Antisense oligonucleotide jacifusen for FUS-ALS: an investigator-initiated, multicentre, open-label case series

Pathogenic variants of fused in sarcoma (FUS) cause amyotrophic lateral sclerosis (FUS-ALS), with evidence of gain of function. Jacifusen is an antisense oligonucleotide targeting FUS pre-mRNA, previously shown to delay neurodegeneration in a mouse model and potentially slow functional decline in a first-in-human study. Here, we sought to further evaluate use of jacifusen as a treatment for FUS-ALS.

This expanded access programme was conducted through a series of single-patient investigational new drug applications at five sites (four hospitals in the USA and one in Switzerland). Participants carried a FUS variant and had clinical evidence of motor neuron disease onset or electrophysiological abnormalities, if not a diagnosis of ALS. Participants were ineligible if chronically ventilated with tracheostomy. Enrolled sequentially, participants received serial intrathecal injections of jacifusen over 2·8-33·9 months. Based on multiple ascending doses of jacifusen (from 20 mg to 120 mg), successive protocols were modified as safety and other data were acquired, with the last participants enrolled receiving 120 mg doses monthly from the start of their treatment. Safety was assessed using the Common Terminology Criteria for Adverse Events version 4.0 and standard cerebrospinal fluid (CSF) metrics. Concentration of neurofilament light chain (NfL) in CSF was used as a biomarker of axonal injury and neurodegeneration, and the ALS Functional Rating Scale-Revised (ALSFRS-R) score was used as an overall measure of motor function. Biochemical analysis and immunohistochemical staining were done on post-mortem CNS tissues to quantify FUS protein expression and assess the burden of FUS pathology.

Between June 11, 2019, and June 2, 2023, we recruited 12 participants (median age 26 years [range 16-45]; seven [58%] were female and five [42%] were male) into the expanded access programme. Transient elevations in cell counts or total protein concentration in CSF (six [50%] participants) were unrelated to treatment duration. The most common adverse events were back pain (six [50%]), headache (four [33%]), nausea (three [25%]), and post-lumbar puncture headache (three [25%]). Two participant deaths were recorded during the programme, both thought to be unrelated to the investigational drug. The concentration of NfL in CSF was reduced by up to 82·8% after 6 months of treatment. Although most participants had continued functional decline (as measured by ALSFRS-R) after starting treatment with jacifusen, one 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 samples from four participants showed reduced FUS protein levels and an apparent decrease in the burden of FUS pathology.

The findings suggest the safety and possible efficacy of jacifusen for treating FUS-ALS. The efficacy of jacifusen is being further evaluated in an ongoing clinical trial.

ALS Association, Project ALS, Ionis Pharmaceuticals, Tow Foundation, Nancy D Perlman and Thomas D Klingenstein Innovation Fund for Neurodegenerative Disease, National Institutes of Health, Angel Fund for ALS Research, Cellucci Fund for ALS Research, Max Rosenfeld ALS Fund, University of Minnesota, and the Muscular Dystrophy Association.