Effects of highly potent octapeptide analogs of somatostatin on growth hormone, insulin and glucagon release

Neural control of pancreatic peptide hormone secretion

Pancreatic peptide hormone secretion is inextricably linked to maintenance of normal levels of blood glucose. In animals and man, pancreatic peptide hormone secretion is controlled, at least in part, by input from parasympathetic (vagal) premotor neurons that are found principally in the dorsal motor nucleus of the vagus (DMV). Iatrogenic (insulin-induced) hypoglycaemia evokes a homeostatic response commonly referred to as the glucose counter-regulatory response. This homeostatic response is of particular importance in Type 1 diabetes in which episodes of hypoglycaemia are common, debilitating and lead to suboptimal control of blood glucose. Glucagon is the principal counterregulatory hormone but for reasons unknown, its secretion during insulin-induced hypoglycaemia is impaired. Pancreatic parasympathetic neurons are distinguishable electrophysiologically from those that control other (e.g. gastric) functions and are controlled by supramedullary inputs from hypothalamic structures such as the perifornical region. During hypoglycaemia, glucose-sensitive, GABAergic neurons in the ventromedial hypothalamus are inhibited leading to disinhibition of perifornical orexin neurons with projections to the DMV which, in turn, leads to increased secretion of glucagon.

Enteroendocrine K Cells Exert Complementary Effects to Control Bone Quality and Mass in Mice

The involvement of a gut-bone axis in controlling bone physiology has been long suspected, although the exact mechanisms are unclear. We explored whether glucose-dependent insulinotropic polypeptide (GIP)-producing enteroendocrine K cells were involved in this process. The bone phenotype of transgenic mouse models lacking GIP secretion (GIP-GFP-KI) or enteroendocrine K cells (GIP-DT) was investigated. Mice deficient in GIP secretion exhibited lower bone strength, trabecular bone mass, trabecular number, and cortical thickness, notably due to higher bone resorption. Alterations of microstructure, modifications of bone compositional parameters, represented by lower collagen cross-linking, were also apparent. None of these alterations were observed in GIP-DT mice lacking enteroendocrine K cells, suggesting that another K-cell secretory product acts to counteract GIP action. To assess this, stable analogues of the known K-cell peptide hormones, xenin and GIP, were administered to mature NIH Swiss male mice. Both were capable of modulating bone strength mostly by altering bone microstructure, bone gene expression, and bone compositional parameters. However, the two molecules exhibited opposite actions on bone physiology, with evidence that xenin effects are mediated indirectly, possibly via neural networks. Our data highlight a previously unknown interaction between GIP and xenin, which both moderate gut-bone connectivity. © 2020 American Society for Bone and Mineral Research.

Relationship between clinical characteristics and survival of gastroenteropancreatic neuroendocrine neoplasms: A single-institution analysis (1995-2012) in South China

BACKGROUND

Gastroenteropancreatic neuroendocrine neoplasm (GEP-NEN) is the most common type of neuroendocrine tumors accounting for 65-75% of neuroendocrine neoplasms (NENs). Given the fact that there are few studies on GEP-NENs among Chinese patients, we performed a retrospective study in South China.

METHODS

Totally 178 patients with GEP-NENs treated at the First Affiliated Hospital of Sun Yat-sen University between January 1995 and May 2012 were analyzed retrospectively.

RESULTS

Pancreas was found the most common site of involvement (34.8%). 149 patients (83.7%) presented as non-functional tumors with non-specific symptoms such as abdominal pain (33.7%); carcinoid syndrome was not found in this study. Several methods are useful for localization of GEP-NENs, yielding varied detection rates from 77.8% to 98.7%. Positive rates of chromogranin A (CgA) and synaptophysin (Syn) immunhistochemically were 69.1% and 90.2%, respectively. 87 patients (51.5%) had G1 tumors, 31(18.3%) G2 tumors and 51 (30.2%) G3 tumors. Neuroendocrine tumor (NET), neuroendocrine carcinoma (NEC) and mixed adenoendocrine carcinoma (MANEC) were 69.8%, 27.2% and 3.0%, respectively. 28.1% of patients presented with distant disease. Surgery was performed in 152 (85.4%) patients, and overall 5-year survival rate was 54.5%. Functionality, G1 grading and NET classification were associated with favorable prognosis in univariate analysis. Distant metastasis contributed to unfavorable prognosis of these tumors.

