A phase 3 for the rarest ALS, not the franchise

In Short. A rare genetic form of ALS (about 0.6% of cases) just cleared a phase 3 hurdle: an RNA drug beat placebo at week 72 on a combined measure of daily function and survival. Partners will now talk to regulators about that tiny genotype. It is not a rewrite of the ALS franchise, and the royalty math stays small.

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Amyotrophic lateral sclerosis (ALS) kills motor neurons and steadily strips people of movement, speech, and breathing. Most cases are not driven by a single named gene. A sliver are. Mutations in FUS, the fused-in-sarcoma gene, cause about 0.6% of ALS. That slice, called FUS-ALS, is often fast in younger patients, and can end in respiratory failure and death within one to two years of symptom onset.

Otsuka and Ionis Pharmaceuticals say their candidate for that genotype has now cleared a phase 3 primary endpoint. The Fusion trial enrolled 89 people with FUS-ALS. In the primary analysis set of 73, an RNA drug that lowers the disease-driving FUS protein, ulefnersen, beat placebo at week 72 on a combined score of daily function and survival. That is a real clinical result for a rare genetic form of the disease. It is not proof that ALS as a whole has a new franchise therapy, and it is not an approval. (Fierce Biotech)

The medicine is simple in concept before the labels. Ionis designed ulefnersen as an RNA-targeted molecule that lowers production of FUS protein in motor neurons. Otsuka paid Ionis $10 million upfront in 2024 for worldwide rights, and took on global regulatory and commercial work.

What Fusion measured matters as much as the fact that it hit. The primary endpoint assessed functional impairment and survival at week 72. Fierce Biotech reports that the endpoint analyzed performance on an ALS functional rating scale, time to rescue and ventilation assistance-free survival. Patients on ulefnersen did significantly better than those on placebo on that combined measure. Secondary results also favored the drug: change from baseline in a neurodegeneration biomarker, and time to death, permanent ventilation, rescue, or withdrawal because of disease progression. The partners called the safety and tolerability profile favorable, with most adverse events mild to moderate.

The money frame is as small as the genotype. Guggenheim Securities, in a September 14 note cited by Fierce, forecasts Ionis royalties of about $14 million by 2030 and about $16 million by 2036. Those are royalty footnotes, not blockbuster math. John Kraus, Otsuka’s chief medical officer, said the company intends to work closely with health authorities to advance ulefnersen with urgency. Talks with regulators are the next step. An approved therapy is not.

A powered rare-ALS primary is worth watching precisely because it is narrow. Fusion tells readers what week-72 function and survival looked like in a few dozen people with FUS mutations. It does not rewrite care for the other 99-plus percent of ALS. Whether the readout becomes a medicine still depends on what regulators decide. Promising, in other words, but not yet definitive.■