TAS0612, a Triple Inhibitor for RSK, AKT, and S6K, Is a Promising Anti-Myeloma Agent Regardless of Types of Cytogenetic/Genetic Abnormality

Introduction: Despite advancements in therapeutic approaches, treatment outcome for multiple myeloma (MM) remains unsatisfactory. One critical factor of the poor outcomes is the complex inter-patient and intra-clonal heterogeneity of cytogenetic, genetic, and molecular abnormalities. To overcome this, the exploration of therapeutic target molecules that play a universal and essential role in MM regardless of the types of molecular profile is crucial even in the post-genomic era. We have previously demonstrated that RSK2 ^Ser227 at the N-terminal kinase domain (NTKD), as the downstream signaling hub of the RAS/ERK pathway, plays essential roles in the proliferation and survival of myeloma cells (Shimura Y, Mol Cancer Ther 2012). We also identified that RSK2-NTKD is constitutively activated by constitutive active PDPK1, the master serine-threonine kinase of >20 AGC kinases, in myeloma cells of approximately 90% of MM patients regardless of cytogenetic abnormalities (Chinen Y, Cancer Res 2014, Tatekawa S, Br J Haematol 2017). Moreover, the combinatory blockade of RSK2 and AKT, one of the other important molecules regulated by PDPK1, exhibited synergistic anti-myeloma efficacy through the modulation of various critical gene sets involved in myeloma pathophysiology, such as MYC (Isa R, Int J Mol Sci 2022). Based upon these, this study investigates the anti-myeloma potency of TAS0612 (Taiho Oncology Inc., Princeton, NJ) that targets RSK, including RSK2, AKT, and S6K, and inhibits the bindings to their respective substrate molecules (Shibata T, Mol Cancer Ther 2020). Methods and Results: Firstly, we examined the activation status of RSK, AKT, and S6K in eight human myeloma-derived cell lines (HMCLs) with diverse genetic and molecular profiles at the protein level. RSK2 ^Ser227 was found to be commonly activated through phosphorylation in all HMCLs examined. In contrast, the activation patterns of p-AKT ^Ser473 and p-S6K ^Thr389 were inconsistent among eight HMCLs, and depending on cell types. Despite these cell type-dependent effects, the exposure to TAS0612 resulted in the accumulation of p-total RSK ^Ser380, p-AKT ^Ser473, and p-S6K ^Thr389 in the majority of HMCLs examined, and this led to the subsequent inactivation of their respective target substrates, such as p-YB1 ^Ser102, p-PRAS ^Thr246, and p-S6 ^Ser235/236, through dephosphorylation. In accompany with these molecular effects, the modified MTT assay showed the dose-dependent growth inhibitory effects of TAS0612 in all cell lines with various types of cytogenetic/genetic abnormalities with IC50 values mostly below 0.5 μM (48 hours). In addition, flow cytometry analyses for the cell cycle analysis and the apoptosis assay by the counterstaining of propidium iodide and Annexin-V showed that TAS0612 induced G1/S cell cycle arrest and apoptotic cell death. Furthermore, patient-derived primary myeloma cells were labeled with anti-CD138 MicroBeads, and were positively isolated from bone marrow mononuclear cells from patients with newly diagnosed MM (n=7) and relapsed/refractory MM (n=11), and, then, were treated by TAS0612. As the results, the ex vivo treatment of 2.5-5.0 μM TAS0612 for 72 hours showed robust cell reductive effects on primary-myeloma cells from all 18 patients examined irrespective of cytogenetic profile. To clarify the precise molecular effects of TAS0612 treatment in myeloma cells, we conducted a comprehensive molecular analysis with TAS0612 treatment by gene expression profiling. The results showed that TAS0612 treatment significantly modulated the gene sets involved in Myc signaling, mTOR signaling, and the core serum response (CSR). Additionally, TAS0612 caused the concomitant upregulation of several bona-fide tumor suppressor genes, including TP53INP, HBP1, FBOX32, and YPEL3, while the downregulation of oncogenes LYAR and HSPA1. Intriguingly, according to MMRF CoMMpass Study, 8 of 18 upregulated genes by TAS0612 in HMCLs were reported to be associated with favorable survival outcomes when highly expressed in myeloma cells. Conclusion: Our findings strongly support the potential of TAS0612 as an effective anti-myeloma agent. Further preclinical investigations are necessary to substantiate these results and facilitate its future clinical development.