Is Retatrutide a Peptide or a Biologic? The 40-Amino-Acid Rule
FDA counts 40 alpha amino acids; Lilly counts 41. How one amino acid decides whether retatrutide is a drug or a biologic, where the court case stands, and what changes either way.
Update History ▾
Research-use-only framing applied throughout in line with Remy editorial standards.
Chemically, retatrutide is a peptide. Legally, whether it is a "drug" or a "biologic" in the United States comes down to one amino acid, and a federal appeals court has not yet ruled. FDA defines a protein, and therefore a biologic, as an alpha amino acid polymer of more than 40 amino acids.[1] Retatrutide has a 39-residue backbone plus a short side chain. FDA counts 40 alpha amino acids and one that is not alpha, so in its view retatrutide is a drug. Lilly counts 41 amino acids and argues it is a biologic.[2]
A district court agreed with FDA on the count in September 2025 but set aside FDA’s decision on a second ground. Lilly’s appeal was argued in the Seventh Circuit on September 24, 2026, and no decision had been published by October 10, 2026.[3][4] Lilly says it plans to file a Biologics License Application in Q1 2027.[5] Retatrutide is not approved anywhere, whichever way the case goes.
Is Retatrutide a Peptide?
Yes, in the chemical sense. Retatrutide (LY-3437943) is a synthetic peptide: a 39-amino-acid backbone with a short side chain attached, given as a once-weekly injection. It activates three hormone receptors, GIP, GLP-1 and glucagon, which is why it is called a triple agonist rather than a GLP-1 drug.[6]
"Peptide" is a chemistry word, though, not a regulatory category. US law sorts products into drugs, approved through a New Drug Application (NDA), and biological products, licensed through a Biologics License Application (BLA). Many peptides are drugs. Tirzepatide, a 39-amino-acid peptide from the same company, is approved as a drug under NDA 215866, and semaglutide under NDA 209637.[7][8] Insulin, a 51-amino-acid protein, was moved from NDAs to BLAs on March 23, 2020.[9]
The 40-Amino-Acid Rule
Congress put "protein" into the definition of a biological product in 2010, and a December 2019 amendment removed the old exception for chemically synthesized polypeptides.[9] FDA then wrote a size line into its regulations in February 2020.[10] The rule, 21 CFR 600.3(h)(6), reads in part:
"A protein is any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size."[1]
The same paragraph says that when two or more chains are associated "in a manner that occurs in nature", their amino acids are counted together. Anything at or below 40 stays a peptide drug. Anything above 40 is a protein and has to be licensed as a biologic.[1]
| Molecule | Size | US pathway |
|---|---|---|
| Semaglutide | Peptide, under 40 amino acids | Drug (NDA 209637) |
| Tirzepatide | 39 amino acids | Drug (NDA 215866) |
| Insulin | 51 amino acids, two chains | Biologic since March 23, 2020 |
| Retatrutide | 39-residue backbone plus side chain | Disputed in court; FDA designated it a drug |
How Do You Count Retatrutide?
Lilly asked FDA in November 2023 to designate retatrutide a biological product, and filed a superseding request in January 2024. Its request described retatrutide as a polymer of 41 amino acids in total: a 39 alpha amino acid backbone plus an associated second chain made of one gamma-glutamate and one 8-amino-3,6-dioxaoctanoic acid (ADO).[2]
FDA counted it differently. It treated the definition as requiring more than 40 alpha amino acids. On that reading retatrutide has, at most, 40 alpha amino acids and one non-alpha amino acid, ADO, so it is not a protein. FDA also decided it was not "analogous to a protein", because having more than 40 alpha amino acids is the defining feature of a protein.[2][3]
| Party | Count | Conclusion |
|---|---|---|
| Lilly | 41 amino acids in total | Greater than 40, so a protein and a biologic |
| FDA | 40 alpha plus 1 non-alpha | Not greater than 40 alpha, so a drug |
The whole question turns on the ADO unit. It is an amino acid, but not an alpha amino acid, and the rule says "alpha amino acid polymer" before it says "greater than 40 amino acids".[3]
What the Courts Have Said
- September 3, 2024. Lilly sued FDA in the Southern District of Indiana over the drug designation.[2]
- September 30, 2025. The district court split the case. It held that the regulation requires more than 40 alpha amino acids, so FDA was right that retatrutide is not a protein. But it vacated FDA’s decision that retatrutide is not "analogous to a protein", finding that FDA’s reasoning effectively read "analogous product" out of the statute, and sent that question back to the agency. It did not order FDA to classify retatrutide as a biologic.[2][3]
- February 17, 2026. Lilly’s appeal was docketed in the Seventh Circuit as No. 26-1301.[11]
- September 24, 2026. The Seventh Circuit heard oral argument.[4]
- October 10, 2026. No appellate decision has been published, and we found no new FDA decision on the remanded "analogous" question. Appeals courts rule on no fixed timetable.
Why the Classification Matters
| Question | As a drug | As a biologic |
|---|---|---|
| Application | NDA | BLA |
| Market exclusivity | 5 years (new chemical entity) | 12 years from first licensure |
| Later competitors | Generic (ANDA) pathway | Biosimilar pathway |
| US compounding | Depends on approval and bulks lists | Not eligible under 503A or 503B |
The exclusivity gap is the commercial stake: five years of new-chemical-entity exclusivity for a drug against twelve years for a reference biologic.[12][13][14] The biologic route would also send any future competitor through the biosimilar pathway rather than the generic one.[9]
Compounding is often mentioned in coverage of the case, but it does not change today’s picture. Biologics can never be compounded under sections 503A or 503B. Retatrutide is not approved and is on no compounding list, so it cannot lawfully be compounded in the US whichever way the court rules.[14]
Where the Filing Stands
Lilly’s July 23, 2026 release says it is completing the chemistry, manufacturing and controls package "required for a Biologics License Application" and plans to submit retatrutide for US approval in Q1 2027.[5] The release does not explain how that plan relates to the pending appeal, and we have not seen Lilly or FDA say which application type FDA will accept.
