PI3Kδ Inhibition Elicits Anti-Leukemic Effects through Bim-Dependent Apoptosis

Introduction: Phosphatidylinositol-3-kinase δ (PI3Kδ)-signaling provides key maintenance, proliferation, and survival cues during both normal and malignant B-lymphocyte development. Consequently, isoform-selective PI3Kδ inhibitors (PI3Kδi) have generated huge interest as a potential treatment for lymphoid malignancies. In particular, PI3Kδi demonstrate impressive clinical efficacy in combination with anti-CD20 monoclonal antibodies (mAbs) for relapsed chronic lymphocytic leukemia (CLL). However, these combinations function primarily to delay disease progression, but are not curative. With the ever-rising number of new targeted therapeutics, the challenge is to identify combinations that will ultimately deliver curative regimes. In order to guide these selections, a detailed mechanistic understanding is required. To date, only limited data are available regarding the exact in vivo therapeutic mechanism of PI3Kδi. Currently, studies identify immunomodulation, inhibition of BCR-, chemokine/cytokine-signaling, and induction of apoptosis as putative therapeutic mechanisms. Here we characterize the molecular mechanisms responsible for PI3Kδi-induced cytotoxicity and determine the relative contribution toward in vivo therapeutic responses utilizing the Eµ-TCL1-Tg mouse model of CLL alongside human CLL samples.

Methods: The molecular mechanisms of PI3Kδi alone or in combination with anti-CD20 mAbs were assessed using the Eµ-TCL1-Tg mouse model, an in vivo model system of CLL. To inhibit PI3Kδ, the δ isoform-selective inhibitor GS-9820 was chosen, as it is highly structurally related to idelalisib, and critically demonstrates improved pharmacokinetic properties in the mouse in comparison to idelalisib. In vitro GS-9820 IC50 are as follows: PI3Kδ 27 nM; PI3Kα 83,424 nM; PI3Kβ 14,899 nM; and PI3Kγ 15,606 nM. Assays to measure its effects on BCR-mediated kinase activation, chemokine signaling/chemotaxis, and cytokine- and cell-mediated support were performed. GS-9820 was administered in vivo at 10 mg/kg per os BID (formulated in 0.5% methylcellulose, 0.05% tween-80), once leukemias were detected, and maintained throughout the treatment period (GS-9820 C_max 3114 nM, C_trough 48.6 nM).

Results: GS-9820 induced substantive in vitro cell death and disruptedBCR-mediated kinase activation, chemokine signaling/chemotaxis, and inhibited both cytokine- and cell-mediated support in murine (Eµ-TCL1) and human CLL cells. In vivo administration of GS-9820 imparted significant therapeutic responses in Eµ-TCL1-bearing animals, reducing leukemic burden by 75% and splenic tumor deposits by 66% 4 weeks posttreatment. GS-9820 appeared well tolerated in recipient animals with no obvious toxicity apparent (e.g. weight loss or behavioral symptoms). When in combination with anti-CD20 mAbs, GS-9820 extended leukemia depletion by several weeks. GS-9820 enhanced overall survival by 66% in comparison with vehicle control-recipient animals and enhanced the survival benefit of anti-CD20 mAb therapy. These therapeutic responses were associated with a 2-fold increase in expression of the pro-apoptotic BH3-only Bcl-2 family member Bim and a 3-fold increase in the extent of Bim/Bcl-2 interaction. Accordingly, Bim^-/- Eµ-TCL1-Tg leukaemias exhibited profound resistance to PI3Kδi-induced cytotoxicity, were refractory to PI3Kδi in vivo, and failed to display combination efficacy with anti-CD20 mAbs. These findings informed the rational design of a GS-9820 + ABT199 (Venetoclax) complementary drug combination strategy. Combinations of GS-9820 and ABT199 were well tolerated with an absence of weight loss or altered behavioral symptoms. The GS-9820 + ABT199 combination effectively halted leukemia progression in vivo with increased efficacy compared to monotherapy regimes, resulting in a 90% reduction in leukemic burden at the end of the treatment period.

Conclusions: Bim-dependent apoptosis represents the key in vivo effector mechanism for PI3Kδi in the Eµ-TCL1-Tg mouse model, both alone and in combination with anti-CD20 mAbs. As such, combinations of PI3Kδ and Bcl-2 inhibitors may represent an efficacious drug combination strategy.