Abstract 1396: Non-competitive inhibition of proteasome by kinase inhibitors

Abstract

Abstract Inhibitors of Bruton's Tyrosine Kinase (BTK) and inhibitors of the proteasome are currently in use for treatment of hematologic malignancies. While the ubiquitin proteasome system is necessary for protein homeostasis in all mammalian cells, BTK is unique to some B cell malignancies. However, BTK inhibitors and LU-102, a specific inhibitor of the β2 site of the proteasome, have previously been shown to synergize in hematologic malignancies which do not express BTK, at a 100-fold higher concentration than is needed for complete inhibition of BTK, suggesting an off-target effect of these BTK inhibitors. Triple Negative Breast Cancer (TNBC), a cancer with poor prognosis and no current targeted therapy, also does not express BTK. We found that LU-102 and a specific BTK inhibitor, CGI-1764, are de-facto synthetically lethal to TNBC cells, and that effect of other BTK inhibitors varied from similar synergy to no synergy. This data further supports the idea that synergy is due to off-target effects of BTK inhibitors.We have now found that CGI-1764 is a non-competitive, allosteric inhibitor of all catalytic subunits of the proteasome 20S proteolytic core and exerts its effect in a unique, dose-dependent manner. RNA sequencing shows that the Unfolded Protein Response pathway is upregulated in TNBC cells response to treatment with the CGI-1746+LU-102 combination. We also found that accumulation of ubiquitin conjugates is synergistic indicating that the combination treatment has a synergistic effect on the inhibition of proteasome activity. These findings may pave the path to the development of more potent allosteric inhibitors of the proteasome and show that kinase inhibitors should be screened for inhibition of the proteasome as potential off-target effect. Citation Format: Alexei F. Kisselev, Olasubomi Akintola, John Smith. Non-competitive inhibition of proteasome by kinase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1396.