ABSTRACT
A cyclic somatostatin analog [structure: see text] (1) has been synthesized. Biological assays show that this compound has strong binding affinities to somatostatin hsst2 and hsst5 receptor subtypes (5.2 and 1.2 nM, respectively, and modest affinity to hsst4 (41.1 nM)). Our conformational analysis carried out in DMSO-d6 indicates that this compound exists as two structures arising from the trans and cis configurations of the peptide bond between Phe7 and N-alkylated Gly8. However, neither conformer exhibits a type II' beta-turn. This is the first report of a potent bioactive somatostatin analog that does not exhibit a type II' beta-turn in solution. Molecular dynamics simulations (500 ps) carried out at 300 K indicate that the backbone of compound 1 is more flexible than other cyclic somatostatin analogs formed by disulfide bonds.
Subject(s)
Peptides, Cyclic/chemistry , Peptides, Cyclic/chemical synthesis , Somatostatin/analogs & derivatives , Amino Acid Sequence , Humans , In Vitro Techniques , Magnetic Resonance Spectroscopy , Models, Molecular , Molecular Structure , Peptides, Cyclic/metabolism , Protein Conformation , Protein Structure, Secondary , Receptors, Somatostatin/metabolism , Solutions , Somatostatin/chemical synthesis , Somatostatin/chemistry , Somatostatin/metabolism , ThermodynamicsABSTRACT
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.
