MariTide Guide: The First Antibody-Peptide Conjugate for Obesity

What Is MariTide?

MariTide — international non-proprietary name maridebart cafraglutide, development code AMG-133 — is an investigational biologic developed by Amgen for chronic weight management in adults with obesity or overweight with weight-related comorbidities. It belongs to a new therapeutic class: the antibody-peptide conjugate, sometimes abbreviated APC.

Functionally, MariTide marries two drug modalities that are usually kept separate. The first is a monoclonal antibody directed against the glucose-dependent insulinotropic polypeptide (GIP) receptor. The second is a pair of peptide agonists of the GLP-1 receptor, chemically linked to the antibody backbone. The result is a hybrid molecule that simultaneously activates GLP-1 signaling while antagonizing GIP signaling — and inherits the long circulating half-life of an antibody.

Amgen first disclosed AMG-133 human pharmacokinetic and pharmacodynamic data in 2023. A dose-ranging Phase 2 study in adults with obesity read out in late 2024 and 2025, showing large, sustained reductions in body weight and triggering the MARITIME Phase 3 program that is actively enrolling as of April 2026. The Phase 3 program is among the largest obesity trials ever designed, reflecting MariTide's positioning as a potential major commercial competitor to semaglutide (Wegovy) and tirzepatide (Zepbound).

How MariTide Works: The GIP Antagonism Question

MariTide's mechanism is unusual. It combines GLP-1 receptor agonism (pushing one accelerator pedal) with GIP receptor antagonism (taking the foot off a different pedal). This is the opposite of tirzepatide, which agonizes both GLP-1 and GIP.

GLP-1 Receptor Agonism

The GLP-1 agonist peptides conjugated to the antibody activate GLP-1 receptors in the brain, pancreas, stomach, and intestine, producing the effects the class is now famous for:

• Appetite suppression — GLP-1 receptors in the hypothalamus and brainstem reduce hunger signaling and increase satiety.
• Delayed gastric emptying — slowing the stomach prolongs fullness.
• Glucose-dependent insulin secretion — pancreatic beta cells release insulin when blood glucose rises, improving glycemic control.
• Reduced glucagon secretion — limits hepatic glucose output.

GIP Receptor Antagonism: The Counterintuitive Part

GIP is the other major incretin hormone. Agonizing it — as tirzepatide does — has clear clinical benefits. But a body of genetic, animal, and human data suggests that antagonizing GIP may also produce weight loss, through partially distinct pathways:

• Genetic variants that reduce GIP receptor function have been associated with lower body weight.
• GIP receptor antagonism may reduce nutrient storage in adipose tissue and improve insulin sensitivity.
• Blocking GIP may dampen chronic overeating signals in hypothalamic circuits sensitive to high-fat diets.
• Some preclinical studies suggest combined GLP-1 agonism plus GIP antagonism produces additive or synergistic weight loss.

The paradox that both GIP agonism and GIP antagonism can produce weight loss is still being actively debated in endocrinology literature. MariTide is the first compound to test pure GIP antagonism alongside GLP-1 agonism in humans at scale.

The Antibody-Peptide Conjugate Advantage

Peptides tend to have short half-lives because kidneys filter them out quickly and enzymes degrade them. Attaching a peptide to an antibody borrows the antibody's slow clearance — antibodies are recycled by neonatal Fc receptors (FcRn) and can persist in circulation for weeks. For MariTide, that means a half-life of roughly 21 days versus ~7 days for semaglutide and ~5 days for tirzepatide. Clinically, this translates to monthly rather than weekly injections, which improves convenience and adherence.

MariTide vs. Semaglutide vs. Tirzepatide vs. Retatrutide

Feature	MariTide	Semaglutide	Tirzepatide	Retatrutide
Molecule type	Antibody-peptide conjugate	Peptide	Peptide	Peptide
Receptor targets	GLP-1 agonist + GIP antagonist	GLP-1 agonist	GLP-1 + GIP agonist	GLP-1 + GIP + glucagon agonist
Manufacturer	Amgen	Novo Nordisk	Eli Lilly	Eli Lilly
Approval status	Phase 3 (MARITIME)	FDA approved	FDA approved	FDA approved (2025)
Dosing interval	Once monthly SC	Once weekly SC	Once weekly SC	Once weekly SC
Half-life	~21 days	~7 days	~5 days	~6 days
Phase 2/3 weight loss*	~20% at 52 weeks	~15% at 68 weeks	~20–22% at 72 weeks	~24% at 48 weeks
Common side effects	Nausea, vomiting, constipation	Nausea, vomiting, diarrhea	Nausea, vomiting, diarrhea	Nausea, vomiting, diarrhea

*Phase 2 or Phase 3 mean body-weight reduction at the highest studied doses in participants without diabetes. Individual response varies widely. Head-to-head trials between most of these agents are still pending.

