The influence of material properties on manufacturability of direct compression tablets

Tablet manufacturing requires both high-quality equipment and powder blend with high flowability and compactability and low segregation tendency. The process is complex and tablet formation process still remains not fully understood. Adequate powder flow is a necessity for the pharmaceutical manufacturing process, i.e., powder flowability and flow properties play a great role when designing manufacturing processes for solid dosage forms. As such, the powder characteristics need to be investigated. However, one property is seldom enough to predict the flowability of a powder in specific processes and different test methods need to be used to fully understand the tableting performance of a particular powder. It is crucial to know how the assessed properties reflect the manufacturing conditions. The need for test batches and the use of empirical testing still exists despite the numerous powder characterization tests available. The main aim of the study was to understand the influence of material properties, flow properties and segregation tendencies on both the processability of a formulation during tablet compression and the critical quality attributes, such as mass, tensile strength and dose uniformity of the final drug product. Additionally, testing of an in-line NIR method to observe the homogeneity of the powder inside the force feeder right before the compression step and transmission Raman as an at-line method for tablet content were also evaluated. A number of powder characterization tests were employed in order to fully understand the impact of the formulation on the process performance. Three formulations with different particle size of the active substance and mannitol were used throughout the study. Both the sifting segregation and fluidization segregation tests’ results predicted the formulations’ tabletability particularly well. Fluidization segregation test predicted the changing composition of the formulation throughout tableting whereas sifting segregation results showed the constantly fluctuating API concentration in the manufactured tablets. Moreover, the Raman results confirmed the tablets of variable content despite the offset caused by the different particle size of the raw materials used. The functionality of the NIR in the force feeder was tested successfully. The residence time distribution could be determined at a sufficient level to point out tablets of a bad quality from the batch on grounds of the NIR data. Results from the powder flow property tests were rather conflicting. Angle of repose, Carr’s index and volume flow rate gave the best characterizing results, whereas the mass flow rate, shear test with higher normal stress in pre-shear gave the worst results, considering the experienced flow character of the formulations. As stated above, different flow property tests may give conflicting result, and hence, it is crucial to know which results are the most relevant ones. Furthermore, the right settings for the test should be known to gain applicable results, best exemplified by the shear cell test.