(4)-4-(3-[F]Fluoropropyl)-l-glutamate

The cystine (dimer of l-cysteine)/glutamate antiporter system x− is a heterodimeric, sodium-independent, and chloride-dependent anionic amino acid (aa) transporter that transports cystine out of the cell and glutamate into the cell at a ratio of 1:1 (1). In general, the cystine concentration within the cell is very low (because intracellular cystine is rapidly cleaved into two molecules of l-cysteine), and the intracellular concentration of glutamate is relatively higher than in the extracellular spaces. Cysteine is not only a component of most proteins; it is also required for the synthesis of glutathione (GSH; a l-γ-glutamyl-l-cysteinyl-glycine tripeptide), an important detoxifier that scavenges and reduces the reactive oxygen or nitrogen species (RO/NS) generated during the metabolic activity of cells (the RO/NS are known to oxidize and denature or modify the activity of proteins, lipids, and DNA in a cell) (2). In addition, GSH influences the intracellular redox signaling pathways during the progression of cell death (2). Therefore, system x− is believed to play an important role in regulating the concentration of GSH in a cell and plays an important role in several physiological processes, such as immune response, inflammation, cellular infection by oncogenic Kaposi’s sarcoma herpesvirus, pathogenesis of disorders of the eye (such as age-related macular degeneration) and central nervous system (such as Alzheimer's disease, epilepsy, and cerebral ischemia), growth and progression of cancer, and the development resistance to anticancer drugs by neoplastic cells; for a detailed discussion on the roles of system x− and GSH in health and disease, see Lewerenz et al., and Franco and Cidlowski (1, 2). Many cancerous tissues have a survival advantage over normal cells because they show a higher expression of system x−, accumulate above-normal levels of l-cysteine and l-glutamate, and maintain high levels of GSH to detoxify the RO/NS efficiently (3). Therefore, a variety of inhibitors that target the x− antiporter have been evaluated for drug sensitization or the treatment of cancer (3). Increased growth and proliferation are the typical characteristics of cancer cells. To maintain these processes, the cells have an increased demand for energy, exhibit increased macromolecules, and metabolize d-glucose and l-glutamine to produce elevated amounts of fatty acids and aa that are required for the survival of the tumor cells (4). There are indications that many tumors do not use the glycolytic pathway and metabolize nutrients such as l-glutamine to produce energy (5). [F]-Fluorodeoxyglucose ([F]-FDG), an analog of glucose that is transported into and metabolized similarly to glucose in the cell (after phosphorylation to [F]-FDG-6 phosphate, it cannot be further metabolized by glycolysis and remains metabolically trapped within the cell), is often used to detect, stage, and monitor cancer therapy with positron emission tomography (PET) [PubMed]. However, a major drawback of PET imaging with [F]-FDG is that, in addition to tumor cells, normal cells in the brain, heart, brown adipose tissue, etc., also utilize above-average amounts of glucose for energy generation, which can lead to the generation of false-positive results (6). Moreover, it is known that [F]-FDG imaging cannot always distinguish between infection, inflammation, and tumors (6). On the basis of the information described above, it was hypothesized that PET agents that target system x− can probably be used to visualize tumors that have an enhanced expression and activity of the antiporter (5). The F-labeled structural analog of l-glutamate 4-[F]fluoro-l-glutamate (BAY 85-8050) was prepared and evaluated with PET for the visualization of tumors in humans (5). studies showed that the labeled compound was transported by both system x− and the sodium-dependent glutamate transporters. In humans, the tumors showed some uptake of the label, but suboptimal PET images were obtained because the labeled compound was defluorinated while it was in circulation (5). Koglin et al. proposed that, to visualize tumors that overexpress system x− with PET, it is necessary to have an agent that is not only metabolically stable but should interact specifically with the system x− transporter (5). To test this, several F-labeled derivatives of l-glutamine were prepared, and it was shown that, among all the F-labeled compounds that were evaluated, only (4S)-4-(3-[F]fluoropropyl)-L-glutamate ([F]FSPG, [F]BAY 94-9392) was stable in mouse blood and could be used with PET to visualize tumors that overexpress system x− in mice and rats (5). In clinical studies, it was shown that [F]FSPG can be used with PET to visualize non-small cell lung cancer (NSCLC) and breast cancer lesions (7), as well as hepatocellular carcinoma (HCC) tumors (8) in humans.