The potential effect of metformin on cognitive and other symptom dimensions in patients with schizophrenia and antipsychotic-induced weight gain: a systematic review, meta-analysis, and meta-regression

Introduction Metformin has shown good efficacy in the management of antipsychotic-induced metabolic syndrome (MetS) in patients with schizophrenia or schizoaffective disorders. Its ability to induce antidepressant behavioural effects and improve cognitive functions has also been investigated: yet information has not been systematized. The aim of this study was therefore to investigate the effects of metformin on cognitive and other symptom dimension in schizophrenic patients treated with antipsychotics through a systematic review and meta-analysis. Methods We searched PubMed, ClinicalTrials.Gov, Embase, PsycINFO, and WHO ICTRP database up to February 2022, Randomised Controlled Trials (RCT) evaluating patients diagnosed with schizophrenia and related disorders, who were treated with metformin as add-on therapy to antipsychotics for the treatment of weight gain and in which changes in psychiatric symptoms and cognitive functions were evaluated. Results A total of 19 RCTs met the inclusion criteria. Meta-analysis was performed on 12 eligible studies. We found a positive trend after 24 weeks of treatment in schizophrenic patients with stable conditions [SMD (95%CI) = -0.40 (−0.82;0.01), OR (95%CI) = 0.5 (−2.4;3.4)]. Better performance was detected in the Brief Assessment of Cognition in Schizophrenia and Positive and Negative Syndrome Scale (PANSS) with low heterogeneity among studies. One study reported changes in BACS-verbal memory subdomain in favour of placebo [MD (95%CI) = -16.03 (-23.65;8.42)]. Gastrointestinal disorders, xerostomia, and extrapyramidal syndrome were the most reported adverse effects. Psychiatric adverse events were also described: in particular, symptoms attributable to a relapse of schizophrenia. Conclusion Some degree of efficacy was found for Metformin in improving cognitive and other symptom dimensions in patients with Schizophrenia. Given the clinical relevance of this potential pharmacological effect, longer specific studies using adequate psychometric scales are strongly recommended. Likewise, how metformin acts in this context needs to be evaluated in order to enhance its efficacy or find more efficacious drugs.


