Aberrant regulation of endogenous survival pathways plays a major role in progression of chronic lymphocytic leukemia (CLL). Signaling via conjugation of surface receptors within the tumor environmental niche activates survival and proliferation pathways in CLL. Of these, the PI3K/AKT pathway appears to be pivotal to support CLL pathogenesis, and pharmacologic inhibitors targeting this axis have shown clinical activity. Here we investigate OSU-T315, a compound that disrupts the Phosphoinositide 3-kinase (PI3K)/AKT pathway in a novel manner. Dose-dependent, selective cytotoxicity by OSU-T315 is noted in both CLL-derived cell lines and primary CLL cells relative to normal lymphocytes. In contrast to the highly successful BTK and PI3K inhibitors that inhibit BCR signaling pathway at proximal kinases, OSU-T315 directly abrogates AKT activation by preventing translocation of AKT into lipid rafts without altering the activation of receptor-associated kinases. Through this mechanism, the agent triggers caspase-dependent apoptosis in CLL by suppressing BCR, CD49d, CD40 and TLR9-mediated AKT activation in an ILK-independent manner. In vivo, OSU-T315 attains pharmacologically active drug levels and significantly prolongs survival in the TCL1 mouse model. Together, our findings indicate a novel mechanism of action of OSU-T315 with potential therapeutic application in CLL.
Chronic Lymphocytic Leukemia (CLL) demonstrates variable reactivity of the B cell receptor (BCR) to antigen ligation, but constitutive pathway activation. Brutons Tyrosine Kinase (BTK) shows constitutive activity in CLL, and is the target of irreversible inhibition by ibrutinib, an orally bioavailable kinase inhibitor that has shown outstanding activity in CLL. Early clinical results in CLL with other reversible and irreversible BTK inhibitors have been less promising, however, raising the question of whether BTK kinase activity is an important target of ibrutinib and also in CLL. To determine the role of BTK in CLL, we utilized patient samples and the E?-TCL1 (TCL1) transgenic mouse model of CLL which results in spontaneous leukemia development. Inhibition of BTK in primary human CLL cells by siRNA promotes apoptosis. Inhibition of BTK kinase activity through either targeted genetic inactivation or ibrutinib in the TCL1 mouse significantly delays the development of CLL, demonstrating that BTK is a critical kinase for CLL development and expansion and thus an important target of ibrutinib. Collectively, our data confirm the importance of kinase-functional BTK in CLL.
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