PET/MR Imaging of a Lung Metastasis Model of Clear Cell Renal Cell Carcinoma with (2S,4R)-4-[¹⁸F]Fluoroglutamine

Metabolic reprogramming plays an important role in the tumorigenesis of clear cell renal cell carcinoma (ccRCC). Currently, positron emission tomography (PET) reporters are not used clinically to visualize altered glutamine metabolism in ccRCC, which greatly hinders detection, staging, and real-time therapeutic assessment. We sought to determine if (2S,4R)-4-[¹⁸F]fluoroglutamine ([¹⁸F]FGln) could be used to interrogate altered glutamine metabolism in ccRCC lesions in the lung.

We generated a novel ccRCC lung lesion model using the ccRCC cell line UMRC3 stably transfected with GFP and luciferase constructs. This cell line was used for characterization of [¹⁸F]FGln uptake and retention by transport analysis in cell culture and by PET/MRI (magnetic resonance imaging) in animal models. Tumor growth in animal models was monitored using bioluminescence (BLI) and MRI. After necropsy, UMRC3 tumor growth in lung tissue was verified by fluorescence imaging and histology.

In UMRC3 cells, [¹⁸F]FGln cell uptake was twofold higher than cell uptake in normal kidney HEK293 cells. Tracer cell uptake was reduced by 60-90% in the presence of excess glutamine in the media and by 20-50% upon treatment with V-9302, an inhibitor of the major glutamine transporter alanine-serine-cysteine transporter 2 (ASCT2). Furthermore, in UMRC3 cells, [¹⁸F]FGln cell uptake was reduced by siRNA knockdown of ASCT2 to levels obtained by the addition of excess exogenous glutamine. Conversely, [¹⁸F]FGln cellular uptake was increased in the presence of the glutaminase inhibitor CB-839. Using simultaneous PET/MRI for visualization, retention of [¹⁸F]FGln in vivo in ccRCC lung tumors was 1.5-fold greater than normal lung tissue and twofold greater than muscle. In ccRCC lung tumors, [¹⁸F]FGln retention did not change significantly upon treatment with CB-839.

We report one of the first direct orthotopic mouse models of ccRCC lung lesions. Using PET/MR imaging, lung tumors were easily discerned from normal tissue. Higher uptake of [¹⁸F]FGln was observed in a ccRCC cell line and lung lesions compared to HEK293 cells and normal lung tissue, respectively. [¹⁸F]FGln cell uptake was modulated by exogenous glutamine, V-9302, siRNA knockdown of ASCT2, and CB-839. Interestingly, in a pilot therapeutic study with CB-839, we observed no difference in treated tumors relative to untreated controls. This was in contrast with cellular studies, where CB-839 increased glutamine uptake.