Abstract
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Objectives Cancer cells exhibit an altered metabolic profile compared to normal cells and can become highly dependent on the amino acid glutamine. ASCT2 (gene symbol SLC1A5) is the primary transporter responsible for glutamine import in many cancers. No imaging probes are currently available that specifically target ASCT2. Given the clinical relevance of ASCT2 in human cancers, this study aims to develop and evaluate ASCT2-targeted PET probes that could be deployed clinically for precision cancer imaging.
Methods High-throughput screening of candidate small molecules was conducted in live cell glutamine uptake assays using C6 and HEK293 cells were carried out in 96-well plates. 3H-glutamine was added concomitantly with inhibitor and incubated for 15 minutes followed by washing with buffer and lysing with NaOH. Scintillation fluid was added and plates were counted on a scintillation counter. Fifty percent inhibitory concentrations (IC50) were calculated using GraphPad Prism. All animal studies were conducted in accordance with federal and institutional guidelines. Prior to administration of imaging probe, animals were fasted between six to eight hours and allowed to acclimate to facility environment for at least fifteen minutes in a warmed chamber at 31.5°C. At the time of imaging, animals were anesthetized under 2% isofluorane and positioned in a microPET Focus 220 (Siemens Preclinical). A 60 min dynamic acquisition was initiated simultaneously with a retro-orbital injection of ASCT2 PET probe.
Results Focused library development led to a novel series of ASCT2 probes with significantly improved potency compared to prior art with our best compound exhibiting a potency of 7.2 micromolar in hASCT2. This represents a 40-fold improvement over previously reported ASCT2 inhibitors. Preliminary in vivo PET studies using a member of this series, showed significant tumor uptake compared to background in models of lung cancer known to be dependent on ASCT2-mediated glutamine uptake.
Conclusions We have developed the most potent inhibitors of ASCT2-mediated glutamine uptake reported to date and members of this class exhibit tractable properties for labeling with the positron-emitting isotope fluorine-18. Our preliminary in vivo evaluation of members of this class demonstrated that ASCT2 is a viable target for PET imaging of cancer.