TB-500 vs BPC-157: One Name, Three Molecular Weights

BPC-157 resolves across three federal records to a single 15-residue sequence at about 1,420 Da, while TB-500 is a trade name under which FDA has documented chemically distinct materials spanning roughly fivefold in molecular weight. Two federal sources do not even agree with each other on what TB-500 weighs, so the two cannot be procured against one incoming specification.

A purchase order that lists TB-500 and BPC-157 on consecutive lines treats the two as one procurement class: two short synthetic peptides, two lyophilized vial formats, one incoming specification covering both. FDA's own compounding record does not support that treatment, and the remainder of this article works through why — starting with what each name resolves to in the federal documentation, and ending with what a certificate of analysis has to state before a receiving department can rely on it.^1,10,2

That asymmetry is not a labeling nuisance for a receiving department. It decides whether an intact-mass number printed on an incoming certificate of analysis can be compared against any reference value at all, and whether a single release document covering both names means anything once a lot is in a quarantine cage.^10,2

One Name, One Sequence: What BPC-157 Actually Specifies

FDA's Pharmacy Compounding Advisory Committee briefing document specifies BPC-157 in its free base form with a molecular formula of C62H98N16O22 and a molecular weight of 1419.5 g/mol. That is a closed description: a named formula, a stated weight, and a stated salt condition. Nothing in it is left to the supplier's interpretation, which means an incoming certificate reporting a different formula for a vial labeled BPC-157 is reporting a different substance, not a different grade of the same one.¹

A companion FDA briefing document states the sequence residue by residue — Gly Glu Pro Pro Pro Gly Lys Pro Ala Asp Asp Ala Gly Leu Val — and identifies it as the N-terminal portion of a naturally occurring BPC protein. Two federal documents, prepared for different agenda items, describe the same fifteen residues in the same order. For a buyer writing an incoming specification, that convergence is the useful part: the sequence can be stated in the specification itself rather than referenced by trade name.¹⁰

The precision.fda.gov GSRS registry entry adds the third record. It lists the same fifteen residues, records an average molecular weight of 1420 Da, and establishes a parent-and-salt relationship between BPC-157 and BPC-157 acetate. That salt relationship is the detail most incoming specifications omit, because it is the point at which counterion content begins to move the number on the balance without changing the identity of the peptide backbone.⁷

• FDA briefing document: C62H98N16O22, 1419.5 g/mol, free base.
• FDA briefing document: fifteen named residues, N-terminal portion of a natural BPC protein.
• precision.fda.gov GSRS: same fifteen residues, 1420 Da average, parent of BPC-157 acetate.
• Spread across the three records: under 1 Da.^1,10,7

One Name, Several Substances: What TB-500 Actually Specifies

FDA's briefing document states plainly that TB-500 and thymosin beta-4 are not the same substance, and notes that several commercial sources use the two terms interchangeably. That sentence has direct purchasing consequences, because it means a catalog entry reading "TB-500 (thymosin beta-4)" is not a synonym pair. It is two candidate materials sharing one line item, and only the certificate's analytical data can say which one arrived.¹⁰

The same document describes TB-500 as a synthetic N-acetylated heptapeptide corresponding to residues 17 through 23 — LKKTETQ — of the 43-amino-acid thymosin beta-4 protein. Seven residues against forty-three. The N-acetylation is part of the description, not an incidental modification, which means a certificate that states only "thymosin beta-4 fragment" has disclosed neither the fragment boundaries nor the terminal chemistry.¹⁰

Any specification language built on the phrase thymosin beta-4 fragment therefore specifies almost nothing on its own. A fragment specification is only closed once it names the residue range, the terminal modifications at both ends, and the salt form. Without those three fields, two lots that both satisfy the written specification can differ in composition by more than a purchasing file can reconcile after the fact.¹⁰

Three Molecular Weights Under One Trade Name

The two federal entries for the heptapeptide disagree. FDA's reproduced certificate-of-analysis figure gives 889.01 g/mol for the free base against formula C38H68N10O14. The GSRS registry gives an estimated formula of C36H66N10O13 and a calculated average molecular weight of 846.981 Da. The gap is roughly 42 Da — close enough to a single acetyl group that it is plainly a chemistry difference rather than a rounding artifact, and large enough that no mass-spectrometric acceptance window of ±1 or ±2 Da can be satisfied against both references at once.^10,2

