Emerging Role of BTK Inhibitors in Multiple Sclerosis: From Immunobiology to Clinical Translation

Background: Multiple sclerosis (MS), an autoimmune disease, involves peripheral immune activation followed by CNS inflammation in a compartmentalized manner. Although high-efficacy disease-modifying therapies (HE-DMTs) have been effective in suppressing relapses in MS patients, they fail to effectively target chronic microglial activation and smoldering lesions in MS patients. Bruton’s tyrosine kinase inhibitors (BTKis), which are orally active and capable of crossing the blood–brain barrier, have been found to be effective in modulating B cells and CNS-resident myeloid cells. Objective: The objective was to assess the efficacy and safety of Bruton’s tyrosine kinase inhibitors in patients with relapsing, secondary, and primary progressive MS. Methods: We performed a systematic review and meta-analysis according to the Cochrane and PRISMA guidelines (PROSPERO registration number: 1323474). We included randomized controlled trials (RCTs) that assessed fenebrutinib, evobrutinib, or tolebrutinib in adult MS patient populations. The main outcome measures were annualized relapse rate, MRI lesion activity, disability progression (EDSS), and hepatotoxicity. The quality of the included trials was assessed for bias by the RoB2 tool. Results: Six RCTs with 3616 participants were included. BTK inhibitors significantly reduced ARR compared with control therapy (pooled RR 0.24; 95% CI 0.15–0.39). MRI activity was reduced (mean difference −1.45 new/enlarging T2 lesions; 95% CI −2.08 to −0.82). Disability progression was unchanged in short-term relapsing MS trials. Serious hepatotoxicity was reported in 11.0% of BTKi-treated patients compared with 13.7% of control patients (pooled RR 0.80; 95% CI 0.66–0.96). However, increased transaminase elevations were reported in placebo-controlled trials, which indicates that hepatotoxicity remains a clinically relevant safety concern for the class. Conclusions: BTK inhibitors reduce inflammatory disease activity in relapsing MS and have emerging efficacy in progressive MS phenotypes; however, continued monitoring for hepatotoxicity is warranted. Optimization of CNS penetrance and pharmacologic selectivity may influence long-term clinical positioning.