Efficacy, pharmacokinetics and tolerability of a somatostatin analogue (L-363,586) in insulin-dependent diabetes mellitus

To assess the pharmacologic properties and possible use in the treatment of diabetes mellitus of a recently developed analogue somatostatin (L-363,586), the analogue (2, 5, 10 or 40 micrograms/hr), somatostatin (200 micrograms/hr), or placebo were infused intravenously for 5 hours in 6 insulin-dependent diabetic subjects who were given a standard meal containing xylose. The metabolic clearance rate of the analogue (approximately 300 ml/min) was 1/6 that previously reported for somatostatin (approximately 2000 ml/min) and its half-life was approximately 20 times as great as that reported for somatostatin (45 vs 2 min). At a dose of 10 micrograms/hr, the analogue produced suppression of plasma glucagon, growth hormone, glucose, xylose and triglyceride responses to meal ingestion which were comparable to those observed when somatostatin was infused at a rate of 200 micrograms/hr. We conclude that L-363,586 is a long-acting and potent analogue of somatostatin, which has the potential for use as an adjunct to insulin in the treatment of diabetes mellitus.     

Role of cholecystokinin in the control of gastric somatostatin in the rat: in vivo and in vitro studies.

Cholecystokinin (CCK) has been shown to be a powerful stimulus for somatostatin release from isolated canine fundic D-cells in short-term culture. The influence of the CCK analogue caerulein on the secretory activity of the D-cell in the intact stomach in vitro and the effect of elevated plasma levels of endogenous CCK on gastric somatostatin stores in vivo were investigated in the rat. Basal somatostatin secretion from the isolated, vascularly perfused rat stomach preparation was not affected by various doses of caerulein. Slight stimulation of somatostatin-like immunoreactivity (SLI) release by epinephrine was significantly inhibited by caerulein, whereas caerulein did not alter half-maximal stimulation of SLI secretion by isoproterenol. Rats with chronically elevated plasma CCK levels induced by experimental exocrine pancreatic insufficiency did not show any change in tissue concentrations of SLI or in D-cell number, both in the antrum and corpus. These data suggest that CCK--in contrast to dogs--is not an important modulator of gastric somatostatin in the rat.     

Effects of the Long-acting Somatostatin Analogue Octreotide on Abomasal Function and Plasma Level of Insulin and Glucagon in Sheep

The effects of the long-acting somatostatin analogue octreotide were studied in sheep. Octreotide was given subcutaneously at a dose of 0.75 ug/kg body-weight and, as a control, 0.9% saline solution was injected in a change-over design. Octreotide inhibited abomasal acid secretion and retarded the turnover time of digesta through the abomasum. The plasma levels of insulin and glucagon decreased due to the octreotide injection, while the plasma glucose level was not affected. The effects of octreotide lasted for 3-4h. There were no significant effects of the saline injection. The effects of octreotide showed similarities with results from previous studies on monogas-tric species.     

In vivo application of [111In-DTPA-D-Phe1]-octreotide for detection of somatostatin receptor-positive tumors in rats.

Radioiodinated somatostatin analogues are useful ligands for the in vitro and in vivo detection of somatostatin receptors. [111In-DTPA-D-Phe1]-octreotide, a somatostatin analogue labeled with a different radionuclide, also binds specifically to somatostatin receptors in vitro. In this study we investigated its in vivo application in the visualization of somatostatin receptor-positive tumors in rats. The distribution of the radiopharmaceutical was investigated after intravenous injection in normal rats and in rats bearing the somatostatin receptor-positive rat pancreatic carcinoma CA 20948. After injection the radiopharmaceutical was rapidly cleared (50% decrease in maximal blood radioactivity in 4 min), predominantly by the kidneys. Excreted radioactivity was mainly in the form of the intact radiopharmaceutical. Ex vivo autoradiographic studies showed that specific accumulation of radioactivity occurred in somatostatin receptor-containing tissue (anterior pituitary gland). However, in contrast to the adrenals and pituitary, the tracer accumulation in the kidneys was not mediated by somatostatin receptors. Increasing radioactivity over the somatostatin receptor-positive tumors was measured rapidly after injection and the tumors were clearly visualized by gamma camera scintigraphy. In rats pretreated with 1 mg octreotide accumulation of [111In-DTPA-D-Phe1]-octreotide in the tumors was prevented. Because of its relatively long effective half-life, [111In-DTPA-D-Phe1]-octreotide is a radionuclide-coupled somatostatin analogue which can be used to visualize somatostatin receptor-bearing tumors efficiently after 24 hr, when interfering background radioactivity is minimized by renal clearance. This is an advantage over the previously used [123I-Tyr3]-octreotide which has a shorter effective half-life and shows high abdominal interference due to its hepato-biliary clearance. Therefore, [111In-DTPA-D-Phe1]-octreotide seems a better alternative for scintigraphic imaging of somatostatin receptor-bearing tumors.     

