S6K1i and MCL-1i synergistically induce apoptosis of ex vivo cultured primary MM plasma cells. (A) Representative plots of flow cytometric analysis of apoptosis induction in CD38⁺ primary MM cells after 48 hours of exposure to the indicated concentrations MCL-1, dexamethasone (DEX), or S6K1i. Gates represent viable (DiOC6⁺/TO-PRO-3^–) and dead (DiOC6^–/TO-PRO-3⁺) cells. (B) Plots comparing expected (EXP) specific apoptosis vs observed (OBS) specific apoptosis induced by 48 hours of exposure to 100 nM DEX or 10 μM S6K1i, in combination with 10 nM or 100 nM MCL-1i in CD38⁺ primary MM cells as indicated in panel A. Statistical analysis was performed by using paired Student t tests. (C) Representative histogram and quantification plot showing S6 phosphorylation on serine residues 240/244 (pS6 S240/244) in CD38⁺ primary MM cells exposed to 100 nM DEX or 10 μM S6K1i for 8 hours, assessed by flow cytometry. The geometric mean fluorescent intensity (gMFI) was normalized to immunoglobulin G isotype control and relative to untreated control cells (ctr).