Tipelukast (KCA 757) is a sulfidopeptide leukotriene receptor antagonist, an orally bioavailable anti-inflammatory agent and used for the treatment of asthma. In Vitro: Tipelukast inhibits the binding of [³H] LTD4 to the LTD4 receptors on pul-monary cell membrane of guinea-pigs (IC₅₀ = 2.3 μmol)^[2]. In Vivo: Fiftheen min after an aerosolized antigen challenge, and UNDW inhaled 5 min later into the guinea pigs, Tipelukast significantly alters the UNDW-induced bronchoconstriction^[1]. Tipelukast (1 and 5 mg/kg) administered intravenously 15 min after antigen challenge reduces the propranolol-induced bronchoconstriction (PIB) in a dose-dependent manner in guinea-pigs^[2].