CONCLUSIONS

Nonfunctional tumors with non-specific symptoms account for the majority of GEP-NENs. Diagnosis depends on pathological classification. Multidisciplinary treatments could help improve the outcome.

Relationship between clinical characteristics and survival of gastroenteropancreatic neuroendocrine neoplasms: A single-institution analysis (1995-2012) in South China

BACKGROUND

Gastroenteropancreatic neuroendocrine neoplasm (GEP-NEN) is the most common type of neuroendocrine tumors accounting for 65-75% of neuroendocrine neoplasms (NENs). Given the fact that there are few studies on GEP-NENs among Chinese patients, we performed a retrospective study in South China.

METHODS

Totally 178 patients with GEP-NENs treated at the First Affiliated Hospital of Sun Yat-sen University between January 1995 and May 2012 were analyzed retrospectively.

RESULTS

Pancreas was found the most common site of involvement (34.8%). 149 patients (83.7%) presented as non-functional tumors with non-specific symptoms such as abdominal pain (33.7%); carcinoid syndrome was not found in this study. Several methods are useful for localization of GEP-NENs, yielding varied detection rates from 77.8% to 98.7%. Positive rates of chromogranin A (CgA) and synaptophysin (Syn) immunhistochemically were 69.1% and 90.2%, respectively. 87 patients (51.5%) had G1 tumors, 31(18.3%) G2 tumors and 51 (30.2%) G3 tumors. Neuroendocrine tumor (NET), neuroendocrine carcinoma (NEC) and mixed adenoendocrine carcinoma (MANEC) were 69.8%, 27.2% and 3.0%, respectively. 28.1% of patients presented with distant disease. Surgery was performed in 152 (85.4%) patients, and overall 5-year survival rate was 54.5%. Functionality, G1 grading and NET classification were associated with favorable prognosis in univariate analysis. Distant metastasis contributed to unfavorable prognosis of these tumors.

CONCLUSIONS

Nonfunctional tumors with non-specific symptoms account for the majority of GEP-NENs. Diagnosis depends on pathological classification. Multidisciplinary treatments could help improve the outcome.

Somatostatin analogs for the treatment of neuroendocrine tumors

Somatostatin is an important regulator of endocrine and exocrine secretion, affecting the release of many hormones. The effects of somatostatin are mediated through its interaction with one of five somatostatin receptors. Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) express multiple somatostatin receptors, making them excellent potential therapeutic targets. Many trials have shown that treatment with somatostatin analogs is associated with disease stabilization and prolonged survival. More recently, somatostatin analogs have been shown to have antiproliferative effects, thus broadening the scope of their uses. In this review, we update the current data on the treatment of GEP-NETs with somatostatin analogs, with particular emphasis on the results of the PROMID study. In addition, we discuss the current state of knowledge of novel therapies against GEP-NETs, including the use of somatostatin analogs with broader receptor binding profiles, chimeric somatostatin-dopamine molecules, combinations of somatostatin analogs with other active chemotherapy agents, and peptide receptor-targeted radionuclide therapy.

Reversible Lipidization Prolongs the Pharmacological Effect, Plasma Duration, and Liver Retention of Octreotide

PURPOSE

Octreotide (OCT) was reversibly lipidized to improve the pharmacological effect and to increase the plasma half-life and the liver retention of OCT for greater therapeutic potential in the treatment of liver cancers such as hepatocellular carcinoma.

METHODS

OCT was chemically modified using reversible aqueous lipidization (REAL) technology. REAL-modified OCT (REAL-OCT) was characterized with high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. A single dose of OCT or REAL-OCT or vehicle only was subcutaneously administered to male Sprague-Dawley rats, and the plasma growth hormone (GH) levels were measured after an intravenous injection of 2.5 microg/kg of growth hormone releasing factor (GRF) to assess the ability of REAL-OCT on GH inhibition. Radio-iodinated Tyr3-OCT (TOC) and REAL-TOC were used for pharmacokinetic studies.

RESULTS

At 0.1 mg/kg, REAL-OCT inhibited the GRF-induced GH surge in rats for a greater than 24-h period in comparison to the 6-h period for OCT. The distribution and elimination half-life for 125I-REAL-TOC were 1.4 h and 6.6 h, respectively, which were significantly longer than those of 125I-TOC. Sustained high blood concentrations and reduced in vivo degradation were observed for 125I-REAL-TOC. In addition, 125I-REAL-TOC appeared to be targeted to the liver with persistent high liver retention.