Introduction
Schizophrenia (SCZ) is a chronic disorder characterized by a combination of psychotic symptoms (i.e., hallucinations, delusions, and disorganization) and motivational and cognitive dysfunctions. It affects about 1% of the world's population and it is considered a highcost disease due to the lifelong clinical course and the need of healthcare resource utilization (1).
Patients with SCZ have a mortality rate 2.6 times higher than that in the general population, mostly due to the occurrence of cardiovascular diseases and metabolic syndrome (MetS). The latter disease frequently arises in patients with SCZ due to a dysregulated and unhealthy lifestyle (2,3), but it is also related to the treatment with second-generation antipsychotics (SGAs) (4)(5)(6). Considering pharmacodynamic implications, occupancies of H1 histaminergic and M1/M3 cholinergic receptors represent risk factors for increased levels of total cholesterol, HDL, LDL, insulin, and triglycerides (7, 8). Therefore, metabolic adverse effects of SGAs contribute to long-term risk of mortality and to short-term risk of obesity and MetS (9,10). Current therapeutic options for weight control consist in dietary support and regular exercise, which, however, may not be sufficient for antipsychotic induced MetS (11,12).
In order to prevent MetS in psychiatric patients, many drugs have been evaluated, especially among antidiabetic drugs, that showed efficacy, good tolerability and compliance (26). One of the most studied drugs for preventing and treating antipsychotic-induced weight gain and MetS is metformin, a biguanide drug used to treat DM2 because of its high efficacy in lowering plasma glucose levels. It also exerts additional metabolic effects such as weight loss, reduction of triglycerides and LDL levels while increasing HDL and sensitivity to insulin (27). There is evidence that metformin can control antipsychotic induced MetS in schizophrenic patients (28). Of importance, its ability to induce antidepressant behavioural effects and improve cognitive functions has also been investigated (29). According to recent preclinical and clinical findings, metformin can penetrate through the Blood-Brain Barrier (BBB) into the central nervous system (CNS) where it promotes neuroprotective, neurotrophic, neurogenetic and anti-inflammatory effects (30). Furthermore, metformin reduces the inflammatory markers p-IKB, IL-1, and VEGF in neuronal cells reducing the neuroinflammation, a driver for neurotoxicity and the development of neuropsychiatric diseases (31,32). A recent review concludes that metformin may activate the AMP Protein Kinase (AMPK), an enzyme that regulates the metabolic process of lipids and carbohydrates, leading to potential cognitive properties (33). Consistent with this hypothesis, many studies conducted in murine models have demonstrated the potential positive effect of metformin on cognition in neurodegenerative disorders (34) as diverse as Alzheimer's disease (35), and traumatic brain injuries (36). At the same time, animal studies have focused on the beneficial effects of metformin on the CNS even across different neuropsychiatric conditions, such as anxiety (37), depression (38), schizophrenia-like symptoms (39) and seizures (40).
In a population-based longitudinal cohort study of diabetic individuals, participants using metformin showed higher performances in neuropsychological tests involving cognitive functions, especially verbal learning, working memory and executive function; even after adjusting for behavioural lifestyle or clinical conditions, these results did not change (41). In diabetic patients metformin had a neuroprotective function for the prevention of dementia (42). An improvement in cognitive function through the use of metformin was observed also in Huntington's disease (43) and in a small sample of patients diagnosed with Fragile X Syndrome (44). In a randomised controlled trial, treatment with metformin showed antidepressive effects in depressive and diabetic patients (45). Other clinical trials and observational studies, however, did not confirm the efficacy of metformin on cognitive function or on prevention against any form of dementia (46); moreover, metformin monotherapy has also been found to have negative effects in diabetic patients increasing the risk of Parkinson disease (47).
Even though data from both preclinical and clinical studies on possible pro-cognitive effects of metformin provide contrasting outcomes no attempts have been done to date to systematize and Frontiers in Psychiatry 03 frontiersin.org weight the available knowledge. We have thus specifically investigated the effects of metformin on psychiatric and cognitive functions through a systematic review of literature and meta-analysis of clinical trials in a selected population composed by schizophrenic patients treated with antipsychotics.

Literature search
We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (48). We submitted our Protocol at the International Prospective Register of Ongoing Systematic Reviews (CRD42021250690). We searched up to February 2022 PubMed, ClinicalTrials.Gov, Embase, PsycINFO, WHO ICTRP database using a search string containing two sets of words referring to 1) schizophrenic patients and 2) metformin that were combined using the Boolean operator "AND. " There was no language, date, document type, or publication status limitations for inclusion of records. Additional articles were collected through the reference lists of reviews and eligible studies we found. We did not plan to contact authors for unpublished data. An example of a search string is fully described in the Supplementary Material S1.
We included only Randomised Controlled Trials (RCT) evaluating people diagnosed with SCZ and related disorders (such as schizoaffective disorder, schizophreniform disorder, and delusional disorder) who were treated with metformin as add-on therapy to antipsychotics. We did not use any criteria for age, nationality or sex of the participants, duration/stage of illness, treatment setting, current clinical state, or symptom clusters. We considered metformin compared to placebo or other types of pharmacological interventions for the treatment of weight gain. We considered behavioural interventions only when combined with a pharmacological intervention. Primary outcomes were the changes in psychiatric and cognitive scales: the psychometric properties of the measuring instrument should have been validated and the measuring instrument should have not been modified for that trial.

Data extraction and processing
All titles and abstracts were assessed independently by two authors (GM, RL) to identify potentially relevant articles. Studies fulfilling the eligibility criteria were included and their full texts were retrieved and reviewed in duplicate (GM, ER). Discrepancies during the check of the two-step independent screening were resolved through the discussion with a third author (VB). Data were extracted by two researchers (RL, VB) and disagreements were resolved by consensus and consultation with the expert group (MP, CC, GC).
For every study the following data were extracted: First author; Year; Study duration; Study type (blinding/design); Number of subjects; diagnosis; number of males; age; antipsychotic(s) used and dose; control/comparator/placebo group; concomitant drugs; additional behavioural interventions; all outcomes of interest; Adverse Drug Reactions. Endpoint data were mainly chosen, mean change data if the former was not available.

