Adsorption, and the dose that stayed on the glass

The diluent question is asked constantly and answered badly. Water for injection contains nothing but water and supports microbial growth as readily as any other nutrient-poor aqueous medium. Bacteriostatic water contains benzyl alcohol at nine parts per thousand, which inhibits the growth of contaminating organisms and is the reason multiple-dose presentations exist at all. Two things follow that are routinely missed: a preservative inhibits growth rather than eliminating existing contamination, and benzyl alcohol is not inert towards every peptide, having been implicated in aggregation of certain protein formulations.

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.

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.¹

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.²

Retest date and expiry date are distinct concepts used interchangeably here. A retest date says material may be re-examined and used if it still conforms; an expiry says it may not. Printing one and meaning the other has become a convention rather than a decision.

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.

Residual moisture is the cheapest determination in this whole subject and the most predictive of shelf life, and it appears on almost no certificate. A cake at one per cent water and a cake at four behave quite differently over a year at the same temperature.

The customs leg remains the part of this story we cannot report properly, and it deserves saying every time the subject comes up: there is a segment of every cross-border journey during which nobody measures and nobody has authority to intervene. Any claim of end-to-end control across that segment is a claim about something unobserved.