Histone deacetylase inhibitors (HDACi) have been reported to increase tumor antigen expression, and have been successfully tested as adjuvants for melanoma immunotherapy in mouse models. In this work, we tested the effects of a pan-HDACi on human lymphocytes and melanoma cell lines. Effects of the pan-HDACi panobinostat (LBH589) on cell viability, cell cycle, apoptosis and DNA damage were determined in peripheral blood mononuclear cells (PBMC) from two healthy donor (HD), thirteen patients with metastatic melanoma (MD), two bone marrow samples from different malignances, and twelve human melanoma cell lines. Intracellular signaling in lymphocytes, with or without cytokine stimulation, was analyzed by phospho-flow cytometry in one of each type. The 50% inhibition concentration (IC50) in PBMC was <20 nM compared to >600 nM in melanoma cell lines; >40% apoptotic cell death in PBMC versus <10% in melanoma cell lines was seen at the same concentration. Phospho-histone variant H2A.X (pH2A.X) increased 2-fold in HD PBMC at 1 nM, while the same effect in the melanoma cell line M229 required 10nM. pH2A.X was slightly inhibited in 3 MD PBMC at 1 nM and M370 at 10 nM. Panobinostat inhibited phospho-STATs 1, 3, 5, 6, -p38, -ERK, -p53, -cyclin D3 and -histone H3 in flow gated HD B- and T-cells, while there was up to six-fold activation in MD and bone marrow samples. In human lymphocytes, panobinostat alters key lymphocyte activation signaling pathways and is cytotoxic at concentrations much lower than that required for melanoma antitumor activity, resulting in an adverse therapeutic window.
- Received October 25, 2013.
- Revision received January 6, 2014.
- Accepted January 20, 2014.
- Copyright © 2014, American Association for Cancer Research.