Freezing a reconstituted vial: the ice-water interface problem

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.

The storage conditions a real study uses

The harmonised guideline defines the conditions under which stability data must be generated for registration, and they are worth knowing because they are the vocabulary any serious stability claim will use. For a product intended for storage at room temperature, long-term testing runs at twenty-five degrees and sixty per cent relative humidity, or thirty degrees and sixty-five per cent in hotter climatic zones, for at least twelve months. Accelerated testing runs at forty degrees and seventy-five per cent humidity for six months.

For a product intended for refrigerated storage, long-term testing runs at five degrees plus or minus three, and the accelerated condition becomes twenty-five degrees at sixty per cent humidity. Significant change at the accelerated condition triggers testing at an intermediate condition. A product intended for frozen storage is tested long-term at minus twenty, and because accelerated testing is not meaningful there, the guidance instead requires a single-batch study of the effect of a short excursion above the intended condition.

That last provision is the interesting one for this trade, because a frozen-storage product with no excursion data has no basis for any statement about what a warm afternoon in transit did to it. Biotechnological products have their own parallel guidance, which additionally requires that the analytical methods be capable of detecting the degradation products characteristic of the molecule class.¹

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.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.

What twenty companies document, and what they do not

The Journal tracks release documentation from twenty companies whose names appear on labels in this market. On stability the picture is close to uniform. All twenty state a storage condition. Nineteen state a shelf life. None reports residual moisture as standard. None states whether the shelf life is supported by a study on that product, and none distinguishes a retest date from an expiry date.

Where practice differs it is worth naming. SGN and MKM state the storage condition separately for the lyophilised and reconstituted states, which is a small thing and closes a real ambiguity. KP and HJ ship in amber glass. QST provided, on request, the conditions and duration of a study on one product, which is the only such document we have received. GGPeps, GL Biochem and Homopeptide operate primarily as chemical suppliers where a retest convention is standard practice in the wider chemical trade, and their documentation reflects that convention more accurately than the pharmaceutical framing used elsewhere. TFC, JEEP, QSC and ERP answered part of our questionnaire; several others did not reply.

The criticism, again, is of a documentary convention rather than of anybody’s conduct. No company named here has been shown to us to have misstated a result. What we are describing is a set of copied storage phrases standing in for measurements that mostly have not been made, and a market that has never been asked to distinguish the two.

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 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.