CAL-101 (GS-1101), a Specific Inhibitor of Phosphatidylinositol-3-Kinase-Delta (PI3Kδ), Disrupts Signals From the Microenvironment, Induces Apoptosis, and Enhances the Antitumor Activity of Everolimus (RAD001), An Inhibitor of Mammalian Target of Rapamycin (mTOR), in Mantle Cell Lymphoma (MCL),
Authors: Meadows Sarah A , Kashishian Adam , Johnson David , Ulrich Roger G , Miller Langdon L , Lannutti Brian J. 
Published: October 2019

Abstract
Abstract 3730

Background:
The PI3K/Akt/mTOR pathway plays a critical role in cellular proliferation and survival through transduction of signals from cell-surface receptors to proteins involved in cell cycling (eg, cyclin D1) and mRNA translation (eg, ribosomal protein S6 and translation initiation factor 4E-binding protein 1 [4E-BP1]). MCL is an aggressive B-cell non-Hodgkin lymphoma. Overexpression of cyclin D1 has prompted clinical evaluation of mTOR inhibitors (everolimus, temsirolimus) in MCL. While efficacy has been observed, the extent and duration of tumor control has been modest, encouraging assessment of additional methods of intervention. Among the several PI3K isoforms (p110α, β, δ, γ), we have previously shown a unique role for PI3Kδ in maintaining the survival of hematological cancers and have also shown that GS-1101, a highly selective oral PI3Kδ-isoform-specific inhibitor with no activity against mTOR, shows promising clinical activity in non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL). These considerations prompted us to assess the specific role of PI3Kδ in MCL and to determine if dual PI3Kδ/mTOR inhibition might enhance antitumor effects.

Results:
In all evaluated MCL cell lines (Jeko, Mino, Granta, and NCEB), PI3Kδ levels were high while expression of PI3Kα, β, and γ were variable. PI3Kδ was functionally active, inducing phosphorylation of Akt (pAkt) in all tested cell lines and also in 4 of 4 primary MCL samples. Single-agent GS-1101 decreased pAkt in cell lines and primary samples. The pAkt decrease was associated with growth suppression and induction of apoptosis in all MCL cell lines. Consistent with these effects, immunoblotting showed that GS-1101 decreased cyclin D1 levels in MCL cell lines. Because signals from the tumor microenvironment are essential for homing, survival, and proliferation of malignant B cells, we investigated the potential role of PI3Kδ in mediating these signals by stimulating MCL lines with CXCL12 (SDF-1), CXCL13 (BCA-1), BAFF, or BCR crosslinking in the presence or absence of GS-1101. These stimuli all resulted in the phosphorylation of Akt, which was inhibited by GS-1101 in a dose-dependent manner. Furthermore, selective inhibition of PI3Kd was able to attenuate the upregulation of Akt phosphorylation and the secreation of CCL17 and CCL22 associated with coculturing MCL cells and stromal cells. Treatment with GS-1101 also decreased S6 phosphorylation, but levels of phospho-4E-BP1 remained high, suggesting that mTOR signaling was not completely inhibited. However, the combination of GS-1101 and everolimus suppressed phosphorylation of both S6 and 4E-BP1, inhibiting MCL viability and enhancing apoptosis relative to treatment with either GS-1101 or everolimus alone.

Conclusions:
Our findings indicate that excessive PI3Kδ activity is characteristic in MCL and GS-1101-mediated PI3Kδ inhibition reduces MCL growth and survival. Combination therapy to address the molecular complexity associated with the convergence of the PI3Kδ-Akt and mTOR pathways may provide a novel treatment approach for MCL.

Disclosures:
Meadows: Gilead Sciences: Employment. Kashishian:Gilead Sciences: Employment. Johnson:Gilead Sciences: Employment. Ulrich:Gilead Sciences: Employment. Miller:Gilead Sciences: Employment. Lannutti:Gilead Sciences: Employment.