Radiotracers for Molecular Imaging

In clinical practice, FDG PET is currently used with a variety of human cancers such as nonsmaU cell lung cancers, colorectal, esophageal, breast, and thyroid carcinomas, melanomas, lymphomas, head and neck cancers, and other tumors (38). 

Because inflammatory cells exhibit low protein metabolism when compared with their glucose metabolism, inflammatory tissues obscure radiolabeled amino acid uptake less than they do FDG uptake (41). Similarly, labeled amino acids may be useful tools to evaluate brain lesions, or lesions that show hypo-or isometabolic uptake of FDG (42). 

The majority of PET studies in oncology have been performed with C-labeled methionine, which is relatively easy to synthesize (41-43). Because of its low uptake in the brain, [ Cjmethionine has been proven to be a sensitive and specific tool for brain tumors, able to differentiate nonneoplastic from malignant lesions (42,44). Methionine uptake reflects the amino acid transport, whereas [ Cjtyrosine has been used to quantify protein synthesis rate in brain tumors (45,46). An I-iodinated tyrosine analog (IMT) also has been proposed to assess tumor cells proliferation by means of SPECT imaging. IMT uptake in glioma cells in vitro (47) and in soft-tissue sarcomas in vivo (48) was shown to be induced by rapid proliferation. 

Choline is another essential substrate for cell membrane synthesis, as it is a precursor for the biosynthesis of phosphatidylcholine (52). In preliminary clinical studies, [ CJcholine has shown potential for detection of mmors in areas where FDG lacks sensitivity (53-55). High tumor to background ratios were found for many malignant brain tumors. However, in these tumors, [ CJcholine PET did not allow for malignancy grade estimation (Table 1.2) (55). 

Because cell growth requires an adequate energy supply, FDG uptake has often been used as an indirect measure of tumor proliferation (57). Correlation between FDG uptake and tumor cells proliferation was confirmed in ghoma (58), meningioma (59), lymphoma (60), ovarian epithelial carcinoma (57), lung cancer (61-63), and squamous-cell carcinoma of the head and neck (64). 1-[ C]Acetate, [ C]choline, and radiolabeled amino acids are other PET tracers used to indirectly measure tumor proliferation, because cell growth and proliferation inevitably necessitate membrane constiments and proteins (65). 

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