Investigation into powder tribo-charging of pharmaceuticals. Part II: Sensitivity to relative humidity.

Environmental conditions can have a profound impact on the bulk behaviour of pharmaceutical powders, including their tribo-charging tendency. Typically, high relative humidity (RH) has been associated to a reduction in the electrostatic charge of the material. However, the occurrence of charge mitigation seems to be related to the quantity of water molecules at the powder surface, which depends on intrinsic material attributes (i.e., water sorption propensity), and external factors (i.e., RH level). In the present study, pharmaceutical powders (i.e., microcrystalline cellulose, D-mannitol, paracetamol and magnesium stearate) were conditioned at three levels of RH, relevant for pharmaceutical operations, and their bulk behaviour, including charging propensity, was analyzed. Depending on the material type, powders sorbed water from the humid atmosphere to different extents, resulting in different charging behaviours. Overall, the charge density of the materials was found to decrease after a certain RH or monotonically decrease with an increase of RH, except for D-mannitol. For this material, a contrasting trend of increase in charging was observed with an increase in RH. Moreover, the powders showed a distinct tribo-charging sensitivity to RH, with paracetamol being the most affected. These findings suggest that a careful consideration on solid material-moisture interactions is needed when using RH as strategy to minimize electrostatic effects in powder processing.

Investigation into powder tribo-charging of pharmaceuticals. Part I: Process-induced charge via twin-screw feeding.

Powder feeding is a crucial unit operation in continuous manufacturing (CM) of pharmaceutical products. Twin-screw feeders are typically employed to ensure the accurate mass flow of pharmaceutical materials throughout the production process. Here, contact and separation of particles can give rise to electrostatic charges, affecting feeder performance and final product quality. The knowledge of the material charging tendency would therefore be beneficial for both formulation and process design. At the early stage of product development, only a limited amount of material is available and the propensity of the powders to charge needs to be assessed on lab test equipment, which not necessarily represent the material state during processing. In this study, the tribo-charging behaviour of a set of common pharmaceutical materials (i.e., microcrystalline cellulose, D-mannitol, paracetamol and magnesium stearate) was experimentally evaluated. To this end, powder materials were let to flow over the stainless-steel pipes of the GranuCharge™ instrument. The resulting charge was compared to the one acquired during twin-screw feeding. In both cases, paracetamol exhibited the highest charging tendency followed by D-mannitol and microcrystalline cellulose and last by magnesium stearate. A good correlation was found for charge values obtained for both methods, despite the different tribo-charging mechanisms involved in the two set-ups. However, these differences in experimental set-ups led to diverse magnitudes and, in one case, polarity of charge. Additionally, an extensive material characterization was performed on the selected powders and results were statistically analyzed to identify critical material attributes (CMAs) affecting powder tribo-charging. A strong correlation was obtained between the measured charge and inter-particle friction. This indicated the latter as one of the most influencing material characteristic impacting the powder tribo-charging phenomenon of the selected materials.

Sensitivity of a continuous hot-melt extrusion and strand pelletization line to control actions and composition variation.

The purpose of this work was to develop a robust hot-melt extrusion and strand pelletization process for manufacturing pellets with an immediate release (IR) of a poorly water-soluble active pharmaceutical ingredient (API), nimodipine. The robustness of pharmaceutical continuous manufacturing processes and of its control strategy is vital for competitiveness to traditional batch-manufacturing. Therefore, first the sensitivity of product quality, process stability, and process monitoring tools to i) parameter changes due to control actions and ii) typical process deviations, i.e., feeding errors, was investigated in a design of experiments (DoE). Thereby, die melt pressure was found to be highly sensitive to composition deviations, i.e. a limiting factor for process stability. Especially critical were deviations to increased HPMC content, since it behaved as a filler in the melt. Pelletization, or pellet size and size distribution respectively, were found to be sensitive to an increased throughput, due to the resulting insufficient strand cooling before the pelletizer. API dissolution in contrast, was found to be robust across the entire investigated range of formulation and process settings. Second, a design space for the production of IR pellets for subsequent tableting was established, and finally, a technical control strategy was developed to ensure a robust process. Near-infrared (NIR) spectroscopy was applied to monitor API content and the sensitivity of the residence time distribution (RTD) was investigated by means of tracer measurements. NIR-based API content monitoring and RTD models for material tracking were found to be at risk after processing melt with high HPMC content, due to a lack of purging by less viscous formulation compositions.

Formulation performance and processability window for manufacturing a dual-polymer amorphous solid dispersion via hot-melt extrusion and strand pelletization.

This work evaluates several compositions of an amorphous solid dispersion (ASD) comprising nimodipine (NMD) as poorly soluble model API in a dual-polymer carrier system. HPMC E5 and Eudragit E were used for the two polymeric carriers. The formulation was designed for hot-melt extrusion (HME) and subsequent strand pelletization. The aim was to identify a formulation window with desired functional ASD performance, i.e. physical stability and immediate API release, as well as processability in strand pelletization. Samples were prepared using small-scale methods, such as vacuum compression molding (VCM) and benchtop extrusion. Miscibility and phase studies were performed for a wide range of polymer ratios and three levels of API content (10-30% w/w). Ternary ASD formulations were phase-separated, yet physically stable upon exposure to elevated temperature/humidity. A study of phase composition showed that the drug molecules were predominantly solubilized in the Eudragit E fraction of the formulation. The miscibility study and Fourier-transform infrared spectroscopy indicated hydrogen (H)bond interactions between NMD and Eudragit E. In HPMC, the amorphous API was dispersed in polymeric matrix and stabilized due to anti-plasticization and the disruption of intermolecular Hbonding between API molecules. Concerning processability in strand pelletization the formulation is limited at high Eudragit E content. NMD and EE-rich phases exhibit low mixture glass transition, low melt stability and brittle breaking behavior upon strand cutting. The high viscosity and yield point of HPMC contributes to the mechanical robustness of the strand at temperatures relevant for processing. Formulation-intrinsic dissolution rates in VCM ASDs developed as an irregular function of polymer ratio, associated with diverse and competitive dissolution mechanisms in the polymers. With regard to the binary system of NMD with HPMC E5, surface crystallization was observed in VCM ASDs. For extruded pellets this was not the case, and a steady trend of formulation-intrinsic dissolution rate across different polymer ratios was observed. These discrepancies indicated a major influence of shear stress during sample preparation on HPMC-based ASD performance. Finally, a feasible formulation window within a polymer ratio of 1:2-2:3 Eudragit E:HPMC was identified in which Eudragit E acts as a dissolution rate enhancer and ASD stabilizer during dissolution.