Abstract 1808: Potentiation of proteasome inhibitor cytotoxicity by co-treatment with novel β1i-selective immunoproteasome inhibitors

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA The FDA-approved proteasome inhibitors bortezomib and carfilzomib have contributed to major improvements in the treatment of multiple myeloma (MM) following their adoption as key therapies over the last 10 years. Despite the sucesses of proteasome inhibitor-based cancer therapies, drug resistance as well as dose-limiting adverse events remain problematic and prevent these agents from devliering durable and complete responses. Our prior research indicated that unlike carfilzomib and bortezomib which inhibit both the β5 and β5i subunits of the proteasome and immunoproteasome, β1i-selective inhibitors are not cytotoxic. Based on an understanding of the functional role of the β1i in cancer cells, we hypothesized that simulatenous administration of a non-toxic β1i inhibitor would potentiate the activity of classic proteasome inhibitors such as bortezomib and carfilzomib. By developing novel peptide epoxyketones with improved selectivity for β1i, we sucessfully demonstrated that such inhibitors are both non-toxic and are capable of potentiating the activity of cytotoxic proteasome inhibitors via β1i-dependent mechanisms. In the future, the co-administration of β1i-targeting agents and cytotoxic proteasome inhibitors, or the administration of novel agents which potently target β5, β5i, and β1i may serve as therapies for multiple myeloma and other cancers with greater efficacy, safety, and tolerabiliy than existing proteasome inhibitor-based therapies. In order to study this potentiating effect, we began by creating analogs of a known β1i-inhibiting tetrapeptide epoxyketone with modest selectivity for β1i over β1. Approximately twenty tetrapeptide and tripeptide epoxyketones were synthesized and evaluated for their activity against multiple proteasome subunits using subunit-selective flurogenic substrates in the presence of purified 20S proteasomes. Several promising analogs were identified and evaluted in cell culture with human MM cell lines including a carfilzozmib-resistant variant of the RPMI 8226 cell line. Cell viability following treatment with various classic and β1i-targeting proteasome inhibitors was evaluated via MTS assay. In summary, several peptide epoxyketones with improved potency against β1i or improved selectivity over β1 were identified. These inhibitors sucessfully inhibited proteasome catalytic activity both in vitro using purified proteasomes as well as in live cells in culture. Despite being non-toxic when administered alone, β1i-selective agents significantly decreased cell viability when combined with sub-IC50 concentrations of cytoxic proteasome inhibitors. Additionally, this potentiating effect was retained in a carfilzomib-resistant human MM cell line. Citation Format: Zachary C. Miller, Ying Wu, Na-Ra Lee, Shuo Zhou, Kyung-Bo Kim. Potentiation of proteasome inhibitor cytotoxicity by co-treatment with novel β1i-selective immunoproteasome inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1808. doi:10.1158/1538-7445.AM2014-1808