MariTide (AMG 133) Guide

MariTide, internally known as AMG 133, is Amgen's investigational anti-obesity drug currently in Phase 3 clinical trials. It is not a small-molecule peptide like semaglutide (Ozempic, Wegovy) or tirzepatide (Mounjaro, Zepbound) — it is a bispecific monoclonal antibody, a fundamentally different drug class that allows for a radically different dosing schedule.

As a large antibody molecule, MariTide has a much longer half-life than peptide-based GLP-1 drugs, enabling once-monthly subcutaneous injection compared to the once-weekly schedule required by all currently approved GLP-1 receptor agonists. This dosing convenience advantage — going from 52 injections per year to just 12 — could be a major differentiator in patient adherence and quality of life.

The drug was named MariTide as it progressed through clinical development, replacing its internal code name AMG 133. Phase 3 is being conducted under the MARITIME trial designation. Amgen has made AMG 133 / MariTide one of its highest-priority pipeline programs, with substantial manufacturing investment suggesting confidence in the Phase 3 outcome.

MariTide's mechanism is distinct from all other approved obesity drugs — and even from tirzepatide, which is superficially similar. To understand why, it helps to review what GIP (glucose-dependent insulinotropic polypeptide) actually does in obesity.

The scientific rationale for blocking GIPR rather than activating it is grounded in the observation that in obesity, GIP signaling becomes dysregulated and may promote adipogenesis (fat cell creation) and fat storage. By antagonizing GIPR, MariTide theoretically prevents GIP from contributing to fat accumulation, while simultaneously activating GLP-1R to suppress appetite. This combined effect — starving fat cells of pro-storage signal while reducing caloric intake — may explain the superior and non-plateauing weight loss observed in Phase 2.

The debate within obesity pharmacology is not fully resolved: tirzepatide's success with GIPR agonism challenges the "GIPR antagonism is better" hypothesis. The leading reconciliation theory is that in the normal-weight state, GIP receptor agonism is beneficial for glucose disposal; in the obese state, the GIP system is already over-active and blocking it may be more beneficial. Both approaches appear to produce clinically significant weight loss through different paths — the question Phase 3 will answer is which produces more durable and profound outcomes.

The Phase 2 results for MariTide, published in the New England Journal of Medicine in late 2024, generated significant excitement in the obesity medicine community. The trial enrolled 595 participants with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity, across multiple dosing regimens and treatment durations.

The most notable finding was the absence of a plateau. Weekly GLP-1 medications (semaglutide, tirzepatide) typically show the most rapid weight loss in the first 6 months with a gradual leveling off by months 12-18, even with continued treatment. MariTide's weight loss trajectory in Phase 2 appeared to continue declining at 52 weeks without flattening, suggesting the maximum effect had not yet been reached — an unprecedented characteristic among obesity drugs studied to date.

Phase 2 also examined MariTide in participants with type 2 diabetes, where results were somewhat attenuated compared to the non-diabetic group — consistent with the pattern seen for semaglutide (where diabetes blunts weight loss response). Glycemic benefits were also observed, with meaningful HbA1c reductions. The safety profile in Phase 2 was similar to weekly GLP-1 medications: nausea was the most common adverse event, occurred predominantly in the dose-escalation period, and was generally mild-to-moderate and transient.

One of the most significant limitations of current GLP-1 medications is weight loss plateau. After an initial period of substantial loss, the body's metabolic adaptations — reduced resting energy expenditure, increased appetite signaling at the new lower body weight, and potential tachyphylaxis to the medication — collectively cause weight loss to stall. Many patients and clinicians find this frustrating, particularly for those with significant obesity who would benefit from additional loss.

MariTide's apparent avoidance of this plateau in Phase 2 (through 52 weeks of observation) is one of its most compelling differentiating features. There are several hypothetical explanations: the GIPR antagonism component may continue to reduce adipogenesis even as the body adapts to GLP-1R signaling; the monthly dosing pattern may produce a different pharmacodynamic profile that prevents receptor desensitization; or MariTide may engage distinct downstream signaling pathways with different adaptation kinetics compared to peptide agonists.

Phase 3 with longer observation periods (likely 104 weeks / 2 years) will be critical to confirm whether the no-plateau characteristic persists. If it does, MariTide would be in a class of its own — offering not just monthly convenience but qualitatively superior durability of weight loss effect compared to all existing therapies.

The MARITIME trial is Amgen's pivotal Phase 3 program for MariTide. Based on what Amgen has disclosed, MARITIME includes multiple substudies enrolling people with obesity with and without type 2 diabetes, examining weight loss, cardiovascular risk factors, and safety endpoints over a minimum of 72 weeks. The trial is global and multicenter, consistent with FDA and EMA requirements for obesity drug approval.

Key questions MARITIME will need to answer include: Does the no-plateau effect persist beyond 52 weeks? What is the optimal monthly dose? How does MariTide compare head-to-head in efficacy and tolerability vs semaglutide and tirzepatide? What is the safety profile in a much larger and more diverse population? Are there any unexpected immunogenicity concerns unique to the bispecific antibody format?

Pivotal Phase 3 efficacy data is expected in 2026. If MARITIME meets its primary endpoints, Amgen would submit a Biologics License Application (BLA) — not a standard NDA, reflecting MariTide's antibody nature — to the FDA. FDA review typically takes 12 months for priority review or longer for standard review. A US launch in 2027-2028 is a plausible scenario if all goes according to plan.

Amgen has significantly expanded its manufacturing capacity for biologics, which analysts interpret as a signal of high internal confidence in MARITIME outcomes. The obesity drug market — already exceeding $20 billion annually in 2025 — is large enough to accommodate multiple winning products, and Amgen has stated publicly that MariTide's differentiated dosing profile is a strategic priority.

When MariTide becomes available, Shotlee will support once-monthly injection tracking — logging your dose, tracking the 30-day interval, monitoring weight loss progress, and logging side effects at each monthly touchpoint. In the meantime, Shotlee supports tracking all currently available GLP-1 medications.

If you are currently on a weekly GLP-1 medication and considering MariTide when it becomes available, Shotlee lets you build a complete treatment history — documenting your entire weight loss journey from first weekly injection through any future transition to monthly therapy. This longitudinal data is invaluable for your prescriber.