TABLE 2.

In vitro, in vivo, prospective, and retrospective studies evaluating the role of metformin in TB: 2007–2019.

Author, Year and journal	Title	Type of study (Country, sample size)	Main objective	Key finding
Alisjahbana et al., 2007, Clinical Infectious Disease	Effect of type 2 diabetes mellitus on presentation and treatment response of pulmonary tuberculosis	Prospective cohort (Indonesia, n = 634)	Investigated clinical characteristics and outcomes in TB patients with and without DM	– T2D associated with: (a) more symptoms but not increased severity of TB (b) negative outcomes following anti-TB treatment – Possible pharmacokinetic interaction between TB therapy and oral hypoglycemic agents
Arai et al., 2010, Journal of Pharmacology and Experimental Therapeutics	Metformin, an antidiabetic agent, suppresses the production of tumor necrosis factor and tissue factor by inhibiting early growth response factor-1 expression in human monocytes in vitro	In vitro study (mononuclear cells from healthy volunteers)	To identify underlying mechanisms of MET inhibition of tumor necrosis factor (TNF) production and tissue factor (TF) expression in human monocytes, stimulated with lipopolysaccharide (LPS) or oxidized low-density lipoprotein (oxLDL)	– MET (10 μM) halted TNF and tissue factor production when stimulated with LPS or oxLDL (p < 0.01) – NF-κB, AP-1, and Egr-1 (p < 0.01) regulate monocyte production of TNF and TF – The inhibitory effect of MET on TNF and TF production not mediated through inhibition of NF-κB pathway or inhibition AP-1 activation – MET inhibited LPS- or oxLDL-induced phosphorylation of Egr-1
Singhal et al., 2014, Science Translational Medicine	Metformin as adjunct anti-tuberculosis therapy	In vitro (Human monocytic cell line – THP-1) In vivo (C57BL/6 mice) Retrospective study of two independent cohorts (TB and diabetic)	To determine if MET can be used as an adjuvant with TB therapy	– In vitro, MET restricts bacterial growth by increasing production of mitochondrial reactive oxygen species (p < 0.0047) –In vivo: (a) MET (500 mg/kg) decreased bacillary count in lung and spleen (p < 0.001) (b) MET enhanced efficacy of INH – showed by reduced bacillary count in mice lungs co-treated with INH + MET compared to mice receiving INH only (p < 0.05) (c) MET (250 mg/kg or 500 mg/kg), MET + INH (10 mg/kg) reduced organ size and tissue lesions – Retrospective cohort 1: (a) MET therapy reduced TB severity and improved clinical outcome (p < 0.001) (b) 109 patients treated with MET had fewer pulmonary cavities (p < 0.041) – Retrospective cohort 2: (a) MET treatment was associated with reduced T-SPOT reactivity (p < 0.05)
Vashisht et al., 2014, Journal of Translational Medicine	Systems level mapping of metabolic complexity in Mycobacterium tuberculosis to identify high-value drug targets	In silico (dynamical mathematical models to understand function of biological systems)	Investigated metabolic mechanisms in Mtb, in the presence of TB therapy that potentiate formation of persister phenotypes	– Identified critical proteins for growth and survival of Mtb – Formulated novel idea of metabolic persister genes - associated predictions with published in vitro and in vivo experimental evidence: (a) NAD biosynthesis results in bacterial persistence in Mtb during metabolic stress induced by anti-TB treatment (b) Suggest persister genes as possible drug targets to advance their effectiveness and competence of existing antibiotics for drug tolerant bacteria
Vashisht and Brahmachari, 2015, Journal of Translational Medicine	Metformin as a potential combination therapy with existing front-line antibiotics for Tuberculosis	In silico	Assessed if MET can be a potential drug candidate for targeting drug tolerant Mtb	– Identified direct re-routing of metabolic fluxes via NAD biosynthesis pathway and respiratory chain complex – I in Mtb: (a) Probable alternate mechanism of ATP generation may facilitate persister phenotype formation, demonstrating antibiotic tolerance (b) Targeting proteins encoding for NDH-I and NAD pathway together with front-line antibiotics provides a strategy to counter drug tolerance (c) Development of new therapeutic intervention for TB therapy (d) MET inhibits both bacterial NDH-1 complex and human mitochondria complex-1
