News|Articles|February 12, 2026

Tolebrutinib Cuts Brain Volume Loss Despite Missing Primary End Point in Phase 3 Primary Progressive MS Study

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Key Takeaways

  • PERSEUS randomized 767 PPMS patients (2:1) to oral tolebrutinib 60 mg daily versus placebo; eligibility included 2017 McDonald PPMS and limited access/tolerance/response to ocrelizumab.
  • Composite 6‑month CDP was not reduced with tolebrutinib, suggesting added functional components (T25FW/9HPT) may have introduced measurement variability versus EDSS-only progression.
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In the phase 3 PERSEUS trial, Bruton tyrosine kinase inhibitor tolebrutinib did not significantly reduce disability progression compared with placebo in patients with primary progressive multiple sclerosis.

New late-breaking data from the phase 3 PERSEUS trial (NCT04458051) showed that tolebrutinib (Sanofi), an investigational Bruton’s tyrosine kinase (BTK) inhibitor, missed its primary end point but was associated with reduced brain volume loss compared with placebo at 6 months in patients with primary progressive multiple sclerosis (PPMS).1,2

“My initial reaction is ‘puzzled,’ because we saw a reduced risk of disability progression with tolebrutinib in HERCULES and GEMINI I/II, so I was expecting to see the same in PERSEUS. So why the difference? For one, the outcome was different: composite progression with PERSEUS rather than Expanded Disability Status Scale (EDSS)," senior author Robert Fox, MD, told NeurologyLive®. “It was anticipated that composite progression would be more sensitive, but that seems not to have been the case. The additional ‘signal’ from the compositive may have been outweighed by additional 'noise.’”

In the study, presented at the recently concluded 2026 Americas Committee for Treatment and Research in Multiple Sclerosis (ACTRIMS) Forum, held February 5-7, in San Diego, California, tolebrutinib failed to significantly reduce 6-month confirmed disability progression (CDP), the primary end point, compared with placebo (36.9% vs 37.7% placebo; HR, 1.01; 95% CI, 0.81-1.26; P = 0.94). Despite this, findings revealed that treatment with tolebrutinib was associated with significant reductions in brain-volume loss (least square mean [LSM] difference, 0.17; 95% CI, 0.40-0.30; P = .01).

“Indeed, the EDSS outcome trended favorably, with HR = 0.86, but not statistically significant. Also, the characteristics of these patients is much different than other PPMS trials such as two large ocrelizumab trials – ORATORIO and O’HAND,” Fox, staff neurologist at Cleveland Clinic’s Mellen Center for Multiple Sclerosis Treatment and Research, added. “Compared to those two ocrelizumab trials, in PERSEUS 3-6 times more participants had been treated with an MS therapy and half as many had gadolinium enhancing lesions at baseline. These participant characteristics may impact whether treatments work, and that will need to be compared across trials.”

READ MORE: BTK Inhibitor Fenebrutinib Shows Noninferiority to Ocrelizumab in Phase 3 Study of Primary Progressive MS

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PERSEUS is a randomized, double-blind, placebo-controlled, parallel-group, multicenter, event-driven trial that assessed the efficacy and safety of tolebrutinib in individuals living with PPMS. Eligible participants were aged 18 to 55 years, had a diagnosis of PPMS according to the 2017 McDonald criteria, an EDSS score of 2.0 to 6.5 at screening, positive cerebrospinal fluid findings, and either no access to, intolerance of, or perceived lack of efficacy with ocrelizumab (Ocrevus; Genentech).

In the trial, participants were randomized in a 2:1 ratio to receive tolebrutinib 60 mg or placebo, administered orally once daily with food. The primary end point is time to onset of 6-month composite CDP (cCDP), defined by worsening on the EDSS, Timed 25-Foot Walk, or 9-Hole Peg Test. Authors noted that secondary end points include additional disability measures, MRI outcomes, and safety assessments.

All told, a total of 767 participants were enrolled into the study. The mean age for participants was 45.3 years, and the mean time since PPMS diagnosis was 4.2 years. Overall, 54% of participants were men and 83% were White. At baseline, the mean EDSS score was 4.9 (median, 5.0), and 59% of participants were treatment naïve. On MRI, 89% of participants had no gadolinium-enhancing lesions at baseline. Among those who underwent susceptibility-sensitive imaging, 63% had at least 1 paramagnetic rim lesion at baseline.

Researchers reported that only 3% of patients with PPMS treated with tolebrutinib experienced liver enzyme elevations more than 5 times the upper limit of normal and 1 patient met Hy’s Law criteria for liver injury. In addition, authors observed that the discontinuation rate was 24.6% of participants in the tolebrutinib arm and 22.7% of patients in the placebo arm. In a company update, Sanofi noted that, based on the phase 3 PERSEUS data, a regulatory submission for tolebrutinib in PPMS is not expected.2

“As for my thoughts regarding BTK inhibitors in general, I remain optimistic that this class will be found helpful in MS, but it’s clear that different drugs within the class work differently. As we look to compare across trials, we’ll want to try to adjust for differences in patient characteristics between the trials,” Fox told NeurologyLive.

Editor’s Note: Fox has disclosed that he has received consulting fees from Sanofi for advisory roles related to the PERSEUS trial.

Click here for coverage of 2026 ACTRIMS Forum.

REFERENCES
1. Reich DS, Bar-Or A, Vermersch P, et al. Efficacy and Safety of Tolebrutinib Versus Placebo in Primary Progressive Multiple Sclerosis: Results From the Phase 3 PERSEUS Trial. Presented at ACTRIMS Forum 2026; February 5-7; San Diego, California. LB1.4.
2. Sanofi provides update on tolebrutinib in primary progressive multiple sclerosis. Sanofi. News release. December 15, 2025. Accessed February 11, 2026. https://www.sanofi.com/assets/dotcom/pressreleases/2025/2025-12-15-06-05-00-3205094-en.pdf

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