Octreotide

Octreotide binds SSTR2 with high affinity and SSTR5 with moderate affinity (SSTR1 and SSTR3 minimally). SSTRs couple through Gi to reduce cAMP, inhibit voltage-gated Ca2+ channels, activate inwardly rectifying K+ channels, and suppress protein phosphorylation. In pituitary somatotrophs, this reduces GH secretion. In neuroendocrine tumor cells, it reduces secretion of serotonin, VIP, gastrin, glucagon, and insulin. SSTR2 activation also exerts direct anti-proliferative effects via cell cycle arrest and apoptosis in tumor cells.

Octreotide reduces splanchnic blood flow, decreases intestinal motility, and inhibits secretion of multiple GI hormones including gastrin, secretin, CCK, motilin, and VIP. It reduces pancreatic exocrine secretion (enzyme and bicarbonate output), reduces portal pressure, and inhibits intestinal absorption. These GI effects underlie its utility in managing carcinoid syndrome symptoms, refractory diarrhea, upper GI bleeding (varices), and pancreatic fistulas.

The GH-suppressing action of octreotide is mediated through SSTR2/5 on pituitary somatotrophs. In acromegaly, SC octreotide reduces mean GH below 2.5 ng/mL and normalizes IGF-1 in ~65% of patients. In research contexts, octreotide has been used to study the growth hormone axis, reduce IGF-1 levels in cancer prevention studies (IGF-1 is a proliferative signal), and probe the GH-dependent versus GH-independent effects of IGF-1.

Octreotide LAR (long-acting repeatable) achieves GH and IGF-1 normalization in 55-65% of acromegaly patients and reduces pituitary tumor size in ~50%. It is first-line medical therapy for acromegaly patients unsuitable for surgery. Predictors of response include SSTR2 expression on tumor (assessable by somatostatin receptor scintigraphy or 68Ga-DOTATATE PET) and GH nadir <1 ng/mL on oral glucose tolerance test.

The PROMID and CLARINET trials established octreotide LAR and lanreotide as antiproliferative agents for well-differentiated gastroenteropancreatic NETs, prolonging progression-free survival. Functional NETs (carcinoid, VIPoma, glucagonoma, insulinoma) benefit from symptom control. Peptide receptor radionuclide therapy (PRRT) with 177Lu-DOTATATE targets SSTR2 expressing NETs, and octreotide forms the scaffold for radiolabeled analogues.

Cancer prevention trials have used octreotide to lower IGF-1 in high-risk populations (familial colon cancer, breast cancer prevention). Reduced IGF-1 signaling may slow cell proliferation in pre-malignant tissues. Longevity research interest exists given associations between lower IGF-1 and longevity in some human populations (e.g., Laron syndrome, Ecuadorian cohorts), though this remains controversial and octreotide is not validated as a longevity intervention.