Robustness of a continuous direct compression line against disturbances in feeding

The aim of the current study was to characterize the robustness of an integrated continuous direct compression (CDC) line against disturbances from feeding, i.e. impulses of API and short step disturbances. These disturbances mimicked typical variations that can be encountered during long-term manufacture. The study included a primary formulation, with API of standard particle size, which was manufactured at 5 and 10 kg/h production rates, and a modified formulation, with API of large particle size, which was manufactured at 5 kg/h production rate. Overall, the CDC line smoothened all the disturbances, fulfilling the USP uniformity of dosage units (UDU) limit for single tablets. However, runs with the modified formulation failed the pharmacopoeia UDU requirements for the entire run due to high variation between tablets. The primary formulation passed the requirements in all cases. The residence time distribution (RTD) results indicated that the primary formulation allowed better smoothening ability, and an increase in production rate led to poorer smoothening due to shorter RTD. The RTDs revealed that a substantial part of back-mixing took place after the blender. Thus, the tablet press has an important role in smoothening disturbances longer than the mean residence time of the blender, which was very short.