Cell-Intrinsic Determinants of Ibrutinib-Induced Apoptosis in Chronic Lymphocytic Leukemia

Abstract

<b>Purpose:</b> Ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor, is approved for the treatment of relapsed chronic lymphocytic leukemia (CLL) and CLL with del17p. Mechanistically, ibrutinib interferes with B-cell receptor (BCR) signaling as well as multiple CLL cell-to-microenvironment interactions. Given the importance of ibrutinib in the management of CLL, a deeper understanding of factors governing sensitivity and resistance is warranted.<b>Experimental Design:</b> We studied 48 longitudinally sampled paired CLL samples, 42 of which were procured before and after standard CLL chemotherapies, and characterized them for well-studied CLL molecular traits as well as by whole-exome sequencing and SNP 6.0 array profiling. We exposed these samples to 0.25 to 5 μmol/L of ibrutinib <i>ex vivo</i> and measured apoptosis fractions as well as BCR signaling by immunoblotting. We disrupted <i>TP53</i> in HG3, PGA1, and PG-EBV cell lines and measured BCR signaling and ibrutinib responses.<b>Results:</b> CLL samples demonstrated a surprisingly wide range of <i>ex vivo</i> sensitivities to ibrutinib, with IC₅₀ values ranging from 0.4 to 9.7 μmol/L. Unmutated IGVH status, elevated ZAP70 expression, and trisomy 12 were associated with heightened sensitivity to ibrutinib treatment. Five CLL samples were substantially more resistant to ibrutinib following relapse from chemotherapy; of these, three had acquired a del17p/<i>TP53</i>-mutated status. A validation sample of 15 CLL carrying <i>TP53</i> mutations, of which 13 carried both del17p and a <i>TP53</i> mutation, confirmed substantially less sensitivity to ibrutinib-induced apoptosis.<b>Conclusions:</b> This study identifies that CLL harboring del17p/<i>TP53</i>-mutated cells are substantially less sensitive to ibrutinib-induced apoptosis than del17p/<i>TP53</i> wild-type cells. <i>Clin Cancer Res; 23(4); 1049-59. ©2016 AACR</i>.