Effect of phosphotidylinositol 3-kinase-delta inhibitor idelalisib (GS-1101) on signaling in primary non-Hodgkin lymphoma cells: Correlative studies from NCT01306643.

Abstract

8579 Background: Studies of GS-1101, an oral phosphatidylinositol 3-kinase (PI3K)d-specific inhibitor, in heavily pre-treated patients with indolent-non-Hodgkin's lymphomas (iNHL) have shown marked clinical activity [Kahl BS, et al., Blood 2010]. We have shown that GS-1101 blocks both constitutive and inducible signaling events proximally downstream of PI3K [Lannutti BJ, et al., Blood 2011]. We hypothesized that flow-cytometric interrogation of PI3Kd-inhibition in iNHL samples might demonstrate signaling differences between patients that can be correlated with clinical outcomes. Methods: Single-cell suspensions of biopsy specimens from patients were incubated with or without GS-1101, B-cell receptor stimulated and stained with lineage markers and phospho-specific antibodies targeting signaling nodes proximally (e.g. pAkt S473) or distally (e.g. pS6 S235/6) downstream, or "parallel" to (e.g. pErk1/2) described PI3K pathways. Results: Healthy and iNHL B cells demonstrated complete or near-complete inhibition of PI3K proximal downstream signaling by GS-1101. PI3K distal downstream signaling was completely inhibited by GS-1101 in healthy B cells. The degree inhibition of distal downstream signaling was variable between iNHL patients. iNHL showed marked variability in the degree of PI3K distal downstream signaling amongst tumor cells within the same sample, suggesting an admixture of "PI3K-dependent“ and "PI3K-independent" tumor cells. We also observe variable PI3K-independence amongst other NHL histologies such as mantle cell lymphoma. Additionally, combining inhibition of PI3K with that of other signaling nodes shows additive distal downstream effects. Conclusions: We demonstrate that GS-1101 blocked both constitutive and invoked signaling proximally downstream of PI3K in primary iNHL cells. By correlating clinical outcomes in the ongoing study with signaling readouts between patients we may develop predictive rules for response. Ongoing studies will focus on studying the change in intratumoral cell subsets which develop in vivo as patients progress on PI3Kd inhibitor therapy.