Introduction

Dementia is a progressive neuropsychiatric disorder that affects memory and other cognitive functions, thus interfering with life function1. Alzheimer disease (AD) is the leading cause of dementia among elderly people2,3, and the prevalence of dementia due to AD is 4.02% among adults aged over 60 years4. Approximately 35.6 million adults worldwide developed dementia in 2010, and the number is projected to increase to 65.7 million in 20305. Worldwide spending on AD was approximately 422 billion US dollars in 20096. The main pharmacologic treatments for patients with AD are cholinesterase inhibitors and the N-methyl-d-aspartate receptor (NMDAR) partial antagonist memantine. However, these treatments have adverse effects, and the response is unsatisfactory7. Mild cognitive impairment (MCI), characterized by objective cognitive impairment, is defined as the predementia stage on the continuum of cognitive decline8,9. Several studies have reported that patients with MCI generally develop AD10,11. However, no treatment has yet been approved for MCI. Therefore, the development of alternative medications is urgently required for AD and MCI.

NMDAR has been demonstrated to play a critical role in controlling synaptic plasticity and memory function12. Studies have reported that overactivation of NMDAR may cause neurotoxicity, especially in the late phase of AD13,14. NMDAR antagonists have been developed for the treatment of AD, and memantine, an uncompetitive NMDAR partial antagonist, has been approved as an antidementia medication for moderate to severe AD15,16; memantine is not approved for mild AD and MCI because of unsatisfactory efficacy17. However, NMDAR antagonists, such as ketamine, have been demonstrated to impair spatial learning and verbal information ability in healthy humans18. Moreover, human studies have reported an age-related decrease in the density of NMDARs in the cerebral cortex and hippocampus19. These findings implicate the hypoactivation of NMDAR in cognitive impairment.

Several trials of NMDAR enhancing agents have been performed to evaluate their effect on cognitive impairment among patients with dementia. Sodium benzoate, an inhibitor of D-amino acids oxidase, can increase d-serine and thus enhance NMDAR activation20,21. A randomized, double-blind, placebo-controlled trial of 60 patients with early-phase AD or MCI reported a larger improvement in the AD Assessment Scale-cognitive subscale (ADAS-cog) score compared with placebo22. D-cycloserine, a partial agonist at the glycine site of NMDAR, has been reported to improve ADAS-cog score in patients with AD23. However, other trials with D-cycloserine did not report significant improvement in cognition24,25. Therefore, we conducted a meta-analysis on the cognitive effects of NMDAR enhancing agents in dementia.

Methods

Search strategy and inclusion criteria

Two independent authors (Chun-Hung Chang and Shaw-Ji Chen) performed a systematic literature search from the study’s inception until August 7th, 2021. We followed the drug keywords used by previous meta-analytic reviews about NMDA receptor positive modulators26,27,28,29. The search strategy comprised the following keywords: (Dementia OR Alzheimer*) AND (acetylcysteine OR α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid OR AMPA OR benzoate OR CX516 OR D-cycloserine OR D-serine OR glutamine OR glutamate OR glutamate carboxypeptidase 2 OR GCP2 OR glycine OR glycine transporter type 1 OR GlyT1 OR glutamate receptor ionotropic kainate OR GRIK OR kynurenine aminotransferase OR KAT OR metabotropic glutamate receptor OR mGluR OR minocycline OR N-acetyl-aspartylglutamate OR NAAG OR N-methyl-d-aspartate OR NMDA OR pregnenolone OR sarcosine) AND controlled trial 26. The authors independently evaluated the titles and abstracts of all retrieved papers for eligibility. Full-text articles of potentially eligible trials were reviewed and compiled in a list of studies for inclusion. Disagreements between the two authors regarding the inclusion of a study were settled by a third reviewer (Professor Tsai). The Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) were followed (Fig. 1, Supplementary Table S1)30,31,32.

