the GLP-1 revolution

The story begins with glucagon-like peptide-1, an endogenous incretin hormone secreted by L-cells in the gut after meals. Native GLP-1 has a half-life of roughly 2 minutes -- it is rapidly degraded by dipeptidyl peptidase-4 (DPP-4) and cleared by the kidneys. That makes the native hormone useless as a drug.

Novo Nordisk solved this with three key modifications to the GLP-1(7-37) backbone. First, an Aib substitution at position 8 blocks DPP-4 cleavage. Second, an arginine substitution at position 34 prevents fatty-acid binding at the wrong site. Third -- and most critically -- a C18 fatty diacid chain at position 26 enables tight albumin binding, extending the half-life to approximately 7 days and making once-weekly subcutaneous dosing viable.

Semaglutide activates the GLP-1 receptor, a class B1 G-protein-coupled receptor (GPCR) expressed across multiple organ systems. In the pancreas, receptor activation stimulates glucose-dependent insulin secretion from beta cells and suppresses glucagon release from alpha cells -- but only when blood glucose is elevated.

In the gastrointestinal tract, semaglutide delays gastric emptying, slowing nutrient absorption and contributing to post-meal satiety. In the central nervous system, GLP-1 receptors in the hypothalamus and brainstem mediate appetite reduction through direct modulation of hunger and reward signaling pathways. This central action is the primary driver of the weight-loss effect.

The STEP program (Semaglutide Treatment Effect in People with Obesity) established semaglutide 2.4 mg weekly as the most effective single-agent pharmacotherapy for weight loss, with mean reductions of approximately 15% body weight in STEP 1. The SELECT trial (17,604 patients with established cardiovascular disease but without diabetes) demonstrated a 20% reduction in major adverse cardiovascular events -- the first time any anti-obesity medication proved cardiovascular benefit.

The SUSTAIN and PIONEER programs (injectable and oral, respectively) built the type 2 diabetes evidence base across HbA1c reduction, weight, and cardiovascular safety. Most recently, the FLOW trial showed semaglutide significantly slowed kidney disease progression in patients with type 2 diabetes and chronic kidney disease -- stopped early for efficacy.