Riluzole, a Glutamate Modulator, Slows Cerebral Glucose Metabolism Decline in Patients With Alzheimer's Disease

Discussion

The results of this pilot double-blind, randomized, placebo-controlled trial of riluzole 50 mg twice daily in patients with Alzheimer's disease have confirmed our primary hypotheses, showing 6 months of riluzole treatment to be associated with less decline in FDG-PET measures of cerebral glucose metabolism compared to placebo. The effect was most robust in PC, but effects were also observed in precuneus, lateral temporal cortex, right hippocampus and frontal cortex. Glutamate levels measures with ¹H MRS, a secondary outcome, showed a significant or trend-level group × visit interaction in PC, whereby the levels of this excitatory amino acid neurotransmitter increased after three months of treatment, suggesting the possibility that riluzole engages the glutamatergic system as its therapeutic target. No changes were found in the ¹H MRS levels of NAA, our second primary outcome measure. A significant correlation was observed between cognitive measures and cerebral metabolism in FDG-PET, a key measure of brain function in Alzheimer's disease. PC glutamate levels also correlated with cognitive performance. Our study provides the first in-human data supporting a potential therapeutic benefit of riluzole in patients with Alzheimer's disease.

The beneficial effects of riluzole on neuronal function and cognition observed in this pilot study could be attributable to one or more of the mechanisms that have been established in rodent models. In this mild Alzheimer's disease population, where amyloid and tau were already well-established, the most likely mechanisms of therapeutic effect by riluzole could have been in preventing further damage to vulnerable pyramidal neurons through modulation of glutamate levels, reduced glutamate-mediated toxicity and increased synaptic activity. The directional favourable cognitive effects are consistent with the prevention of age-related cognitive decline found in rodent models.^[22,26]

Similar effects on glucose metabolism were observed with riluzole as those reported for memantine in patients with Alzheimer's disease over the same time period,^[65] with both studies finding greatest effects in PC and precuneus. Memantine acts on the glutamatergic system through NMDA receptor partial antagonism, reducing calcium ion influx and related toxicity.^[66] Riluzole has shown similar mitigating effects on sodium and calcium ion influx, through protein kinase C (PKC) inhibition or by regulating glutamate transporter and modulation of ion channels leading to potential decreased glutamate overflow to extrasynaptic space rather than direct NMDA interaction.^[67] Given memantine's effects on glucose metabolism, this mitigation may also have contributed to the effects of riluzole. The numerous effects of riluzole demonstrated preclinically may support additional therapeutic benefit in Alzheimer's disease.

FDG-PET was chosen as the main primary outcome measure in this study because it is a well-established biomarker of neuronal function in Alzheimer's disease,^[64,68] and progressive hypometabolism in Alzheimer's disease-relevant regions strongly correlates with clinical progression.^[62–64] The PC, in which the most robust treatment effect was observed in this study (Figure 3A–C), is a hub network region and one of the earliest and most strongly affected regions in Alzheimer's disease.^[69,70] The slower decline in cerebral glucose metabolism with riluzole was observed in both younger and older groups, males and females, and APOE ɛ4 carriers and non-carriers (Figure 3D).

The observed lessening of metabolic decline in the riluzole group compared to placebo in several Alzheimer's disease-related regions of interest (Figure 4) suggested an effect in Alzheimer's disease-related networks. Consistent with this, the Alzheimer's disease Progression Classifier score, previously validated in ADNI participants,^[51] showed a trend-level slower disease progression in the riluzole-treated group than in the placebo group (Figure 5A). This preservation supports further exploration of the apparent disease-modifying effect of riluzole in larger, longer, full-efficacy clinical trials, with assessments of clinical and neuroimaging changes at multiple time points to map the trajectory of therapeutic response. The less robust though significant effects in the Alzheimer's disease-related regions other than PC and precuneus may reflect technical factors including greater variability due to rotational head motion and differences in slice location from the reference region.

The trend-level differences in longitudinal Alzheimer's disease progression pattern expression between APOE ɛ4 carriers and non-carriers merits further study. A potential explanation may be due to baseline neural hyperexcitability in APOE ɛ4 carriers^[71,72] that could be more responsive to glutamatergic modulation. It would be of interest to characterize patients with Alzheimer's disease for analysis stratification, as the population is highly heterogeneous with regard to clinical manifestations, rate of disease progression, tau burden, comorbidities and possibly treatment response. This heterogeneity was present in our study.

We observed a strong correlation between the FDG-PET Alzheimer's disease Progression Classifier score and ADAS-cog at baseline and in treatment change from baseline to 6 months (Figure 5B). This is consistent with previous findings in the ADNI and other populations,^[51] and suggests that FDG-PET progression scores are predictive of and aligned with cognitive changes. Examining placebo and riluzole groups separately, correlations at baseline were significant for both groups. Additional associations between FDG-PET glucose metabolism and cognitive measures at baseline were observed (Figure 6). Use of biomarkers to predict cognitive effects in smaller trials could potentially lower clinical trial costs and play an important role in identifying which therapies should be advanced to larger trials.

