Enhanced anti-tumor activity and cytotoxic effect on cancer stem cell population of metformin-butyrate compared with metformin HCl in breast cancer

Microbiota-Derived Natural Products Targeting Cancer Stem Cells: Inside the Gut Pharma Factory

Cancer stem cells (CSCs) have drawn much attention as important tumour-initiating cells that may also be crucial for recurrence after chemotherapy. Although the activity of CSCs in various forms of cancer is complex and yet to be fully elucidated, opportunities for therapies targeting CSCs exist. CSCs are molecularly distinct from bulk tumour cells, so they can be targeted by exploiting their signature molecular pathways. Inhibiting stemness has the potential to reduce the risk posed by CSCs by limiting or eliminating their capacity for tumorigenesis, proliferation, metastasis, and recurrence. Here, we briefly described the role of CSCs in tumour biology, the mechanisms involved in CSC therapy resistance, and the role of the gut microbiota in cancer development and treatment, to then review and discuss the current advances in the discovery of microbiota-derived natural compounds targeting CSCs. Collectively, our overview suggests that dietary intervention, toward the production of those identified microbial metabolites capable of suppressing CSC properties, is a promising approach to support standard chemotherapy.

Gut Metabolites and Breast Cancer: The Continuum of Dysbiosis, Breast Cancer Risk, and Potential Breast Cancer Therapy

The complex association between the gut microbiome and cancer development has been an emerging field of study in recent years. The gut microbiome plays a crucial role in the overall maintenance of human health and interacts closely with the host immune system to prevent and fight infection. This review was designed to draw a comprehensive assessment and summary of recent research assessing the anticancer activity of the metabolites (produced by the gut microbiota) specifically against breast cancer. In this review, a total of 2701 articles were screened from different scientific databases (PubMed, Scopus, Embase and Web of Science) with 72 relevant articles included based on the predetermined inclusion and exclusion criteria. Metabolites produced by the gut microbial communities have been researched for their health benefits and potential anticancer activity. For instance, the short-chain fatty acid, butyrate, has been evaluated against multiple cancer types, including breast cancer, and has demonstrated anticancer potential via various molecular pathways. Similarly, nisin, a bacteriocin, has presented with a range of anticancer properties primarily against gastrointestinal cancers, with nominal evidence supporting its use against breast cancer. Comparatively, a natural purine nucleoside, inosine, though it has not been thoroughly investigated as a natural anticancer agent, has shown promise in recent studies. Additionally, recent studies demonstrated that gut microbial metabolites influence the efficacy of standard chemotherapeutics and potentially be implemented as a combination therapy. Despite the promising evidence supporting the anticancer action of gut metabolites on different cancer types, the molecular mechanisms of action of this activity are not well established, especially against breast cancer and warrant further investigation. As such, future research must prioritise determining the dose-response relationship, molecular mechanisms, and conducting animal and clinical studies to validate in vitro findings. This review also highlights the potential future directions of this field.

Breast and gut microbiome in health and cancer

The microbiota plays essential roles in health and disease, in both the intestine and the extra-intestine. Dysbiosis of the gut microbiota causes dysfunction in the intestine, which leads to inflammatory, immune, and infectious diseases. Dysbiosis is also associated with diseases beyond the intestine via microbial translocation or metabolisms. The in situ breast microbiome, which may be sourced from the gut through lactation and sexual contact, could be altered and cause breast diseases. In this review, we summarize the recent progress in understanding the interactions among the gut microbiome, breast microbiome, and breast diseases. We discuss the intestinal microbiota, microbial metabolites, and roles of microbiota in immune system. We emphasize the novel roles and mechanisms of the microbiome (both in situ and gastrointestinal sourced) and bacterial products in the development and progression of breast cancer. The intestinal microbial translocation suggests that the gut microbiome is translocated to the skin and subsequently to the breast tissue. The gut bacterial translocation is also due to the increased intestinal permeability. The breast and intestinal microbiota are important factors in maintaining healthy breasts. Micronutrition queuine (Q) is derived from a de novo synthesized metabolite in bacteria. All human cells use queuine and incorporate it into the wobble anticodon position of specific transfer RNAs. We have demonstrated that Q modification regulates genes critical in tight junctions and migration in human breast cancer cells and a breast tumor model. We further discuss the challenges and future perspectives that can move the field forward for prevention, diagnosis, and treatment of breast diseases.

