FormalPara Key Summary Points

Exploratory post hoc analyses of a masupirdine phase 2 study on cognition were carried out based on the use of memantine dosage forms and memantine plasma concentrations.

The treatment effect of masupirdine with concomitant treatment with donepezil + Namenda was numerically superior to that with concomitant treatment with donepezil + Namenda XR when compared with placebo.

Less worsening in cognition was observed with masupirdine treatment as compared with placebo in patients whose trough memantine plasma concentrations were ≤ 100 ng/mL.

The results suggest potentially important effects of interactions among drugs used to treat Alzheimer’s disease.

Introduction

Dementia is a syndrome characterized by disturbance of multiple brain functions, including memory, thinking, orientation, comprehension, calculation, learning capacity, language, and judgment. Impairments of cognitive function are commonly accompanied, and occasionally preceded, by deterioration in emotional control, social behavior, or motivation. Alzheimer’s disease (AD), the most common form of dementia, is a progressive and debilitating neurodegenerative disorder characterized by cognitive and memory deterioration in addition to progressive impairment in activities of daily living (ADL), and contributes to 60–70% of cases of dementia. Other types of dementia include vascular dementia, dementia with Lewy bodies, and a group of diseases that contribute to frontotemporal dementia. Approximately 50 million people worldwide are living with AD and other dementias, and an estimated 6.2 million Americans aged 65 years and older were living with AD dementia in 2021 [1]. By 2060, the number of people aged 65 years and older with AD dementia in the United States is projected to reach 13.85 million [2].

Commonly used symptomatic treatments available today for AD dementia do not slow or stop the damage and destruction of neurons that causes AD symptoms and makes the disease fatal. Though these treatment options delay the cognitive decline [3, 4], their effect is modest and use tends to be short-term. The U.S. Food and Drug Administration (FDA) has currently approved six drugs or regimens for the treatment of AD—rivastigmine, galantamine, donepezil, memantine, memantine combined with donepezil, and aducanumab [1]. The last is an anti-amyloid monoclonal antibody that was shown to reduce amyloid plaque burden in the brain; trials show preliminary evidence of slowing of cognitive decline [5]. The modest efficacy and side effects associated with the current treatment options underline the need to develop more effective and safer treatment options.

The serotonin-6 (5-HT6) receptor is a G protein-coupled receptor localized almost exclusively in the central nervous system including the cerebral cortex and hippocampus, and may play role in learning and memory [6,7,8,9,10,11]. Several 5-HT6 receptor antagonists have been evaluated in clinical settings for their potential therapeutic utility in the treatment of cognitive impairment associated with AD either alone or in combination with donepezil. Intepirdine was studied both as monotherapy and in combination with donepezil [12]. As a monotherapy, no benefit of intepirdine was observed on measures of cognition or global impression of change; however, as an adjunct to donepezil, intepirdine treatment was associated with sustained improvement in cognition and ADL compared with treatment with donepezil alone. In a phase 2 clinical trial, the addition of idalopirdine to donepezil showed promising beneficial effects on cognition [13]. However, positive observations from phase 2 studies of intepirdine could not be replicated in phase 3 studies [14]. Similarly, preliminary signs of efficacy for idalopirdine were not confirmed in phase 3 trials [15]. A phase 2 clinical trial evaluating the efficacy and safety of SAM-760 30 mg once daily (QD) for 12 weeks in patients with AD on a stable regimen of donepezil 5 to 10 mg QD was terminated before planned completion, as the study met the futility criteria [16]. The potential utility of 5-HT6 receptor antagonists in the treatment of AD has been evaluated either as monotherapy or as an add-on therapy to donepezil. No previous trial has reported results for the evaluation of a 5-HT6 receptor antagonist as an add-on therapy to both donepezil and memantine, and treatment with memantine was an exclusion for participation in most of the trials.

