Piclamilast (RP 73401) is a selective PDE4 inhibitor with IC50 of 1 nM at PDE4 from human neutrophils. Inhibition of PDE4 blocks hydrolysis of cAMP thereby increasing levels of cAMP within cells. cAMP suppresses the activity of immune and inflammatory cells. Piclamilast also inhibits LTB4 synthesis in human neutrophils with IC50 of 2 nM. PDE4 inhibitor Piclamilast (RP 73401) displays >19,000-fold selectivity over other PDE isoenzymes. It is comparable to other PDE4 inhibitors for its anti-inflammatory effects. Piclamilast pre-treatment significantly inhibited the changes in 23 genes via mechanisms involving AP-1 activation and c-Jun phosphorylation at Ser63.