Histone deacetylase inhibitors (HDACi) were identified nearly four decades ago based on their ability to induce cellular differentiation. However, the clinical development of these compounds as cancer therapies has focused on their capacity to induce apoptosis in hematologic and lymphoid malignancies, often in combination with conventional cytotoxic agents. In many cases, HDACi doses necessary to induce these effects result in significant toxicity. Since osteosarcoma cells express markers of terminal osteoblast differentiation in response to DNA methyltransferase inhibitors, we reasoned that the epigenetic reprogramming capacity of HDACi might be exploited for therapeutic benefit. Here, we show that continuous exposure of osteosarcoma cells to low concentrations of HDACi LBH589 (Panobinostat) over a three-week period induces terminal osteoblast differentiation and irreversible senescence without inducing cell death. Remarkably, transcriptional profiling revealed that HDACi therapy initiated gene signatures characteristic of chondrocyte and adipocyte lineages in addition to marked upregulation of mature osteoblast markers. In a mouse xenograft model, continuous low dose treatment with LBH589 induced a sustained cytostatic response accompanied by induction of mature osteoblast gene expression. These data suggest that the remarkable capacity of osteosarcoma cells to differentiate in response to HDACi therapy could be exploited for therapeutic benefit without inducing systemic toxicity. 1. Introduction Osteosarcoma is a malignant mesenchymal neoplasm characterized by primitive osteoblastic cells [1] that represents the most prevalent primary tumour of bone, mainly arising in adolescents and in adults over the age of 50 [2]. Despite advances in surgical techniques and neoadjuvant chemotherapy, it remains the second leading cause of cancer-related death in children and young adults, and it contributes significantly to the health care burden of our society [3]. Approximately 20% of patients present with metastases and of the remaining 80%; a further 25%–50% will develop metastatic disease during their treatment [4, 5]. The use of adjuvant chemotherapy in osteosarcoma has significantly increased the 5-year survival rate from 10% to 70% for nonmetastatic disease [6]. However, cure rates for patients with metastatic or relapsed disease are poor, with a 5-year survival rate of <20% [7, 8]. The stagnation of these survival rates since the introduction of adjuvant chemotherapy three decades ago highlights the urgent need for new and improved therapeutic approaches
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