Lyophilisation Explained

What is lyophilisation?

Lyophilisation is commonly called freeze-drying.

In a typical process, a solution is:

1. frozen;
2. exposed to reduced pressure;
3. dried by sublimation;
4. subjected to further drying to reduce remaining bound or adsorbed water.

Sublimation is the direct transition of ice into water vapour without first becoming liquid water.

The process is usually divided into:

• freezing;
• primary drying;
• secondary drying.

Freezing

During freezing, water forms ice and the remaining dissolved components become concentrated in the unfrozen phase.

Freezing conditions can affect:

• ice-crystal size;
• pore structure;
• drying behaviour;
• concentration gradients;
• physical form;
• product appearance.

The material's composition influences how it behaves during freezing.

Primary drying

During primary drying, chamber pressure is reduced and heat is carefully supplied.

Ice sublimes through the dried layer and leaves the product as vapour.

Important parameters can include:

• shelf temperature;
• chamber pressure;
• product temperature;
• condenser performance;
• duration;
• equipment loading.

Excessive product temperature can damage the physical structure of the dried material.

Secondary drying

Primary drying removes most frozen water, but some water remains associated with the material.

Secondary drying uses controlled temperature and low pressure to remove additional adsorbed or bound water.

The finished material normally retains some residual moisture. Lyophilisation does not mean that every water molecule has been removed.

Why lyophilisation is used

For some materials, converting a solution into a dry solid can reduce certain degradation pathways and make storage more practical.

Potential advantages may include:

• reduced molecular mobility;
• reduced hydrolytic degradation;
• improved storage stability;
• easier transport in some circumstances;
• rapid redissolution where a validated procedure exists.

These benefits depend on the formulation and process. Lyophilisation is not automatically beneficial for every peptide.

Process-related risks

Lyophilisation can also introduce stress.

Potential concerns include:

• freezing stress;
• concentration of solutes;
• pH shifts during freezing;
• oxidation;
• aggregation;
• incomplete drying;
• excessive residual moisture;
• over-drying;
• collapse or shrinkage;
• stopper or container problems;
• contamination during an inadequately controlled process.

The process therefore requires defined parameters and suitable finished-material testing.

Residual moisture

Residual moisture can affect:

• chemical stability;
• physical stability;
• powder flow;
• aggregation;
• glass-transition behaviour;
• long-term storage.

Water content may be measured using a suitable method such as Karl Fischer analysis, where appropriate.

A visually dry cake is not proof of an acceptable water-content result.

Cake appearance

A lyophilised material may appear as:

• a uniform cake;
• a loose powder;
• a shrunken cake;
• a fragmented cake;
• material attached to the vial wall.

Appearance can provide useful manufacturing information, but it cannot establish identity, purity, content or stability by itself.

A visually attractive cake can still fail an analytical specification.

Container closure

The vial and stopper help protect the material from:

• atmospheric moisture;
• oxygen;
• contamination;
• physical damage;
• loss of vacuum or inert atmosphere where applicable.

Container-closure integrity and storage conditions therefore remain important after drying.

Lyophilised does not mean sterile

Freeze-drying is not, by itself, a sterilisation process.

A sterile lyophilised material requires an appropriate sterile-manufacturing strategy, contamination control and supporting testing.

A non-sterile material does not become sterile merely because water has been removed.

Lyophilised does not mean indefinitely stable

Shelf life and retest periods require stability evidence.

Factors affecting stability may include:

• peptide sequence;
• chemical form;
• formulation;
• residual moisture;
• oxygen exposure;
• light;
• temperature;
• packaging;
• shipping excursions.

Generic statements should not replace product- and batch-specific information.