Synthesis and Characterization of Lipophilic Salts of Metformin to Improve Its Repurposing for Cancer Therapy

Epidemiological evidence has accentuated the repurposing of metformin hydrochloride for cancer treatment. However, the extreme hydrophilicity and poor permeability of metformin hydrochloride are responsible for its poor anticancer activity in vitro and in vivo. Here, we report the synthesis and characterization of several lipophilic metformin salts containing bulky anionic permeation enhancers such as caprate, laurate, oleate, cholate, and docusate as counterions. Of various counterions tested, only docusate was able to significantly improve the lipophilicity and lipid solubility of metformin. To evaluate the impact of the association of anionic permeation enhancers with metformin, we checked the in vitro anticancer activity of various lipophilic salts of metformin using drug-sensitive (MYCN-2) and drug-resistant (SK-N-Be2c) neuroblastoma cells as model cancer cells. Metformin hydrochloride showed a very low potency (IC50 ≈ >100 mM) against MYCN-2 and SK-N-Be2c cells. Anionic permeation enhancers showed a considerably higher activity (IC50 ≈ 125 μM to 1.6 mM) against MYCN-2 and SK-N-Be2c cells than metformin. The association of metformin with most of the bulky anionic agents negatively impacted the anticancer activity against MYCN-2 and SK-N-Be2c cells. However, metformin docusate showed 700- to 4300-fold improvement in anticancer potency compared to metformin hydrochloride and four- to five-fold higher in vitro anticancer activity compared to sodium docusate, indicating a synergistic association between metformin and docusate. A similar trend was observed when we tested the in vitro activity of metformin docusate, sodium docusate, and metformin hydrochloride against hepatocellular carcinoma (HepG2) and triple-negative breast cancer (MDA-MB-231) cells.

Save your gut save your age: The role of the microbiome in stem cell ageing

The tremendous importance of microbiota in microbial homoeostasis, alterations in metabolism and both innate and adaptive immune systems has been well established. A growing body of evidence support that dysbiosis or compositional changes in gut microbiota is linked to the ageing of stem cells in terms of dysregulations of metabolism, aberrant activation of the immune system as well as promoting epigenetic instability of stem cell. In this concise review, we elucidate recent emerging topics on microbiotic alterations and underlying mechanisms in stem cell ageing.

The Biguanides Metformin and Buformin in Combination with 2-Deoxy-glucose or WZB-117 Inhibit the Viability of Highly Resistant Human Lung Cancer Cells

The biguanides metformin (MET) and to a lesser extent buformin (BUF) have recently been shown to exert anticancer effects. In particular, MET targets cancer stem cells (CSCs) in a variety of cancer types but these compounds have not been extensively tested for combination therapy. In this study, we investigated in vitro the anticancer activity of MET and BUF alone or in combination with 2-deoxy-D-glucose (2-DG) and WZB-117 (WZB), which are a glycolysis and a GLUT-1 inhibitor, respectively, in H460 human lung cancer cells growing under three different culture conditions with varying degrees of stemness: (1) routine culture conditions (RCCs), (2) floating lung tumorspheres (LTSs) that are enriched for stem-like cancer cells, and (3) adherent cells under prolonged periods (8-12 days) of serum starvation (PPSS). These cells are highly resistant to conventional anticancer drugs such as paclitaxel, hydroxyurea, and colchicine and display an increased level of stemness markers. As single agents, MET, BUF, 2-DG, and WZB-117 potently inhibited the viability of cells growing under RCCs. Both MET and BUF showed a strong synergistic effect when used in combination with 2-DG. A weak potentiation was observed when used with WZB-117. Under RCCs, H460 cells were more sensitive to MET and BUF and WZB-117 compared to nontumorigenic Beas-2B cells. While LTSs were less sensitive to each single drug, both MET and BUF in combination with 2-DG showed a strong synergistic effect and reduced cell viability to similar levels compared to the parental H460 cells. Adherent cells growing under PPSS were also less sensitive to each single drug, and MET and BUF showed a strong synergistic effect on cell viability in combination with 2-DG. Overall, our data demonstrates that the combination of BGs with either 2-DG or WZB-117 has “broad-spectrum” anticancer activities targeting cells growing under a variety of cell culture conditions with varying degrees of stemness. These properties may be useful to overcome the chemoresistance due to intratumoral heterogeneity found in lung cancer.

