Lyophilization is not a freeze-dried technology

It has been more than a hundred years since Bordas and d'Arsonval demonstrated that a fragile substance could be stabilized by drying its solution from the frozen state. Additionally, for more than sixty years, lyophilization has been used extensively to prepare instant human plasma. Antibiotics, steroids, and bacterial and virus vaccines have also been prepared through freeze-drying by pioneers including Flosdorf, Greaves and Henaff.

Today, despite a somewhat depressed market in the early 1970s because of the development of pure, stable crystallized chemicals, lyophilization is still an almost unavoidable technique when preparing delicate proteins, complex immunological factors and sensitive products resulting from genetic engineering. Many different laboratories are engaged in this research and they often face rather difficult issues. Five of these key areas are discussed in this article.

Product formulation

Most products cannot be dried because they are either in very small amounts (per therapeutic unit) or because they need some kind of "protection" during the different stages of the process. Product formulation is thus a fundamental issue and requires the appropriate optimum association of cryoprotectors, bulking agents, moisture stabilizers and co-solvents. Quite obviously, the great number of different "formulations" that result from these studies deserve specific case by case investigations to determine — at the experimental and pilot levels — ad hoc freeze-drying cycles since there are no all-purpose recipes available. An interesting alternative to bulking agents could be the adsorption of the active substrate on a neutral porous substrate from which it is extracted at the time of reconstitution.

Product safety

As most freeze-dried products are used as injectables, it is compulsory to avoid their contamination during processing to ensure their sterility and prevent the presence of undesirable micronic or even sub-micronic particles.

Accordingly, a substantial amount of research has been devoted to developing special glassware, devoid of any surface contaminant and having quartz-like, non-adsorbing internal walls (e.g., Type I Plus [www.formavitrum.ch]) as well as new polymer vials resistant to heat-sterilization. Another interesting approach has also been initiated by GORE with the introduction of sterile containers for bulk-drying sealed with water vapour permeable membranes, which are impermeable to bacteria and micronic particles (Lyoguard [www.gore.com]).

Automation

The ever growing size of freeze-drying operations results in the need to conduct large-scale operations, involving tens of thousands of vials, making it necessary to use automation. Equipment manufacturers as well as pharmaceutical operators have developed clever and reliable designs for automatic filling, loading, unloading, capping and final release of the finished products.

Key points

Compliance and controls

Regulatory authorities require full and appropriate validation of the freeze-drying process. The major issue relates to sterility at the end of the drying cycle, which can never be taken for granted when dealing with batches of 100000 or more vials in several multi-shelf freeze-drying cabinets. Recently, radiation has been introduced as a way of ensuring absolute sterility. This includes cobalt-based gamma irradiators, a new generation of powerful, high-voltage electron-beams and high-energy X-ray units.

Another major issue is the control of the residual moisture in the plug. A full range of different instrumentation may be used including classical Karl Fischer determination of chemical water thermogravimetry, nuclear magnetic resonance, near-infrared technology and, more recently, non-invasive water vapour pressure measurements.

Optimum residual moisture should always be achieved and it depends entirely on the product. Some freeze-dried preparations need to be on the very low side (0.1%) while some others require a much higher value (5% or more) to avoid denaturation of the tertiary structure of sensitive proteins, which must not lose their constitutional water-content.

PAT and new developments

In view of the nature of its complexity, it might be desirable to apply FDA's process analytical technology (PAT) to lyophilization.

PAT could be applied in many segments of the freeze–drying cycle, which could then be controlled and monitored on-line (e.g., filling, loading, unloading and capping.) It is, unfortunately, much more difficult to effectively implement PAT in the heart of the process — drying under vacuum — since this is a unit operation where direct access to representative samples of product is difficult.

This does not mean, however, that it is impossible, but special equipment will need to be developed. It is a challenging issue and, as we proposed several years ago, the solution may lie within the use of semi-continuous to continuous operations, which are common practice in the food industry (e.g., instant coffee).

A continuous freeze-dryer, coupled with both on-line conditioning equipment and a final post-sterilization irradiation unit (also on-line), could be the answer to successfully implementing PAT.

This is a challenge for all parties and requires close collaboration between equipment manufacturers, freeze-drying operators and compliance officers so that regulatory requirements remain in line with technical and economical possibilities.

If we add, on top of these issues, the possibilities offered by the use of organic solvents or co-solvents in formulation, and even the potential application of selected mineral solvents such as NH₃ and CO₂, we see that lyophilization remains a very important technology.

Louis Rey is scientific advisor at the AERIAL Technological Resources Center, Illkirch (France).

Florence Lhospice is manager of pharmaceutical operations, Innate Pharma, Marseille (France).