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......