Is AMG 133 Peptide Real? MariTide Results and Bloodwork

Every weight loss drug you have tracked bloodwork on so far has been weekly. Semaglutide, tirzepatide, retatrutide: inject Sunday, draw blood whenever suits you, and the number you get is roughly the number you would have got on any other day. Steady state does that for you.

MariTide breaks that assumption, and almost nobody using it has noticed yet.

What MariTide actually is

MariTide is the trade name. The generic is maridebart cafraglutide. It spent its early life as AMG 133, and plenty of forum posts still call it that.

Structurally it is not a peptide, which is where most of its interesting properties come from. It is a monoclonal antibody against the GIP receptor, with two GLP-1 analogue peptides chemically bolted on. A peptide-antibody conjugate. That construction is why it behaves pharmacokinetically like an antibody rather than like semaglutide, and it is the entire reason monthly dosing is possible.

The half-life reflects that. In the Phase 1 work, the intact conjugate ran roughly 14 to 16 days and the total antibody species roughly 21 to 24 days (Véniant et al., 2024). Semaglutide is about a week. Tirzepatide is about five days.

Why nothing sold as AMG 133 peptide can be real

Two of the most common things people type into Google about this drug are "amg 133 peptide for sale" and "amg 133 peptide where to buy". Both rest on a category error, and it is worth understanding why, because the same reasoning protects you from a whole class of scams.

Grey-market peptides are made by solid-phase peptide synthesis. You anchor an amino acid to a resin bead and add the next one, and the next, chemically. It is well understood, the equipment is obtainable, and it works reliably up to roughly 50 amino acids. That is why the underground peptide market exists at all, and why it is populated by the molecules it is: BPC-157 is 15 amino acids, semaglutide 31, tirzepatide and retatrutide 39 each. All comfortably inside what the chemistry can do.

An antibody is not in that league. An IgG is around 1,300 amino acids spread across four separate chains, roughly 150 kilodaltons, and it only works if it folds correctly, forms the right disulfide bridges, and carries the right sugars. You cannot build that on a resin bead. It requires engineered mammalian cells grown in bioreactors, then protein A chromatography to purify, then quality control that most of the peptide market has never attempted. MariTide then adds a further step: site-specific conjugation of two GLP-1 analogue peptides onto that antibody.

That is a biologics manufacturing plant, not a lab in a spare room. The gap is not one of scale or willingness. It is a different branch of chemistry.

Amgen has not licensed, sold or released supply to anyone. Phase 3 material is controlled trial stock. Until an approval happens, every gram of real MariTide in existence is accounted for, and none of it is for sale.

Why MariTide blocks the receptor tirzepatide switches on

Here is the part that should bother you, because it bothers the people who develop these drugs.

Tirzepatide is a GIP receptor agonist. It turns the receptor on. MariTide is a GIP receptor antagonist. It blocks it. Both are paired with GLP-1 agonism, and both produce substantial weight loss in humans.

Two drugs doing opposite things at the same receptor should not both work. One of them should be wrong.

The leading explanation, and why you should hold it loosely

The most popular resolution is receptor desensitisation. The argument runs that chronic, high-level GIP receptor agonism eventually exhausts the receptor, so sustained agonism becomes functionally indistinguishable from blocking it. Amgen's own researchers demonstrated exactly this: prolonged GIP exposure produced near-complete desensitisation of GIPR signalling in adipocytes (Killion et al., 2020).

That is a real finding. It is also a finding in cell culture and rodents. Nobody has shown this happens in a person taking tirzepatide. The doses differ, tirzepatide is a partial and deliberately imbalanced GIP agonist rather than a full one, and the receptors that appear to drive appetite suppression are in the brain, not fat tissue, where desensitisation may behave completely differently (Zhang et al., 2021).

Two papers, same journal, opposite conclusions

If you want a sense of how unresolved this is, look at what the journal Diabetes published in August 2025.

Pages 1326 to 1333: A Contemporary Rationale for Agonism of the GIP Receptor in the Treatment of Obesity, from Lilly-affiliated authors (Samms and Sloop, 2025).

Pages 1334 to 1338, immediately following: GIP Receptor Antagonists in the Pharmacotherapy of Obesity, with Amgen affiliation (Rosenkilde et al., 2025).

