CYN 154806 (TFA), a cyclic octapeptide, is a potent and selective somatostatin sst2 receptor antagonist, with pIC50 values of 8.58, 5.41, 6.07, 5.76 and 6.48 for human recombinant sst2, sst1, sst3, sst4 and sst5 receptors respectively.