A vial can be 99.4 per cent pure and not sterile in the same afternoon

We began looking at this seriously after a reader sent us a certificate with a query attached. The certificate reported 99.2 per cent purity for a lyophilised peptide, and the reader wanted to know whether that meant the remaining 0.8 per cent could be bacterial. It is a reasonable question and the answer is that the figure has nothing to do with bacteria in either direction. A vial containing a substantial microbial burden would very probably return a purity figure indistinguishable from a clean one, because the mass involved is far below the integration threshold of any method in commercial use.

Two questions that share a piece of paper

Consider what happens physically when a certificate is produced. A few milligrams of lyophilised powder are weighed, dissolved in an aqueous mobile phase with an organic modifier, injected onto a reversed-phase column and separated over a programmed gradient while an ultraviolet detector records absorbance. The output is a trace. Software integrates the areas beneath its features, and the main peak area as a proportion of the total becomes the purity figure.

Every step of that procedure is blind to microbial contamination. A bacterial cell contributes no ultraviolet-absorbing peak at any retention time a peptide method would record. Endotoxin, a lipopolysaccharide, is not usefully detected at the wavelengths used for peptide bond absorbance and would in any case be present at a mass fraction several orders of magnitude below any integration threshold in commercial use. A vial holding a hundred colony-forming units and a vial holding none produce chromatograms that no analyst could distinguish.

This is not a defect of the method. Reversed-phase chromatography is an excellent way of determining what proportion of the chromatographically visible material is the intended species, and that is what it is being asked. The defect is in the reading. A document answering one question is being filed as evidence about five.

What endotoxin is, and why sterility does not address it

Endotoxin is a structural component of the outer membrane of Gram-negative bacteria: a lipopolysaccharide with a lipid A anchor that is the pyrogenic moiety, a core oligosaccharide, and a variable O-antigen chain. It is shed during growth and released in quantity on cell lysis, which means that killing a bacterial population does not remove its endotoxin and may increase the free concentration.

Three physical properties make it a separate discipline. It is thermally robust, surviving autoclave conditions with little loss of pyrogenicity, so terminal sterilisation is not a depyrogenation step. It is small and amphipathic, forming aggregates that pass a 0.22 micron membrane without difficulty, so sterilising filtration is not a depyrogenation step either. And it is active in humans at very low mass — the threshold pyrogenic dose corresponds to something in the region of a nanogram per kilogram of body weight.

The practical consequence is stark. A vial can pass a sterility test, contain no viable organism of any kind, and carry an endotoxin burden many times a defensible parenteral limit, because the organisms responsible died somewhere upstream in a water system, a holding tank or a poorly stored component.¹

Recombinant factor C, and the end of an awkward supply chain

The lysate on which conventional endotoxin testing depends is harvested from horseshoe crabs, which are bled and returned to the sea with a mortality that is disputed and not negligible. The assay also inherits the biological variability of a natural product: lysate lots differ, and the cascade includes a branch responsive to beta-glucans, which is a common source of false positives in the presence of cellulose filter residues.

Recombinant factor C reagents replace the harvested cascade with an expressed enzyme, activated by lipopolysaccharide and read chromogenically or fluorimetrically. The response is specific to endotoxin and insensitive to the glucan branch, lot-to-lot consistency is a manufacturing rather than an ecological question, and comparative studies across a wide range of matrices have found agreement with conventional methods well within the variability of the conventional methods themselves.

The reagent has been available for well over a decade and its slow adoption was a regulatory rather than a scientific matter: for years it sat in an alternative-method chapter, obliging users to validate it as a departure. That has now changed, with dedicated chapters in both the United States and European compendia treating recombinant reagents as methods in their own right, and the Journal expects the harvested lysate to become the historical option within this decade.²³

Low endotoxin recovery, and the clean result that is wrong

The most uncomfortable finding in endotoxin testing over the past fifteen years is that certain formulation matrices cause added endotoxin to become undetectable over time. Spike a known quantity of reference endotoxin into a solution containing a non-ionic surfactant and a chelating buffer, hold it, and the recoverable endotoxin declines — sometimes to a small fraction of what was added — while nothing has been removed. The lipopolysaccharide aggregate state has changed, and the assay cannot see what it cannot bind.

