Mechanistic investigation on pressure dependency of Heckel parameter

Successful Formulation Window for the design of pharmaceutical tablets with required mechanical properties

Pharmaceutical tablet formulations combine the active ingredient with processing aids and functional components. This paper evaluates compressibility based predictive models for binary and ternary formulations to establish an acceptable range of tablet compression parameters that satisfy prescribed quality target criteria for tablets including minimum tablet strength and processing constraints such as maximum ejection stress and maximum compaction pressure. The concept of Successful Formulation Window (SFW) is introduced. A methodology is proposed to determine the SFW for a given formulation based on compaction simulator data collected for individual formulation components. The methodology is validated for binary and ternary mixtures and lubricated formulations. The SFW analysis was developed to support tablet formulation design to meet mechanical requirements.

Insights into the role of tooling characteristics on compressibility evolution in non-flat faced tablets

The robustness of tablet manufacturability largely depends on compressibility behavior of a powder. The compressibility assessment is traditionally conducted on cylindrical flat-faced compacts in contrast to the fact that marketed tablets are majorly produced using non-flat faced or shaped toolings. The present work demonstrates the feasibility of quantifying average compressibility on shaped toolings through a proof-of-concept study by investigating the central band portion and the entire volume of the tablet, which led to several notable findings. Firstly, the yield stress (deformability) was found independent of type of tooling for a given powder in the in-die condition, but for the same tooling it conversely spanned over a wide range in the out-die condition due to characteristic elastic recovery. Secondly, the yield stress parameter correlated with the change in band volume of the shaped tablet with applied compaction pressure, thereby establishing an orthogonal approach to assess compressibility on non-flat faced toolings. The study emphasizes that tooling characteristics may affect compressibility and tablet robustness of a same powder, which should be practiced cautiously in drug product manufacturing.

Multivariate data analysis to evaluate commonly used compression descriptors

Numerous studies elucidated material behavior based on compression analyses. Especially compressibility, compactibility and tabletability were in the focus of these investigations. In the present study, a comprehensive multivariate data analysis was performed using principal component analysis method. Twelve pharmaceutically used excipients were selected for direct compression tableting and subsequent evaluation of several compression analyses. Material properties, tablet properties, tableting parameters and parameters from compression analyses were used as input variables. The materials could successfully be grouped using principal component analysis. Of the tableting parameters, the compression pressure showed the greatest influence on the results. The tabletability was found to be the most important compression analysis in the material characterization. Compressibility and compactibility only played a minor role in the evaluation. Some important insights have been gained for a deeper understanding of the tableting process using the multivariate approach to evaluate the variety of compression data.

Studies on the influence of high-shear granulation process on the compressibility of microcrystalline cellulose

Microcrystalline cellulose (MCC) is a diluent for oral solid dosage forms. The wet granulation process was selected to prepare losartan potassium tablets using MCC as a model for a predictive study. It was found that the hardness of the tablets could not satisfy the quality standards. In this study, the effect of the high-shear granulation process on the compressibility of MCC was characterized by plotting the compression characteristics curve, as well as the mechanism of the effect from the perspectives of mechanical properties, powder properties. The solid-state properties were also analyzed. Combined with the Heckel equation, the Ryshkewitch-Duckworth equation, the energy method, PXRD, SEM, and other evaluation methods, the results suggest that the high-shear granulation process reduced the compressibility of MCC, which may be caused by the reduced plastic deformation capacity of MCC and the change of the particle morphology structure. The method applied in this study can also be applied to other excipients, which is an important guideline for solving possible problems and process selections during the preparation stage of solid formulations.

Compression Modulus and Apparent Density of Polymeric Excipients during Compression-Impact on Tabletability

The present study focuses on the compaction behavior of polymeric excipients during compression in comparison to nonpolymeric excipients and its consequences on commonly used Heckel analysis. Compression analysis at compaction pressures (CPs) from 50 to 500 MPa was performed using a compaction simulator. This study demonstrates that the particle density, measured via helium pycnometer (ρpar), of polymeric excipients (Kollidon®VA64, Soluplus®, AQOAT®AS-MMP, Starch1500®, Avicel®PH101) was already exceeded at low CPs (<200 MPa), whereas the ρpar was either never reached for brittle fillers such as DI-CAFOS®A60 and tricalcium citrate or exceeded at CPs above 350 MPa (FlowLac®100, Pearlitol®100SD). We hypothesized that the threshold for exceeding ρpar is linked with predominantly elastic deformation. This was confirmed by the start of linear increase in elastic recovery in-die (ERin-die) with exceeding particle density, and in addition, by the applicability in calculating the elastic modulus via the equation of the linear increase in ERin-die. Last, the evaluation of “density under pressure” as an alternative to the ρpar for Heckel analysis showed comparable conclusions for compression behavior based on the calculated yield pressures. However, the applicability of Heckel analysis for polymeric excipients was questioned in principle. In conclusion, the knowledge of the threshold provides guidance for the selection of suitable excipients in the formulation development to mitigate the risk of tablet defects related to stored elastic energy, such as capping and lamination.

