23.0 ± 5.7% and 22.1 ± 4.4% decay-corrected radiochemical yield, respectively, for [18F]AlF 1 and [18F]AlF 2. Both demonstrated excellent affinity and selectivity for integrin αvβ6 by ELISA (IC50(αvβ6) = 3-7nM vs IC50(αvβ3) > 10μM) and in cell binding studies (51.0 ± 0.7% and 47.2 ± 0.7% of total radioactivity bound to DX3puroβ6 cells at 1h, respectively, vs. ≤ 1.2% to DX3puro for both compounds). The radiotracer [18F]AlF 1 bound to human serum at 16.3 ± 1.9%, compared to 67.5 ± 1.0% for the ABM-containing [18F]AlF 2. In vivo studies confirmed the effect of the ABM on blood circulation (≤ 0.1% ID/g remaining in blood for [18F]AlF 1 as soon as 1h p.i. vs. > 2% ID/g for [18F]AlF 2 at 6h p.i.) and higher αvβ6(+) tumor uptake (4h: DX3puroβ6; [18F]AlF 1: 3.0 ± 0.7% ID/g, [18F]AlF 2: 7.2 ± 0.7% ID/g; BxPC-3; [18F]AlF 2: 10.2 ± 0.1% ID/g). Both compounds were prepared using standard chemistries; affinity and selectivity for integrin αvβ6 in vitro remained unaffected by the albumin binding moiety. In vivo, the albumin binding moiety resulted in prolonged circulation and higher αvβ6-targeted uptake.