Somatostatin inhibition of insulin secretion in insulin-producing tumors

Somatostatin analogues for the treatment of hyperinsulinaemic hypoglycaemia

Hyperinsulinaemic hypoglycaemia (HH) is a biochemical finding of low blood glucose levels due to the dysregulation of insulin secretion from pancreatic β-cells. Under normal physiological conditions, glucose metabolism is coupled to β-cell insulin secretion so that blood glucose levels are maintained within the physiological range of 3.5-5.5 mmol/L. However, in HH this coupling of glucose metabolism to insulin secretion is perturbed so that insulin secretion becomes unregulated. HH typically occurs in the neonatal, infancy and childhood periods and can be due to many different causes. Adults can also present with HH but the causes in adults tend to be different. Somatostatin (SST) is a peptide hormone that is released by the delta cells (δ-cells) in the pancreas. It binds to G protein-coupled SST receptors to regulate a variety of location-specific and selective functions such as hormone inhibition, neurotransmission and cell proliferation. SST plays a potent role in the regulation of both insulin and glucagon secretion in response to changes in glucose levels by negative feedback mechanism. The half-life of SST is only 1-3 min due to quick degradation by peptidases in plasma and tissues. Thus, a direct continuous intravenous or subcutaneous infusion is required to achieve the therapeutic effect. These limitations prompted the discovery of SST analogues such as octreotide and lanreotide, which have longer half-lives and therefore can be administered as injections. SST analogues are used to treat different forms of HH in children and adults and therapeutic effect is achieved by suppressing insulin secretion from pancreatic β-cells by complex mechanisms. These treatments are associated with several side effects, especially in the newborn period, with necrotizing enterocolitis being the most serious side effect and hence SS analogues should be used with extreme caution in this age group.

Differentiation of insulin secretion patterns in insulinoma

BACKGROUND

In patients with insulinoma, biochemical proof of inappropriately elevated insulin secretion during hypoglycemia is required prior to surgery. Because circulating insulin levels usually vary widely, we have used the combined OGTT-fasting test to define new normative criteria for a retrospective systematic analysis.

METHODS

We retrospectively analyzed insulin concentrations from OGTT-fasting tests of 64 patients with surgically removed insulinomas. In addition, the response to intravenous somatostatin infusions was estimated. Normative criteria were defined to obtain comparable estimates of insulin concentrations: basal, glucose-stimulated maximum, postglucose plateau, and secretory bursts.

RESULTS

Three types of insulin secretion patterns were identified: (1) the autonomous secretion pattern (type 1, N=17) with basal and post-OGTT plateau insulin concentrations of approximately 50 mU/L, suppression after OGTT by 41%, virtual absence of distinctive secretory bursts, and resistance to somatostatin-mediated suppression (25 %); (2) the inadequate suppression pattern (type 2, N=28) with moderately elevated basal and post-OGTT insulin concentrations of approximately 20 mU/L, suppression after OGTT by 73%, absence of secretory bursts, and incomplete somatostatin-induced suppression (56 %); (3) the late-burst secretion pattern (type 3, N=19) with similar basal and post-OGTT insulin concentrations of 17 mU/L, suppression after OGTT by 76%, true insulin bursts of Delta 13+/-11 mU/L (184%), and nearly complete somatostatin-induced suppression by 64%.

CONCLUSIONS

By means of a new normative analysis of the combined OGTT-fasting test, three different patterns of insulin secretion can be described in patients with insulinoma: the autonomous secretion type, the inadequate suppression type, and the late-burst secretion type.

Case report: a glucose responsive insulinoma--implication for the diagnosis of insulin secreting tumors

