Preclinical Evaluation of the Proteasome Inhibitor MLN9708 (MLN2238) In Chronic Lymphocytic Leukemia Cells

Abstract 4634

The proteasome is an important therapeutic target in multiple hematological malignancies. The proteasome inhibitor bortezomib has demonstrated significant clinical activity in multiple myeloma and mantle cell lymphoma. However, clinically bortezomib has failed to demonstrate efficacy in chronic lymphocytic leukemia (CLL); the exact reason for this remains unknown. MLN9708 (Millennium Pharmaceuticals, Inc., Cambridge, MA) is a proteasome inhibitor which has a shorter proteasome dissociation half life than bortezomib and is currently in Phase I clinical development. Upon exposure to aqueous solutions or plasma, MLN9708 rapidly hydrolyzes to MLN2238, the biologically active form. MLN2238 was used for all of the studies reported here, in which we evaluated the antileukemic effects of MLN2238 in primary CLL cells from 16 patients. MLN 2238 induced a time and dose dependent decrease in CLL cell viability in 12 (75%) patient cells in vitro. Cell death was shown to be due to the induction of apoptosis confirmed by Annexin V staining of CLL cells and cleavage of PARP-1, an indicator of apoptosis. Biochemical analysis showed that caspase 3 and 9 were activated in these cells, indicating that MLN2238 induces cell death through the intrinsic apoptosis pathway. We noted increased mitochondrial outer membrane permeability (MOMP) in the presence of MLN2238, which suggests the engagement of the mitochondria in MLN2238 induced apoptosis in CLL cells. Evaluation of non-responding CLL patient samples demonstrated that increased expression of PSMB5 is associated with lack of sensitivity to MLN2238. Furthermore, incremental addition of autologous serum in responding CLL samples resulted in upregulation of PSMB5 levels, resulting in protection against MLN2238 induced cell death. Our preclinical observations demonstrate for the first time that MLN2238 can induce death in primary CLL cells, and support further investigation in CLL.