Highly potent melanocortin MC4 receptor antagonist (Ki values are 0.29, 18.6, 5.45 and 3.29 nM for cloned human MC4, MC1, MC3 and MC5 receptors respectively). Increases food intake, and blocks α-MSH- and MTII-induced hypotension and bradycardia in rats, following central administration in vivo. MW (observed): 1266.5.