Characterizing the metabolic fate of (2S, 4R)-4-[18F]fluoroglutamine and its applications in cancer imaging and treatment response monitoring

Abstract

Glutaminolysis is a crucial pathway to provide the nitrogen source in addition to the carbon source from the glycolysis pathway for proliferating cells. (2S, 4R)-4-[18F]fluoroglutamine (4-[18F]fluoroglutamine) is potentially useful for Positron Emission Tomography (PET) imaging glutamine metabolism of cancer in vitro and in vivo. Despite initial promising results, including clinical trials, further metabolic characterisation of this probe is required to fully understand the metabolic fate of 4-[18F]fluoroglutamine in cancer cells. The overall aims of this project are to characterise the metabolic fate of 4-[18F]fluoroglutamine, to investigate whether it can be used as a non-invasive PET imaging method for [18F]FDG undetected tumours or for patients who have difficulty with the fasting requirement of [18F]FDG and to apply 4-[18F]fluoroglutamine for the monitoring of glutamine uptake changes in cancer following therapeutic strategies dual mTOR1/2 inhibitor AZD2014 and Paclitaxel, or Cdk2 kinase inhibitor CYC065. In this study, we employed the stable isotope version of this PET tracer and [19F]-Magnetic Resonance Spectroscopy (19F-MRS) to evaluate the metabolic fate of the 4-[18F]fluoroglutamine PET tracer. Similar to L-glutamine metabolism, glutaminase inhibition significantly increased the cellular 4-[19F]fluoroglutamine concentration and decreased the cellular level of 4-[19F]fluoroglutamate. Following alanine aminotransferase inhibition, the cellular concentration of 4[19F]fluoroglutamate increased in a similar way as L-glutamate. However, the rate of free F secretion increased, which is contrary to the hypothesis of the study. Therefore, this study proved that the metabolism of the 4-[18F]fluoroglutamine PET tracer follows the same pathway as L-glutamine in cancer cells. There are more than one pathway for the defluorination of 4-[18F]fluoroglutamine beside the catalysis by alanine aminotransferase enzyme. A comparison uptake study between 4-[18F]fluoroglutamine and [18F]FDG was performed in different cancer cell lines and tumour models. The results presented here indicate specific cancer cell types have a lower avidity for [18F]FDG than for 4-[18F]fluoroglutamine and suggest 4-[18F] fluoroglutamine can have potential future application to pateints who have difficulties with the fasting requirement of [18F]FDG. Finally, the changes in in vitro 4-[18F]fluoroglutamine uptake of which an increase following AZD2014 and a decrease following CYC065 treatments were achieved provide a necessary validation step towards establishing 4-[18F]fluoroglutamine as a metabolic imaging radiotracer of treatment response in cancer.