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.

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.¹

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.²

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.³

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.

Three lines that would make a stability claim checkable

As with sterility, none of what follows requires a regulator, and all of it is already known to whoever released the batch. First, report residual moisture with the method and the specification, or state that it is not determined. Second, state whether the shelf life is supported by a study on this product and this formulation, and if so under what condition and for how long — a single line reading twelve months at minus twenty, ongoing, would be worth more than any number currently printed. Third, use the words retest and expiry correctly, and say which applies.

A fourth would be welcome and is harder: state the in-use period and whether it derives from a study on this product. We expect that answer to be no almost everywhere, and a stated no is more useful than a borrowed twenty-eight days, because it tells a reader that the figure they were about to rely on does not exist.

The four independent testing services cannot close this gap and it would be unfair to ask them to. Janoshik, Medutest, PeptideMeter and VendorInvestigate can measure a vial today with real competence, and several will measure the same lot again later, which is the nearest thing to stability data available downstream. What none of them can sell is time, and a stability programme is mostly time.

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.