Bruton tyrosine kinase inhibitors (BTKI΄S) are emerging class of therapeutics that blocks the activity of a protein called Bruton tyrosine kinase (BTK), which plays an important role in the development and maturation of B cells, the type of white blood cell that makes antibodies. BTK inhibitors bind to the BTK protein, which is found on most B cells and some types of cancer cells. <br />The BCR is constitutively active through both ligand-dependent and ligand-independent mechanisms, resulting in continuous Bruton tyrosine kinase (BTK) signaling activation, which provides a survival and proliferation advantage to the beta lymphocytes<br />BTK is expressed in B-cells and myeloid cells, key progenitors of which include dendritic cells, microglia and macrophages, integral effectors of multiple sclerosisn (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).<br />BTKThey are classified into three generations: Ibrutinib was the first BTK inhibitor, but since its approval in 2013, progress has been made to advance the field. Acalabrutinib and zanubrutinib have also become commercially available. All three drugs work in the same way to inhibit the BTK enzyme by irreversibly binding to it.<br />The FDA granted accelerated approval in December 2023 to a new kind of BTK inhibitor pirtobrutinib (Jaypirca), for adults with CLL (chronic lymphocytic leukemia) or small lymphocytic lymphoma (SLL) who have had at least two prior lines of therapy.<br />BTK inhibitors are effective against: Chronic lymphocytic leukemia (CLL), Follicular lymphoma, Mantle cell lymphoma (MCL), Marginal zone lymphoma, Small lymphocytic lymphoma, Waldenstrom macroglobulinemia, Other selective B cell malignancies, and Chronic graft-versus-host disease.<br />MS is a chronic, immune-mediated disease of the CNS, leading to demyelination and axonal damage. This translates into clinical disability for people living with MS. Second-generation BTK inhibitors are under clinical development for relapsing MS (RMS), relapsing-remitting MS (RRMS), primary progressive MS (PPMS) and one in secondary progressive MS (SPMS). In MS pathogenesis, T-cells, B-cells and myeloid cells of the immune system play a key role in damaging the CNS and, as such, they have been of much interest as a therapeutic target in the development of disease-modifying treatments (DMTs). Immune-mediated mechanisms of inflammation are of much importance in MS progression; thus, targeting these mechanisms is integral in the development of prospective DMTs for multiple MS subtypes. <br />A recent study was conducted by Laura et al, on April 2024 to study the effectiveness of BTK Inhibitors againt MS mentioned that inhibitors of BTK have several proposed mechanisms of action, which may be relevant to the pathogenesis of MS. Pre-clinical models have been utilized to investigate the role of BTK in MS. Treatment with the BTK inhibitor evobrutinib (Merck KGaA) dose-dependently inhibited maturation of B-cells, antigen-triggered activation and the release of pro-inflammatory cytokines in animal models of MS. This resulted in reduced disease severity in mice, highlighting the potential importance of BTK signalling pathways in MS pathogenesis. Furthermore, leptomeningeal inflammation, which was associated with poor clinical outcomes in MS, has been modelled in experimental autoimmune encephalomyelitis (EAE) mice. Mice treated with evobrutinib showed a significant improvement in meningeal inflammation, compared with vehicle, exhibiting a 30% reduction and a 5% increase, respectively. Remibrutinib (Novartis) treatment in EAE models strongly reduced B-cell-dependent inflammation and exhibited BTK occupancy in the brain and peripheral immune system. <br />Despite the promising safety profiles of second-generation BTK inhibitors observed to date in MS trials, there are several adverse reactions that have previously been associated with first-generation BTK inhibitor administration. While not affecting all people taking BTK inhibitors, several adverse reactions are of concern when long-term administration is considered. Most notably, these include atrial fibrillation, dermatological toxicities, spontaneous bleeding and invasive fungal infection. <br />So far, the benefit–risk profile of second-generation BTK inhibitors in the treatment of MS is encouraging. BTK inhibitors can enter the CSF, thus highlighting the potential for access to cells of the adaptive and innate immune system, both in the CNS and periphery concurrently. Furthermore, preclinical models have suggested modulation of immune cells in demyelination and myelin repair following BTK inhibition, but this is yet to be understood in humans and may be illuminated by future clinical studies. The introduction of BTK inhibition was perceived as a major breakthrough in oncology; it is hoped that these drugs will bring a similar incremental value to the therapeutic armamentarium in fighting MS.<br />