PET imaging and biodistribution analysis of an acetylated [⁶⁸Ga]GLP-1 analogue in a xenograft insulinoma mouse model

Objectives Glucagon-like peptide-1 receptor (GLP-1R), which is highly expressed in beta cells, is a promising biomarker for early diagnosis of type 1 diabetes and is also expressed in insulinomas. In this study we aim to demonstrate that Ga-68 labelled acetylated GLP-1 is an improved PET radiotracer, compared to the non-acetylated analogue, for imaging of GLP-1R expressing insulinoma.

Methods Stable GLP-1 analogues [D-Ala⁸,Lys(DOTA)³⁷]GLP-1-(7-37) amide (1) and [D-Ala⁸,Lys²⁶(Ac), Lys³⁴(Ac),Gly³⁶,Lys(DOTA)³⁷]GLP-1-(7-37) amide (2) were synthesized using solid-phase Fmoc peptide synthesis. IC₅₀ values were determined from competitive displacement assays, against [¹²⁵I]exendin, using the CHO/GLP-1R cell line. Ga-68 radiolabelling was performed using a 20 mCi Ge-68/Ga-68 generator and an automated modular lab unit. The radiochemical purity was determined by HPLC. PET imaging and biodistribution studies were performed in mice bearing INS-1 xenograft insulinoma that highly express GLP-1R.

Results Gallium coordinated peptides had strong GLP-1R affinity with IC₅₀ values for [^69/71Ga]-1= 8.1 nM and [^69/71Ga]-2 = 76 nM. Radiolabelling of 2 provided [⁶⁸Ga]-2 in >95% radiochemical purity. PET imaging showed acetylated [⁶⁸Ga]-2 uptake in the insulinoma tumor, with biodistribution data (90 min p.i.) indicating that tumor uptake (1.08% ± 0.12 ID/g) was higher than in other collected tissues except liver and kidney. The ratio of tumor/blood and tumor/muscle was 1.22 and 9.85 respectively. High urine and kidney values illustrated predominately renal clearance. Acetylation resulted in a decrease in kidney retention compared to non-acetylated [⁶⁸Ga]-1.

Conclusions Acetylated [⁶⁸Ga]GLP-1, while displaying similar potency, exhibits an improved pharmacokinetic profile when compared to the non-acetylated GLP-1 analogue, as demonstrated by insulinoma tumor uptake and a reduction in kidney uptake.