Susan Aldridge

The main challenge of getting a biotech drug from this stage to the patient is the delicate and unpredictable nature of protein molecules. According to Geoff Helleswell of Micropharm (UK), the manufacture of their anti-sera and polyclonal antibody products takes a couple of days, but formulation, filling and signing off a batch can take 4–6 weeks. The cost of formulation and filling, as well as ensuring stability, is a big issue for the company as it produces antivenoms for countries, such as Nigeria, that cannot afford expensive freeze drying, which is a popular formulation technology for protein drugs.

"There are several stages involved in the formulation and filling of a biotech drug", says Helleswell. Some of them are similar to those for small molecule drugs, but regulatory authorities often demand more data for a protein drug because of its inherent instability, and various tests must be done on the bulk product, with bioburden (microbial content), protein concentration, and virus removal or inactivation being the most important.

At this stage, the question arises as to whether the product can be terminally sterilized. For a biologic, the answer is usually 'no' as protein molecules unravel when exposed to the heat of a sterilization operation. Therefore, filtration into a previously sterilized container will be necessary, with a recommended second filtration step immediately prior to filling. "Double filtration is very important for biologics," says Helleswell.

The author says...

Most biopharmaceutical products are injected. This means preparing either a freeze-dried solid product that is reconstituted in the clinic or solutions that are contained in either capped vials or sealed ampoules. Prefilled syringes, for ease of patient use, are an increasingly attractive alternative in some applications. Additives, such as salts or preservatives, might be necessary to stabilize the protein and keep it stable for whatever storage times and conditions are appropriate to the final end use. A great deal of development work is needed to select the most appropriate additives to ensure they do not give any untoward clinical side-effects.

Inspection

The filled product must be inspected for complete closure and for visible and subvisible contamination. Meanwhile, sterility is checked with samples that are representative of the whole batch. Other tests, such as protein concentration, pH, osmolality and activity may also be required.

Visual inspection, which can be performed by eye by a validated operator or with an automated visual inspection machine using lasers, is an important check. Regulatory authorities have requirements for clarity and place limits on even subvisible particles in a product. Proteins tend to precipitate, adsorb onto surfaces, or aggregate — all of which can cause visible or invisible deposits.

The clinical significance of aggregation processes — particularly whether they can cause unwanted immune side-effects — is currently a subject of some debate. No protein product can be guaranteed to be 100% free of aggregation, but it should be understood and controlled as much as possible for a particular process and product. Aggregation often goes undetected and the underlying causes can be quite complex. pH and ionic strength of the formulation mix tend to be important factors and freeze drying often causes aggregation in some products when they are reconstituted. Aggregation can also occur during transport of a product by the use of pumps and even certain types of packaging. Additionally, shaking a protein solution is less likely to cause aggregation than stirring it.

Visual inspections are important for assessing formulation stability as well as protein aggregation. There are a number of approaches for assessing the particle content of a protein solution. Light extinction methods are used to work out the actual size of the particle, but they are not applicable if the particle is translucent. Sometimes the sample must be diluted, which can cause interfering air bubbles, and may, in itself, cause aggregation.