• Jennifer A Woyach, Richard R Furman, Ta-Ming Liu, Hatice Gulcin Ozer, Marc Zapatka, Amy S Ruppert, Ling Xue, Daniel Hsieh-Hsin Li, Susanne M Steggerda, Matthias Versele, Sandeep S Dave, Jenny Zhang, Ayse Selen Yilmaz, Samantha M Jaglowski, Kristie A Blum, Arletta Lozanski, Gerard Lozanski, Danelle F James, Jacqueline C Barrientos, Peter Lichter, Stephan Stilgenbauer, Joseph J Buggy, Betty Y Chang, Amy J Johnson, and John C Byrd.
• From the Division of Hematology, Department of Internal Medicine (J.A.W., T.-M.L., A.S.R., S.M.J., K.A.B., A.L., A.J.J., J.C. Byrd), the Department of Biomedical Informatics (H.G.O., A.S.Y.), and the Department of Pathology (G.L.), Ohio State University, Columbus; the Department of Medicine, Division of Hematology-Oncology, Weill Cornell Medical College, New York (R.R.F.); the Division of Molecular Genetics, German Cancer Research Center, Heidelberg (M.Z., P.L.), and the Department of Internal Medicine III, University of Ulm, Ulm (S.S.) - both in Germany; Pharmacyclics, Sunnyvale, CA (L.X., D.H.-H.L., S.M.S., D.F.J., J.J.B., B.Y.C.); the Duke Cancer Institute, Duke University, Durham, NC (S.S.D., J.Z.); the Division of Hematology-Oncology, Department of Medicine, Hofstra North Shore-LIJ School of Medicine, New Hyde Park, NY (J.C. Barrientos); and Janssen Research and Development, Beerse, Belgium (M.V.).
• N. Engl. J. Med. 2014 Jun 12; 370 (24): 228622942286-94.

BackgroundIbrutinib is an irreversible inhibitor of Bruton's tyrosine kinase (BTK) and is effective in chronic lymphocytic leukemia (CLL). Resistance to irreversible kinase inhibitors and resistance associated with BTK inhibition have not been characterized. Although only a small proportion of patients have had a relapse during ibrutinib therapy, an understanding of resistance mechanisms is important. We evaluated patients with relapsed disease to identify mutations that may mediate ibrutinib resistance.MethodsWe performed whole-exome sequencing at baseline and the time of relapse on samples from six patients with acquired resistance to ibrutinib therapy. We then performed functional analysis of identified mutations. In addition, we performed Ion Torrent sequencing for identified resistance mutations on samples from nine patients with prolonged lymphocytosis.ResultsWe identified a cysteine-to-serine mutation in BTK at the binding site of ibrutinib in five patients and identified three distinct mutations in PLCγ2 in two patients. Functional analysis showed that the C481S mutation of BTK results in a protein that is only reversibly inhibited by ibrutinib. The R665W and L845F mutations in PLCγ2 are both potentially gain-of-function mutations that lead to autonomous B-cell-receptor activity. These mutations were not found in any of the patients with prolonged lymphocytosis who were taking ibrutinib.ConclusionsResistance to the irreversible BTK inhibitor ibrutinib often involves mutation of a cysteine residue where ibrutinib binding occurs. This finding, combined with two additional mutations in PLCγ2 that are immediately downstream of BTK, underscores the importance of the B-cell-receptor pathway in the mechanism of action of ibrutinib in CLL. (Funded by the National Cancer Institute and others.).

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