Association Between Metformin and Alzheimer's Disease: A Systematic Review and Meta-Analysis of Clinical Observational Studies

Pathology and Treatments of Alzheimer’s Disease Based on Considering Changes in Brain Energy Metabolism Due to Type 2 Diabetes

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder with cognitive dysfunction, memory decline, and behavioral disturbance, and it is pathologically characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. Although various hypotheses have been proposed to explain the pathogenesis of AD, including the amyloid beta hypothesis, oxidative stress hypothesis, and abnormal phosphorylation of tau proteins, the exact pathogenic mechanisms underlying AD remain largely undefined. Furthermore, effective curative treatments are very limited. Epidemiologic studies provide convincing evidence for a significant association between type 2 diabetes and AD. Here, we showed energy metabolism using glucose, lactate, ketone bodies, and lipids as energy substrates in a normal brain, and changes in such energy metabolism due to type 2 diabetes. We also showed the influences of such altered energy metabolism due to type 2 diabetes on the pathology of AD. Furthermore, we comprehensively searched for risk factors related with type 2 diabetes for AD and showed possible therapeutic interventions based on considering risk factors and altered brain energy metabolism due to type 2 diabetes for the development of AD.

Repurposing antidiabetic drugs for Alzheimer's disease: A review of preclinical and clinical evidence and overcoming challenges

Repurposing antidiabetic drugs for the treatment of Alzheimer's disease (AD) has emerged as a promising therapeutic strategy. This review examines the potential of repurposing antidiabetic drugs for AD treatment, focusing on preclinical evidence, clinical trials, and observational studies. In addition, the review aims to explore challenges and opportunities in repurposing antidiabetic drugs for AD, emphasizing the importance of well-designed clinical trials that consider patient selection criteria, refined outcome measures, adverse effects, and combination therapies to enhance therapeutic efficacy. Preclinical evidence suggests that glucagon-like peptide-1 (GLP-1) analogs, dipeptidyl peptidase-4 (DPP4) inhibitors, metformin, thiazolidinediones, and sodium-glucose co-transporter-2 (SGLT2) inhibitors exhibit neuroprotective effects in AD preclinical models. In preclinical studies, antidiabetic drugs have demonstrated neuroprotective effects by reducing amyloid beta (Aβ) plaques, tau hyperphosphorylation, neuroinflammation, and cognitive impairment. Antidiabetic drug classes, notably GLP-1 analogs and SGLT2 inhibitors, and a reduced risk of dementia in patients with diabetes mellitus. While the evidence for DPP4 inhibitors is mixed, some studies suggest a potential protective effect. On the other hand, alpha-glucosidase inhibitors (AGIs) and sulfonylureas may potentially increase the risk, especially in those experiencing recurrent hypoglycemic events. Repurposing antidiabetic drugs for AD is a promising therapeutic strategy, but challenges such as disease heterogeneity, limited biomarkers, and benefits versus risk evaluation need to be addressed. Ongoing clinical trials in mild cognitive impairment (MCI) and early AD patients without diabetes will be crucial in determining the clinical efficacy and safety of the antidiabetic drugs, paving the way for potential treatments for AD.

Therapeutic Effects of Metformin on Central Nervous System Diseases: A Focus on Protection of Neurovascular Unit

Metformin is one of the most commonly used oral hypoglycemic drugs in clinical practice, with unique roles in neurodegeneration and vascular lesions. Neurodegeneration and vasculopathy coexist in many diseases and typically affect the neurovascular unit (NVU), a minimal structural and functional unit in the central nervous system. Its components interact with one another and are indispensable for maintaining tissue homeostasis. This review focuses on retinal (diabetic retinopathy, retinitis pigmentosa) and cerebral (ischemic stroke, Alzheimer's disease) diseases to explore the effects of metformin on the NVU. Metformin has a preliminarily confirmed therapeutic effect on the retinal NUV, affecting many of its components, such as photoreceptors (cones and rods), microglia, ganglion, Müller, and vascular endothelial cells. Since it rapidly penetrates the blood-brain barrier (BBB) and accumulates in the brain, metformin also has an extensively studied neuronal protective effect in neuronal diseases. Its mechanism affects various NVU components, including pericytes, astrocytes, microglia, and vascular endothelial cells, mainly serving to protect the BBB. Regulating the inflammatory response in NVU (especially neurons and microglia) may be the main mechanism of metformin in improving central nervous system related diseases. Metformin may be a potential drug for treating diseases associated with NVU deterioration, however, more trials are needed to validate its timing, duration, dose, clinical effects, and side effects.

