A13: Antidotes in Depth

INTRODUCTION

Octreotide is a long-acting, synthetic octapeptide analog of somatostatin that inhibits pancreatic insulin secretion. It currently is the essential complement to dextrose for the treatment of refractory hypoglycemia induced by overdoses of insulin secretagogues (eg, sulfonylureas) and quinine. Although not formally approved by the US Food and Drug Administration (FDA) for this indication, octreotide currently is used in toxicology for the treatment of hypoglycemia from insulin secretagogues.

HISTORY

Somatostatin is a collective term for shorter fragments (SRIF-28, SRIF-25, and SRIF-14) cleaved by tissue specific enzymes from preprosomatostatin (116 amino acids) and prosomatostatin (92 amino acids).¹⁴ Somatostatin was identified in 1973, during the search for growth hormone releasing factor.⁸ In addition to its effects on growth hormone and insulin secretion, somatostatin has far-reaching effects as a central nervous system (CNS) neurotransmitter and as a modulator of hormonal release.^45,54 The importance of somatostatin on insulin secretion led to the need to create an analog as a therapeutic tool as somatostatin is limited because of its short duration of action.

Octreotide was synthesized in 1982 in an effort to develop a longer-acting analog of somatostatin.⁵ Octreotide is currently approved by the FDA for the treatment of acromegaly, carcinoid tumors, and vasoactive intestinal peptide tumors. It is also used therapeutically for the treatment of pituitary adenomas, pancreatic islet cell tumors, portal hypertension, esophageal varices, and secretory diarrhea.⁴⁵ Octreotide is being investigated for its inhibitory effects on tumor cell proliferation through stimulation of apoptosis, antiangiogenesis, immunomodulatory effects, and the suppression of tumor-stimulating growth factors.^42,45,55

PHARMACOLOGY

Mechanism of Action on Insulin Secretion and Other Hormones

The effects of somatostatin are mediated by high-affinity binding to membrane receptors on target tissues. Five different somatostatin receptor subtypes that belong to a superfamily of G-protein coupled receptors are identified and assigned numbers (SSTR1–SSTR5) according to their order of discovery.¹⁴ Octreotide, lanreotide, and vapreotide have high binding affinity for subtype SSTR2, a lower affinity for SSTR5 and SSTR3, and almost no affinity for SSTR1 and SSTR4.⁴⁵ The pancreas contains all five subtypes, but in mice SSTR5 is more prevalent in the β-cells and SSTR2 is more prevalent in the α-cells in animal studies.⁶⁹ SSTR5 is found in many tissues, including the brain, pituitary, stomach, intestine, thyroid, and adrenal gland; SSTR2 is found in the brain, pituitary, stomach, liver, kidney, lung, intestine, spleen, thymus, uterus, prostate, and adrenal gland.^45,50,58,69 A variety of pituitary and gastroenteropancreatic tumors contain varying percentages of the SSTR subtypes. Recently in comparison to studies in mice SSTR2 was found to be the functionally dominant somatostatin receptor in human pancreatic β- and α-cells.³⁶

Octreotide, similar to the natural hormone somatostatin, but with a longer duration of ...