Investigation of Fluorine-18 Labelled Peptides for Binding to Cholecystokinin-2 Receptors with High Affinity

Abstract

Many cancers of neuroendocrine origin overexpress cholecystokinin-2 receptors (CCK-2R) including medullary thyroid cancer, small cell lung cancer and other lung carcinoids. Fluorine-18 labelled peptides targeting CCK-2R enable direct visualization and quantification of this receptor in vivo using positron emission tomography imaging. CP04 1 and MG11 2 are two previously described truncated peptides derived from the native CCK-2R hormone ligand, gastrin. The N-terminus of the MG11 2 octopeptide was chemically modified with various fluorine containing aromatic (4-fluorobenzoate), heterocyclic (6-fluoronicotinate) and aliphatic (2-fluoropropionate) moieties. To assess the impact these modifications had on CCK-2R binding, ligand-binding assays were conducted using A431 cells overexpressing human CCK-2R. MG11 2 modified by 4-fluorobenzoate (FB-MG11 3) demonstrated the highest binding affinity (0.20 nM) followed by MG11 2 modified by 6-fluoronicotinate (FNic-MG11 4; 0.74 nM) and 2-fluoropropionate (FP-MG11 5; 1.80 nM), respectively. Whilst indirect labelling of MG11 2 using fluorine-18 labelled activated esters of fluorobenzoate and 6-fluoronicotinate was unsuccessful, direct fluorine-18 labelling at the N-terminus modified with 6-nitronicotinate afforded a 47.6% radiochemical yield of [18F]FNic-MG11. Unfortunately, [18F]FNic-MG11 4 was chemically unstable, decomposing slowly through defluorination, thereby impeding any further work with this radiotracer.

Development of the First 18F-Labeled Radiohybrid-Based Minigastrin Derivative with High Target Affinity and Tumor Accumulation by Substitution of the Chelating Moiety

“…In 2021, Khan et al reported the first attempts to introduce scaffolds into the peptide structure of the CCK-2R-targeting minigastrin analog MG11 (glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH 2 ) that enables direct radiofluorination via a nucleophilic aromatic substitution of a nitro group by [ 18 F]fluoride (K 2 CO 3 , kryptofix 2.2.2, azeotropic drying). However, rapid de-fluorination due to the poor chemical stability of these compounds was observed, which illustrates the need for optimized alternatives [ 14 ]. In addition, the earlier studies of Good et al demonstrated a poor metabolic stability for MG11 in vivo [ 15 , 16 ], aggravating the use of this basic structure as a scaffold for radiopharmaceuticals, which is why alternative strategies are desired.…”