Hide and Seek: The Game Between Chronic Lymphocytic Leukaemia Cells and B Cell Receptor Signalling Inhibitors

The emergence of B cell receptor (BCR) kinase inhibitors has recently changed the treatment landscape in chronic lymphocytic leukaemia (CLL). The inhibitors that selectively target potential kinases downstream from BCR (particularly Bruton’s tyrosine kinase [BTK] and phosphoinositide 3-kinase [PI3K]) have replaced conventional chemotherapy for high-risk CLL. Ibrutinib and idelalisib are the respective first-in-class BTK and PI3K-δ inhibitors that are US Food and Drug Administration (FDA) approved for CLL treatment, with promising second-generation molecules under development. Differing from idelalisib, duvelisib (IPI-145) inhibits both delta and gamma isoforms of PI3K. Kinase inhibitors have gained popularity in the clinic primarily due to their ability to induce remissions in the vast majority of patients, even in patients with high-risk disease features, without causing haematotoxicity. In particular, they interfere with the homing capabilities of CLL cells residing in their respective microenvironments and cause lymphocytosis via redistribution of tissue-resident CLL cells into the peripheral blood. Thereby, BCR inhibitors can seek out and target hiding CLL cells in the lymph node and marrow niches. In this review, we discuss laboratory and clinical aspects of the BCR inhibitors that have recently advanced the treatment of B cell malignancies, with a particular emphasis on CLL. Despite the excitement about this new class of compounds targeting BCR signalling, single agent therapy with kinase inhibitors has limitations, requiring continuous kinase suppression to maintain remissions, which generally are partial remissions, indicating that combination strategies will become important for moving the field forward.

Differential regulatory functions of three classes of phosphatidylinositol and phosphoinositide 3-kinases in autophagy

Autophagy is an evolutionarily conserved and exquisitely regulated self-eating cellular process with important biological functions. Phosphatidylinositol 3-kinases (PtdIns3Ks) and phosphoinositide 3-kinases (PI3Ks) are involved in the autophagic process. Here we aim to recapitulate how 3 classes of these lipid kinases differentially regulate autophagy. Generally, activation of the class I PI3K suppresses autophagy, via the well-established PI3K-AKT-MTOR (mechanistic target of rapamycin) complex 1 (MTORC1) pathway. In contrast, the class III PtdIns3K catalytic subunit PIK3C3/Vps34 forms a protein complex with BECN1 and PIK3R4 and produces phosphatidylinositol 3-phosphate (PtdIns3P), which is required for the initiation and progression of autophagy. The class II enzyme emerged only recently as an alternative source of PtdIns3P and autophagic initiator. However, the orthodox paradigm is challenged by findings that the PIK3CB catalytic subunit of class I PI3K acts as a positive regulator of autophagy, and PIK3C3 was thought to be an amino acid sensor for MTOR, which curbs autophagy. At present, a number of PtdIns3K and PI3K inhibitors, including specific PIK3C3 inhibitors, have been developed for suppression of autophagy and for clinical applications in autophagy-related human diseases.

PKC-β as a therapeutic target in CLL: PKC inhibitor AEB071 demonstrates preclinical activity in CLL. Blood. 2014;124:1481-1491. [PMID: 25001469 DOI: 10.1182/blood-2014-05-574830]

Targeting B-cell receptor (BCR) signaling in chronic lymphocytic leukemia (CLL) has been successful with durable remissions observed with several targeted therapeutics. Protein kinase C-β (PKC-β) is immediately downstream of BCR and has been shown to be essential to CLL cell survival and proliferation in vivo. We therefore evaluated sotrastaurin (AEB071), an orally administered potent PKC inhibitor, on CLL cell survival both in vitro and in vivo. AEB071 shows selective cytotoxicity against B-CLL cells in a dose-dependent manner. Additionally, AEB071 attenuates BCR-mediated survival pathways, inhibits CpG-induced survival and proliferation of CLL cells in vitro, and effectively blocks microenvironment-mediated survival signaling pathways in primary CLL cells. Furthermore, AEB071 alters β-catenin expression, resulting in decreased downstream transcriptional genes as c-Myc, Cyclin D1, and CD44. Lastly, our preliminary in vivo studies indicate beneficial antitumor properties of AEB071 in CLL. Taken together, our results indicate that targeting PKC-β has the potential to disrupt signaling from the microenvironment contributing to CLL cell survival and potentially drug resistance. Future efforts targeting PKC with the PKC inhibitor AEB071 as monotherapy in clinical trials of relapsed and refractory CLL patients are warranted.

Phosphatidylinositol 3-kinase (PI3K) inhibitors as cancer therapeutics

Phosphatidylinositol 3-kinases (PI3Ks) are lipid kinases that regulate diverse cellular processes including proliferation, adhesion, survival, and motility. Dysregulated PI3K pathway signaling occurs in one-third of human tumors. Aberrantly activated PI3K signaling also confers sensitivity and resistance to conventional therapies. PI3K has been recognized as an attractive molecular target for novel anti-cancer molecules. In the last few years, several classes of potent and selective small molecule PI3K inhibitors have been developed, and at least fifteen compounds have progressed into clinical trials as new anticancer drugs. Among these, idelalisib has advanced to phase III trials in patients with advanced indolent non-Hodgkin's lymphoma and mantle cell lymphoma. In this review, we summarized the major molecules of PI3K signaling pathway, and discussed the preclinical models and clinical trials of potent small-molecule PI3K inhibitors.