In vivo activity and characterization of the mechanism of action of a novel class of proteasome inhibitors in Ewing’s sarcoma

The Ewing’s sarcoma family of tumors (EFT) is an aggressive childhood malignancy with dismal outcomes for patients with metastatic and recurrent disease. In an effort to identify novel agents against EFT and to provide new insights into critical pathways relied upon by these tumors, we screened a library of 309,898 chemicals for agents that selectively inhibit the growth of well-characterized EFT cell lines positive for the EWS-FLI1 fusion. By taking advantage of chemical genomic and computational biology techniques, we identified a set of highly active, structurally related agents that may represent a novel class of proteasome inhibitors.

Bortezomib, the only current FDA approved proteasome inhibitor, is highly effective in liquid tumors such as multiple myeloma and mantle cell lymphoma, and its use in first line therapy has led to improved outcomes for these patients. Significant susceptibility to proteasome inhibition has been repeatedly demonstrated in numerous in vitro and in vivo solid tumor models. However, the clinical efficacy of bortezomib in patients with solid tumors has been poor, mandating the need for novel proteasome inhibitors for clinical development.

Our goal in this proposal is to determine the efficacy of this new class of proteasome antagonists in murine xenograft models of EFT. In parallel, we will attempt to identify the cellular protein(s) bound and inhibited by these agents using affinity proteomics and quantitative mass spectrometry. If we confirm that these agents do indeed interact with specific proteasome subunits, we will then perform a quantitative assessment of the ubiquitin-modified proteome using mass spectrometry.

The plans outlined in the proposal are essential for the further development of these agents, which may serve as therapeutically active agents (alone or in combination with existing therapy) for Ewing’s sarcoma family of tumors, as well as other more common solid tumor types.