Identification of tRNA-derived small noncoding RNAs as potential biomarkers for prediction of recurrence in triple-negative breast cancer

Triple‐negative breast cancer (TNBC), an intrinsic subtype of breast cancer, is characterized by aggressive pathology and shorter overall survival. Yet there is no effective therapy for these patients. Breast cancer stem cells (BCSCs) in TNBC may account for treatment failure. It is urgent to identify new therapeutic targets for TNBC. tRNA‐derived small noncoding RNAs (tDRs) represent a new class of small noncoding RNAs (sncRNA), which have been reported in some human diseases and biological processes. However, there is no detailed information about the relationship between tDRs and BCSCs. In this study, a population of CD44⁺/CD24^−/low cells was isolated and identified by reliable BCSC surface markers. tDR expression profiles in TNBC and non‐TNBC CSCs were performed by RNA sequencing. A total of 1327 differentially expressed tDRs contained 18 upregulated and 54 downregulated in TNBC group. Furthermore, the selected tDRs were validated by quantitative reverse transcription‐polymerase chain reaction (qRT‐PCR). tDR‐000620 expression level was consistently lower in TNBC cell lines CSCs (all P < 0.05) and serum samples (t = 2.597, P = 0.013). tDR‐000620 expression was significant association with age (P = 0.018), node status (P = 0.026) and local recurrence (P = 0.019) by chi‐square test. Kaplan‐Meier method with log‐rank test for comparison of recurrence curves. The results showed that the tDR‐000620 expression (P = 0.002) and node status (P = 0.001) group were statistically significant with recurrence‐free survival. Furthermore, multivariate Cox regression demonstrated that lymphatic metastasis (HR, 3.616; 95% CI, 1.234‐10.596; P = 0.019) and low tDR‐000620 expression (HR, 0.265; 95% CI, 0.073‐0.959; P = 0.043) were two independent adverse predictive factors for recurrence‐free survival. Finally, we found that tDR‐000620 participated in some important biological processes though GO and KEGG analysis. Taken together, our study reveals the expression profiles of tDRs in TNBC and non‐TNBC CSCs. It offers helpful information to understand the tDR‐000620 expression is responsible for the aggressive phenotype of BCSCs. It may provide predictive biomarkers and therapeutic targets for TNBC recurrence.

Inhalational supplementation of metformin butyrate: A strategy for prevention and cure of various pulmonary disorders

The management of chronic lung diseases such as cancer, asthma, COPD and pulmonary hypertension remains unsatisfactory till date, and several strategies are being tried to control the same. Metformin, a popular anti-diabetic drug has shown promising effects in pre-clinical studies and has been subject to several trials in patients with debilitating pulmonary diseases. However, the clinical evidence for the use of metformin in these conditions is disappointing. Recent observations suggest that metformin use in diabetic patients is associated with an increase in butyrate-producing bacteria in the gut microbiome. Butyrate, similar to metformin, shows beneficial effects in pathological conditions found in pulmonary diseases. Further, the pharmacokinetic data of metformin suggests that metformin is predominantly concentrated in the gut, even after absorption. Butyrate, on the other hand, has a short half-life and thus oral supplementation of butyrate and metformin is unlikely to result in high concentrations of these drugs in the lung. In this paper, we review the pre-clinical studies of metformin and butyrate pertaining to pathologies commonly encountered in chronic lung diseases and underscore the need to administer these drugs directly to the lung via the inhalational route.

Current applications of antibody microarrays

The concept of antibody microarrays is one of the most versatile approaches within multiplexed immunoassay technologies. These types of arrays have increasingly become an attractive tool for the exploratory detection and study of protein abundance, function, pathways, and potential drug targets. Due to the properties of the antibody microarrays and their potential use in basic research and clinical analytics, various types of antibody microarrays have already been developed. In spite of the growing number of studies utilizing this technique, few reviews about antibody microarray technology have been presented to reflect the quality and future uses of the generated data. In this review, we provide a summary of the recent applications of antibody microarray techniques in basic biology and clinical studies, providing insights into the current trends and future of protein analysis.

