Development of a novel radiofluorinated riboflavin probe for riboflavin receptor-targeting PET imaging

Riboflavin receptor 3 (RFVT3) is a key protein in energetic metabolism reprogramming and is overexpressed in multiple cancers involved in malignant proliferation, angiogenesis, chemotherapy resistance, and immunosuppression. To enable non-invasive real-time quantification of RFVT3 in tumors, we sought to develop a suitable PET probe that would allow specific and selective RFVT3 imaging in vivo. A novel radiofluorinated riboflavin probe (18F-RFTA) based on riboflavin was synthesized and characterized in terms of radiochemical purity, hydrophilicity, binding affinity, and stability. Positron emission tomography (PET) imaging of 18F-RFTA was performed in U87MG tumor-bearing mice. Immunohistochemistry staining was carried out to determine the expression of RFVT3 in U87MG tumors. 18F-RFTA was characterized by high radiochemical purity and RFVT3 binding affinity, and remarkable stability in vitro and in vivo. Small-animal PET imaging with 18F-RFTA revealed significantly higher uptake in RFVT3-expressing U87MG tumors than in muscle. In conclusion, we have developed the first radiofluorinated riboflavin-based PET probe that is suitable for imaging RFVT3-positive tumors. The new target/probe system can be leveraged for extensive use in the diagnosis and treatment of RFVT3 overexpressing diseases, such as oncologic, cardiovascular, and neurodegenerative diseases.