Back-to-back. Consecutive pages. One argues for switching the receptor on, the next argues for switching it off, and the editors printed them together because neither side can currently prove the other wrong.

A separate review that year concluded the same thing: both development programmes rest on plausible, partially supported models, and no human study has adjudicated between them (Douros et al., 2025).

What the Phase 2 trial actually found

The Phase 2 results were published in the New England Journal of Medicine in June 2025 (Jastreboff et al., 2025). 592 participants across 11 groups and two cohorts, 52 weeks, primary endpoint percent change in body weight.

Dosing arms tested:

• 140mg, 280mg or 420mg every 4 weeks, no dose escalation
• 420mg every 8 weeks, no escalation
• 420mg every 4 weeks with 4-week escalation
• 420mg every 4 weeks with 12-week escalation

That an 8-week arm exists at all tells you the dosing interval is a design variable rather than a fixed property.

Results at 52 weeks:

HbA1c in the diabetes cohort fell 1.2 to 1.6 percentage points against a 0.1 point rise on placebo. That is a serious glycaemic effect, comparable to what you would expect from the established drugs in this class.

The detail worth sitting with: weight loss had not plateaued at 52 weeks. The curve was still going down when the trial ended.

The cost, and it is a real one

Gastrointestinal adverse events were common: nausea, vomiting, constipation. The trial found they were less frequent with dose escalation and a lower starting dose, which is exactly what you would expect, and exactly the problem with monthly dosing.

On a weekly drug you titrate gently. You take a small dose, feel it, and step up. On a monthly drug you take a dose and live with it for four weeks. There is no gentle version of that first injection, and the trial data shows the adverse events cluster there.

When to draw blood on a once-monthly drug

This is the part no other MariTide article covers, and it is the part that will actually change your numbers.

Why weekly drugs let you be sloppy

Semaglutide has a half-life of about a week and is dosed weekly. After a month or so you are at steady state: the amount you inject each week roughly equals the amount you clear, and your plasma concentration ripples gently around a stable average. Draw on day 2 or day 6 and you get much the same answer.

Why MariTide does not

MariTide peaks around 4 to 7 days after injection and has a half-life of roughly 21 days against a 28-day interval (Véniant et al., 2024).

Work that through. From peak at roughly day 5 to the next injection at day 28 is about 23 days, which is a little over one half-life. So trough sits somewhere near half of peak.

I want to be precise about the status of that number: it is my arithmetic from the published half-life and time-to-peak, not a figure any trial has reported. No published peak-to-trough ratio exists for the monthly regimen. Treat "roughly twofold" as a reasonable inference, not a measurement.

There is a second wrinkle. With a 21-day half-life, MariTide does not reach steady state within a single 28-day cycle at all. It keeps accumulating dose over dose through the early months. So in your first few injections you are not only sampling a different point in the cycle, you are sampling a different overall exposure level.

Which markers this actually affects

The Phase 1 study measured markers at day 29, day 85 and day 169, the last being after the drug had washed out (Véniant et al., 2024). The pattern is informative.

Timing-sensitive. These tracked drug exposure and drifted back as levels fell:

• Fasting glucose fell dose-responsively by day 29 and held at day 85, then "returned toward or above baseline" by day 169
• Fasting insulin dropped at day 29 but was not maintained at day 85 or 169
• Fasting glucagon fell at the higher doses and normalised during follow-up
• Free fatty acids rose on drug and returned to or below baseline by day 169

Not timing-sensitive.

• HbA1c integrates roughly three months of glycaemia through red cell lifespan. A single cycle's peak or trough cannot meaningfully move it. Draw it whenever.
• C-peptide showed no notable trend across the timeline.
• ALT and AST track cumulative hepatic fat loss over months rather than plasma drug level. Extrapolated from semaglutide MASH data rather than measured on MariTide (Ajmera et al., 2026).
• eGFR moves on a multi-year slope, not a monthly one (Perkovic et al., 2024).

Genuinely unreliable. Phase 1 saw transient falls in total cholesterol, LDL and triglycerides between days 29 and 78 in the placebo group as well as on drug. At that sample size the lipid signal cannot be separated from noise. Do not read much into a single lipid panel on this drug.