Metformin use and cervical cancer risk in female patients with type 2 diabetes

This study evaluated whether metformin may affect the risk of cervical cancer. The reimbursement databases of the Taiwan's National Health Insurance were used. Female patients with type 2 diabetes at an onset age of 25-74 years during 1999-2005 and newly treated with metformin (n=132971, "ever users of metformin") or other antidiabetic drugs (n=6940, "never users of metformin") were followed for at least 6 months until December 31, 2011. The treatment effect of metformin (for ever versus never users, and for tertiles of cumulative duration of therapy) was estimated by Cox regression incorporated with the inverse probability of treatment weighting using propensity score. Analyses were also conducted in a 1:1 matched pair cohort based on 8 digits of propensity score. Results showed that the respective numbers of incident cervical cancer in ever users and never users were 438 (0.33%) and 38 (0.55%), with respective incidences of 68.29 and 121.38 per 100,000 person-years. The overall hazard ratio suggested a significantly lower risk in metformin users (0.558, 95% confidence intervals: 0.401-0.778). In tertile analyses, the hazard ratios (95% confidence intervals) for the first (<23.0 months), second (23.0-47.9 months) and third (>47.9 months) tertile of cumulative duration were 1.272 (0.904-1.790), 0.523 (0.366-0.747) and 0.109 (0.070-0.172), respectively. Findings were supported by the analyses in the matched cohort. In conclusion, metformin may significantly reduce the risk of cervical cancer, especially when the cumulative duration is more than 2 years.

Targeting cancer stem cells in the clinic: Current status and perspectives

Resistance to chemotherapy and cancer relapse are major clinical challenges attributed to a sub population of cancer stem cells (CSCs). The concept of CSCs has been the subject of intense research by the oncology community since evidence for their existence was first published over twenty years ago. Emerging data indicates that they are also able to evade novel therapies such as targeted agents, immunotherapies and anti-angiogenics. The inability to appropriately identify and isolate CSCs is a major hindrance to the field and novel technologies are now being utilized. Agents that target CSC-associated cell surface receptors and signaling pathways have generated promising pre-clinical results and are now entering clinical trial. Here we discuss and evaluate current therapeutic strategies to target CSCs.

In vitro and in vivo characterization of stem-like cells from canine osteosarcoma and assessment of drug sensitivity

Cancer stem cell (CSC) self-renewing and drug resistance cause treatment failure and tumor recurrence. Osteosarcoma is an aggressive bone tumor characterized by biological and molecular heterogeneity, possibly dependent on CSCs. CSC identification in osteosarcoma and their efficient targeting are still open questions. Spontaneous canine osteosarcoma shares clinical and biological features with the human tumors, representing a model for translational studies. We characterized three CSC-enriched canine osteosarcoma cultures. In serum-free conditions, these CSC cultures grow as anchorage-independent spheroids, show mesenchymal-like properties and in vivo tumorigenicity, recapitulating the heterogeneity of the original osteosarcoma. Osteosarcoma CSCs express stem-related factors (Sox2, Oct4, CD133) and chemokine receptors and ligands (CXCR4, CXCL12) involved in tumor proliferation and self-renewal. Standard drugs for osteosarcoma treatment (doxorubicin and cisplatin) affected CSC-enriched and parental primary cultures, showing different efficacy within tumors. Moreover, metformin, a type-2 diabetes drug, significantly inhibits osteosarcoma CSC viability, migration and self-renewal and, in co-treatment with doxorubicin and cisplatin, enhances drug cytotoxicity. Collectively, we demonstrate that canine osteosarcoma primary cultures contain CSCs exhibiting distinctive sensitivity to anticancer agents, as a reliable experimental model to assay drug efficacy. We also provide proof-of-principle of metformin efficacy, alone or in combination, as pharmacological strategy to target osteosarcoma CSCs.

Effect of metformin and celecoxib on cytotoxicity and release of GDF-15 from human mesenchymal stem cells in high glucose condition