Set the full-length protein beside them and the spread becomes structural. Thymosin beta-4 is documented at 4,982 Da with an isoelectric point of 5.1 in FDA's briefing document, and the same figures appear independently in the peer-reviewed literature cited above. From 846.981 Da to 4,982 Da is a factor of nearly six. A receiving lab running intact-mass confirmation on a vial labeled only TB-500 has no way to declare a result out of specification, because the specification has three candidate values and the trade name does not select among them.^10,2,5

The acetate figure compounds the problem in the opposite direction. FDA's certificate-of-analysis figure lists the acetate salt form at 949.1 g/mol against 889.01 for the free base, a 60 g/mol difference attributable to counterion content rather than to peptide. A gravimetric fill of a stated vial strength therefore delivers different peptide mass depending on salt form, and a certificate silent on counterion content leaves that difference undisclosed.¹⁰

FDA's Own Words: 'Common Name, Not a USAN'

FDA does not treat this ambiguity as a buyer's misreading. Its briefing document states that TB-500 is a common name rather than a United States Adopted Name, and that the agency has encountered multiple salts and derivatives — including different active moieties — sold under that same common name. "Different active moieties" is the operative phrase. It describes materials that differ in the peptide itself, not merely in the counterion attached to it.¹⁰

The agency uses parallel language for BPC-157 in its companion briefing document, noting that BPC-157 is likewise a common name rather than a USAN and that multiple salts and derivatives have been sold under it. The difference between the two compounds is not that one is governed by a formal name and the other is not. Neither is. The difference is that the underlying sequence behind BPC-157 is consistently documented, while the material behind TB-500 is not.¹

FDA states directly that inconsistent naming conventions for TB-500-related bulk drug substances create difficulty in determining which bulk drug substance a given reference standard refers to. That is a regulator describing its own reference-standard problem. A commercial buyer comparing a supplier's certificate against a catalog reference faces the same reference-standard ambiguity described above, typically without the fuller submission file FDA had before it reached that assessment.¹⁰

• Because a common name does not fix a molecular formula, an incoming certificate for TB-500 must state formula and weight explicitly rather than rely on the product name.
• Because a common name does not fix a salt form, stated vial strengths require counterion disclosure before they can be compared across suppliers.
• Because a common name does not fix an active moiety, a certificate must confirm which analyte was tested, not merely that testing occurred.
• Because a common name does not identify a reference standard, a certificate must name the reference value a mass-spectrometry result was checked against.^10,1

When a Fixed Sequence Still Arrives Without Proof

A stable sequence is necessary for a defensible incoming specification. It is not sufficient. FDA's evaluation of the nominated BPC-157 bulk drug substance found the nominated material inconsistent with the information listed in its own submitted certificate of analysis. The nomination package contained a certificate; the certificate did not match the substance described alongside it. That is a documentation failure occurring on a compound whose sequence is not in dispute anywhere in the federal record.¹

The second FDA review goes further. For one nomination, reviewers found no certificate of analysis at all for BPC-157 in free base form, and the certificate material available in the public domain lacked impurity limits, aggregation data, and microbial bioburden and bacterial endotoxins testing results. Those are not optional annexes. They are the fields that distinguish a release document from a purity headline.¹⁰

• Impurity limits: absent from the public-domain certificate material FDA reviewed.
• Aggregation data: absent.
• Microbial bioburden results: absent.
• Bacterial endotoxins testing results: absent.
• Certificate of analysis for the free base form in one nomination: not located by FDA reviewers.¹⁰

Net peptide content belongs on the same list of omissions, for the same reason the aggregation field does. A chromatographic purity percentage describes the ratio of one peak to the total peak area; net peptide content describes how much of the powder in the vial is peptide rather than counterion, water, and residual salts. The two numbers answer different questions and neither substitutes for the other.^7,10

The Case Report Where Both Failures Combined

FDA's BPC-157 briefing document cites a published case report involving an injection product labeled as containing both BPC-157 and TB-500 at 5 mg and 5 mg, distributed under a "research purposes only" label. A single container, two trade names, one label statement, and — by the labeling alone — no indication of which analytical method established the identity of either component or which of the candidate TB-500 materials the second figure referred to.¹

