PDE4A and psoriatic arthritis: Typical embodiments are (S)-2-[1-(3-eth­oxy-4-meth­oxy­phen­yl)-2-(meth­yl­sulfon­yl)eth­yl]-4-acetamido­isoindoline-1,3-dione] with the generic name apremilast (AP), which is an inhibitor of phosphodiesterase 4 (PDE4) and is indicated for the treatment of adult patients with active psoriatic arthritis (Gottlieb et al., 2008 ▸; Man et al., 2009 ▸; Duplantier et al., 1996 ▸).