Highly potent κ opioid receptor antagonist (Ki = 0.18 nM for human cloned κ receptors expressed in CHO cells). Displays 208- and 799-fold selectivity over μ and δ receptors respectively. Reduces feeding behavior in rats with a much higher potency (300-30,000-fold) and a shorter duration of action than nor-binaltorphimine (Cat.No. 0347).