Risk of bias assessment
This study was designed as an Intention-To-Treat (ITT) analysis. The risk of bias of included studies was assessed by three authors (GM, RL, ER) by using the Cochrane risk-of-bias tool for randomised trials (RoB 2) (49). Disagreements were resolved by consensus among them and a further consultation with the expert group (CC and VB). We planned to conduct a sensitivity analysis excluding studies rated with a high risk of bias if the number of remaining studies exceeds three.

Meta-analysis
A meta-analysis was performed by using the generic inverse variance method with a random effect model combining psychiatric scales reported by each study, which were Brief Psychiatric Rating Scale (BPRS) (50), Positive and Negative Syndrome Scale (PANSS) (51), Clinical Global Impression Scale (CGI) (52), Global Assessment of Functioning (GAF) (53), Scale for the Assessment of Negative Symptoms (SANS) and Scale for the Assessment of Positive Symptoms (SAPS) (54), Brief Assessment of Cognition in SCZ (BACS) (55) and Patient Health Questionnaire-9 (PHQ9) (56). Among scales, when more than one tool was available, a priority order was defined considering their impact on cognitive assessment. The procedure is clearly described in Supplementary Material S2. Thus, the final priority order adopted was as follows: BACS composite T score > BACS verbal memory T score > PANSS> BPRS > GAF > CGI > SANS and SAPS > PHQ9.
Assumptions were made regarding missing SDs using data of similar studies in terms of population, number of patients, and the point estimate. Forest plots were created for the main outcomes. Sensitivity analyses were performed excluding these studies to check their influence in the results.
In order to help the reader in the interpretation of results we also provided Mean Differences (MDs) of subgroup analyses concerning single scales. In these analyses, we included all studies reporting results of the scale of interest, we did not follow the priority order reported above.
RevMan 5 was the chosen tool to perform the meta-analysis (57).

Meta-regression
It is known that the amount of adipose tissue of the patients impacts on the pharmacokinetic properties of antipsychotics and thus on their therapeutic effect (58)(59)(60)(61).
We then explored the influence of the Body Mass Index (BMI) at baseline on the treatment effect (SMD) of our main metaanalysis to evaluate if a better response is more related to a better response to the psychiatric treatment than the efficacy of metformin in the regulation of psychiatric and cognitive symptoms. A random-effects meta-regression model with Knapp-Hartung method was performed. Data are provided by a regression bubble plot. The [meta] R package was used to perform metaregression (62).

Post-hoc analysis
As specified in our protocol, we planned to check for eventual useful analyses that were not previously considered. We therefore decided to perform a sub-group analysis of studies reporting data at 12 and 24 weeks.
Since six studies reported an additional dietary and physical exercise control to patients, we also investigated the role of Lifestyle interventions in changes of scales with a sensitivity analysis.

Assessment of heterogeneity
We interpreted I 2 estimate greater than or equal to 50% together with a statistically significant Chi 2 statistic as evidence of substantial heterogeneity. We also visually inspected graphs to investigate the possibility of statistical heterogeneity and discussed it in the proper section.

Differences between protocol and review
We clearly state down below the deviations from the original protocol registered in PROSPERO: • We included those studies in which less than 10% of patients were diagnosed with bipolar disorder. • Our aim is to whether metformin as add-on therapy improved or not SCZ symptoms, with a particular interest in cognitive functions. Therefore, within psychiatric scales, when more than one tool was available in the study, we applied a priority order as described in the statistical analysis section. This was due to the presence of more than one outcome in several studies. • We changed the statistical method of the meta-analysis due to the type of outcomes reported in the literature. We preferred the generic invariance methods instead of the Mantel-Haenszel because data were all reported in continuous variables (endpoint data or mean changes) and it wasn't possible to convert them into dichotomous. However, we decided to provide Odds ratios (OR) by converting the results of total • We provided sub-group analyses for each scale.
• Due to the paucity of studies retrieved, we could not perform neither funnel plots, nor analyses on the effects of high risk of bias and the role of diabetes.

