Potent and selective inhibitor of phosphodiesterase (PDE) 4 (IC50 = 12 nM for native PDE4). Competitively inhibits all PDE4 isoenzymes (IC50 values are 4, 9, 9 and 45 nM for PDE4A, PDE4B, PDE4C and PDE4D human recombinant isoforms expressed in yeast).