Phase 2 Clinical Data: What MariTide Achieved

Amgen's Phase 2 study enrolled approximately 592 adults across two cohorts: participants with obesity but without type 2 diabetes, and participants with obesity and type 2 diabetes. Dosing was explored across multiple subcutaneous dose levels administered every four or every eight weeks, with a 52-week treatment period.

Weight Loss in Adults with Obesity (No Diabetes)

At the highest doses, adults without diabetes achieved mean body-weight reductions of approximately 20 percent after 52 weeks. Amgen emphasized that weight loss curves had not yet plateaued at trial end, suggesting further reduction was likely with continued dosing. For context, semaglutide and tirzepatide's Phase 3 weight-loss curves plateau between weeks 60 and 72.

Weight Loss in Adults with Obesity and Type 2 Diabetes

In the diabetes cohort, 52-week mean weight reductions reached approximately 17 percent — somewhat attenuated relative to the non-diabetes cohort, which is a pattern seen with all GLP-1-based therapies. HbA1c and fasting glucose improvements were robust and comparable to other GLP-1 class agents.

Cardiometabolic Markers

• Systolic and diastolic blood pressure reductions in the range of 6–10 mmHg at higher doses.
• Triglyceride reductions of 15–25 percent.
• LDL cholesterol and hsCRP reductions consistent with the GLP-1 class.
• Glycemic improvement in diabetic participants comparable to class-leading agents.

The MARITIME Phase 3 Program

Amgen announced the full Phase 3 program — branded MARITIME — in 2024, and the studies are actively enrolling throughout 2025–2026. MARITIME is notable for its size and breadth:

Core Phase 3 Studies

• MARITIME-1 — obesity without diabetes; primary endpoint is weight change at 72 weeks versus placebo. This is the core efficacy study for a weight-management indication.
• MARITIME-2 — obesity with type 2 diabetes; co-primary endpoints of weight change and HbA1c reduction.
• MARITIME-OSA — obstructive sleep apnea in adults with obesity; tests whether weight loss with MariTide reduces apnea-hypopnea index.
• MARITIME-HFpEF — heart failure with preserved ejection fraction; tests effects on symptoms and exercise capacity.

Cardiovascular Outcomes Trial

A large dedicated cardiovascular outcomes trial (CVOT) is planned or underway to assess MACE — major adverse cardiovascular events — a requirement for a modern obesity drug to compete on the cardiometabolic label. The semaglutide SELECT trial and tirzepatide SURMOUNT programs have set the precedent.

Estimated Regulatory Timeline

• 2026–2027: Phase 3 trial readouts begin.
• 2027–2028: FDA Biologics License Application (BLA) submission expected.
• 2028–2029: Potential FDA approval and launch, pending data and review.

MariTide Dosing Protocols in Clinical Trials

MariTide is administered as a subcutaneous injection. Because the molecule is a biologic with a long half-life, dosing is spaced monthly or every two months rather than weekly.

Phase 2 Dose Ranges

• Q4W (every 4 weeks): 140 mg, 280 mg, 420 mg
• Q8W (every 8 weeks): 280 mg, 420 mg, 560 mg
• Titration: Most cohorts used dose escalation over the first 2–4 doses to reduce gastrointestinal intolerance before reaching maintenance dose.

Injection Sites and Technique

Phase 2 injections were administered into the abdomen or thigh. Because MariTide is an antibody-peptide conjugate, injection volumes are larger than small-peptide GLP-1 agonists. Delivery devices for Phase 3 and potential commercial use include prefilled pens and autoinjectors designed to handle these larger volumes.

Side Effects and Safety Profile

Across the Phase 2 program, MariTide's safety profile was largely consistent with the GLP-1 class. Because the molecule is novel and clinical exposure is still limited, long-term safety is still being characterized in Phase 3.

Gastrointestinal Effects

The most common adverse events mirrored the GLP-1 class:

• Nausea: Reported in a majority of participants at higher doses, most often during the first 1–2 dose cycles. Intensity decreased with continued treatment.
• Vomiting: Less common than nausea, typically transient.
• Constipation: Reported notably in MariTide trials; somewhat more prominent than in semaglutide or tirzepatide trials, possibly related to GIP antagonism.
• Diarrhea: Reported at rates similar to other GLP-1 agents.

