ERK1/2 activation promotes prosurvival signaling in ibrutinib-resistant MYD88-mutated WM and ABC DLBCL cells. Immunoblotting studies depicting BTK, PLCγ2, ERK1/2, and p90RSK signaling in nontransduced, vector only, BTK^WT, or BTK^Cys481Ser (BTK^C481S) MYD88-mutated WM (BCWM.1) and ABC DLBCL (TMD8) cells following treatment with vehicle control, ibrutinib, or ERK1/2 inhibitors (ulixerinib, GDC-0994) alone or with ibrutinib for 2 hours. GAPDH was used as protein loading control. Data for ulixertinib are shown in panel A and for GDC-0994 in panel B. Cellular apoptosis determined by Annexin V-FITC and PI staining for BTK^WT or BTK^C481S expressing WM and ABC DLBCL cells following treatment with vehicle control, ibrutinib (1.0 µM), or ulixertinib (2.0 µM) alone or in combination. Percent of cells staining for both Annexin V and PI is depicted in each panel. Results are representative from studies performed in triplicate (C). Dose-dependent survival determined by CellTiter-Glo Luminescent cell viability assay for vector only, BTK^WT, or BTK^C481S transduced MYD88-mutated WM and ABC DLBCL cells following treatment with ibrutinib and ulixertinib at pharmacologically relevant dosimetry for 72 hours. Synergism was assessed by combination index (CI) analysis, with the heat maps depicting the CI values at varying dosimetry for ibrutinib and ulixertinib. CI values <1 denote synergistic interactions (D). IB, ibrutinib.