Twenty-eight days is a number from somebody else’s product

Reconstitution is the point at which a stable product becomes a perishable one, and the change is not incremental. Deamidation, hydrolysis and disulphide exchange all require water as a participant or a medium; aggregation requires molecular mobility that the dried matrix denies. Adding two millilitres of diluent restores all of it at once. The consequence is that a product with a two-year shelf life as a cake may have an in-use period measured in weeks, and the second figure is not derivable from the first by any calculation.

What the drying cycle actually does

A lyophilisation cycle has three stages and the differences between them explain most of what can go wrong. Freezing solidifies the solution, converting bulk water into ice crystals and concentrating everything else into an interstitial amorphous phase. Primary drying holds the product below the temperature at which that amorphous phase would soften, reduces the chamber pressure, and sublimes the ice directly to vapour. Secondary drying raises the shelf temperature to desorb water that remains bound to the solid matrix.

The rate-limiting stage is primary drying, and it is the stage under commercial pressure, because sublimation is slow and freeze-dryer time is expensive. Raising the shelf temperature accelerates it and risks carrying the product above its collapse temperature; shortening it leaves ice in the cake, which then melts during secondary drying and produces a partially collapsed plug with elevated moisture.

Secondary drying is the stage most often truncated, and truncation is invisible in the finished appearance. A cake can look entirely correct and carry three or four per cent residual water because the final desorption step was cut by six hours. The only way to detect it is to measure the water, which is why residual moisture is a release test in regulated manufacture and why its absence from a certificate is a substantive omission rather than a formatting one.¹

Hydrolysis, isomerisation and the slower routes

Backbone hydrolysis cleaves an amide bond outright and produces two fragments, each of which is a distinct chromatographic species and each of which is detectable by mass. It is generally slower than deamidation at ordinary storage conditions but becomes dominant at low pH and elevated temperature, which is one reason accelerated stability data for peptides extrapolates so poorly: the pathway that dominates at forty degrees may be irrelevant at five.

Certain positions are much more labile than others. Aspartate-proline and aspartate-glycine bonds hydrolyse relatively readily under acidic conditions. N-terminal glutamine can cyclise to pyroglutamate, losing ammonia. Peptides with an N-terminal sequence of the right geometry can form a diketopiperazine and shed the first two residues as a cyclic dipeptide, a route that is fast enough at neutral pH to matter for some sequences.

Racemisation at susceptible residues produces epimers that are chemically identical in composition and differ only in stereochemistry. They are among the hardest impurities to detect, requiring either a chiral method or a sufficiently discriminating reversed-phase separation, and they are essentially never reported. A vial can be nominally pure by every measurement on its certificate and contain a percentage of a diastereomer with unknown biological behaviour.

What makes a method stability-indicating

A stability study is only as good as the analytical method behind it, and the requirement has a name: the method must be stability-indicating, meaning it must resolve the parent compound from its degradation products and quantify the change. Establishing that is done by forced degradation — deliberately stressing the material with acid, base, oxidant, heat and light — and demonstrating that the resulting products are separated from the parent and from each other with adequate peak purity.

Almost nothing sold as a purity determination in this market has been validated that way. A generic peptide gradient run for twelve minutes may perfectly well resolve the parent from its two largest process impurities and entirely fail to resolve it from its isoaspartate isomer or a closely related oxidation product. The number it returns is a purity figure, not a stability measurement, and using a series of such figures to argue that a product has not degraded is a category error.

The compendial guidance on analytical validation is explicit about specificity, and about demonstrating it against the degradation products the molecule can actually form. The gap between that expectation and practice in this trade is not a matter of dishonesty. It is that the method being sold was designed for a different purpose and is being asked a question it was not built to answer.²

Eleven days nobody logged

Our ninth parcel entered a customs facility on a Thursday and left it eleven days later, released without explanation and without any record of the conditions in between. The logger, configured with a memory sufficient for a five-day journey at five-minute sampling, stopped recording on day five with the temperature at twenty-two degrees and rising through the afternoon. What happened over the following six days is unknown and cannot be reconstructed.

This is the least documented step in the supply chain and, when it occurs, routinely the longest. Examination facilities are not temperature-controlled, dwell time is not committed to, and neither shipper nor recipient receives a record. It follows that any claim of end-to-end temperature control on an international parcel of this kind cannot be true, because there is a segment during which nobody is measuring and nobody has authority to intervene.

The Journal reports this as a structural feature rather than a scandal. Customs authorities are performing a lawful function and are under no obligation to preserve the thermal history of a research chemical. But the consequence deserves to be stated plainly, because it is stated nowhere else: for a cross-border shipment, the shipper controls the first leg, the carrier controls the second, and there is a third leg over which nobody has visibility at all. Buyers reasoning about cold chain should reason about that leg.

