Does glucagon receptor agonism cost glycaemic control at the top doses?

This matters because it predicts what a related molecule should do.

I keep seeing this stated as a fact with no explanation attached, and unexplained facts make me suspicious.

My background is quantitative but not chemical, so I can follow an equation more easily than a hand-wave.

Why does this happen, and what would falsify the usual explanation?

The honest position is that the balance is empirical: it has been found by dose-ranging rather than derived from first principles.

Retatrutide is a triple agonist at the GIP, GLP-1 and glucagon receptors; survodutide is a dual glucagon and GLP-1 agonist. The compositions are different and so are the trial programmes.

On the detail: the glycaemic penalty from the glucagon limb is dose-dependent and is the practical constraint on how far the limb can be pushed in a diabetic population.

Expect a slightly larger heart-rate effect than with a pure GLP-1 agonist.

More usefully, the relevant design parameter is the ratio between the limbs. Too much glucagon and glycaemia deteriorates; too little and the metabolic-rate benefit disappears.

Because the mechanism is partly independent of appetite, the weight loss is less strongly coupled to reported food intake, which makes the trial data harder to interpret rather than easier.

Glucagon acts primarily on hepatocytes to promote glycogenolysis, gluconeogenesis and fatty-acid oxidation. The last of those is the therapeutic target; the first two are the reason a counterbalancing incretin limb is required.

The energy-expenditure effect of glucagon receptor agonism is established from human infusion studies and from indirect calorimetry within trials.

The ratio between the limbs is the whole design problem.

edited 16 Nov 2024 by Dr_Ravi_Selvarajah — added the method parameters

It helps to be literal here: this is the mechanistic reason the newer multi-agonists report weight reductions beyond what GLP-1 agonism alone achieves.

Hepatic fat reduction with glucagon-containing agonists in phase 2 has been substantial, which is why the MASH programmes in this class exist at all.

Research-use material is not approved for human use, and a multi-agonist with a glycaemic penalty is the worst possible candidate for uncontrolled use.

Energy expenditure up, hepatic fat down, glycaemia under pressure. That is the glucagon limb in one line.

Answer first: adding glucagon receptor agonism raises energy expenditure and hepatic fat oxidation, and the cost is a glycaemic penalty that the GLP-1 limb has to cover.

Amino-acid handling changes too: glucagon drives hepatic ureagenesis, so the liver–alpha-cell axis is part of the picture and shows up as changes in circulating amino acids.

The liver–alpha-cell axis linking glucagon signalling to hepatic amino-acid metabolism is well described and predicts the amino-acid changes seen with these agents.

The mechanism is not appetite-mediated, which makes it interesting and makes it harder to monitor.