Highlights
Efficacy Results
Fenebrutinib demonstrated a 69% relative reduction in the total number of new T1 gadolinium-enhancing (Gd+) lesions compared to placebo (p=0.0022) at 12 weeks.
Long-term Stability
Data from the open-label extension (OLE) through week 48 showed a sustained annualized relapse rate (ARR) of 0.04, with 96% of participants remaining relapse-free.
Pharmacological Profile
As a noncovalent and reversible inhibitor, fenebrutinib offers high selectivity for Bruton’s tyrosine kinase (BTK), potentially minimizing off-target effects compared to covalent inhibitors.
Safety Profile
While generally well-tolerated, hepatic enzyme elevations occurred in 6% of the fenebrutinib group, necessitating ongoing monitoring in larger phase 3 programs.
Background: The BTK Inhibition Landscape in MS
Multiple sclerosis (MS) is a chronic inflammatory and neurodegenerative disease of the central nervous system (CNS). While current high-efficacy disease-modifying therapies (DMTs), particularly anti-CD20 monoclonal antibodies, excel at depleting peripheral B cells and reducing relapses, they have a limited impact on the compartmentalized inflammation within the CNS that drives progressive disability. Bruton’s tyrosine kinase (BTK) has emerged as a compelling therapeutic target because it is expressed not only in B cells but also in myeloid cells, including microglia and macrophages, which are implicated in the chronic smoldering inflammation characteristic of MS.
Recent clinical trials involving BTK inhibitors have yielded inconsistent results, leading to questions regarding the optimal magnitude and sustainability of BTK inhibition. Fenebrutinib, a highly selective, noncovalent, and reversible inhibitor, was designed to provide potent inhibition of the BTK pathway while maintaining a safety profile that allows for long-term administration. The FENopta study was designed to evaluate whether this specific pharmacological approach could translate into rapid and sustained suppression of focal MRI activity in patients with relapsing MS (RMS).
The FENopta Trial: Methodology and Study Design
FENopta was a multicenter, double-blind, randomized, placebo-controlled, phase 2 trial conducted across 18 sites in Europe and North America. The study enrolled 109 patients aged 18–55 years with relapsing MS, defined by an Expanded Disability Status Scale (EDSS) score of 0.0–5.5 and evidence of recent disease activity (either a relapse within the previous year or at least one T1 Gd+ lesion within the previous 6 months).
Participants were randomized in a 2:1 ratio to receive either oral fenebrutinib (200 mg twice daily) or a matching placebo for 12 weeks. Randomization was stratified based on the presence or absence of T1 Gd+ lesions at screening. Following the double-blind phase, participants could enter an optional open-label extension (OLE) to receive fenebrutinib for up to 192 weeks.
The primary efficacy endpoint was the total number of new T1 Gd+ lesions on brain MRI at weeks 4, 8, and 12. Secondary endpoints included safety, CSF drug concentrations, and long-term efficacy markers such as the annualized relapse rate (ARR) during the OLE.
Key Findings: Rapid and Robust Radiographic Impact
At the conclusion of the 12-week double-blind phase, the trial met its primary endpoint. The combined number of new T1 Gd+ lesions at weeks 4, 8, and 12 was significantly lower in the fenebrutinib group (0.077; 95% CI 0.043–0.135) compared to the placebo group (0.245; 95% CI 0.144–0.418). This translates to a 69% relative reduction in lesion activity (p=0.0022).
The reduction in MRI activity was observed early, with differences emerging as soon as the first post-baseline scan at week 4. Furthermore, fenebrutinib significantly reduced the total number of new or enlarging T2-hyperintense lesions compared to placebo, suggesting a broad suppression of focal inflammatory activity.
