<sup>18</sup>F-Fluciclovine PET Imaging of Glutaminase Inhibition in Breast Cancer Models
Rong Zhou, Hoon Choi, Jianbo Cao, Austin R. Pantel, Mamta Gupta, Hsiaoju Lee, David A. Mankoff
Abstract
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, <sup>18</sup>F-fluciclovine, as a PET imaging biomarker for detecting the pharmacodynamic response to GLSi. <b>Methods:</b> Uptake of <sup>18</sup>F-fluciclovine into human breast cancer cells was studied in the presence and absence of inhibitors of glutamine transporters and GLSi. To allow <sup>18</sup>F-fluciclovine PET to be performed on mice, citrate in the tracer formulation is replaced by phosphate-buffered saline. Mice bearing triple-negative 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<sub>D</sub>) of <sup>18</sup>F-fluciclovine was estimated by Logan plot analysis. <b>Results:</b> Our data showed that cellular uptake of <sup>18</sup>F-fluciclovine is mediated by glutamine transporters. A significant increase in V<sub>D</sub> 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<sub>D</sub> were corroborated with changes in GLS activity measured in tumors treated with CB839 versus vehicle, as well as with changes in V<sub>D</sub> of <sup>18</sup>F-(2S,R4)-fluoroglutamine, which we previously validated as a measure of cellular glutamine pool size. A moderate, albeit significant, decrease in <sup>18</sup>F-FDG PET signal was observed in HCC1806 tumors after CB839 treatment. <b>Conclusion:</b><sup>18</sup>F-fluciclovine PET has potential to serve as a clinically translatable pharmacodynamic biomarker of GLSi.