Selective, high affinity tachykinin NK1 receptor antagonist (Ki = 0.69 nM) that displays 320-fold selectivity for human over rat NK1 receptors. Inhibits substance P-induced contraction of isolated guinea pig trachea (IC50 = 32 nM) and inhibits substance P-induced airway constriction in vivo following i.v. and oral administration. Purity 98%. MW (observed): 588.69