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Clinical Infection & Immunity

Editorial

Volume 8, Number 1, March 2023, pages 1-4

Metformin as Antineoplastic Agent Against Glioblastoma

Derived from the legume Galega officinalis, metformin (MTF) was originally an antiviral drug used in influenza, and its hypoglycemic properties were just only one of its side effects [1]. It is currently used as the standard treatment for type 2 diabetes mellitus (T2DM) as it decreases glucose production in the liver [2]. MTF’s primary effect is at the level of the cellular respiratory chain [3], which helps explain its effects on various cell types [4].

Besides being widely used to treat T2DM and metabolic syndrome, MTF has immunomodulatory activity that reduces the production of pro-inflammatory cytokines by macrophages and neutrophils [5]. In fact, MTF’s anti-inflammatory effect, regardless of diabetes status, has shown to be beneficial in patients with COVID-19 [5-7]. MTF is also the first drug of choice for lowering glucose in diabetic patients with active tuberculosis, which is characterized with harmful inflammation that destroys granuloma architecture [8].

Besides its anti-inflammatory properties, MTF also has antioxidant, immunomodulatory, and antiviral capabilities, MTF has shown to induce cell cycle arrest in keratinocytes [9]. MTF has also been shown to inhibit the growth of human glioblastoma (HGBM) cells and to enhance the therapeutic response to this neoplasia.

MTF has been associated with improved survival in patients with various types of cancer. Although its exact mechanism of action is yet to be determined, numerous retrospective studies have identified a trend toward improved survival in glioblastoma patients treated with MTF [10-14] (Table 1).

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Table 1. Retrospective Studies of Survival in Glioblastoma Patients Treated With MTF

Overall survival (OS) has been the gold standard for demonstrating the end-point efficacy of a cancer drug [11, 12, 15]. An initial retrospective study on survival in diabetic glioblastoma patients showed MTF as one of the most important predictors of survival on multivariate analysis [11]. A systematic review of five studies analyzed MTF’s potential as an antineoplastic agent in brain tumors, with clinical outcomes assessed using progression-free survival (PFS) and OS values [16]. Although these studies illustrated prolonged survival in primary or secondary glioblastoma patients, one of the five studies did not demonstrate that the use of MTF was statistically associated with PFS or OS. The variation in results could have originated from differences in MTF dosage, duration of therapy and patient population [13]. In another study, although no negative influence of diabetes on progression and survival was detected, diabetic patients with MTF demonstrated prolonged progression-free intervals [14].

An in vitro study evaluating the antiproliferative activity of MTF on cultures isolated from four HGBMs showed statistically significant inhibition of cell viability after 24 h of MTF treatment, with a maximal reduction of cell viability after 72 h [17]. MTF exerted a cytostatic inhibition of growth with reduced number of cell divisions and, at higher concentrations, cytotoxic effects were observed.

One of the major drawbacks of in vitro studies suggesting MTF’s inhibitory effects on glioma cells is that MTF doses used are significantly higher than the concentrations measured in the brain of patients [10]. Also, the dosage of MTF that was administered was much higher than a standard antidiabetic dose [13]. Also, in vitro studies often use MTF doses in the millimolar range [17], whereas MTF doses in the brain of diabetic patients have been measured in the micromolar range [18]. Optimal dosing regimen of MTF as an adjunct antineoplastic agent is still under investigation.

The aforementioned systematic review discovered that the dosage of MTF for each study was either different or not indicated at all [16].

MTF has recently been postulated to decrease mammalian target of rapamycin (mTOR) signaling, a potent cellular proliferation and protein anabolism activator, via several mechanisms. One mechanism is the inhibition of the electron transport chain complex 1, which decreases adenosine triphosphate (ATP) concentration and changes the adenosine monophosphate (AMP)/ATP ratio within cells, exacerbating oxidative stress in the cell. Such AMP/ATP ratio also causes cellular adenosine-monophosphate-activated protein kinase (AMPK), a ser/thr kinase, to maintain a phosphorylated state by blocking phosphatase access to the phosphorylation site of AMPK [19], which in turn inhibits mTOR signaling complex allowing the cell to maintain a cytostatic state [20-22]. The end result is a halt of tumor growth progression.

MTF has also been shown to inhibit mTOR through mechanisms not involving AMPK [23]. This signaling, along with promoting a catabolic state in the cell, also increases mitochondrial biogenesis [24], a chain of cellular events happening in glioblastoma multiforme (GBM) cells [21]. Phosphorylated and activated p53 signaling downstream of AMPK due to the effects of MTF decreases unregulated cell growth. One particular mechanism of MTF on non-AMPK signaling is enhancement of the REDD1 protein. REDD1 allows mTOR to be sensitized to hypoxic changes within the cell, allowing mTOR to inactivate and for cell growth to arrest, especially in glioma cells [21].

In certain cancer cell types such as melanoma cells, MTF has also been shown to increase autophagy, reducing melanoma cell number in a dose-dependent fashion [25]. Through mTOR inhibition, markers of autophagy were increased in these melanoma cells, particularly LC3, a protein involved in autophagosome formation and synthesis.

In vivo and in vitro studies have demonstrated a synergistic antitumor role of MTF. One particular drug, temozolomide (TMZ), methylates DNA strands at guanine residues causing DNA replication errors, with ensuing apoptosis. TMZ has been well characterized in GBM. TMZ also activates upstream pathways that activate AMPK. The 6-O-methylguanine adducts formed by TMZ, are thought to activate p53, which activates downstream signaling molecules involving AMPK. There is evidence that MTF is able to potentiate the effects of TMZ in its pro-apoptotic effects [26, 27].

MTF has anti-inflammatory and antioxidant properties in murine nervous system [28]. MTF can protect against neuronal apoptotic cell death caused by trauma or sepsis as well. This is thought to be due to decreased nuclear factor-κB (NF-κB) translocation in the cell, and decreased production of inflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6 [29]. MTF decreases the activity of M1 macrophages as well, and allows differentiation to continue in the M2 or anti-inflammatory subtype, decreasing reactive oxygen species (ROS) production. ROS production inhibition is thought to be due to AMPK activation by the drug [30]. ROS can also cause DNA damage and mutagenesis in cells, which can be attenuated by MTF. MTF treatment has shown less DNA damage, and less mutagenesis of human fibroblasts [31]. MTF can induce apoptosis in many types of cancer cells via ROS by interfering with mitochondrial physiology. The central theme in tumor arrest is that MTF inhibits glycolysis in tumor cells which starves the cell. Different cancer cell types, nevertheless, respond in unique ways to MTF.

We are still discovering the versatility of MTF on central nervous system (CNS) tumors. Future knowledge may allow us to incorporate it into our therapeutic arsenal.

Acknowledgments

None to declare.

Financial Disclosure

None to declare.

Conflict of Interest

None to declare.

Author Contributions

JC contributed to conceptualization and introduction. DH contributed to clinical trials data organization. AK contributed to mechanistic aspects.

Data Availability

Any inquiries regarding supporting data availability of this study should be directed to the corresponding author.

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