The mechanism of degradation of cyclo(-Asn-Phe-Phe-D-Trp-Lys-Thr-Phe-Gaba-) and the relative stabilities of this and other octapeptide somatostatin analogues in rat intestinal juice

The mechanism of degradation of the somatostatin analogue cyclo(-Asn-Phe-4-[3H]-Phe-D-Trp-Lys-Thr-Phe-Gaba-) in rat intestinal juice in vitro has been studied by isolation and identification of cleavage products. The analogue is much more stable than somatostatin but is nevertheless degraded in minutes in dilute intestinal juice. There is a single primary cleavage site at Lys-Thr and the linear peptide thus formed is subsequently rapidly degraded to smaller peptides. Analogues with a modified lysine are markedly stabilised relative to the octapeptide analogue.     

Secretory diarrhea in villous adenoma of rectum: effect of treatment with somatostatin and indomethacin.

The effects of treatment with the synthetic long-acting somatostatin analogue SMS-201-995 were studied in a patient with a fluid and electrolyte secreting villous adenoma of the rectum. The effects of SMS-201-995 on rectal fluid volume and electrolyte loss, and local and general prostanoid production were compared with those of treatment with indomethacin. During treatment with the somatostatin analogue iso-osmolar rectal fluid production increased about 25%; the quantity of prostaglandin E2 in the rectal fluid rose almost 20-fold. Prostaglandin F2 alpha, 6-keto-prostaglandin F1 alpha and 13,14-dihydro-15-keto-prostaglandin F2 alpha output showed similar, though less impressive increments during somatostatin treatment. The somatostatin analogue did not affect urinary prostanoid excretion except for levels of 2,3-dinor-thromboxane B2, which doubled. With indomethacin treatment diurnal rectal fluid production dropped by about 50% and all prostanoids measured in urine and rectal fluid decreased below control values. It appears that the somatostatin analogue SMS-201-995 has a marked stimulatory effect on the in vivo prostanoid production by the villous adenoma. Perhaps this stimulation is not confined to the tumor only, but also affects thromboxane synthesis.     

Synergistic regulation of cytosolic Ca2+ concentration by somatostatin and alpha 1-adrenergic agonists in mouse astrocytes.

The effects of somatostatin and alpha 1-adrenergic receptor agonists on cytosolic Ca2+ in striatal astrocytes from the embryonic mouse in primary culture have been investigated by microfluorimetry. Methoxamine or somatostatin induced a transitory increase in cytosolic Ca2+, but their combined addition led to a sustained increase in cytosolic Ca2+ which seems to be due to a Ca2+ influx since it was not observed in the absence of external Ca2+. Voltage-independent Ca2+ channels contribute to this process. Indeed, voltage-operated calcium channels are not involved since neither dihydropyridines nor La3+ were effective in suppressing the sustained cytosolic Ca2+ elevation. Moreover, depolarization by 50 mM KCl, which was ineffective alone, suppressed the effect of somatostatin observed in the presence of the alpha 1 agonist, methoxamine. The implication of arachidonic acid in the observed potentiation is suggested by the following observations: 1) arachidonic acid induced a sustained elevation of cytosolic Ca2+ similar to that evoked by the co-application of methoxamine and somatostatin; 2) the addition of ETYA, an inactive and non-metabolizable analogue of arachidonic acid suppressed the calcium plateau produced by the agonists. In addition, direct activation of PKC by an exogeneous diacylglycerol analogue allowed somatostatin alone to evoke a sustained elevation of cytosolic Ca2+. Therefore, methoxamine through the successive activation of PLC and PKC could allow a lipase, probably PLA2, to be stimulated by somatostatin. Since arachidonic acid has already been shown to trigger the opening of K+ channels and the formation of inositol phosphates, somatostatin, through the arachidonic acid-mediated hyperpolarization could increase the Ca2+ driving force and thus improve Ca2+ influx through the inositol phosphate gated channels.     

