A noncompetitive, irreversible α-adrenergic receptor antagonist (IC50 = 34.2 nM for α1; Kis = 0.06, 0.01, and 0.06 µM for α2A, α2B, and α2C, respectively) with antihypertensive activity; covalently binds the adrenergic receptor, resulting in a blockade with slow onset and highly prolonged duration.