The Novel Potent Pan-Deacetylase (DAC) Inhibitor, Panobinostat (LBH589), Markedly Enhances the Anti-Myeloma Effects of Chemotherapy In Vitro and In Vivo.

Deacetylase (DAC) inhibitors represent a new class of anti-cancer therapeutics that inhibit DAC enzymes and have been shown to have multiple effects in tumor cell lines including decreased oncoprotein expression (Bcr-Abl, HER-2), decreased angiogenesis, induction of apoptosis, induction of cell-cycle arrest, and decreased tumor cell motility and invasion. Panobinostat (LBH589), a novel cinnamic hydroxamic acid analogue with potent histone DAC inhibitor activity, has recently been shown to have the potential to treat a wide range of solid and hematological malignancies including multiple myeloma (MM). In this study, we first evaluated the in vitro anti-MM effects of panobinostat alone and in combination with doxorubicin or melphalan using the MM cell lines RPMI8226, U266 and MM1S. Cells treated with the combinations of panobinostat + doxorubicin and panobinostat + melphalan showed marked synergistic anti-MM effects as determined by measuring proliferation with the MTS assay compared to treatment with single agent and untreated cells. Next, we evaluated the anti-MM effects of panobinostat alone and in these combinations in vivo using one of our SCID-hu mouse models of human MM, LAGλ-1. Each SCID mouse was implanted with a 2.0 - 4.0 mm³ LAGλ-1 into the left superficial gluteal muscle. In our panobinostat single agent study, tumors were allowed to grow for 7 days at which time human IgG levels were detectable in the mouse serum, and mice were blindly assigned into panobinostat treatment groups. Panobinostat was administered via intraperitoneal (i.p.) injection once daily five times per week at 5, 10 and 20 mg/kg. Control mice were given sterile normal saline as vehicle. Mice receiving panobinostat showed marked inhibition of tumor growth (10 mg/kg, P < 0.003; 20 mg/kg, P < 0.009) and reduction of paraprotein levels (10 mg/kg, P < 0.0025; 20 mg/kg, P < 0.015) compared to mice receiving vehicle. Next, we evaluated the combination of low-dose panobinostat (5 mg/kg) with low doses of either liposomal doxorubicin (1 mg/kg) or melphalan (3 mg/kg) i.p. in mice bearing LAGλ-1. Tumors were allowed to grow for 10 days at which time human IgG levels were detectable in the mouse serum, and mice were blindly assigned into treatment groups. Panobinostat was administered as above, and liposomal doxorubicin was injected once daily for three consecutive days weekly and melphalan once weekly. Mice treated with the combination of panobinostat + liposomal doxorubicin showed markedly smaller tumors and reduced hIgG levels compared to treatment with the DAC inhibitor alone, and treatment with liposomal doxorubicin as a single agent produced no anti-MM effects. Mice bearing LAGλ-1 treated with the combination of low-dose panobinostat + low-dose melphalan also showed markedly smaller tumors and decreased hIgG levels compared to treatment with panobinostat alone whereas mice receiving melphalan alone showed similar results to vehicle-treated animals. These promising results support the further clinical development of panobinostat and suggest that combining this DAC inhibitor with low-dose chemotherapy (liposomal doxorubicin or melphalan) may enhance the efficacy of this novel agent for the treatment of MM patients.