Open-Label Extension: Sustaining Clinical Stability
Results from the open-label extension provided evidence of sustained clinical efficacy. Through 48 weeks of fenebrutinib treatment (including both the double-blind and OLE phases), the unadjusted ARR was 0.04. Impressively, 95 out of 99 patients (96%) remained relapse-free during this period. The MRI data in the OLE continued to show low levels of lesion activity, reinforcing the notion that the early radiographic benefits observed in the first 12 weeks are maintained with continued therapy.
Safety, Tolerability, and Hepatic Considerations
Fenebrutinib was generally well-tolerated. During the 12-week double-blind phase, adverse events (AEs) were slightly more frequent in the fenebrutinib group than in the placebo group. The most notable AEs included hepatic enzyme elevations (6% in the fenebrutinib group vs. 0% in the placebo group), headache (4% vs. 3%), and nasopharyngitis (3% vs. 0%).
The hepatic enzyme elevations were asymptomatic and reversible upon treatment discontinuation or, in some cases, resolved while continuing treatment. There were no serious adverse events (SAEs) or deaths reported during the trial. However, given that other BTK inhibitors have been associated with drug-induced liver injury (DILI) in larger trials, the hepatic signal in FENopta remains a point of clinical focus for the ongoing Phase 3 programs (FENhance and FENtrepid).
Expert Commentary: Mechanistic Insights and Clinical Context
Fenebrutinib’s performance in FENopta is particularly significant given the recent setbacks for other BTK inhibitors, such as evobrutinib, which failed to meet its primary endpoints in Phase 3 trials. Experts suggest that fenebrutinib’s high selectivity and its noncovalent, reversible binding mechanism may be key differentiators. By binding more selectively to BTK, fenebrutinib may minimize off-target inhibition of other kinases (such as TEC or EGFR), which are often linked to the side effects observed with earlier-generation covalent inhibitors.
Furthermore, fenebrutinib achieves high concentrations in the cerebrospinal fluid (CSF). Given that MS progression is driven largely by CNS-resident cells, the ability of a BTK inhibitor to cross the blood-brain barrier and modulate microglia is a critical requirement for impacting long-term disability progression. While FENopta focused on relapsing MS, the real test for fenebrutinib will be its efficacy in non-relapsing progressive MS, where existing therapies are largely ineffective.
Study limitations include the relatively small sample size and the short duration of the placebo-controlled phase. While the MRI results are robust, radiographic suppression does not always correlate perfectly with long-term disability outcomes. The medical community awaits the Phase 3 results to confirm if these radiographic gains translate into significant reductions in disability accumulation.
Conclusion: The Future of Fenebrutinib in MS Care
The FENopta trial successfully demonstrated that fenebrutinib provides early and robust suppression of focal brain lesions in patients with relapsing MS. Its favorable safety profile and sustained efficacy during the open-label extension position it as a promising candidate in the evolving MS treatment paradigm. If Phase 3 trials confirm these findings and demonstrate an impact on disability progression, fenebrutinib could offer a vital new option for patients seeking therapies that target both peripheral and CNS-compartmentalized inflammation.
Funding and ClinicalTrials.gov
This study was funded by F. Hoffmann-La Roche. The trial is registered with ClinicalTrials.gov (NCT05119569) and EudraCT (2021-003772-14).
References
1. Bar-Or A, Dufek M, Budincevic H, et al. Safety and efficacy of fenebrutinib in relapsing multiple sclerosis (FENopta): a multicentre, double-blind, randomised, placebo-controlled, phase 2 trial and open-label extension study. Lancet Neurol. 2025 Aug;24(8):656-666. doi: 10.1016/S1474-4422(25)00174-7.
2. Krämer J, Bar-Or A, Nowane S, et al. Bruton tyrosine kinase inhibitors for multiple sclerosis. Nat Rev Neurol. 2023;19(11):675-704.
3. Montalban X, Arnold DL, Weber MS, et al. Placebo-Controlled Trial of an Oral BTK Inhibitor in Multiple Sclerosis. N Engl J Med. 2019;380(25):2406-2417.
