A mouse study from the University of Cambridge has mapped how GIPR brain regions produce weight loss through two entirely different mechanisms, depending on whether the receptor is activated or blocked. The research, published in Nature Metabolism, offers the clearest explanation yet for a puzzle that has sat at the centre of obesity pharmacology: how can drugs that do opposite things to the same receptor both help people lose weight?
The answer, the Cambridge team found, is location. Activating the glucose-dependent insulinotropic polypeptide receptor (GIPR) in the brainstem reduced appetite directly. Blocking the same receptor in the hypothalamus had a different effect: it removed what the researchers describe as a kind of ‘brake’ on fullness signals, allowing the body to respond more strongly to the sensation of being full. Same receptor, different brain region, different physiological route.
How the researchers isolated the effect of GIPR brain regions
The team used genetically engineered mice in which GIPR had been selectively removed from specific areas. One group lacked the receptor in the brainstem, the region at the base of the brain just above the spinal cord. Another group lacked it in the hypothalamus. A third group of unmodified mice served as controls. The animals were then treated with a GIPR agonist (which activates the receptor), a GIPR antagonist (which blocks it), or a GLP-1 drug, in various combinations. Food consumption, body weight, fat mass, blood sugar control and brain activity were all monitored.
The results traced each drug type to its region. GIPR agonists worked primarily through the brainstem, reducing appetite and body weight. GIPR antagonists worked through the hypothalamus, lifting the brake on satiety and letting fullness signals register more forcefully. The researchers also found evidence that blocking GIPR could enhance the effects of medicines targeting the amylin receptor, a finding that broadens the possible scope of GIPR antagonism beyond its current applications.
Dr Jo Lewis, the study’s first author from the Institute of Metabolic Science at the University of Cambridge, said: ‘Understanding which brain circuits respond to these medications, and how they do so, could help us design better drugs that produce more weight loss with fewer side effects, and which might work in combination with other obesity medicines to even greater effect. Our work also strengthens the idea that the brain is central to obesity treatment. Obesity drugs are not acting simply on the gut or pancreas. Instead, they have important effects on specific, identifiable brain circuits that regulate appetite and food intake.’
What this means for MariTide and combination drug design
The findings carry direct relevance for MariTide, a drug that combines GIPR antagonism with GLP-1 receptor agonism and is currently in phase 3 clinical trials. According to Superpower, Amgen’s phase 3 programme for MariTide was initiated in 2024 and 2025. The Cambridge research helps explain, at a mechanistic level, why that combination approach might be effective: the two components appear to act on distinct brain circuits, which raises the possibility that their effects could complement rather than merely overlap.
Earlier clinical data add weight to the interest in MariTide. In a phase 2 trial, the drug rapidly enrolled nearly 600 participants with obesity, with or without diabetes, across 12 countries, as reported by Springer Nature Research Communities. A separate analysis published on PubMed reported that in multiple ascending dose cohorts, weight loss was maintained for up to 150 days after the last dose, a durability figure that distinguishes it from some other agents in the class.
In the current phase 3 programme, according to Drugs.com, participants are randomised to target doses beginning at 21 mg, escalating to 35 mg and then to 70 mg over an eight-week titration period. Whether the brain-circuit evidence from Cambridge translates into measurably stronger clinical outcomes at those doses remains to be tested in humans.
The Cambridge study was funded by the Medical Research Council and Wellcome. Obesity affects more than a billion people worldwide and raises the risk of type 2 diabetes, cardiovascular disease and cancer. The researchers say that understanding how GIPR brain regions interact with GLP-1 pathways could inform drug combinations that produce greater weight loss with fewer side effects. Phase 3 results from MariTide will provide the first large-scale human test of whether the antagonist route lives up to that promise.
