Key points

Introduction

Although chronic lymphocytic leukemia (CLL) usually responds well to initial chemotherapy, the disease inevitably relapses, and remains incurable by conventional therapy. It has been hypothesized that after treatment, sanctuary sites such as the lymph nodes and bone marrow may harbor residual CLL cells that can later lead to relapse. Indeed, the protective role of the CLL microenvironment may be a key to understanding why stroma-exposed CLL cells are protected from undergoing apoptosis in response to treatment. The B-cell receptor (BCR) pathway has been particularly identified as a key mediator of prosurvival signals in CLL cells. Several novel kinase inhibitors are now in development to target various components of the BCR pathway. A class effect of these BCR inhibitors is a “lymphocyte redistribution” phenomenon, whereby a majority of patients initially develop a transient lymphocytosis while simultaneously achieving nodal reduction. This observation has led to the hypothesis that these agents may achieve their efficacy, at least in part, by mobilizing CLL cells out of sanctuary sites and into the peripheral blood, where they more readily die or can be killed by combination therapy.

Of the new agents targeting the BCR pathway, phosphoinositide 3′-kinase (PI3K) inhibitors are among the most promising. This article reviews the scientific rationale underlying PI3K inhibition in CLL, as well as data from recent and ongoing clinical trials of PI3K inhibitors in CLL. Also discussed are potential biological predictive markers for PI3K clinical response, as well as where PI3K inhibitors may fit into the evolving landscape of CLL therapy.

Introduction

Although chronic lymphocytic leukemia (CLL) usually responds well to initial chemotherapy, the disease inevitably relapses, and remains incurable by conventional therapy. It has been hypothesized that after treatment, sanctuary sites such as the lymph nodes and bone marrow may harbor residual CLL cells that can later lead to relapse. Indeed, the protective role of the CLL microenvironment may be a key to understanding why stroma-exposed CLL cells are protected from undergoing apoptosis in response to treatment. The B-cell receptor (BCR) pathway has been particularly identified as a key mediator of prosurvival signals in CLL cells. Several novel kinase inhibitors are now in development to target various components of the BCR pathway. A class effect of these BCR inhibitors is a “lymphocyte redistribution” phenomenon, whereby a majority of patients initially develop a transient lymphocytosis while simultaneously achieving nodal reduction. This observation has led to the hypothesis that these agents may achieve their efficacy, at least in part, by mobilizing CLL cells out of sanctuary sites and into the peripheral blood, where they more readily die or can be killed by combination therapy.

Of the new agents targeting the BCR pathway, phosphoinositide 3′-kinase (PI3K) inhibitors are among the most promising. This article reviews the scientific rationale underlying PI3K inhibition in CLL, as well as data from recent and ongoing clinical trials of PI3K inhibitors in CLL. Also discussed are potential biological predictive markers for PI3K clinical response, as well as where PI3K inhibitors may fit into the evolving landscape of CLL therapy.

Biology of PI3K in CLL

Signaling cascades from several major pathways converge on PI3K, which serves as a key node regulating B-cell function and survival. Although there are 3 classes of PI3K isoforms, only class I isoforms are thought to be directly related to oncogenesis. Within class I, PI3K isoforms can be further subdivided into class IA (PI3K-α, -β, and -δ) and class IB (PI3K-γ). Although having shown that activating mutations in PI3K are very rare in CLL, we did identify amplification of PIK3CA in about 3.5% of CLL. Furthermore, even in the absence of genetic activation, CLL cells generally express high levels of active PI3K (in particular the δ isoform ), and great interest has therefore focused on elucidating the role played by PI3K in the pathogenesis of the disease.

The 3 best characterized pathways that activate PI3K include the BCR, receptor tyrosine kinases (RTKs), and cytokine/chemokine receptors ( Fig. 1 ). Of these, the BCR pathway is thought to play a dominant role in CLL. The BCR usually becomes activated in the presence of antigen (although tonic signaling has also been described ). Activated BCR recruits other kinases such as spleen tyrosine kinase (Syk) and Lyn kinase, which phosphorylate immunoreceptor tyrosine–based activation motifs (ITAMs) on the cytoplasmic immunoglobulin domains of the receptor. Stimulated RTKs, cytokine, and chemokine receptors also cause autophosphorylation of the tyrosine residue on the ITAMs and subsequent PI3K activation in immune cells, although the importance of these pathways in CLL is variable.

