[(18)F](2S,4S)-4-(3-Fluoropropyl)glutamine as a tumor imaging agent

Although the growth and proliferation of most tumors is fueled by glucose, some tumors are more likely to metabolize glutamine. In particular, tumor cells with the upregulated c-Myc gene are generally reprogrammed to utilize glutamine. We have developed new 3-fluoropropyl analogs of glutamine, namely [¹⁸F](2S,4R)- and [¹⁸F](2S,4S)-4-(3-fluoropropyl)glutamine, 3 and 4, to be used as probes for studying glutamine metabolism in these tumor cells. Optically pure isomers labeled with ¹⁸F and ¹⁹F (2S,4S) and (2S,4R)-4-(3-fluoropropyl)glutamine were synthesized via different routes and isolated in high radiochemical purity (≥95%). Cell uptake studies of both isomers showed that they were taken up efficiently by 9L tumor cells with a steady increase over a time frame of 120 min. At 120 min, their uptake was approximately two times higher than that of l-[³H]glutamine ([³H]Gln). These in vitro cell uptake studies suggested that the new probes are potential tumor imaging agents. Yet, the lower chemical yield of the precursor for 3, as well as the low radiochemical yield for 3, limits the availability of [¹⁸F](2S,4R)-4-(3-fluoropropyl)glutamine, 3. We, therefore, focused on [¹⁸F](2S,4S)-4-(3-fluoropropyl)glutamine, 4. The in vitro cell uptake studies suggested that the new probe, [¹⁸F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, is most sensitive to the LAT transport system, followed by System N and ASC transporters. A dual-isotope experiment using l-[³H]glutamine and the new probe showed that the uptake of [³H]Gln into 9L cells was highly associated with macromolecules (>90%), whereas the [¹⁸F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, was not (<10%). This suggests a different mechanism of retention. In vivo PET imaging studies demonstrated tumor-specific uptake in rats bearing 9L xenographs with an excellent tumor to muscle ratio (maximum of ∼8 at 40 min). [¹⁸F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, may be useful for testing tumors that may metabolize glutamine related amino acids.