Metformin: an old drug with new potential

Abstract

Metformin is the most commonly prescribed antidiabetic oral agent. It has also been used off-label for polycystic ovarian syndrome, steatohepatitis, and HIV-associated metabolic abnormalities. However, this oldie is a newbie for the oncologist. Population studies have suggested that metformin decreased the incidence and mortality rates of cancer in diabetic patients. With better understanding of its mechanisms of antitumor activity, metformin may become a new drug for cancer in combination with chemotherapy or targeted therapy.

Keywords::

• cancer
• diabetes
• HIV
• metformin

1. Introduction

Metformin is one of the most prescribed drugs in the offices of internists and endocrinologists, with over half a century of use as an antidiabetic agent. Recently, this pharmaceutical old-timer has become a new kid on the block for oncologists. In this editorial, we discuss this old-but-new drug from the perspectives of an endocrinologist, an internist, and an oncologist.

2. Background

The chemical basis of metformin (1-(diaminomethylidene)-3,3-dimethylguanidine) is derived from Galega officinalis (goat's rue or French lilac) [1]. Metformin is a member of the biguanide class of compounds. Like other biguanides, it was developed as a therapeutic for hyperglycemia and diabetes mellitus in the 1950s [2-5]. Metformin absorption occurs mainly in the small intestine, it has an oral bioavailability of 50 – 60%, and it is excreted unmetabolized from the kidneys. Its half-life is about 5 h [6,7].

3. Clinical use in diabetes mellitus

One-third of adults in the United States have pre-diabetes [8-10]. Type 2 diabetes mellitus occurs in 3.7% of adults between 20 and 44 years in age, but the prevalence reaches 27% in adults over 65 years [11]. Metformin use has been associated with about a 31% reduced risk of developing type 2 diabetes mellitus in a high-risk population [12]. In the UK Prospective Diabetes Study, metformin was superior to sulfonylureas and insulin in reducing the incidence of macrovascular and microvascular complications in patients with type 2 diabetes with lower risk of hypoglycemia. Furthermore, metformin use in overweight patients was associated with 36% lower risk of all-cause mortality compared to patients treated with diet alone [13]. In a separate prospective trial, adding metformin to insulin therapy ameliorated insulin-induced weight gain, improved diabetes control, as well as reduced macrovascular complications (Figure 1) [14].

Figure 1. Mechanism of action and clinical uses of metformin.

Metformin therapy improves insulin sensitivity and reduces the fasting plasma glucose and insulin concentrations. It decreases hepatic glucose output by inhibiting gluconeogenesis and enhances peripheral glucose uptake by muscles and adipose tissues. Additionally, it increases the intestinal use of glucose and decreases fatty acid oxidation. When compared with sulfonylurea, metformin was associated with lower risk of decline in kidney function. This effect was independent of weight, blood pressure, and glycated hemoglobin level [15]. Rarely does metformin cause hypoglycemia since it neither stimulates insulin secretion nor modulates glucose-counter-regulatory mechanisms [16].

The anti-hyperglycemic effects of metformin are complex and likely multifactorial. In the past decade, many details have emerged about metformin's molecular mechanisms of action. The key function of metformin is reducing hepatic gluconeogenesis but the exact molecular targets are elusive. The activation of AMP-activated protein kinase (AMPK) in hepatocytes was thought to be the key mechanism to explain metformin's antidiabetic activity [17], but this intriguing theory was challenged by other alternative mechanisms that were uncovered to explain metformin's effects. Metformin was found to reduce the activity of glucose 6 phosphatase, and thus, reduced hepatic glucose production. Also, the inhibiting cell respiration by modulating mitochondrial respiratory chain complex-1 is gaining traction as the most likely explanation for metformin effects [18,19].

Simultaneously, metformin indirectly enhances glucose-dependent insulin secretion by increasing the secretion of glucagon-like peptide-1 (GLP-1) from intestinal L cells and possibly by reducing GLP-1 degradation [20,21]. Recently, metformin was also shown to antagonize the hyperglycemic effect of glucagon in the liver by increasing intracellular AMP leading to the inhibition of key intracellular enzymes (such as protein kinase A and adenylate cyclase) [22].

4. Clinical use for non-diabetes indications

4.1 Polycystic ovarian syndrome

Metformin has also been used off-label for the treatment of polycystic ovarian syndrome (PCOS), a condition that may include hirsutism and reduced fertility, in the context of obesity and insulin resistance [23]. Metformin improves hirsutism and acne in patients with PCOS [24]. In patients with PCOS, metformin normalizes the quality and activation of molecules that regulate insulin-dependent glucose transporter (GLUT4). Metformin reverses GLUT4 endocytosis and increases GLUT4 mRNA expression, which, in turn, improves endometrial metabolic function and reproductive outcomes (Figure 1) [25,26].

