Combined Inhibition of Phosphatidylinositol 3-Kinase (PI3K) Isoform α and δ By the Pan-Class I PI3K Inhibitor SAR245409 (XL765) in Primary Chronic Lymphocytic Leukemia Cells Blocks Survival, Adhesion and Proliferation

CLL cells are highly dependent on B- cell receptor (BCR) signaling and on stimuli from the microenvironment for survival and proliferation. New drugs targeting PI3K downstream of BCR signaling have emerged as promising treatment options for patients with CLL. Among four PI3K catalytic subunits, the PI3Kd isoform is crucial for downstream BCR signaling, but the relative importance of the PI3Kα isoform in CLL is less clear. Impressive clinical activity of idelalisib in CLL and indolent NHL patients was recently reported. Idelalisib, a PI3Kd specific inhibitor, inhibits chemotaxis and adhesion of leukemia cells, resulting in rapid lymphocytosis followed by a decrease in lymphadenopathy. However, idelalisib has no direct impact on leukemic cell survival [1], raising the potential risk of residual clones responsible for the development of resistance. In this study, we evaluated the impact of a pan-class I PI3K inhibitor (SAR245409/XL765), a PI3Kα-specific inhibitor (BYL719) and a PI3Kd specific inhibitor (idelalisib) on PI3K/mTOR signaling, apoptosis, cell adhesion, CD40-induced survival and proliferation in primary patient derived leukemic cells.

Phosphorylation of the downstream effector of mTOR, S6RP, was completely blocked by SAR245409 but not by BYL719 or idelalisib. SAR245409 induced apoptosis in unstimulated CLL cells (IC50= 0.86µM) in contrast to BYL719 or idelalisib (IC50 >10µM), demonstrating that targeting multiple PI3K isoforms is required to completely block the PI3K/Akt/mTOR pathway (table 1). Importantly, SAR245409 also induced apoptosis in p53 or ATM dysfunctional CLL samples. SAR245409, as well as idelalisib, and in contrast to BYL719 completely inhibited BCR-mediated adhesion to fibronectin [2]. Similarly, SAR245409 inhibited CD40L-mediated survival [3], and induced upregulation of the pro-apoptotic protein BIM. All 3 PI3K inhibitors inhibited CD40 ligation + IL-21-mediated CLL proliferation [4].

This study revealed that the pan-class I PI3K inhibitor SAR245409 is more cytotoxic to primary CLL cells than PI3Kα or PI3Kd specific inhibitors. Furthermore, combined inhibition of PI3Kα and d can block signaling pathways that are critical for CLL survival, adhesion and proliferation in the LN microenvironment (see table 1). This work provides a rationale for the evaluation of SAR245409 in CLL patients either as monotherapy or in combination therapies.

[1] Hoellenriegel et al. The phospoinositide 3'-kinase delta inhibitor, CAL-101, inhibits B-cell receptor signaling and chemokine networks in chronic lymphocytic leukemia. Blood 2011;(118):3603-3612

[2] de Rooij et al. The clinically active BTK inhibitor PCI-32765 targets B-cell receptor- and chemokine-controlled adhesion and migration in chronic lymphocytic leukemia. Blood 2012;(119):2590-2594.

[3] Smit et al. Differential Noxa/Mcl-1 balance in peripheral versus lymph node chronic lymphocitic leukemia cells correlates with survival capacity. Blood 2007;(109):1660-1668.

[4] Pascutti et al. IL-21 and CD40L signals from autologous T cells can induce antigen-independent proliferation of CLL cells. Blood 2013;(122):3010-3019.