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.

Pamapimod inhibited p38, but inhibition of JNK was not detected. Pamapimod also inhibited lipopolysaccharide (LPS)-stimulated tumor necrosis factor (TNF) α production by monocytes, interleukin (IL)-1β production in human whole blood, and spontaneous TNFα production by synovial explants from RA patients[1].

In murine collagen-induced arthritis, pamapimod reduced Clinicalal signs of inflammation and bone loss at 50 mg/kg or greater. In a rat model of hyperalgesia, pamapimod increased tolerance to pressure in a dose-dependent manner, suggesting an important role of p38 in pain associated with inflammation. Pamapimod suppresses spontaneous production of TNFα by synovial explants from RA patients. LPS- and TNFα-stimulated production of TNFα and IL-6 in rodents also was inhibited by pamapimod[1].