The Potent PI3K-δ,γ Inhibitor IPI-145 Exhibits Differential Activity In Diffuse Large B-Cell Lymphoma (DLBCL) Cell Lines

Diffuse large B-cell lymphoma (DLBCL) is the most common form of non-Hodgkin lymphoma (NHL). Despite aggressive chemoimmunotherapy, ∼40% of patients die of their disease. Molecular differences in the tumor genome are likely to be major contributors to the heterogeneity of clinical response. Several investigators have identified molecular subtypes, including a “cell-of-origin” (COO) signature with activated B-cell-like (ABC) and germinal center B-cell-like (GCB) sub-types (Alizadeh, et al. Nature 2000), and a “consensus clustering” (CC) signature with oxidative phosphorylation, B-cell receptor (BCR)/proliferation, and host response sub-types (Monti, et al. 2005).

Phosphoinositide-3 kinases (PI3Ks) are key cellular signaling proteins that act as a central node, relaying signals from cell surface receptors to downstream mediators such as AKT. PI3K-δ and PI3K-γ isoforms are preferentially expressed in normal and malignant leukocytes where they are critical for cell differentiation, migration, and proliferation. IPI-145 is a potent, oral PI3K-δ,γ inhibitor that has shown clinical activity in the Phase 1 trial (IPI-145-02) in patients with advanced hematologic malignancies (ClinicalTrials.gov NCT01476657). Activation of the PI3K pathway is an important component of normal BCR signaling and has been implicated in the pathogenesis of DLBCL. To further explore the role of PI3K signaling in DLBCL, a panel of more than 10 DLBCL cell lines of varying molecular profile was treated with IPI-145.

PI3K-δ and PI3K-γ were expressed at varying levels across the DLBCL cell lines, without evidence of a correlation with molecular subtype. In a cellular growth inhibition assay, 5 cell lines including 4 GCB (SU-DHL-4, SU-DHL-6, OCI-LY-8, and WSU-DLCL-2) and 1 ABC (Ri-1) subtype were sensitive to IPI-145 treatment in the micromolar (μM) range or below, and 5 cell lines (OCI-LY-3, OCI-LY-7, Pfeiffer, Toledo and U2932) were insensitive to IPI-145 (IC₅₀ >25μM). There was no evidence of a correlation between IPI-145 sensitivity and COO or CC molecular profile in this panel. IPI-145 sensitivity did correlate with evidence of PI3K pathway inhibition as measured by reduction in phospho (p)-AKT. To characterize the kinetics of pathway modulation, phosphorylation of AKT, PRAS40, and S6 was examined following a time-course of IPI-145 treatment in selected cell lines. P-AKT and p-PRAS40 were modulated by 30 minutes, whereas modulation of p-S6 was not detected until after 8 hours. Upon BCR stimulation via antibody-induced crosslinking, some cell lines exhibited enhanced AKT phosphorylation, which was inhibited with IPI-145. The GCB cell line OCI-LY-8 was moderately sensitive to IPI-145 without BCR crosslinking (low μM range) and exhibited enhanced sensitivity to IPI-145 with BCR crosslinking (sub-μM range). These results suggest that intact BCR pathway signaling contributes to IPI-145 sensitivity in DLBCL cell lines. Current efforts are focusing on identification of biomarkers that define IPI-145 responsive DLBCL subsets.

IPI-145 activity was also explored in combination with ibrutinib, an irreversible inhibitor of Bruton agammaglobulinemia tyrosine kinase (BTK). Interestingly, in the setting of BCR crosslinking, OCI-LY-8 cells exhibited a robust increase in p-AKT which was completely inhibited by IPI-145 but only partially inhibited by ibrutinib. In other cell lines, such as SU-DHL-4, robust inhibition of p-BTK was observed with ibrutinib treatment but not with IPI-145. These findings suggest a potential mechanistic rationale for combination of PI3K-δ,γ and BTK inhibition. Supporting this hypothesis, a combination effect was observed in a cellular growth inhibition assay with IPI-145 plus ibrutinib in the SU-DHL-4 cell line and in the OCI-LY-8 cell line with BCR crosslinking. Studies are ongoing to explore this combination in additional DLBCL cell lines as well as in in vivo DLBCL models.

Taken together, these findings suggest that a subset of DLBCL patients might benefit fromPI3K-δ,γ inhibition. Additional work is necessary to determine how to prospectively identify IPI-145-sensitive DLBCL. In addition, preliminary evidence suggests that there may be opportunities to improve targeted therapy options for DLBCL patients with the combination of IPI-145 and ibrutinib.