Normal insulin secretagogues, including glucose, usually have little influence on insulin secretion from insulinomas. Therefore, insulinomas typically cause fasting hypoglycemia with relative hyperinsulinemia. This report describes a patient with hyperinsulinemia due to an islet cell adenoma with microadenomatosis, which, upon provocative in vivo testing, was found to be profoundly responsive to hypoglycemic and hyperglycemic stimuli. A 72 hr fast followed by brisk exercise resulted in a gradual reduction of serum glucose and insulin concentrations, but did not provoke symptomatic hypoglycemia. Oral glucose tolerance testing resulted in a prompt 10-fold increase in serum insulin accompanied by a mildly symptomatic and gradual fall in serum glucose to 30 mg/dl 90 minutes after glucose ingestion. An intravenous glucose challenge caused an acute increase in serum insulin to more than 1200 microU/ml with a resulting serum glucose of 11 mg/dl 25 minutes later, associated with loss of consciousness. Although a prolonged fast has proven to be the best diagnostic test for insulin secreting tumors, many other provocative tests that use normal insulin secretagogues have been somewhat useful in this regard. The patient in this report supports the concept that insulinomas vary widely in their response to a number of normal physiologic regulators of insulin secretion, including the serum glucose concentration. A variety of provocative tests may be needed to fully evaluate the rare patient in whom there is a strong clinical suspicion of insulinoma but who has a nondiagnostic prolonged fast.

Evaluation of a euglycaemic clamp procedure as a diagnostic test in insulinoma patients

In 15 patients with insulinoma, six patients after successful removal of this tumour, two patients with previous pancreas resection because of hypoglycaemia elsewhere, and 10 control subjects, the diagnostic usefulness of euglycaemic clamp procedures (without exogenous insulin) was assessed in comparison with prolonged starvation. Only insulinoma patients developed sustained hypoglycaemia (less than or equal to 2.3 mmol l-1) within 2-44 h without caloric intake, because of inappropriately elevated immunoreactive insulin (IR-insulin) concentrations. IR-proinsulin values were elevated in most (7 out of 10), but not in all insulinoma patients. The steady-state glucose infusion rate necessary to maintain a stable plasma glucose concentration of 4.4-5.0 mmol l-1 was significantly (P less than or equal to 0.001) higher in insulinoma patients (2.5 +/- 0.6 mg kg-1 min-1) than in pancreas resected patients (0.6 +/- 0.2 mg kg-1 min-1), or in control subjects (0.5 +/- 0.1 mg kg-1 min-1). Due to a considerable degree of overlap, sensitivity (0.44) and specificity (0.95) were too low for such a procedure to qualify as a diagnostic test. There was no correlation of glucose infusion rates to IR-insulin values (r = 0.024, P = 0.461). One reason for this was the development of insulin resistance in some, but not in all insulinoma patients. When, in analogy to insulin/glucose ratios, a diagnostic index was derived by multiplying the steady state glucose infusion rate by the steady state IR-insulin concentration, the diagnostic accuracy was greatly increased (sensitivity and specificity 0.94, respectively), but still lower than that of 'amended' insulin/glucose ratios in fasting plasma or at the time of discontinuation of prolonged fasts (1.00). Somatostatin infusions inhibited insulin secretion (IR-C-peptide plasma concentrations) by 52-88% in subjects without insulinoma and in those insulinoma patients whose tumour cells ultrastructurally contained plenty of normal secretory granules, and to a lesser degree when only abnormal or virtually no secretory granules were present, i.e. in more de-differentiated tumours. In contrast to this significant (P = 0.036) association, malignancy, i.e. the presence of metastases, could not be predicted from whether or not insulin secretion was resistant to the inhibitory action of somatostatin. In conclusion, euglycaemic clamp experiments are less reliable for detecting or excluding a functioning insulinoma than the relation of glucose and insulin values during starvation. The inhibition of insulin secretion by somatostatin depends on the presence of normal beta-granules, and does not distinguish adenomas from carcinomas.

The hyperinsulinaemic hypoglycaemias in infancy: a study of six cases

The aim of the present study was to evaluate various functional tests for the differentiation of hyperinsulinaemic hypoglycaemia. The pathophysiological and histological findings in six infants, aged 2-7 months, with persistent hyperinsulinaemic hypoglycaemia are described. Islet cell adenoma was found in four infants and pancreatic nesidioblastosis in two others. Circulating levels of blood glucose (BG), immunoreactive insulin and C-peptide immunoreactivity were measured under basal conditions and during both stimulation and suppression. The diagnosis of hyperinsulinaemia was made by estimation of the BG/serum insulin ratio, which was the most important diagnostic criterion of hyperinsulinism. Control subjects of comparable age showed a ratio of 8.3 +/- 4.4 (range 4.1-13.3), whereas the six patients had values between 0.3 and 5.1. At least four determinations with ratios lower than 2.6 were necessary for confirming the diagnosis. Preoperatively we performed oral glucose tolerance, diazoxide infusion, somatostatin infusion and C-peptide suppression tests. It is suggested that the various function tests, especially the suppression tests, do not differentiate hyperinsulinism caused by an adenoma from that caused by diffuse pancreatic nesidioblastosis.

