Ibrutinib, sold under the brand name Imbruvica among others, is a small molecule drug that inhibits B-cell proliferation and survival by irreversibly binding the protein Bruton's tyrosine kinase (BTK). Blocking BTK inhibits the B-cell receptor pathway, which is often aberrantly active in B cell cancers. Ibrutinib is therefore used to treat such cancers, including mantle cell lymphoma, chronic lymphocytic leukemia, and Waldenström's macroglobulinemia.