ptr-3173 and Pituitary-Neoplasms

Somatostatin (SST), an inhibitory polypeptide with two biologically active forms SST14 and SST28, inhibits GH, prolactin (PRL), TSH, and ACTH secretion in the anterior pituitary gland. SST also has an antiproliferative effect inducing cell cycle arrest and apoptosis. Such actions are mediated through five G-protein-coupled somatostatin receptors (SSTR): SSTR1-SSTR5. In GH-secreting adenomas, SSTR2 expression predominates, and somatostatin receptor ligands (SRLs; octreotide and lanreotide) directed to SSTR2 are presently the mainstays of medical therapy. However, about half of patients show incomplete biochemical remission, but the definition of resistance per se remains controversial. We summarize here the determinants of SRL resistance in acromegaly patients, including clinical, imaging features as well as molecular (mutations, SSTR variants, and polymorphisms), and histopathological (granulation pattern, and proteins and receptor expression) predictors. The role of SSTR5 may explain the partial responsiveness to SRLs in patients with adequate SSTR2 density in the cell membrane. In patients with ACTH-secreting pituitary adenomas, i.e. Cushing's disease (CD), SSTR5 is the most abundant receptor expressed and tumors show low SSTR2 density due to hypercortisolism-induced SSTR2 down-regulation. Clinical studies with pasireotide, a multireceptor-targeted SRL with increased SSTR5 activity, lead to approval of pasireotide for treatment of patients with CD. Other SRL delivery modes (oral octreotide), multireceptor-targeted SRL (somatoprim) or chimeric compounds targeting dopamine D2 receptors and SSTR2 (dopastatin), are briefly discussed.

Topics: Adenoma; Adrenocorticotropic Hormone; Apoptosis; Cell Cycle Checkpoints; Dopamine; Drug Resistance, Neoplasm; Hormones; Human Growth Hormone; Humans; Ligands; Pituitary Neoplasms; Prolactin; Receptors, Somatostatin; Signal Transduction; Somatostatin; Thyrotropin

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.

Topics: Adenoma; Cells, Cultured; Human Growth Hormone; Humans; Membrane Proteins; Octreotide; Pituitary Gland; Pituitary Neoplasms; Prolactin; Receptors, Somatostatin; Somatostatin; Tumor Cells, Cultured