Bruton's Tyrosine Kinase Inhibitors in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic, inflammatory disorder of the central nervous system (CNS) characterized by overlapping relapsing and progressive pathologies. Although they exist along a continuum, pathology in relapsing MS (RMS) is primarily driven by the peripheral adaptive immune system, while progressive MS (PMS) pathology is underpinned by innate immune activity and other pathologic processes compartmentalized within the CNS. While currently approved therapies are highly effective in targeting aspects underpinning relapsing pathology, they have limited efficacy in PMS due to either inability to cross the blood brain barrier (BBB) or inability to modulate pathologies driving PMS. Bruton's tyrosine kinase (BTK) inhibitors are an emerging class of oral agents that represent a novel approach to MS treatment. These agents target BTK, an enzyme which plays a pivotal role in both adaptive and innate immune signaling. As small molecules, some BTK inhibitors can cross the BBB at biologically relevant concentrations. Therefore, BTK inhibitors may potentially modulate both relapsing and progressive MS pathology. Data from Phase 2 and Phase 3 trials have been promising in this regard. The unique pharmacological profile of each BTK inhibitor may underpin observed differences in efficacy and safety outcomes to date. In particular, tolebrutinib has demonstrated efficacy against disability progression in non-relapsing secondary progressive MS, while fenebrutinib has recently shown promise in both relapsing and primary progressive MS. However, adverse events include elevated liver enzymes, which can reach life-threatening levels. This review highlights the role of BTK in immune signaling and the rationale for BTK inhibition in MS, discusses the evolution of BTK inhibitors for MS and other indications, summarizes findings from Phase 2 and Phase 3 trials in RMS and PMS, and explores practical questions around the potential use of BTK inhibitors in real-world practice.