Targeting Aminopeptidases by Tosedostat (TST) (CHR2797), Alone and with LBH589, Induces Significant Cytotoxicity Against Human Multiple Myeloma (MM) Cells

Abstract 1847

Aminopeptidases (AP) are necessary for the growth and development of malignant cells and have a selectively important role in the maintenance of intracellular amino acid (AA) levels in neoplastic cells. CHR2797 is a novel, low nanomolar inhibitor of the M1 family of AP, a group of metalloenzymes containing a central Zn²⁺ ion. CHR2797 has antiproliferative and apoptotic effects against MM in vitro by inducing the AA deprivation response (AADR). TST, an oral, chronically administered agent with a good safety profile has demonstrated activity in patients with relapsed/refractory AML and is currently under study as part of combination therapy for untreated elderly patients with AML. At the epigenetic regulatory level, Zn-dependent histone deacetylase (HDAC) cause the deacetylation of histone and non-histone cellular proteins which are critical for gene expression, inducing apoptosis and cell cycle arrest in cancer cells. LBH589 (Panobinostat) is an established pan-HDAC inhibitor with potent in vitro anti-cancer activity in many hematological malignancies. The clinical efficacy of Panobinostat is currently being studied in several Phase II/III clinical trials with particular promise seen in the treatment of MM. Here we examined the potential therapeutic effect of CHR2797, alone and with LBH589, against MM cells. Using MTS and CTG assays, CHR2797, at clinically achievable concentrations, decreased survival and proliferation in MM1S and IL-6-dependent ANBL6 cells, in the presence or absence of bone marrow stromal cells following 72 hours incubation. CHR2797 induces apoptosis in MM cells via activation of Caspase 3/7 and 9 but not Caspase 8. Significantly, CHR2797 (10 μM) induced apoptosis in patient MM cells, as seen by % of annexin V and PI from 22 + 1.5% to 39 + 2.3% after 48h incubation. Combined treatment with CHR2797 and LBH589 in MM cells (MM1S, ANBL6, and INA6) further reduced cell viability following 72 hour incubation when compared with CHR2797 treatment alone, as determined by CTG viability luminescent assay. Both drugs together also augmented growth inhibitory effects when compared with single agent alone, after 72 hours incubation followed by MTS assay. Importantly, the combination of both drugs increased caspase 3/7- & 9-mediated apoptosis than CHR2797 alone in these MM cells following 24h-treatment. Cell cycle analysis (CHR2797 at 1μM; LBH589 at 1 nM) showed an increased growth arrest in G0/G1 cells in MM1R cells treated with both drugs versus CHR2797 alone after 24 hours: 68.5±3.3% versus 36±2.5%. Furthermore, CHR2797 inhibited anti-apoptotic protein Mcl-1 in MM1R and U266 MM cells by immunoblottings. Combined treatment with CHR2797 and LBH589 further blocked Mcl-1 when compared with either treatment alone after 24 hours incubation. Together, these results show that the combination of CHR2797 and LBH589 enhanced anti-myeloma effects when compared with either drug alone. This combination, which also has the potential of being without overlapping clinical toxicities, provides a promising novel approach to anti-myeloma therapy.