In-use stability, and where the numbers come from

In-use stability is established by a dedicated study: the finished product reconstituted as intended, at the intended concentration, in the intended container, stored at the intended temperature, sampled at intervals, and analysed by stability-indicating methods for related substances and by a size-based method for aggregates. The output is a period, and the period belongs to that formulation in that container and to nothing else.

The in-use periods circulating in this market are not derived that way. They are, in the Journal’s experience of tracing them, borrowed from the labelling of marketed pen presentations, which are different formulations at different concentrations with different preservative systems in different primary containers. Marketed in-use periods for the incretin pens run from four weeks to eight depending on product and storage condition, and none of those figures transfers to a reconstituted research vial by any argument we can construct.

What can be said generally is directional rather than numerical. Degradation in solution proceeds orders of magnitude faster than in the cake. Lower temperature helps substantially. Repeated warming and cooling of an opened vial is worse than steady storage. Preservative-containing diluent addresses microbial growth and does nothing about chemical degradation. And in the absence of a study on the actual product, any specific number quoted for an in-use period is an assumption wearing a specification’s clothes.

One further loss is routinely mistaken for degradation. Peptides adsorb to glass and polymer surfaces, and the relationship runs the awkward way: the more dilute the solution, the larger the proportion a given surface area removes.³

Bacteriostatic water, sterile water, and what each is for

Sterile water for injection contains water and nothing else. It is sterile when the container is opened and it has no capacity to remain so, and it supports the growth of any organism introduced subsequently. It is the appropriate diluent for a single-use presentation and the wrong one for anything intended to be entered more than once.

Bacteriostatic water for injection contains benzyl alcohol at nine parts per thousand. Benzyl alcohol inhibits microbial growth, which is what makes a multiple-dose presentation coherent, and it is important to be exact about what that means: a preservative suppresses the proliferation of organisms introduced during use. It does not sterilise a contaminated solution, it does not act instantly, and its effectiveness against a given organism is established by a specific compendial test rather than assumed.

Two further points get lost. Benzyl alcohol is not universally compatible; it has been implicated in the aggregation of certain protein formulations, and compatibility with a given peptide is a question for data rather than for convention. And a preservative system has its own stability: preservative content declines over an in-use period, which is one of the attributes a proper in-use study measures. A diluent choice is therefore a formulation decision with chemical consequences, not a matter of preference between two clear liquids.⁴

Freezing a solution is not storing it

Freezing a reconstituted vial to extend its life is a common inference and a poor one, for reasons that have nothing to do with temperature and everything to do with what happens during the phase change. As ice forms, solutes are excluded from the crystal lattice and concentrated into a shrinking unfrozen fraction. Local concentration, ionic strength and pH in that fraction can shift dramatically — buffer components crystallise at different points, and a phosphate buffer is notorious for a large pH excursion on freezing.

The ice-water interface is itself a denaturing surface, and interfacial area increases with the number of freeze-thaw cycles. Each cycle presents the peptide with a fresh opportunity to unfold at that interface and aggregate. This is why formulations intended for frozen storage contain cryoprotectants and why lyophilisation exists as a technique at all: the point of drying is to avoid keeping a peptide in a partially frozen aqueous system.

The Journal states the mechanism and declines the recommendation, as this department’s practice requires. What can be said without advising anybody is that freezing a reconstituted solution is a different chemical operation from freezing a dried cake, that its effects are formulation-dependent and not predictable from first principles, and that no in-use study we have seen in this market has examined it. A reader treating the freezer as a pause button is relying on an assumption nobody has tested for that product.

A note on method and sourcing

The regulatory framework in this article is taken from the harmonised guidelines on stability testing and on biotechnological products, read in the original, and from the current compendial chapters on storage definitions, distribution of temperature-sensitive products and stability in dispensing practice. The degradation chemistry is drawn from the peptide and pharmaceutical sciences literature, and where a claim is a generalisation across sequences this piece says so, because sequence dependence is the rule rather than the exception.

The shipment data is ours. Nine parcels, ordered at catalogue prices as ordinary customers, with calibrated loggers placed inside the insulated payload and sampling at five-minute intervals. Eight complete traces and one truncated by a customs hold. We disclose that nine parcels is not a survey, that we did not control the packing operation, and that a single logger cannot characterise a payload with a thermal gradient across it.

Nothing in this department is a recommendation about storing, reconstituting or administering anything. The compounds discussed are sold for research use only and are not approved for human use in any jurisdiction. Corrections and disputes go to standards@compoundjournal.com; documents, traces and certificates readers would like examined go to letters@compoundjournal.com, and we do not identify the source of anything sent to us.

Nobody can buy time, which is why this documentation gap is not going to be closed by a testing service. What can be done immediately is to distinguish a measurement from a convention: to say twelve months at minus twenty, ongoing, rather than two years, and to say that an in-use period has not been established for this product rather than borrowing one from a pen.