TD-5471 hydrochloride is a potent and selective full agonist of the human β2-adrenoceptor. TD-5471 is potent, with slow onset in an in vitro guinea pig trachea model. TD-5471 has potency with pEC50 of 8.7 in the guinea pig tracheal ring assay.TD-5471 is structurally differentiated from Milveterol and its long duration of action is consistent with a correlation with hydrophobicity observed in other long-acting β2-agonist discovery programs. TD-5471 shows a dose-dependent and long duration of action in an in vivo guinea pig model of bronchoprotection. In a guinea pig model of in vivo bronchoprotection, TD-5471 shows excellent potency (measured 1.5 h after nebulized dosing). Because a high dose of Salmeterol (300 μg/mL) provides bronchoprotection lasting ≥24 h in this model, a 72 h time point is selected to provide confidence that TD-5471 will be suitable for once-daily dosing in man. TD-5471-mediated bronchoprotection at 72 h is dose-dependent and is significantly greater than that of Salmeterol at nebulizer concentrations of ≥30 μg/mL.