Figure 1
figure 1

PRISMA flow diagram of the search and identification of included studies. Database: PubMed (n = 304), Cochrane Central Register of Controlled Trials (n = 208), Cochrane Database of Systematic Reviews (n = 7). Keyword: (Dementia OR Alzheimer*) AND (acetylcysteine OR α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid OR AMPA OR benzoate OR CX516 OR D-cycloserine OR D-serine OR glutamine OR glutamate OR glutamate carboxypeptidase 2 OR GCP2 OR glycine OR glycine transporter type 1 OR GlyT1 OR glutamate receptor ionotropic kainate OR GRIK OR kynurenine aminotransferase OR KAT OR metabotropic glutamate receptor OR mGluR OR minocycline OR N-acetyl-aspartylglutamate OR NAAG OR N-methyl-d-aspartate OR NMDA OR pregnenolone OR sarcosine) AND controlled trial. Date: date available to August 7th, 2020.

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Eligibility criteria

The inclusion criteria were as follows: (a) studies on participants who received a diagnosis of dementia, (b) studies including randomized placebo-controlled trials, and (c) studies on the use of NMDAR enhancing agents as a monotherapy or adjunctive treatment to concomitant antidementia drugs. The exclusion criteria were as follows: (1) abstracts from ClinicalTrials.gov, (2) conference abstracts, (3) reviews and comments, (4) studies on treatment other than NMDAR enhancing modulators, (5) studies on a different population or outcome, and (6) trial protocols.

Outcome measures

The effects of glutamate positive modulators on cognitive deficits in patients with dementia were investigated. Overall cognitive function (primary outcome) was compared between patients receiving NMDAR enhancing agents and those receiving placebo. The common cognitive measures in dementia are the Mini-Mental State Examination (MMSE)33 and the Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-cog)34. The MMSE is a cognitive test commonly used to screen for dementia and measure cognitive impairment in older adults35. The MMSE total score ranges from 0 to 30 (highest to lowest level of cognitive impairment). The ADAS-cog consists of 11 tasks, and its score ranges from 0 to 70 (lowest to highest level of cognitive impairment).

Data extraction

Two independent reviewers (CH Chang and SJ Chen) screened and identified articles. We recorded information including the first author, published year, number of participants, gender proportion, mean age, duration, NMDA receptor enhancing agents, and summary of neurocognitive measures (Table 1). We followed the PRISMA guidelines. The primary outcome was the difference in effect sizes between the NMDAR enhancing agent and control groups (calculated in terms of the standard mean difference [SMD] with the corresponding 95% confidence interval [CI] and P value). The primary author of a study was contacted to request the original data if these data were not available in the corresponding article. If no relevant data were reported in the article, other compatible statistical parameters (e.g., P value, sample size, or odds ratio) were used to estimate the effect size, following the Comprehensive Meta-Analysis manuals protocol and the Comprehensive Meta-Analysis website guide (https://www.meta-analysis.com/downloads/Meta-analysis%20Converting%20among%20effect%20sizes.pdf); the estimated effect sizes were then converted and pooled into SMDs. Variables of interest were extracted when possible, including first author, year, sample size, proportion of men, mean age, therapy duration, add-on NMDAR enhancing agents, and cognitive outcome measures.

Table 1 Summary of the characteristics of studies included in the meta-analysis.
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Quality assessment

Two authors (CH Chang and SJ Chen) evaluated the methodological quality of the included studies. The Jadad scoring system was used for randomized controlled trials (RCTs)36. The Jadad scoring system contains three items that assess randomization (2 points), blinding (2 points), and an account of all patients (1 point). Therefore, the score ranges from 0 to 5. A higher score indicates higher methodological quality.