Somatostatin analogue (SMS 201-995) in the management of gastroenteropancreatic tumors and diarrhea syndromes

SMS 201-995 (Sandostatin) was studied using low doses (50 to 100 micrograms) administered subcutaneously every 12 hours. A single 50-micrograms dose of SMS 201-995 effectively controlled gastric acid and blood gastrin levels for 12 hours in three patients with benign gastrinomas and was useful in their perioperative management. Higher doses of the agent (500 to 800 micrograms per day) had no effect on metastases in one of two patients with metastatic gastrinoma. In the other patient, one tumor shrank but the other continued to grow after three months of treatment while serum gastrin levels did not change. Cultured metastatic tumor tissue from this patient released different forms of gastrin; growth rates varied, independent of uptake of SMS 201-995, and gastrin release increased. A neonate with nesidioblastosis maintained normal blood glucose levels while receiving SMS 201-995 therapy following a 95 percent pancreatic resection. In two elderly patients with organic hypoglycemia--one with a single benign adenoma and one with multiple adenomatosis--the somatostatin analogue did not prolong the hypoglycemia-free interval. In nine patients with carcinoid syndrome, flushing was uniformly controlled with 50 micrograms of SMS 201-995 administered every eight to 12 hours. One of the nine required exocrine pancreatic replacement. After six months of treatment, three of the nine had no change in tumor size and one had remission of symptoms and stopped treatment. In two patients with vipoma, SMS 201-995 controlled diarrhea and reduced levels of vasoactive intestinal peptide; tumor necrosis occurred in one patient. In a patient with diabetic diarrhea unresponsive to all treatments, SMS 201-995 therapy controlled the diarrhea but did not interfere with control of the diabetes.

Inhibition of growth of pancreatic carcinomas in animal models by analogs of hypothalamic hormones

Using animal models of acinar and ductal pancreatic cancer, we investigated the effect of analogs of hypothalamic hormones on tumor growth. In Wistar/Lewis rats bearing the acinar pancreatic tumor DNCP-322, chronic administration of [L-5-Br-Trp8]somatostatin-14 significantly decreased tumor weights and volume. Somatostatin-28 and the cyclic hexapeptide analog of somatostatin cyclo(Pro-Phe-D-Trp-Lys-Thr-Phe) failed to influence the growth of this tumor. The agonistic analog of luteinizing hormone-releasing hormone [D-Trp6]LH-RH also significantly decreased tumor weight and volume in this model and reduced testosterone levels and the weights of the ventral prostate and tests. In Syrian hamsters bearing ductal type of pancreatic carcinoma, chronic administration of [L-5-Br-Trp8]somatostatin diminished tumor weights and volume. The percentage change in tumor volume was significantly decreased when compared to control animals. In one experiment, cyclic hexapeptide of somatostatin also inhibited growth of this tumor. [D-Trp6]LH-RH, given twice daily or injected in the form of microcapsules for constant controlled release, significantly decreased tumor weight and volume and suppressed serum testosterone levels. Hamsters castrated 4 days after transplantation of the pancreatic tumors showed a significant decrease in weight and volume of these tumors. This suggests that pancreatic cancers may, at least in part, be sex hormone sensitive. [D-Trp6]LH-RH may decrease the growth of pancreatic carcinomas by suppressing androgens. Somatostatin analogs reduce the growth of pancreatic ductal and acinar cancers, probably by inhibiting the release or stimulatory action of gastrointestinal hormones on tumor cells (or both). Inhibition of animal models of pancreatic tumors by chronic administration of somatostatin analogs and [D-Trp6]LH-RH suggests that these compounds should be considered for the development of a new hormonal therapy for cancer of the pancreas.

