Why One Peptide Has Five Names: Codes, Trade Names, and CAS Numbers

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

The same research peptide can appear in a catalog, a journal article, a regulatory database and a customs form under four or five different names, and the names do not always point to exactly the same molecule. This article explains where each kind of name comes from, which identifiers are unambiguous, and how to check that a label, a certificate and a paper are all talking about the same substance, using TB-500 and PT-141 as worked examples.

The five kinds of name a peptide can carry

A peptide can be named by its sequence, by a laboratory or company development code, by an International Nonproprietary Name (INN) assigned through the WHO, by a trade name owned by a company, and by registry identifiers such as a CAS Registry Number or an FDA Unique Ingredient Identifier (UNII). Only the sequence and the registry identifiers describe a specific molecule; codes and trade names describe a product or a program, and they can drift.

The WHO describes INNs as unique, globally recognized names for pharmaceutical substances that are public property and serve as the basis for generic names in pharmacopoeias, labelling and the scientific literature. Each INN is built around a stem that signals a pharmacological or chemical family; the WHO publishes these stems in its INN Stem Book, and most peptide INNs end in a peptide-class stem such as "-tide" or "-relin". A 2022 mAbs review of the INN system notes that the scheme evolves as new molecule classes appear, which is why older peptides sometimes carry names that predate the current rules.

Worked example one: TB-500, thymosin beta-4 and timbetasin

"TB-500" is a product name, not a chemical one, and the chemistry it refers to has not been consistent. Thymosin beta-4 is a 43-residue, N-terminally acetylated peptide first sequenced from calf thymus in a 1981 PNAS paper by Low, Hu and Goldstein, with a calculated mass of 4,982 Da. A 1991 Journal of Biological Chemistry study by Safer, Elzinga and Nachmias showed that thymosin beta-4 is identical to Fx, an actin-sequestering peptide isolated from platelets, which is why the molecule is described in both immunology and cytoskeleton literature under two historical names.

The regulatory record adds a third name. The FDA Global Substance Registration System lists TIMBETASIN (UNII 2D5MRE3SSY) and TIMBETASIN ACETATE (UNII 945CRJ0XZ0) as substance records, with CAS 77591-33-4 associated with the full-length peptide. Timbetasin is the nonproprietary name used in clinical development programs for synthetic thymosin beta-4; it is not an approved drug in the United States.

What makes TB-500 unusual is that the product name has been attached to more than one molecule. A 2012 Drug Testing and Analysis paper by Esposito and colleagues analyzed a product sold as TB-500 by high-resolution mass spectrometry and identified it as Ac-LKKTETQ, the N-terminally acetylated residues 17 to 23 of thymosin beta-4, a seven-residue fragment rather than the full 43-residue protein. PubChem now carries that fragment as its own record (CID 62707662, CAS 885340-08-9, UNII QHK6Z47GTG, formula C₃₈H₆₈N₁₀O₁₄), separate from thymosin beta-4 itself. A buyer who sees "TB-500" on a label therefore cannot know from the name alone whether the vial holds a 4,982 Da peptide or a 889 Da fragment. The identity line on the certificate, which should state the observed mass, is what settles it. US Prime's TB-500 10 mg product page posts the lot-matched certificate with that observed mass before purchase, and the TB-500 and thymosin beta-4 research overview covers the literature for the full-length peptide and the fragment separately.

Worked example two: PT-141, bremelanotide and Vyleesi

PT-141 is a company development code that later received an INN, and then a trade name, without the molecule changing. A 2004 Current Opinion in Investigational Drugs profile describes PT-141 as a synthetically modified analog of PT-14 under development by Palatin Technologies. The INN bremelanotide was assigned to that molecule: a cyclic heptapeptide derived from alpha-melanocyte-stimulating hormone, with the lactam bridge between the aspartate and lysine side chains and a D-phenylalanine at position four of the ring. PubChem records it as CID 9941379, formula C₅₀H₆₈N₁₄O₁₀, molecular weight 1,025.2, CAS 189691-06-3, UNII 6Y24O4F92S, with the development codes PT-141, PT 141 and PT141 all listed as synonyms of the same record.

In 2019 the FDA approved bremelanotide as the drug product Vyleesi, as documented in the "first approval" summary by Dhillon and Keam in Drugs. That approval attaches to one finished product made by the license holder, not to the substance name. A research vial labeled PT-141 or bremelanotide is not that product and is not an approved drug. The PT-141 research overview walks through the receptor literature; US Prime's PT-141 10 mg listing carries the sequence, the expected mass and the lot certificate so the molecule can be matched to the PubChem record rather than to the code.

CAS numbers and UNIIs: the identifiers that do not drift

A CAS Registry Number designates one substance and one substance only, which is why regulators and customs forms rely on it. The CAS Registry describes the number as numeric, up to ten digits, written in three hyphen-separated parts with a check digit at the end, and states that a single CAS RN identifies only one substance across its database of more than 290 million disclosed substances. Two consequences follow for peptides. First, a free peptide and its salt are different substances and may carry different CAS numbers, so an acetate salt record will not match a record for the free base. Second, a fragment is a different substance from its parent, which is exactly the TB-500 situation above.

The FDA's Global Substance Registration System (GSRS) serves a similar role in US regulatory filings. Its UNII is a code generated by GSRS for a substance, used in electronic drug listing and visible in FDA drug-labeling records. The FDA page states that UNIIs can be generated at any point in the regulatory life cycle for any substance, from an atom to an organism. Like CAS, GSRS issues separate records for salts, which is why timbetasin and timbetasin acetate have different UNIIs. When a certificate lists a CAS number or UNII, that single line pins the identity more tightly than any amount of marketing copy.

