Receptor

A biological receptor is a specialized protein located on the plasma membrane or inside the cell (cytoplasm/nucleus) that recognizes and binds endogenous signaling molecules (hormones, neurotransmitters) or exogenous drugs with high affinity and stereospecificity. The overwhelming majority of therapeutic peptide targets are G-protein coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), or cytokine receptors.

Understanding receptor distribution explains both drug efficacy and side effects. For example, GLP-1 receptors in the pancreas stimulate insulin, while GLP-1 receptors in the area postrema of the brain induce nausea and satiety.

Receptors explain why a peptide does what it does, and why it does other things too. GLP-1 receptors are found in the pancreas, gastrointestinal tract, heart, lungs, kidneys and brain. Receptors in the pancreas and brain account for much of the blood sugar and body weight effects of liraglutide and semaglutide, while receptor activity in the gut and brain is linked to the nausea many people experience.

Receptor targets are a useful test of a peptide claim. For approved drugs the target is usually well defined: semaglutide acts on the GLP-1 receptor, and tirzepatide on both the GIP and GLP-1 receptors. When a product is said to act on many unrelated systems at once, with no identified receptor and no human data, that is a sign its mechanism has not actually been established.

Many of the most important drug targets belong to one receptor family. G protein-coupled receptors (GPCRs) are the most intensively studied drug targets; a 2017 analysis found that 475 drugs, about 34% of all FDA-approved drugs, act on 108 different GPCRs. Peptide hormones such as GLP-1, GIP, glucagon and GHRH signal through GPCRs, which is why so much peptide drug development focuses on them.

“Each receptor is found in only one organ”

Most receptors are expressed in several tissues. That is why a drug designed for one purpose often has effects elsewhere. The GLP-1 receptor, for example, is present in the pancreas, gut, heart, kidneys and brain, so a GLP-1 drug affects appetite and digestion as well as blood sugar, and side effects can appear far from the intended target.

“Binding a receptor always switches it on”

Molecules that bind receptors can fully activate them (agonists), partially activate them (partial agonists) or block them (antagonists). Others bind at a separate site and change how the receptor responds to its natural signal. Knowing that a peptide binds a receptor does not reveal which of these it does, or whether the effect is strong enough to matter in the body.

“A peptide that hits the same receptor as an approved drug will work the same way”

Two molecules can share a receptor yet differ in potency, how long they remain active, which signalling pathways they favour and which tissues they reach. These differences can change both benefits and side effects. Each compound needs its own evidence; sharing a target with an approved medicine is not a substitute for clinical trials.

What is the GLP-1 receptor?

The GLP-1 receptor is a class B G protein-coupled receptor that responds to glucagon-like peptide-1, a hormone released from the gut after eating. Activating it increases insulin secretion when blood sugar is raised, suppresses glucagon, slows stomach emptying and reduces food intake. Semaglutide and liraglutide act on this receptor, and tirzepatide acts on it as well as on the GIP receptor.

What is the difference between an agonist and an antagonist?

An agonist binds a receptor and activates it, mimicking the body’s natural signal. An antagonist binds the receptor without activating it and blocks the natural signal from acting. Semaglutide is a GLP-1 receptor agonist. Some drugs fall in between, as partial agonists that produce a weaker maximum response than the natural hormone.

Why do peptide drugs cause side effects?

Mostly because their receptors exist in more than one place. A drug that activates GLP-1 receptors to improve blood sugar also reaches receptors in the gut and brain, which is linked to nausea and reduced appetite. Side effects can also come from impurities, immune reactions to the peptide, or effects on other receptors the drug was not designed to hit.

What are G protein-coupled receptors?

G protein-coupled receptors (GPCRs) are a large family of proteins that span the cell membrane and pass signals inside the cell through G proteins when a hormone or drug binds them. They are the most intensively studied drug targets, and around a third of FDA-approved drugs act on them. Receptors for GLP-1, GIP and GHRH are all GPCRs.