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

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 (GLS) enzyme represents the first and rate-limiting step of this pathway, and a target for drug development. Indeed, a novel GLS inhibitor (GLSi) has been developed and tested in clinical trials, but with limited success, suggesting the potential for a biomarker to select patients that could benefit from this novel therapy. Here, we study a non-metabolized amino acid analog, ¹⁸F-Fluciclovine (Axumin®) as a PET imaging biomarker for detecting the pharmacodynamic response to GLSi. We show that glutamine transporters mediate the uptake of ¹⁸F-Fluciclovine into human breast cancer cells. To allow ¹⁸F-Fluciclovine PET to be performed in mice, citrate in the tracer formulation is replaced by PBS. Mice bearing TNBC (HCC38, HCC1806 and MBA-MD-231) and ER-positive (MCF-7) breast cancer xenografts were imaged with dynamic PET at baseline and after a 2-day treatment of GLSi (CB839 (Telaglenastat)) or vehicle. Kinetic analysis suggested reversible uptake of the tracer and the distribution volume (VD) of ¹⁸F-Fluciclovine was estimated by Logan plot analysis. A significant increase of VD was observed after CB839 treatment in TNBC models exhibiting high GLS activity (HCC38 and HCC1806), but not in TNBC or MCF-7 exhibiting low GLS. Changes of VD were corroborated with changes in GLS activity measured in CB839- versus vehicle-treated tumors, as well as with changes of VD of [¹⁸F]-(2S,R4)-fluoroglutamine, which we previously validated as a measure of cellular glutamine pool size. A moderate, albeit significant decrease of [¹⁸F]fluorodeoxyglucose (FDG) PET signal was observed in HCC1806 tumors after CB839 treatment. In conclusion, ¹⁸F-Fluciclovine PET has potential to serve as a clinical translatable, pharmacodynamic biomarker of GLSi.