Radiosensitizing potential of Phenformin in a preclinical colorectal cancer model.

Background and Purpose: Radiotherapy is the treatment of choice for colorectal cancer. Tumor hypoxia is a negative prognostic factor, due in part to its role in causing resistance to radiotherapy. Hypoxia arises in tumor regions distal to blood vessels as oxygen is consumed by more proximal tumor cells. Reducing the rate of oxygen consumption is therefore a potential strategy to reduce tumor hypoxia. We hypothesize that Phenformin, a biguanide, which reduces oxygen consumption through inhibition of mitochondrial complex I, would improve radiation. Moreover, may Phenformin exert antitumor effects via activation of AMPK. In this study we investigated the beneficial effect of combination of Phenformin and radiotherapy on colon cancer.

Material and Methods: Two human (DLD1 and HCT116) and one mice (CT26) colorectal cancer cell lines were subjected to metabolic hypoxia and analyzed for radiosensitivity by clonogenic assay. Seahorse and expression of AMPK and mTOR measured their oxygen consumption by Western blotting CT26 cells were injected in Balb/c mice and treated with Phenformin through oral gavage in combination or alone with radiation therapy.

Results: Phenformin radiosensitizes all the three cell lines subjected to metabolic hypoxia. It seems that the radiosensitizing effect of Phenformin is associated with oxygen sparing, but not AMPK activation. In vivo we observed tumor growth delay in the CT26 tumors after treatment with Phenformin + radiation, there was also some tumor growth delay after treatment with Phenformin alone. The treatment of Phenformin with radiation even cured some mice, which could mean that Phenformin has an immune modulating effect in vivo.

Conclusion: Phenformin has a clear radiosensitizing effect both in vitro as in vivo on colorectal cancer cell lines. The main mechanism of this effect in vitro is possible through oxygen sparing, but in vivo immune modulation could play a role