Triple Agonists: Why Three Receptors Might Beat Two

Somebody took a drug that activates two hormone receptors, noticed it worked very well, and decided to build one that activates three, including one whose main day job is raising blood sugar. That is the triple agonist: a single engineered peptide that switches on the GLP-1 receptor, the GIP receptor and the glucagon receptor at the same time. Retatrutide is the famous one (it has its own pages on this site, several of them). A newer entrant, UBT251, is described in its phase 1a/1b study as “a novel triple agonist peptide targeting the GLP-1, GIP, and glucagon receptors.”

Why might three beat two? GLP-1 and GIP mostly work on the eating side, glucagon works on the burning side, and the two incretins are supposed to cancel glucagon’s habit of pushing up blood sugar. No head-to-head trial in front of me settles whether three actually beats two. What exists is a cross-trial picture. A review in Annals of Internal Medicine covered 26 randomized trials and 15,491 adults without diabetes. It put retatrutide at 24.2% weight loss at 12 mg weekly, ahead of tirzepatide at 17.8% and injectable semaglutide at 13.9%. That is a respectable gap. It is not a proof.

The sensible objection to a triple agonist

Let us establish terms with appropriate rigor. A drug that activates one receptor is a mono-agonist. A drug that activates two is a dual agonist. A drug that activates three is a triple agonist. The reader who has followed so far has demonstrated the ability to count to three, and should hold the ability to count to four in reserve, because it will matter later.

Now the objection. This looks like razors. For years the razor companies kept adding blades (at some point the handle was mostly a delivery mechanism for blades), and eventually everyone understood that blade count was a marketing number more than a shaving number. Tirzepatide was already nicknamed the “twincretin” (the field rarely passes up a pun), a 39-amino-acid peptide hitting the GIP and GLP-1 receptors, and a pharmacodynamics review puts dual GIP/GLP-1 agonists like it at roughly 15 to 20% weight loss in trials. So is the third receptor just a third blade?

To be fair, that is a reasonable suspicion about any pharmaceutical franchise. But the third receptor is not more of the same. It does something the other two don’t, it also does something you very much don’t want, and the whole design is an argument that the other two can cancel the bad part. That is weirder than a blade. It is like adding a blade that nicks you, plus two blades whose job includes applying the bandage.

Why put a blood-sugar hormone in a metabolic drug

Glucagon tells the liver to release glucose. If you are designing a drug for a population full of type 2 diabetes, it is the last hormone you would invite. The case for inviting it anyway is energy expenditure. A review of next-generation multi-agonists says dual GLP-1/glucagon agonism “increases energy expenditure by stimulating fat oxidation and glucose production, distinguishing it from the appetite-focused GLP-1/GIP approach.” (Note that “glucose production” is listed as part of the mechanism, not as a side issue.)

In plain words: the glucagon part turns up the furnace, and the GLP-1 and GIP parts follow it around mopping up the sugar the furnace spills. The furnace has an official name, adaptive thermogenesis, and researchers have proposed that these agonists drive it by changing amino acid metabolism and immune cells in brown and beige fat. The mopping is spelled out in a review of glucagon signaling, which describes mazdutide, survodutide and retatrutide as glucagon-based multi-agonists that “incorporate GLP-1 receptor-activating moieties to counteract glucagon’s hyperglycaemic effects.”

Here is a made-up household version. You want to burn more fuel, so you run the heater all winter and the air conditioner at the same time, and the living room stays at 70 degrees. The thermostat looks normal. The fuel tank drains. Your neighbor, impressed, installs a second heater and a second air conditioner, and her house is also 70 degrees, and the utility company starts sending both of you holiday cards. The mechanism is not fully settled, either. One mouse study found that bile acid binding resins boosted glucagon-driven weight loss, which suggests bile acids are part of the furnace too.

How one peptide gets three receptors to say yes

Getting a single molecule to hook up with three choosy receptors, on terms each one finds attractive, is the actual chemistry problem. Retatrutide’s development reportedly took advances in peptide design, amino acid modification and medicinal chemistry. You can imagine the meeting. “Hey, what if it also hits glucagon?” “Doesn’t that raise blood sugar?” “Sure, but we have two other receptors for that.”

The best public window into the tuning comes from dual GLP-1/glucagon agonists. A study titled Structural analysis of the dual agonism at GLP-1R and GCGR used cryo-EM to image both receptors bound to three of them: peptide 15, cotadutide and SAR425899. The three had different activation profiles at each receptor. The middle of each peptide grips the receptor’s outer loops, and selectivity comes down to the very front: the study found “differences in side chain orientations within the first three residues” that point to “distinct engagements in the deep binding pocket.” (I write jokes about drug pipelines, so I will take their word on the deep binding pocket.)

The point is that the first three amino acids help decide which receptor gets more of the action. A triple agonist is not three drugs taped together. It is one chain whose front end has to be acceptable to three locks, in proportions someone chose on purpose.

What is actually in development

A network meta-analysis of 14 randomized trials of glucagon receptor agonists ranked the field by weight loss against placebo. Retatrutide came first at 13.44 kg, survodutide at 10.74 kg and mazdutide at 6.47 kg (the last two are glucagon-plus-GLP-1 duals, not triples). Retatrutide also had the largest HbA1c reduction and was the only one whose HbA1c effect reached statistical significance. On the other hand, the confidence intervals for retatrutide and survodutide overlap, which is the statistician’s way of saying slow down.

UBT251 is the other triple with real data. In a phase 2 trial of 205 adults with obesity over 24 weeks, it produced weight loss of 13.6% to 19.7% depending on dose, against 2.0% on placebo. One headline on the results called it “Novo Nordisk’s triple agonist.” Nearly a fifth of body weight in under six months is, by most standards, a fair amount.

The duals keep feeding the triple story. Cotadutide, a GLP-1/glucagon dual, has early data suggesting benefits on albuminuria and cardiorenal markers. Survodutide is aimed at the liver, and has its own article, etc. Anyway, the triple is mostly a bet that glucagon’s extras survive having GIP added on top.

Where counting receptors stops being simple

Here is where the reserved skill comes in. Amycretin, Novo’s single-molecule GLP-1 and amylin drug, is described in one outlet as an agonist of GLP-1, amylin and calcitonin receptors, which is three targets. Elsewhere it is a two-hormone molecule. Whether it is a triple agonist depends on who is counting, and on whether calcitonin gets its own line in the slide deck or whatever.

The stranger problem is GIP. Tirzepatide activates the GIP receptor. Amgen’s maridebart cafraglutide, a bispecific antibody, blocks it while activating GLP-1. Both add weight loss on top of GLP-1. A review called The Premise of the Paradox describes this as “a central unresolved scientific question in incretin pharmacology.” The triple agonist’s logic assumes each receptor you switch on contributes something. For GIP, switching it off also contributes something.

Which means the logical next molecule turns the GIP receptor on and off at the same time, and the slide deck counts it as four.