Metformin As A Cancer Drug

I was thinking about a couple of one line quips I'd seen about oxygen based v. sugar based metabolism of cancer cells and the potential of Vitamin C as a cancer therapy yesterday, and why neither had been exploited, when it occurred to me that the drug metformin usually used for other purposes might be a good cancer drug based on those theories. 

I googled the idea, and what did I get?

Metformin is a standard clinical drug used to treat type 2 diabetes mellitus (T2DM) and polycystic ovary syndrome. 

Recently, epidemiological studies and meta-analyses have revealed that patients with T2DM have a lower incidence of tumor development than healthy controls and that patients diagnosed with cancer have a lower risk of mortality when treated with metformin, demonstrating an association between metformin and tumorigenesis. In vivo and in vitro studies have revealed that metformin has a direct antitumor effect, which may depress tumor proliferation and induce the apoptosis, autophagy and cell cycle arrest of tumor cells. 

The mechanism underpinning the antitumor effect of metformin has not been well established. Studies have demonstrated that reducing insulin and insulin-like growth factor levels in the peripheral blood circulation may lead to the inhibition of phosphoinositide 3-kinase/Akt/mechanistic target of rapamycin (mTOR) signaling or activation of AMP-activated protein kinase, which inhibits mTOR signaling, a process that may be associated with the antitumor effect of metformin. The present review primarily focuses on the recent progress in understanding the function of metformin in tumor development.

Fuming Zi, "Metformin and cancer: An existing drug for cancer prevention and therapy" 15(1) Oncol Lett. 683 (January 2018) (published online November 14, 2017) (emphasis added) 

doi: 10.3892/ol.2017.7412