Crystallisation for the continuous downstream processing of proteins in the pharmaceutical industry

Introduction to Crystallization

• Focus: Crystallization for the downstream processing of proteins in the pharmaceutical industry.

  • Comparison with chromatography due to its high resin and buffer costs.

  • Crystallization allows for high purity and stable solid forms.

Importance of Crystallization

• Advantages over chromatography:

  • Achieves very high purity.

  • Solid forms provide stability.

• Continuous crystallization:

  • Potential to reduce operating costs and accommodate higher volumes.

  • Currently predominately conducted in batch processes.

Understanding Supersaturation

• Definition of Supersaturation:

  • An unstable condition with a concentration greater than equilibrium, leading to potential crystallization.

• How to achieve supersaturation:

  • Heating a saturated solution can increase saturation levels.

  • Slowly cooling a heated solution can maintain a supersaturated state.

• Metastable state:

  • Ready to crystallize upon disturbance or addition of a seeding agent.

Key Conditions for Crystallization

• Factors affecting supersaturation:

  • pH, temperature, salts, and seeding crystals.

• Balancing between:

  • Being not too soluble (left of supersaturation) and not reaching precipitation (right).

• Understanding nucleation states:

  • Importance of primary and secondary nucleation in achieving desired crystallization.

Types of Continuous Reactors for Crystallization

Tubular Crystallizer

• Features:

  • Preferred for high yield and short residence times.

  • Operates under mild conditions and laminar flow to prevent disruption of crystallization.

• Flow systems:

  • Types include normal plug flow, slug flow, and oscillatory baffled systems.

• Scalability:

  • Scaling achieved with parallel reactors for higher yield.

Mixed Suspension Mixed Product Removal (MSMPR) Crystallizer

• Characteristics:

  • Functions similarly to a Continuous Stirred Tank Reactor (CSTR) with vigorous mixing.

  • Better suited for longer residence times.

• Disadvantages:

  • Lower surface-to-volume ratio leads to typically lower yields compared to tubular crystallizers.

Challenges in Crystallization of Proteins

• Less common than chromatography for downstream processing of proteins.

• Issues encountered:

  • Slower crystallization rates for proteins compared to small molecules (clogging, longer residence times).

  • Lack of understanding of crystallization for many proteins.

  • Need for phase diagrams to elucidate specific conditions.

• Model systems:

  • Often overlook contaminants, necessitating better models for protein testing.

• Yield concerns:

  • Tubular crystallizers often yield lower amounts than batch processes.

  • Possible enhancements through recycling streams and integrating filtration methods.

Conclusion

• Crystallization is pivotal for determining crystal structures and achieving high yields in pharmaceuticals.

• Improved understanding and processes for protein crystallization could facilitate greater use and efficacy in downstream processing.

• Potential future advancements depend on addressing contamination and yield challenges, particularly compared to established chromatography methods.