Bruton's Tyrosine Kinase Inhibitors in Multiple Sclerosis: Mechanistic Considerations Across Relapsing and Progressive Disease.

Multiple sclerosis (MS) reflects a dynamic interplay between peripheral immune activation and compartmentalized inflammation within the central nervous system (CNS). While current disease-modifying therapies effectively reduce relapse activity driven by transient peripheral immune infiltration, their impact on progressive disability remains limited, prompting interest in strategies targeting CNS-resident immune mechanisms. Bruton's tyrosine kinase (BTK), expressed in B cells and myeloid-derived cells, including microglia, serves as a shared intracellular signaling node linking adaptive and innate immune pathways. Second-generation BTK inhibitors, including evobrutinib, tolebrutinib, fenebrutinib, remibrutinib, and orelabrutinib, have advanced through Phase II-III development in MS. These agents differ in binding mode, selectivity, pharmacokinetics, CNS penetration, and safety profiles, distinctions that may influence stage-specific therapeutic performance. Recent trials across relapsing and progressive phenotypes have yielded heterogeneous outcomes. Divergent signals in primary and secondary progressive MS reflect underlying biological heterogeneity and suggest that therapeutic responsiveness may depend on residual inflammatory activity, lesion biology, and pharmacologic characteristics. Emerging biomarker frameworks further emphasize the importance of stratifying inflammatory activity and degenerative progression when interpreting trial data. This review integrates molecular pharmacology and the most recent clinical evidence available through 2026 to examine how pharmacologic properties translate into stage-dependent therapeutic positioning. We also consider safety constraints within a disease-stage-specific benefit-risk framework, aiming to clarify the evolving role of BTK inhibition in MS.