Histone deacetylases (HDACs) are enzymes involved in the epigenetic regulation of gene expression through chromatin remodelling, which often upregulated in cancer. Inhibition of HDACs by several chemical classes of HDAC inhibitors (HDIs) has emerged as a potential treatment strategy in cancer. The FDA-approved hydroxamate-based HDI, suberoylanilide hydroxamic acid (SAHA), is effective against cutaneous T-cell lymphoma, and yet is moderately effective against a range of other solid tumors. Combinations of SAHA, with other anticancer drugs such as TRAIL, sulindac (survivin inhibitor), and Idarubicin (DNA topoisomerase inhibitor), have been reported to increase SAHA anti-cancer efficacy. In the present study, we sought to identify additional small molecules that may be used as SAHA enhancers, using an automated, high-throughput robotic screening system. A molecule was identified as a SAHA enhancer if its administration with SAHA decreased the cell viability by > 40 % relative to the effect of SAHA alone.
Screening of a 10,560 random compounds from the Walter and Eliza Hall Institute compound library on SAHA-resistant breast cancer cells, MDA-MB-231, identified 21 SAHA-enhancer molecules. Further testing on neuroblastoma (e.g. BE(2)-C, and SY5Y) and medulloblastoma (e.g. DAOY) cells validated 2 compounds, A1 and B1, as enhancer molecules. Importantly, the two lead compounds elicited limited (ca. ~ 35%) cytotoxicity on normal human fibroblasts, whilst synergistically enhancing SAHA cytotoxicity (CI ~0.6 – 0.8) on SY5Y and DAOY at ≤ 10 µM. Development of A1- and B1-focused library led to identification of a more potent SAHA-enhancer molecule, B2-XI. Combination of SAHA (1 μM) with B2-XI (5 μM) synergistically reduced the cell viability, and proliferation, and induced cellular apoptosis in a range of neuroblastoma cells, but not in the normal human fibroblasts. On-going studies are aimed at determining the molecular mechanisms for the SAHA-enhancing effects of the lead molecules.