Strategies to improve plasma half life time of peptide and protein drugs

Summary. Due to the obvious advantages of long-acting peptide and protein drugs, strategies to prolong plasma half life time of such compounds are highly on demand. Short plasma half life times are commonly due to fast renal clearance as well as to enzymatic degradation occurring during systemic circulation. Modifications of the peptide/protein can lead to prolonged plasma half life times. By shortening the overall amino acid amount of somatostatin and replacing l-analogue amino acids with d-amino acids, plasma half life time of the derivate octreotide was 1.5 hours in comparison to only few minutes of somatostatin. A PEG_2,40 K conjugate of INF-α-2b exhibited a 330-fold prolonged plasma half life time compared to the native protein. It was the aim of this review to provide an overview of possible strategies to prolong plasma half life time such as modification of N- and C-terminus or PEGylation as well as methods to evaluate the effectiveness of drug modifications. Furthermore, fundamental data about most important proteolytic enzymes of human blood, liver and kidney as well as their cleavage specificity and inhibitors for them are provided in order to predict enzymatic cleavage of peptide and protein drugs during systemic circulation.     

Metabolism of somatostatin and its analogues by the liver

The rate of degradation of ¹²⁵I-labelled [Tyr¹¹]somatostatin by isolated rat hepatocytes was similar to that of unlabelled somatostatin. Reaction was dependent upon cell concentration and temperature, being rapid at 37°C and negligible at 0°C. The apparent K_m for the overall degradation process was approximately the same for degradation by hepatocytes and by partially-purified liver plasma membranes. Extracellular breakdown of somatostatin, by proteases released from cells into the incubation medium, represented less than 10% of the cell-associated degradation. Homogenization of hepatocytes resulted in a 10–20-fold increase in the degrading ability of the cells. After incubation of ¹²⁵I-labelled [Tyr¹¹]somatostatin and ¹²⁵I-labelled [Tyr¹]somatostatin with hepatocytes, ¹²⁵I-labelled tyrosine was the major radioactive product identified in the incubation medium. The rate of release of ¹²⁵I-labelled tyrosine from the labelled [Tyr¹] analogue was approximately 11 times greater than from the labelled [Tyr¹¹] analogue. ¹²⁵I-labelled [Tyr¹¹]somatostatin bound to the cells in a non-saturable manner and approx. 70% of the cell-associated radioactivity could be dissociated by dilute acid. The rate of degradation of somatostatin was unchanged by reagents that inhibit the internalisation and lysosomal degradation of polypeptides by cell suspensions but was reduced by reagents that inhibit sulphydryl-dependent proteases. It is proposed that plasma-membrane associated proteolysis, involving both endo- and exopeptidases may represent the predominant degradative pathway of somatostatin in vivo.     

Postprandial glycaemic effects of a long-acting somatostatin analogue (octreotide) in non-insulin dependent diabetes mellitus

Postprandial changes in blood glucose, insulin and glucagon were examined in 7 non-insulin dependent diabetic patients, before and after 3 days' treatment with the somatostatin analogue, octreotide (50 ug injected subcutaneously thricedaily). After octreotide injection, postprandial rises in plasma insulin and glucagon were significantly flattened. The postprandial glycaemic rise was delayed but the area under the glycaemic curve was not increased. Animal studies have suggested that octreotide inhibits growth hormone and glucagon secretion much more powerfully than native somatostatin, while relatively sparing insulin secretion. However, the present findings suggest that this analogue is not sufficiently selective to be therapeutically useful in non-insulin dependent diabetes.     

Somatostatin 1-28 circulates in human plasma

The gel filtration profile of immunoreactive somatostatin in human plasma in the fasting state is not well established as a consequence of insufficient sensitivity of the combined chromatography and radioimmunoassay procedures usually employed. We here report the gel filtration profiles of plasma samples after somatostatin concentration by batchwise immunoaffinity chromatography. The results clearly and reliably document the presence of a circulating peptide in human plasma with a gel permeation chromatography profile identical to the one of synthetic somatostatin 1-28. Approximately 46% of the total somatostatin-like immunoreactivity in plasma is due to this component.     

Somatostatin inhibits neutrophil elastase release in vitro

The influence of the long-acting somatostatin analogue, SMS 201-995, on FMLP-induced neutrophil elastase release in vitro has been investigated. Doses from 150 ng/ml upwards inhibited elastase release, with 100% inhibition by 2500 ng/ml. Inhibition was demonstrated both by an assay measuring elastase immunometrically and by an assay based on its enzyme activity. The demonstration that SMS 201-995 inhibits protease release from polymorphonuclear leukocytes may have implications for the long-term clinical use of this somatostatin analogue.     