4.2 Steatohepatitis

In non-diabetic pediatric patients with non-alcoholic steatohepatitis (NASH), metformin enhances insulin sensitivity, reduces liver fat, improves liver function, and enhances quality of life [27]. By reducing accumulation of free fatty acid in liver significantly, metformin suppresses oxidation of fatty acids which is responsible for hepatocyte inflammation and injury. However, it is noteworthy to observe that improvement in liver histology is linked to weight reduction contributed by metformin (Figure 1) [28,29].

Metformin-activated AMPK is a major cellular regulator of lipid and glucose metabolism. This activation leads to inhibition of glucose production reduction, induction of oxidation of fatty acid, and suppression of expression of lipogenic enzymes. Additionally, metformin treatment leads to suppression of expression of SREBP-1, a key lipogenic transcription factor in rat primary hepatocytes [17]. More recent studies, using genetic-loss-of function models, show that metformin also has AMPK-independent effects on the counter-regulatory hormone glucagon [30]. Further study is needed in this area to clarify how to incorporate metformin into currently available treatments for NASH.

4.3 HIV-associated metabolic abnormalities

Among HIV-infected patients – particularly in HIV-infected patients on treatment with protease inhibitors and nucleoside analogs – abnormalities in lipid and glucose metabolism, as well as lipodystrophy and lipoatrophy, are increasingly recognized. Elevated glucose, triglyceride, total cholesterol, and low-density lipoprotein (LDL) levels are commonly observed. In HIV-associated lipodystrophy, metformin leads to a reduction in insulin area under the curve and insulin levels 120 min after oral glucose tolerance test, suggesting an overall improvement in insulin sensitivity. There is also a decrease in visceral abdominal fat, which is increased in patients with HIV and is considered a risk factor for coronary artery disease in non-HIV-infected patients [31]. A potential benefit of use of metformin in combination with exercise training in this population has been suggested by a randomized trial; however, another randomized trial suggested that metformin use be reserved for patients with adequate peripheral fat and marked insulin resistance (Figure 1) [32,33].

4.4 Cardiovascular protective effect

By positively affecting cardiac risk factors, metformin offers cardiovascular protection. Metformin lowers glucose level and hence reduces oxidative stress and lipid oxidation. It lowers total cholesterol, LDL, and triglyceride levels while maintaining or increasing high-density lipoprotein levels [4,34]. In both diabetic and non-diabetic patients, metformin decreases the level of plasminogen activator inhibitor (PAI-1), thus improving fibrinolysis and reducing clot formation. Moreover, it reverses platelet aggregation [35,36]. Metformin improves vascular function and has acute sympathetic inhibitory actions in spontaneously hypertensive rats (Figure 1) [37,38].

5. Metformin and cancer

5.1 Mechanisms of action

Several mechanisms of action of metformin have been proposed for its activities as an antineoplastic agent (Figure 1). Activation of AMPK by metformin plays an important role in mediating its effects. AMPK activation results in phosphorylation and activation of tuberous sclerosis complex 2 (TSC2), a tumor suppressor gene, which exerts an inhibitory effect on the mammalian target of rapamycin (mTOR), a serine-threonine kinase involved in initiating mRNA translation [39]. Ultimately, AMPK activation results in inhibition of cellular protein synthesis and cell growth, making metformin an effective treatment for cancer [2,40,41].

Metformin-induced activation of AMPK has also been shown to disrupt crosstalk between insulin/IGF-1 receptor and G-protein-coupled receptor signaling in pancreatic cancer cells and inhibits the growth of these cells in xenograft models [42]. Another explanation of the antiproliferative effect of metformin is that it reverses hyperinsulinemia and thus decreases the stimulation that insulin exerts on tumor cells via the insulin receptor. Thereby, metformin decreases intracellular signaling through the phosphoinositide 3-kinase inhibitor/AKT/mTOR leading to the inhibition of cancer cell growth and proliferation. Metformin further decreases intracellular signaling through the activated Ras/Raf/MEK/ERK pathway, which is known to increase cell proliferation and inhibit apoptosis [43,44].

Dowling et al. demonstrated that metformin treatment led to a 30% decrease in global protein synthesis in MCF-7 breast cancer cells, which express liver kinase B1 (LKB1) [2]. However, metformin had no effect on protein synthesis in MDA-MB-231 breast cancer cells, which do not express LKB1 [2]. In MCF-7 cells, metformin treatment led to a dose-dependent increase in the phosphorylation of AMPK and a strong decrease in the phosphorylation of S6K and 4E-BP1, the two major downstream effectors of mTOR. Interestingly, metformin treatment had no effect on the phosphorylation of AMPK, S6K, and 4E-BP1 in MDA-MB-231 cells. This provides strong evidence that LKB1 is required for AMPK activation and mTOR inhibition by metformin, as well as for metformin's inhibition of mRNA translation [2].