Masupirdine, a selective 5-HT6 receptor antagonist, modulates neurotransmitters that play an important role in learning and memory [17] and has shown pro-cognitive potential in various behavioral assessments in animal models and in electroencephalography studies. Studies in animal models demonstrated that co-administration of masupirdine with donepezil and memantine resulted in pro-cognitive effects significantly greater than those observed with the combination of donepezil and memantine [17]. Based on the evident potential benefits of masupirdine for cognition in animal models with co-administration of donepezil and memantine, and the wide use of this combination in clinical practice, masupirdine was evaluated as an add-on therapy to donepezil and memantine in patients with moderate AD dementia.

The study did not meet its primary objective, and additional exploratory analyses were conducted to understand the trial outcomes. [18]. An important issue given its unique role in this trial was the impact of memantine on cognitive outcomes. Three different forms of memantine were allowed in study participants: (1) memantine 10 mg twice a day (BID) (Namenda®); (2) memantine 28 mg QD (Namenda XR®); and (3) the fixed combination tablet of memantine 28 mg with 10 mg donepezil QD (Namzaric®). Compliance with donepezil and memantine treatment was monitored by measurement of trough plasma levels that were analyzed after study completion. The analyses revealed differential exposure to memantine. Thus, a post hoc analysis was conducted to evaluate the effect of plasma exposure of memantine on the efficacy of masupirdine.

Methods

The analysis reported here is based on the data from a previously reported randomized, double-blind, placebo-controlled, parallel-group, multicenter study of masupirdine in patients with moderate AD who were on stable doses of donepezil and memantine [18]. Recruitment, enrollment, all characterizations, and analysis of patients in this study were approved by the central or local institutional review boards for all study sites. Informed consent was obtained from all participants at the time of enrollment. All procedures were designed and conducted in accordance with the tenets of the Declaration of Helsinki.

The study recruited ambulatory or ambulatory-aided male or female patients aged between 50 and 85 years (both inclusive) with a diagnosis of probable AD based on the National Institute of Neurological and Communicative Diseases and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria [19], with moderate severity of dementia, defined as having a Mini-Mental State Examination (MMSE) score of 12 to 20 [20]. A total of 564 patients were randomized in a 1:1:1 ratio to placebo, 50 mg masupirdine, or 100 mg masupirdine treatment for up to 6 months. Study participants were allowed to use one strength of donepezil (10 mg QD) and one of the three forms of memantine: (1) memantine 10 mg BID; (2) memantine 28 mg QD (Namenda XR®); or (3) the fixed combination tablet of memantine 28 mg with 10 mg donepezil QD (Namzaric®). The primary efficacy outcome of the study was the 11-item cognitive subscale of the Alzheimer’s Disease Assessment Scale (ADAS-Cog 11), which has historically been used for evaluating cognitive measures in randomized clinical trials. Secondary efficacy measures were based on the Clinical Dementia Rating Scale—Sum of Boxes (CDR-SB), MMSE, 23-item Alzheimer’s Disease Cooperative Study Activities of Daily Living (ADCS-ADL) scale, and 12-item Neuropsychiatric Inventory (NPI-12).

This exploratory post hoc analysis of the primary study data was conducted to evaluate the impact of memantine regimen and exposure on efficacy of masupirdine with regard to cognition (ADAS-Cog 11 and MMSE) in AD patients. For the purpose of this post hoc analysis, randomized patients were divided in two categories: patients taking memantine 10 mg BID or patients taking memantine 28 mg QD. For further analysis, patients were divided as patients with trough memantine concentrations ≤ 100 ng/mL and > 100 ng/mL at week 26. A responder analysis was carried out to determine the number of responders with active treatment versus placebo.

Statistical Analysis

All statistical tests were two-sided and were performed at the 5% level of significance. The analyses were conducted using either SAS® version 9.3 or GraphPad Prism version 7.3. All efficacy analyses were performed on the modified intent-to-treat (mITT) population. Proportions of responders were subjected to the chi-square test. Adjustments for multiple comparisons were not performed in these post hoc hypothesis-generating analyses.