The classification fight does not touch the clinical evidence. TRIUMPH-2, the Phase 3 trial in adults with obesity and type 2 diabetes, was published in The Lancet on September 29, 2026, and describes retatrutide as still under investigation.[6] For the trial data and current approval status, see the TRIUMPH trial tracker and Is Retatrutide Approved?
What This Page Does Not Cover
This page explains a US classification question. It is not legal advice, and it does not predict how the Seventh Circuit will rule. Other regulators do not use FDA’s 40-amino-acid line, so the US outcome does not decide how retatrutide is classified elsewhere. For the compound itself, see the retatrutide research profile and the guide to the "reta" peptide name.
Our Research Standards
This article relies on the regulation’s text, court records, FDA’s own transition documents and the sponsor’s release, each checked on the day of publication. Where the record is silent, we say so. It is not legal advice, and no therapeutic, human-use or veterinary-use claim is made here. Read our editorial policy →
Retatrutide Classification FAQ
Is retatrutide a peptide?
Yes. Chemically, retatrutide is a synthetic peptide with a 39-amino-acid backbone and a short side chain attached. Whether US law treats it as a drug or a biologic is a separate question that is still in court.[2]
How many amino acids does retatrutide have?
It depends on how you count. Lilly counts 41 amino acids in total: a 39-residue alpha amino acid backbone plus a second chain of one gamma-glutamate and one ADO unit. FDA counts 40 alpha amino acids and one non-alpha amino acid, ADO.[2]
Is retatrutide a biologic?
Not under FDA’s current designation. FDA classified it as a drug because it does not have more than 40 alpha amino acids. A district court upheld that count in September 2025, but set aside FDA’s separate finding that retatrutide is not "analogous to a protein". Lilly’s appeal was argued on September 24, 2026, and no decision had been published by October 10, 2026.[3][4]
What is the 40-amino-acid rule?
FDA’s regulation, 21 CFR 600.3(h)(6), defines a protein as "any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size." Proteins are licensed as biologics; smaller peptides are approved as drugs.[1]
Is retatrutide a GLP-1?
It is more than a GLP-1. Retatrutide activates the GIP, GLP-1 and glucagon receptors, so it is described as a triple agonist. Tirzepatide activates two of those receptors and semaglutide one.[6]
Why does it matter whether retatrutide is a drug or a biologic?
Can retatrutide be compounded?
No. Retatrutide is not approved and is on no compounding list, so it cannot lawfully be compounded in the US whatever the court decides.[14]
When will retatrutide be approved?
No approval date exists. Lilly says it plans to submit a Biologics License Application to FDA in Q1 2027; FDA’s review can only start after that.[5]
Sources
- Electronic Code of Federal Regulations. 21 CFR 600.3(h)(6), definition of "protein" (text as of October 1, 2026). ecfr.gov ↩
- U.S. District Court for the Southern District of Indiana. Eli Lilly and Company v. Becerra et al., No. 1:24-cv-01503, order on cross-motions for summary judgment, September 30, 2025. govinfo.gov ↩
- Food and Drug Law Institute. Case summary: Eli Lilly & Co. v. Kennedy, 2025 U.S. Dist. LEXIS 196211 (S.D. Ind. 2025). June 2026. fdli.org ↩
- U.S. Court of Appeals for the Seventh Circuit. Oral argument recording, No. 26-1301, Eli Lilly and Company v. Robert Kennedy, Jr., argued September 24, 2026. media.ca7.uscourts.gov ↩
- Eli Lilly and Company. Lilly’s triple agonist, retatrutide, successful in two additional Phase 3 obesity trials (TRIUMPH-2, TRIUMPH-3); BLA submission planned for Q1 2027. July 23, 2026. investor.lilly.com ↩
- Bellido V, le Roux CW, Ekinci EI, et al. Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2): a double-blind, parallel-group, randomised, placebo-controlled, phase 3 trial. Lancet. 2026;408(10563):1455-1472. doi: 10.1016/S0140-6736(26)01861-1 · PMID: 42810372 ↩
- U.S. Food and Drug Administration. Drugs@FDA: Mounjaro (tirzepatide), NDA 215866, Eli Lilly and Company. accessdata.fda.gov ↩
- U.S. Food and Drug Administration. Drugs@FDA: Ozempic (semaglutide), NDA 209637, Novo Nordisk. accessdata.fda.gov ↩
- U.S. Food and Drug Administration. The "Deemed to be a License" Provision of the BPCI Act (transition of protein products from NDAs to BLAs on March 23, 2020). fda.gov ↩
- U.S. Food and Drug Administration. Definition of the Term "Biological Product". Final rule, Federal Register, February 21, 2020 (Doc. 2020-03505). federalregister.gov ↩
- Justia Dockets. Eli Lilly and Company v. Robert Kennedy, Jr., et al., No. 26-1301 (7th Cir.), filed February 17, 2026. dockets.justia.com ↩
- Legal Information Institute. 21 U.S.C. § 355(j)(5)(F)(ii): five-year exclusivity for new chemical entities. law.cornell.edu ↩
- Legal Information Institute. 42 U.S.C. § 262(k)(7): exclusivity for reference biological products (12 years from first licensure). law.cornell.edu ↩
- Mirasol F, Robinson D. Amino Acid Count Determines Retatrutide’s Biologic or Drug Classification. BioPharm International, October 2026. biopharminternational.com ↩
For the clinical record, continue to the TRIUMPH trial tracker and approval status page. For editorial standards, see the research standards page.