Lyophilisation

Lyophilisation, also called freeze-drying, is a dehydration process in which a solution is frozen and the ice is then removed by sublimation at a pressure below the triple point of water, leaving a porous solid whose volume approximates that of the original fill. It is the standard final step in the isolation of synthetic peptides after preparative purification, and the reason a research peptide is distributed as a dry cake in a sealed vial rather than as a solution.^[1]

The process is used because peptides in aqueous solution degrade by hydrolytic routes — deamidation of asparagine and glutamine, backbone cleavage, aggregation — whose rates depend strongly on molecular mobility and on the availability of water. Removing water and holding the residue in an amorphous glass suppresses those routes by orders of magnitude, converting a product with a shelf life of days at ambient temperature into one with a shelf life measured in years.^[2][3]

A lyophilisation cycle has three phases: freezing, primary drying in which ice is sublimed, and secondary drying in which water sorbed to the solid is desorbed. Each phase is bounded by a physical constraint. Freezing determines the ice-crystal structure and therefore the resistance the vapour must overcome later; primary drying must be conducted below the temperature at which the freeze-concentrated matrix loses rigidity; secondary drying must reach a residual moisture low enough for stability without over-drying a solid that may require a small amount of water for conformational integrity.^[4][5]

The finished cake is characterised by appearance, residual moisture, reconstitution time and the chemical purity of the reconstituted solution. None of these is visible from a purity figure alone, which is one reason a certificate of analysis that reports only area percent purity leaves the physical quality of the vial undocumented.^[6]

Water can pass directly from solid to vapour only at pressures below its triple point, which lies at 611.657 Pa and 0.01 °C. Above that pressure, warming ice produces liquid; below it, warming ice produces vapour. Every lyophilisation cycle therefore operates at a chamber pressure well beneath 611 Pa, typically between 5 and 20 Pa, so that heat supplied to the product drives sublimation rather than melting.^[1]

The driving force for sublimation is the difference between the vapour pressure of ice at the product temperature and the partial pressure of water vapour in the chamber. Because the vapour pressure of ice falls steeply with temperature, small changes in product temperature produce large changes in drying rate.

Values follow the standard formulations reviewed by Murphy and Koop.^[7] The table explains a practical asymmetry of the process: a cycle run with the product at −40 °C rather than −30 °C loses roughly two-thirds of its driving force, and a cycle run 5 °C warmer than intended may dry substantially faster while risking the structural failure described below.

Sublimation is strongly endothermic. The enthalpy of sublimation of ice is approximately 2.83 MJ·kg⁻¹, about 13% greater than the enthalpy of vaporisation of liquid water, so the shelf must supply a large heat flux for a modest mass flow. That heat must cross a gas gap between shelf and vial base, and the efficiency of that transfer — conventionally expressed as a vial heat transfer coefficient — is itself a function of chamber pressure, because conduction through rarefied gas falls as pressure falls.^[8]

Two consequences follow and recur throughout the article. First, product temperature is not a set point but a result: it emerges from the balance between heat supplied and heat consumed by sublimation, and it is lower than the shelf temperature throughout primary drying. Second, anything that changes the resistance of the dried layer to vapour flow changes the product temperature, which is why the freezing step — which sets that resistance — governs the behaviour of the two steps that follow.^[9]

Cooling an aqueous solution below 0 °C does not immediately produce ice. Nucleation requires supercooling, and in clean pharmaceutical solutions supercooling of 10–20 °C below the equilibrium freezing point is routine. The temperature at which nucleation happens to occur determines the number and size of the ice crystals formed: nucleation close to 0 °C produces few, large crystals and a coarse, low-resistance dried structure, whereas deep supercooling produces many small crystals and a fine structure that impedes vapour escape.^[9][10]

Because nucleation is stochastic, vials within one batch nucleate at different temperatures and therefore dry at different rates. This is a principal source of vial-to-vial variability in residual moisture and cake appearance, and it is the reason controlled-nucleation techniques — depressurisation, ice fog, or brief pressure cycling — have been developed to force all vials to nucleate within a narrow window.^[10]

As ice forms, solutes are excluded from the crystal lattice and concentrate in the remaining liquid. This freeze concentration continues until the residual phase either crystallises at a eutectic temperature or becomes so viscous that it vitrifies. The temperature at which the maximally freeze-concentrated amorphous phase undergoes its glass transition is written Tg′ and is a formulation property, not a process setting.

