MEC1^MUT and OSU^MUT cells have attenuated sensitivity to idelalisib which is reversed by MAPK pathway inhibition. (A) Cell counts of MEC1 cells overexpressing WT or mutant MAP2K1, using CountBright absolute counting beads at 72 hours (n = 6). (B) CellTiter-glo luminescent cell viability was assessed at 96 hours on WT and mutant MAP2K1-expressing OSU-CLL cells. The cells were treated with the indicated doses of idelalisib (idela), a MEK1/2 inhibitor (CI-1040 or trametinib), or a combination of idelalisib with either CI-1040 or trametinib. (C) Proliferation of WT and mutant MAP2K1-expressing MEC1 cells was assessed following 48-hour serum starvation and labeling with the CellTrace Violet Proliferation Kit. The labeled cells were cultured on anti-IgM–coated plates for 7 days with vehicle control, idelalisib (1 μM or 10 μM), CI-1040 (1 μM or 10 μM), or the combination of idelalisib and CI-1040 (1 μM each or 10 μM each), and proliferation index was measured (n = 4). (D) OSU-CLL cells expressing WT and mutant MAP2K1 were synchronized by 48-hour serum starvation and labeled using the CellTrace Violet Proliferation Kit. The labeled cells were cultured with the indicated doses of vehicle control, idelalisib, CI-1040, trametinib, or the combination of idelalisib with either CI-1040 or trametinib and the percentage of divided cells was measured (n = 4). Data represented as mean ± SEM. ***P < .001; **P < .01; *P < .05.