Results

Patient Demographics and Baseline Characteristics

A total of 564 patients were randomized according to the planned ratio to receive masupirdine 50 mg (190 patients), masupirdine 100 mg (185 patients), or placebo (189 patients). The mITT population comprised 543 patients.

The median age was similar among participants in all three treatment arms (masupirdine 50 mg: 75.0 years; masupirdine 100 mg: 76.0 years; placebo: 74.0 years). The majority of participants (at least 85.8%) were over the age of 65 years, with similar age distribution among participants in the treatment arms. Overall, a slightly higher proportion of female than male participants were enrolled in the study (54.7% female vs. 45.3% male participants). This proportion of female and male participants was similar across all treatment arms. A total of 357 participants (65.7%) used memantine 10 mg BID and 186 participants (34.3%) used either Namenda XR or Namzaric (memantine 28 mg QD), and the treatment formulations were evenly distributed among all three treatment arms.

A summary of patient demographics and baseline disease characteristics is presented in Table 1.

Table 1 Baseline characteristics
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Subgroup Analyses of Primary Endpoint

Treatment Response by Concomitant Medication Use

The outcomes on the primary endpoint and subgroup analysis of ADAS-Cog 11 scores by concomitant medication use in the mITT population are presented in Figs. 1 and 2, respectively.

Fig. 1
figure 1

ADAS-Cog 11 score: total population

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Fig. 2
figure 2

ADAS-Cog 11 score: memantine 10 mg BID vs. Memantine 28 mg QD

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In the mITT population, the treatment effect of masupirdine with concomitant treatment with donepezil + Namenda was numerically superior to that with concomitant treatment with donepezil + Namenda XR when compared with placebo at week 26. In patients who were on donepezil + Namenda, the mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 50 mg and placebo at week 26 was −1.0 (95% CI −2.6, 0.5; p = 0.192). The mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 100 mg and placebo was −0.2 (95% CI −1.8, 1.4; p = 0.805). The treatment differences were numerically greater when the analysis was done on the evaluable population (per-protocol population). At week 26, the mean difference between masupirdine 50 mg and placebo was −1.5 (95% CI −3.3, 0.2; p = 0.081) and the mean difference between masupirdine 100 mg and placebo was −0.6 (95% CI −2.3, 1.2; p = 0.517) in patients taking donepezil + memantine (10 mg BID). In patients in the mITT population who were on donepezil + Namenda XR (28 mg QD), at week 26, the mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 50 mg and placebo was 0.5 (95% CI −1.6, 2.6; p = 0.620). The mean difference between masupirdine 100 mg and placebo was 0.2 (95% CI −2.1, 2.4; p = 0.894). These observations suggest that participants treated with memantine 28 mg, alone or as part of the fixed combination, responded less well to treatment with masupirdine.

Treatment Response by Sex

In the mITT population, the treatment effect of masupirdine was numerically superior in male patients to that in female patients when compared with placebo at week 26. In male patients, the mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 50 mg and placebo at week 26 was −1.6 (95% CI −3.5, 0.2; p = 0.081). The mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 100 mg and placebo was −1.2 (95% CI −3.1, 0.7; p = 0.225). In female patients, at week 26, the mean difference in change from baseline in ADAS-Cog 11 scores between masupirdine 50 mg and placebo was 0.3 (95% CI −1.4, 2.0; p = 0.746). The mean difference between masupirdine 100 mg and placebo was 0.7 (95% CI −1.0, 2.4; p = 0.411).

Treatment Response by Memantine Concentration

Based on the observed trends in the effects of masupirdine on cognitive endpoint (ADAS-Cog 11) with respect to memantine dosage forms and the literature indicating differences in memantine exposure with these dosage forms, further analyses were conducted based on memantine exposure levels at week 26.

The median steady-state trough concentration of memantine was 104 ng/mL at week 26 in patients taking memantine 10 mg BID (immediate-release [IR] tablets), whereas it was 130 ng/mL for patients taking Namenda XR, approximately 25% higher in those taking the long-acting formulation.