Srujitha et al., 2017, The Brazilian Journal of Infectious Diseases	Protective effect of metformin against tuberculosis infections in diabetic patients: an observational study of South Indian tertiary healthcare facility	Observational study (South Indian diabetics with TB n = 152 and without TB n = 299)	To determine the protective effect of MET against TB in T2D patients To investigate the relationship between poor glycemic control and TB	– Poor glycemic control (HbA1c > 8) observed in experimental (51.7%) vs. control groups (31.4%) – HbA1c < 7 associated with TB protection – 3.9-fold protection against TB with MET in diabetics
Degner et al., 2017, American Thoracic Society	The Effect of Diabetes and Comorbidities on Tuberculosis Treatment Outcomes	Retrospective cohort study (Patients > 13 years with culture confirmed drug-susceptible pulmonary TB, undergoing treatment)	To assess the effect of DM and poor glycemic control on mortality during TB treatment and 2-month TB sputum culture conversion	– TB associated mortality in DM and poor glycemic control (23.6%) vs. mortality in DM and good glycemic control (10.9%) (p < 0.001) – 1.9 times greater odds of death during TB therapy than non-diabetic patients (CI 1,37–2.66, p < 0.01) – 1.7 times greater odds of remaining culture positive at 2 months in DM vs. non-DM (CI 1.16–3.69, p < 0.01) – Diabetic patients had a significant association between MET use and decreased death during TB therapy (CI 0.10–0.65, p < 0.01)
Dutta et al., 2017, Antimicrobial Agents Chemotherapy	Metformin Adjunctive Therapy Does Not Improve the Sterilizing Activity of the First-Line Antitubercular Regimen in Mice	In vivo (BALB/c mice)	To investigate bactericidal and sterilizing activities of human-like exposures of MET when given in combination with the first-line regimen against TB	– 53.3%, 20%, and 6.6% of mice treated with conventional TB therapy only reverted after therapy at 3.5, 4.5, and 5.5 months, respectively – MET adjunct treatment did not significantly alter reversion proportions, as 46.6% (p = 0.52), 20% (p = 1.0), and 0% (p = 1.0) of mice reverted following therapy for 3.5, 4.5, and 5.5 months, respectively – Mice treated with MET showed no obvious signs of toxicity during treatment period
Degner et al., 2018, Clinical Infectious Diseases	Metformin Use Reverses the Increased Mortality Associated with Diabetes Mellitus During Tuberculosis Treatment	Retrospective cohort study (patients aged ≥ 13 years undergoing treatment for culture-confirmed, drug-susceptible pulmonary TB, n = 2416)	(1) To assess the effect of DM on all-cause mortality during TB treatment and 2- and 6-month TB sputum-culture conversion rates (2) To evaluate the effect of metformin use on survival during TB treatment	– 2416 patients undergoing TB therapy were adjusted for age, sex, chronic kidney disease, cancer, hepatitis C, tobacco use, cavitary disease, and treatment adherence – During TB treatment: (a) 29.0% of patients with DM and 13.7% patients without DM experienced the primary clinical outcome, death (p < 0.01) (b) 23.9% of patients with DM and 14.2% patients without DM had a 2-month sputum culture positive for Mtb (p < 0.01) (c) No difference in 6-month TB sputum culture positivity between DM (0.3%) and non-DM (0.8%) patients (p < 0.39) (d) Overall death was 17.6% among metformin users and 31.3% among non-metformin users (hazard ratio, 0.56 [95% CI, 0.39–0.82]) (e) Survival was significantly higher in the metformin group in a log-rank test of Kaplan–Meier survival distributions (p < 0.01)
