. Both studies investigated potential benefits of suppressing the toxic activity in cells of a mutant gene (SOD1) that encodes superoxide dismutase 1 (SOD1) in patients with ALS.
One study investigated the antisense oligonucleotide (ASO) tofersen (Biogen); the other study examined viral vector–mediated gene suppression.
The studies’ promising results signal “the beginning of a new precision medicine–based approach towards treating ALS,” said Orla Hardiman, BSc, MB, BCh, BAO, MD, a consultant neurologist and professor of neurology at Trinity College and Beaumont Hospital in Dublin, Ireland. Dr. Hardiman co-authored an editorial that accompanied the two studies, which were published July 9 in the New England Journal of Medicine.
Genetic culprits
ALS is a disorder of progressive degeneration of upper and lower motor neurons. It typically leads to death from ventilatory failure within 5 years of symptom onset.
Genetic factors are responsible for about half the risk variance of ALS. In populations of European origin, variants in SOD1 account for an estimated 13% to 20% of familial ALS, although this rate varies around the world. Although SOD1 is not the most common variant in ALS, it is the one that researchers are most familiar with and has been studied in an animal model.
In the first study, investigators evaluated the safety, pharmacokinetics, and pharmacodynamics of the ASO tofersen in adults with ALS.
An ASO is a small piece of nucleic acid that enters neurons in the spinal cord and brain, explained co-investigator Toby A. Ferguson, MD, PhD, vice president and head of the neuromuscular development unit at Biogen.
ASO binds to the SOD1 gene and knocks down the SOD1 protein, which is the “toxic engine [that] drives the disease, kills neurons, and causes patients to have loss of function and eventually to die,” said Dr. Ferguson. “The ASO turns off the motor that produces that toxic protein,” he added.
Animal studies have shown that ASOs that target SOD1 messenger RNA transcripts prolong survival, improve motor performance, and reduce SOD1 protein concentrations.
The new phase 1/2 double-blind study included 50 adults at 18 sites in the United States, Canada, and four European countries. All had muscle weakness attributed to ALS and a documented SOD1 mutation. Participants were randomly assigned to receive one of four doses of tofersen—20, 40, 60, or 100 mg—or placebo. Treatment was administered via a lumbar intrathecal bolus injection. The study included a screening period followed by a 12-week intervention period and a 12-week follow-up.
Adverse events
A primary outcome was the incidence of adverse events (AEs) and serious AEs. Results showed that all participants reported one or more AEs. The most common AEs were headache, pain at the injection site, post–lumbar puncture syndrome, and falls. Three deaths occurred, one in the placebo group, one in the 20-mg dose group, and one in the 60-mg dose group. There were no serious AEs in the 100-mg group.
Although the investigators found an increase in cerebrospinal fluid (CSF) protein and white cell counts, there was no clear association between these observations and higher doses of tofersen or longer duration of exposure.
“We don’t know the implications of this, and it’s something we need to keep an eye on as we move these studies forward,” Dr. Ferguson said.
None of the AEs or CSF abnormalities led to trial discontinuation.
A secondary outcome was change in SOD1 protein concentration in CSF at day 85. The study showed that SOD1 concentrations decreased by 36% among the participants who received tofersen 100 mg and by lesser amounts in the patients who received lower doses. Concentrations in the placebo group were reduced by 3%.
The 36% reduction in the highest dose group is likely meaningful and “foundational to the concept of what this molecule can do,” Dr. Ferguson said.
“If the number one cause of SOD1 ALS is accumulation of toxic SOD1 protein, then the demonstration that we can reduce SOD1 protein in the CSF ... is saying that’s the first step on the way to showing the molecule is doing what it should do,” he added.