Literature search
The study selection and screening process is presented in the PRISMA 2020 flowchart (Figure 1). The electronic search identified 9,605 records from literature databases and 743 trials in study registers. After duplicates removal, 9,455 records were screened. Thirty-eight records were retrieved by manual search in the reference lists of relevant reviews and included studies for full-text analysis. Nine-teen studies eventually met our eligibility criteria and were included in the review: 12 were eligible to perform meta-analysis, 2 reported only qualitative data, and 5 were only present in trial registers without any result. These latter trials will be described in a separate section.

Characteristics of the included studies (n = 14)
Full description of trials is available in Table 1 and Supplementary Material S1. All studies were designed as comparison between metformin and placebo, except two: Mondal et al. (64) 64), reported any other chronic disease (such as thyroid, liver or renal dysfunction, cardiovascular disease) and pregnancy among the exclusion criteria. The mean age at diagnosis was between 21 and 26 years. The disease duration varied from months (first episode) to more than two decades. Clozapine, olanzapine, aripiprazole, and risperidone were the most used antipsychotics. In 3 trials the concomitant use of mood stabilizers, benzodiazepines and antidepressants was permitted (65,69,75). Lifestyle intervention involving diet and physical exercise was provided in 6 trials (65,66,68,72,76,77). All studies included validated psychometric scales for the clinical assessment; PANSS (65,70,(72)(73)(74)77) and BPRS (66-69, 71, 76) were the most used.

Scales
The following numerical results must be read as "metformin compared to placebo. " In the analysis of 12 studies (65-74, 76, 77), metformin resulted in a favorable position against placebo (Figure 2), even if not

Metformin vs. topiramate
Only one study (64) compared metformin to another drug used to control the increase of weight in schizophraenic patients. The Authors did not report any quantitative result, they only state that no differences were found in SAPS and SANS scales among groups after 24 weeks.

Adverse events
The general adverse events that were reported by authors were related to gastrointestinal discomfort, xerostomia, and extrapyramidal syndrome. Several psychiatric adverse events were also described, particularly some symptoms attributable to a relapse of SCZ (psychotic relapse/exacerbation, unstable/worsening of illness), others to mood alteration (depression, suicidality, irritated/bad mood), and finally some unspecific symptoms such as insomnia and agitation (Supplementary Table S2). The PRISMA 2020 flowchart.

Studies in trial registers (n = 5)
Five trials were registered on clinicaltrial.gov (78). NCT01654640 was terminated because they were not able to recruit enough patients; in NCT02140788, the Principal Investigator left the Institution, and the trial was interrupted. NCT03271866, reported as "unknown status, " focuses on the effect of metformin on cognitive impairment. NCT03708549 is a phase 4 trial that is still recruiting; the aim of the study is to compare berberine and metformin and the evaluation of the PANSS is among the secondary outcomes. NCT04865835 is a phase I trial that has been completed and it is likely under review; however, the aim of this study is to evaluate in pharmacokinetic of a novel substance compared to metformin, which is not specifically our outcome of interest.

Risk of bias
Risk of Bias of the included studies is shown in Figure 6. In general, most of the studies reported high risk of bias (8/14). However, the randomisation process was favorably assessed in all studies and the "Deviations from intended interventions" domain was the one that highly influenced the general results because of the Per Protocol analysis used in 6 studies (64,67,68,71,72,76). Three studies were considered with low-risk of bias in all domains (65,70,74), and only one study was at real high-risk of bias, since none of the fields were assessed without any concern (64).