Lee et al., 2018, Korean Journal of Internal Med	The effect of metformin on culture conversion in tuberculosis patients with diabetes mellitus	Retrospective cohort study (patients with culture-positive pulmonary TB diagnosed between 2011 and 2012)	To examine the anti-TB treatment effects of metformin on sputum Mtb culture conversion after 2 months of TB treatment	– Baseline characteristics, except for chronic renal disease, were not significantly different between the groups – MET treatment had no significant effect on sputum culture conversion (p = 0.60) and recurrence within 1 year after TB treatment completion (p = 0.39) – MET improved sputum culture conversion rate in patients with cavitary pulmonary TB, who have higher bacterial loads (odds ratio, 10.8; 95% confidence interval, 1.22 to 95.63) – drug resistance was significantly higher in patients with failure to achieve conversion (10.1% vs. 36.1%, p < 0.01) – Patients with cavitary pulmonary TB, experienced higher 2-month culture conversion rates in the MET group compared to the non-MET group (OR, 10.80; 95% CI, 1.22 to 95.63; p = 0.03)
Novita et al., 2019, Indian journal of tuberculosis	Metformin induced autophagy in diabetes mellitus – Tuberculosis co-infection patients: A case study	Observational clinical study (T2D patients newly diagnosed with TB)	To measure the levels of microtubule-associated Protein 1 light chain 3B (MAP1LC3B) (autophagy associated), superoxide dismutase (SOD), interferon and interleukin-10, and smear reversion in DM-TB co-infected patients	– All patients in the MET group had sputum smear reversion after 2 months of intensive phase TB therapy – Increases in MAP1LC3B, SOD, and interferon before and after the observation period were significant following MET treatment (p < 0.005) – During the intensive period of anti-TB therapy MAP1LC3B and interferon displayed significant changes (p < 0.005), and SOD showed no significant changes – MAP1LC3B is representative of autophagy whilst SOD induces autophagy – Interferon is responsible for protecting against TB infection
Lachmandas et al., 2019, The journal of infectious diseases	Metformin alters human host responses to Mycobacterium tuberculosis in healthy subjects	In vitro (human monocytes)	To investigate the effects of MET on mTOR signaling, p38 and protein kinase B in non-diabetic individuals	Outcome 1 (L1) – MET significantly decreased Mtb lysate–induced cytokine production (p < 0.05) – MET inhibited mTOR (strongly influences cellular growth and cytokine production) (p < 0.01) causing decreased cellular growth (p < 0.01) and cytokine production (p < 0.05) Outcome 2 (L2) – MET increased lactate production and glucose consumption (p < 0.01) in Mtb lysate–stimulated PBMCs from healthy individuals Outcome 3 (L3) – p-AMPK was increased in both unstimulated and Mtb lysate–stimulated PBMCs after MET intake – MET increased phagocytosis of Mtb in macrophages – In response to the innate host responses to Mtb – MET has beneficial effects (regulates inflammatory cytokine messenger RNA stability, cell proliferation, and apoptosis) on cellular metabolism and immune function
Novita et al., 2018, Indian Journal of Tuberculosis	A case risk study of lactic acidosis risk by metformin use in type 2 diabetes mellitus tuberculosis coinfection patients	Observational clinical study Type 2 DM newly TB coinfection outpatients Surabaya Paru Hospital	This study aimed to understand the effect of MET as an adjuvant therapy in TB and insulin simultaneous therapy	– Among 42 participants showed no case of lactic acidosis – No evidence that MET therapy induced lactic acidosis event nor that it increased lactate blood level among individuals with TB pulmonary disease – MET use in type 2 DM TB co-infection did not induce lactic acidosis – Contributes to our understanding on the clinical effect of MET use in type 2 DM TB co-infection.

DM, diabetes mellitus; MET, metformin; Mtb, Mycobacterium tuberculosis; TB, tuberculosis.