Meta-analysis procedure

A random-effects model was used for the meta-analysis37. The SMD with 95% CI was selected to compare the ESs of our primary outcome, rather than the difference in means, because each study was presumed to use different units. Furthermore, when the SMD value was higher than 0, the effect size was defined as indicating a positive effect in the NMDAR enhancing agent group compared with the placebo group. After data from included trials were extracted, Comprehensive Meta-Analysis software version 3 (Biostat, Englewood, NJ, USA) was used to perform meta-analyses. Statistical significance was defined as a two-tailed P < 0.05. Of note, the correction of multiple comparison was not made in this study due to this was an exploratory study rather than confirmatory study.

Heterogeneity and publication bias

The I2 statistic was used as a measure of heterogeneity37, which was assessed using the Cochran Q test and corresponding P value38. Publication bias was assessed using funnel plots39 and Egger's regression test40 If publication bias was discovered, Duval and Tweedie's trim-and-fill procedure, a validated model for ES estimation, was used41.

Sensitivity test

Studies were individually removed from the meta-analysis to verify that the results were not caused by outliers in the included studies.

Subgroup meta-analysis and meta-regression

Subgroup analyses were performed when at least two independent data sets were available41. Samples were segmented by variables, including diagnosis of dementia, cognitive measure, study design, NMDAR enhancing agent, drug pathway in enhancing glutamatergic neurotransmission, duration of therapy, and age range. Furthermore, meta-regression analyses were performed for each potential moderator, including baseline mean ADAS-cog total score, proportion of men, and mean age.

Results

Characteristics of included studies and patients

The final quantitative analysis included 2224 participants from 14 trials22,23,24,25,42,43,44,45,46,47,48,49,50,51. The PRISMA flow diagram is displayed in Fig. 1, and the study characteristics are summarized in Table 1 and Supplementary Table S2. The mean number of subjects in each study was 158.86 ± 188.73 (range: 10–544), and the mean duration of the studies was 20.21 ± 25.69 weeks (range: 2–104 weeks). The mean age of the subjects was 74.10 ± 5.14 years. The mean proportion of men was 47.27% ± 16.02%. Nine NMDAR enhancing agents were investigated. The numbers of studies and subjects for each compound investigated were as follows: benzoate, two studies, n = 157; cycloserine, two studies, n = 443; D-cycloserine, four studies, n = 147; LY451395, one study, n = 181; minocycline, one study, n = 544; monosodium L-glutamate, one study, n = 159; N-acetylcysteine (NAC), one study, n = 43; S47445, one study, n = 520; and sarcosine, one study, n = 30.

Meta-analyses of overall cognitive function

Overall cognitive function data were reported in 11 studies22,23,24,25,42,43,44,48,49,51,52. Effects were conventionally categorized as small (SMD = 0.2), moderate (SMD = 0.5) or large (SMD = 0.8), with positive values indicating improvements in cognitive function. Overall, NMDAR enhancing modulators had a small positive significant effect on overall cognitive function among patients with dementia (SMD = 0.1002, 95% CI 0.0105–0.1900, P = 0.02860; Fig. 2a).

Figure 2
figure 2figure 2

Meta-analyses of (a) overall cognitive function, (b) diagnosis of dementia groups, (c) cognitive measure groups, (d) study design groups, (e) NMDAR enhancing agent groups, (f) drug pathway in enhancing glutamatergic neurotransmission groups, (g) duration of therapy groups, and (h) age range groups.

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Subgroup analyses

Diagnosis of dementia

This meta-analysis which pooled eight trials22,23,24,25,43,44,48,51 showed that the patients with AD revealed a significant but small effect (SMD = 0.1042, 95% CI 0.0076–0.2007, P = 0.03451; Fig. 2b).

Cognitive measure as the primary outcome

This meta-analysis which summarized seven trials22,23,24,25,43,49,51 that used the ADAS-cog as their cognitive measure reported small effects (SMD = 0.1267, 95% CI 0.0145–0.2388, P = 0.2686), whereas two trials44,48 that used the MMSE as their cognitive measure reported nonsignificant effects (SMD = 0.0323, 95% CI − 0.1391–0.2037, P = 0.71201; Fig. 2c).