A summary of subgroup analysis of ADAS-Cog 11 scores by observed memantine plasma concentrations at week 26 in the mITT population is presented in Table 2 and Fig. 3.

Table 2 ADAS-COG 11: memantine plasma concentrations ≤ 100 ng/mL
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Fig. 3
figure 3

ADAS-Cog 11 score: memantine concentrations ≤ 100 ng/mL vs. > 100 ng/mL

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Patients with memantine concentrations ≤ 100 ng/mL showed a significant treatment difference in change from baseline in ADAS-Cog 11 scores when comparing the masupirdine 50 mg arm and the placebo arm. The mean difference between masupirdine 50 mg and placebo was −3.08 (95% CI −4.98, −1.18; p = 0.0008) at week 26, favoring masupirdine. The mean difference between masupirdine 100 mg and placebo was −1.98 (95% CI −3.91, −0.05; p = 0.04), favoring masupirdine. In the evaluable population the results were similar to those in the mITT population. The effect size (Cohen’s d) observed with 50 mg masupirdine was 0.52 and with 100 mg masupirdine was 0.35.

Patients with memantine concentrations of > 100 ng/mL did not show any beneficial effect of masupirdine compared with the placebo. At week 26, the least-squares mean difference between the masupirdine 100 mg arm and placebo arm was 1.3 (95% CI −0.6, 3.1; p = 0.180), favoring placebo.

A positive sign was also observed in the MMSE scores in the patients whose memantine concentrations were ≤ 100 ng/mL The mean difference between masupirdine 50 mg and placebo was 0.98 (95% CI −0.03, 1.99; p = 0.0587) at week 26, favoring masupirdine. The mean difference between masupirdine 100 mg and placebo was 1.03 (95% CI 0.02, 2.04; p = 0.0458), favoring masupirdine (Fig. 4).

Fig. 4
figure 4

MMSE score: memantine concentrations ≤ 100 ng/mL vs. > 100 ng/mL

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Responder Analysis

A summary of responder analysis for ADAS-Cog 11 scores in the population with memantine concentrations ≤ 100 ng/mL is presented in Table 3. Patients who showed no deterioration based on the ADAS-Cog 11 during the study were considered responders for this analysis.

Table 3 Summary of responder analysis (population with memantine concentrations ≤ 100 ng/mL)
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In patients with memantine plasma concentrations ≤ 100 ng/mL, the percentage of responders on the ADAS-Cog 11 with masupirdine treatment was 49% versus 23% of participants on placebo. The percentage of responders was slightly higher in the 50 mg masupirdine arm than in the 100 mg masupirdine arm (53 vs. 45%).

Discussion

Masupirdine, a 5-HT6 receptor antagonist, was evaluated in participants with moderate AD (MMSE range of 12–20). In this phase 2 study, based on the potential beneficial effects of co-administration of masupirdine with donepezil and memantine on cognition observed in animal models, and the common use of memantine along with acetylcholinesterase inhibitors (AChEIs) in this population, patients on stable background therapy of donepezil and memantine were recruited. The evaluation of masupirdine is the first of its kind where this class of compounds was evaluated in combination with double background therapy. The phase 2 study with masupirdine did not show a drug–placebo difference on the primary endpoint. In an effort to understand the effects of the combination treatment, an exploratory post hoc analysis was carried out using data generated in a phase 2 study of masupirdine [18]. In the trial, participants were allowed to use either 10 mg BID or 28 mg QD memantine. Due to the variability in the memantine dosage forms used in the trial population, it was hypothesized that the effect of masupirdine would vary with the use of different dosage forms of memantine. Therefore, this post hoc analysis was carried out to understand the influence of memantine with respect to the effects of masupirdine on cognitive decline in patients with moderate AD.

In the phase 2 study, although no statistical significance between drug and placebo effects was observed at the end of 26 weeks of treatment, there were numerical differences observed in the ADAS-Cog 11 scores between the placebo and masupirdine treatment arms. A trend of slower decline was observed at week 26 [18] in patients treated with masupirdine. However when the population was segregated based on memantine use, the difference in the change from baseline scores in ADAS-Cog 11 between placebo and masupirdine was greater in patients taking memantine 10 mg BID than those who were on memantine 28 mg QD.