Emerging role of Metformin in Alzheimer's disease: A translational view

Alzheimer's disease (AD) constitutes a major public-health issue of our time. Regrettably, despite our considerable understanding of the pathophysiological aspects of this disease, current interventions lead to poor outcomes. Furthermore, experimentally promising compounds have continuously failed when translated to clinical trials. Along with increased population ageing, Type 2 Diabetes Mellitus (T2DM) has become an extremely common condition, mainly due to unbalanced dietary habits. Substantial epidemiological evidence correlates T2DM with cognitive impairment as well. Considering that brain insulin resistance, mitochondrial dysfunction, oxidative stress, and amyloidogenesis are common phenomena, further approaching the common features among these pathological conditions. Metformin constitutes the first-choice drug to preclude insulin resistance in T2DM clinical management. Experimental evidence suggests that its functions might include neuroprotective effects, in addition to its hypoglycemic activity. This review aims to summarize and discuss current knowledge of experimental data on metformin on this path towards translational medicine. Finally, we discuss the controversial data of responses to metformin in vitro, and in vivo, animal models and human studies.

Metformin Lysosomal Targeting: A Novel Aspect to Be Investigated for Metformin Repurposing in Neurodegenerative Diseases?

Metformin is a widely employed drug in type 2 diabetes. In addition to warranting good short- and long-term glycemic control, metformin displays many intriguing properties as protection against cardiovascular and neurodegenerative diseases, anti-tumorigenic and longevity promotion. In addition to being a low-cost drug, metformin is generally well tolerated. However, despite the enthusiastic drive to aliment these novel studies, many contradictory results suggest the importance of better elucidating the complexity of metformin action in different tissues/cells to establish its possible employment in neurodegenerative diseases. This review summarises recent data identifying lysosomal-dependent processes and lysosomal targets, such as endosomal Na+/H+ exchangers, presenilin enhancer 2 (PEN2), the lysosomal pathway leading to AMP-activated protein kinase (AMPK) activation, and the transcription factor EB (TFEB), modulated by metformin. Lysosomal dysfunctions resulting in autophagic and lysosomal acidification and biogenesis impairment appear to be hallmarks of many inherited and acquired neurodegenerative diseases. Lysosomes are not yet seen as a sort of cellular dump but are crucial in determining key signalling paths and processes involved in the clearance of aggregated proteins. Thus, the possibility of pharmacologically modulating them deserves great interest. Despite the potentiality of metformin in this context, many additional important issues, such as dosing, should be addressed in the future.

Metformin beyond type 2 diabetes: Emerging and potential new indications

Metformin is best known as a foundational therapy for type 2 diabetes but is also used in other contexts in clinical medicine with a number of emerging and potential indications. Many of its beneficial effects may be mediated by modest effects on weight loss and insulin sensitivity, but it has multiple other known mechanisms of action. Current clinical uses beyond type 2 diabetes include: polycystic ovarian syndrome; diabetes in pregnancy/gestational diabetes; prevention of type 2 diabetes in prediabetes; and adjunct therapy in type 1 diabetes. As metformin has been in clinical use for almost 70 years, much of the underpinning evidence for its use in these conditions is, by definition, based on trials conducted before the advent of contemporary evidence-based medicine. As a result, some of the above-established uses are 'off-label' in many regulatory territories and their use varies accordingly in different countries. Going forward, several current 'repurposing' investigational uses of metformin are also being investigated: prevention of cancer (including in Li Fraumeni syndrome), renal protection, Alzheimer's disease, metabolic dysfunction-associated steatotic liver disease and promotion of healthy ageing. Despite the longevity of metformin and its important current roles beyond type 2 diabetes in clinical medicine, it has further potential and much research is ongoing.

Neuroprotective effects of dipeptidyl peptidase 4 inhibitor on Alzheimer's disease: a narrative review

Insulin resistance in brain and amyloidogenesis are principal pathological features of diabetes-related cognitive decline and development of Alzheimer's disease (AD). A growing body of evidence suggests that maintaining glucose under control in diabetic patients is beneficial for preventing AD development. Dipeptidyl peptidase 4 inhibitors (DDP4is) are a class of novel glucose-lowering medications through increasing insulin excretion and decreasing glucagon levels that have shown neuroprotective potential in recent studies. This review consolidates extant evidence from earlier and new studies investigating the association between DPP4i use, AD, and other cognitive outcomes. Beyond DPP4i's benefits in alleviating insulin resistance and glucose-lowering, underlying mechanisms for the potential neuroprotection with DPP4i medications were categorized into the following sections: (Ferrari et al., Physiol Rev, 2021, 101, 1,047-1,081): the benefits of DPP4is on directly ameliorating the burden of β-amyloid plaques and reducing the formation of neurofibrillary tangles; DPP4i increasing the bioactivity of neuroprotective DPP4 substrates including glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and stromal-derived factor-1α (SDF-1α) etc.; pleiotropic effects of DPP4is on neuronal cells and intracerebral structure including anti-inflammation, anti-oxidation, and anti-apoptosis. We further revisited recently published epidemiological studies that provided supportive data to compliment preclinical evidence. Given that there remains a lack of completed randomized trials that aim at assessing the effect of DPP4is in preventing AD development and progression, this review is expected to provide a useful insight into DPP4 inhibition as a potential therapeutic target for AD prevention and treatment. The evidence is helpful for informing the rationales of future clinical research and guiding evidence-based clinical practice.

