Sanofi just halted another late-stage trial, and it's not the first pipeline disappointment this year. With Dupixent's patents expiring around 2031 and its top successor underwhelming Wall Street, the pressure to find a Plan D is getting very real.
Imagine you're a restaurant with one legendary dish. It brings in 90% of your customers. So you start developing new recipes to survive the day that dish gets copied. Reasonable plan. Except now, three of your new recipes have flopped in a row.
That's roughly where Sanofi finds itself after halting yet another late-stage clinical trial.
Another One Bites the Dust
On June 10, Sanofi pulled the plug on MOBILIZE, a Phase 3 trial testing a drug called riliprubart in patients with CIDP (chronic inflammatory demyelinating polyneuropathy, a rare condition where the immune system attacks the nerves). An independent data monitoring committee reviewed interim results and concluded the drug was unlikely to work well enough to justify continuing.
No safety problems were flagged. The drug just didn't do what it needed to do.
Riliprubart works by blocking a protein called C1s in the complement pathway, part of the immune system's attack machinery. The idea was to calm that overactive response in patients who hadn't responded to standard treatments. On paper, it was a smart bet. In practice, the data said otherwise.
Sanofi is still deciding whether to continue a second riliprubart trial (called VITALIZE) in a different group of CIDP patients. But the outlook isn't exactly rosy.
The Real Story: Death by a Thousand Setbacks
If this were an isolated miss, nobody would panic. Drug development is hard; roughly 90% of clinical-stage drugs fail. But riliprubart isn't an isolated miss. It's the latest in a string of disappointments that have rattled investor confidence in Sanofi's ability to build a life after Dupixent.
Let's rewind. Dupixent is Sanofi's crown jewel, a blockbuster anti-inflammatory biologic that pulled in roughly €13.1 billion in recent annual sales. It treats everything from eczema to asthma to chronic sinus problems, and the company is pushing it into even more conditions. By 2031, analysts project sales could exceed .
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$25 billion
But Dupixent's core patents start expiring around 2030 to 2031. When that happens, biosimilar competitors will flood the market, and Sanofi's golden goose will start laying smaller eggs. The company needs replacements. Several of them.
The Heir That Stumbled
Sanofi's most prominent successor candidate was amlitelimab, a biologic that targets a different part of the immune system (OX40L, which helps regulate T-cell activity). Wall Street had essentially crowned it as the next Dupixent. Analysts at Barclays described it as the asset investors had "anointed as Sanofi's leading post-Dupixent asset."
Then the Phase 3 data came in. Amlitelimab worked, but not as well as Dupixent. JPMorgan analysts confirmed the drug appeared "less effective than Dupixent," which is a polite way of saying the crown prince didn't live up to the hype.
Sanofi's stock cratered roughly 8 to 10% in a single day, erasing nearly $13 billion in market cap. That's like watching an entire mid-cap biotech company vanish because of one data readout.
To be fair, amlitelimab still has legs. Its safety profile looks clean, and it can be dosed just once every 12 weeks (compared to Dupixent's every-two-week schedule). For patients who hate needles, that's a meaningful perk. Jefferies analysts noted it could still carve out a role, just not the premium, first-line franchise everyone had hoped for.
Meanwhile, in Neurology
Sanofi's troubles aren't limited to immunology. Tolebrutinib, a BTK inhibitor being developed for multiple sclerosis, failed its Phase 3 trial in primary progressive MS. On top of that, the FDA delayed its decision on tolebrutinib for other MS types, pushing the timeline into early 2026. Liver toxicity concerns have been lurking in the background, adding another layer of uncertainty.
The stock dropped another 4 to 6% on that news. Jefferies called the MS failure a "negative surprise," noting that the biggest commercial upside had been expected in primary progressive MS specifically.
What Sanofi Has Left in the Tank
It's not all doom. Sanofi still has a deep bench, and some recent results have been genuinely encouraging.
Amlitelimab, duvakitug, and lunsekimig are immunology bets that analysts collectively project could generate over $1.9 billion by 2031. That's real money, though it's a fraction of what Dupixent produces. Venglustat, a drug for rare lysosomal storage disorders like Gaucher disease, posted positive Phase 3 results and could become a meaningful specialty franchise.
On the commercial side, Sanofi's recent launches (including cancer drug Sarclisa, RSV antibody Beyfortus, and hemophilia treatment Altuviiio) grew 34% collectively in 2025. The company has also been on an acquisition spree, picking up companies like Blueprint Medicines ($9.1 billion) and Dren Bio ($1.9 billion).
The strategy is clear: since no single drug can replace Dupixent, Sanofi is betting on a portfolio approach. Think of it like diversifying your investments instead of putting everything in one stock.
The "Show Me" Stock
Analysts increasingly describe Sanofi as a "show me" story. Translation: investors are done giving the benefit of the doubt. They want clean, convincing late-stage data before they'll pay up for the stock again.
The "cliff stock" label keeps getting louder. Multiple analyst firms argue that Sanofi's valuation should trade at a discount to peers until the company proves it can replace Dupixent's earnings power. UBS described amlitelimab's efficacy as "solid but clearly inferior to Dupixent," which is the kind of compliment that makes you wince.
Sanofi insists the riliprubart setback doesn't change its 2026 financial guidance. That's technically true; CIDP was never a major revenue driver in consensus models. But the symbolic damage matters. Each late-stage failure chips away at the narrative that Sanofi's pipeline is strong enough to weather the Dupixent cliff.
The Clock Is Ticking
Sanofi has roughly four to five years before biosimilar pressure hits Dupixent in earnest. That's not nothing. Several key data readouts are expected through 2026, including additional amlitelimab trials, venglustat in Fabry disease, and new Dupixent label expansions.
But the margin for error keeps shrinking. Every failed trial narrows the path. And when your Plan B (amlitelimab) underwhelms and your Plan C (riliprubart) gets shelved, investors start asking an uncomfortable question: what's Plan D?
Sanofi better have a good answer. The clock doesn't care about your pipeline slides.
Clinical & Regulatory4 min read
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