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Nonclassical nucleation via mesoscale clusters in crystallisation of polymorphic active pharmaceutical ingredients
Date
2025
Abstract
The nucleation and prenucleation stages of crystallisation play a crucial role in determining the characteristics of the resulting crystal. However, the molecular mechanisms underlying these early stages of crystallisation remain unknown. Even though nonclassical nucleation via mesoscale clusters elegantly explains the nucleation mechanism, experimental evidence for its applicability to active pharmaceutical ingredients (APIs) is limited. Furthermore, from the standpoint of the pharmaceutical industry, a thorough mechanistic understanding would aid in the rational design of the crystallisation process, which would be a step forward from the current empirical design. Hence, this thesis investigates nonclassical nucleation via mesoscale clusters of two APIs, flufenamic acid (FFA) and griseofulvin (GSF), in different solvents. This work focuses on the critical properties of mesoscale clusters in these API solutions and their susceptibility to sample treatment parameters, namely filtration, agitation, and thermal pretreatment. This is accomplished through the use of nanoscale detection methods, including dynamic light scattering, nanoparticle tracking analysis, small-angle X-ray scattering, and transmission electron microscopy.
FFA solutions, ranging from undersaturated to supersaturated, were found to form mesoscale clusters in ethanol, where the cluster size and number concentration were independent of solute concentration. Under filtration stress, the mesoscale cluster peak disappeared from the size distribution regardless of the pore size of polytetrafluoroethylene (PTFE) or nylon membranes used, with no detectable change in concentration. In contrast, similarly sized FFA nanoparticles were removed by filtration, causing a significant change in solute concentration and size distribution. Hence, mesoscale clusters of FFA in ethanol constitute only a tiny fraction of the total solute concentration and possess poor light-scattering properties, lower mass density than solid particles of similar size, and have no clear phase boundary.
It is well established that process parameters such as filtration, agitation, and thermal pre-treatment impact the nucleation behaviour of API solutions. In this work, the impact of these process parameters on the mesoscale cluster properties was determined and thus a correlation between mesoscale cluster properties and nucleation behaviour was experimentally established. Depending on the solute concentration, filtration via a polyethersulfone (PES) membrane affected mesoscale cluster properties and altered their thermal stability.
Furthermore, higher agitation rates promoted the formation of a higher number of denser and smaller-sized mesoscale clusters. This behaviour correlated well with the observed faster induction times at higher agitation rates, providing experimental evidence for shear-promoted nucleation via mesoscale clusters for an organic compound.
Mesoscale clusters were detected in both undersaturated and supersaturated solutions of griseofulvin (GSF) in acetonitrile, n-butyl acetate, and methanol. These clusters exhibited varying responses to filtration, with PTFE filters allowing their passage depending on the solvent. Molecular dynamics simulations were then used to examine the clustering tendency of GSF in different solvents, revealing that acetonitrile had the highest propensity for cluster formation, followed by n-butyl acetate and methanol. Furthermore, the impact of agitation was investigated, showing that mesoscale clusters tended to reform in methanol. Overall, these findings correlated well with nucleation kinetics, where griseofulvin nucleated fastest in acetonitrile, followed by n-butyl acetate, and slowest in methanol.
Lastly, electron beam-induced nucleation in highly undersaturated solutions was used in LPTEM experiments to investigate the effect of solution history on the FFA nucleation pathway. High temporo-spatial imaging of samples incubated at various temperatures and times revealed that FFA molecules in solution took a very distinct route to nucleation. A freshly prepared FFA solution nucleated to crystals with uniform electron density, which likely corresponds to Form I, based on previous work. On the other hand, a thermally pre-treated sample nucleated to a new phase with an inhomogeneous electron density and no distinct habit. Hence, thermally pre-treated samples exhibited a distinct nucleation tendency compared to freshly prepared samples, highlighting the influence of sample treatment on electron-beam-assisted nucleation, similar to its role in conventional solution crystallisation.
Overall, this thesis demonstrates that sample treatment parameters, filtration, agitation, and thermal pre-treatment, significantly influence cluster population in API solutions and nucleation behaviour and confirms with experimental evidence the role of mesoscale clusters in the nucleation of APIs in organic solvents.
Supervisor
Hudson, Sarah
Rasmuson, Åke
Rasmuson, Åke
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Publisher
University of Limerick
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Sustainable Development Goals
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Attribution-NonCommercial-ShareAlike 4.0 International