Abstract
279
Introduction: Currently, hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related death worldwide. Natural amino acids such as methionine (Met) was radiolabeled with C-11 for cancer imaging based on upregulated amino acid transport and protein synthesis in different tumor types. However, the short half-life of C-11 decay limited further clinical development of [11C]-Met for liver cancer applications. The non-natural amino acids are advantageous for their ability to incorporate longer-lived radionuclides such as F-18; and the lack of radiolabeled metabolite formation. Synthetic amino acid analog trans-1-amino-3-[18F]fluoro-cyclobutane-1-carboxylic acid (anti-[18F]-FACBC, or FACBC) was developed and FDA-approved for PET imaging of recurrent prostate cancer. This study investigated FACBC when “repurposed” for PET imaging of primary liver cancer such as HCC.
Methods: FACBC was purchased from a commercial outlet. [11C]-Met was synthesized according to a published procedure. A clinically relevant animal model of spontaneously developed HCC in the woodchucks after chronic viral hepatitis infection was used for PET imaging experiments with the clinical PET/CT scanner. The expression level of amino acid transporters responsible for the uptake of radiotracers in HCC was analyzed using the RNA-seq data from The Cancer Genome Atlas (TCGA) for human HCC vs. the surrounding liver tissues; similar analysis was performed with the gene expression data from applying a customized woodchuck microarray onto the woodchuck tissue samples. Dynamic PET scans of [11C]-Met and FACBC were acquired from the same animal within one week. Standardized Uptake Value (SUV) was calculated for regions of interest (ROIs) defined over the HCC and a liver background region. The animals were euthanized after PET imaging for liver tissues harvested. Some of the tissues were fixed for H&E staining as well as immunohistochemical (IHC) staining for proliferation status. Other tissues were fresh-frozen for use in qRT-PCR.
Results: Both [11C]-Met and FACBC were obtained with satisfactory radiochemical yield and purity. Analyses of the human data from TCGA and the woodchuck microarray data showed a higher level of expression for ASCT2 and LAT1, which are highly homologous among the species (human and woodchuck), in HCC than in the surrounding liver tissues. PCR results validated this differential expression. Both Met and FACBC displayed higher uptake in liver cancer than in the liver tissues. Comparison between the two radiolabeled amino acids for their uptake and retention in HCC revealed some differences between the two. The uptake of Met in HCC reached a plateau with an SUV of 3.5 after 10 min post-injection, and maintained a contrast uptake from that in the liver. The uptake of FACBC in HCC also reached peak around 10 min with an SUV of 3.5, and then declined over the time along with that in the surrounding liver tissues. Pathological assessment revealed the liver tumor as moderately-differentiated as well as proliferative, similar to the human HCC.
Conclusions: Both FACBC and methionine displayed uptake in HCCs as shown through the use of a clinically relevant animal model of spontaneously developed HCC in the woodchucks. The uptake and retention of FACBC and methionine depends on tracer metabolism, and also relies possibly on different principal amino acid transporters, ASCT2 mainly for FACBC and LAT1 for methionine, which could lead to different uptake patterns of the two radiotracers for the same HCCs. Once further developed and validated, PET imaging with FACBC will potentially help a number of clinical management decisions.