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

Abstract

Purpose: The α_vβ₆-BP peptide selectively targets the integrin α_vβ₆, 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.

Procedures: The peptides NOTA-α_vβ₆-BP (1) and NOTA-K(ABM)-α_vβ₆-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 [α_vβ₆(+)]/DX3puro [α_vβ₆(-)], and for [¹⁸F]AlF 2, BxPC-3 [α_vβ₆(+)] cell xenografts (PET imaging, biodistribution).

Results: 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 α_vβ₆ by ELISA (IC₅₀(α_vβ₆) = 3-7 nM vs IC₅₀(α_vβ₃) > 10 μM) and in cell binding studies (51.0 ± 0.7 % and 47.2 ± 0.7 % of total radioactivity bound to DX3puroβ6 cells at 1 h, 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 1 h p.i. vs. > 2 % ID/g for [¹⁸F]AlF 2 at 6 h p.i.) and higher α_vβ₆(+) tumor uptake (4 h: 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).

Conclusion: Both compounds were prepared using standard chemistries; affinity and selectivity for integrin α_vβ₆ in vitro remained unaffected by the albumin binding moiety. In vivo, the albumin binding moiety resulted in prolonged circulation and higher α_vβ₆-targeted uptake.

Keywords: Albumin; Albumin binding moiety; Aluminum [18F]fluoride; Blood circulation; Integrin αvβ6; PET imaging; Peptide.