Biogen's Wild Day: An FDA Gold Star and a Rejection Letter, Both for SMA

Salanersen: The Sequel Biogen Is Betting On

Now the good news, and it's genuinely exciting.

Salanersen (also known as BIIB115) earned Breakthrough Therapy Designation, which is the FDA's way of saying: "This looks promising enough that we want to help you get it across the finish line faster." The designation is reserved for drugs that show preliminary evidence of substantial improvement over existing treatments. It's not a guarantee of approval, but it's the closest thing the FDA offers to a thumbs-up emoji.

So what makes salanersen special? It works through the same basic mechanism as Spinraza: both are antisense oligonucleotides (essentially synthetic snippets of genetic material) injected into spinal fluid to boost production of SMN protein, the stuff motor neurons need to survive. SMA patients don't make enough of it, which leads to progressive muscle weakness and, in severe cases, death.

But salanersen is engineered to be more potent, with the potential for once-yearly dosing. Compare that to Spinraza's current schedule of injections every four months (after a loading phase), and you can see why doctors and patients might prefer the upgrade. Fewer spinal injections per year is a meaningful quality-of-life improvement when you're talking about kids and young adults who've been getting poked regularly for years.

The Phase 1 data backed up the hype. All 24 treated participants improved on at least one endpoint, and half achieved at least one new motor milestone. In patients with elevated neurofilament light chain (a marker of nerve damage), salanersen delivered roughly a 75% reduction at six months, suggesting real neuroprotective effects. Biogen selected the 80 mg dose and is now launching a Phase 3 program that includes multiple global studies: STELLAR-1 for presymptomatic infants and SOLAR for teens and adults.

The SMA Hunger Games, 2026 Edition

Biogen isn't competing with itself here. It's competing with everyone.

The SMA market in 2026 is a three-player oligopoly. Spinraza (Biogen) was the first approved treatment. Zolgensma (Novartis) is a one-time gene therapy given intravenously, the "one-and-done" option with a famously astronomical price tag. Evrysdi (Roche) is an oral daily pill, the most convenient of the three. Together, these companies control over 85% of global SMA revenue.

Each has a distinct competitive moat. Zolgensma wins on durability (one dose, potentially lifelong benefit) but is limited to younger patients. Evrysdi wins on convenience (swallow a pill at home). Spinraza wins on breadth (usable across all ages) but loses on procedural burden; nobody loves repeated spinal injections.

Salanersen could reshuffle the deck. Once-yearly dosing would dramatically narrow the convenience gap with Evrysdi while keeping the proven antisense oligonucleotide mechanism that doctors trust. If Phase 3 data holds up, Biogen could essentially replace its own aging blockbuster with a better version, a rare pharmaceutical trick that's part product upgrade, part franchise defense.

And the competitive pressure isn't just coming from established players. Scholar Rock has a late-stage SMA asset in development, targeting different aspects of the disease biology. The cozy three-company club may not stay cozy for long.

What This Means for Biogen's Playbook

The BTD and CRL tell a clear story about Biogen's SMA strategy: protect the present, bet on the future.

The high-dose Spinraza CRL is a temporary headache that should resolve with a quick resubmission. The salanersen breakthrough designation is a longer-term signal that Biogen's pipeline has real legs.

But the real takeaway isn't about Biogen's stock price. It's about the patients. SMA is a devastating disease that, just a decade ago, had zero approved treatments. Now there are three on the market, a high-dose regimen working through regulatory hiccups, and a next-generation therapy sprinting toward Phase 3 with the FDA's blessing.

For families navigating SMA, the question is shifting from "Is there a treatment?" to "Which treatment is best for my child?" That's the kind of problem biotech should aspire to create.