The ATP-Competitive mTOR Inhibitor AZD8055 Reduces Cell Proliferation and Tumour Load in Chronic Lymphocytic Leukaemia

Chronic lymphocytic leukaemia (CLL), often diagnosed in the elderly, is incurable with current therapeutic regimes. Recent studies demonstrate that the microenvironment within CLL patient lymphoid organs protects leukaemic cells from chemotherapy induced apoptosis. This highlights the need for novel therapies that can overcome such cytoprotective signals. Serine/threonine protein kinase mammalian target for rapamycin (mTOR) is a key regulator of cell survival and proliferation and is commonly deregulated in cancer. mTOR is active in two complexes mTORC1 and mTORC2. We demonstrate that mTORC1 and mTORC2 substrates, including S6 (pS6^S235/236) and Akt (pAkt^S473) respectively, are active although differentially regulated, in CLL cells from distinct prognostic subsets ex vivo, which suggests that mTOR may represent a valid therapeutic target for CLL. Initial studies with everolimus, an analogue of rapamycin, showed only limited antitumor activity in recurrent/refractory CLL patients, possibly because mTORC1 inhibition releases the "brake" that normally active mTORC1 has on mTORC2, thus enabling mTORC2 mediated signalling. To test the impact of inhibiting both mTORC1 and mTORC2 on cell proliferation and survival, we treated primary CLL cells and an aggressive CLL mouse model with the novel ATP-competitive mTOR inhibitor AZD8055 in vitro and in vivo. CLL proliferation and mTORC1/2 activity were significantly reduced upon addition of AZD8055 to NTL-CD154/IL4 co-cultures, which mimic pro-proliferative conditions present in the microenvironment of lymphoid organs. However, in the same conditions use of AZD8055 did not induce apoptosis at clinically-achievable drug concentrations (<150 nM), likely due to ineffective inhibition of Mcl-1 upregulation mediated by the NTL-CD154/IL4 co-cultures. In contrast, B cell receptor (BCR)-mediated survival was inhibited by AZD8055 treatment, inducing significant levels of apoptosis, concomitant with a decreased Mcl-1 expression. These findings suggest that the BCR and CD154/IL4 mediated pathways regulate Mcl-1 expression by distinct mechanisms. In vivo studies using the PKCa-KR-induced CLL mouse model revealed that AZD8055 treatment efficiently reduced tumour burden in the organs (spleen and bone marrow) and blood of mice with established CLL. This was reflected by in vitro studies showing that use of AZD8055 increased apoptosis and reduced proliferation in mouse CLL cells. Overall these data indicate that AZD8055 inhibits CLL proliferation, in vitro and in vivo, and disrupts BCR mediated survival, by downregulating Mcl-1 protein levels. Therefore dual mTOR inhibitors through reduction of tumour burden, show promise as a future therapy for CLL.