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

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. 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 and measured apoptosis fractions as well as BCR signaling by immunoblotting. We disrupted in HG3, PGA1, and PG-EBV cell lines and measured BCR signaling and ibrutinib responses. CLL samples demonstrated a surprisingly wide range of sensitivities to ibrutinib, with IC 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/-mutated status. A validation sample of 15 CLL carrying mutations, of which 13 carried both del17p and a mutation, confirmed substantially less sensitivity to ibrutinib-induced apoptosis. This study identifies that CLL harboring del17p/-mutated cells are substantially less sensitive to ibrutinib-induced apoptosis than del17p/ wild-type cells. .