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

Abstract

Purpose Ibrutinib, a Bruton’s tyrosine kinase (BTK) inhibitor, is approved for the treatment of relapsed CLL and CLL with del17p. Mechanistically, ibrutinib interferes with 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. Experimental Design 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 μM – 5 μM of ibrutinib ex vivo and measured apoptosis fractions as well as BCR signaling by immunoblotting. We disrupted TP53 in HG3, PGA1 and PG-EBV cell lines and measured BCR signaling and ibrutinib responses. Results CLL samples demonstrated a surprisingly wide range of ex vivo sensitivities to ibrutinib with IC50 values ranging from 0.4 μM – 9.7 μM. 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/TP53 mutated status. A validation sample of 15 CLL carrying TP53 mutations, of which 13 carried both del17p and a TP53 mutation confirmed substantially less sensitivity to ibrutinib-induced apoptosis. Conclusions This study identifies that CLL harboring del17p/TP53 mutated cells are substantially less sensitive to ibrutinib-induced apoptosis than del17p/TP53 wild type cells.

Unravelling the suboptimal response of TP53‐mutated chronic lymphocytic leukaemia to ibrutinib

“…We demonstrate that, following in vitro incubation with different drug concentrations, ibrutinib has a significantly lower capacity of inducing apoptosis and inhibiting proliferation on TP53‐ M CLL cells compared to TP53 ‐WT cases. The reduced activity on TP53 ‐M CLL cells, already observed after 24 h of incubation, was significant after 48 and 72 h. In line with our results, a recent paper has demonstrated that CLL cells carrying a TP53 disruption show a reduced in vitro sensitivity to ibrutinib‐induced apoptosis (Amin et al , ). These observations point towards a clinically relevant ibrutinib resistance mechanism in TP53 ‐M CLL, that is independent of BTK itself or its immediate downstream target PLCG2, whose mutations have been linked to the acquired resistance to ibrutinib (Woyach et al , ).…”

Microenvironmental agonists generate de novo phenotypic resistance to combined ibrutinib plus venetoclax in CLL and MCL

“…Although this variability could reflect genetic heterogeneity in these patients 25 , we hypothesized that the de novo resistance could also be generated by variability in the in vivo environment of cancer cells 26–32 . Neoplastic B cells receive growth/survival signals through interaction with components of the microenvironment such as stromal cells, cytokines, antigens, and circulating microbial/cellular DNA 16,26–29,33–42 ; the latter three are found in lymph node (LN) and bone marrow (BM) as well as in circulation.…”

USP7 inhibition alters homologous recombination repair and targets CLL cells independently of ATM/p53 functional status

“…[6][7][8][9][10] Although these agents act independently of the ATM/p53 pathway, in vitro studies have shown that CLL cells harboring del(17p) or TP53 mutations are less sensitive to the BCR signaling inhibitor ibrutinib. 11 Moreover, BCR signaling inhibitors are not curative, and patients with del(17p) CLL continue to exhibit inferior clinical outcomes with these agents, indicating that alternative treatment strategies and therapeutic combinations are still needed. 6,10,12 DDR-deficient cells rely on cooperating DNA repair pathways for survival.…”