Development of novel HDAC inhibitors to selectively co-inhibit HDAC8 and HDAC10 in childhood cancer

Introduction:

High HDAC8 and HDAC10 expression are predictors of poor outcomes in neuroblastoma and high-risk (INSS stage 4) neuroblastoma, respectively. The prognosis of high-risk neuroblastoma remains poor with current treatment regimens. Broad-spectrum HDAC inhibitors, approved for hematological malignancies, are associated with frequent toxicities, which are mainly attributed to HDAC1, 2 and 3 inhibition. Thus, the aim of this study is to develop and characterize a selective HDAC8/10 co-inhibitor for the treatment of high-grade neuroblastoma.

Methods:

After initial structural screening, we tested ten optimized novel small-molecule inhibitors in cell-free assays measuring enzymatic activity of different HDACs in vitro. Employing human pediatric cancer and non-transformed cell lines, we assessed HDAC8/10 specificity and antitumoral efficacy in analyses of target protein acetylation, functional assays (e.g. acidic vesicle staining to evaluate HDAC10 inhibition) as well as combination studies with the differentiating agent retinoic acid.

Results:

The most promising selective inhibitor induced a cytoplasmic accumulation of acidic vesicular organelles and significantly reduced high grade neuroblastoma cell growth in vitro, synergizing with retinoic acid. This drug combination was tested both in vitro and in vivo and no relevant toxicity could be observed.

Conclusion:

The presented selective HDAC8/10 co-inhibitor bears promising anti-tumoral effects in vitro. It effectively prevents high-grade neuroblastoma cell survival in vitro without relevant toxicity to non-transformed cells. In order to assess suitability for future clinical trials, the novel HDAC8/10 inhibitor is currently being tested in combination with retinoic acid in an in vivo xenograft model.