Effect of a single administration of somatostatin analogue (SMS 201-995) on GH, TSH and insulin secretion in patients with acromegaly

The effect of a long-acting somatostatin analogue SMS 201-995 on GH secretion was investigated. Eleven acromegalic patients received a single dose of 50 micrograms SMS 201-995 administered subcutaneously, and plasma GH, IGF-I, GRF, TSH, IRI and blood glucose were determined at regular intervals. Nine of 11 patients had elevated basal plasma GH levels above 5 ng/ml. In all patients, plasma GH levels fell immediately from 39.5 +/- 17.3 ng/ml (mean +/- SEM) to 4.3 +/- 1.6 ng/ml (P less than 0.05) with a maximal inhibition of 82.9 +/- 3.3% of the basal levels and the suppression persisted for about 6 h of the observation period. IGF-I and GRF levels were not apparently altered. TSH and IRI levels also rapidly fell. Blood glucose levels fell slightly by 0.5 h. Ten of 11 patients had pain at injection sites. Except for this, no side effects were observed. Our results show that the new somatostatin analogue SMS 201-995 may inhibit GH hypersecretion in acromegalic patients for significant periods, suggesting that this agent can be a useful clinical tool for the treatment of acromegaly.     

Distribution and metabolism of exogenous somatostatin in the rat

The mechanisms of clearance and degradation of injected [3H]somatostatin have been studied in the rat using octadecasilyl-silica extraction and HPLC separation methods.. Three apparent consecutive plasma half-lives of 1, 3 and 20 min were estimated following administration of a pharmacological dose. The initial rapid clearance was due to uptake by various tissue beds, mainly the large peripheral tissue masses muscle, skin and intestine which together accounted for 70% injected radioactivity at 1 min. By contrast the amounts taken up by liver and kidney were relatively small (less than 10%) despite the accumulation of higher concentrations. Massive degradation occurred following uptake and small fragments and amino acids were released into the circulation almost immediately. Inactivation by blood itself was negligible. A slow phase of decline observed at later times suggested a return of intact peptide from extravascular storage sites to sustain plasma concentrations.     

Age-dependent levels of plasma neuropeptides in normal children

Several neuropeptides are secreted in high amounts in pediatric tumors such as neuroblastoma and have been used as markers of residual or recurrent disease. Plasma levels of neuropeptides might be expected to change during development, but have not been determined in normal children. We have obtained fresh plasma from cord blood of six full-term infants and from peripheral blood in 41 healthy children, ages 1 month to 21 years. Levels of six neuropeptides, vasoactive intestinal peptide (VIP), somatostatin, gastrin releasing peptide (GRP), substance P, pancreastatin and neuropeptide Y (NPY) were measured by radioimmunoassay along with insulin-like growth factor-1 (IGF-1) whose plasma levels are known to vary during development. A child with neuroblastoma was treated with the somatostatin analogue, octreotide, and the effect on plasma neuropeptides quantified. Octreotide doses of 2-3 microg/kg daily resulted in a 40-60% decrease in plasma levels of IGF-1, pancreastatin and GRP. These results are the first publication of plasma neuropeptide levels in normal children.     

Secretory diarrhea in villous adenoma of rectum: Effect of treatment with somatostatin and indomethacin

The effects of treatment with the synthetic longacting somatostatin analogue SMS-201-995 were studied in a patient with a fluid and electrolyte secreting villous adenoma of the rectum. The effects of SMS-201-995 on rectal fluid volume and electrolyte loss, and local and general prostanoid production were compared with those of treatment with indomethacin.During treatment with the somatostatin analogue iso-osmolar rectal fluid production increased about 25%; the quantity of prostaglandin E₂ in the rectal fluid rose almost 20-fold. Prostaglandin F_2α, 6-keto-prostaglandin F_1α and 13,14-dihydro-15-keto-prostaglandin F_2α output showed similar, though less impressive increments during somatostatin treatment. The somatostatin analogue did not affect urinary prostanoid excretion except for levels of 2,3-dinor-thromboxane B₂, which doubled. With indomethacin treatment diurnal rectal fluid production dropped by about 50% and all prostanoids measured in urine and rectal fluid decreased below control values.It appears that the somatostatin analogue SMS-201-995 has a marked stimulatory effect on the in vivo prostanoid production by the villous adenoma. Perhaps this stimulation is not confined to the tumor only, but also affects thromboxane synthesis.     

