The first therapy targeted at modifying a mutant gene associated with amyotrophic lateral sclerosis (ALS), approved in early 2023, has offered reassurance that the biology of ALS, when known, is targetable. Historically, the disease has been considered a clinical diagnosis, but the
Despite a narrow indication, the only therapy targeted at an ALS-associated gene so far, SOD1 ALS, supports the premise that the biology of ALS can be modified, according to Christina N. Fournier, MD, an associate professor in the Department of Neurology, Emory University, Atlanta, Georgia.
Dr. Christina N. Fournier
Rather than a single pathological entity, ALS is best understood as the end result of many different pathological processes. Each might require its own targeted therapy in order to interrupt the upstream biological pathways that drive disease.
About 15% of ALS Has An Identifiable Genetic Cause
A family history of ALS is present in about 10% of cases. A genetic cause can be identified in approximately 15%. Cases without an identifiable genetic etiology are considered sporadic. So far, the only approved therapy that modifies the function of a gene associated with ALS is tofersen (Qalsody, Biogen), an antisense oligonucleotide. Tofersen inhibits RNA transcription of the superoxide dismutase 1 (SOD1) gene to decrease production of the SOD1 protein.
This first gene therapy for ALS is a breakthrough, but it is indicated for only a small proportion of ALS patients. Even though SOD1 gene mutations represent the second most common genetic cause of ALS after the C90rf72 gene, the proportion of patients who are candidates for tofersen is low. Efficacy is expected only in about 1% of those with familial ALS and 1% of those with sporadic ALS, or about 2% of all patients with ALS.
The evidence of benefit from a treatment with a specific target has provided the basis for concluding that “we are onto something,” Dr. Fournier said. An expert in ALS, she sees reason for excitement about the prospects in treatment with the growing focus on the underlying pathways of disease rather than the downstream consequences.
“The hope is that new gene-targeted therapies will be developed in the future to treat the broader ALS population,” said Dr. Fournier, explaining that the move toward rationally targeted treatments, whether related to gene mutations or independent molecular pathways of ALS progression, has created excitement in the field.
Numerous Disease Processes Are Potentially Targetable
As treatments are developed to address nongenetic molecular processes that contribute to the risk or progression of ALS, such as neuroinflammation or abnormal protein misfolding and aggregation, individualized treatment is likely to become key. Just as not all genetic cases share mutations in the same gene, the key molecular drivers of disease are likely to differ between patients. If so, it is hoped that biomarkers reflective of this underlying biology can be identified to appropriately target treatments.
“The excitement behind the newer targets in clinical trials is based on both basic science and early clinical data that support treatment based on specific drivers of disease,” Dr. Fournier said.
In 2023 and just prior to the FDA approval of tofersen, a set of expert consensus guidelines were published calling for genetic testing to be offered to all patients with ALS. These recommendations suggested that SOD1, C9orf72, FUS, and TARDBP should be included routinely into the panel of genes evaluated, calling for additional genes to be added as they emerge as potential therapeutic targets.
Even before these guidelines were released, genetic testing was already being offered at many centers with expertise in ALS. The rationale was to differentiate ALS with a genetic etiology from that with a nongenetic etiology, as well as to counsel family members when genetic risk was identified, but genetic testing has now assumed new urgency. In addition to the potential for offering a specific treatment for SOD1-related ALS, patients with other genetic forms of disease might be candidates for genetically focused clinical trials.
Genetic testing should be performed as soon as a diagnosis of ALS is made, according to Dr. Fournier. Although not all patients have accepted genetic testing, particularly in the past when there was no immediate clinical gain from establishing the presence of a genetic mutation, she said there is no longer any controversy about clinical relevance.
Genetic Testing Is Key to Genetic Therapies
“We do not want to miss the opportunity to treat patients when we have the chance,” said Dr. Fournier, referring to both the likely advantage of an early start of the approved gene therapy as well as the opportunity to participate in a clinical trial with other gene therapies in development.
Prior to the approval of tofersen, riluzole and edaravone had been the only disease-modifying agents in widespread use, but these drugs are nonspecific. There are no established biomarkers for establishing which patients are most likely to benefit.
In the case of riluzole, a pivotal trial conducted 30 years ago showed a survival benefit relative to placebo at 12 months (74% vs. 58%; P = 0.014). In a retrospective study published in 2022 that evaluated survival in a database of 4778 ALS patients of whom 3446 received riluzole, early diagnosis of ALS and prompt treatment with riluzole was associated with longer survival than delayed treatment. The benefit of edaravone has been validated with clinical measures, such as the revised Amyotrophic Lateral Sclerosis Functional Scale (ALSFRS-R).
The retrospective study of riluzole provides the basis for predicting better benefits from disease-modifying therapies if started earlier in the course of ALS. The same premise will be explored with newer therapies that target ALS-associated genes.