Discussion
Since its approval in 1958, metformin has become one of the most widely used therapy for DM2 and it still represents the first-line therapy. While improved mitochondrial metabolism and insulin signaling are generally suggested as mechanisms underlying beneficial pro-cognitive effects of antidiabetic drugs, other factors such as active adenosine 5′-monophosphate-activated protein kinase (AMPK) activation, modulation of microglial phenotype, mTOR inhibition, and increased autophagy in the brain might be involved (79). Because of these multiple mechanisms, many studies have already described potential effects of metformin in treating conditions other than diabetes (80-83); here we assessed for the first time through a systematic review the potential effects of metformin on cognitive functions and psychiatric symptoms in schizophrenic patients treated with antipsychotics.
A general positive trend was seen after 24 weeks of treatment [SMD (95%CI) = −0.40 (−0.82;0.01), OR (95% CI) = 0.5 (−2.4; 3.4)] in patients who were generally considered in stable conditions. Unfortunately, the relatively short period of investigation of the included studies (only one study up to 36 weeks) could mask the neuroprotective effects of metformin since in previous RCTs they seemed to emerge after long-term use (6-8 years) (42). A better improvement was related to those scales allocated in the higher positions of our priority scale (BACS composite T score > BACS verbal memory T score > PANSS) with low heterogeneity among studies, then worsening while going further with the other psychiatric tools (BPRS > GAF > CGI > SANS and SAPS > PHQ9). Furthermore, among all cognitive domains assessed by the BACS (verbal memory, working memory, motor speed, attention, executive functions, and verbal fluency), only verbal memory was in favour of placebo [MD (95%CI) = −16.03 (−23.65; 8.42)] (66). This finding could indicate a greater influence of metformin on cognitive rather than psychiatric symptoms, but it is not possible to draw any conclusions since only one study (66) reported results for the BACS composite t-score [MD (95% CI) = 1.26 (−0.42; 2.94)]. This scale is specifically designed for the evaluation of cognitive functions, but the small sample size of this trial could be an important limit for the power of the performed analysis. It is interesting to note that we were able to retrieve another trial that was registered in 2017 (84): the aim of this study was to investigate the impact of metformin on cognitive impairment in schizophrenic patients with or without MetS. This 24-week trial should recruit 80 patients and compare metformin group versus controls on PANSS Scale, Calgary Depression Scale for SCZ (Chinese version) and MATRICS Consensus Cognitive Battery. Unfortunately, the last version of the protocol was submitted in 2020 and the recruiting status is unknown. We could not therefore include their findings in our analysis. However, some indirect clinical evidence on a potential enhancement of cognitive function may come from neurodegenerative disorders: metformin has shown potential therapeutic benefit against mild cognitive impairment and Alzheimer's disease among diabetic patients (85), even if the use of metformin for prevention of dementia in older non-diabetic adults is not currently recommended (42).
No correlation was seen with the patients' BMI at baseline, thus indicating no potential differences in the use of metformin in firstepisode psychosis or under chronic treatment with SGAs. Literature findings report how the earlier the onset of SCZ and the longer its duration, the worse is the clinical response to antipsychotics (86). One of the hypotheses behind this evidence-based finding is that progressive brain tissue loss occurs in schizophrenic patients, and this neurobiological alteration would interfere with the effectiveness of metformin as much as antipsychotic therapy (87). Among the studies included in our analysis, only four used a first episode psychosis as an inclusion criterion (65,69,73,77). Therefore, despite missing data, we can assume that most patients were enrolled after a duration of illness that could impact negatively on the efficacy of pharmacotherapy. Disease duration ranged from 6.8 months to 27.8 years and in seven studies it was not reported (64, 67-70, 72, 76). Further studies including disease onset and duration information or that include only first-episode patients are therefore recommended.
Regarding the antipsychotic drugs that were used in the included studies, all patients were mainly treated with SGAs, while only three studies (66, 69, 76) reported concurrent treatment with firstgeneration antipsychotics, confirming the known strong association between weight gain and SGAs (4). Among them, clozapine and olanzapine were responsible for the highest incidence of MetS, consistently with a recent network meta-analysis on glyco-metabolic adverse effects of antipsychotics (7).
As all the other drugs available on the market, metformin might cause adverse effects, although the most frequent ones are considered mild enough to recommend maintaining the use of metformin unless renal/hepatic function deterioration arises (88). Metformin doses that were used in all the included trials were in line with the latest recommendation (89) and no high-concerning adverse event was therefore reported. Gastrointestinal disorders were the most described events; this is not surprising as they are known to be very common at Frontiers in Psychiatry 11 frontiersin.org the start of the therapy and can be minimized by dose reduction, slower dose titration and after-meal administration (89). Physical symptoms, namely xerostomia, headache and extrapyramidal syndrome were also reported; this indicates that it is worth recommending caution and careful patients' counseling before starting metformin, as adverse events may represent an additional risk factor for dropping out of the overall psychiatric treatment (90).
Somehow unexpectedly, few psychiatric adverse events were reported, these were essentially from relapse of SCZ mood alteration, insomnia, and agitation. Based on the known mechanism of action of metformin a clear causal relationship between psychiatric symptoms and metformin appear improbable. Rather, it is likely that they arose due to the chronic course of the underlying psychiatric disease.
However, considering the observed adverse effects, it is important to assess the risk-benefit ratio of an add-on therapy with metformin. Metformin has proved its efficacy on cardiometabolic complications, which cause a three-times higher mortality risk in SCZ patients than that of the general population (91). When there is balance between the odds of therapeutic effects and the risk of adverse events, metformin administration in these patients seems beneficial, especially if metformin might exert improvements in pro-cognitive functions, which is of clinical relevance. However, such evidence is not yet solid enough and it is premature to propose a change in current clinical practice and in medical prescription at this stage. Further studies considering the benefit/risk ratio are warranted. Metformin compared to Placebo considering all scales. Sub-group analysis at twelve weeks. Psychiatry  12 frontiersin.org Risk of Bias of the included studies.