Figures are collated from the freeze-drying literature and vary with concentration and with the measurement method.^[1][4] The presence of sodium chloride is significant for peptide formulations because its low eutectic temperature drags the critical temperature of the whole system downward, forcing a colder and therefore longer primary drying step. A formulation carrying appreciable trifluoroacetate or phosphate salt from purification behaves similarly.

Annealing — holding the frozen product for a period above Tg′ but below the melting point — allows small ice crystals to grow at the expense of smaller ones and permits crystallising excipients such as mannitol to complete crystallisation before drying begins. Its benefit is a more uniform and less resistant dried structure; its cost is cycle time and, for some proteins, additional exposure to the ice-water interface.^[10]

Primary drying removes the ice, which for a typical aqueous fill is 90–98% of the water present. Its duration is set by the sublimation rate, which in the simplest treatment follows from an energy balance across the vial base: the mass flow equals the heat flow divided by the enthalpy of sublimation.

A worked estimate for a single 6R vial illustrates the magnitudes involved. Taking a vial cross-sectional area of 3.14 cm², a vial heat transfer coefficient of 20 W·m⁻²·K⁻¹, a shelf temperature of −10 °C and a product temperature of −30 °C, the heat flow is

Q = 20 W m^-2 K^-1 × 3.14 × 10^-4 m² × 20 K = 0.126 W

and the sublimation rate is

dm/dt = 0.126 W ÷ 2.83 × 10⁶ J kg^-1 = 4.4 × 10^-8 kg s^-1 ≈ 0.16 g h^-1

A 2.0 mL fill therefore requires approximately 12.5 hours of primary drying under these conditions, before any safety margin is added. Doubling the shelf-to-product temperature difference halves that figure, which is why the choice of critical temperature — and how close the cycle is permitted to approach it — dominates cycle economics.^[4][8]

Vials do not all behave alike. Those at the edge of a shelf receive additional radiant heat from chamber walls and door, sublime faster, and reach higher product temperatures; this edge-vial effect can amount to several degrees and is the usual reason a cycle validated on a laboratory dryer fails at production scale.^[8]

Two distinct structural failures bound primary drying. In a system whose freeze-concentrate crystallises, exceeding the eutectic melting temperature produces liquid, and the vial dries as a shrunken residue or a glassy shell — usually described on inspection as melt-back. In an amorphous system, exceeding a temperature a few degrees above Tg′ allows viscous flow of the freeze-concentrated matrix into the pores that the sublimed ice has vacated. The dried layer loses its structure, and the result is a shrunken, sometimes glassy or foamed cake. The temperature at which this becomes observable by freeze-drying microscopy is the collapse temperature, Tc, conventionally 1–3 °C above Tg′ for small-molecule and peptide systems.^[4]

Collapse is not necessarily a chemical failure. A collapsed cake may reconstitute and may assay within specification. Its documented consequences are elevated residual moisture, because the collapsed structure traps water; slower and sometimes incomplete reconstitution; and a cake appearance that fails a visual specification. For proteins, collapse has been associated with reduced storage stability, attributed to increased molecular mobility in the denser matrix.^[2]

Conservative practice therefore sets the target product temperature 2–5 °C below Tc and accepts the longer cycle. Aggressive cycles that run within a degree of Tc exist and are defensible when the critical temperature has been measured for the specific formulation rather than assumed from a table.^[4]

Primary drying ends when the last ice sublimes, and the transition is detectable because the vapour load on the condenser falls. Several signals are used. Comparative pressure measurement exploits the difference between a capacitance manometer, which reads total pressure, and a Pirani gauge, whose calibration depends on gas composition: as water vapour is replaced by nitrogen, the Pirani reading converges on the manometer reading, and the convergence marks the endpoint. Pressure-rise testing isolates the chamber briefly and measures the rate at which pressure recovers. Product thermocouples show a rise toward shelf temperature once no ice remains to consume heat, though a thermocouple perturbs nucleation in the vial it occupies and reads that vial rather than the batch.^[11]

The practical difficulty is that the last vials to finish are not the ones being monitored. Endpoint criteria are therefore normally applied with a hold period appended, and the batch is held under primary-drying conditions for a defined time after the signal is observed.^[11]

