MS BTK Race Over: Fenebrutinib Won, Tolebrutinib CRL'd

Multiple sclerosis is the quiet neurology market everyone forgot while Alzheimer’s grabbed the headlines, and that is exactly why it is tradeable. The setup most investors think is still unfolding, a three-way race to bring the first oral BTK inhibitor across the finish line, is already over. Roche’s fenebrutinib swept all three of its Phase 3 trials. Sanofi’s tolebrutinib got a complete response letter from the FDA and is limping toward Europe. Merck KGaA’s evobrutinib flamed out years ago. The market has barely noticed, which is the whole point. Here is the market, the race as it actually ran, and the one trade still left.

The market and the moat

About 2.9 million people worldwide live with multiple sclerosis, a market worth roughly $25 billion a year and growing. It splits into two pricing regimes. Relapsing forms, roughly 85% of patients, are crowded and well served. Progressive forms are the unmet need, where disability accumulates without relapses and almost nothing works. Only Ocrevus is approved for primary progressive MS, which is why a noninferior oral option there is worth real money even without superiority. The incumbent standard of care is the anti-CD20 B-cell depleters: Roche’s Ocrevus and Novartis’ Kesimpta. Ocrevus did CHF 7.0 billion in 2025, about $7.8 billion, making it Roche’s biggest drug. Kesimpta did $4.4 billion, up 37%, and is the fastest-growing franchise in Novartis’ neuroscience shelf.

That moat is the whole reason the BTK class mattered. The thesis was that an oral, brain-penetrant BTK inhibitor could target the smoldering inflammation that drives progression, something the injectable CD20 drugs only partially address. The prize for the first BTK inhibitor to prove a benefit in progressive MS was a slice of the $25 billion market with no direct oral competitor. The base rate was zero, for reasons covered in the why Alzheimer’s drug trials fail framework, which applies to every neurology disease where progression outruns the endpoints. For the endpoint definitions readers need here, EDSS, annualized relapse rate, and MRI lesion counts, see the neuroscience endpoints guide.

The race, already run

Fenebrutinib won. The Roche FENtrepid readout in primary progressive MS, the hardest form of the disease, met its primary endpoint of noninferiority to Ocrevus on time to 12-week confirmed disability progression, with a hazard ratio of 0.88 (95% CI 0.75 to 1.03). It numerically reduced the risk of disability progression by 12% versus Ocrevus, and the largest effect was on upper limb function, a 26% reduction on the nine-hole peg test (HR 0.74, 95% CI 0.56 to 0.98). Then the two relapsing MS trials hit. FENhance 1 cut the annualized relapse rate 51% versus teriflunomide, and FENhance 2 cut it 59%. Three registrational trials, three primary endpoints met, and a regulatory submission planned across both progressive and relapsing forms. Fenebrutinib is now positioned to be the first oral, brain-penetrant treatment across the full MS spectrum.

One caveat worth keeping in mind: fenebrutinib’s relapsing wins came against teriflunomide, a low-efficacy oral that is the industry’s default weak comparator, and beating it does not prove fenebrutinib beats Kesimpta or Ocrevus on relapses. The trial that matters is FENtrepid in progressive disease, because there the comparator was Ocrevus itself, and there fenebrutinib was noninferior, not superior. That distinction is the difference between a moat-protecting oral and a moat-destroying one.

Tolebrutinib lost in the United States. Sanofi’s BTK inhibitor had positive HERCULES data in non-relapsing secondary progressive MS and a Breakthrough Therapy designation, but the FDA issued a complete response letter on December 24, 2025. Sanofi has said it expects further guidance and remains committed to a path forward, and Europe’s CHMP has since recommended approval, so tolebrutinib is not dead. But in the market that matters for the trade, the US, it was rejected. That is a meaningful signal about how the FDA is treating the BTK class in progressive MS, and it raises the bar for everyone else.

Evobrutinib never made it out of the starting gate. Merck KGaA’s EVOLUTION trials, evolutionRMS 1 and 2, missed their primary endpoints on annualized relapse rate back in December 2023, with evobrutinib roughly matching teriflunomide (0.15 versus 0.14 and 0.11 versus 0.11, both nonsignificant). The program is effectively dead. That leaves the BTK field with exactly one credible winner, fenebrutinib, and one wounded challenger in Sanofi.

The next mechanism, and why it still matters

The differentiated play left on the board is Sanofi’s frexalimab, a CD40L antibody, not a BTK inhibitor, with a non-depleting mechanism that blocks T-cell and B-cell cross-talk without wiping out B cells the way Ocrevus and Kesimpta do. Phase 2 data in relapsing MS was strong enough to publish in the New England Journal of Medicine, and Phase 3 trials in relapsing MS (NCT06141473) and non-relapsing secondary progressive MS are enrolling. This is the contrarian angle in a market that just watched its BTK hype get cut down: the winning mechanism may not be BTK at all, it may be CD40L, and Sanofi is the only one running it at Phase 3 scale. What actually moves these stocks from here is not the BTK race but the frexalimab Phase 3 readouts and the fenebrutinib label, and both are 2027 stories at the earliest.

The risks and the verdict

The specific risk in fenebrutinib is safety, not efficacy. Across the two relapsing MS trials there were eight fatal cases in the fenebrutinib arms versus one in the teriflunomide arm, a signal Roche says it is still analyzing, plus a Hy’s law liver case in each arm of FENhance 1 and liver enzyme elevations in 13.3% of FENtrepid patients versus 2.9% on Ocrevus. The FDA has already shown with tolebrutinib’s CRL that it is in a cautious mood on this class. A mortality signal is exactly the thing that turns an approval into a restricted label. That is the same reason the FDA handed tolebrutinib a CRL despite positive HERCULES data: progressive MS is a chronic, decades-long disease, and the agency will not accept even a modest liver or mortality question mark on a drug patients take every day.

Ranked by risk-reward, the trade is thinner than the hype suggested. Roche at a $360 billion market cap owns the winner, fenebrutinib, but the drug is immaterial to a company that already owns Ocrevus, the very franchise fenebrutinib would cannibalize. This is Gilead-competing-with-Gilead all over again, an oral follow-on that protects the moat rather than extends it, and it will not move a $360 billion stock. Novartis at roughly $300 billion has the fastest-growing incumbent in Kesimpta, and a non-superior oral challenger does not dislodge a $4.4 billion injectable that is compounding at 37%. Sanofi at roughly $109 billion is the only name with a genuine shot at differentiation through frexalimab, but a rejected tolebrutinib and a Phase 3 antibody years from data mean the catalyst is distant and the stock is immaterial. Merck KGaA is out.

The contrarian takeaway is this: the BTK revolution everyone was waiting to trade already happened, and it did not change much. The winner is an immaterial asset inside the company that already owns the market. The incumbents keep their moat. The only place real optionality still lives is frexalimab’s non-depleting mechanism, and that is a multi-year wait on a $109 billion company. If you want exposure to this shelf, own Roche or Novartis for the recurring franchise revenue and treat the BTK race as a settled footnote, not a catalyst. The broader neuroscience catalyst map still lists the real binary trades elsewhere, and the lesson from the BioVie bezisterim SUNRISE-PD readout is the same here: in neurology, the base rate is zero until a drug actually proves otherwise. Fenebrutinib is close to proving it. The market is not paying for it, because for a $360 billion company, it barely matters.