Radiosynthesis and biological evaluation of fluorine-18 labeled N-acetylgalactosamine derivative [ ¹⁸ F]FPGalNAc for PET imaging of asialoglycoprotein receptor-positive tumors

Introduction: The asialoglycoprotein receptor(ASGPR) is abundantly expressed on the surface of hepatocytes where it recognizes and endocytoses glycoproteins with galactosyl and N-acetylgalactosamine groups. ASGPR not only express on the surface of hepatocytes, but also express in several tumor cells (HepG2, A549 and HCT116). The purpose of this study was to develop a ASGPR-specific radiofluorinated ligand for positron emission tomography (PET) imaging in several tumor models.

Methods: The radiosynthesis of [¹⁸F]FPGalNAc was initiated with fluorine-18 and 5-(p-toluenesulfonyl)-1-yne. The obtained 5-[¹⁸F]fluoro-1-pentyne intermediate was then reacted with 2-acetamido-2-deoxy-β-d-galactopyranosyl azide using "click chemistry" to produce the final product. The K_d of the product was determined in HepG2 cells at a range of concentrations of [¹⁸F]FPGalNAc. Cellular uptake and blocking experiments were also performed. In vivo biodistribution studies were performed in nude mice bearing HCT116 tumor and micro positron emission tomography/computed tomography (PET/CT) evaluations were then performed in tumor-bearing mice (HepG2, HCT116) models.

Results: The radiosynthesis of [¹⁸F]FPGalNAc required 50 min with 5-6% RCY (radiochemical yield). The K_d of [¹⁸F]FPGalNAc to ASGPR in HepG2 cells was 0.25 ± 0.02 mM. Uptake values of 0.29% were observed within 30 min of incubation with HepG2 cells, which could be blocked by 200 mM d(+)-galactose (< 0.13%). The data of biodistribution revealed that the uptake of [¹⁸F]FPGalNAc was higher in kidneys and liver, and lower in muscle, bone and brain. In vivo micro PET studies, both HCT116 and HepG2 tumors showed high uptake for [¹⁸F]FPGalNAc, the radio of tumor/muscle (T/M) was 3.7 and 3.91, respectively.

Conclusions: In vitro assays and in vivo PET/CT imaging and biodistribution studies showed that [¹⁸F]FPGalNAc represents a promising tumor imaging agent that can provide insight into ASGPR related disease.

Keywords: ASGPR; Click chemistry; N-acetyl-galactosamine derivative; PET tumor imaging agent; [(18)F]FPGalNAc.