Glutaminase inhibitor CB-839 increases radiation sensitivity of lung tumor cells and human lung tumor xenografts in mice

PURPOSE. The purpose of this study was to translate our in vitro therapy approach to an in vivo model. Increased glutamine uptake is known to drive cancer cell proliferation, making tumor cells glutamine-dependent. Studying lymph-node aspirates containing malignant lung tumor cells showed a strong correlation between glutamine consumption and glutathione (GSH) excretion. Subsequent experiments with A549 and H460 lung tumor cell lines provided additional evidence for glutamine's role in driving synthesis and excretion of GSH. Using stable-isotope-labeled glutamine as a tracer metabolite, we demonstrated that the glutamate group in GSH is directly derived from glutamine, linking glutamine utilization intimately to GSH syntheses. MATERIALS AND METHODS. To understand the possible mechanistic link between glutamine consumption and GSH excretion, we studied GSH metabolism in more detail. Inhibition of glutaminase (GLS) with BPTES, a GLS-specific inhibitor, effectively abolished GSH synthesis and excretion. Since our previous work, several novel GLS inhibitors became available and we report herein effects of CB-839 in A427, H460 and A549 lung tumor cells and human lung tumor xenografts in mice. RESULTS. Inhibition of GLS markedly reduced cell viability, producing ED(50) values for inhibition of colony formation of 9, 27 and 217 nM in A427, A549 and H460, respectively. Inhibition of GLS is accompanied by ~30% increased response to radiation, suggesting an important role of glutamine-derived GSH in protecting tumor cells against radiation-induce injury. In subsequent mouse xenografts, short-term CB-839 treatments reduced serum GSH by >50% and increased response to radiotherapy of H460-derived tumor xenografts by 30%. CONCLUSION. The results support the proposed mechanistic link between GLS activity and glutathione synthesis and suggest that GLS inhibitors are effective radiosensitizers.