Improved radiolytic stability of a68ga‐labelled collagelin analogue for the imaging of fibrosis

Abstract

There is an unmet medical need for non‐invasive, sensitive, and quantitative methods for the assessment of fibrosis. Herein, an improved collagelin analogue labelled with gallium‐68 for use with positron emission tomography (PET) is presented. A cyclic peptide, c[CPGRVNleHGLHLGDDEGPC], was synthesized by solid‐phase peptide synthesis, conjugated to 2‐(4,7‐bis(2‐(tert‐butoxy)‐2‐oxoethyl)‐1,4,7‐triazonan‐1‐yl)acetic acid, and labelled with gallium‐68. High performance liquid chromatography (HPLC) was used for the quality and stability assessment of the collagelin analogue. Non‐specific organ distribution, blood clearance, and excretion rates were investigated in healthy mice and rats using ex vivo organ distribution analysis and dynamic in vivo PET/CT. Mice with carbon tetrachloride (CCl4) induced liver fibrosis were used for the investigation of specific binding via in vitro frozen section autoradiography, ex vivo organ distribution, and in vivo PET/CT. A non‐decay corrected radiochemical yield (48 ± 6%) of [68Ga]Ga‐ NOTA‐PEG2‐c[CPGRVNleHGLHLGDDEGPC] ([68Ga]Ga‐NO2A‐[Nle13]‐Col) with a radiochemical purity of 98 ± 2% was achieved without radical scavengers. The68Ga‐labelling was regioselective and stable at ambient temperature for at least 3 h. The autoradiography of the cryosections of fibrotic mouse liver tissue demonstrated a distinct heterogeneous radioactivity uptake that correlated with the fibrosis scores estimated after Sirius Red staining. The blood clearance and tissue washout from the [68Ga]Ga‐NO2A‐[Nle13]‐Col was fast in both normal and diseased mice. Dosimetry investigation in rats indicated the possibility for 4–5 PET/CT examinations per year. Radiolytic stability of the collagelin analogue was achieved by the substitution of methionine with norleucine amino acid residue without a deterioration of its binding capability. [68Ga]Ga‐NO2A‐ [Nle13]‐Col demonstrated a safe dosimetry profile suitable for repeated scanning.