Mesenchymal stem cells (MSCs) have therapeutic potential for treatment of diabetes. However, in vitro behavior of MSCs in high glucose condition as well as presence of glucose lowering agents is not fully understood. Because MSCs have an important role in tissue repair, we examined the effects of metformin and celecoxib on viability of MSCs in different glucose conditions. MSCs, from umbilical cord blood, were cultured in normoglycemic (glucose 5.5 mM), midglycemic (glucose 10 mM), and hyperglycemic (glucose 25 mM) conditions, and the cell viability was evaluated by MTT assay. The cytotoxicity and secretion of GDF-15 were further tested in MSCs treated with metformin and celecoxib in various glucose concentrations. Our results showed that high glucose condition lowered viability of MSCs. Metformin treatment also inhibited proliferation of MSCs, but its toxicity was not changed in high glucose condition. Celecoxib induced cytotoxicity in MSCs, and the toxicity was increased in high glucose condition. Metformin and celecoxib induced release from MSCs; however, high glucose inhibited the metformin-induced GDF-15 release. These findings suggested that metformin did not increase the cytotoxicity of high glucose condition in MSCs. Moreover, celecoxib treatment in diabetic condition can reduce the viability of MSCs to proliferate and regenerate perhaps via change in release of GDF-15.

Reprogramming to developmental plasticity in cancer stem cells

During development and throughout adult life, sub-populations of cells exist that exhibit phenotypic plasticity - the ability to differentiate into multiple lineages. This behaviour is important in embryogenesis, is exhibited in a more limited context by adult stem cells, and can be re-activated in cancer cells to drive important processes underlying tumour progression. A well-studied mechanism of phenotypic plasticity is the epithelial-to-mesenchymal transition (EMT), a process which has been observed in both normal and cancerous cells. The epigenetic and metabolic modifications necessary to facilitate phenotypic plasticity are first seen in development and can be re-activated both in normal regeneration and in cancer. In cancer, the re-activation of these mechanisms enables tumour cells to acquire a cancer stem cell (CSC) phenotype with enhanced ability to survive in hostile environments, resist therapeutic interventions, and undergo metastasis. However, recent research has suggested that plasticity may also expose weaknesses in cancer cells that could be exploited for future therapeutic development. More research is needed to identify developmental mechanisms that are active in cancer, so that these may be targeted to reduce tumour growth and metastasis and overcome therapeutic resistance.

Metformin Use Is Associated with Reduced Incidence and Improved Survival of Endometrial Cancer: A Meta-Analysis

Studies have suggested that metformin can potentially decrease the incidence of cancer and improve survival outcomes. However, the association between metformin use and the incidence and survival of endometrial cancer (EC) remains controversial. So, a meta-analysis was performed. An electronic search was conducted using PubMed, EMBASE, and Web of Science. The outcome measures were relative risks (RRs) or hazard ratios (HRs) with 95% confidence intervals (CIs) comparing the EC incidence and survival in patients treated with and without metformin. Eleven studies involving 766,926 participants were included in this study. In the pooled analysis of five studies which evaluated the association of metformin use with the incidence of EC, we found that metformin use was associated with a 13% reduction in EC risk among patients with diabetes (RR = 0.87, 95% CI: 0.80–0.95; p = 0.006). In the pooled analysis of six retrospective cohort studies evaluating the effect of metformin on the survival of EC patients, we found that, relative to nonuse, metformin use significantly improved the survival of EC patients (HR = 0.63, 95% CI: 0.45–0.87; p = 0.006). This study showed that metformin use was significantly associated with a decreased incidence of EC in diabetes and a favorable survival outcome of EC patients.

Colon cancer modulation by a diabetic environment: A single institutional experience

Background

Multiple observational studies suggest an increased risk of colon cancer in patients with diabetes mellitus (DM). This can theoretically be the result of an influence of the diabetic environment on carcinogenesis or the tumor biologic behavior.

Aim

To gain insight into the influence of a diabetic environment on colon cancer characteristics and outcomes.

Material and methods

Retrospective analysis of clinical records in an academic tertiary care hospital with detailed analysis of 81 diabetic patients diagnosed of colon cancer matched with 79 non-diabetic colon cancer patients. The impact of streptozotocin-induced diabetes on the growth of colon cancer xenografts was studied in mice.

Results

The incidence of DM in 1,137 patients with colorectal cancer was 16%. The diabetic colon cancer cases and non-diabetic colon cancer controls were well matched for demographic and clinical variables. The ECOG Scale Performance Status was higher (worse) in diabetics (ECOG ≥1, 29.1% of controls vs 46.9% of diabetics, p = 0.02), but no significant differences were observed in tumor grade, adjuvant therapy, tumor site, lymphovascular invasion, stage, recurrence, death or cancer-related death. Moreover, no differences in tumor variables were observed between patients treated or not with metformin. In the xenograft model, tumor growth and histopathological characteristics did not differ between diabetic and nondiabetic animals.

