API Purification

What are the currently preferred methods of purification for synthetic APIs and API bioprocessing?

In processes that use a series of chemical reaction steps to synthesise the API, the removal of reaction by-products, including colour bodies and metals, is critical to produce high quality pharmaceuticals. The preferred methods for removing residual metal catalysts are distillation, crystallisation and precipitation. A distillation collects the pure API as a distillate, leaving the non-volatile compounds in the residue, while crystallisation and precipitation steps both generate solid material that can be physically removed by selecting a filtration step. In addition, both chromatography and activated carbon powder treatments are used to exploit charge and adsorptive technologies for impurity removal.

Peter Koklitis

In bioprocessing, whereby APIs are isolated and purified following clarification steps of mammalian or bacterial cell harvests, a combination of affinity and ion-exchange chromatography are used in conjunction with filtration steps. Protein A coupled to an agarose matrix is now widely used as an effective affinity column for the purification of monoclonal antibodies, while anion-exchange chromatography is used for host cell impurity removal.

What are the latest significant innovations in API purification?

In synthetic API manufacture, the use of metal catalysts, such as Palladium, has increased from using convergent reaction processes to prepare chemical fragments in parallel, which are coupled together to form the API. In these processes, there is a choice to use either homogeneous (non-immobilised and dissolved) or heterogeneous (immobilised) metal catalyst systems. For example, many reactions use Palladium bound to carbon (Pd/C), as these offer high surface area supports which are relatively easily and inexpensively prepared.

Several companies have specialised in providing heterogeneous catalysts, such as using highly cross-linked microporous matrices, which retain catalytic activity and enable the metal to be reclaimed and reused, but there are other examples where specific functional groups on a polymer backbone or alumina have been used to attach the metal catalyst. Heterogeneous metal catalysts bound to supports are easily separated by filtration after the reaction (e.g., Suzuki, Heck and Sonogashu), although leaching of the metal may occur. The removal of metal residuals has resulted in the need to develop rapid screening devices to identify suitable adsorbents. There are a wide range of adsorbents to consider for metal removal, e.g. activated carbon, functionalised polymer resins, silica, diatomaceous earth, alumina and clay—the choice is dependent on the degree of selectively required and cost of ownership.

In API bioprocess applications, there have been several recent innovations to provide single-use systems that enable modularisation. For example, large-depth filter systems used in cell harvest clarification are now being considered as single use because of developments in encapsulation technology. The desire to engineer modular processing units also exists for API synthetic processes; a modular platform approach provides faster and easier scale up for new processes, with clear visibility of capital and variable costs. These innovations also provide operational flexibility, while reducing equipment-cleaning issues at the same time.

What are the process challenges involved in API filtration and purification?

The increased need to fast track APIs to market and to minimise validation activities, although challenging, has presented significant opportunities for new product developments in purification. The requirement to remove any residual metal catalyst from API synthetic process streams co-exists with the drive to establish modular platforms for flexibility and scale-up; thus, once an effective metal adsorbent is identified, the process scale-up requirements must be considered. The use of adsorbents in bulk powder form has inherent disadvantages, particularly when scaling up for production operation (i.e., dust handling, personal protection and equipment cleaning). In some instances, the metal adsorbent selected for pilot scale operations is found to be unsuitable at a larger production scale because of cost considerations or fundamental changes in process step conditions, for example. The adsorption of metal species is influenced by temperature, pH and solvent composition; therefore it is often necessary to re-consider the choice of adsorbent if any or all of these conditions change for scale up following process optimisation. The use of chemically aggressive solvents, such as acetone, tetrahydrofuran and dimethylformamide, in API manufacture, although not preferred, may also be required for catalytic synthetic steps. Consequently, these can present compatibility challenges to the material components of the systems.

The move to having flexible scalable systems for process filtration has also influenced the development of filter products to provide modular solutions. The use of filter media impregnated by adsorbent powder, such as activated carbon modules, to avoid the use of bulk powder handling has been widened to consider how other adsorbent materials or speciality metal scavengers can be similarly immobilised. This approach can only be successful when the adsorptive properties are not impaired by the immobilisation procedure. Laboratory and pilot laboratory testing can confirm efficacy and also accurately size area requirements for process scale up. The cost of implementation can be offset by the removal of what will become redundant existing filtration steps, previously used for the bulk powder removal.

In API bioprocessing, host cell impurity removal at clarification and in downstream processing is an area of focus and the use of charged membranes as encapsulated capsules could decrease expensive chromatography column usage. The adoption of single-use systems in bioprocessing process steps is generally not restricted through chemical compatibility issues, as seen with synthetic API processes. However, alkali-resistant capsules are required in bioprocessing manufacturing if caustic solutions are used for validated depyrogenation procedures.

How has the trend towards single-use technologies impacted API filtration processes?

As mentioned previously, there has been much innovation in the area of single-use systems. However, although such systems are being used in API filtration, particularly in API bioprocessing, disposable solutions do not necessarily suit all situations. The investment and maintenance costs of all process steps need to be calculated and then considered for both disposable and stainless steel systems. The calculations that contribute to forming a decision should include workers time, water, cleaning chemicals and waste disposal costs. The adoption of plastic encapsulated filters is also restricted when chemically aggressive solvents are used. Future process design technologies may yield novel reaction pathways, which use less challenging solvent systems, and such advances would contribute to widening the adoption of single-use components.

Single-use systems are also being used for highly potent APIs. These ingredients require special containment, but a range of solutions have been developed in this area, such as encapsulated filter media that can clarify harvests.

What are the latest trends in API filtration equipment/services?

Bioprocessing companies are achieving higher titres for their target API proteins, typically monoclonal antibody fragments. A consequence of this is to allow smaller bioreactors to be used with smaller facility design specifications in order to obtain the required amounts of product for clinical trials. These smaller throughput requirements have contributed to the increased use of single-use technologies for the purification process, as well as for the bioreactor. Consequently, demand has grown for single-use sterile connectors, membrane-chromatographic adsorbers, sensors and mixers, as well as production-scale filter capsules, containers and the bioreactors themselves. The standardisation of all these single-use components, with respect to materials and dimensions of connectors and fittings, is a user consideration. However, many of the design features and materials of these components will remain proprietary to specific manufacturers.

Peter Koklitis is a technical filtration specialist at 3M's Purification Division.