Once ice has gone, water remains sorbed to the solid — hydrogen-bonded to the peptide and to any amorphous excipient. Removing it requires desorption rather than sublimation, and desorption is driven by raising the product temperature well above the primary-drying value, typically to between 20 and 40 °C, while holding chamber pressure low.^[1]

Desorption kinetics are fast at first and then slow markedly, so the last fraction of a percent of moisture is disproportionately expensive in time. Secondary drying is consequently the step most often truncated, and truncation is a common explanation for a cake that meets appearance specifications but exceeds its moisture specification.^[2]

The heating ramp matters because the glass transition temperature of the partially dried solid rises as water leaves it. Early in secondary drying the solid is plasticised by residual water and its glass transition may be only slightly above the product temperature; heating too quickly can carry the product above the moving glass transition and cause the same collapse phenomenon seen in primary drying, at a much higher temperature. Conventional practice ramps shelf temperature at 0.1–0.5 °C per minute for this reason.^[4]

Residual moisture specifications for lyophilised peptides commonly sit below 3% w/w, and frequently below 1% for products intended for long ambient storage. Lower is not always better. For some proteins a monolayer of water appears to be required for conformational stability, and over-dried preparations have shown reduced stability relative to preparations dried to an intermediate moisture. The relationship between residual moisture and degradation rate is not monotonic and is formulation-specific.^[3][2]

Stoppers contribute to the final moisture in a way that is easy to overlook. Elastomeric closures absorb water during steam sterilisation and release it slowly into the sealed headspace, so a vial can gain moisture during storage from its own closure. Post-sterilisation drying of stoppers, and the choice of a low-moisture-transmission elastomer, are the usual mitigations; the phenomenon is discussed further at vial.^[12]

A pure peptide dried from water alone often produces an unsatisfactory solid: a thin film at the vial base, a cake that shrinks away from the wall, or a residue that is difficult to see and easy to mistake for an empty vial. Formulation addresses cake structure, chemical stability and reconstitution behaviour simultaneously.

Bulking agents

Mannitol and glycine crystallise on freezing and give an elegant, mechanically robust cake. Because they crystallise, they contribute little to the amorphous phase and therefore raise the permissible product temperature. Mannitol is the most widely used.

Cryoprotectants and lyoprotectants

Sucrose and trehalose remain amorphous and protect by forming a glassy matrix in which the peptide is immobilised, and by hydrogen bonding to the solute in place of the water removed. The two mechanisms — vitrification and water replacement — are both supported by evidence and are not mutually exclusive.^[5][3]

Buffers

Buffer salts can crystallise selectively during freezing and shift pH substantially. Sodium phosphate is the classic example: crystallisation of the disodium salt during freeze concentration has been reported to drive pH down by several units, a change more than sufficient to accelerate hydrolytic degradation. Histidine and citrate buffers are less prone to this behaviour.^[5]

Surfactants

Polysorbate 20 or 80 at low concentration reduces adsorption at the ice-water and air-water interfaces, a recognised route to aggregation during freezing.^[2]

For research peptides the formulation is frequently minimal — the peptide, its counterion, and whatever residue survives purification — because the material is sold as a chemical rather than as a dosage form. This has two consequences an encyclopedia should record. The cake is often small and poorly formed relative to a pharmaceutically formulated product, so cake appearance carries little information about quality; and the absence of a bulking agent means the mass in the vial is dominated by peptide, counterion and water, making peptide content rather than fill weight the meaningful quantity.^[6]

Peptides purified by reverse-phase chromatography with trifluoroacetic acid as ion-pairing agent are isolated as trifluoroacetate salts unless a salt exchange is performed. Trifluoroacetate is hygroscopic, contributes appreciable mass, and lowers the pH of the reconstituted solution. Its presence also depresses the critical temperature of the freeze concentrate, lengthening the cycle. Acetate exchange is the usual remedy and is one of the routine differences between material intended as an active pharmaceutical ingredient and material sold for laboratory use.^[1]

The attributes recorded for a lyophilised product are appearance, residual moisture, reconstitution time and the assay of the reconstituted solution. Each answers a different question, and none substitutes for another.

Appearance is judged against a description in the specification — for instance a white to off-white cake occupying substantially the volume of the fill, free of melt-back and of visible particulate. Appearance detects gross process failure and is the only attribute observable without instruments. It does not detect chemical degradation, and a cosmetically perfect cake carries no information about purity.

