Tirzepatide 10mg (Mounjaro)

Tirzepatide is a synthetic analogue of gastric inhibitory polypeptide (GIP), developed for its ability to stimulate insulin release and so to address type 2 diabetes and non-alcoholic fatty liver disease together. At 39 amino acids it is a relatively large peptide, and it prompts the pancreas to release insulin by binding both the GIP and the GLP-1 (glucagon-like peptide-1) receptors. Taken over longer periods it also increases adiponectin levels by as much as 26%[1]. Research shows that it reduces feelings of hunger, lowers insulin levels and increases insulin sensitivity. Taken together those effects produce significant weight loss of 11 kg (25 lbs), improve glucose tolerance, decrease fat (adipose) tissue and reduce cardiovascular risk.

Tirzepatide is a single 39 amino acid peptide that activates two receptors: the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon-like peptide-1 receptor (GLP-1R). It is not a mixture of two drugs. One chain carries both activities, which is what the term unimolecular dual agonist describes, and the backbone is derived from native GIP rather than from GLP-1.

The two activities are not balanced. In the discovery work the molecule behaved as a full agonist at the GIP receptor but as a biased partial agonist at the GLP-1 receptor, favouring cyclic AMP generation over beta-arrestin recruitment. That bias matters because beta-arrestin recruitment drives receptor internalisation and desensitisation, so a molecule that under-recruits it may keep signalling where a balanced full agonist would blunt itself.

Like other weekly incretin analogues, it carries a C20 fatty diacid that binds albumin and extends the half-life to about five days. The pharmacological question the molecule was built to answer is whether adding GIP receptor agonism to GLP-1 receptor agonism improves metabolic outcomes beyond what the GLP-1 arm achieves alone, and the head-to-head trial against semaglutide was designed to test exactly that.

At its simplest, tirzepatide increases the release of insulin from the pancreas and glucose control improves as a result. In individuals with type 2 diabetes it decreases hemoglobin A1c (HbA1c) levels by 2.4% after six months. Weight loss follows a dose-dependent relationship as well, reaching as much as 11 kg (25 lbs) over the same six months[1], [2].

There is evidence that the peptide improves the function of the pancreatic beta cells themselves, the cells that make and release insulin. Making those cells more effective at processing insulin raises circulating insulin levels and, at the same time, reduces the stress the beta cells are under, which may in turn slow the progressive course that type 2 diabetes normally takes.

The rise in insulin appears to happen only in response to a rise in blood glucose. During fasting the peptide lowers insulin instead, which over time helps increase insulin sensitivity. It also decreases fasting glucagon levels, which are thought to exacerbate hyperglycemia by interfering with hepatic glucose metabolism[3].

Tirzepatide is a dual agonist at the gastric inhibitory polypeptide receptor and the glucagon-like peptide-1 receptor. Acting at both appears to be synergistic, and that is what makes it more effective than the strict GLP-1 agonists already approved for the treatment of type 2 diabetes.

Gastric inhibitory polypeptide is synthesized naturally in the small intestine. Binding to the GIP receptor inhibits gastric acid secretion and gastrin release while stimulating insulin release; that last is the receptor's primary function and the main reason insulin levels rise following a meal.

Glucagon-like peptide-1 receptors sit on beta cells and on neurons in the brain. Activating them increases insulin synthesis and insulin release both; in the brain, the same activation lowers appetite. It also appears to increase beta cell density in the pancreas by enhancing beta cell survival[4].

At the GLP-1R, tirzepatide favours cAMP production over beta-arrestin recruitment. That departure from what endogenous GLP-1 does appears to activate the receptor without increasing its physiological internalization, which leaves GLP-1R activity higher than either endogenous GLP-1 or other synthetic GLP-1R agonists achieve[5].

Tirzepatide use is associated with substantial weight loss over a six-month interval. Set against insulin degludec, the once-daily basal insulin it was compared with in the SURPASS-3 trial, the difference is striking: tirzepatide causes a dose-dependent decrease in weight over time where degludec causes weight gain[12].

The long-term effect on weight appears to come from the GIP agonism. GIP acts directly on the insulin sensitivity of adipocytes, and that is the likely mechanism by which tirzepatide moves adiponectin levels.

GIP signalling in the central nervous system regulates the hypothalamic feeding centres, which decreases food intake and improves glucose handling, and body weight falls as a result[13]. Tirzepatide therefore works on weight from two directions at once: adiponectin signalling directly in adipose tissue, and GIPR signalling in the brain that reduces hunger.

Tirzepatide alters adiponectin levels. Low adiponectin has been associated with atherosclerosis, obesity and heart disease, while increased levels have been associated with a decreased risk of all three. Research in humans with type 2 diabetes has shown that the peptide improves lipoprotein biomarkers, lowering levels of triglycerides, apoC-III and a handful of other lipoproteins[8].

Increases in adiponectin raise HDL levels while decreasing triglycerides, both of which are associated with lower risk of heart disease. The peptide hormone appears to go further than that, reducing scavenger receptors in macrophages and increasing cholesterol efflux, which offers considerable protection against atherosclerosis[9].

GLP-1 signalling has been shown to induce relaxation of blood vessels, which decreases blood pressure and enhances end organ perfusion. It also appears to decrease inflammation, through reduced NF-kB signalling, decreased MMP-9 activity and inhibited synthesis of inflammatory cytokines. Those effects have been reported to last as long as three months after a single dose of a GLP-1R agonist[10]. A clinical trial is under way to further evaluate tirzepatide's medium-term effects in individuals with heart failure[11].

For years GIP receptor agonism was considered an unpromising target in metabolic disease. GIP action appeared blunted in type 2 diabetes, and there was a well-supported argument that GIP receptor antagonism, not agonism, would be the useful direction, since GIP promotes fat storage.

Tirzepatide is the empirical answer to that debate rather than a theoretical resolution of it. Adding GIP receptor agonism to GLP-1 receptor agonism produced larger effects than the GLP-1 arm alone in a direct comparison, which is difficult to reconcile with the antagonism hypothesis but does not by itself explain why.

The mechanism of the GIP contribution is still being characterised. Proposals include improved adipose lipid buffering, central effects on nausea that permit greater exposure, and altered receptor desensitisation. None of these is settled, and the clinical result stands ahead of the explanation.

At the GLP-1 receptor, tirzepatide favours cyclic AMP generation over beta-arrestin recruitment. Beta-arrestin recruitment is the step that leads to receptor internalisation and desensitisation, so a ligand that recruits less of it may sustain signalling longer than a balanced full agonist would.

This is a mechanistic argument derived from cell-based pharmacology, not a clinical finding. It is a plausible contributor to the efficacy difference observed against a full GLP-1 receptor agonist, but the trials were not designed to test it and it should not be reported as an established explanation.

SURMOUNT-4 is worth separating from the other trials because its design answers a different question. Participants first lost weight in an open-label lead-in, then were randomised either to continue treatment or to switch to placebo.

Because both groups had already lost weight before randomisation, the comparison isolates the effect of continued exposure rather than the effect of starting treatment. The regain observed in the placebo group is therefore evidence about what withdrawal does, which no standard placebo-controlled design can show.

Tirzepatide is supplied for laboratory work as a lyophilised powder and is far more stable dry than in solution. Its two Aib residues are non-proteinogenic, which is worth noting when interpreting sequencing or digestion data, since standard proteolytic mapping behaves differently around them.

As with other acylated peptides, related substances include des-acyl and mis-acylated species that share the parent mass closely enough to complicate interpretation. A stated purity figure is only as informative as the method that produced it, so the analytical method and lot identifier on a certificate carry real weight.