Why codes and trade names drift

Development codes drift because they name a program rather than a molecule, and programs change. A company may advance an analog, switch the salt form, or move from one formulation to another while keeping the code in its press releases; the "PT-14" to "PT-141" step in the Palatin history is one example of a code that survived a chemical modification. Vendor names drift for a different reason: they are chosen for recognition, not accuracy, and once a name like TB-500 is widely searched there is commercial pressure to put it on whatever is being sold under it. The 2012 Esposito analysis is the clearest published case: the label said TB-500, and only mass spectrometry revealed which molecule that meant.

Trade names are the most stable of the three in one sense, because trademark law prevents two companies from using the same one for the same class of goods, but they are also the narrowest. Vyleesi is a specific formulation at a specific strength from a specific license holder. The WHO page on the INN Programme warns that trademarks should not be derived from INNs or contain INN stems, specifically to keep the two layers from being confused.

How to reconcile a label, a certificate and a paper

The practical rule is to reconcile on the molecule, not on the name. The sequence, the expected monoisotopic or average mass, and the CAS number or UNII are the three fields that can be compared across a product label, a certificate of analysis and the methods section of a paper. If the label says TB-500, the certificate's identity line should state the observed mass, and that number tells you whether you have the 4,982 Da protein or the 889 Da fragment; the paper you are comparing against will have used one or the other, and its methods section usually says which. The guide to reading a peptide sequence explains the notation for caps, D-residues and salt forms that make two sequence strings look different when they describe the same molecule, and the FDA status reference table lists which research peptides also exist as approved drugs under an INN or trade name.

Three checks cover most cases. First, look up the CAS number or UNII on the certificate in PubChem or GSRS and confirm the formula matches the sequence on the label. Second, compare the observed mass on the certificate with the mass PubChem computes for that record; a mismatch of more than the expected salt or water contribution means the names do not describe the same thing. Third, when a paper uses a development code, find the sentence in its methods that gives the sequence or supplier, because the code alone may refer to an earlier version of the molecule.

Frequently Asked Questions

Is TB-500 the same thing as thymosin beta-4?

Not necessarily. Thymosin beta-4 is a defined 43-residue peptide with its own CAS number, 77591-33-4, and the INN timbetasin. TB-500 is a vendor name that has been applied both to full-length thymosin beta-4 and, in a 2012 analytical study, to the seven-residue fragment Ac-LKKTETQ, which has a separate PubChem record. Only the certificate's observed mass tells you which one is in a given vial.

What is the difference between PT-141 and bremelanotide?

They are the same molecule under two kinds of name. PT-141 is the development code Palatin Technologies used during the program; bremelanotide is the INN assigned to the substance. Vyleesi is the trade name of the drug product approved by the FDA in 2019. A research vial labeled with either name is not that approved product.

Why do peptides have more than one CAS number?

Because CAS assigns one number per substance, and a salt, a hydrate or a fragment counts as a different substance from the parent peptide. An acetate salt and the free peptide can therefore carry different CAS numbers even though the peptide chain is identical. The same logic applies to UNIIs in the FDA's GSRS, where timbetasin and timbetasin acetate are separate records.

How do I find the CAS number or UNII for a research peptide?

Search PubChem by any synonym and read the identifiers section of the compound record, which lists CAS numbers, UNIIs and development codes together. For regulatory substance records, search the public FDA GSRS by name or UNII. Compare the molecular formula shown there with the sequence on your label to confirm the record describes the same molecule.

Citations

1. Low TL, Hu SK, Goldstein AL. Complete amino acid sequence of bovine thymosin beta 4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations. Proc Natl Acad Sci U S A. 1981;78(2):1162-1166. PMID 6940133
2. Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. J Biol Chem. 1991;266(7):4029-4032. PMID 1999398
3. Esposito S, Deventer K, Goeman J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal. 2012;4(9):733-738. PMID 22962027
4. Hedlund P. PT-141 Palatin. Curr Opin Investig Drugs. 2004;5(4):456-462. PMID 15134289
5. Dhillon S, Keam SJ. Bremelanotide: First Approval. Drugs. 2019;79(14):1599-1606. PMID 31429064
6. Guimaraes Koch SS, Thorpe R, Kawasaki N, et al. International nonproprietary names for monoclonal antibodies: an evolving nomenclature system. mAbs. 2022;14(1):2075078. PMID 35584276
7. World Health Organization. International Nonproprietary Names Programme and Classification of Medical Products. who.int
8. World Health Organization. INN Stem Book 2018: The use of stems in the selection of International Nonproprietary Names for pharmaceutical substances. WHO/EMP/RHT/TSN/2018.01. who.int
9. CAS, a division of the American Chemical Society. CAS REGISTRY. cas.org
10. U.S. FDA. FDA's Global Substance Registration System (GSRS), home of the Unique Ingredient Identifier (UNII). fda.gov
11. NCATS/FDA GSRS. Substance records: TIMBETASIN (UNII 2D5MRE3SSY) and TIMBETASIN ACETATE (UNII 945CRJ0XZ0). gsrs.ncats.nih.gov
12. PubChem. Bremelanotide, CID 9941379. pubchem.ncbi.nlm.nih.gov
13. PubChem. TB-500 (Ac-LKKTETQ), CID 62707662. pubchem.ncbi.nlm.nih.gov
14. PubChem. Thymosin beta 4, CID 45382195. pubchem.ncbi.nlm.nih.gov

This article is for laboratory research reference only. US Prime Peptides products are supplied strictly for in vitro research. Not for human consumption.

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