Additionally, Dowling et al. also demonstrated that TSC2 is necessary for metformin's inhibitory effects [2]. Metformin inhibited the growth of TSC2^+/+ mouse embryonic fibroblasts (MEFs) by 53% but did not affect TSC2^-/- MEFs. Treatment of cells with an AMPK inhibitor prevented metformin's inhibitory effect on translation. These observations, taken together, provide proof that metformin's effect on translation is mediated by the activation of AMPK, which ultimately results in mTOR pathway inhibition and in inhibited translation and cell growth [2].

Additionally, nuclear factor-κB, a regulatory factor for MDR1 gene transcription, is suppressed by metformin, leading to decreased expression of P-glycoprotein and reduced drug resistance [45-47].

In vitro, metformin inhibits proliferation in multiple cancer cell lines, an effect likely mediated through activation of AMPK. Tumor types that have been studied in vitro include breast, colon, and prostate [40,44,48,49]. Metformin, in combination with a histone deacetylase inhibitor, exerted an antitumor effect against an osteosarcoma cell line in vivo and in vitro [50]. Moreover, phenformin, another biguanide, induced apoptosis in LKB1-mutant non-small cell lung cancer cells [51].

5.2 Cancer prevention

Population studies have suggested that metformin may decrease the incidence and mortality rate of cancer in diabetic patients [52-54]. In a retrospective study of early-stage breast cancer, the rate of pathologic complete response was higher in diabetic patients taking metformin (24%) than in the non-metformin diabetic group (8%) or in patients without diabetes (16%). Metformin was independently predictive of a pathologic complete response [55].

5.3 Metformin in cancer treatment

It would be naïve to assume that metformin alone may be enough to treat various types of cancer. However, metformin can be combined with chemotherapy or targeted therapy. These combinations should be driven by preclinical findings. Suggested cancers include, but not limited to, endometrial, breast, prostate, non-small cell lung cancer, and hepatocellular cancers. For example, metformin is being considered as a novel drug for treating endometrial cancer [56]. Metformin reversed progestin resistance and sensitized endometrial cancer cells to chemotherapy by repressing expression of glyoxalase 1 enzyme [56,57]. Metformin has been shown to potentiate the effect of paclitaxel in endometrial cancer cells through inhibition of cell proliferation and modulation of the mTOR pathway [58,59]. It has been shown that metformin promotes progestin receptor expression via inhibition of mTOR in endometrial cell lines; therefore, the combination of metformin with an mTOR inhibitor should be investigated as a synergistic therapeutic option for endometrial carcinoma [60]. It would be of interest to observe whether there is a clinical response to metformin in patients with TSC2 mutation or LKB1 mutation.

6. Side effects, precautions, and contraindications

The most common side effects of metformin are diarrhea, nausea, vomiting, metallic taste, bloating, flatulence, and anorexia [61]. Chronic treatment with metformin impairs absorption vitamin B12 and increases its accumulation in the liver, leading to a decrease in its accumulation in kidney and circulation. However, clinical vitamin B12 deficiency is not common [62].

The most serious toxicity of metformin is thought to be lactic acidosis [63]; however, there is still controversy as to whether use of metformin truly increases the risk of this condition. In a systematic review of over 200 trials, the incidence of lactic acidosis was 5.1 cases per 100,000 individuals treated with metformin, compared with 5.8 cases per 100,000 non-metformin-treated individuals [64]. Even if the risk exists, it can be mitigated when metformin use is avoided in patients with hepatic, cardiac, or renal failure and in patients aged 80 years or older.

Metformin is contraindicated in lactating women and in patients with diabetic ketoacidosis, renal failure or dysfunction, hepatic insufficiency or alcohol abuse, heart failure, and conditions of tissue hypoxia, such as myocardial infarction or sepsis. Metformin is also contraindicated with the use of iodinated contrast dye (1 day before and 2 days after the imaging procedure) [65,66].

7. Conclusions

Metformin is widely used in management of diabetes. It is frequently used as an off-label treatment for polycystic ovarian disease. Further clinical trials are needed to establish how to incorporate metformin into current treatment of NASH and when to use metformin for HIV-associated metabolic abnormalities.

In oncology, mechanisms of action of metformin are increasingly better understood. Retrospective and meta-analyses of trials showed that metformin had preventive effects against cancer. Neoadjuvant trials demonstrated anticancer activities in prostate and endometrial cancers. In the clinical setting, there is a trend toward increased use of metformin in non-diabetic cancer patients owing to emerging data on metformin's effects against cancer. Early-phase clinical trials combining metformin with a chemotherapy regimen or targeted therapy are needed to evaluate toxicity and efficacy and determine a recommended Phase II trial dose. Ultimately, these early trials should lead to randomized Phase II trials in mutation- or organ-specific cancers, which could result in transforming the newly discovered translational results for this old drug into practical use as a new cancer agent.

Declaration of interest

The authors state no conflict of interest and have received no payment in preparation of this manuscript.