MariTide (Maridebart Cafraglutide / AMG 133): The Once-Monthly GLP-1 Obesity Drug Explained

The headline: monthly dosing changes the user experience

Every approved GLP-1 drug today — semaglutide (Ozempic, Wegovy, Rybelsus), tirzepatide (Mounjaro, Zepbound), liraglutide (Saxenda, Victoza), dulaglutide (Trulicity), exenatide (Bydureon) — requires a weekly injection at the longest interval, and several are daily. MariTide is different: one injection per month. That single change is enough to make it the most-watched drug in the obesity-medicine pipeline.

The reason monthly dosing matters is not the convenience of a monthly versus a weekly needle — it is what happens between doses. Weekly GLP-1s reach stable blood levels and stay there as long as adherence holds, but adherence is the single biggest determinant of real-world outcomes. Monthly dosing dramatically simplifies the cadence, reduces injection-site fatigue, and structurally pushes adherence closer to 100%. It is also a different pharmacologic strategy: MariTide is built on an antibody backbone with a half-life measured in weeks rather than days, which is what allows the monthly schedule in the first place.

What MariTide actually is (the molecule)

MariTide's generic name is maridebart cafraglutide — a name that took some time to settle, with the molecule referred to as AMG 133 through most of the early literature. The drug is being developed by Amgen.

Structurally, it is a bispecific peptide-antibody conjugate: two GLP-1 receptor agonist peptides chemically linked to a human monoclonal antibody that binds the GIP receptor. The antibody scaffold serves two purposes. It blocks the GIP receptor (the antibody is an antagonist, not an agonist), and it dramatically extends the molecule's half-life — antibodies persist in the blood for weeks because of their recycling via the neonatal Fc receptor.

The practical consequence: a single injection produces meaningful drug exposure for a month or more, and the molecule decays slowly enough that the off-target tail extends well beyond the dosing interval. The same long half-life is what allows monthly dosing in the first place — and is likely why Phase 2 patients who stopped dosing did not immediately rebound.

The mechanism: GIP antagonism, not agonism

Here is where MariTide diverges most interestingly from the existing dual-agonist class. Tirzepatide — the current best-in-class for weight loss — is a GLP-1 receptor agonist and a GIP receptor agonist. Activating both receptors deepens weight loss compared with GLP-1 alone, and is the proof-of-concept that drove the field toward dual agonists.

MariTide does the opposite on the GIP arm. It is a GLP-1 receptor agonist and a GIP receptor antagonist. The biological rationale comes from human genetics — loss-of-function variants in the GIP receptor are associated with lower body mass in population studies — and from animal models showing that blocking GIP signaling can produce weight loss on its own. The Amgen position is that simultaneously activating GLP-1 and blocking GIP can be at least as effective as activating both.

This is not a settled question. Tirzepatide's dual-agonism advantage is empirically robust, and there is a real scientific debate about how much of the metabolic benefit attributed to GIP agonism actually reflects partial receptor desensitization that effectively mimics antagonism. The MariTide trials are the first head-to-head test in humans of whether the antagonist direction can match or exceed the agonist direction — and the Phase 2 magnitudes are in the same neighborhood as tirzepatide's, which is a clue that both directions of GIP modulation may work.

What the Phase 2 trial showed

The pivotal Phase 2 trial of MariTide enrolled adults with obesity (with or without type 2 diabetes) and tested the drug at multiple doses against placebo over 52 weeks. The results, reported in late 2024 and discussed widely through 2025, were the reason MariTide became the most-discussed obesity drug of the cycle:

• Approximately 20% mean weight loss at the highest dose in patients without diabetes over 52 weeks — a magnitude in the same neighborhood as tirzepatide's SURMOUNT-1 result.
• Approximately 17% mean weight loss in patients with type 2 diabetes — also strong, given that weight loss in diabetics on GLP-1 drugs is typically modestly less than in non-diabetics.
• Glycemic improvement in the diabetic group consistent with the rest of the class.
• Side-effect profile qualitatively similar to other GLP-1 drugs — primarily nausea, vomiting, and constipation, concentrated in the dose-escalation phase.

