Chemotherapy has contributed to improvements in the outcome of colorectal cancer (1,2), yet the control of metastatic lesions is crucial for improving outcomes. The metastatic mechanisms of colon cancer are still not known in detail; however, previous studies by us and others have identified the key molecules related to liver metastasis in colon cancer (3–5), and pathological findings of budding and venous invasion of tumors are known to be important in predicting the outcome of colorectal cancer (6,7). Clinically, ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG-PET/CT) fusion imaging is a useful tool for evaluating the stage, recurrence, outcome, and effectiveness of treatment in human cancers (8–11). Some studies have reported that PET/CT is useful in colon cancer (12–14). However, PET/CT imaging has a limitation in revealing hepatic metastasis due to normal uptake in the liver (15).

Many studies have shown that in vivo studies with subcutaneous xenografts in nude mice and severe combined immunodeficient (SCID) mice are useful for analyzing human cancers (16–19), whereas they are limited in evaluating internal organ metastases associated with human cancers. Immunodeficient mice commonly show poor metastatic lesions in vivo. We reported that distant metastatic lesions were easily reproduced by human cancer cell lines in newly developed NOD/Shi-scid/IL-2Rγ^null (NOG) mouse models employing systemic injections (20–23). It was confirmed that the experimental metastasis model of human cancer using NOG mice was more sensitive and easier to achieve than that using SCID mice due to their multiple immunological dysfunctions not only in cytokine production capability, but also in the functional competence of T, B and NK cells (24,25). We previously developed a reliable new model for assaying hepatic metastasis with superimmunodeficient NOG mice (26,27).

Some in vivo studies have evaluated metastatic lesions of internal organs in mice with conventional modalities such as bio-luminescence imaging (BLI), but difficulties were found in revealing the mechanisms of metastasis (28). Therefore, a new in vivo system to detect internal organ metastatic lesions is required to simulate the clinical situation of metastatic lesions using reliable and quantitative approaches.

In this study, we examined whether ¹⁸F-FDG-PET/CT scans could semi-quantitatively reveal abnormal FDG uptakes in hepatic metastasis of human colon cancer cell line HCT116 in NOG mice, thereby overcoming the above described limitations encountered when evaluating metastatic lesions in the liver. Moreover, liver metastasis in NOG mice was studied by histopathological analysis. We showed here that the evaluation of metastatic lesions in NOG mice by PET/CT may be an extremely useful in vivo metastatic model.

In this study, we clearly and quantitatively demonstrated increased multiple ¹⁸F-FDG uptakes in hepatic metastases of human colonic cancer cell line HCT116 in NOG mouse models using a small animal PET/CT system. We previously reported that ¹⁸F-FDG-PET/CT clinically is a very useful imaging modality to evaluate human cancers, including rare carcinoma cases (31–34). Many reports have shown that animal PET imaging is useful for evaluating human cancer xenografts in vivo. It is usually difficult to reveal metastases as conventional in vivo models are limited in their evaluation of metastases associated with human cancers (28,35). It is easy to produce experimental liver metastases in immunocompromised NOG mice because of their multiple immunological dysfunctions in cytokine production capability as well as functional competence of T, B and NK cells (24–27). However, quantitative evaluation of experimental liver metastases is still being developed. We previously reported the use of liver weight as an index of metastatic hepatomegaly or microvessel counts to reflect tumor cell activity (23,36). We present herein that the model system with NOG mice and a small animal PET/CT system has the feasibility of a quantitative metastasis model to simulate clinical situations. Several studies have reported the development of new anticancer therapies and novel drugs for human cancers using animal PET (37,38). This model system presented here with NOG mice and a small animal PET/CT system may be useful for developing new anticancer therapies and novel drugs for hepatic metastases of human cancers.

Although PET imaging with ¹⁸F-FDG is routinely used for tumor detection and therapy monitoring, several limitations have been reported with ¹⁸F-FDG, such as high uptake in normal tissues of the brain and inflammatory tissues (15,39). In contrast, positron-labeled amino acids and their analogs are considered to be useful because of the low rate of amino acid use in normal control tissues. Natural and unnatural artificial labeled amino acids have been reported (40,41). This hepatic metastasis model, using intrasplenic injection of small numbers of cancer cells into NOG mice, is reliable and quantitative, and more closely mimics in vivo conditions (26). New compounds, which are more sensitive and specific for the detection of human cancers, are expected to replace ¹⁸F-FDG (42,43). The hepatic metastasis model system in NOG mice combined with small animal PET/CT analysis will be useful in developing new labeling compounds for PET.

In conclusion, the hepatic metastasis model system in NOG mice combined with PET/CT has the feasibility to simulate clinical situations, and this model system is useful for analyzing mechanisms of metastasis and developing new therapeutic approaches for metastatic lesions of human cancers.