¹H MRS has shown metabolic differences in Alzheimer's disease compared to matched normal individuals;^[73] however, the results have not been sufficiently consistent to enable their use as reliable outcome measures in Alzheimer's disease in a manner analogous to FDG-PET measures of cerebral glucose metabolism. For example, in a study of memantine that showed no MRS effects despite favourable glucose metabolism effects, the authors noted that MRS results were affected by variability and patient-induced artefacts.^[74] With riluzole an established glutamate modulator, this study sought to derive objective evidence of riluzole treatment target engagement by using ¹H MRS as a secondary outcome measure to measure in vivo brain levels of glutamatergic compounds. Our finding of a significant or trend-level group × visit interaction for PC glutamate, with its levels rising between 3 and 6 months of treatment (Figure 7A) suggest potential engagement of the glutamatergic system by riluzole. This interpretion should be made with caution as it is a significance in a group × visit interaction and preliminary while there were no significant differences between groups from baseline to 6 months as originally hypothesized. It has been proposed that enhancing the efficiency of glutamatergic synaptic activity, which riluzole is postulated to accomplish, leads to an increase in intracellular glutamate levels,^[75] which was probably the major contributor to the detected ¹H MRS glutamate signal. We sought to assess the effects of riluzole on neuronal viability and function through simultaneous ¹H MRS measurement of the putative neuronal marker NAA, but did not detect changes in NAA. Negative findings have also been reported in previous longitudinal ¹H MRS studies of Alzheimer's disease and ALS, which revealed cross-sectional but not longitudinal NAA changes, and attributed this to inter-participant variability and technical factors.^[76,77] Depleted ¹H MRS levels of NAA and glutamate have been reported in Alzheimer's disease compared to healthy controls,^[73,78,79] which could not be assessed in this study due to the lack of a normal comparison group. We observed an exploratory correlation between glutamate levels and cognitive measures in PC such as MMSE and ADAS-cog (Figure 7C), with higher ¹H MRS glutamate levels associating positively with higher cognitive performance. The positive correlation between NAA and glutamate levels (Figure 7B) was consistent with previous reports in the normal brain,^[80] potentially underpinned by tight coupling of the two compounds in NAA synthesis in neuronal mitochondria by addition of glutamate-derived aspartate to an acetyl group derived from acetyl-CoA, in a reaction catalysed by L-aspartate-N-acetyltransferase.^[81] Conversely, NAA has been postulated to serve as a reservoir of glutamate synthesis.^[82] As an exploratory analysis, we observed a significant or trend group × visit interaction with increased GABA levels in left hippocampus with riluzole treatment. GABA is reportedly depleted in Alzheimer's disease and mild cognitive impairment.^[83,84] Treatment-related increases in GABA levels (Supplementary Figure 2B) suggest that riluzole may at least partially alleviate this reported deficit, with possible benefit given the positive correlation of GABA with memory performance in this study (Supplementary Figure 2C). There was general agreement between the ratios of metabolite level peak ratios relative to unsuppressed water (W) and tCr in our study, but greater consistency in ratios measured relative to W. Caution has been urged in interpreting ratios relative to tCr because levels have been reported to change in a number of neuropsychiatric^[85] and neurological disorders.^[86] We provided both ratios for comparison with literature.

Riluzole was generally well tolerated, with no significant difference in side effects compared to placebo. Riluzole has been used for decades in the treatment of ALS. However, larger, longer duration studies are necessary to have a comprehensive evaluation of safety and efficacy of riluzole in the Alzheimer's disease population, and should precede its use in Alzheimer's disease outside of a monitored clinical trial.

This study has several limitations. First, the sample size was relatively small, and results require replication in larger-sample studies. A second limitation was the lack of amyloid characterization. However, the presence of an FDG-PET Alzheimer's disease pattern helped to confirm clinical diagnosis and has been shown to have a high degree of agreement with the presence of tau pathology.^[87] Third, ¹H MRS is unable to differentiate neurotransmitter or vesicular and metabolic pools of glutamate or GABA, limiting interpretation. ¹H MRS data acquisition techniques require relatively large voxels for reliable quantification of metabolites, potentially leading to partial volume effects and masking of group effects. As noted in other studies, longitudinal MRS measurements can be subject to technical variability and participant head motion. The study was not powered for neuropsychological outcome measurement, and clinical changes must be viewed only as directional and exploratory.

In conclusion, a slower decline in cerebral glucose metabolism was observed in riluzole-treated patients with Alzheimer's disease than in placebo in multiple Alzheimer's disease-relevant brain regions, which correlated with cognitive performance. These findings support future fully powered clinical trials to further evaluate riluzole as a potential pharmacological therapy for Alzheimer's disease.

Brain. 2022;144(12):3742-3755. © 2022 Oxford University Press