Role of the Bruton tyrosine kinase pathway in multiple sclerosis

Multiple sclerosis (MS) is a chronic, immune-mediated, neurodegenerative condition that results in progressive accumulation of disability over the course of the disease. MS presents heterogeneously, and, as the disease progresses, patients develop a range of physical and neurologic problems that include reduced mobility, cognitive impairment, weakness, fatigue, pain, and defects in speech or vision. Economically, MS is costly, including both direct costs stemming from clinical care and medications and the indirect costs of productivity losses. These costs pose a substantial burden to patients, families, caregivers, employers, and society. There are 21 approved disease-modifying therapies for MS across several drug classes. The importance of early MS treatment has been confirmed, and progress has been made in the treatment of relapsing-remitting MS, although this progress has not been replicated for progressive presentations of the disease. Ongoing research continues to elucidate the exact mechanisms of disease in MS as well as potential new treatment strategies that may better address current gaps, such as disability progression in secondary progressive MS without activity. One of the novel pathways under investigation is the inhibition of Bruton tyrosine kinase, a cytoplasmic tyrosine kinase, which is expressed in B cells and other potentially targetable hematopoietic lineage cells. This review examines emerging hypotheses that targeting both B cells and myeloid cells within the periphery and central nervous system could yield clinical effects in key areas of MS pathophysiology that are currently unaddressed.