Conclusion

Our findings point towards a mild or negligible effect of the diabetes environment on colon cancer behavior, once cancer has already developed.

A story of metformin-butyrate synergism to control various pathological conditions as a consequence of gut microbiome modification: Genesis of a wonder drug?

The most widely prescribed oral anti-diabetic agent today in the world today is a member of the biguanide class of drugs called metformin. Apart from its use in diabetes, it is currently being investigated for its potential use in many diseases such as cancer, cardiovascular diseases, Alzheimer's disease, obesity, comorbidities of diabetes such as retinopathy, nephropathy to name a few. Numerous in-vitro and in-vivo studies as well as clinical trials have been and are being conducted with a vast amount of literature being published every day. Numerous mechanisms for this drug have been proposed, but they have been unable to explain all the actions observed clinically. It is of interest that insulin has a stimulatory effect on cellular growth. Metformin sensitizes the insulin action but believed to be beneficial in cancer. Like -wise metformin is shown to have beneficial effects in opposite sets of pathological scenario looking from insulin sensitization point of view. This requires a comprehensive review of the disease conditions which are claimed to be affected by metformin therapy. Such a comprehensive review is presently lacking. In this review, we begin by examining the history of metformin before it became the most popular anti-diabetic medication today followed by a review of its relevant molecular mechanisms and important clinical trials in all areas where metformin has been studied and investigated till today. We also review novel mechanistic insight in metformin action in relation to microbiome and elaborate implications of such aspect in various disease states. Finally, we highlight the quandaries and suggest potential solutions which will help the researchers and physicians to channel their research and put this drug to better use.

PRPS1 silencing reverses cisplatin resistance in human breast cancer cells

PRPS1 (phosphoribosyl pyrophosphate synthetase 1), which drives the nucleotide biosynthesis pathway, modulates a variety of functions by providing central building blocks and cofactors for cell homeostasis. As tumor cells often display abnormal nucleotide metabolism, dysregulated de-novo nucleotide synthesis has potential impacts in cancers. We now report that PRPS1 is specifically and highly expressed in chemoresistant (CR) cancer cells derived from cisplatin-resistant human breast cancer cell lines SK-BR-3 and MCF-7. The inhibition of PRPS1 activity in CR cells by genetic silencing reduces cell viability and increases apoptosis in vitro, both of which can be further potentiated by cisplatin treatment. Significantly, such down-regulation of PRPS1 in CR cells when administered to nude mice enhanced the survival of those animals, as demonstrated by decreased tumor growth. Knockdown of PRPSI may cause these effects by potently inducing autonomous activation of caspase-3 and inhibiting the proliferation in the engrafted CR tumors. As a result, cisplatin sensitivity in a xenograft model of CR cancer cells can be restored by the down-regulation of PRPS1. Thus, PRPS1 inhibition may afford a therapeutic approach to relapsed patients with breast cancer, resistant to chemotherapy.

Metformin reduces gastric cancer risk in patients with type 2 diabetes mellitus

This retrospective cohort study investigated whether metformin may reduce gastric cancer risk by using the reimbursement databases of the Taiwan's National Health Insurance. Patients with type 2 diabetes diagnosed during 1999-2005 and newly treated with metformin (n=287971, "ever users of metformin") or other antidiabetic drugs (n=16217, "never users of metformin") were followed until December 31, 2011. The effect of metformin (for ever versus never users, and for tertiles of cumulative duration of therapy) was estimated by Cox regression incorporated with the inverse probability of treatment weighting using propensity score. Results showed that the respective numbers of incident gastric cancer in ever and never users were 759 (0.26%) and 89 (0.55%), with respective incidences of 55.26 and 122.53 per 100,000 person-years. The overall hazard ratio (95% confidence intervals) of 0.448 (0.359-0.558) suggested a significantly lower risk among ever users. In tertile analyses, hazard ratios (95% confidence intervals) for the first (＜21.47 months), second (21.47-45.97 months) and third (＞45.97 months) tertile of cumulative duration was 0.973 (0.773-1.224), 0.422 (0.331-0.537) and 0.120 (0.090-0.161), respectively, while compared to never users. In conclusion, metformin significantly reduces gastric cancer risk, especially when the cumulative duration is more than approximately 2 years.