Efficacy of Panobinostat (LBH589) in Multiple Myeloma Cell Lines and In Vivo Mouse Model: Tumor-Specific Cytotoxicity and Protection of Bone Integrity in Multiple Myeloma.

Panobinostat (LBH589) is a highly potent oral pan-deacetylase (DAC) inhibitor currently undergoing clinical development in hematologic and solid malignancies. Here we report the effects of panobinostat on multiple myeloma (MM) cells in vitro and in a murine xenograft model in vivo. Panobinostat exhibited potent cytotoxic activity (IC₅₀ <10 nM) against 8 MM cell lines (KMS-12PE, KMS-18, LP-1, NCI H929, KMS-11, RPMI8226, OPM-2, and U266). Panobinostat has been shown to affect signals involved in MM cell-cycle arrest and cell death, and to induce apoptosis via mitochondrial perturbation. In addition, panobinostat has been shown to selectively induce cell death of plasma cells isolated from MM patients without toxicity to normal lymphocytes or granulocytes. To investigate the effect of panobinostat in vivo, a disseminated luciferized MM.1S xenograft mouse model was treated with vehicle or panobinostat 15 mg/kg by intraperitoneal (i.p.) administration qd×5 for 3 weeks. Panobinostat treatment reduced the burden of MM.1S tumor cells to 22% treated over control (T/C) relative to vehicle-treated animals. In addition, MM.1S tumor-bearing mice treated with panobinostat displayed reduced trabecular and cortical bone damage relative to vehicle-treated animals. The mean ± SEM trabecular bone density and cortical bone density (% Bone Volume/Total Volume) of panobinostat-treated animals was 14.5% ± 2.0 and 98.1% ± 0.4, respectively, compared with 2.2% ± 0.3 and 89.1% ± 1.5 in vehicle-treated animals. In combination with the proteosome inhibitor bortezomib (BZ), panobinostat displayed significant synergistic cytotoxicity without additional toxicity to normal bone marrow stromal cells in vitro. In the MM.1S-luciferase tumor mouse model, combined treatment with panobinostat at 10 mg/kg i.p. qd×5 for 4 weeks and BZ at 0.2 mg/kg intravenously 1qw for 4 weeks reduced tumor burden to 7% T/C relative to vehicle, panobinostat alone (31% T/C), or BZ alone (44% T/C). Disease progression, measured as median time to endpoint (TTE) was improved from 37 to 54 days (P<0.05) by panobinostat and to 46 days by BZ (P<0.05). The combination treatment further improved clinical outcome relative to both single-agent treatment groups (P<0.05), extending the TTE to 73 days. In contrast to BZ, the immunomodulatory drug thalidomide (TH) had no significant single-agent activity at 150 mg/kg p.o. qd for 4 weeks. However, combination activity (18% T/C) was observed when TH was combined with a sub-efficacious dose of panobinostat (5 mg/kg, 64% T/C). Combination of panobinostat and TH increased the TTE to 50 days, compared with 37.5, 43, and 39.5 days (P<0.05), respectively, for the vehicle, panobinostat, or TH as single agents. These data demonstrate that panobinostat exhibits significant anti-proliferative and anti-tumor activities on MM cells both in vitro and in vivo. Panobinostat, as a single agent or in combination with BZ or TH, is a promising therapy for MM, and these studies may provide the rationale for clinical evaluation of panobinostat and BZ combination in the treatment of MM.