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Animal brain-tumor models have demonstrated a synergistic interaction between radiation therapy and a ketogenic diet (KD). Sheba Medical Center single-institution phase I clinical trial was performed to assess tolerability and feasibility of ketogenic diet (KD) combined with radiotherapy in gliomas (Porper et al. 2021). In this clinical trial, Radiotherapy was either 60 or 35 Gy over 6 or 2 weeks, for newly diagnosed (n=6) and recurrent (n=7) gliomas, respectively. The dietary intervention consisted of a Modified Atkins Diet (ModAD) supplemented with high-dose or low-dose metformin with the goal of increasing blood ketone concentration (beta hydroxybutyrate, bOHB). Factors associated with blood ketone levels were investigated, and unpublished clinical results indicated a positive correlation between b-OHB and blood Adiponectin (Adp). Adp is an adipocyte-secreted hormone that regulates lipid and glucose metabolism, and its receptors are differentially expressed in gliomas. Previous research indicated Adp protects fibroblasts, but not prostate or colorectal cancer cells, from radiation-induced death (Kosmacek & Oberley-Deegan 2020). We used a clonogenic assay to test the hypothesis that Adp and/or b-OHB affect response of cells in GL261 mouse glioma model to radiation.

GL261 cells were seeded in triplicate (600 cells/well in control, 1000 cells/well for treatment) in DMEM medium at either Low glucose levels (1g/L) or high glucose levels (4.5g/L). Following 24h incubation with either Adp (1ng/ml), b-OHB (5mM ), both Adp and b-OHB or medium only, cells were exposed to 0 ,2 or 4 Gray of radiation (Kimtron Polaris Biological Irradiator). Cells were incubated to form colonies for 2 weeks, after which cells were fixed, stained with crystal violet (0.5% v/w) and counted under a microscope. A colony was defined as consisting of at least 50 cells.  

Adiponectin or ketone treatments did not significantly affect the fraction of surviving cells, while the level of glucose in the medium affected both plating efficiency (PE) and order of magnitude of surviving fraction of cells. PE in High Glucose ranged between 45-65% while Low Glucose PE was 18-25%. Under High Glucose conditions, more than 10% of cells irradiated with 4 Gray created colonies, regardless of drug treatment, while in Low Glucose only 1% of cells created colonies regardless of drug treatment.  

Metabolic background was more effective in predicting response to radiation than metabolic treatments. While it is known that high blood glucose levels predispose cancer development, we show in a mouse model that high glucose levels also limit the effects of radiation treatment regardless of Adp or b-OHB levels. If replicated in human cell culture models and animal in vivo models, this work has implications for development of efficient metabolic treatments to synergize radiation therapy for cancer.