Study design

This meta-analysis which pooled eight RCTs22,42,43,44,48,49,50,51 reported significant positive effects (SMD = 0.0971, 95% CI 0.0046–0.1896, P = 0.03971). Three trials23,24,25 were crossover RCTs and discovered nonsignificant effects (SMD = 0.1514, 95% CI − 0.2198–0.5226, P = 0.42409; Fig. 2d).

NMDAR enhancing agents

Furthermore, we performed a subgroup meta-analysis of the studies with add-on NMDAR enhancing agents to evaluate the overall cognitive effect. Nine NMDAR enhancing agents were investigated: benzoate, cycloserine, D-cycloserine, LY451395, minocycline, monosodium L-glutamate, NAC, S47445, and sarcosine. Subgroup meta-analyses for each NMDAR enhancing agent revealed small positive but nonsignificant differences in effect on overall cognitive function (Fig. 2e).

Pathway through which the drugs enhance glutamatergic neurotransmission

Glycine allosteric site of NMDARs

A small positive but nonsignificant SMD was observed in the effect of drugs (benzoate, monosodium L-glutamate, sarcosine, NAC, and D-cycloserine) and placebo on overall cognition (SMD = 0.1560, 95% CI − 0.0244–0.3364, P = 0.09004; Fig. 2f).

α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors

A small positive but nonsignificant SMD was observed in the effects of drugs (minocycline, S47445, and LY451395) and placebo on overall cognition (SMD = 0.0819, 95% CI − 0.0216–0.1854, P = 0.12087; Fig. 2f).

Treatment duration

This meta-analysis which summarized two trials22,51 with 24-week treatment duration discovered small positive significant effect (SMD = 0.1585, 95% CI 0.0226–0.2944, P = 0.02222; Fig. 2g).

Patient age

This meta-analysis which pooled eight trials22,23,25,42,43,48,49,51 enrolling patients aged 70–79 years obtained nonsignificant small effect (SMD = 0.0931, 95% CI − 0.0018–0.1880, P = 0.05438; Fig. 2h).

Meta-regression analyses of overall cognitive function

Higher baseline mean ADAS-cog total score was associated but nonsignificant with higher proportion of men, higher mean age, and weaker effect of NMDAR modulators on overall cognitive function (baseline mean ADAS-cog score, slope =  − 0.0174, P = 0.3984; proportion of men, slope =  − 0.2973, P = 0.3580; age, slope =  − 0.0044, P = 0.7029; Fig. 3).

Figure 3
figure 3

Meta-regression of effects of NMDAR enhancing agents on overall cognitive function in relation to (a) baseline mean ADAS-cog total score, (b) proportion of men, and (c) mean age.

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Heterogeneity and publication bias

No significant heterogeneity was observed in overall cognitive function among the studies (Q = 3.784, df = 10, I2 = 0.000%, P = 0.957). Egger’s test revealed no significant publication bias regarding the overall cognitive effects (P = 0.4207). The funnel plots for the effect sizes of overall cognitive function are displayed in Fig. 4.

Figure 4
figure 4

Funnel plots of overall cognitive function SMD.

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Sensitivity analysis

The positive effect of NMDAR enhancing agents on overall cognitive function became nonsignificant when the study by Bernard51 or Lin22 was removed.

Methodological quality of the included studies

Among the fourteen RCTs, the average Jadad score was 3.14 with a standard deviation (SD) of 0.36 (Supplementary Table S2).

Discussion

To our knowledge, this is the first study to investigate the efficacy of NMDA enhancing agents on cognition among patients with dementia. The main findings of this analysis are that (1) NMDAR enhancing agents had small positive significant effects on overall cognitive function compared with placebo, (2) trials on patients with AD obtained small positive significant effects, (3) trials using the ADAS-cog as the primary outcome obtained small positive significant effects.

