Brain tumor PET imaging in a transgenic medulloblastoma mouse model using a novel 18F- labeled tryptophan tracer

63 Objectives: Abnormal tryptophan metabolism via the kynurenine pathway (KP) has been discovered in a range of neurological diseases, including brain tumors, epilepsy, and autism. Previously, α-[11C] methyl-L-tryptophan (11C-AMT) positron emission tomography (PET) tracer showed highly increased tryptophan metabolism and uptake through KP in patients with brain tumors, tuberous sclerosis, and epilepsy. However, the short half-life of carbon-11 isotope has limited the broad applications of 11C-AMT in clinical research and diagnostics. There have been significant efforts in developing 18F-labeled tryptophan PET tracers. Recent in vitro and in vivo studies suggest that a novel PET tracer, 1-(2-[18F]fluoroethyl)-L-tryptophan (L-1-[18F]FETrp), might have the same pathophysiological and metabolic properties as 11C-AMT and could be used to assess abnormal tryptophan metabolism via KP in human brain. In this study, we used L-1-[18F]FETrp for the PET imaging of brain tumors in Smo-Smo mice, a transgenic mouse model of the Sonic hedgehog (Shh) subgroup of medulloblastoma, the most common malignant pediatric brain tumor that usually develops in the cerebellum. Methods: L-1-[18F]FETrp was synthesized according to literature procedures on a PETCHEM automatic synthesizer. The tracer for injection was sterile filtered and formulated in 5% ethanol in sodium acetate buffer (0.05 M, pH = 5.5). PET imaging studies for both Smo-Smo transgenic mice bearing medulloblastoma tumors (N = 5) and normal control mice (N = 5) were performed on a Perkin-Elmer G4 PET-X-Ray scanner. PET images were acquired 1 hour after injection of the tracer through tail vein. The tumor standardized uptake values (SUVs) for L-1-[18F]FETrp were calculated by using VivoQuant 3.5 software. Tumor tissue samples were collected to analyze KP related enzyme expression levels using western blotting and immunofluorescence using antibodies for indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan-2,3-dioxygenase 2 (TDO2). Results: The decay-corrected L-1-[18F]FETrp tracer radiochemical yields were in the range of 10 - 20% (N = 5). The tracer had good chemical and radiochemical purities (>95% and > 99%, respectively), as well as good enantiomeric excess (ee) values (>95%) and specific activity (100 - 170 GBq/µmol). L-1-[18F]FETrp showed an increased tumor accumulation (SUV = 2.2 - 4.0) compared to normal brain tissues (SUV = 0.3 - 1.0). The tracer uptake ratio of tumor to normal brain tissue was in the range of 4.0 - 8.8. Brain tissue analyses showed strong TDO2 immunofluorescence in tumor compared to surrounding cerebellum. Western blotting showed strong TDO2 positive bands in tumor, while weak bands were observed for IDO1.Conclusions: L-1-[18F]FETrp showed clear retention in medulloblastoma tumors of Smo-Smo mice. Our preliminary medulloblastoma tumor tissue analysis indicates high expression of TDO2 enzyme, which suggests the tracer uptake reflects TDO2-mediated KP. These preclinical studies suggest that TDO2 may be a therapeutic target for medulloblastoma and L-1-[18F]FETrp has potential for PET imaging of medulloblastoma.