The development of drug substances heavily depends on a thorough and precise understanding of its different processes. One of these critical steps is crystallization, which has a significant impact on the properties of active pharmaceutical ingredients (APIs). By optimizing the approaches to crystallization, we can improve the stability, solubility, and bioavailability of APIs, which are essential factors in the successful development of effective drugs.
What is Crystallization?
Crystallization refers to the transformation of molecules or atoms from a liquid or gaseous state into a solid state. This process results in the formation of crystals, which are pure substances with atoms arranged in a well-defined and rigid crystal lattice to minimize their energy state.
The properties of the solid state, including the size, shape, and internal structure (morphology) of the crystals, can have a significant impact on the performance of a drug.
In drug development, various types of crystallization processes are utilized, such as cooling, evaporation, and reactive crystallization. Each of these methods offers distinct advantages. The selection of the appropriate process depends on factors such as the nature of the drug, required purity levels, and specific physical and chemical properties of the active pharmaceutical ingredient (API).
Challenges in Crystallization
Crystallization, an essential aspect of pharmaceutical manufacturing, presents a range of obstacles. Its complex nature, combined with the need for precise outcomes, demands specialized expertise and resources.
- Ensuring reproducibility in the crystallization process
Consistency is vital for the development of active pharmaceutical ingredients (APIs). It is necessary to achieve consistent results in successive crystallization runs conducted under identical conditions. However, due to the process’s sensitivity to even slight variations in factors like temperature, supersaturation levels, and stirring rates, maintaining reproducibility is often difficult.
To achieve reliable results, your contract manufacturing partner must possess expertise in the crystallization process. This expertise enables them to select the appropriate process parameters, facilitating consistent production of crystals with the desired size, shape, and purity while minimizing downstream processing issues.
- Anticipating and controlling crystal morphology
The shape and size of crystals, known as crystal morphology, can significantly impact a drug’s physical and chemical properties, including solubility, stability, and bioavailability. However, predicting and controlling crystal morphology is a complex task that necessitates a profound understanding of the solute-solvent interaction, nucleation and growth rates, and the conditions of the crystallization process.
- Managing impurities and their impact on the crystallization process
Even trace amounts of impurities can disrupt the crystallization process, leading to issues such as alterations in crystal structure, hindered crystal growth, and even failed crystallization. Controlling impurities originating from both the solute and the solvent is a crucial challenge in pharmaceutical crystallization.
- Scaling to commercial manufacturing without compromising quality
Laboratory-scale crystallization often does not seamlessly translate to larger-scale production due to variations in parameters like mixing and heat transfer. The scale-up process typically requires iterative adjustments and is time-consuming, as careful optimization is necessary to maintain product quality and process efficiency.
- Control over polymorphisms
Polymorphism refers to a compound’s ability to exist in multiple crystal structures, each with distinct physical properties. While polymorphism offers opportunities for optimizing drug properties, it also presents challenges in controlling which polymorphic form is obtained during crystallization. Undesired polymorphic transformations can occur during storage or processing, negatively impacting the drug’s properties.
Crystallization Process Steps
Each step in the complex crystallization process plays an essential role in shaping the final product.
- Solvent selection
Selection of the appropriate solvent is the key first step in the process. You must consider solubility and safety when choosing the proper solvent.
- Supersaturation
Crystallization begins once supersaturation, a state where the concentration of the solute surpasses the solvent’s ability to dissolve it under specific conditions, is achieved. This imbalance is typically attained by manipulating temperature, reaction, evaporation, or pressure to exceed the solute’s natural solubility in the solvent. The delicate dance of this manipulation is integral, as the degree of supersaturation influences both the rate of nucleation and crystal growth that follows.
- Nucleation
Following supersaturation, the solution undergoes nucleation. Here, solute molecules or atoms dispersed in the solvent start to congregate into stable clusters, forming a nucleus. This aggregation is the seed from which a crystal will grow. Precise control over nucleation is crucial, as the number of nuclei formed will dictate the number of crystals and inversely affect their size.
- Crystal growth
Once the nucleus forms, the stage of crystal growth commences. Additional solute molecules continue to deposit onto the existing nuclei. As they arrange in a defined, repeating pattern, the crystal begins to grow.
This stage is key to determining the crystal’s size, shape, and quality. Several factors can influence this growth, such as temperature, agitation, and the rate of supersaturation reduction. Striking a balance between development and nucleation is often the key to achieving the desired crystal size distribution.
- Product removal
The final step in the crystallization process is removing and isolating the crystals from the remaining solution. This isolation is typically accomplished using filtration or centrifugation, followed by a drying step to remove residual solvent. It’s important to handle the crystals gently during these steps to prevent damage or alteration to their carefully grown structures.
Common Crystallization Parameters and Transformations
Understanding the parameters involved in crystallization is vital for process control and optimization. Some of the common parameters include supersaturation levels, temperature, and pH. Monitoring and managing these parameters helps ensure optimal conditions for crystal formation.
Transformations are changes in solid forms during or after crystallization. They could involve polymorphic transformations (changes in crystal structure), amorphous to crystalline transformations, and desolvation (loss of solvent from solvate crystals).
Neuland’s Expertise in Crystallization
Our research team at Neuland focused on enhancing the physicochemical properties of Voxelotor, an API utilized for treating sickle cell disease. Voxelotor is a Class II drug with poor solubility. To address this issue, we employed crystallization techniques, resulting in significant improvements.
The outcomes of our study, titled “Physicochemical aspects and comparative analysis of Voxelotor and its salt and cocrystal,” were recently published in the Journal of Molecular Structure. The cocrystal of Voxelotor with Succinic acid exhibited notable enhancements in solubility and stability compared to the free base form. This work highlights our unwavering commitment to advancing drug development by employing innovative crystallization approaches.
Crystallization impacts multiple aspects of a drug’s quality and performance. Navigating the complexities of this process requires a deep understanding of crystallization chemistry, expertise in process control, and the ability to solve inherent challenges.
Contact us today if you want to improve stability, solubility, and bioavailability.