Two things made the result notable beyond the raw magnitude. First, it was achieved with monthly rather than weekly injections — one-quarter the number of needle events. Second, and more surprising, was what happened after the last dose.

The sustained-effect finding

In follow-up analysis of the Phase 2 patients, those who completed the trial and then stopped MariTide injections did not begin regaining weight at the rate that typically follows discontinuation of weekly GLP-1 drugs. Several patients continued to lose weight for months after the last dose, and most held the losses they had achieved for a substantial period before any rebound began.

This is unusual. With semaglutide, tirzepatide, and liraglutide, the standard observation is that weight regain begins within weeks of stopping the drug and most patients return close to baseline within a year — a pattern documented across multiple discontinuation studies and the reason GLP-1 therapy is now framed as long-term metabolic management rather than a finite weight-loss course.

The sustained-effect signal with MariTide is almost certainly driven by the molecule's pharmacology. The antibody scaffold gives MariTide a half-life much longer than weekly peptides, so "stopping" the drug is less of a step function and more of a slow taper that may extend over months. Whether there is also a durable biological adaptation — a reset of appetite or energy expenditure that persists after drug levels fall — is an open and important question. If real, it would change how the entire class is positioned.

The honest caveat: Phase 2 patient numbers are small, and "sustained" needs longer follow-up than what has been reported so far. Phase 3 will be the real test.

Phase 3: what to watch for

The MARITIME Phase 3 program is enrolling now and runs across multiple trials in different populations — obesity without diabetes, type 2 diabetes, sleep apnea, and cardiovascular outcomes. Lead readouts are expected to begin in 2026-2027, with regulatory submission likely in 2027 and U.S. approval most plausibly in 2028.

The metrics worth watching:

• Weight loss magnitude at 72 weeks — the timeframe used for SURMOUNT-1 (tirzepatide) and STEP-1 (semaglutide), allowing direct comparison.
• Tolerability in larger populations — Phase 2 nausea profile looked manageable, but Phase 3 reveals real-world rates of discontinuation.
• Whether the sustained-effect finding replicates — perhaps the highest-impact unknown. An arm with planned discontinuation and longitudinal follow-up would directly answer this.
• Cardiovascular outcomes — like SELECT (semaglutide) and the pending SURPASS-CVOT (tirzepatide), MariTide will need its own outcomes trial to be positioned as a cardiovascular-and-metabolic agent rather than a weight-loss drug.

How MariTide fits into the pipeline

By 2028 the obesity-pharmacology landscape will plausibly look quite different from the semaglutide-or-tirzepatide choice of 2026. Several next-generation drugs are converging:

• Retatrutide (Lilly): triple agonist (GIP/GLP-1/glucagon), weekly, Phase 3, deepest weight loss of any GLP-1-class drug in Phase 2 so far.
• CagriSema (Novo): semaglutide + cagrilintide combination, weekly, Phase 3 ongoing.
• Amycretin (Novo): a single molecule combining GLP-1 and amylin activity, Phase 1/2 striking, oral and injectable forms in development.
• Orforglipron (Lilly): oral small-molecule (non-peptide) GLP-1 agonist, daily, Phase 3 readouts expected through 2026.
• MariTide (Amgen): monthly, GLP-1 + GIP antagonist, the only one of the group on a non-weekly schedule.

MariTide's distinct positioning is the combination of monthly dosing and the unique mechanism. Even if its weight-loss magnitude lands slightly below retatrutide's, the dosing convenience and the potential sustained effect are differentiating features that no other late-stage drug currently shares.

The honest read

MariTide is the most pharmacologically interesting drug in the late-stage obesity pipeline. The Phase 2 magnitude is strong, the monthly dosing is genuinely novel, and the sustained-effect finding — if it survives Phase 3 — would meaningfully change how the class is used. Approval is still two-plus years away, and the usual Phase 2-to-Phase 3 caveats apply: magnitudes often shrink modestly at scale, and rare side effects only become visible in larger populations.

For someone choosing among currently-available drugs in 2026, MariTide should not change today's decision — it is not approved, not available, and the most realistic timeline puts it years out. For someone tracking the field, it is the drug whose readouts will most likely move the field's center of gravity between now and 2028.