Ibrutinib is an orally bioavailable Bruton's tyrosine kinase (BTK) inhibitor indicated for the treatment of mantle cell lymphoma (MCL) patients that previously received at least one therapy. The drug was jointly developed by Janssen Biotech and Pharmacyclics. Ibrutinib selectively binds to Cys-481 residue in the allosteric inhibitory segment of BTK (TK/SH1 domain), and irreversibly blocks its enzymatic activity thus preventing B-cell activation and signaling, totally blocking the B-cell receptor and cytokine receptor pathways. This leads to an inhibition of the growth of malignant B cells that overexpress BTK. Apart from mantle cell lymphoma Ibrutinib is approved for the treatment of chronic lymphocytic leukemia and Waldenstrom Macroglobulinemia.