Correcting the biology that drives the symptoms of autism and Fragile X syndrome.

The causes of autism number in the hundreds and there is tremendous inter-individual heterogeneity in symptom severity and comorbidity (e.g. intellectual disability, verbal fluency, irritability). The challenges to therapeutic development include identification of patients most likely to respond to a particular treatment and capturing benefits that manifest differently across individuals.

Multiple genetic and environmental risk factors for human autism have been studied in animal models. These studies have shown that varied causes of ASD converge onto a limited number of pathophysiological processes. Targeting the pathophysiology rather than a single symptom is a potential path to disease-modifying treatment.

Silencing of the FMR1 gene, encoding a protein called FMRP, is the cause of autism and intellectual disability in Fragile X Syndrome. Studies of animals in which this gene has been knocked out have shown that symptoms arise from disruption of synaptic protein synthesis, increased neuronal electrical excitability, and diminished inhibitory tone in the brain. This same pathophysiology is observed in animal models of numerous genetic and environmental causes of ASD. We discovered that arbaclofen corrects this pathophysiology in Fragile X, and subsequent studies showed that treatment improves multiple disease phenotypes in 10 different animal models of autism. Further, in humans with idiopathic ASD, several disease biomarkers have been corrected by arbaclofen.

Glutamate and GABA are the main excitatory and inhibitory neurotransmitters in the brain. Glutamate drives protein synthesis at excitatory synapses, and this response is exaggerated in Fragile X syndrome. One consequence of this molecular disturbance is elevated neuronal excitability which, in addition to impaired inhibitory synaptic transmission, increases the ratio of excitation (E) to inhibition (I) in the brain. Altered E/I and protein synthesis regulation are believed to be common causes of symptoms in ASD.

One class of GABA receptor, the GABA-B receptor, is uniquely found in the membranes of both the excitatory presynaptic boutons that release glutamate and the postsynaptic dendrites of excitatory neurons. Arbaclofen selectively activates these pre- and post-synaptic GABA-B receptors, and by doing so both normalizes protein synthesis and dampens excessive excitability.

The foundational discoveries came initially from studies in the Fmr1 knockout mouse model of Fragile X syndrome. Fragile X is caused by transcriptional silencing of the FMR1 gene and loss of the encoded FMRP protein. Fragile X is the leading known heritable cause of autism in humans, so insights gained by studying this disease are likely to apply to ASD more broadly.

Mark Bear and his students at Brown and MIT discovered that synaptic protein synthesis, and protein synthesis-dependent long-term synaptic depression (LTD), are triggered by activation of a metabotropic glutamate receptor (mGluR5), and that this response is elevated in the Fmr1 knockout mouse. This finding led to the theory that exaggerated protein synthesis in response to mGluR5 activation by glutamate underlies multiple symptoms of Fragile X. Numerous studies have confirmed this theory, showing that inhibition of signaling downstream of mGluR5 corrects diverse disease phenotypes across multiple animal models of Fragile X. These findings launched human clinical trials of mGluR5 negative allosteric modulators for Fragile X, but these unfortunately failed due to the rapid development of drug tolerance (acquired treatment resistance). Research conducted under Bear's supervision at Seaside Therapeutics identified GABA-B receptors as a potential alternative therapeutic target, and studies in the Fmr1 knockout mouse showed that arbaclofen treatment normalized excessive protein synthesis, reversed alterations in synaptic function and structure, prevented audiogenic seizures, and corrected multiple behavioral phenotypes, without the development of tolerance.

The scientific foundation for arbaclofen does not rest solely on animal studies. As mentioned, a feature of FXS pathophysiology is neuronal hyperexcitability. This phenotype is shared by neurons derived from both Fmr1 knockout mice and patients with FXS. In both species, it is significantly improved by treatment with arbaclofen.

In humans with ASD of unknown cause (termed idiopathic), researchers have established objective neurophysiological biomarkers of altered brain physiology. These include altered EEG signatures and visual processing deficits, ascribed to elevated E/I. Arbaclofen treatment rescues these phenotypes, bringing them closer to values in neurotypical children of the same age.

Across multiple Phase 2 and 3 trials and open-label extensions, arbaclofen has been studied in children, adolescents, and adults with FXS, ASD, and 16p11.2 deletion syndrome. These studies have shown that arbaclofen is safe and well tolerated. This was expected, as arbaclofen is the active ingredient of Lioresal (racemic baclofen), which has been in clinical use for decades with a well-known safety profile.

Arbaclofen studies were the first good-clinical-practice (GCP) compliant, randomized placebo controlled trials (RCTs) conducted in Fragile X and 16p11.2 microdeletion syndrome, and among the first in ASD. The clinical development program reflects iterative learning regarding dose selection, treatment duration, population stratification, and outcome measure sensitivity. Early findings informed the design of later trials and contributed to a more refined understanding of the appropriate patient populations and the endpoints that measure clinically meaningful benefits of treatment for FXS and ASD. This work has culminated in alignment with the FDA on the path to approval.

A hiatus in development occurred when Seaside Therapeutics closed in 2013, but progress continued with the sponsorship of the Simons Foundation. Allos Pharma was founded to complete what was started by Seaside, and acquired the exclusive license to develop arbaclofen for Fragile X in 2020 and for all indications in 2025.

In the last iterations of Seaside-sponsored trials, the minimal effective dose of arbaclofen was determined, it was recognized that divergence from placebo continued with increasing trial duration, and the age range when effects were most readily detectable was defined. These studies also revealed that additional steps were needed to mitigate placebo effects. Due to resource limitations, study FX302 in 5 to 11-year-olds could not be fully enrolled. Nevertheless, the study in 172 patients revealed a clear dose-response relationship and improvements across symptom domains. Responder analyses based on published meaningful within-subject change thresholds revealed statistically significant improvements that aligned with clinician and caregiver reports. Children felt more comfortable in their own skin, showed improved coping skills, and were able to engage in social activities. The FDA has agreed that a single confirmatory trial would be sufficient for a New Drug Application.

Exploratory studies in ASD performed by Seaside showed promising results. The Phase 2 RCT AS208 (N = 150, 5 to 17-year-olds) employed knowledge gained from FXS studies regarding dose and duration (12 weeks). Although no statistically significant difference from placebo was detected on the primary endpoint in the overall study population, pre-specified and post hoc analyses demonstrated clinically meaningful improvements among participants with fluent language. The post hoc analysis also confirmed findings from FXS that treatment benefits those with problematic levels of irritability at baseline.

These learnings were incorporated into the study design of coordinated, parallel studies of arbaclofen in Europe (the Innovative Medicines Initiative funded arbaclofen ASD clinical trial, AIMS-2-TRIALS-CT1) and Canada (the Province of Ontario funded Canadian arbaclofen ASD clinical trial, ARBA), in which the dose was increased by up to 50% and the duration extended to 16 weeks. Global statistical tests applied to these combined studies show statistically significant and extraordinarily broad improvements in measures nominated by autism expert clinicians to reflect clinically significant benefit. This is the result one would expect from a therapy targeting pathophysiology rather than symptoms, and is consistent with disease modification.

• ✓Validation in multiple animal models
• ✓Rescue of human biomarkers and cellular pathophysiology
• ✓Significant and meaningful improvements in multiple behavioral domains
• ✓Excellent safety profile

Arbaclofen checks all the boxes.