Utilising terahertz pulsed imaging to analyse the anhydrous-to-hydrate transformation of excipients during immediate release film coating hydration

Pharmaceutical tablets are routinely film-coated to improve appearance, reduce medication errors and enhance storage stability. Terahertz pulsed imaging (TPI) can be utilised to study the liquid penetration into the porous tablet matrix in real time. Using polymer-coated flat-faced tablets with anhydrous lactose or mannitol, we show that when the tablet matrix contains anhydrous material, the anhydrous form transforms to the solid-state hydrate form in the tablet core while the immediate release coating dissolves. The process starts at the interface between the coating and the core. It commences inward for more than 7min, resulting in a shell of hydrate form on the order of a few hundred microns thicknesses immediately underneath the dissolving coating. TPI is sensitive enough to detect and spatially resolve this process based on the subtle change in the refractive index of the two materials. TPI can further resolve tablets with different components using the time-domain waveforms in reflection and provide insights into the presence of hydrate material (lactose monohydrate) in altering the water mass transport mechanism. The kinetics data obtained from TPI fit the power law model (y=kt^m), whose constants enable us to infer the different mass transport mechanisms.