These findings accord with those of most related studies. Two of the studies reported significant improvement in overall cognitive function in patients with AD22,23. Lin and colleagues treated patients with mild AD with 250–750 mg/d of sodium benzoate or placebo for 24 weeks, and the patients treated with sodium benzoate displayed significantly greater improvement in ADAS-cog score (P = 0.0031)22. Tsai and colleagues treated 17 patients with AD with 50 or 100 mg/d of D-cycloserine or placebo for 4 weeks; the patients treated with D-cycloserine displayed significantly greater improvement in ADAS-cog score (placebo: 24.05 ± 10.99; 50 mg of D-cycloserine: 23.86 ± 10.19; 100 mg of D-cycloserine: 21.12 ± 8.82)23. Seven studies reported nonsignificant improvement in overall cognitive functions24,25,43,44,48,50,51. For example, Howard reported that patients treated with minocycline had mean MMSE score 0.1 points higher than those treated with placebo; however, the mean was nonsignificantly different (95% CI − 1.1–1.2; P = 0.90)48. Bernard reported that patients treated with S47445 had nonsignificant improvement in ADAS-cog score; the adjusted mean change in ADAS-cog total score from baseline to week 24 was 0.35 ± 5.53 in the placebo group, − 0.20 ± 4.91 in the 5 mg of S47445 group, − 0.74 ± 5.43 in the 15 mg of S47445 group, and − 0.05 ± 5.82 in the 50 mg of S47445 group51.

Subgroup analyses revealed that the meta-analysis which pooled eight trials enrolling patients with AD22,23,24,25,43,44,48,51 showed a small positive significant effect (SMD = 0.1042, 95% CI 0.0076–0.2007, P = 0.03451; Fig. 2b), while trials enrolling patients with non-AD dementia did not. This finding may imply that patients with AD may more benefit from NMDAR enhancing agents than patients with non-AD dementia. Lin and colleagues treated patients with mild AD receiving with 250–750 mg/d of sodium benzoate or placebo for 24 weeks; those treated with sodium benzoate displayed significantly greater improvement in ADAS-cog score (P = 0.0031)22. Their findings suggest that patients with mild AD may benefit more from NMDAR enhancing agents than patients with moderate to severe AD. However, in our meta-regression, no significant association between effect size and mean ADAS-cog was observed. Further studies are needed to investigate these findings.

Moreover, we noted that this meta-analysis summarizing seven trials22,23,25,43,49,51 in which the ADAS-cog was employed as the cognitive measure revealed small positive significant effects (SMD = 0.1267, 95% CI 0.0145–0.2388, P = 0.02686), whereas two trials44,48 using the MMSE reported nonsignificant effects (SMD = 0.0323, 95% CI − 0.1391–0.2037, P = 0.71201; Fig. 2c). The MMSE and ADAS-cog are most common cognitive measures in these trials. The MMSE is often employed to screen for dementia and measure cognitive impairment in older adults35. The MMSE total score ranges from 0 to 30 (highest to lowest level of cognitive impairment) while the ADAS-cog consists of 11 tasks, with a total score ranging from 0 to 70 (lowest to highest level of cognitive impairment)53. MMSE and ADAS-cog have shown to be relatively psychometrically poor at detecting small changes54,55,56. Therefore, researchers may use more sensitive instruments like Addenbrooke's Cognitive Examination57 or Montreal Cognitive Assessment58,59 to explore minor changes in cognition in the early stages of dementia receiving NMDAR enhancing agents.