Diabetes, antidiabetic medications and risk of dementia: A systematic umbrella review and meta-analysis

AIMS

The objective of this umbrella review and meta-analysis was to evaluate the effect of diabetes on risk of dementia, as well as the mitigating effect of antidiabetic treatments.

MATERIALS AND METHODS

We conducted a systematic umbrella review on diabetes and its treatment, and a meta-analysis focusing on treatment. We searched MEDLINE/PubMed, Embase, PsycINFO, CINAHL and the Cochrane Library for systematic reviews and meta-analyses assessing the risk of cognitive decline/dementia in individuals with diabetes until 2 July 2023. We conducted random-effects meta-analyses to obtain risk ratios and 95% confidence intervals estimating the association of metformin, thiazolidinediones, pioglitazone, dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors, meglitinides, insulin, sulphonylureas, glucagon-like peptide-1 receptor agonists (GLP1RAs) and sodium-glucose cotransporter-2 inhibitors (SGLT2is) with risk of dementia from cohort/case-control studies. The subgroups analysed included country and world region. Risk of bias was assessed with the AMSTAR tool and Newcastle-Ottawa Scale.

RESULTS

We included 100 reviews and 27 cohort/case-control studies (N = 3 046 661). Metformin, thiazolidinediones, pioglitazone, GLP1RAs and SGLT2is were associated with significant reduction in risk of dementia. When studies examining metformin were divided by country, the only significant effect was for the United States. Moreover, the effect of metformin was significant in Western but not Eastern populations. No significant effect was observed for dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors, or insulin, while meglitinides and sulphonylureas were associated with increased risk.

CONCLUSIONS

Metformin, thiazolidinediones, pioglitazone, GLP1RAs and SGLT2is were associated with reduced risk of dementia. More longitudinal studies aimed at determining their relative benefit in different populations should be conducted.

Pharmacological Approaches Using Diabetic Drugs Repurposed for Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) are chronic, progressive disorders affecting the elderly, which fosters global healthcare concern with the growing aging population. Both T2DM and AD have been linked with increasing age, advanced glycosylation end products, obesity, and insulin resistance. Insulin resistance in the periphery is significant in the development of T2DM and it has been posited that insulin resistance in the brain plays a key role in AD pathogenesis, earning AD the name "type 3 diabetes". These clinical and epidemiological links between AD and T2DM have become increasingly pronounced throughout the years, and serve as a means to investigate the effects of antidiabetic therapies in AD, such as metformin, intranasal insulin, incretins, DPP4 inhibitors, PPAR-γ agonists, SGLT2 inhibitors. The majority of these drugs have shown benefit in preclinical trials, and have shown some promising results in clinical trials, with the improvement of cognitive faculties in participants with mild cognitive impairment and AD. In this review, we have summarize the benefits, risks, and conflicting data that currently exist for diabetic drugs being repurposed for the treatment of AD.

Metformin as a Potential Prevention Strategy for Alzheimer's Disease and Alzheimer's Disease Related Dementias

Background

Metformin is a safe and effective medication for type 2 diabetes (T2D) that has been proposed to decrease the risk of aging related disorders including Alzheimer's disease (AD) and Alzheimer's disease related disorders(ADRD).

Objective

This review seeks to summarize findings from studies examining the association of metformin with AD/ADRD related outcomes.

Methods

This is a narrative review of human studies, including observational studies and clinical trials, examining the association of metformin with cognitive and brain outcomes. We used PubMed as the main database for our literature search with a focus on English language human studies including observational studies and clinical trials. We prioritized studies published from 2013 until February 15, 2024.

Results

Observational human studies are conflicting, but those with better study designs suggest that metformin use in persons with T2D is associated with a lower risk of dementia. However, these observational studies are limited by the use of administrative data to ascertain metformin use and/or cognitive outcomes. There are few clinical trials in persons without T2D that have small sample sizes and short durations but suggest that metformin could prevent AD/ADRD. There are ongoing studies including large clinical trials with long duration that are testing the effect of metformin on AD/ADRD outcomes in persons without T2D at risk for dementia.

Conclusions

Clinical trial results are needed to establish the effect of metformin on the risk of AD and ADRD.

