Side Chain Optimization Remarkably Enhances the in Vivo Stability of ¹⁸F-Labeled Glutamine for Tumor Imaging

Similar to glycolysis, glutaminolysis acts as a vital energy source in tumor cells, providing building blocks for the metabolic needs of tumor cells. To capture glutaminolysis in tumors, ¹⁸F-(2S,4R)4-fluoroglutamine ([¹⁸F]FGln) and ¹⁸F-fluoroboronoglutamine ([¹⁸F]FBQ) have been successfully developed for positron emission tomography (PET) imaging, but these two molecules lack stability, resulting in undesired yet significant bone uptake. In this study, we found that [¹⁸F]FBQ-C₂ is a stable Gln PET tracer by adding two more methylene groups to the side chain of [¹⁸F]FBQ. [¹⁸F]FBQ-C₂ was synthesized with a good radiochemical yield of 35% and over 98% radiochemical purity. [¹⁸F]FBQ-C₂ showed extreme stability in vitro, and no defluorination was observed after 2 h in phosphate buffered saline at 37 °C. The competitive inhibition assay results indicated that [¹⁸F]FBQ-C₂ enters cells via the system ASC and N, similar to natural glutamine, and can be transported by tumor-overexpressed ASCT2. PET imaging and biodistribution results indicated that [¹⁸F]FBQ-C₂ is stable in vivo with low bone uptake (0.81 ± 0.20% ID/g) and can be cleared rapidly from most tissues. Dynamic scan and pharmacokinetic studies using BGC823-xenograft-bearing mice revealed that [¹⁸F]FBQ-C₂ accumulates specifically in tumors, with a longer half-life (101.18 ± 6.50 min) in tumor tissues than in other tissues (52.70 ± 12.44 min in muscle). Biodistribution exhibits a high tumor-to-normal tissue ratio (4.8 ± 1.7 for the muscle, 2.5 ± 1.0 for the stomach, 2.2 ± 0.9 for the liver, and 17.8 ± 8.4 for the brain). In conclusion, [¹⁸F]FBQ-C₂ can be used to perform high-contrast Gln imaging of tumors and can serve as a PET tracer for clinical research.