The ubiquity of the tabletability flip phenomenon

Elucidating critical factors driving the tabletability flip phenomenon

Tabletability is a key property that determines a powder's ability to form tablets under applied stresses, typically represented by a plot of tablet tensile strength versus pressure. Tablet tensile strength reflects the contributions of interparticulate bonding area (BA) and bonding strength (BS) between adjacent particles in a tablet. BA is influenced by mechanical properties, particle characteristics, and tableting conditions, while BS is governed by molecular packing and intermolecular interactions. The "tabletability flip" phenomenon (TFP) occurs when, for a pair of solid forms of an active pharmaceutical ingredient (API), the form with a higher tabletability as a pure powder exhibits a lower tabletability when formulated with an excipient. Factors affecting either BA or BS can also impact the occurrence and extent of TFP, but their impact has not been systematically evaluated. In this work, we evaluated the impact of API loading, excipient type, particle size, and tableting speed on TFP. Our results indicate that TFP is likely to occur when the plasticity of an excipient is comparable to the softer API, particularly at intermediate drug loadings and under high compaction pressures. Additionally, the particle size of the excipient significantly influences both the occurrence and extent of TFP, while API particle size and tableting speed have only a marginal impact.

Simultaneously improving tabletability and solubility of diclofenac by cocrystallization with picolinamide

Diclofenac (DIC) is a nonsteroidal anti-inflammatory drug with poor tabletability and water solubility. In the present study, a new diclofenac-picolinamide cocrystal (DIC-PIC) was prepared to simultaneously improve its tabletability and solubility. The cocrystal was characterized using multiple techniques, such as X-ray diffraction, thermal methods and spectral analyses. The tabletability of DIC-PIC was significantly improved over DIC, which is attributed to the larger bonding area between crystals due to the higher plasticity of DIC-PIC, demonstrated by the lower in-die mean yield pressure, Py,i, of DIC-PIC (59.5 ± 0.6 MPa) than DIC (86.6 ± 1.4 MPa). The higher plasticity of DIC-PIC is consistent with the existence of a slip plane (001) in its crystal structure. The solubility of DIC-PIC is significantly higher than that of DIC (112 times higher in water and 22 times higher in pH = 6.8 buffer solution). Hence, the simultaneous improvement in tabletability and solubility of DIC-PIC overcomes two main barriers in developing DIC tablets, which makes it a promising candidate for developing a DIC tablet with improved performance.

Understanding the roles of compaction pressure and crystal hardness on powder tabletability through bonding area - Bonding strength interplay

Bonding area (BA) and bonding strength (BS) interplay dictates tensile strength of a tablet and, hence, tabletability. Using a series of alkali halides with mechanical properties spanning more than one order of magnitude, the role of compaction pressure and mechanical properties on tabletability is systematically investigated and explained using the BA-BS interplay. Results reveal that BA dominates the BA-BS interplay at low pressures, where more plastic powders attain higher tensile strength due to larger BA. In contrast, BS dominates the interplay at high pressures, when difference in BA between powders is minimized. Under the typical compaction pressures of 100-300 MPa, tablet tensile strength is the highest for materials with intermediate hardness, or plasticity, due to an optimal BA-BS interplay.

A new insight into the mechanism of the tabletability flip phenomenon

Tabletability is an outcome of interparticulate bonding area (BA) - bonding strength (BS) interplay, influenced by the mechanical properties, size and shape, surface energetics of the constituent particles, and compaction parameters. Typically, a more plastic active pharmaceutical ingredient (API) exhibits a better tabletability than less plastic APIs due to the formation of a larger BA during tablet compression. Thus, solid forms of an API with greater plasticity are traditionally preferred if other critical pharmaceutical properties are comparable. However, the tabletability flip phenomenon (TFP) suggests that a solid form of an API with poorer tabletability may exhibit better tabletability when formulated with plastic excipients. In this study, we propose another possible mechanism of TFP, wherein softer excipient particles conform to the shape of harder API particles during compaction, leading to a larger BA under certain pressures and, hence, better tabletability. In this scenario, the BA-BS interplay is dominated by BA. Accordingly, TFP should tend to occur when API solid forms are formulated with a soft excipient. We tested this hypothesis by visualizing the deformation of particles in a model compressed tablet by nondestructive micro-computed tomography and by optical microscopy when the particles were separated from the tablet. The results confirmed that soft particles wrapped around hard particles at their interfaces, while an approximately flat contact was formed between two adjacent soft particles. In addition to the direct visual evidence, the BA-dominating mechanism was also supported by the observation that TFP occurred in the p-aminobenzoic acid polymorph system only when mixed with a soft excipient.