Titration Strategy

Based on the Phase 2 dose-response curves, MARITIME Phase 3 incorporates longer dose-titration sequences. The design hypothesis is that stepping the dose up over 2–3 months substantially reduces gastrointestinal intolerance without blunting peak efficacy.

Immunogenicity

Because MariTide contains an antibody component, clinical development closely monitors anti-drug antibody (ADA) formation. In Phase 2, ADA rates were low and did not appear to impact efficacy or safety. Ongoing Phase 3 monitoring will clarify long-term immunogenicity.

Class Effects to Watch

• Pancreatitis: A theoretical GLP-1 class concern; incidence in MariTide Phase 2 was not different from background.
• Gallbladder disease: Cholelithiasis and cholecystitis occur at slightly elevated rates with rapid weight loss on GLP-1 drugs, regardless of mechanism.
• Thyroid C-cell tumors: Preclinical rodent signal common across GLP-1 agents; not confirmed in humans.
• Lean mass loss: Like all weight-loss pharmacotherapy, 20–30 percent of total weight lost is typically lean mass; resistance exercise and adequate protein intake mitigate this.

Why MariTide Matters for the Obesity Pharmacology Landscape

MariTide is strategically important for several reasons that go beyond its weight-loss numbers.

Once-Monthly Dosing Is a Real Adherence Story

Real-world persistence on weekly GLP-1 injections is lower than clinical trials suggest — roughly half of commercial Wegovy patients discontinue within a year. A once-monthly injection reduces cognitive and logistical friction substantially. If MARITIME replicates Phase 2 efficacy, MariTide would offer a compelling package: similar or better weight loss delivered with one quarter of the injection burden.

Proof-of-Concept for Antibody-Peptide Conjugates

MariTide is a platform success. If the molecule achieves approval, expect antibody-peptide conjugate technology to expand rapidly into other metabolic and hormonal indications — potentially enabling quarterly or semi-annual dosing of therapies that currently require weekly injections.

Resolving the GIP Question

The endocrinology field has spent years debating whether GIP should be agonized or antagonized in the context of obesity. Phase 3 head-to-head (or real-world) comparisons of MariTide versus tirzepatide will help resolve this, with implications for future compound design.

Competitive Dynamics

Amgen is entering a market dominated by Novo Nordisk and Eli Lilly. MariTide's differentiation — the monthly cadence and novel mechanism — is designed to carve out meaningful share rather than compete purely on weight-loss percent.

MariTide and Research Peptides: Important Differences

Because MariTide is a biologic — not a simple peptide — it is not available in the research chemical market. Unlike small peptides such as semaglutide or retatrutide, which can be synthesized by specialty manufacturers, antibody-peptide conjugates require sophisticated cell-line expression, purification, and conjugation chemistry that is effectively impossible to reproduce outside a biologics manufacturing facility.

Any product marketed as "MariTide" or "AMG-133" outside of Amgen's clinical trial program should be assumed to be mislabeled, counterfeit, or something else entirely. Researchers interested in adjacent GLP-1 or GLP-1/GIP biology have more legitimate options in the small-peptide space covered elsewhere on WolveStack.

Research Peptide Vendors (Related Compounds)

Disclosure: MariTide itself is not available for research use. The vendors below offer related research peptides in the GLP-1 and metabolic class for researchers and clinicians whose work involves the broader incretin research landscape.

MariTide FAQ

Conclusion: A Platform Shift for Obesity Medicine

MariTide represents more than another weight-loss drug. It is the clinical proof of a platform — antibody-peptide conjugates — that could fundamentally change how long-acting biologics are designed across metabolic and hormonal disease. If MARITIME Phase 3 replicates the Phase 2 efficacy (roughly 20 percent mean weight loss with no plateau at 52 weeks), Amgen will enter the obesity market with a product that is competitive on weight loss and differentiated on dosing cadence. Even if Phase 3 attenuates the Phase 2 signal somewhat, once-monthly dosing alone is likely to carve out meaningful share.

For researchers following GLP-1 biology, MariTide's key contribution may be mechanistic rather than commercial. Its Phase 3 results will be the first large-scale test of GIP antagonism — a neat scientific contrast to tirzepatide's GIP agonism — and will inform the next generation of multi-agonist and multi-modality compounds.

For clinicians and individuals tracking the obesity space, the practical watch list is straightforward: MARITIME Phase 3 readouts beginning in 2026–2027, BLA submission in 2027–2028, potential approval in 2028–2029, and a cardiovascular outcomes trial that will determine how broadly MariTide is labeled beyond weight management.