**5. Discussion**

Any discussion of a therapeutic agent is incomplete without covering its toxicity, side-effects and drug interactions. In this regard, metformin is probably one of the safest drugs in use, especially when compared with standard anti-cancer agents in its context as a potential cancer preventative or therapeutic. With its long history of widespread use, its pharmacokinetics and toxicity profile are well established. The most common side-effect is mild to moderate gastrointestinal discomfort or diarrhea which is usually self-limited and can be minimized if metformin is taken with food, while its most serious side-effect of lactic acidosis usually due to overdose is relatively rare, occurring once per 100,000 years of use or 3 case per 1,000,000 after long term treatment [97]. As in the case of all medications, it should be dispensed carefully in elderly patients and in those with impaired renal, cardiac, and hepatic function. For practical purposes, it needs to be emphasized that metformin as an antidiabetic and as monotherapy does not cause hypoglycemia or weight gain, unlike insulin or sulfonylureas. For cancer, because of its very common use in diabetics, it has practically seen combined use with most oncologic agents in the diabetic cancer patient and remarkably no serious interactions with standard cancer anti-cancer agents have been reported. The minimum toxic dose of metformin is not well defined, but rare case reports of severe toxicity has only been reported after ingestion of 25 to 35 grams of metformin by adults.

A treatment for any condition is ideal if it relatively non-toxic and scientifically well evidenced, as well as low in cost and convenient to administer. Metformin fits all the above criteria. It is apparent from our review that metformin has ample scientific evidence from bench to the bedside as a repurposed drug for cancer. In fact, it is safe to say that it is currently the most well evidenced repurposed drug for cancer. Also, its wide-spread and decades of experience of clinical use and low observed toxicities alone or in combination with other agents, as well as very low cost also marks it as an optimal therapeutic agent. Finally, the versatility it possesses against various cancers and its applicability from prevention to treatment further distinguishes it as an ideal or model repurposed drug for cancer.

Of course, there remains limitations and challenges to metformin's use as an anticancer. The first obstacle we have in translating *in vitro* results of metformin to the clinical arena revolves around its dosage. The usual dosage of metformin in cancer trials is the same range as that prescribed normally for T2DM which is from 1000 mg – 2000 mg per day. Treatment is usually started at the lower dose with dose escalations weekly to the maximum dose which means starting at 500 mg of the immediate release version twice daily or 850 mg of the extended release versions once daily, with 500 mg increments weekly as tolerated, to a maximum of 2000–2550 mg per day for either immediate or extended release versions. It may not be apparent at first glance, but the concentration of metformin at 10–100 microM when used clinically at 1000-2000 mg per day is much less than the concentration of >2–5 mM demonstrated for its anti-cancer effects *in vitro* where metformin was usually experimented at concentrations between 5 to 20 mM, which is 2 000–10 000 times more concentrated than achieved with clinical dosing [98]. Fortunately, many clinical studies still yielded positive results at the much lower metformin in concentrations achieved with clinical dosing, but it may also explain why the

clinical results of metformin in cancer may not be as dramatic as demonstrated in preclinical studies, and why it is never intended to be used as monotherapy for cancer treatment. Related to dosing is a possible dose-dependent effect of metformin on cancer risk [99], which raises the question of attempting higher doses of metformin in future clinical trials of metformin in cancer, this while taking into account that there are no cases of acute metformin overdose leading to death found in which patients with a peak serum metformin concentration is under 50 microg/ mL [100]. Beyond dosing, another issue with the literature to date on metformin and cancer is that most of the clinical studies so far are retrospective that mainly involve observations in the T2DM patient population and thus subject to selection bias. However, many cohort studies in the non-diabetic is planned, and despite methodological limitations, it is apparent that the overwhelming evidence so far is in favor of potential benefits and a high benefit to risk and benefit to cost ratios for metformin's application in cancer.