The combination is what makes the case instructive for a procurement file rather than merely cautionary. One of the two names on that label maps to a documented 15-residue sequence. The other maps to a name FDA has stated covers multiple salts, derivatives, and active moieties. A release document that treats both as equally determinate names has silently applied the specification discipline of the stable compound to the unstable one.^1,10

A multi-component line item carrying both names is only as well identified as its least-documented component, and here that component is TB-500. A buyer accepting a combined release document is accepting a certificate whose identity section cannot be independently reconstructed from the document itself.¹⁰

Category 2 Status and the Identity-Testing Precedent

FDA currently places both BPC-157 and TB-500 — the latter identified on the list as the thymosin beta-4 fragment LKKTETQ — in Category 2 of its bulk drug substances list, the category for substances raising significant safety risks. The stated grounds include peptide-related impurities and incomplete API characterization data. Incomplete characterization is the agency's assessment of the supply base generally, not of one shipment, which means the documentation gap described in the sections above is the baseline condition rather than the exception.³

The enforcement precedent on identity sits in the same file. FDA's April 1, 2020 warning letter to Tailor Made Compounding LLC, which listed BPC-157 among the firm's compounded products, states that the firm failed to conduct at least one test to verify the identity of each drug component. Not each shipment. Not each supplier. Each component.³

That phrasing is the standard an incoming specification should be written against. For a product line in which one named component resolves to three documented molecular weights across two federal databases, "at least one test to verify the identity" means a test whose reference value is stated in the specification and whose result is reported as a number rather than as a pass. Peptide identity testing by mass spectrometry produces that number; an identity line reading "conforms" does not.^3,10

Writing Two Incoming Specifications Instead of One

The practical output of the whole record is that these two materials require separate incoming specification documents, with different mandatory fields, even when they are quoted on the same purchase order and warehoused in the same freezer. The TB-500 specification exists to force disclosure of which substance arrived. The BPC-157 specification exists to force disclosure of how much peptide arrived.^10,1

TB-500: identity fields the certificate must carry

• Full residue sequence written out, not the trade name — FDA states TB-500 is a common name rather than a USAN and that multiple active moieties have been sold under it.
• Terminal modification stated explicitly, since FDA describes the material as an N-acetylated heptapeptide corresponding to residues 17-23 of thymosin beta-4.
• Salt form named, with the free base and salt molecular weights both reported; FDA's certificate figure separates 889.01 g/mol free base from 949.1 g/mol acetate.
• Observed intact mass by mass spectrometry reported as a measured figure against a named reference value, given the roughly 42 Da disagreement between FDA's 889.01 g/mol figure and the GSRS calculated average of 846.981 Da.
• A positive statement that the material is not full-length thymosin beta-4 at 4,982 Da, since FDA records commercial use of the two names interchangeably.^10,2

BPC-157: content fields the certificate must carry

• Sequence stated as the fifteen residues documented by FDA and by the GSRS registry, with molecular formula C62H98N16O22 and molecular weight reported against the 1419.5 g/mol figure.
• Parent or salt form declared, since the GSRS registry records BPC-157 acetate as a distinct salt of the same parent.
• Net peptide content by amino acid analysis or an equivalent quantitative method, reported separately from chromatographic purity.
• Peptide counterion content reported as a percentage, so gravimetric vial strengths are reconcilable across suppliers.
• The four fields FDA found missing from public-domain certificate material: impurity limits, aggregation data, microbial bioburden, and bacterial endotoxins results.^1,7,10

Both specifications close the same loop from opposite ends. TB-500 needs a sequence written into the document because the name does not supply one. BPC-157 needs a content number written into the document because the sequence, settled as it is across three federal records, says nothing about how much peptide is in the vial. A supplier able to produce one set of fields but not the other has not failed at purity. It has failed at the two separate questions these two materials ask.^10,1,7,2

The July 23, 2026 PCAC meeting placed BPC-157-related and TB-500-related bulk drug substances on the same agenda, alongside KPV- and MOTS-c-related substances. Shared agenda placement is not shared identity, and a procurement file that mirrors the agenda structure rather than the chemistry will carry one specification where the record requires two.¹⁰