Previously we have demonstrated that limiting glutamine availability through nutrient deprivation and disruption of amino acid transporters renders lung cancer cell lines exquisitely sensitive to radiation in vitro. Subsequent work with a pharmacologic inhibitor of glutaminase, CB-839 (Calithera Biosciences, USA), yielded similar results in vitro. We hypothesized that concurrent treatment with radiation therapy in vivo would exhibit improved tumor control and establish glutaminase inhibition as an effective radiosensitizer. A549 (lung adenocarcinoma cell line, ATCC# CCL-185) tumors were established in the bilateral hind legs of nude mice. Tumors were matched for size after three weeks and randomized into treatment groups of PBS (control) or 200 mg/kg CB-839 (experimental). On days 1-5 of treatment, mice received PBS or CB-839 by oral gavage. Four hours later, mice were anesthetized and irradiated in a caudal hemibody field encompassing the tumor + 1cm. Radiation was delivered via a 6MV beam with 1.5cm bolus, dosimetrically calculated to deliver 200 cGy to midplane. Mice were treated on sequential days to a total dose of 1000 cGy to the tumor. The populations were then monitored for 6 weeks with tumor dimensions [(L x W x W)/2] measured biweekly. Tumors were established in 9 mice, ranging in size from 72-880 mm3. The average tumor sizes by treatment group were 427 and 435 mm3 for controls and experimental, respectively (p=0.963) One mouse in the experimental group died unexpectedly between Day 1 and 2 of treatment, with no overt cause identified, and was omitted from further analysis. By 6 weeks post-completion of therapy, every tumor in the control group increased in size, with a range from +36% to +123%. In the CB-839 experimental group, several tumors decreased in size (-27 to -85%) and the remainder enlarged modestly (+24 to +90%). The mean tumor change across each group was 84% and 14%, respectively. An unpaired t-test yielded a two-tailed p value of 0.0164. We have established that concurrent delivery of oral glutaminase inhibitor sensitizes tumors to radiation. Repeat experiments are being performed to confirm these findings and elucidate a mechanism in vivo. Given the favorable safety profile of CB-839 in other disease sites, we anticipate a clinical trial for appropriately selected advanced stage lung cancer patients may be feasible in the near future.