In a study comparing memantine exposure between 28 mg QD and 10 mg BID, the maximum plasma concentration (Cmax) and area under the concentration–time curve over a 24-h dosing interval (AUC0–24) were 48% and 33% higher for the XR dosage regimen, respectively [21]. In the current study, the median steady-state trough concentrations of memantine were 104 ng/mL at week 26 in patients taking memantine 10 mg BID (IR tablets), whereas it was 130 ng/mL for patients taking Namenda XR, 25% higher. According to the literature and exposure values from the current study, the mean minimum concentration (Cmin) values range from 95 to 105 ng/mL for memantine IR tablets. The differential exposures between memantine dosage forms prompted the post hoc evaluation of the effects of exposure on clinical efficacy variables.

Overall, the results of the post hoc analyses suggest that masupirdine performance was better with less exposure to memantine—those receiving memantine BID and those with lower trough levels of memantine. The effect sizes of 0.52 (50 mg) and 0.35 (100 mg) suggest a clinically meaningful impact of therapy. Effect sizes may be more appropriate for analyses of this type involving small patient populations  than p values [22].

Sex may also affect memantine trough levels. When male patients were compared with female patients, the male patients responded better on clinical measures. On review of memantine exposure values, men had lower trough memantine levels than women (104 ng/mL vs. 121 ng/mL).

Serotonin 5-HT6 receptors are expressed on γ-aminobutyric acid (GABA)-ergic spiny neurons of the striatum and are co-localized with glutamic acid decarboxylase in the cerebral cortex and hippocampus [23]. Experimental evidence suggests that 5-HT6 receptor antagonists may modulate cholinergic and/or glutamatergic systems via disinhibition of GABAergic neurons. In agreement with observations in the literature, masupirdine produced significant increases in acetylcholine and glutamate levels [17].

The current study involved combinations of masupirdine with donepezil and memantine, agents acting on two different neurotransmitter systems. Although the increase in acetylcholine by masupirdine is in accord with the mechanism of donepezil, the mechanism of action of memantine is divergent from the glutamate increase observed with masupirdine. Glutamate plays a prominent role in neural circuits involved with synaptic plasticity to shape learning and memory through activation of glutamate receptors, whereas memantine blocks the N-methyl-d-aspartate (NMDA) receptor to stop the current flow through its channels. NMDA receptor antagonists also have varying effects on cognition. Memantine improves memory in AD patients and in some (but not all) studies in animals [24, 25], whereas other NMDA receptor antagonists like MK-801, phencyclidine, and ketamine cause memory deficits.

Studies with idalopirdine and intepirdine in combination with AChEIs showed no clinical benefit. These agents have shown drug/placebo differences in phase 2 trials. The negative trials may have many explanations, including more heterogeneous populations with global trials, unknown adherence to donepezil, and changes in population inclusion parameters between phase 2 and phase 3 trials. Each 5-HT6 receptor antagonist may also have different levels of pharmacologic activity at the receptor level.

While the results here are from post hoc analyses, they suggest potentially important effects of interactions among drugs used to treat AD. These data indicate that further exploration in a prospective clinical trial is warranted to evaluate masupirdine with donepezil alone or with low-dose memantine. From a safety point of view, masupirdine treatment had a good safety and tolerability profile when added to background therapy of donepezil and memantine.

Limitations

This is a post hoc analysis; therefore, the hypothesis generated needs to be tested in a randomized clinical trial.

Conclusions

The use of different memantine products may have affected the potential efficacy of masupirdine in AD patients in the phase 2 trial. These exploratory observations merit further investigation. If proven in a follow-up randomized clinical trial (treatment with masupirdine with a low dose of memantine or without memantine), masupirdine can be used alone or in combination with AchI and/or with low-dose memantine.