The rule

Pick one point in the dosing cycle and always draw there. Either 3 to 5 days after injection, near peak, or the day before your next injection, near trough. Write it down. Then compare like with like every time.

If you draw at day 5 in March and day 26 in April and your fasting insulin looks better, you have not learned anything about your metabolic health. You have measured your calendar.

The lean mass question nobody has answered

This is the question this audience cares about most, and MariTide has not answered it.

No DXA or body composition substudy has been published for MariTide. The Phase 1 and Phase 2 papers report body weight, BMI and waist circumference. That is all. Anyone quoting you a MariTide muscle retention figure is extrapolating and probably not telling you so.

What we can say comes from the neighbours:

• Tirzepatide, SURMOUNT-1 DXA substudy (n=160), week 72: body weight down 21.3%, fat mass down 33.9%, lean mass down 10.9%. Roughly 75% of the weight lost was fat and 25% was lean, and that ratio held on placebo too (Look et al., 2025)
• Semaglutide, SUSTAIN 8 substudy: fat mass down 3.4kg, lean mass down 2.3kg over 52 weeks (McCrimmon et al., 2020)
• Class meta-analysis of 7 trials, 821 patients: pooled lean mass loss 1.74kg, with very high heterogeneity. Worth reading the authors' own conclusion alongside that number: because total weight fell further than lean mass did, lean mass as a share of body weight actually rose about 1.8%, and they argue lean loss should not be treated as a limitation of these drugs (Laverde et al., 2026). Whether you find that reassuring depends on whether you care about the ratio or the kilograms.

Whether MariTide behaves the same is unknown, and there is a specific reason to withhold judgement: it acts on GIP in the opposite direction to tirzepatide, and GIP has effects on adipose tissue that could plausibly change the fat-to-lean ratio of the weight you lose. Could go either way. Nobody has measured it.

In the meantime, resistance train and eat protein, and watch creatinine drifting down over months as a crude proxy for losing muscle mass.

What we still do not know

Being straight about the gaps is more useful than pretending they are filled.

• No within-cycle sampling study exists. Nobody has drawn bloods from the same person at week 1 and week 4 of a MariTide cycle and compared them. Everything in the timing section above is inferred from between-visit trends in a small Phase 1 trial.
• No published peak-to-trough ratio for the monthly regimen.
• No body composition data, as above.
• No detailed lab panel from Phase 2. The published results cover weight and HbA1c. Lipids, liver and kidney markers at 52 weeks have not been reported.
• Amylase and lipase: two isolated elevations in single participants across the entire Phase 1 programme, both resolving. Too few to establish a pattern, but worth watching given this drug class and pancreatitis.
• Whether the 8-week arm behaves differently on any of this has not been reported separately.

Practical recommendations

If you are following the science rather than taking the drug, which is where most readers should be right now, the interesting thing to watch is not the weight number. It is whether the Phase 3 programme publishes body composition data, and whether anyone resolves the GIP direction question.

If you are somehow taking it:

1. Standardise your draw day to a fixed point in the cycle. Trough is the more honest default.
2. Use HbA1c as your anchor marker. It is the one number immune to your dosing schedule.
3. Do not over-read a single lipid panel. The Phase 1 data suggests lipids are noisy on this drug, in both directions, including on placebo.
4. Get a DXA scan at baseline and every six months. Since the trials have not published body composition, yours is the only data you will have.
5. Expect the first injection to be the worst one. That is where the GI events cluster, and you cannot titrate your way around a monthly dose.

Key takeaways

• MariTide is a peptide-antibody conjugate that agonises GLP-1 and blocks GIP, the opposite of tirzepatide's action at that receptor. Both work, and nobody currently knows why both work.
• Phase 2 (n=592, 52 weeks) produced 12.3% to 16.2% weight loss without diabetes and 8.4% to 12.3% with it, and the curve had not plateaued.
• HbA1c fell 1.2 to 1.6 percentage points in the diabetes cohort.
• The half-life is roughly 21 days against a 28-day interval, so fasting glucose, insulin, glucagon and free fatty acids depend on where in the cycle you draw. Standardise your draw day.
• HbA1c, ALT, AST and eGFR are not timing-sensitive. Lipids are too noisy on this drug to read from a single panel.
• No body composition data exists for MariTide. Any muscle retention figure you are quoted is borrowed from tirzepatide or semaglutide.
• Bone density was specifically measured in Phase 2 and showed no signal. This was a genuine open question after a Phase 1 signal at the 420mg dose moved Amgen's share price in late 2024, and it is worth watching in Phase 3.