The mechanism of the positive effect of NMDAR enhancing agents on cognition remains unclear. Meta-analytic studies have investigated NMDAR enhancing agents in patients with schizophrenia. Tsai and colleagues reported that the overall effect of NMDAR enhancing agents (glycine, D-serine, and sarcosine) on cognitive symptoms was 0.28 (95% CI 0.10–0.47, P = 0.002, 13 studies, n = 485)29, whereas two other meta-analytic studies did not report a significant effect on overall cognition in schizophrenia26,28. An updated meta-analysis enrolling 25 trials and 1951 participants revealed no significant effect of NMDA-enhancing agents on overall cognition. However, subgroup analysis suggested that NMDAR-enhancing agents may benefit young patients with schizophrenia, and NAC may have effect on working memory27. Human studies have reported an age-related decrease in the density of NMDARs in the cerebral cortex and hippocampus19. NMDAR activation improves memory function12. Several trials with NMDAR enhancing agents, such as benzoate and D-cycloserine, have reported promising effects, especially in early-stage AD22,23.

Side effects

The side effects were mostly mild and relieved after discontinuing the agents. The side effect rate of NMDAR enhancing agents ranged from 0 to 61.1%. Minocycline was reported side effects including dermatologic symptoms (hyperpigmentation, photosensitivity, rash), gastrointestinal symptoms (diarrhea, nausea, sore mouth, vomiting), neurologic symptoms (headache, visual or auditory disturbances, dizziness), infections (oral or genital candidiasis, vaginitis, anal irritation, bacterial enteritis, staphylococcal, or Clostridium difficile). Six skin toxic effects were considered severe48. The most common adverse effects of the study with AMPA modulator S47445 are nasopharyngitis, blood creatine phosphokinase increased, diarrhea, fall, headache, type 2 diabetes mellitus, abdominal pain51. D-cycloserine was reported no side effects23,25. Detailed information about the adverse events addressed in the included studies is summarized in Supplementary Table S2. The attrition rate ranged from 0 to 0.49. The effect size in this exploratory meta-analysis was very small. Most side effects were mild. The benefit may weigh up against the side effects.

Implication

Cognitive impairment is a critical problem in central nerve disease such as dementia. AD is the leading cause of dementia. In 2010, the number of patients with AD was estimated at 36.6 million and is projected to double every 20 years2. Dementia is a considerable economic burden on both patients and society6. However, understanding of the complex mechanisms and treatment response remains unsatisfactory3. Therefore, developing alternative therapeutic approaches is crucial. Our meta-analysis revealed a positive and significant effect of NMDAR positive modulators on overall cognition among patients with dementia. Further trials with better design and larger samples are needed to explore NMDAR enhancing agents in MCI or early-phase dementia.

Strength

The present study had several strengths. First, this is the first meta-analytic study to evaluate the treatment effect of NMDAR positive modulators among patients with dementia. Second, nine compounds were included in this meta-analysis. Third, we noted that NMDAR enhancing agents may have a small positive significant effect on overall cognitive function compared with placebo.

Limitations

There are several limitations in the present study. First, this is an exploratory systematic review and meta-analysis of randomized controlled trials. We could observe related phenomena but not clarify the underlying pathophysiology because of the methodological limitations of meta-analyses. Therefore, caution should be exercised when drawing conclusions. Second, the number of participants was small in some included trials. Third, some trials were conducted over a short period (less than 6 months). Therefore, the long-term cognitive effects of NMDAR enhancing agents and the persistent effects after treatment remain unclear. Kouzuki and colleagues reported improvement in the Touch Panel-type Dementia Assessment Scale total score during a 4-week follow-up assessment50. Fourth, the trials used different standard cognitive tests. Fifth, nine NMDAR enhancing agents were investigated. These agents may have different mechanisms of action involving the N-methyl-d-aspartate (NMDA) system. Further studies are needed to explore the relationships between neurocognitive effects and specific NMDAR enhancing mechanisms. Some modulators may be involved in mechanisms other than the NMDA system. Sixth, the effects of the stage of dementia and concomitant antidementia drugs remain unclear.

Conclusion

The findings from this meta-analysis indicated that NMDAR enhancing agents showed a very small positive effect on overall cognitive function in patients with dementia. Further studies with a larger sample are warranted to explore the role of the NMDA system on specific cognitive domains in subgroups of patients with early-stage dementia.