[Octreotide, analog of somatostatin. Pharmacological properties and therapeutic applications in pituitary endocrine tumors].

Octreotide, the first somatostatin analogue developed, has pharmacological properties (it inhibits pituitary GH and TSH secretion, pancreatic and digestive tract endocrine secretion and various gastrointestinal functions) which are qualitatively similar to those of the natural 14 amino acids peptide. Enzymatic structural modification had resulted in a peptide which is very resistant to degradation and has a 90-110 min half-life permitting subcutaneous administration. Following a subcutaneous injection of 50 to 100 micrograms, absorption is rapid and complete. The peak plasma concentration is obtained within 20 to 30 minutes. The drug is degraded mostly in the liver. Among pituitary tumours, GH- and TSH-secreting adenomas constitute the primary indication. Octreotide has been widely tested in the treatment of acromegaly; 50 to 80 percent of patients with that disease respond to a discontinuous administration (usually 3 subcutaneous injections per day), and the IGF1 level is normalized in 50 percent of the cases. No long-term desensitization and no rebound phenomenon after drug withdrawal have been observed. Clinical improvement is obvious, even in cases with partial response to treatment. The dose required to obtain maximum response may vary from one patient to another, and resistance to the peptide has been observed in a few patients. This resistance does not seem to be related solely to the absence of somatostatin receptors on the tumours. The principal side-effect of octreotide treatment is the occurrence of gallstones. In some patients, continuous subcutaneous injection gives a better result with a lower dose. Other ways of administration (nasal or oral) have been tested or are being evaluated.