OR04-6 Comprehensive Profiling of the MELK Inhibitor OTSSP167's Action and Its Effect on Sensitization to Wee1 Inhibitor AZD1775 in a Spectrum of ACC Models

Adrenocortical carcinoma (ACC) is an aggressive cancer with a significant risk of recurrence and high mortality rates. ACC has limited response to currently used treatments and there is an unmet clinical need to identify new treatment options. Disease rarity coupled with absence of preclinical models of disease heterogeneity contributed to limited therapeutic progress in ACC. We previously established new in vitro and in vivo ACC models (CUACC1, CUACC2, CUACC9) with variable genomic alterations commonly seen in ACC tumors. Prior work demonstrated that maternal leucine embryonic kinase (MELK) is an oncogenic kinase in ACC, and that it can be targeted with the MELK inhibitor OTSSP167. To further evaluate the anti-tumor efficacy and mode of action of OTSSP167, this study used genetically diverse ACC cell lines (CUACC1, CUACC2, H295R), patient derived xenografts (PDX) (CUACC1 and CUACC9) and a cell line xenograft model (H295R) of ACC. ACC PDXs treated with 10 mg/kg OTSSP167 had significantly reduced MELK expression, tumor growth inhibition of more than 80%, reduced mitosis, and increased tumor tissue necrosis. Transcriptomic and phospho-proteomic analysis at multiple treatment timepoints in in vitro models revealed that OTSSP167 induced wide transcriptomic and proteomic changes involving effectors of signal transduction, cell cycle, protein translation, and epigenetic regulation to exert its anti-tumorigenic effects. Evaluation of OTSSP167's role in protein synthesis via a puromycin based SUnSET (surface sensing of translation) and kinase assay established that OTSSP167 inhibits de-novo protein synthesis, and targets RSK1 and its substrate rpS6 (S235/S236). Varying the degree of MELK silencing in H295R cell lines using a doxycycline inducible shRNA construct, inhibited RSK1 phosphorylation at T573 and S380 indicating a MELK-RSK1 axis in ACC. Additionally, OTSSP167's effect on the DNA damage pathway was validated, which activated the G2/M checkpoint protein Wee1, increased inhibitory phosphorylation of CDK1, causing an accumulation of cyclin B1 and G2/M arrest independent of TP53 status. Combination treatment with the Wee1 inhibitor AZD1775 and OTSSP167 enhanced OTSSP167 mediated cytotoxic effects in in-vitro models with an increase in mitotic entry and caspase mediated apoptosis. In summary, OTSSP167 exerted its anti-tumorigenic effects via inhibition of the protein translation machinery, DNA damage and cell cycle dysregulation causing drug sensitization. Results from these studies are a first demonstration of the anti-cancer effects of OTSSP167 in sensitization to Wee1 inhibition in ACC. These data suggest that OTSSP167 is a potential therapeutic option for patients with ACC and could be combined with Wee1 inhibitors in a clinical trial using a synthetic lethal approach. Presentation: Saturday, June 11, 2022 12:45 p.m. - 1:00 p.m.