Noncovalent inhibition of C481S Bruton tyrosine kinase by GDC-0853: a new treatment strategy for ibrutinib-resistant CLL.

The clinical success of ibrutinib validates Bruton tyrosine kinase (BTK) inhibition as an effective strategy for treating hematologic malignancies, including chronic lymphocytic leukemia (CLL). Despite ibrutinib's ability to produce durable remissions in patients, acquired resistance can develop, mostly commonly by mutation of C481 of BTK in the ibrutinib binding site. Here, we characterize a novel BTK inhibitor, GDC-0853, to evaluate its preclinical efficacy in ibrutinib-naive and ibrutinib-resistant CLL. GDC-0853 is unique among reported BTK inhibitors in that it does not rely upon covalent reaction with C481 to stabilize its occupancy within BTK's adenosine triphosphate binding site. As with ibrutinib, GDC-0853 potently reduces B-cell receptor signaling, viability, NF-κB-dependent transcription, activation, and migration in treatment naïve CLL cells. We found that GDC-0853 also inhibits the most commonly reported ibrutinib-resistant BTK mutant (C481S) both in a biochemical enzyme activity assay and in a stably transfected 293T cell line and maintains cytotoxicity against patient CLL cells harboring C481S BTK mutations. Additionally, GDC-0853 does not inhibit endothelial growth factor receptor or ITK, 2 alternative targets of ibrutinib that are likely responsible for some adverse events and may reduce the efficacy of ibrutinib-antibody combinations, respectively. Our results using GDC-0853 indicate that noncovalent, selective BTK inhibition may be effective in CLL either as monotherapy or in combination with therapeutic antibodies, especially among the emerging population of patients with acquired resistance to ibrutinib therapy.

The BTK Inhibitor ARQ 531 Targets Ibrutinib-Resistant CLL and Richter Transformation.

Targeted inhibition of Bruton tyrosine kinase (BTK) with the irreversible inhibitor ibrutinib has improved outcomes for patients with hematologic malignancies, including chronic lymphocytic leukemia (CLL). Here, we describe preclinical investigations of ARQ 531, a potent, reversible inhibitor of BTK with additional activity against Src family kinases and kinases related to ERK signaling. We hypothesized that targeting additional kinases would improve global inhibition of signaling pathways, producing more robust responses. In vitro treatment of patient CLL cells with ARQ 531 decreases BTK-mediated functions including B-cell receptor (BCR) signaling, viability, migration, CD40 and CD86 expression, and NF-κB gene transcription. In vivo, ARQ 531 was found to increase survival over ibrutinib in a murine Eµ-TCL1 engraftment model of CLL and a murine Eµ-MYC/TCL1 engraftment model resembling Richter transformation. Additionally, ARQ 531 inhibits CLL cell survival and suppresses BCR-mediated activation of C481S BTK and PLCγ2 mutants, which facilitate clinical resistance to ibrutinib.Significance: This study characterizes a rationally designed kinase inhibitor with efficacy in models recapitulating the most common mechanisms of acquired resistance to ibrutinib. Reversible BTK inhibition is a promising strategy to combat progressive CLL, and multikinase inhibition demonstrates superior efficacy to targeted ibrutinib therapy in the setting of Richter transformation. Cancer Discov; 8(10); 1300-15. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 1195.

The regulation of tumor-suppressive microRNA, miR-126, in chronic lymphocytic leukemia.

The introduction of miR profiling of chronic lymphocytic leukemia (CLL) patients with different cytogenetic profiles and responses to therapy has allowed incorporation of important miR-mRNA interactions into the understanding of disease biology. In this study, we performed miR expression analysis using NanoString nCounter to discover differentially regulated miRs after therapy with the Bruton tyrosine kinase inhibitor ibrutinib. Of the differentially regulated miRs in the discovery set, miR-29c and miR-126 were confirmed using real-time PCR to be upregulated in CLL patient cells with ibrutinib therapy. In the validation set, an inverse correlation was observed between miR-126 levels and expression of its putative target p85ß, an isoform of the phosphoinositide 3-kinase p85 regulatory subunit. We found that mRNA for the host gene EGFL7, primary unprocessed miR-126, and mature miR-126 are all downregulated in CLL cells compared to normal B cells. Patients in later stages of disease have a greater decrease in miR-126 expression compared to treatment-naive patients, indicating that lower miR-126 levels may associate with disease progression. Overexpression of miR-126 in leukemia cell lines significantly downregulates p85ß expression and decreases activation of prosurvival mitogen-activated protein kinase (MAPK) signaling. These results implicate miR-126 in the pathology of CLL.