PTR-3173 (Somatoprim™), a novel somatostatin analog with affinity for somatostatin receptors 2, 4 and 5 is a potent inhibitor of human GH secretion

Abstract

SS, a natural cyclic tetradecapeptide, is a potent suppressor of pituitary GH and TSH secretion. At least five distinct SS receptor (SSTR) subtypes have been cloned and termed SSTRs 1–5. Both SSTR2 and SSTR5 regulate human GH and TSH secretion. Recently, a novel enzymatically stable SS analog, PTR-3173 (Somatoprim™), with affinity for human SSTR2, SSTR4 and SSTR5, has been identified. This cyclic heptapeptide analog suppressed rat GH in vivo with no effect on insulin and minimal effect on glucagon secretion. Using primary cultures of human fetal pituitaries (20–24- week gestation) and GH-secreting adenomas, we studied the in vitro inhibitory effects of PTR-3173 on human pituitary secretion. PTR-3173 suppressed GH release from both fetal pituitaries (maximal suppression of 54% with 10 nM) and cultures of GH-cell adenomas (35% suppression with 100 nM). Octreotide and PTR-3173 had comparable inhibitory effects on GH secretion from fetal human pituitaries. TSH was mildly suppressed by PTR-3173, whereas ACTH secretion was not affected in fetal pituitary cultures. In cultures of eight GH-secreting adenomas, octreotide was superior to PTR-3173 in suppressing GH from two adenomas, PTR-3173 was more potent in three other tumors, and three adenomas did not respond significantly to either analog. PTR-3173 suppressed PRL in several mixed GH-PRL adenomas. In conclusion, PTR-3173, a novel SS analog with a unique SSTRs binding combination, is a potent in vitro suppressor of human GH. Combining this inhibitory effect with the lack of effect on insulin secretion, it is suggested that PTR-3173 may be clinically useful for the treatment of acromegaly.