Pamapimod (R1503) (R-1503, Ro4402257) is a novel, selective inhibitor of p38 mitogen-activated protein kinase. It inhibits p38α and p38β enzymatic activity with IC50 values of 0.014±0.002 and 0.48± 0.04 microM, respectively with no activity against p38delta or p38 gamma isoforms.