Repositioning of Anti-Diabetic Drugs against Dementia: Insight from Molecular Perspectives to Clinical Trials

Insulin resistance as a hallmark of type 2 DM (T2DM) plays a role in dementia by promoting pathological lesions or enhancing the vulnerability of the brain. Numerous studies related to insulin/insulin-like growth factor 1 (IGF-1) signaling are linked with various types of dementia. Brain insulin resistance in dementia is linked to disturbances in Aβ production and clearance, Tau hyperphosphorylation, microglial activation causing increased neuroinflammation, and the breakdown of tight junctions in the blood-brain barrier (BBB). These mechanisms have been studied primarily in Alzheimer's disease (AD), but research on other forms of dementia like vascular dementia (VaD), Lewy body dementia (LBD), and frontotemporal dementia (FTD) has also explored overlapping mechanisms. Researchers are currently trying to repurpose anti-diabetic drugs to treat dementia, which are dominated by insulin sensitizers and insulin substrates. Although it seems promising and feasible, none of the trials have succeeded in ameliorating cognitive decline in late-onset dementia. We highlight the possibility of repositioning anti-diabetic drugs as a strategy for dementia therapy by reflecting on current and previous clinical trials. We also describe the molecular perspectives of various types of dementia through the insulin/IGF-1 signaling pathway.

Mapping new pharmacological interventions for cognitive function in Alzheimer's disease: a systematic review of randomized clinical trials

Background and Objective: Alzheimer's disease (AD) is a progressive neurodegenerative disorder, that is, characterized by cognitive decline. To date, there are no effective treatments for AD. Therefore, the objective of this study was to map new perspectives on the effects of pharmacological treatment on cognitive function and the overall psychological state in patients with AD. Methods: Two independent researchers searched for randomized clinical trials (RCTs) exploring new pharmacological approaches related to cognition in Alzheimer's disease in adults from 2018 to 2023 in PubMed, Web of Science, Scopus, and Cochrane Library databases. A total of 17 RCTs were included in this review. Results: The results show that in recent years, new drugs have been tested in patients with Alzheimer's disease, including masitinib, methylphenidate, levetiracetam, Jiannao Yizhi, and Huannao Yicong formulas. Most studies have been conducted in populations with mild to moderate Alzheimer's disease. Conclusion: Although some of the drugs found suggested improvement in cognitive function, the scarcity of available studies highlights the need for further research in this area. Systematic review registration: [www.crd.york.ac.uk/prospero], identifier [CRD42023409986].

Long-term use of metformin and Alzheimer's disease: beneficial or detrimental effects

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by extracellular deposition of amyloid beta (Aβ) leading to cognitive decline. Evidence from epidemiological studies has shown the association between type 2 diabetes mellitus (T2DM) and the development of AD. T2DM and peripheral insulin resistance (IR) augment the risk of AD with the development of brain IR with inhibition of neuronal insulin receptors. These changes impair clearance of Aβ, increase secretion of Aβ_1-42, reduce brain glucose metabolism, and abnormal deposition of Aβ plaques. Insulin-sensitizing drug metformin inhibits aggregation of Aβ by increasing the activity of the insulin-degrading enzyme (IDE) and neprilysin (NEP) levels. Additionally, different studies raised conflicting evidence concerning long-term metformin therapy in T2DM patients, as it may increase the risk of AD or it may prevent the progression of AD. Therefore, the objective of this review was to clarify the beneficial and detrimental effects of long-term metformin therapy in T2DM patients and risk of AD. Evidence from clinical trial studies revealed the little effect of metformin on AD. Various animal studies showed that metformin increases Aβ formation by activation of amyloid precursor protein (APP)-cleaving enzymes with the generation of insoluble tau species. Of note, the metformin effect on cognitive function relative to AD pathogenesis is mostly assessed in animal model studies. The duration of metformin therapy was short in most animal studies, this finding cannot apply to the long-term duration of metformin in humans. Therefore, large-scale prospective and comparative studies involving long-term metformin therapy in both diabetic and non-diabetic patients are required to exclude the effect of T2DM-induced AD.

Clinical antidiabetic medication used in Alzheimer's disease: From basic discovery to therapeutics development

Alzheimer's disease (AD) is a common neurodegenerative disease. Type 2 diabetes mellitus (T2DM) appears to increase and contributing to the risk of AD. Therefore, there is increasing concern about clinical antidiabetic medication used in AD. Most of them show some potential in basic research, but not in clinical research. So we reviewed the opportunities and challenges faced by some antidiabetic medication used in AD from basic to clinical research. Based on existing research progress, this is still the hope of some patients with special types of AD caused by rising blood glucose or/and insulin resistance.