PI3K signaling and molecular interactions in the CLL cell. Three important pathways converge on PI3K signaling, including the B-cell receptor (BCR), thought to be the dominant pathway in CLL, as well as receptor tyrosine kinases (RTKs) and cytokine/chemokine receptors. The BCR usually becomes activated in the presence of antigen (although tonic signaling has also been described). Activated BCR recruits other kinases such as spleen tyrosine kinase (Syk) and Lyn kinase, which phosphorylate immunoreceptor tyrosine–based activation motifs (ITAMs) on the cytoplasmic immunoglobulin domains of the receptor. RTKs can be stimulated by a variety of ligands, and also lead to autophosphorylation of the tyrosine residue on the ITAM. Stimulated cytokine and chemokine receptors can activate Janus-activated kinase (JAK) tyrosine kinases, which phosphorylate tyrosine residues in proteins such as gp130 and inflammatory response system (IRS) family members. These tyrosine-phosphorylated proteins interact with SH2 domains of the p85 subunit of PI3K, thereby activating the p110 kinase activity, which is normally inhibited in the p85-p110 complex. This activation results in the conversion of phosphatidylinositol-4,5-biphosphate (PIP2) to phosphatidylinositol-3,4,5-triphosphate (PIP3), which has several downstream effects, many of which are mediated by AKT. The net result of PI3K activation is to promote CLL cell survival and proliferation, largely through activation of nuclear transcription factors such as nuclear factor κB, nuclear factor of activated T cells (NFAT), and serum response factor (SRF). These effects can be modulated by nearby stromal cells including T cells, nurse-like cells (NLC), and bone marrow stromal cells (BMSC). mTOR, mammalian target of rapamycin; PTEN, phosphatase and tensin homolog.

Stimulation of each of these 3 pathways sets off a chain of downstream molecular interactions, the net result of which is to create Src homology 2 (SH2)-binding domains capable of binding the p85 regulatory subunit of PI3K. Once this binding occurs, p85 can no longer inhibit the p110 catalytic domain of PI3K, thereby leading to PI3K activation. One of the primary functions of activated PI3K in B cells is to convert phosphatidylinositol-3,4-biphosphate into phosphatidylinositol-3,4,5-triphosphate, leading to AKT phosphorylation, which then can go on to activate a wide variety of downstream kinases. In addition to AKT, activated PI3K also promotes calcium mobilization and activation of other downstream kinases such as protein kinase C (PKC)-β, mammalian target of rapamycin (mTOR), and MAP kinase (ERK). These events promote increased proliferation of B cells, largely mediated by the upregulation of transcription factors such as nuclear factor κB (NF-κB) and nuclear factor of activated T cells (NFAT).

Although substantial evidence indicates that PI3K activation inhibits both the extrinsic and intrinsic pathways of apoptosis, the precise mechanism of these interactions remains incompletely understood. Activated AKT likely interferes with FasL expression, thereby decreasing levels of this primary mediator of extrinsic apoptosis. Activated AKT has also been hypothesized to affect the intrinsic mitochondrial pathway of apoptosis by increasing the amount of the proapoptotic protein BAD that is sequestered by 14-3-3, a regulatory protein that by binding BAD limits its ability to promote apoptosis, thereby pushing the cell farther from the threshold of apoptosis (ie, decreasing “priming” for apoptosis). Other interactions between AKT and the mitochondrial pathway of apoptosis are likely, and this remains an active area of investigation.

Beyond its direct effects on promoting B-cell proliferation and inhibiting apoptosis, activated PI3K also has a profound influence on B-cell trafficking by promoting CLL cell chemotaxis toward CXCL12/13, migration beneath stromal cells, and upregulation of CLL cell chemokine secretion. Once CLL cells enter the stromal microenvironment, they become bathed in a variety of protective mediators such as CD40L, fibronectin, and B-cell activating factor, all of which likely send prosurvival signals through PI3K. We have shown that the net effect of these stromal interactions is to decrease CLL cell mitochondrial apoptotic priming, which may lead to resistance to a wide variety of therapies.

Inhibition of PI3K in CLL

Given the key role that PI3K plays in CLL pathophysiology, the potential efficacy of small-molecule PI3K inhibitors has been widely recognized. Several different PI3K inhibitors are in various stages of development, and can be divided into 2 main categories: δ-isoform specific and pan-PI3K inhibitors. A third category of dual inhibitors targeting both PI3K and mTOR are also being investigated.