Synergistic HDAC4/8 Inhibition Sensitizes Osteosarcoma to Doxorubicin via pAKT/RUNX2 Pathway Modulation.

Osteosarcoma is a highly aggressive bone malignancy, particularly challenging in metastatic cases, with a 5-year survival rate remaining under 30%. Although doxorubicin (doxo) is a standard first-line chemotherapeutic agent, its clinical utility is often hindered by the development of drug resistance and associated systemic toxicity. Emerging evidence highlights the role of epigenetic alterations, particularly those involving histone deacetylases (HDACs), in promoting chemoresistance. In this context, the present study aimed to evaluate the therapeutic potential of combining doxo with the selective HDAC inhibitors, tasquinimod (Tas, targeting HDAC4) and PCI-34051 (PCI, targeting HDAC8), in SJSA-1 osteosarcoma cells. Utilizing both 2D and 3D in vitro models, the combination treatment (referred to as the T4 group) significantly reduced cell viability by 57.69% in 2D cultures and decreased spheroid volume by 35.19% in 3D models. The apoptotic response was markedly enhanced, with late apoptosis reaching 64.59% and necrosis at 32.07%, both surpassing the effects observed with doxo alone. Furthermore, wound healing assays demonstrated a 37.74% inhibition of migration, accompanied by a decreased expression of the matrix metalloproteinases MMP9 and MMP13. Mechanistically, the combination therapy led to the downregulation of protein kinase B (pAKT) and RUNX2, along with upregulation of apoptotic markers, including caspase 8, caspase 3, and cleaved caspase 3, indicating a disruption of key survival pathways. These findings suggest that dual HDAC inhibition with Tas and PCI can potentiate doxo efficacy by enhancing apoptosis, inhibiting proliferation, and reducing metastatic potential, thus offering a promising strategy to overcome chemoresistance in osteosarcoma. Further preclinical and clinical studies are required to validate these therapeutic benefits.