Strength and limitations
Chronic treatment with SGAs is essential in the control of psychotic symptoms and the prevention of relapses in SCZ. For this reason, it is widely adopted in clinical settings, even though it can increase the risk of MetS and negatively impact cognitive performance thus worsening the therapeutic compliance, already impaired by the pathology itself. Therefore, the identification of a treatment that can contrast dysmetabolism and cognitive impairment in psychiatric patients would have a high impact in psychiatric clinical practice. Not only has metformin previously shown to be effective in reducing MetS, but it is also considered a low-cost drug, with a well-known safety profile. Our primary aim was to verify the hypothesis, previously emerged from several preclinical and clinical studies, that metformin may exert pro-cognitive effects also in psychotic patients, with or without DM2. This meta-analysis, in addition to its clinical relevance, represents an original perspective in the current literature background. Sub-group analysis at twenty-four weeks. The influence of BMI at baseline, meta-regression.
Frontiers in Psychiatry 13 frontiersin.org The first obstacle in investigating our primary objective was that only one study (66) used a specific assessment instrument for cognitive function, the BACS. This is the most widely adopted and validated scale that assesses cognition's domains most impaired and correlated with outcome of SCZ (55,92). Unfortunately, it is still underused in clinical practice, while the clinical course and functioning of SCZ are usually assessed by several validated psychometric scales, the main ones being BPRS, PANSS, CGI, GAF. Most of these latter scales contain specific items concerning the patient's cognitive asset. Thus, since partial scores of these items were not available in the analyzed studies, we applied the priority order described above, that is an original method in order not to neglect valuable information for our primary aim. However, further studies with appropriate scales are warranted.
Another limit of our analysis is the relatively short period of investigation (only one study up to 36 weeks) while neuroprotective effects of metformin observed in previous RCT seem to emerge after long-term use (8 and 6 years) (42). Only 3 RCTs were assessed with low risk of bias, and we could not perform any sensitive analysis excluding those with high risk. However, considering that our aim was defining changes in measurements, the most important domains for our results were the quality of the randomisation process (domain 1) and the measurement of the outcome (domain 4), which were both considered at low risk of bias in the 64% of the included studies.

Conclusion
Metformin has been previously shown to reduce weight gain and the risk of MetS in schizophrenic patients treated with SGA; our systematic review suggests that it may also improve psychiatric and cognitive symptoms in the same population. Given the clinical relevance of this potential pharmacological effect of metformin, longer specific studies exploring cognitive performance and using adequate psychometric scales are strongly recommended.

Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions
VB, GC, RL, ER, GM, BB, MP, MN, SR, CC, BD'O, and EC contributed to the study conception and design. Material preparation and data collection were performed by VB, RL, GM, and ER. Data analyses were performed by VB, ER, and CC. The first draft of the manuscript was written by VB, RL, GC, and ER. All authors contributed to the article and approved the submitted version.

Funding
This work was supported by Università degli Studi di Milano (Piano di Sostegno alla Ricerca, LINEA 3 to CC) which are gratefully acknowledged. The funding public institutions had no role in any part of the work.