Ulefnersen's Phase 3 Win in FUS-ALS: First Genetic-Cause Trial to Clear the Bar, But Effect Size Remains the Unanswered Question

Ionis' Ulefnersen Achieves Positive Phase 3 FUSION Results in FUS-ALS

Ionis Pharmaceuticals, Inc. and Otsuka Pharmaceutical Development & Commercialization, Inc. announced positive topline results from the Phase 3 FUSION trial evaluating ulefnersen, an investigational RNA-targeted medicine for FUS-ALS. The study met its primary endpoint, demonstrating a statistically significant improvement in functional impairment and survival (p=0.0005) compared to placebo. This marks the first placebo-controlled clinical study targeting the underlying genetic cause of FUS-ALS to show such a benefit, supporting ulefnersen's potential to modify disease progression. The drug also showed significant improvements in secondary endpoints and a favorable safety profile. Discussions with the U.S. FDA and global health authorities for expedited regulatory submissions are planned.

• The FUSION trial successfully met its primary endpoint, demonstrating a statistically significant improvement (p=0.0005) in a prespecified joint rank analysis. This analysis combined assessments of time to death or permanent ventilation, time to rescue, and change in ALS Functional Rating Scale Revised (ALSFRS-R) score from baseline to Day 505, providing robust evidence of ulefnersen's efficacy in FUS-ALS.
• Beyond the primary endpoint, ulefnersen showed statistically significant improvements across important secondary endpoints, including changes from baseline in serum neurofilament light chain (NfL) and time to death, permanent ventilation, rescue, or withdrawal due to disease progression. The treatment also exhibited a favorable safety and tolerability profile, with most adverse events being mild or moderate in severity.
• Ulefnersen is the first investigational medicine to target the underlying genetic cause of FUS-ALS and demonstrate the potential to modify disease progression in a Phase 3 trial. Ionis and Otsuka plan urgent discussions with the U.S. FDA and other global health authorities to explore expedited regulatory submission pathways for approval, aiming to address the critical unmet need for this rare, rapidly progressive, and fatal form of ALS.

Ulefnersen's Groundbreaking Phase 3 FUSION Results for FUS-ALS

One recent investigator-initiated, multicentre, open-label case series evaluated jacifusen, an antisense oligonucleotide targeting FUS pre-mRNA, in participants with FUS-ALS through an expanded access programme conducted at five sites (four in the USA and one in Switzerland). Between June 11, 2019, and June 2, 2023, 12 participants were enrolled (median age 26 years [range 16–45]; seven [58%] female and five [42%] male). Participants received serial intrathecal injections of jacifusen over 2·8–33·9 months, with doses escalating from 20 mg to 120 mg across successive protocols; the last participants enrolled received 120 mg doses monthly from the start of treatment.

From a safety standpoint, the most common adverse events were back pain (six [50%] participants), headache (four [33%]), nausea (three [25%]), and post-lumbar puncture headache (three [25%]). Transient elevations in cell counts or total protein concentration in CSF occurred in six (50%) participants and were assessed as unrelated to treatment duration. Two participant deaths were recorded during the programme, both thought to be unrelated to the investigational drug.

On the efficacy side, CSF neurofilament light chain (NfL) concentration — used as a biomarker of axonal injury and neurodegeneration — was reduced by up to 82·8% after 6 months of treatment. Although most participants had continued functional decline as measured by the ALS Functional Rating Scale-Revised (ALSFRS-R), one participant showed unprecedented, objective functional recovery after 10 months, and another remained asymptomatic with documented improvement in electromyographic abnormalities. Biochemical and immunohistochemical analysis of CNS tissue from four participants demonstrated reduced FUS protein levels and an apparent decrease in the burden of FUS pathology. The efficacy of jacifusen is being further evaluated in an ongoing clinical trial.

Designing the FUSION Trial to Target FUS-ALS

The primary clinical evidence for jacifusen in FUS-ALS derives from an investigator-initiated, multicentre, open-label expanded access programme conducted across five sites (four in the USA, one in Switzerland). Participants carried a pathogenic FUS variant with clinical or electrophysiological evidence of motor neuron disease, and were enrolled sequentially between June 11, 2019, and June 2, 2023.

Ulefnersen's FUSION Success: A New Era for Genetic ALS?

The recent announcement regarding ulefnersen's positive Phase 3 FUSION trial results marks a pivotal moment for the ALS community, particularly for those affected by FUS-ALS. This investigational RNA-targeted medicine has demonstrated a statistically significant improvement in functional impairment and survival, a breakthrough in a disease historically characterized by rapid progression and limited therapeutic options. For patients with FUS mutations, this represents the first time a placebo-controlled study targeting the underlying genetic cause has shown such a profound benefit, offering a tangible hope for disease modification.

This success underscores the growing promise of antisense oligonucleotide (ASO) technology in addressing challenging neurodegenerative disorders. By silencing FUS expression, ulefnersen directly targets the genetic driver of this aggressive ALS subtype, moving beyond symptomatic management to potentially altering the disease's trajectory. This validation of the ASO platform could have far-reaching implications, de-risking other ASO programs in development and attracting further investment into this innovative therapeutic modality.

However, the path forward is not without its considerations. The delivery of ASOs to the central nervous system often necessitates intrathecal administration, which, while effective, can be burdensome for patients and healthcare providers. Furthermore, ulefnersen's specificity to FUS-ALS means its impact, while significant for this rare population, will not extend to the broader ALS patient community. As discussions with regulatory authorities commence, the focus will undoubtedly shift to ensuring long-term safety and efficacy, alongside strategies to optimize patient access and adherence for this specialized treatment.