CONCLUSIONS

REAL-OCT has a significantly enhanced pharmacological effect, and this is most likely due to the favorable changes in the pharmacokinetic parameters upon lipidization. The observed liver targeting effect of REAL-TOC suggests that REAL-OCT might be advantageous over OCT in treating liver cancers.

A novel somatostatin mimic with broad somatotropin release inhibitory factor receptor binding and superior therapeutic potential

A rational drug design approach, capitalizing on structure-activity relationships and involving transposition of functional groups from somatotropin release inhibitory factor (SRIF) into a reduced size cyclohexapeptide template, has led to the discovery of SOM230 (25), a novel, stable cyclohexapeptide somatostatin mimic that exhibits unique high-affinity binding to human somatostatin receptors (subtypes sst1-sst5). SOM230 has potent, long-lasting inhibitory effects on growth hormone and insulin-like growth factor-1 release and is a promising development candidate currently under evaluation in phase I clinical trials.

Endocrine effects of new bombesin/gastrin-releasing peptide antagonists in rats

Four new and specific pseudononapeptide bombesin/gastrin-releasing peptide (GRP) receptor antagonists, containing the D-forms of Trp or Trp analogue (Tpi) at position 6, were studied for their effects on the endocrine pancreas and GRP-(14-27)-induced gastrin release in pentobarbital-anesthetized rats. One of the analogues, D-Tpi6,Leu13-psi (CH2NH)Leu14-bombesin-(6-14) (RC-3095), was injected into the lateral brain ventricle just preceding intracerebroventricular administration of GRP-(14-27) to evaluate its antagonistic effect on GRP-induced serum growth hormone (GH) suppression. Analogues RC-3095, D-Trp6,Leu13-psi (CH2NH)Leu14-bombesin-(6-14) (RC-3125), and D-Trp6,Leu13-psi (CH2NH)Phe14-bombesin-(6-14) (RC-3420), but not D-Tpi6,Leu13-psi (CH2NH)Phe14-bombesin-(6-14) (RC-3105), significantly (P < 0.01) inhibited GRP-(14-27)-stimulated serum gastrin secretion. Analogues RC-3095, RC-3420, and RC-3105, but not RC-3125, demonstrated significant (P < 0.05) antagonistic activities on GRP-(14-27)-stimulated plasma glucagon secretion. Intracerebroventricular injection of RC-3095 (10 micrograms) immediately before GRP-(14-27) (1 microgram) completely prevented the GRP-(14-27)-induced serum GH suppression. These results indicate that 1) marked differences exist in the ability of these analogues to antagonize GRP-(14-27)-induced gastrin or glucagon release, suggesting the existence of different bombesin/GRP receptor subtypes, and 2) the central effect of bombesin/GRP on GH release from the pituitary is probably mediated through specific bombesin/GRP receptors.

Permeability of the murine blood-brain barrier to some octapeptide analogs of somatostatin

Analogs of somatostatin are being investigated clinically for the treatment of various malignancies, including brain tumors. We studied the ability of three therapeutically promising radioactively labeled somatostatin octapeptide analogs, RC-160, RC-121, and RC-161, to cross the blood-brain barrier (BBB) after peripheral or central injection. After i.v. injection, intact RC-160 was recovered from the blood and the brain. The entry rates were different from each compound but were generally low. By contrast, entry across the intact BBB increased 220 times when RC-160 was given in a serum-free perfusate. This suggests that some serum-related factor, probably the previously described protein binding or an aggregation-promoting factor, is the main determinant in limiting the blood-to-brain passage of somatostatin analogs. Entry into the brain was not inhibited by the addition of unlabeled analog to the perfusate, showing that passage was probably by diffusion across the membranes that comprise the BBB rather than by saturable transport. By contrast, a saturable system was found to transport peptide out of the central nervous system (CNS). The clearance from the CNS of RC-160 and RC-121, but not RC-161, was faster than could be accounted for by reabsorption of cerebrospinal fluid. Transport of radioactively labeled RC-160 out of the CNS was inhibited by unlabeled RC-160 or somatostatin but was not affected by some other peptide known to cross the BBB by their own transport systems. More than 80% of the radioactivity recovered from the blood after intracerebroventricular injection of RC-160 was eluted by HPLC at the position of the labeled analog, showing that the peptide had crossed the BBB in intact form. Our results indicate the presence of a saturable transport system in one direction across the BBB for some superactive analogs of somatostatin.

