Freeze-drying is often used to increase the shelf life and stability of drugs. However, it can be an expensive and time-consuming process which can result in a substandard product if it is not done correctly. World Pharmaceutical Frontiers explores the best practices for overcoming the disadvantages of this technique and looks at the latest technology in the freeze-drying field with Sajal Patel, senior scientist at MedImmune.

World Pharmaceutical Frontiers: Why is freeze-drying a popular process for biopharmaceutical drugs?

Sajal Patel: Freeze-drying is routinely used to stabilise an unstable molecule and achieve the required commercial shelf life. In an aqueous solution, physical and chemical reactions are much faster due to higher molecular mobility than lyophilised products, leading to unacceptable stability. Lyophilisation provides low-temperature and low-pressure conditions to remove the solvent (usually water) to convert the material in the dry state. Freeze-drying has been used in one form or the other since the dawn of civilisation. However, industrial freeze-drying processes were used during World War II to preserve blood serum, which was used to treat the wounded soldiers.

Explain the process of lyophilisation. What are the critical steps?

Freeze-drying is usually carried out in three steps:

  • freezing: conversion of water into ice
  • primary drying: sublimation of ice at low temperature and low pressure
  • secondary drying: removal of unfrozen water at high temperature and low pressure.

Typically the residual water is less than 1% at the end of secondary drying. Freezing may or may not include an annealing step depending on the formulation and process optimisation. If there are excipients in the formulation that tend to crystallise, and are serving as bulking agents, then an annealing step is included to ensure complete crystallisation during the freezing part of the process. In which case, the annealing time and temperature becomes critical to ensure crystallisation during the freezing step. If these excipients stay amorphous during freeze drying then they tend to crystallise during storage potentially impacting stability and shelf life.

Routinely, annealing is also included for a completely amorphous formulation matrix to reduce primary drying time and also to minimise the heterogeneity within the batch due to random ice nucleation during the freezing step.

For what types of drugs is lyophilisation most suited to?

In general, the lyophilisation process is suited to any molecule (small molecules, monoclonal antibodies, bispecifics, peptides, fusion proteins, vaccines, enzymes) that have stability challenges in solution state. However, freezing and the drying process could be detrimental to the stability of the molecule. Hence, a delicate balance between formulation and process optimisation is needed to achieve acceptable stability for commercial shelf life.

What are some important considerations for developing stable drug formulations by lyophilisation?

It is critical during early development, that formulation and process development are considered in parallel rather than in sequence. Formulation selection defines the limits for lyophilisation process design and development, whereas selection of lyophilisation process parameters governs the end-product quality. Lastly, developing a robust freeze-drying process is critical for successful scale-up and tech transfer of the process from lab scale to manufacturing scale, between different dryers or between different fill sites.

Run us through the major challenges of the technique. How common is it for protein drugs to suffer irreversible damage, and why does this happen?

The major challenges with lyophilised drug product development can be classified into two categories – formulation and process.

Formulation: since the process of freezing and thawing itself could be detrimental for biologics, cryoprotectants and lyoprotectants are needed to maintain stability during processing. Selection of excipients and the right ratio of these excipients relative to other excipients/active is critical. Selection of buffer and buffer species needs due consideration, since some reagents tend to crystallise during freezing – causing a pH shift, which results in loss of activity or purity during the freezing process.

If crystallising excipients are used in the formulation then the ratio of crystallising excipients to other total solids in the formulation is important. Protein instability may happen during freeze drying or during reconstitution. Generally accepted theories for protein stabilisation during freeze-drying suggest identifying formulations that preserve native structure and/or immobilising the protein in a glassy matrix. Thus, the formulation really needs to be designed to develop a successful lyophilised drug product. Occasionally, the widely accepted general rule of thumbs are not followed which ends up costing unnecessary time, material, and efforts.

Process: Lyophilisation process design depends on formulation selection. I believe there is no such thing as process design and optimisation for a poorly designed formulation, since the limits imposed by the formulation selection don’t allow for any optimisation, for example, formulations with low-glass transition temperature (Tg) and collapse temperature (Tc), wherein the criteria for process parameter selection would be to run as cold as possible and as long as possible. Truly, in such cases, there is no such thing as process optimisation, and one would just hope that there are no malfunctions or freeze-dryer breakdowns and the batch comes out with an acceptable product quality. This is in line with quality-by-accident (QbA) and not quality-by-design (QbD), where the latter aims at building quality within the formulation and process rather than merely hoping that the product will meet the desired quality at the end.

Additionally, the lyophilisation process is well-established for freeze drying in vials. However, more work needs to be done to address the challenges associated with freeze drying in non-vial containers, namely dual-chamber syringes, cartridges and ampoules.

What are the critical new developments in the field of freeze-drying and what has been your experience with them? How do they compare with older techniques?

There are several advancements in the field of freeze-drying – both in formulation and process development. Advancements in the biophysical characterisation techniques have enhanced our understanding of why certain excipients have greater stabilising effects than certain others. New technologies have made it possible to make measurements of physical and chemical properties that were not possible before. Neutron backscattering has shed light on a long-debated question of whether it’s global mobility (α-relaxation) or fast dynamics (β-relaxation), or both that govern protein stability.

Solid state hydrogen deuterium exchange (ssHDX) now provides peptide level details to understand the effect of formulation composition on stability. Techniques such as Raman allows in-situ determination of protein secondary structure in the ice phase; whereas near infrared (NIR) allows online monitoring of the freeze-drying process, namely to monitor completion of freezing, primary and secondary drying.

With the process analytical tools (PAT), the lyophilisation process can be monitored in real time to build the desired quality attributes within the process. Techniques such as TDLAS (tuneable diode laser absorption spectroscopy) and MTM (manometric temperature measurement) provide insight in to process performance, which is tremendously useful for process optimisation, scale-up and tech transfer. Several techniques are now available to address the issue of heterogeneity within the batch due to random and stochastic ice nucleation temperature during the freezing step. Of course more work needs to be done before the industry can really adopt and put these techniques into routine manufacturing.

Over the past three decades, the understanding gained in the area of lyophilisation (formulation and process) along with advancement in PAT tools, has transformed lyophilisation from being an art to being science based. Applying QbD principles to other unit operations may seem challenging, but it lends itself really well to lyophilisation, which is reflected in increasing number of publications describing QbD-based approach to lyophilised drug product development.

How important do you predict the technique will be in the next ten years?

In the next ten years, lyophilisation will continue to be an important unit operation that will enable delivering lifesaving drugs to the market. The entire process can be carried out aseptically and the resulting end product, with the right formulation and process, is usually much more stable than the solution. With companies trying to enter the developing countries, there might be a need for room temperature stable products – in which case lyophilisation will be the way to go forward. Lastly, the complex molecules that are engineered to manage the life-threatening diseases are highly unstable in solution and hence would need to be lyophilised to achieve acceptable shelf life to be marketed as a commercial product.