Somatostatin, also known as
somatotropin release-inhibiting factor (SRIF), is a natural
cyclic peptide inhibitor of pituitary, pancreatic, and gastrointestinal secretion. Its long-acting analogs are in clinical use for treatment of various endocrine syndromes and gastrointestinal anomalies. These analogs are more potent inhibitors of the endocrine release of GH,
glucagon, and
insulin than the native SRIF; hence, they do not display considerable physiological selectivity. Our goal was to design effective and physiologically selective SRIF analogs with potential therapeutic value. We employed an integrated approach consisting of screening of backbone
cyclic peptide libraries constructed on the basis of molecular modeling of known SRIF agonists and of high throughput receptor binding assays with each of the five cloned human SRIF receptors (hsst1-5). By using this approach, we identified a novel, high affinity, enzymatically stable, and long-acting SRIF analog,
PTR-3173, which binds with nanomolar affinity to human SRIF receptors hsst2, hsst4, and hsst5. The hsst5 and the rat sst5 (rsst5) forms have the same nanomolar affinity for this analog. In the human
carcinoid-derived cell line BON-1,
PTR-3173 inhibits
forskolin-stimulated cAMP accumulation as efficiently as the
drug octreotide, indicating its agonistic effect in this human cell system. In
hormone secretion studies with rats, we found that
PTR-3173 is 1000-fold and more than 10,000-fold more potent in inhibiting GH release than
glucagon and
insulin release, respectively. These results suggest that
PTR-3173 is the first highly selective somatostatinergic analog for the in vivo inhibition of GH secretion, with minimal or no effect on
glucagon and
insulin release, respectively.