Differentiation between the somatostatin inhibition and the post-somatostatin rebound observed on growth hormone secretion in vitro

A rebound in growth hormone secretion following somatostatin treatment has been shown in several systems where somatostatin suppresses secretion of the hormone. We have developed an in vitro system in which isolated and cultured pituitary cells were perfused after mild trypsinization. After washing, these cells retained their sensitivity and secreted growth hormone (GH) in response to physiological activators (norepinephrine, dopamine, serotonin) or inhibitors (somatostatin) as well as pharmacological activators (PGE2). The variation in GH secretion occurred within a minute after commencement of the infusion and was as rapidly reversible and repeatable minutes later. During somatostatin infusion the GH secretion was not totally suppressed (residual secretion (mean +/- S.D.) 34 +/- 7%). After the infusion a rapid rebound in GH secretion occurred, reaching levels in excess of the pretreatment value of 138 +/- 13%. This rebound effect occurred at doses higher than (10(-10)M) but not at lower doses, even when significant inhibition was observed. The inhibitory effect is of greater magnitude than the rebound effect (rebound = inhibition X 57 +/- 7% (mean +/- S.D.)). Furthermore, rebound was not enhanced by prolongation of somatostatin infusion. These latter results indicate that the rebound in secretion cannot be explained on the sole basis of storage of intracellular GH during somatostatin infusion and in fact suggest the involvement of a process of GH degradation and/or an inhibition of GH synthesis.

[Islet cell cancer with organic hyperinsulinism. Clinical aspects, diagnosis and therapy]

About 8%-15% of the patients with organic hyperinsulinism have an islet cell carcinoma (13% in our series). In addition to a history of complaints of relatively recent onset, the patients present clinically the typical intermittent neurologic-psychiatric symptoms concurrently associated with hypoglycemia. The diagnosis is established biochemically on the basis of hypoglycemia, with inadequate incrementation of the insulin concentration subsequent to suppression and provocation tests. Elevated serum proinsulin and, in most patients, an increased insulin secretion rate are usually found after administration of agents such as glucose or leucine. Localization of the tumors is achieved by selective coeliacography as well as abdominal computerized axial tomography. The islet cell carcinoma is found most frequently in the tail of the pancreas, less frequently in the body and head of the pancreas. Metastatic spread is seen early into adjacent lymph nodes and especially in the liver. The treatment of choice is surgical resection of the tumor. Even in cases with advanced metastatic involvement, surgical intervention appears indicated. Medical treatment includes the administration of diazoxide, long-acting glucagon as well as the cytostatic agent streptozotocin. The average survival time is 30-40 months after diagnosis (in our series 79 months). Thus, the prognosis of patients with islet cell carcinoma appears relatively favorable, especially when compared with adenocarcinoma of the pancreas.

Effects of somatostatin in a case of severe hypoglycemia due to nesidioblastosis

The Authors have studied the effects of somatostatin (SRIF) treatment in an infant affected by hypoglycemia due to nesidioblastosis. During iv infusion with SRIF we observed a marked increase of blood glucose levels; concomitantly insulin secretion was almost completely suppressed. In contrast, during treatment with protamine zinc-somatostatin (PZ-SRIF), a long acting SRIF preparation, the blood glucose levels did not rise and insulin concentrations were inappropriately elevated. Therefore in this case the long term treatment of hypoglycemia due to nesidioblastosis with PZ-somatostatin was unfeasible.

Effects of somatostatin on insulin and glucagon in patients with insulinoma

Effects of somatostatin on fasting and arginine-or tolbutamide-stimulated insulin release were studied in four patients with insulinoma. Somatostatin (bolus or bolus + infusion) reduced fasting insulin values in all patients; insulin response to tolbutamide was partially reduced in two patients; somatostatin bolus impaired the insulin response to arginine. Fasting glucagon levels and glucagon response to arginine were also reduced by somatostatin. These results indicate the potential usefulness of somatostatin in the diagnosis of insulinoma even if its effect on insulin is only partial.

Failure of somatostatin to diagnose organic hyperinsulinism

In four patients with organic hyperinsulinism (two with surgically proven beta-cell adenomas of the body of the pancreas) a standard tolbutamide test during continuous somatostatin infusion (5 microgram/min) was carried out. Tolbutamide induced insulin release was completely inhibited by somatostatin as in normal subjects. These results suggest that the inhibition test with somatostatin does not seem to be a better or safer way of diagnosing insulin producing tumours.