Bruton's tyrosine kinase (BTK) inhibitors are an emerging class of therapeutics in multiple sclerosis (MS). BTK is expressed in B-cells and myeloid cells, key progenitors of which include dendritic cells, microglia and macrophages, integral effectors of MS pathogenesis, along with mast cells, establishing the relevance of BTK inhibitors to diverse autoimmune conditions. First-generation BTK inhibitors are currently utilized in the treatment of B-cell malignancies and show efficacy in B-cell modulation. B-cell depleting therapies have shown success as disease-modifying treatments (DMTs) in MS, highlighting the potential of BTK inhibitors for this indication; however, first-generation BTK inhibitors exhibit a challenging safety profile that is unsuitable for chronic use, as required for MS DMTs. A second generation of highly selective BTK inhibitors has shown efficacy in modulating MS-relevant mechanisms of pathogenesis in preclinical as well as clinical studies. Six of these BTK inhibitors are undergoing clinical development for MS, three of which are also under investigation for chronic spontaneous urticaria (CSU), rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Phase II trials of selected BTK inhibitors for MS showed reductions in new gadolinium-enhancing lesions on magnetic resonance imaging scans; however, the safety profile is yet to be ascertained in chronic use. Understanding of the safety profile is developing by combining safety insights from the ongoing phase II and III trials of second-generation BTK inhibitors for MS, CSU, RA and SLE. This narrative review investigates the potential of BTK inhibitors as an MS DMT, the improved selectivity of second-generation inhibitors, comparative safety insights established thus far through clinical development programmes and proposed implications in female reproductive health and in long-term administration.

A review of Bruton’s tyrosine kinase inhibitors in multiple sclerosis Why was this study done? This study was done to find out about current knowledge on a type of drug, called Bruton’s tyrosine kinase inhibitors, or BTK inhibitors. There are currently six BTK inhibitors being studied as a possible new drug for treating multiple sclerosis (MS). Some of these six drugs are also being studied as a possible new drug for chronic spontaneous urticaria, rheumatoid arthritis and systemic lupus erythematosus. These are all autoimmune conditions, where the immune system mistakenly attacks parts of the body. Clinician scientists wanted to understand what is currently known about BTK inhibitors, how they work in the laboratory and how safe they could be for treating autoimmune conditions. This could help us understand more about BTK inhibitors in MS.What did the scientists do? The scientists assessed existing research on these six BTK inhibitors, through a process known as a literature review. These were results from ongoing clinical trials, and information collected through studying BTK inhibitors in laboratories. The researchers pieced together all these findings, to produce this paper that summarizes the results.What did the scientists find? The scientists found that most studies of BTK inhibitors for MS are still ongoing. So far, BTK inhibitors seem to show reasonable safety in most studies, but it is too early to know. The researchers also found out about how BTK inhibitors work in the lab, about what could happen if the drugs are taken for a long time and how they could impact female reproductive health.What do these findings mean? These findings will help other scientists learn more about BTK inhibitors in MS. Trials with BTK inhibitors for MS are still ongoing, but piecing together all the current findings gives a picture of what we know and what still needs to be done.

Brain and Mind Center University of Sydney Sydney NSW Australia

Brigham Multiple Sclerosis Center Harvard Medical School Boston MA USA

Department of Neurology Keio University School of Medicine Tokyo Japan

Department of Neurology Medical Faculty Heinrich Heine University Düsseldorf Germany

Department of Neurology Palacký University Olomouc Olomouc Czech Republic

Department of Neurology University Hospital Muenster Albert Schweitzer Campus 1 Building A 1 Muenster 48149 Germany

Department of Neurology University of Texas Southwestern Medical Center Dallas TX USA

Division of Clinical Neurosciences University of Turku Turku Finland

Joi Life Wellness MS Center Atlanta GA USA

Mellen Center for MS Neurological Institute Cleveland Clinic Cleveland OH USA

Neurocenter Turku University Hospital Turku Finland

Neurology Section VA North Texas Health Care System Dallas TX USA

Novartis Pharma AG Basel Switzerland

Peter O'Donnell Brain Institute University of Texas Southwestern Medical Center Dallas TX USA

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