Process-analytical technology-based approaches for the monitoring and control of size and polymorphic form in pharmaceutical crystallisation processes

Pharmaceutical crystallisation operation is often critical because it determines product properties, such as the crystal size distribution (CSD) and polymorphic form, that can influence the subsequent downstream operations and the product therapeutic performance. Driven by the United States Food and Drug Administration s (FDA) Process Analytical Technology (PAT) initiative and the Quality-by-Design (QbD) concept, the development of control approaches, which can improve the manufacturing of products with desired properties, has become of significant interest. This thesis presents the development and application of PAT-based approaches for the monitoring and control of pharmaceutical crystallisation operations that will ensure consistent production of active pharmaceutical ingredients (APIs) with the desired size and polymorphic form. The approaches utilised Lasentec focused beam reflectance measurement (FBRM) and attenuated total reflectance ultraviolet (ATR-UV) spectroscopy as the in situ monitoring and control tools. Crystallisations of the APIs that posses multiple polymorphs are both critical and challenging. This was illustrated in this work by the crystallisations of sulfathiazole polymorphs using literature methods. The processes were monitored using FBRM and ATR-UV spectroscopy to define the design range of the process parameters. The defined range could be used as a recipe to reproduce the same quality of crystals. The obtained crystals were characterised using various techniques (optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetry, hot-stage microscopy (HSM), Fourier Transform infrared spectroscopy and powder X-ray diffractometry) to assess the success of the crystallisation processes. The combined results of the techniques showed that all methods were able to produce the desired pure polymorphs. As a contribution to the technique of investigating polymorphism, a combined approach of DSC-HSM with image analysis, was introduced. Results show the capability of the approach to provide a unique insight into the polymorphic transformations and thermal behaviour exhibited by the model compound. The novel direct nucleation control (DNC) approach was introduced to control the CSD. The approach utilises information on nucleation, provided by FBRM, in a feedback control strategy that adapts the process variables, so that the desired CSD of product is achieved. It also provides in situ fines removal through the operating policy, rather than having additional equipment and external recycle loops. The approach does not require concentration measurement and has the advantage of being a model-free approach, requiring no information on nucleation or growth kinetics in order to design an operating curve; the system automatically and adaptively detects the boundary of the operating curve. Experimental results, using glycine in water-ethanol mixture as a model system, show the benefits of DNC to produce larger crystals with narrower CSD compared to uncontrolled operations. The capability of seeded cooling crystallization with temperature cycling approach to control crystal size uniformity and polymorphic purity was evaluated. Using sulfathiazole in n-propanol and in water as model systems, the method was found to accelerate the growth and enhance the size uniformity of the crystals, in comparison with runs using a linear temperature profile, by promoting Ostwald ripening. Although the approach is conceptually capable of controlling polymorphic purity of a system, the effect of solvent-mediated nucleation/growth can be more dominant, as shown by the results of the experiments. The insights into this behaviour of sulfathiazole crystals were captured very well by the FBRM. The study also demonstrated the successful use of a simple non-linear function as a calibration model to relate temperature and absorbance data, obtained using the ATR-UV spectroscopy, to solute concentration during the crystallisation process. The effect of temperature cycling, performed during seeded cooling crystallisation, on the surface features of sulfathiazole crystals was investigated using FBRM and ex situ optical microscopy, SEM and atomic force microscopy. It was observed during the initial stage of the process, the heating phases produced crystals with smooth surfaces, whilst the cooling phases promoted growth of features on the surfaces. These changes detected by the FBRM as an increase in the number of coarse counts during heating and a drop during cooling. Laser beam spreading caused by the surface roughness, and signal/chord splitting due to sharp edges are offered as an explanation for the FBRM results. This shows the capability of the FBRM to provide useful information about the changes on the surface of the crystalline products. The information can be used to avoid problems in the downstream operations, or in the final product property due to variations in flowability and friability, which are influenced by the surface property.