The phenomenon is called low endotoxin recovery, and it matters because the combination of polysorbate with citrate or phosphate is extremely common in peptide and protein formulations. A hold-time study — spiking the actual product matrix and measuring recovery across the intended sample storage interval — is the standard mitigation, and it is now expected as part of method suitability for products in that formulation space.

The implication for a reader is narrow but worth stating. A negative endotoxin result on a surfactant-containing formulation, reported without a hold-time recovery study, is weaker evidence than it appears. A negative result on a reconstituted lyophilised peptide in plain water for injection, tested promptly, is considerably stronger.⁴

Documentation practice, named and criticised

The Journal tracks the release documentation of twenty companies. On sterility and endotoxin the picture is close to uniform. Every one publishes a purity figure. A minority publish an identity confirmation. Two publish peptide content. On the microbiological attributes, the standard document is silent, and the silence is not annotated: there is no line reading that sterility has not been determined, which would at least be informative.

Some of the practices we would like to see are already in use somewhere in the group. SSA reports peptide content on its certificates, having begun after correspondence with this publication. CPC and SWB describe their fill environment in general terms on request. QST and BCH answered our five questions in full. WXT and FGP declined on the grounds that research-use products are not represented as sterile injectables, which is a legally sound answer that concedes the point of the exercise. Several others did not respond, and we record non-response as non-response rather than as evasion.

What we are criticising is a documentary convention, not the conduct of any company named here. None of the twenty has been shown to us to have misrepresented anything. The convention is that a chemistry certificate stands in for a release package, and it is a convention this trade adopted collectively and could abandon the same way.

What this article is not asserting

Precision about the scope of a criticism is part of the job, so it is worth spending a paragraph on what is not being said. This piece does not allege that any company named in it has sold contaminated material. It does not allege that any of them has concealed a result, falsified a document or misrepresented a test. It does not claim that the products discussed are dangerous, and it does not claim that they are safe, because neither claim is supportable from the evidence we have.

What the piece asserts is documentary. A certificate describing chemistry is being read as a release package covering microbiology. That mismatch is created by the format of the document rather than by anybody’s intent, and it is closed by adding lines rather than by changing behaviour.

There is also a legal point the Journal has no wish to elide. Research-use-only material is not approved for human use in any jurisdiction, is not required to meet parenteral standards, and is not represented by its sellers as meeting them. Everything in this article about endotoxin limits and sterility assurance describes the framework that would apply to a parenteral medicine. Applying that framework to a research chemical is a comparison, not a compliance requirement, and readers should hold both halves of that sentence at once.

A note on method and sourcing

The compendial material in this article is drawn from the current general chapters of the United States Pharmacopeia and the European Pharmacopoeia, read in the original rather than in summary, and from the international standards on aseptic processing and on laboratory competence. Where a chapter has changed status recently — as the recombinant reagent chapters have — we say so, because a reader consulting an older edition will find a different framing.

Where the Journal reports a number it obtained itself, it states the laboratory’s accreditation status, the method family, whether method suitability was established, and the number of determinations. Where we report what a company told us, we distinguish an answer from a refusal and a refusal from a non-response, because those three things are routinely collapsed in coverage of this trade and they are not the same.

Corrections to this department are handled by the standards desk, which reads every letter and records the outcome in the log. Readers who believe a paragraph here overstates its evidence are asked to write to standards@compoundjournal.com; readers with documents to send, including certificates they would like read, should write to letters@compoundjournal.com. We do not publish correspondents’ names without permission and we do not identify the source of a certificate.

Readers should hold two facts together, uncomfortable as the combination is. The frameworks described here — endotoxin limits, sterility assurance, particulate ceilings — govern licensed parenteral medicines, and research-use material is under no obligation to meet them. That is a legal fact about obligation. It is not a reassurance, and nobody involved in the trade has ever suggested it was.