Bacteriostatic water, sterile water, and the 0.9 per cent that makes the difference

An in-use period is a specific piece of experimental work. It requires the actual formulation, at the actual reconstituted concentration, in the actual container, held at the intended storage temperature, sampled at intervals, and analysed by methods capable of detecting the relevant degradation products — which in practice means a chromatographic method for related substances and a size-based method for aggregates. The output is a period over which the material remains within specification. Numbers circulating in this trade are, in the Journal’s experience, borrowed from the labelling of marketed pens, which are different formulations in different containers with different preservative systems.

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

Residual moisture, and how it is measured

Two methods dominate. Karl Fischer titration determines water specifically, by a stoichiometric reaction with iodine, and is the reference method; the coulometric variant works on the small sample masses a single vial provides. Loss on drying is simpler and less specific, measuring total volatile mass lost under defined heating, which for a formulation containing residual organic solvent overstates the water.

Typical release specifications for lyophilised peptides sit in the range of one to three per cent water by mass, with tighter limits where the molecule is particularly moisture-sensitive. The relationship to stability is not linear. Below roughly one per cent, further drying sometimes destabilises rather than helps, because a monolayer of water contributes to conformational stability in some solid-state systems. Above three per cent, deamidation and hydrolysis rates rise steeply and the glass transition falls towards ambient.

None of the twenty companies the Journal tracks reports residual moisture as a standard release test. Two will provide a figure on request. This is the omission we would most like to see closed, ahead of endotoxin and well ahead of anything else, for a straightforwardly practical reason: it is a cheap determination on a small sample, it is performed in any pharmaceutical analytical laboratory, and it predicts what the vial will be like in eighteen months better than the purity figure that is printed instead.

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.

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.

The finding the Journal least wanted and most trusts is that the transit excursions everybody worries about are probably not the largest exposure in this supply chain. A dried cake at low moisture tolerates a warm afternoon. A reconstituted vial in a refrigerator door for six weeks, with no in-use study behind the number that justified the six weeks, is a different proposition, and it attracts almost no attention at all.