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References

1. Jastreboff, A. M., Ryan, D. H., Bays, H. E., Ebeling, P. R., Mackowski, M. G., Philipose, N., Ross, L., Liu, Y., Burns, C. E., Abbasi, S. A., & Pannacciulli, N. (2025). Once-monthly maridebart cafraglutide for the treatment of obesity: A phase 2 trial. New England Journal of Medicine, 393(9), 843-857. PubMed

2. Véniant, M. M., Lu, S. C., Atangan, L., Komorowski, R., Stanislaus, S., Cheng, Y., Wu, B., Falsey, J. R., Hager, T., Thomas, V. A., ... & Strande, J. L. (2024). A GIPR antagonist conjugated to GLP-1 analogues promotes weight loss with improved metabolic parameters in preclinical and phase 1 settings. Nature Metabolism, 6(2), 290-303. PubMed

3. Killion, E. A., Chen, M., Falsey, J. R., Sivits, G., Hager, T., Atangan, L., ... & Lloyd, D. J. (2020). Chronic glucose-dependent insulinotropic polypeptide receptor (GIPR) agonism desensitizes adipocyte GIPR activity mimicking functional GIPR antagonism. Nature Communications, 11(1), 4981. PubMed

4. Zhang, Q., Delessa, C. T., Augustin, R., Bakhti, M., Colldén, G., Drucker, D. J., ... & Müller, T. D. (2021). The glucose-dependent insulinotropic polypeptide (GIP) regulates body weight and food intake via CNS-GIPR signaling. Cell Metabolism, 33(4), 833-844. PubMed

5. Samms, R. J., & Sloop, K. W. (2025). A contemporary rationale for agonism of the GIP receptor in the treatment of obesity. Diabetes, 74(8), 1326-1333. PubMed

6. Rosenkilde, M. M., George, J. T., Véniant, M. M., & Holst, J. J. (2025). GIP receptor antagonists in the pharmacotherapy of obesity: Physiologic, genetic, and clinical rationale. Diabetes, 74(8), 1334-1338. PubMed

7. Douros, J. D., Mowery, S. A., & Knerr, P. J. (2025). The premise of the paradox: Examining the evidence that motivated GIPR agonist and antagonist drug development programs. Journal of Clinical Medicine, 14(11), 3812. PubMed

8. Look, M., Dunn, J. P., Kushner, R. F., Cao, D., Harris, C., Gibble, T. H., Stefanski, A., & Griffin, R. (2025). Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism, 27(5), 2720-2729. PubMed

9. McCrimmon, R. J., Catarig, A. M., Frias, J. P., Lausvig, N. L., le Roux, C. W., Thielke, D., & Lingvay, I. (2020). Effects of once-weekly semaglutide vs once-daily canagliflozin on body composition in type 2 diabetes: A substudy of the SUSTAIN 8 randomised controlled clinical trial. Diabetologia, 63(3), 473-485. PubMed

10. Laverde, L. P., Muñoz-Velandia, O. M., Alfonso, D., & Gómez Medina, A. M. (2026). Effect of GLP-1 receptor agonists at doses for obesity management on muscle health: Systematic review and meta-analysis of randomized controlled trials. International Journal of Obesity, 50(8), 1638-1646. PubMed

11. Ajmera, V., Vuppalanchi, R., Khalili, M., Sheikh, M. Y., Risser, J., Klein, S., ... & Loomba, R. (2026). Clinical trial: Semaglutide versus placebo in NIT-assessed MASH: A multicenter randomised placebo-controlled trial (SAMARA). Alimentary Pharmacology & Therapeutics, 63(8), 1080-1088. PubMed

12. Perkovic, V., Tuttle, K. R., Rossing, P., Mahaffey, K. W., Mann, J. F. E., Bakris, G., ... & Pratley, R. (2024). Effects of semaglutide on chronic kidney disease in patients with type 2 diabetes. New England Journal of Medicine, 391(2), 109-121. PubMed