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Understanding the Importance of Crystallization Processes Craig Callahan Project Scientist Understanding the importance of crystallization processes A Cambrex webinar overview Dr. Craig Callahan, Project Scientist at Cambrex Edinburgh, discusses why crystallization is such a crucial aspect of a molecule’s development and how this technique can be used to avoid unnecessary cost, risk and development delays. Crystallization is the formation of an ordered solid phase from a liquid phase. It is a process common across numerous industries, including cosmetics, foods, oil and gas, and metals, not just pharmaceuticals. The key word here is ‘ordered’ – a molecule needs to adopt a structured repeating unit to form a stable solid phase that can be considered crystalline. There are 2 key steps to forming crystals: nucleation and growth. Nucleation is where the solute molecules cluster together to form a larger structure of a certain critical size and marks the point where the first crystals have formed. This act of nucleation should occur through the bulk of the solution. It provides the sites for further solute molecules to adhere to and grow into large, discrete particles – which are the products that need to be isolated from the crystallization processes. The molecules of the active pharmaceutical ingredient (API) become particles of API, which can then be isolated and taken forward into the formulation. Nucleation and growth are both driven by supersaturation, which is the difference between the solubility and the concentration of the solution. This needs to be directed in such a way as to enable control over the outcome of the crystallization experiment. Supersaturation relates to 2 of the main transfer processes that go on in the crystallization vessel: the mass transfer and the heat transfer. The mass transfer dictates how the solute molecules are transferred from the liquid phase into the solid phase. This is critical for several reasons, not least the need to obtain as high a yield as possible from the liquors. If the solute molecules stay in solution, they will be lost to the filtration. However, it is often the case with crystallization that the real skill comes in balancing the rate of mass transfer and the quality of the crystals obtained against the overall process time. Crystals may have different faces that can grow at different rates and, if this growth can be controlled, Cambrex webinar: Understanding the importance of crystallization processes. June 2020. 1 Contributor Dr. Craig Callahan, Project Scientist at Cambrex Edinburgh Dr. Callahan joined Cambrex following the acquisition of Solid Form Solutions, part of Avista Pharma Solutions, in January 2019. Dr Callahan’s role at Cambrex’s Edinburgh facility is to develop crystallization processes for client APIs with a view to obtaining robust and reliable isolation of particles with consistent physical properties. Prior to joining Cambrex, Dr Callahan achieved a PhD from the Chemical Engineering department of Heriot-Watt University, Edinburgh, where his research focused on understanding the nucleation mechanisms involved in various types of crystallization vessels. Dr Callahan’s previous work experience lies in continuous crystallization process development of APIs. so can the final particle morphology. The growth rate can, therefore, be used to not only control the size of the particles but also limit any impurities from the crystals that we are trying to grow. The heat transfer is also significant because the temperature of the solution is key to driving the supersaturation. Ideally, the molecules will have a high solubility at an elevated temperature and a low solubility at a low temperature. This solubility gradient can then be used to generate and control the supersaturation in the solution and, as the crystals grow, to guide the nucleation and growth of these particles. Crystallization in pharmaceutical manufacturing In common with many other types of synthesis, an API will be derived from a reaction, typically involving two or more components. It is usually performed in the presence of a solvent, and there may be a catalyst – either metallic or organic – involved too. However, the reaction scheme is also likely to create many impurities that can contaminate the target product to varying degrees. Typically, the API is a very high-value commodity; the aim is to have that in the solid phase, with the impurities remaining in the liquid phase so that they can be filtered off. Effectively, crystallization is being used as a purification technique that allows the product to be separated from the impurities and process solvent. API crystallization becomes one of the most critical steps of the drug substance manufacturing process because it offers a chance to purge all of the other components from the preceding reaction. It is the crystallization scientist’s responsibility to ensure that the crystallized API meets the required specifications before it goes into the formulation and, ultimately, on into the patient. Mixing plays a vital role in facilitating good heat and mass transfer, and the other physical processes that compete with the nucleation and growth during crystallization. If the crystallization is designed appropriately, the growth and dissolution can be used advantageously to control the particle size distribution of the material, while being mindful of the detrimental risks of agglomeration and attrition. Agglomeration is often caused by a crystallization that is too rapid and can lead to very high Cambrex webinar: Understanding the importance of crystallization processes. June 2020. 2 Understanding the importance of crystallization processes residual solvent contents when solvent molecules are mechanically trapped in the voids between the particles. It can also result in an unsatisfactory particle size distribution – something that can cause a batch to fail a quality specification. Attrition results from the breakage of particles during the crystallization. It can cause the opposite effect of agglomeration: a mix of different particle sizes where, as well as the target particle size from the crystallization, other smaller particles have broken away from the parent crystals. During mixing, particles experience shear forces. Higher shear rates in the mixing vessel will result in too high an attrition rate, while a shear rate that is too low could cause agglomeration. So these need to be balanced carefully against the mass transfer. Generation of supersaturation One of the most significant considerations when devising a crystallization process is generating the supersaturation. There are several methods for this. One approach, the one favored by Cambrex, is a cooling crystallization where the starting solution is simply cooled towards the metastable limit. This approach is undoubtedly the easiest to control; simply applying a temperature gradient to the system avoids the need to modify any composition of the reaction matrix. The second option is the use of an anti-solvent addition. This involves changing the solvent composition of the system so that the solubility for a known solution concentration is driven down. This can be exploited similarly to cooling, but scaling it up presents more challenges. A third option is an evaporative crystallization. Evaporating or distilling out some of the solvents increases the concentration of the solution, thereby driving the supersaturation. Although this is one of the more laborious processes to control at scale, it is by no means impossible. With careful control over the rate of solvent removal some high-performance crystallization processes can be achieved this way. The final method is reactive crystallization. This is typically required in situations where the free form of the API is highly soluble, but the desired salt to be crystallized is less soluble – often significantly less soluble than the free form of the API. In these cases, a counter ion is added in much the same way as an anti-solvent. As the counter ion is charged to perform the salt formation, supersaturation is increased and particle growth can be driven on an established seedbed within the crystallization vessel. With careful consideration of seed loads, addition rates, and solution concentrations, high-performing crystallizations can be developed. Crystallization process development Apart from allowing control over the impurity profile and process yield, a crystallization process can offer physical benefits to API manufacturing. The ability to control supersaturation and the rate at which it is generated in the crystallization vessel will have a direct impact on the nucleation and growth of the particles. Exploiting this allows the targeting of specific particle sizes that facilitate good filtration. Filtration and drying are often overlooked as critical steps in the drug substance isolation, but two crops of the same API that have undergone two different Cambrex webinar: Understanding the importance of crystallization processes. June 2020. 3 Understanding the importance of crystallization processes crystallization processes can have two completely different outcomes with regards to filtration rate and drying time. An imperfect crystallization process means the formulation does not perform as anticipated. With a well-developed crystallization process, filtration and drying work very well, resulting in a uniform particle that formulates exactly as intended. Cambrex at a glance So how does Cambrex fit into this picture? As the small molecule company for drug substance, drug product, and analytic services,
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