Inhibition of ERK phosphorylation was used as a pharmacodynamic biomarker for inhibition of MEK1/2 by selumetinib in ex vivo-stimulated PBMCs, a commonly used surrogate for tumor tissue in clinical trials.6,29 Blood samples were collected before selumetinib administration and 2 and 5 h after selumetinib administration (Figure 1A). Here, MAP2K1 is linked to neoplasm.