Abstract 3087: KZR-8834: A novel, small molecule inhibitor of Sec61-dependent protein secretion with anti-tumor activity

The majority of secreted and transmembrane (TM) proteins, including growth factors and TM oncogenes, require translocation through Sec61 into the ER for further processing. Sec61, therefore, represents a unique drug discovery opportunity through blockade of typically “undruggable” targets and modulation of protein homeostasis. Multiple inhibitors of protein secretion have been described that directly target Sec61 and have anti-tumor activity but lack adequate pharmaceutical properties and/or tolerability to be considered clinical candidates. We aimed to discover novel, selective inhibitors of Sec61 with increased tolerability as a potential treatment for various malignancies. Sec61 inhibitors were identified using HEK293 cell lines stably expressing secreted or TM proteins of interest fused to a luciferase reporter. Selective anti-tumor activity was assessed via HEK293 reporter cell viability in comparison to tumor cell lines with known levels of sensitivity to published Sec61-modulating compounds. An initial screen resulted in KZR-4152 (4152), which blocked secretion of several target proteins but had no impact on tumor cell viability. Following a medicinal chemistry campaign, KZR-8834 (8834) was identified, which exhibited a 60-fold increase in potency against target proteins relative to 4152. On-target activity of 8834 was demonstrated via protein secretion assays in cells overexpressing a previously described Sec61 mutant with resistance to published compounds. In addition, 8834 did not affect the viability of HEK293 cells or endotoxin- or TCR-stimulated PBMCs. 8834 induced cell death in H929 multiple myeloma cells in vitro (IC50 = 98 nM) with rapid activation (≤8 hours) of caspase 3/7 via dual activation of caspases 8 and 9. When assessed in a panel of 346 human cancer cell lines across 25 different tumor types, head and neck squamous cell carcinoma, acute lymphocytic leukemia, non-Hodgkin’s lymphoma, melanoma, multiple myeloma, and prostate cancer were among the most sensitive tumor types. Weekly and twice-weekly administration of 8834 was effective in an H929 mouse xenograft model at doses that resulted in >90% tumor growth inhibition without significant body weight loss or clinical signs of toxicity. Anti-tumor activity was >50% at doses 1/3rd of the maximum tolerated dose, suggesting a broad therapeutic index. 8834 was also effective in the B16F0 melanoma model, showing tumor growth inhibition similar to that of anti-PD1 therapy. KZR-8834 is a nov