Lyophilization: Why Research Peptides Are Freeze-Dried

What lyophilization is

Lyophilization, also called freeze-drying, removes water from a frozen solution without passing through the liquid state. The water leaves as vapour by sublimation, straight from ice to gas, under reduced pressure. What remains is a dry, porous solid, usually called a cake, which keeps roughly the shape of the frozen material.

Dry solids are a widely used way to give delicate molecules a longer shelf life. The review literature on protein pharmaceuticals puts it simply: many proteins have limited stability in solution, so they are made into solid form, and freeze-drying is the most common way to do that.^[1] Those reviews are written mainly about proteins, but the underlying chemistry of water, mobility and degradation applies equally to peptides.

The three stages of a cycle

A lyophilization cycle is a programme of shelf temperature and chamber pressure over time. It has three stages.^[2]

1. Freezing. The solution is cooled until the water becomes ice and the solutes are concentrated in the spaces between ice crystals. The way freezing is carried out affects ice crystal size and so the structure of the cake, the time drying takes, how much moisture remains, and how fast the cake dissolves again.^[3]
2. Primary drying. Pressure is lowered and gentle heat is supplied so that ice sublimes. Product temperature has to stay below a critical temperature, often called the collapse temperature, or the structure of the cake can fail.^[2]
3. Secondary drying. Temperature is raised to remove water that is still bound to the solid after the ice has gone. The residual moisture left at the end is one of the main factors in how long the dry material keeps.^[4]

Why drying slows degradation

Water is a reactant in hydrolysis, and it also lets molecules move. Removing it slows several routes by which peptides change chemically; the main ones are covered in storage stability and degradation pathways. A study of a model hexapeptide in lyophilized formulations found that residual moisture, temperature and the type of bulking agent all had measurable effects on how fast the peptide reacted in the solid state.^[4] In other words, dry does not mean inert; it means slower, and by an amount that depends on how dry and how cold.

What the cake looks like, and what that means

• A white, even, porous cake. The typical appearance of a well-run cycle with enough solid in the vial.
• A thin film or a few flecks. Very small amounts of material may form almost no visible cake. Appearance on its own is not a measure of how much is in the vial.
• Shrunk, collapsed or glassy material. This can be a sign that the product warmed above its critical temperature during drying or that moisture was taken up later. It is a reason to look closer, not a measurement.
• Cake that has come off the vial wall. Often cosmetic. Cakes can fracture during handling or transport.

Appearance is an observation, not an analysis. Content, purity and moisture each need their own measurement.

Excipients and salts in the cake

Some freeze-dried products contain bulking agents, buffers or stabilizers, which are solids that give the cake body and protect the molecule through freezing and drying; formulation reviews describe how these are chosen.^[5] Peptides prepared by chemical synthesis also carry counter-ions from purification; see counter-ions in synthetic peptides. Whether a given material contains anything besides the peptide is a question for its specification. If a specification does not say, treat the composition as unknown and consider that the weighed mass includes more than peptide.

Residual moisture and hygroscopic cakes

A dry cake can take water back from the air, and many are hygroscopic. Residual moisture is measured by methods such as Karl Fischer titration or thermogravimetric analysis; results are given as percent water by weight. Because water adds to the weighed mass, it is one of the reasons a gross mass is larger than the mass of peptide; see purity versus net peptide content.

Handling practices that follow from this are straightforward laboratory habits:

• Equilibrate before opening. A cold vial opened in warm room air collects condensation on the cake. Let it reach room temperature, still sealed, first.
• Keep the closure intact. The rubber stopper and seal keep the dry atmosphere inside the vial. Once pierced or opened, the sealed environment is gone.
• Minimize open time. Weigh quickly, reseal, and return material to its specified storage condition.
• Use a desiccant policy that matches your facility, and record when a container was opened.

Dissolving a dry cake

Making a solution from a cake is a matter of choosing a solvent and a target concentration, then doing unit arithmetic. The solution concentration calculator does that arithmetic from numbers you enter. The choice of solvent itself depends on the peptide; see solubility and aggregation basics. Freeze-dried material starts to change again once it is in solution, which is why in-use windows are set by your own protocol and records, not by the fact that the starting material was dry.

Limits of what lyophilization does

Freeze-drying is a stress as well as a protection. Freezing concentrates solutes, and drying removes the water that holds large molecules in shape; the reviews describe both freezing and drying stresses on proteins and the use of protective excipients to limit them.^{[1, 3]} For short peptides there is far less structure to lose, but chemical reactions and physical changes can still occur. A lyophilized label describes a physical form. It says nothing by itself about purity, content or identity.