Effect of somatostatin analogs on gastric acid secretion in dogs and rats

The effects of several superactive analogs of somatostatin on gastric acid response to various exogenous and endogenous stimulants were investigated in conscious dogs and rats with gastric fistulae (GF). The inhibition was compared to that induced by somatostatin-14 (S-S-14) at two dose levels. Several octapeptide analogs of somatostatin including D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160) and D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2 (RC-121), which were superactive in tests on suppression of GH levels, were 4-5 times more potent than S-S-14 in inhibiting desglugastrin-stimulated gastric acid secretion in GF dogs. The analog RC-160 also reduced the rise in serum gastrin levels and gastric acid secretion induced by sham feeding (SF) in dogs with gastric and esophageal fistulae (EF), but did not decrease food consumption. Gastric acid secretion induced by histamine (80 micrograms/kg/h) in dogs was not affected by 1-5 micrograms/kg/h of analog RC-121 or by 5 micrograms/kg/h of S-S-14. Analogs RC-160, RC-121, and RC-98-I (D-Trp-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-NH2) and others also powerfully inhibited desglugastrin-induced potent as S-S-14 in dogs but its activity was higher in rats. The results indicate that octapeptide analogs which are superactive in GH-inhibition tests are also more potent than S-S-14 in suppressing gastric acid secretion. These findings may be of clinical value.

Radioimmunoassay for octapeptide analogs of somatostatin: measurement of serum levels after administration of long-acting microcapsule formulations

The development of a long-acting delivery system for D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160), an octapeptide analog of somatostatin, required the establishment of a method for determining the concentration of this analog in serum during treatment. A sensitive and specific radioimmunoassay (RIA) for RC-160 was developed and used for following the rate of liberation of this peptide from microcapsules of poly(DL-lactide-coglycolide). Antibodies were generated in a rabbit against RC-160 conjugated to bovine serum albumin with glutaraldehyde. At an antiserum dilution of 1:100,000, the antibodies bound approximately 25% of added radiolabeled RC-160. Somatostatin octapeptide analogs that had a disulfide bridge showed crossreactivity with the antiserum, but analogs without the disulfide bridge and other peptides tested did not crossreact. The minimum detectable dose of RC-160 was 10 pg. Intra- and interassay coefficients of variation ranged from 9.1% to 12.8% and from 14% to 30%, respectively. The RIA was suitable for direct determination of RC-160 in serum. Eleven prototype batches of microcapsules were tested in rats, and the rate of release of the analog from the microcapsules was followed. An improved batch of microcapsules made from RC-160 pamoate maintained high serum levels of RC-160 for more than 30 days after intramuscular injection. The RIA should be of value for monitoring levels of this analog in serum during long-term therapy.

Superactive somatostatin analog decreases plasma glucose and glucagon levels in diabetic rats

The action of the new analog of somatostatin, D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Trp-NH2 (RC-160), on plasma glucagon and glucose levels was evaluated in streptozotocin-diabetic rats. The effect of this analog on the insulin-induced hypoglycemia in diabetic rats was also investigated in order to evaluate the risk of exacerbating hypoglycemia. Administration of analog RC-160, in a dose of 25 micrograms/kg b. wt. SC, inhibited plasma glucagon secretion and decreased plasma glucose levels. This effect also occurred when plasma glucagon and glucose levels were first elevated by arginine infusion, 1000 mg/kg/hr for 30 min. Subcutaneous injection of regular insulin, 15 U/kg b. wt., produced hypoglycemia with a progressive increase in glucagon levels. Analog RC-160 completely suppressed the hypoglycemia-induced glucagon release for up to 150 min after injection of the analog or insulin. A greater decrease in the plasma glucose level was observed in the group treated with insulin and the analog than in the group injected only with insulin. These results indicate that somatostatin analog RC-160 can produce a marked and prolonged inhibition of glucagon release and a decrease in the plasma glucose level in diabetic rats. This analog may be useful as an adjunct to insulin in the treatment of diabetic patients, although caution should be exercised, to prevent hypoglycemia when using somatostatin analogs together with insulin.