Mean yield pressure from the in-die Heckel analysis is a reliable plasticity parameter

Despite the ability to characterize the plasticity of powders in a material-sparing and expedited manner, the in-die Heckel analysis has been widely criticized for its sensitivity to several factors, such as particle elastic deformation, tooling size, lubrication, and speed. Using materials exhibiting a wide range of mechanical properties, we show that the in-die P_y correlates strongly with three established plasticity parameters obtained from the out-of-die Heckel analysis, Kuentz-Leuenberger analysis, and macroindentation. Thus, the in-die P_y is a reliable parameter for quantifying powder plasticity in a material-sparing and expedited manner.

Assessment of a Once-Daily Controlled-Release Ibuprofen Matrix Tablets Prepared Using Eudragit®E100/Carbopol®971P NF Polymers and Their Salts Combinations

INTRODUCTION

Hydrophilic polymers that swell or dissolve in aqueous media can have the potential to prepare controlled/sustained dosage forms for weakly acidic, poorly soluble drugs.

OBJECTIVE

The main objective of this study is to utilize Eudragit®E100 (EE) and Carbopol®971P NF (Cp) polymers and their salt forms in the preparation of a once-daily controlled-release matrix tablet for model drug, Ibuprofen (IB).

METHODS

Combinations of the polymers in their base forms (EE)/(Cp) or their salt forms (EEHCl/CpNa) were compressed with (IB) into single layer matrix tablets, or otherwise into bilayer tablets. Dissolution profiles were constructed using three different consecutive stages (pH 1.2, 4.8, and 6.8).

RESULT

It was found that the incorporation of (EEHCl) modified the release rates of (IB) from (Cp) based matrix tablets. However, a major enhancement of (IB) release rates occurred when the polymers were combined in their salt forms at a 1:1 ratio by weight. In addition, a bilayer tablet was prepared wherein a relatively rapidly disintegrating layer composed of polymers salts (EEHCl and CpNa), and a second layer containing only (Cp) polymer in its base form at a 1:2 weight ratio possessed excellent release properties, and mechanical strengths.

CONCLUSION

It was concluded that the prepared bilayer tablet could be of promise use in controlling the release rates of (IB) in an extended manner to allow once-daily administration with an improved pH-independent release behavior.

How can single particle compression and nanoindentation contribute to the understanding of pharmaceutical powder compression?

The deformation behaviour of a powder and, thus, of the individual particles is a crucial parameter in powder compaction and affects powder compressibility and compactibility. The classical approach for the characterization of the deformation behaviour is the performance of powder compression experiments combined with the application of mathematical models, such as the Heckel-Model, for the derivation of characteristic compression parameters. However, the correlation of these parameters with the deformation behaviour is physically often not well understood. Single particle compression and nanoindentation enables the in-depth investigation of the deformation behaviour of particulate materials. In this study, single particle compression experiments were performed for the characterization of the deformation behaviour of common pharmaceutical excipients and active pharmaceutical ingredients (APIs) with various, irregular particle morphologies of industrial relevance and the findings are compared with the results from powder compression. The technique was found useful for the characterization and clarification of the qualitative deformation behaviour. However, the derivation of a quantitative functional relationship between single particle deformation behavior and powder compression is limited. Nanoindentation was performed as complementary technique for the characterization of the micromechanical behavior of the APIs. A linear relationship between median indentation hardness and material densification strength as characteristic parameter derived by in-die powder compression analysis is found.

