18F-Fluciclovine PET Imaging of Glutaminase Inhibition in Breast Cancer Models

Aggressive cancers such as triple-negative breast cancer (TNBC) avidly metabolize glutamine as a feature of their malignant phenotype. The conversion of glutamine to glutamate by the glutaminase enzyme represents the first and rate-limiting step of this pathway and a target for drug development. Indeed, a novel glutaminase inhibitor (GLSi) has been developed and tested in clinical trials but with limited success, suggesting the potential for a biomarker to select patients who could benefit from this novel therapy. Here, we studied a nonmetabolized amino acid analog, F-18-fluciclovine, as a PET imaging biomarker for detecting the pharmacodynamic response to GLSi. Methods: Uptake of F-18-fluciclovine into human breast cancer cells was studied in the presence and absence of inhibitors of glutamine transporters and GLSi. To allow F-18-fluciclovine PET to be performed onmice, citrate in the tracer formulation is replaced by phosphate-buffered saline. Mice bearing triplenegative breast cancer (TNBC) xenografts (HCC38, HCC1806, and MBA-MD-231) and estrogen receptor-positive breast cancer xenografts (MCF-7) were imaged with dynamic PET at baseline and after a 2-d treatment of GLSi (CB839) or vehicle. Kinetic analysis suggested reversible uptake of the tracer, and the distribution volume (V-D) of F-18-fluciclovine was estimated by Logan plot analysis. Results: Our data showed that cellular uptake of F-18-fluciclovine is mediated by glutamine transporters. A significant increase in V-D was observed after CB839 treatment in TNBC models exhibiting high glutaminase activity (HCC38 and HCC1806) but not in TNBC or MCF-7 exhibiting low glutaminase. Changes in V-D were corroborated with changes in GLS activity measured in tumors treated with CB839 versus vehicle, as well as with changes in V-D of F-18-(2S,R4)-fluoroglutamine, which we previously validated as a measure of cellular glutamine pool size. A moderate, albeit significant, decrease in F-18-FDG PET signal was observed in HCC1806 tumors after CB839 treatment. Conclusion: F-18-fluciclovine PET has potential to serve as a clinically translatable pharmacodynamic biomarker of GLSi.