68Ga-Labeled Glycopeptides as Effective Tools for Liver Function Imaging.

[^99mTc]Tc-GSA, an albumin-based glycoprotein, is routinely used in Japan to measure the asialoglycoprotein receptor (ASGR) density via single photon emission tomography. Here we describe the development of ⁶⁸Ga-labeled peptide-based alternatives. Peptides were assembled on a solid support using a fragment coupling strategy. Glycosylation was carried out via a click chemistry approach resulting in a set of three peptides with increasing amounts of d-galactose (n = 3, 6, and 9) as well as one glycopeptide bearing nine N-acetylgalactosamine residues. ⁶⁸Ga-labeling of all compounds could be achieved in high radiochemical yields (>95%). Radiotracers exhibited high hydrophilicity, good metabolic stability in human serum and protein binding between 12 and 22%. The IC₅₀ values improved in the series tri-, hexa-, and nonamer with an IC₅₀ of 50 ± 30 pM for the latter one. In analogy, the in vivo biodistribution studies revealed increased liver uptake in the series of [⁶⁸Ga]Ga-NODAGA-TriLysan (9.4 ± 2.0% ID/g, 30 min p.i.), [⁶⁸Ga]Ga-NODAGA-HexaLysan (55.5 ± 7.4% ID/g, 30 min p.i.), and [⁶⁸Ga]Ga-NODAGA-NonaLysan (79.6 ± 8.0% ID/g, 30 min p.i.). [⁶⁸Ga]Ga-NODAGA-GalNAc-NonaLysan reached comparable liver uptake to [⁶⁸Ga]Ga-NODAGA-NonaLysan, but showed higher accumulation in nontarget organs. The impressive imaging properties of [⁶⁸Ga]Ga-NODAGA-NonaLysan were also confirmed by the PET/MR imaging studies in mice. Hence, [⁶⁸Ga]Ga-NODAGA-NonaLysan represents a new PET radiopharmaceutical with even better imaging properties than [^99mTc]Tc-GSA.