Purity >95%. CTAP is a water soluble, cyclic octapeptide which acts as a receptor antagonist that is selective for the μ opioid receptor (IC50 = 3.5 nM) over the Delta receptor (IC50 = 4,500 nM). It is a poor antagonist of the somatostatin receptor (IC50 = 14.3 μM). CTAP is at least 10-fold more potent than naltrexone as an antagonist of diverse compounds which have antinociceptive effects through the μ opioid receptor. It resists enzymatic metabolism in the blood and enters the brain and cerebrospinal fluid.