Predicting Treatment Response to Memantine in Autism Using Magnetic Resonance Spectroscopy

Background and Rationale

Autism Spectrum Disorder (ASD) is a behaviorally defined complex neurodevelopmental disorder characterized by early childhood onset of marked deficiencies in social and communication skills and presence of restrictive, repetitive behaviors. Its phenotypic heterogeneity makes studying ASD and its treatment a challenging task. Currently, only two FDA approved drugs, risperidone and aripiprazole, are available to treat ASD that manage irritability associated with the disorder. None have shown conclusive benefit for core features of social and communication deficits.

Research indicates that a disrupted balance between excitation (glutamate) and inhibition (gamma amino-butyric acid, GABA) is likely to be one of the underlying mechanisms of ASD. This hypothesis is supported by a number of magnetic resonance spectroscopy studies that report an excess of glutamatergic excitation and/or a reduction in GABAergic inhibition observed in different brain regions including the limbic system and the anterior cingulate cortex. Similar findings have also been reported in post mortem studies of the ASD brain.Thus, it is expected that drugs targeting this imbalance could be beneficial. For example, the GABAB agonist, arbaclofen, was tested in ASD for this reason and showed promising results but a large trial failed to show improvement on the primary endpoints of lethargy and social withdrawal.

Memantine, a moderate affinity N-methyl-D-aspartate (NMDA) receptor antagonist, is known to improve communication in patients with Alzheimer's disease and is an FDA approved treatment for that disease. Examination of the efficacy of memantine in treating ASD has also been driven by the excitatory (glutamate) - inhibitory (gamma amino-butyric acid, GABA) imbalance hypothesis of ASD pathology. Memantine binds to the glutamatergic NMDA receptors and attenuates glutamatergic excitation. This reduction in excessive glutamatergic activation is thought to decrease the "noise" in the neural network system and facilitate learning and memory. A few small studies have shown effectiveness for memantine in the treatment of social and communication aspects of ASD spectrum disorder (ASD) as well. However, one large clinical trial using memantine in ASD reported a failure to respond. These discrepancies in results could possibly be due to the heterogeneity in the excitatory-inhibitory imbalance between ASD patients thereby causing a variation in response. In such a scenario, having a biomarker to predict an individual's treatment response would be invaluable.

MR Spectroscopy is a non-invasive tool used to examine the biochemical profile of brain tissue associated with different psychiatric and neurological conditions including ASD. Specifically, 1H-MRS studies in ASD demonstrated the excitatory-inhibitory imbalance in various cortical and subcortical regions of the brain, a finding also reported in post mortem brain tissue studies. The use of this technique to determine if Glutamate/GABA concentrations in certain regions of the brain can predict response to treatment with memantine would be an innovative breakthrough in providing more effective individualized treatment for patients since very little research has been done to explore this possibility.

A similar approach has been adopted in a recent ongoing study that uses proton MR spectroscopy to map changes in glutamate and GABA following use of riluzole in ASD. Riluzole is a drug commonly used in amyotrophic lateral sclerosis (ALS) and is a glutamate antagonist that works by blocking presynaptic glutamate release and noncompetitive inhibition of NMDA receptors. However, riluzole is not clinically used in the treatment of ASD. To our knowledge, only one study so far (apart from the ongoing study mentioned) has examined the effects of riluzole in ASD, but as an adjunct to risperidone. The primary outcomes of the Ghaleiha et al. study were irritability and repetitive motor behaviors and not core social and communication deficits as in the present proposal. The purpose of riluzole in this MR spectroscopy study is a proof of principle for its effects on spectroscopy based on its glutamate antagonistic effects. By contrast, memantine, which will be studied in the present proposal, has some documented evidence from small studies of being beneficial in treating social and communication deficits associated with ASD in clinical settings, despite the failure of the larger trial.

The investigator's lab has previously utilized MR spectroscopy techniques to map these different compounds in a group of 14 subjects with ASD and identify glutamate and GABA alterations in the cerebellum, discovering a relationship to cerebrocerebellar connectivity and behavioral outcomes in individuals with ASD.

Specific Aims:

Specific Aim 1: The investigator proposes a pilot, exploratory, clinical follow-on study using proton magnetic resonance spectroscopy (1H-MRS) to examine whether the following measures can be used to predict treatment response to memantine in ASD:

1. Glx and/or GABA and the Glx/GABA ratio in the anterior cingulate cortex,
2. Glx levels in the dorsolateral prefrontal cortex and the cerebellum
3. Markers of neuronal integrity, N-acetyl aspartate (NAA), myo-inositol, choline and creatine/phosphocreatine levels in the anterior cingulate, dorsolateral prefrontal cortex and cerebellum

Specific Aim 2: To examine whether resting state functional connectivity between the dorsolateral prefrontal cortex, anterior cingulate and the cerebellum, holds predictive value for treatment response to memantine in ASD.

The anterior cingulate cortex, dorsolateral prefrontal cortex and the cerebellum have been have been chosen as regions of interest (ROIs) due to known atypical glutamate and GABAergic profiles in these regions associated with ASD. The approach of limiting to selected ROIs has been adopted in order to ensure patient comfort during the imaging process by reducing the time spent in the scanner. Since altered levels for markers of neuronal integrity, such as NAA, choline, creatine/phosphocreatine and myoinositol have also been reported in ASD, we will be mapping these biochemicals as well in order to explore their effect on treatment response. Resting state functional connectivity will also be analyzed for possible association with treatment outcome prediction since as mentioned before, a previous study from the investigator's has shown a link between alterations in resting state cerebrocerebellar connectivity, excitatory-inhibitory ratio in the cerebellum and behavioral outcome in ASD using MR spectroscopy.