Residual moisture is measured by Karl Fischer titration or, less specifically, by loss on drying. A discrepancy between the two on the same lot is informative: loss on drying substantially exceeding the Karl Fischer figure suggests residual solvent, whereas the reverse suggests water bound too tightly to be driven off at the drying temperature.

Reconstitution time and clarity detect collapse, over-concentration and incipient aggregation. A cake that requires prolonged agitation, or that yields a hazy solution, is a documented indicator of physical instability even when chemical assay is within specification.^[2]

Headspace composition is set at stoppering. Vials sealed under partial vacuum or under nitrogen exclude oxygen and thereby suppress methionine oxidation during storage; vials sealed at atmospheric pressure in air do not. Headspace pressure is measurable non-destructively by laser-based headspace analysis, which is also used as a container-closure integrity method under USP <1207>.^[6]

The practical significance of lyophilisation for distribution is that it removes the dominant degradation pathway. Hydrolytic reactions require water and molecular mobility; a dry amorphous glass held below its glass transition temperature provides neither in quantity. Reported consequences are large. Degradation rates for peptides in the dry state are commonly two to three orders of magnitude below rates for the same peptide in solution at the same temperature, and the temperature sensitivity of the residual degradation is correspondingly lower.^[3][2]

This is the basis for the common practice of shipping lyophilised research peptides at ambient temperature without refrigeration, and for manufacturers' statements that a lyophilised product tolerates transient warming. It is not a licence to disregard temperature entirely. The glass transition temperature of a dry peptide-sucrose matrix may be only 40–70 °C; a parcel left in a vehicle in summer can plausibly approach that range, and above it the matrix devitrifies and mobility rises sharply. Moisture ingress through a compromised seal has the same effect, because water plasticises the glass and lowers its transition temperature by tens of degrees.^[3]

The evidence available for any particular unregulated product is usually nil. A manufacturer's stability programme conducted under cold-chain and ICH conditions produces data that support a labelled storage statement; material distributed without such a programme carries a storage recommendation that is, at best, an inference from the behaviour of similar compounds.^[13] Community-collated observations of material that appeared to perform normally after warm transit exist but are self-reported, unblinded, and without pre-excursion analytical baselines, and cannot establish the absence of degradation.^[14]

A cycle developed on a laboratory dryer frequently fails when transferred, and the reasons are well characterised. Radiative heat load differs between a small chamber with a glass door and a large chamber with cold walls, so the distribution of product temperatures differs. Condenser capacity and duct conductance limit the maximum sublimation rate a large dryer can sustain; when the rate approaches that limit, flow through the duct becomes choked and chamber pressure rises above its set point, warming every vial simultaneously. Shelf-temperature uniformity and the flatness of shelves both affect the vial heat transfer coefficient.^[8][4]

Quality-by-design approaches address this by constructing a design space in terms of the two variables that actually constrain the process — the maximum sublimation rate the equipment can support, and the shelf temperature at which the product would reach its critical temperature — and locating the operating point inside both boundaries. Presented graphically, this is conventionally called a design-space or primary-drying diagram, and it makes explicit that a cycle is equipment-specific.^[4]

For an encyclopedia the transferable point is narrower: a statement that a product is lyophilised conveys almost nothing about how it was lyophilised, and two vials of nominally identical peptide from different suppliers may have experienced cycles differing by tens of degrees in product temperature and by a factor of three in duration. Where cake appearance, moisture and reconstitution behaviour differ visibly between lots of the same compound, process difference is a more parsimonious explanation than difference in the peptide itself.^[1]

• Reconstitution of lyophilised peptides
• Karl Fischer titration
• Vial
• Vial filling and stoppering
• Peptide aggregation
• Cold chain
• Loss on drying

• Rey L, May JC (eds). Freeze-Drying/Lyophilization of Pharmaceutical and Biological Products. 3rd edition, CRC Press (2010) — the standard multi-author reference.
• Franks F. "Freeze-drying of bioproducts: putting principles into practice." European Journal of Pharmaceutics and Biopharmaceutics 45(3):221–229 (1998).
• Pikal MJ. "Freeze-drying of proteins: process, formulation, and stability." ACS Symposium Series 567:120–133 (1994).

• ISO 8362 series (catalogue entry) — Standards for injection containers, closures and freeze-drying closures. Paywalled; abstracts are public.