Piperacetazine Directly Binds to the PAX3::FOXO1 Fusion Protein and Inhibits Its Transcriptional Activity.

ABSTRACT

The tumor-specific chromosomal translocation product, PAX3FOXO1, is an aberrant fusion protein that plays a key role for oncogenesis in the alveolar subtype of rhabdomyosarcoma (RMS). PAX3FOXO1 represents a validated molecular target for alveolar RMS and successful inhibition of its oncogenic activity is likely to have significant clinical applications. Even though several PAX3FOXO1 function-based screening studies have been successfully completed, a directly binding small-molecule inhibitor of PAX3FOXO1 has not been reported. Therefore, we screened small-molecule libraries to identify compounds that were capable of directly binding to PAX3FOXO1 protein using surface plasmon resonance technology. Compounds that directly bound to PAX3FOXO1 were further evaluated in secondary transcriptional activation assays. We discovered that piperacetazine can directly bind to PAX3FOXO1 protein and inhibit fusion protein-derived transcription in multiple alveolar RMS cell lines. Piperacetazine inhibited anchorage-independent growth of fusion-positive alveolar RMS cells but not embryonal RMS cells. On the basis of our findings, piperacetazine is a molecular scaffold upon which derivatives could be developed as specific inhibitors of PAX3FOXO1. These novel inhibitors could potentially be evaluated in future clinical trials for recurrent or metastatic alveolar RMS as novel targeted therapy options. SIGNIFICANCE: RMS is a malignant soft-tissue tumor mainly affecting the pediatric population. A subgroup of RMS with worse prognosis harbors a unique chromosomal translocation creating an oncogenic fusion protein, PAX3FOXO1. We identified piperacetazine as a direct inhibitor of PAX3FOXO1, which may provide a scaffold for designing RMS-specific targeted therapy.