Single-photon emission computed tomography of spontaneous liver metastasis from orthotopically implanted human colon cancer cell line stably expressing human sodium/iodide symporter reporter gene

A recombinant plasmid containing a constitutively driven NIS gene (pcDNA3-NIS) was transfected into the human colon cancer cell line HCT116, and stable cell lines were established. The stable cells were subcutaneously injected into the nude mice. When the diameter reached 10？mm, the xenografts were excised, cut into small fragments, and orthotopically implanted into the cecal walls of another nude mice. 99mTcO4？ SPECT/CT imaging was initiated 8？weeks later and repeated every 1 to 2？weeks.The production and function of NIS protein was confirmed in vitro by Western blotting and 99mTcO4？ uptake assay. On SPECT/CT imaging, focal 99mTcO4？ uptake was detected in the liver. Necropsy revealed local growth of the orthotopic colon xenografts with extensive invasion, microscopic serosal metastasis, and metastatic foci in the corresponding hepatic regions showing focal 99mTcO4？ uptake. Immunohistochemistry revealed high levels of NIS expression in cells forming liver tumor, indicating that the liver tumor cells originated from the orthotopic colon xenografts.The present proof-of-concept study provided a rationale for employing a radionuclide reporter gene for the specific visualization of spontaneous liver metastasis in living mice. This unique animal model of clinically relevant and externally detectable liver metastasis will be a powerful tool for investigating tumor biology and developing novel therapies for cancer metastasis.Research using animal models that represent all processes of metastasis formation is essential. However, the currently used rodent tumor models, such as the subcutaneous (ectopic) tumor graft models in nude mice, do not sufficiently represent clinical cancer, particularly with regard to metastasis formation, since the subcutaneous micro-environment for visceral tumors is considerably different from their original environment [1,2]. Experimental liver metastasis can be induced by intrasplenic [3] or direct intraportal [4] injection of cancer cells in the