The development of optimised process for freeze-dried product is described. New available analytical tools are reviewed and different steps from early phase to life cycle management are covered.
Freeze-drying is used by the pharmaceutical industry for years. It’s a way to protect sensitive molecules from degradation, allowing their shelf life to be extended from days or months to years. Moreover, the technology is perfectly integrated inside industrial GMP (Good Manufacturing Practice) production giving a high degree of confidence in product sterility. However, the process has some drawbacks: it incurs huge costs and is very complex. Other techniques exist to stabilise molecules, but they are not so well accepted in the pharmaceutical industry.
Freeze-drying may be seen as the relationship between the following three parts:
The advancements in science now allow us to produce more and more complex molecules to target specific diseases. The complexity of these molecules makes it very difficult to stabilise them. In this context, freeze-drying appears as a method of choice to stabilise the molecules. Consequently, there’s a need to rationalise the way process development is done.
The product development process consists of different steps. First, cryoprotective molecules such as sugars, salts and polymers are chosen to protect the targeted active ingredients at the liquid state before and during freeze-drying. Then, a freeze-drying cycle is designed to process the formulation. Finally, the process is scaled-up for clinical production and is later transferred to industrial production facilities.
In the past, freeze-drying process development consisted of finding a dedicated process for each product. As there were no technological tools to follow the process, the release criteria were mainly used. Generally, a satisfying cycle was reached by the trial and error method.
With the advances in the scientific knowledge of new molecules, various online and offline tools have been used to improve the process to meet the requirements of the industry.
The following example illustrates how stabilisers are selected from various options keeping in mind the business interests. The first screening shows equivalence between two different stabilisers for biological activity. One of the two stabilisers has a higher collapse temperature. If chosen, this stabiliser will allow developing a more robust and quick (as the allowed temperature is higher, the product can be dried faster) freeze-drying cycle. It is better to run a three freeze-drying cycles a week than two freeze-drying cycles a week as it increases the capacity by 33 per cent. On the other hand if the right stabiliser is chosen at the beginning of the development it allows you to obtain the desired result.
The development of process is generally well defined and involves several steps. If the stabiliser is a well-known substance, former development and some modelisation will help to quickly define the safety margin of the process and necessary trials are planned accordingly. For branding new kind of products, the available tools will be reviewed and applied.
In this category some new equipment have appeared during the last years. Previously, developers only had two online tools to monitor the process, an invasive temperature probe placed in a vial and the view port, through which they could have a look at their product during drying. Now, micro balance can follow the loss of weight of the product during the whole cycle allowing easy optimisation. Cold plasma system can measure the water vapour flux inside the freeze-drying chamber during development phase of the stabiliser.
Another step of process development is to make the cycle robust with Design Space study. The input and output parameters of the process are reviewed. Input parameters for freeze-drying are: shelf temperature, pressure in the system and time. The output parameters can be: residual moisture, potency, content, drying kinetics. The goal is to obtain the same range for output (i.e. good potency) at all times by varying on a wide range the input parameters (i.e. pressure 10 and 100 microbars). If one can reach this target they end up with a robust product where they are confident about the scale-up. The freeze-drying cycle development directly integrates safety margin to assure a smooth transfer to larger scale. For regulatory purpose it’s far easier when freeze-drying cycle is not modified between different clinical phases. Those studies are not performed on a trial and error basis anymore, but use the power of DOE (Design of Experiment). This approach is perfectly suitable for freeze-drying since there are only a few well-characterised input parameters for this closed system process.
A further step is clinical development. All the development trials are performed at small scale (below 5,000 units) to avoid wastage of raw materials. Once clinical consistency is established, the trials are performed at larger scale.. This is the first scale-up of the product towards larger scale. At least three batches are produced with the target value of the developed cycle. They are deeply analysed for visual appearance, moisture content and moisture distribution. Statistical analysis is done on the result to underline any deviation compared to early development results.
Transferring for large-scale production
The last step is the transfer to production unit. It requires people with a good knowledge of clinical and industrial freeze-dryers. The engineering department should receive guidance before buying new freeze-dryers. An exotic industrial freeze-dryer may require upgrade of some technical part, since it will run out of the defined range. The transfer involves a multidisciplinary team in terms of department and competencies. Freeze-drying is not a stand-alone technology; the formulation, filling and some specific tests are equally transferred. For example, with each product a kit of known visual defects is provided to the manufacturing department. Summary of the process evaluation and process validation is set up for transfer.
Commercial consistency batches are produced in the targeted factory. Specific sampling procedure is developed to assure homogeneity inside the load for specific attributes of the new product. Like at clinical step, statistical analysis is realised on each batch and a comparison between batches of different freeze-dryers is done. After transfer a support of the product is kept. It allows detecting any unexpected behaviour of the product at industrial scale. These learnings act as feedback during the development of other new products in order to continually improve the scaling-up model.
Freeze-drying process development is a robust, well-defined way to assure the success of new products. Even if it can be seen as time-consuming, the knowledge of the product and process acquired in the first step of development will avoid a lot of costly burden at production scale with better product understanding.
A freeze-drying cycle may be divided into three part
The product is first frozen at a low temperature to reach a vitreous state. The nature and structure of the frozen plug allows a good protection Then, it undergoes sublimation under vacuum maintaining the product temperature below its glass transition temperature or collapse temperature. That specific temperature is determined by the stabilisers chosen and is measured with specific tools such as the cryomicroscope. This value is of paramount importance for the product elegance. If the product is sublimated at too high a temperature it ends up to be a melted product that will be rejected, creating burden (write-off) at industrial scale
After primary drying, 10 to 20 per cent water is still inside the formulation adsorbed by the active ingredient. Secondary drying, often called desorption, removes that bound water from the product. This step is carried out at a higher temperature to remove the absorbed water. Removal of water is done by diffusion and not anymore by sublimation. Since diffusion kinetics is slower, it explains why secondary drying can take one-third of process length to remove only 10 to 20 per cent of the total water. The maximum temperature is defined in function of the product, but usually stays below 40°C for biopharmaceutical molecules.