Pharmacotherapeutic group: Somatostatin and analogues.
ATC code: H01CB02
Octreotide is a synthetic octapeptide derivative of naturally occurring somatostatin with similar pharmacological effects, but with a considerably prolonged duration of action. It inhibits pathologically increased secretion of growth hormone (GH) and of peptides and serotonin produced within the GEP endocrine system.
In animals, Octreotide is a more potent inhibitor of GH, glucagon and insulin reliease than somatostatin is, with greater selectivity for GH and glucagon suppression.
In healthy subjects, octreotide has been shown to inhibit:
• Release of GH stimulated by arginine, exercise- and insulin-induced hypoglycaemia
• Postprandial release of insulin, glucagon, gastrin, other peptides of the GEP endocrine system, and arginine-stimulated release of insulin and glucagon.
• Thyrotropin-releasing hormone (TRH)-stimulated release of thyroid-stimulating hormone (TSH).
Unlike somatostatin, octreotide inhibits GH secretion preferentially over insulin and its administration is not followed by rebound hypersecretion of hormones (i.e. GH in patients with acromegaly).
In acromegalic patients octreotide lowers plasma levels of GH and IGF-1. A GH reduction by 50% or more occurs in up to 90% patients, and a reduction of serum GH to <5 ng/mL can be achieved in about half of the cases. In most patients, octreotide markedly reduces the clinical symptoms of the disease, such as headache, skin and soft tissue swelling, hyperhidrosis, arthralgia, paraesthesia. In patients with a large pituitary adenoma, octreotide treatment may result in some shrinkage of the tumour mass.
In patients with functional tumours of the GEP endocrine system, octreotide, because of its diverse endocrine effects, modifies a number of clinical features. Clinical improvement and symptomatic benefit occur in patients who still have symptoms related to their tumours, despite previous therapies, which may include surgery, hepatic artery embolization, and various chemotherapies, e.g. streptozocin and 5-fluorouracil.
Octreotide’s effects in the different tumour types are as follows:
Carcinoid tumours
Administration of octreotide may result in improvement of symptoms, particularly of flush and diarrhoea. In many cases, this is accompanied by a fall in plasma serotonin and reduced urinary excretion of 5-hydroxyindole acetic acid.
VIPomas
The biochemical characteristic of these tumours is overproduction of vasoactive intestinal peptide (VIP). In most cases, administration of octreotide results in alleviation of the severe secretory diarrhoea typical of the condition, with consequent improvement in quality of life. This is accompanied by an improvement in associated electrolyte abnormalities, e.g. hypokalaemia, enabling enteral and parenteral fluid and electrolyte supplementation to be withdrawn. In some patients, computed tomography scanning suggests a slowing or arrest of progression of the tumour, or even tumour shrinkage, particularly of hepatic metastases. Clinical improvement is usually accompanied by a reduction in plasma VIP levels, which may fall into the normal reference range.
Glucagonomas
Administration of octreotide results in most cases in substantial improvement of the necrolytic migratory rash which is characteristic of the condition. The effect of octreotide on the state of mild diabetes mellitus which frequently occurs is not marked and, in general, does not result in a reduction of requirements for insulin or oral hypoglycaemic agents. Octreotide produces improvement of diarrhoea, and hence weight gain, in those patients affected. Although administration of octreotide often leads to an immediate reduction in plasma glucagon levels, this decrease is generally not maintained over a prolonged period of administration, despite continued symptomatic improvement.
Gastrinomas/Zollinger-Ellison syndrome
Therapy with proton pump inhibitors or H2 receptor blocking agents generally controls gastric acid hypersecretion. However, diarrhoea, which is also a prominent symptom, may not be adequately alleviated by proton pump inhibitors or H2 receptor blocking agents. Octreotide can help to further reduce gastric acid hypersecretion and improve symptoms, including diarrhoea, as it provides suppression of elevated gastrin levels, in some patients.
Insulinomas
Administration of octreotide produces a fall in circulating immunoreactive insulin, which may, however, be of short duration (about 2 hours). In patients with operable tumours, octreotide may help to restore and maintain normoglycaemia pre-operatively. In patients with inoperable benign or malignant tumours, glycaemic control may be improved without concomitant sustained reduction in circulating insulin levels.
Complications following pancreatic surgery
For patients undergoing pancreatic surgery, the peri- and post-operative administration of Octreotide reduces the incidence of typical post-operative complications (e.g. pancreatic fistula, abscess and subsequent sepsis, post-operative acute pancreatitis).
Bleeding gastro-oesophageal varices
In patients presenting with bleeding gastro-oesophageal varices due to underlying cirrhosis, octreotide administration in combination with specific treatment (e.g. sclerotherapy) is associated with better control of bleeding and early re-bleeding, reduced transfusion requirements, and improved 5-day survival. While the precise mode of action of octreotide is not fully elucidated, it is postulated that octreotide reduces splanchnic blood flow through inhibition of vaso-active hormones (e.g. VIP, glucagon).
Treatment of TSH-secreting pituitary adenomas
The treatment effects of octreotide were prospectively observed in 21 patients and pooled with series of 37 published cases. Among 42 patients with evaluable biochemical data, there were 81% of patients (n-=34) with satisfactory results (at least 50% reduction of TSH and substantial reduction of thyroid hormones), whereas 67% (n=28) had normalisations of TSH and thyroid hormones. In these patients, the response was maintained throughout the duration of treatment (up to 61 months, mean, 15.7 months).
Regarding clinical symptoms, a clear improvement was reported in 19 out of 32 patients with clinical hyperthyroidism. Tumour volume reduction greater than 20% was observed in 11 cases (41%) with a decrease greater than 50% in 4 cases (15%). The earliest reduction was reported after 14 days of treatment.