Abstract 960: Mechanisms underlying primary ibrutinib sensitivity in CLL
Authors: Chen Shih-Shih , Ravichandran1 Priyadarshini , Kieso Yasmine , Barrientos Jacqueline C. , Burger Jan , Rai Kanti R. , Chiorazzi Nicholas 
Published: July 2017

Abstract
The Bruton’s tyrosine kinase (BTK) inhibitor ibrutinib is efficacious in B cell malignancies including chronic lymphocytic leukemia (CLL). Although the majority of CLL patients respond to ibrutinib, CLL clones with unmutated IGHV gene (U-CLL) are significantly more sensitive to ibrutinib than clones with mutated IGHV (M-CLL). About 77% of U-CLL but only 33% M-CLL patients experience a partial or complete response. Since ibrutinib resistance occurs, understanding the molecular mechanisms underlying this difference will improve clinical practice. Here we hypothesized that the difference in ibrutinib sensitivity between U-CLL and M-CLL was due to a greater vulnerability of U-CLL B cells to the loss of environmental prosurvival signals.
We first evaluated 28 treatment naive CLL cases receiving ibrutinib. We defined not only the characteristic CLL B cell but also T cell lymphocytosis and the latter resolved faster in patients with good prognosis. After 1 treatment cycle, CLL T cells failed to proliferate upon mitogenic stimulation and to home to solid tissues after transfer into NSG mice. Without BCR signaling and T cell support, the CLL B cell proliferative fraction was diminished and the remaining B cells had elevated but dysfunctional surface membrane (sm) CXCR4 based on impaired recycling, internalization and signaling. Ibrutinib also inhibited CXCR4 phosphorylation at ser324/325/339 at a greater level in U-CLL.
We then investigated ibrutinib mediated changes in microenvironment in a xenograft mouse model using primary cells from 3 U-CLL and 3 M-CLL cases. Ibrutinib significantly inhibited CLL cell growth in spleen; the treated CLL cells developed elevated but dysfunctional smCXCR4. The inhibition on tumor growth and impaired CXCR4 occurred to much greater extent in U-CLL. Notably, ibrutinib also significantly inhibited T cell growth only in U-CLL cases. Thus, ibrutinib inhibited CXCR4 signaling and CLL B-T cell crosstalk essential for tumor growth in tissue niches, especially in U-CLL cases that has limited B-lymphocytosis.
Finally, molecules controlling the vulnerability of U-CLL B cells to death in the absence of environmental prosurvival signals were examined in 15 patients. Ibrutinib reduced BCL2 protein levels in U-CLL but not M-CLL; levels of BCL-XL and MCL-1 were unchanged. Consistent with these results, U-CLL but not M-CLL cell survival in vitro was promoted by T cell stimulation that led to upregulation of prosurvival BCL2. These findings suggest enhanced CLL B cell death by inhibiting BCL2 in U-CLL after the loss of environmental prosurvival signals.
Altogether, ibrutinib inhibits CLL-microenvironment crosstalk by blocking CXCR4 signaling and T cell support. U-CLL cells had a greater vulnerability to the loss of environmental prosurvival signals after ibrutinib inhibition. Our data demonstrate mechanisms underlying the differences in primary ibrutinib sensitivity between U- and M- CLL patients, supporting the use of inhibitors of BTK and BCL2 in CLL.
Note: This abstract was not presented at the meeting.
Citation Format: Shih-Shih Chen, Priyadarshini Ravichandran1, Yasmine Kieso, Jacqueline C. Barrientos, Jan Burger, Kanti R. Rai, Nicholas Chiorazzi. Mechanisms underlying primary ibrutinib sensitivity in CLL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 960. doi:10.1158/1538-7445.AM2017-960