Pharmacological characterization of a recombinant, fluorescent somatostatin receptor agonist

Somatostatin (SST) is a peptide neurotransmitter/hormone found in several mammalian tissue types. Apart from its natural importance, labeled SST/analogues are utilized in clinical applications such as targeting/diagnosis of neuroendocrine tumors. We report on the development and characterization of a novel, recombinant, fluorescent somatostatin analogue that has potential to elucidate somatostatin-activated cell signaling. SST was genetically fused with a monomeric-red fluorescent protein (mRFP) as the fluorescent label. The attachment of SST to mRFP had no detectable effect on its fluorescent properties. This analogue's potency to activate the endogenous and transfected somatostatin receptors was characterized using assays of membrane potential and Ca(2+) mobilization and immunocytochemistry. SST-mRFP was found to be an effective somatostatin receptor agonist, able to trigger the membrane hyperpolarization, mobilization of the intracellular Ca(2+) and receptor-ligand internalization in cells expressing somatostatin receptors. This complex represents a novel optical reporter due to its red emission spectral band suitable for in vivo imaging and tracking of the somatostatin receptor signaling pathways, affording higher resolution and sensitivity than those of the state-of-the-art radiolabeling bioassays.     

Glucose tolerance during long term treatment with a somatostatin analogue.

Seven patients with active acromegaly were treated with SMS 201-995, an analogue of somatostatin, for one year, the maximum dose being 100 micrograms three times a day. Three patients had impaired glucose tolerance before treatment, due to insulin resistance in two and insulin deficiency in one. In all patients treatment with the analogue slightly increased postprandial glucose concentrations and suppressed insulin concentrations for two to two and a half hours after each injection; growth hormone concentrations decreased progressively with treatment. The patient with impaired glucose tolerance due to insulin deficiency developed diabetes mellitus after four months'' treatment; concomitant treatment with glibenclamide resulted in a decreased glucose concentration and increased insulin concentration. This analogue of somatostatin had only minor side effects on glucose tolerance in patients with acromegaly and may be used in patients with impaired glucose tolerance provided that glucose concentrations are monitored closely.     

Increased sensitivity to somatostatin in obese Zucker rats

The release of somatostatin from the pancreas and stomach following the ingestion of a meal and its increase in the peripheral circulation elicits an attenuation of postprandial hormone secretion such as insulin, pancreatic polypeptide and gastrin and retards the rate at which nutrients enter the circulation. Reduced tissue somatostatin content and/or an attenuated somatostatin release is associated with hyperinsulinism and obesity in certain animal models. In the obese Zucker rat, however, tissue somatostatin levels are increased and therefore the present study was designed to determine the effect of synthetic somatostatin on basal and postprandial arterial insulin levels in obese and lean Zucker rats. Synthetic somatostatin was infused at doses of 0.25, 0.5, 1 and 5 ng/kg X min before and after the intragastric instillation of a liver extract/sucrose test meal. In the obese rats somatostatin at a dose of 5 ng/kg X min reduced basal plasma insulin levels significantly, whereas no effect of somatostatin was observed on basal insulin levels in the lean animals at all doses employed. The integrated postprandial insulin response was reduced during 0.25, 0.5, 1 and 5 ng/kg X min somatostatin in the obese animals, whereas only 0.5 ng/kg X min and higher doses had an inhibitory effect in the lean rats. The degree of inhibition in relation to the postprandial insulin response during saline infusions was 35-230% in the obese and 30-100% in the lean Zucker rats within the range of somatostatin infusions employed.(ABSTRACT TRUNCATED AT 250 WORDS)     

The nature of big plasma somatostatin: implications for the measurement of somatostatin-like immunoreactivity in human plasma

Big plasma somatostatin (BPS) represents an artifact of measurement. High-molecular-weight globulins (α, β, and γ) in human plasma inhibit, in a concentration-dependent manner, the binding of radiolabeled somatostatin analogs to antibody directed against somatostatin. The magnitude of inhibition varies with antibody and plasma sample and is greatest for the α-globulin fraction. The mechanism of inhibition involves binding of plasma globulins to antibody, thereby blocking tracer-binding sites, and does not involve inhibition by somatostatin bound noncovalently to plasma proteins or tracer degradation. Thus BPS arises from a property of plasma rather than of somatostatin and so it is suggested that this mechanism may account for the presence of other “big” forms of hormones in plasma.