Fenspiride-d5 is intended for use as an internal standard for the quantification of fenspiride by GC- or LC-MS. Fenspiride is an antagonist of histamine H1 receptors and a non-steroidal anti-inflammatory drug (NSAID). It inhibits histamine-induced contraction of isolated guinea pig trachea but not histamine-induced inotropy of isolated guinea pig heart. It also inhibits phosphodiesterase 4 (PDE4), PDE5, and PDE3 (IC50s = 69, ~158, and 363 µM, respectively, in isolated human bronchi derived from patients with lung cancer). It is selective for these phosphodiesterases over PDE1 and PDE2, where it provides less than 25% inhibition. Fenspiride potentiates the airway relaxant effects of isoproterenol and sodium nitroprusside indicating an effect on cAMP and cGMP phosphodiesterases, respectively. Aerosolized fenspiride (1 mg ml) reverses bronchoconstriction induced by capsaicin and, when used at aerosolized concentrations ranging from 1-10 mg ml, reduces cough induced by citric aci......