What lyophilized means, and why peptides ship as powder

Freezing, sublimation and desorption in plain terms: why a lyophilized cake looks the way it does, what residual moisture and a collapsed cake mean analytically, and why the label mass is nominal.

By Touchstone PeptidesSeptember 22, 20266 minute read

Lyophilized means freeze-dried. The peptide is dissolved, filled into vials as a solution, frozen solid, and then the ice is removed by sublimation under vacuum, leaving a porous solid cake with very little water in it. Research peptides ship in this form because almost every reaction that degrades a peptide needs water and molecular motion, and a dry glassy solid has neither.

The three stages

Process papers describe pharmaceutical freeze-drying in three parts. Tchessalov and colleagues, in their 2023 update on scientific design of the process, define them as freezing, primary drying, which is "ice sublimation," and secondary drying, which is "desorption of unfrozen water."

Freezing. The filled vials sit on temperature-controlled shelves and the solution is taken well below zero. Water crystallizes as ice; everything else, the peptide, its counter-ion and any excipient, is concentrated into the liquid between the ice crystals until that liquid becomes too viscous to flow and freezes into a glass. The temperature at which that glass softens is called Tg′, and it is the number the rest of the process is designed around.

Primary drying. The chamber is evacuated and the shelves are warmed slightly. At low pressure ice passes directly from solid to vapour without melting, and the vapour is trapped on a cold condenser. This is sublimation, and it takes hours to days because the heat must arrive through the vial base while the product temperature stays below the point at which the glassy solid would soften. Tchessalov's rule: "Product temperature should always be several degrees below Tc in order to obtain a dry product with acceptable appearance," where Tc is the collapse temperature, "usually a few degrees above the Tg′" in the words of Cheng and colleagues.

Secondary drying. With the ice gone, the cake still holds water bound to the solid. The shelf temperature is raised further and this unfrozen water desorbs. Tchessalov reports that for many products "secondary drying can be completed in 3-6 h at shelf temperature 40 or 50 °C, with water content below 0.5 wt% commonly achieved."

Why the cake looks the way it does

A good cake is the skeleton left behind when the ice leaves. Each ice crystal becomes a pore, so the cake has the volume of the original frozen fill and a fraction of its mass, which is why a 5 mg vial can appear to hold a substantial white plug. Cheng's group describes the target as a "mechanistically strong and elegant cake", and names collapse and "meltback" among the ways a cake fails.

The peptide alone is often not enough to build that skeleton. Bulking agents are added in pharmaceutical products for exactly this reason: "crystalline bulking agents such as mannitol and glycine should be considered when there is a challenge in forming mechanistically strong and elegant cake." A research peptide solid of the kind Bachem's content statement describes is the peptide, its counter-ion and residual moisture, with no bulking agent, so the cake reflects the peptide's own salt form. AmbioPharm notes that acetate salts usually give "a better lyophilizate cake, in contrast to some difficult to handle, 'fluffy' peptides" from TFA salts, which is one of the reasons the counter-ion is worth reading off a certificate. A fluffy, loose or fragmented cake is not by itself evidence of a bad lot; it is consistent with a low-solids fill or a trifluoroacetate salt, and only the assay can say more.

Collapse and melt-back, and why they matter analytically

Collapse is what happens when the product warms past Tc during primary drying. Cheng defines it as "the temperature above which the lyophilized drug product loses macroscopic structure and collapses during lyophilization." The glassy walls between the pores soften and flow, the pores close, and the cake shrinks into a dense, glassy or gummy mass. Melt-back is the related fault in which secondary drying begins before all the ice has sublimed, so ice melts inside the cake and the wet region dries into a dense skin.

Both matter for the same reason: water. A collapsed cake traps moisture that the vapour path can no longer carry away, and the same paper lists, among the consequences of incomplete primary drying, "decreased protein drug product stability" and "high levels of residual moisture that can cause chemical degradation through hydrolysis". The hydrolysis in question is the aspartate and asparagine chemistry described in freeze-thaw, aliquots and the residues that oxidize, which runs in a wet solid where it would not run in a dry one. Tchessalov's group also describes "microcollapse", a change in cake morphology short of visible failure, so appearance alone is not a verdict; it is a reason to look at the moisture number.

Residual moisture and Karl Fischer

The water left in a cake is measured by the Karl Fischer method, a coulometric determination in which the sample's water reacts stoichiometrically with iodine and the charge consumed gives the water mass. Bachem's care note is the reason the number matters to a buyer: the peptide content of a research solid "may range from 70 to 90% as peptides contain counter ions (e.g. acetate, trifluoroacetate) and residual moisture." Water is part of the mass on the balance and not part of the peptide, so a certificate that reports water content, counter-ion content and net peptide content together lets a laboratory convert a nominal milligram figure into moles of peptide. Net peptide content walks through that conversion.

Moisture can also arrive after drying. GenScript's guide states that "peptides containing Asp, Glu, Lys, Arg, or His are prone to moisture absorption from the air," and a cold cake exposed to humid air collects condensation, which is why Bachem's guideline mentions a desiccator at all. A dry glassy cake that takes up water becomes, locally, a concentrated solution.

Why a dry solid is the research format

Every manufacturer's guide says the same thing about solutions. GenScript: "The shelf-life of peptides in solution is very limited, much shorter than that of lyophilized peptides." Bachem: long-term storage of solutions is not recommended, particularly for peptides carrying Asn, Gln, Cys, Met or Trp. The reasons are the reactions that need water as a reagent (hydrolysis, deamidation), the reactions that need dissolved oxygen (oxidation of Met, Cys and Trp), the physical stresses of every freeze and thaw, and, in an unpreserved solution, microbial growth. Lyophilization removes the water, and with it the solvent for all four. What the dry solid then tolerates in transit is a separate question with its own published data (heat and transit).

The vial is therefore a different kind of object from a formulated product. A lyophilized research vial contains the peptide and its counter-ion and nothing else; a factory-made solution contains a buffer, a tonicity agent and a preservative chosen and tested for that peptide. The vial is the raw material; the spray is a product built from it. Vial, spray or strip sets out the three catalog formats side by side.

Fill by volume, then dry: why the label mass is nominal

A lyophilizer does not weigh powder into vials. It receives vials filled with a measured volume of solution at a known concentration, and the solid left in each vial is the product of those two numbers, plus whatever water and counter-ion stay behind. The label mass is a target set at the filling step. What that means for the milligram figure, and how far a vial can stray from it, is covered in is the vial actually 5 mg?

Sources

For laboratory research use only. Not a drug, not a supplement, and nothing here is a claim about what any of this material does in a person or an animal.

Two things before you come in.