A monthly obesity shot cut weight up to 16% by blocking a receptor rivals activate

Maridebart cafraglutide, an experimental obesity drug given as a once-monthly injection, cut body weight by between 12.3% and 16.2% over a year in a phase 2 trial, against 2.5% on placebo [s1]. Its interest is as much mechanistic as numerical: the drug, known as MariTide, blocks the GIP receptor that the market-leading drug tirzepatide switches on — two opposite actions on the same receptor, both producing weight loss.

That paradox is the reason this trial is worth reading closely. MariTide is a peptide-antibody conjugate that pairs agonism of the GLP-1 receptor with antagonism of the glucose-dependent insulinotropic polypeptide (GIP) receptor [s1]. Tirzepatide, the drug behind the real-world tirzepatide-versus-semaglutide comparisons, is a GIP receptor agonist [s2]. How activating and blocking the same receptor can both aid weight loss is unresolved, and MariTide's results are a piece of evidence that the GIP arm of these drugs is doing something more complicated than the simple "more agonism is better" story suggests.

What the trial did

The phase 2, double-blind, placebo-controlled, dose-ranging trial, funded by Amgen, enrolled 592 participants across 11 treatment groups arranged as two cohorts [s1]. The obesity cohort — 465 adults, 63% female, mean age 47.9 years, mean body-mass index 37.9 — was randomised to maridebart at 140, 280, or 420 mg every four weeks, to 420 mg every eight weeks, to two escalation schedules, or to placebo [s1]. A second cohort of 127 adults who had both obesity and type 2 diabetes (42% female, mean age 55.1, mean BMI 36.5) received 140, 280, or 420 mg every four weeks or placebo [s1]. The primary endpoint was the percentage change in body weight from baseline to week 52 [s1].

What it found

In the obesity cohort, mean weight loss at 52 weeks under the treatment-policy estimand — the conservative measure that counts everyone as randomised, including those who stopped the drug — ranged from 12.3% (95% CI 9.7 to 15.0) to 16.2% (95% CI 13.5 to 18.9) across the dose groups, against 2.5% (95% CI 4.2 to 0.7) on placebo [s1]. In the obesity-diabetes cohort, where weight loss is typically blunted, the range was 8.4% (95% CI 5.7 to 11.0) to 12.3% (95% CI 9.2 to 15.3) versus 1.7% (95% CI 0.6 to 2.9) on placebo [s1]. That cohort also saw glycated haemoglobin — the three-month blood-sugar average — fall by 1.2 to 1.6 percentage points on maridebart against a 0.1-point rise on placebo [s1].

Two features stand out. The weight loss had not clearly plateaued by 52 weeks in the higher-dose groups, hinting the ceiling may be higher with longer exposure [s1]. And the every-eight-weeks schedule still produced substantial loss, raising the prospect of a drug taken roughly six times a year rather than weekly — a meaningful change for adherence if it holds up. The diabetes cohort's result is worth weighing on its own terms: an 8.4% to 12.3% weight loss alongside a 1.2- to 1.6-point drop in glycated haemoglobin is a large metabolic effect in a group whose diabetes usually blunts both, though the cohort was small at 127 people and the confidence intervals correspondingly wide [s1].

The safety signal

Gastrointestinal side effects — nausea, vomiting — were common, as they are across this drug class, but were less frequent when the dose was escalated gradually from a lower starting point, and no unexpected safety signals emerged in the trial [s1]. That "no unexpected signals" language is a company-and-investigator characterisation of a phase 2 dataset of a few hundred people over a year; it is not the same as the long-term safety record built from the far larger, longer trials behind approved drugs.

How to read it

This is a phase 2 trial: dose-ranging, designed to find a schedule worth testing further, not to prove one. Its results cannot be laid directly alongside tirzepatide's or semaglutide's headline figures, which come from larger phase 3 trials with different populations and designs — the pitfalls of that kind of cross-trial comparison are set out in the updated systematic review of GLP-1 weight-loss data. What the trial does establish is that a GLP-1 agonist paired with GIP antagonism, dosed monthly, can produce weight loss in the same broad range as the weekly agonist-based drugs — enough to justify the phase 3 programme now under way [s1]. The site's broader treatment of what sustained weight loss does and does not achieve is in weight loss as disease modification, and an earlier attempt to preserve muscle while cutting fat in the bimagrumab-plus-semaglutide trial.

What to watch

The phase 3 trials will decide whether the monthly and every-other-month schedules hold their effect at scale, whether the gastrointestinal tolerability improves enough with escalation to keep people on the drug, and whether blocking GIP carries any long-run consequence that a one-year phase 2 study could not see. Until those report, MariTide is a promising mechanism with a striking early number, not an established therapy.

This article describes research and is not medical advice. Decisions about obesity treatment are for patients and their treating clinicians.