One vial, four weeks, and the data that does not exist

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.

Deamidation, and the isomer with the same mass

Deamidation of asparagine proceeds through nucleophilic attack by the backbone nitrogen of the following residue on the asparagine side-chain carbonyl, forming a five-membered succinimide intermediate which then hydrolyses to a mixture of aspartate and isoaspartate, conventionally in a ratio favouring the isomer roughly three to one. Glutamine deamidates by an analogous route, far more slowly, through a six-membered intermediate.

Three factors govern the rate. Sequence is dominant: the residue immediately following the asparagine determines how readily the intermediate forms, and asparagine-glycine is the fastest motif known, with serine, histidine and alanine following. Solution pH matters, with the rate minimal in the mildly acidic region and rising steeply above neutrality as the backbone nitrogen becomes more nucleophilic. Temperature and water activity set the overall pace, which is why the solid state helps so much.

The analytical problem is that isoaspartate has the same elemental composition and therefore the same molecular mass as the parent. Identity confirmation by molecular ion alone cannot distinguish them, and a preparation that is substantially deamidated will present as the intended compound. The isomers usually separate on a sufficiently shallow reversed-phase gradient, and specific methods exist, but only a method designed for the question will find the answer.¹

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

Why accelerated data extrapolates badly for peptides

The temptation with any stability programme is to run the accelerated condition, fit an Arrhenius relationship to the rate constants, and extrapolate to the intended storage temperature. For a single reaction with a temperature-independent mechanism that is sound. For peptides it frequently is not, and the reason is that different pathways have different activation energies.

Suppose a peptide degrades at five degrees principally by deamidation and at forty degrees principally by hydrolysis, with the second having a higher activation energy. Measuring total degradation at forty degrees measures mostly hydrolysis; extrapolating that rate down to five degrees predicts almost nothing about the deamidation that will actually dominate. Aggregation is worse still, because it is frequently nucleated by interfaces and mechanical stress rather than by thermal energy alone, and does not obey a simple temperature relationship at all.

The practical rule the Journal applies when reading a stability claim is to ask what condition the data was generated at and whether the degradation products were identified as well as quantified. Accelerated data that shows which products form is genuinely useful as a warning of what to watch for. Accelerated data reduced to a single percentage and extrapolated to a shelf life is a projection dressed as a measurement, and for this class of molecule it is a poor projection.

Light, and the exposure nobody records

Photostability has its own guideline, its own defined light source options and its own exposure requirement expressed in lux hours of visible light and watt hours per square metre of near ultraviolet. Products are tested in the immediate container, and where they fail, in the marketing pack, and where they fail again the label carries a protection instruction. The chemistry is real: tryptophan and tyrosine absorb in the near ultraviolet and photo-oxidise, and photolytic disulphide cleavage is well documented.

Nothing about light exposure is recorded anywhere in the research-peptide supply chain. Vials are frequently supplied in clear glass. Photographs for listings are taken under studio lighting. Parcels are opened on kitchen counters. A reconstituted vial may sit on a shelf under a window for weeks. The cumulative exposure is unknown and unknowable, and it is plausibly a larger contributor to degradation than the transit excursions that attract all the attention.

The Journal makes one narrow observation rather than a recommendation, because recommendations are not this publication’s business. Amber glass, or a secondary carton, costs a fraction of a cent per unit and removes an uncontrolled variable entirely. Several of the twenty companies we track already ship in amber vials; most do not, and the ones that do not have not, as far as we can establish, generated any data suggesting it does not matter.³

The unfashionable finding: dried peptide is fairly tough

The alarmed version of this story would end with the excursions and leave the reader frightened. The evidence does not support that ending, and the Journal would rather publish the awkward finding than the satisfying one. A lyophilised peptide at low residual moisture, stored below its glass transition, has very little molecular mobility available for degradation. Short warm excursions in that state cost comparatively little, and the published solid-state stability literature is consistent on the point: dried peptides tolerate transient thermal insult far better than solutions do.

Two caveats keep this from being a licence. First, the protection depends on the cake being genuinely dry, which is the unmeasured variable this article keeps returning to. A cake at four per cent moisture has a much lower glass transition and much less margin. Second, repeated cycling is worse than a single excursion, particularly where a warm interval permits moisture redistribution within the cake or condensation inside the container on cooling.

The reordered risk list, on our reading, puts the reconstituted vial first, the cake with unknown residual moisture second, the multi-week domestic storage of an opened vial third, and the four hours at thirty-eight degrees in a courier van somewhere well below all of them. That ordering is not what the anxiety in this market reflects, and we think it is the more defensible one.⁴

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.⁵

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.⁶

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.