Investigation on the impact of different proportions of components in formulations on stability of a moisture sensitive drug

Physicochemical and mechanical properties of tablets are largely dictated by formulation compositions. Different excipients possess different tableting and moisture sorption behaviors. Therefore, this study was designed to elucidate the relative influence of the proportion of components in formulations on tablet properties. Acetylsalicylic acid (ASA) tablets containing different proportions of starch, microcrystalline cellulose (MCC) and calcium hydrogen phosphate dihydrate (DCP) were prepared. The excipients were evaluated for their moisture sorption properties. Mechanical strength of the tablets was determined alongside with ASA stability, by storing the tablets at 75% RH, 25 °C. The stability study showed the importance of drug loading level on its stability. For a fixed ASA proportion, formulations with more starch were able to absorb more moisture and possessed larger areas of hysteresis loop in their moisture sorption isotherms. The presence of starch contributed positively to ASA stability although increasing proportions of starch compromised the tablet mechanical properties. Contrastingly, MCC produced mechanically stronger tablets as its plastically deforming and fibrous properties contributed to a good structural network. The findings provide a deeper understanding of the dichotomous effect by the proportion of components in formulations containing a moisture sensitive drug on drug stability and mechanical strength of the resultant tablets.

Ockham's Razor Applied on Pharmaceutical Powder Compaction Models

This investigation contains a critical survey of a collection of five well-known and often used standard models in pharmaceutical technology. The basic idea is to use the recognised Ockham's razor as a tool in search for simpler methods or explanations for these models. The study includes the indirect tensile strength of tablets, Pitt's equation for tensile strength of biconvex tablets, Adams' model for strength of agglomerates, Weibull's distribution for variability of strength measurements and Heckel's equation for compressibility. In all these cases simpler and equally valid solutions and explanations are presented and subsequently preferred rather than the original.

Impact of Hot-Melt-Extrusion on Solid-State Properties of Pharmaceutical Polymers and Classification Using Hierarchical Cluster Analysis

The impact of hot-melt extrusion (HME) on the solid-state properties of four methacrylic (Eudragit® L100-55, Eudragit® EPO, Eudragit® RSPO, Eudragit® RLPO) and four polyvinyl (Kollidon® VA64, Kollicoat® IR, Kollidon® SR, and Soluplus®) polymers was studied. Overall, HME decreased Tg but increased electrostatic charge and surface free energy. Packing density decreased with electrostatic charge, whereas Carr’s and Hausner indices showed a peak curve dependency. Overall, HME reduced work of compaction (Wc), deformability (expressed as Heckel PY and Kawakita 1/b model parameters and as slope S′ of derivative force/displacement curve), and tablet strength (TS) but increased elastic recovery (ER). TS showed a better correlation with S′ than PY and 1/b. Principal component analysis (PCA) organized the data of neat and extruded polymers into three principal components explaining 72.45% of the variance. The first included Wc, S′ and TS with positive loadings expressing compaction, and ER with negative loading opposing compaction; the second included PY, 1/b, and surface free energy expressing interactivity with positive loadings opposing tap density or close packing. Hierarchical cluster analysis (HCA) assembled polymers of similar solid-state properties regardless of HME treatment into a major cluster with rescaled distance Cluster Combine Index (CCI) < 5 and several other weaker clusters. Polymers in the major cluster were: neat and extruded Eudragit® RSPO, Kollicoat® IR, Kollidon® SR, Soluplus®, and extruded Eudragit® L100-55. It is suggested that PCA may be used to distinguish variables having similar or dissimilar activity, whereas HCA can be used to cluster polymers based on solid-state properties and pick exchangeable ones (e.g., for sustain release or dissolution improvement) when the need arises.

Mechanical properties of starch esters at particle and compact level - Comparisons and exploration of the applicability of Hiestand's equation to predict tablet strength

Hydrophobic starch esters have potential as tablet matrix formers in controlled drug delivery. The mechanical properties of native starch (SN), starch acetate (SA) and starch propionate (SP) were studied at particle and compact level. Particle microhardness and modulus of elasticity were evaluated by nanoindentation. Force-displacement data of compressed powder were analyzed using Heckel in conjunction with piecewise regression, Kuentz-Leuenberger, Kawakita and Adams models, and yield pressure parameters were derived. Starches were characterized for chemical structure by Raman spectroscopy, crystallinity from powder x-ray diffraction (PXRD) patterns and surface energy from apparent contact angle measurements. A-type starch reflections were absent in the PXRDs of esters indicating greater amorphicity. Consequently, the particle microhardness of starch esters decreased leading to greater deformation during compaction and lower values of yield pressure parameters. These parameters increased with microhardness and ranked the starches in the order: SP < SA < SN. Fitting the experimental data into Hiestand's bonding index equation, a linear correlation (R² = 0.902) was established between experimental and calculated tablet strength describing results of all starches, when Adams (τ_ο') yield pressure was used as the 'effective compression pressure' in the above equation.

A Mathematical Approach to Consider Solid Compressibility in the Compression of Pharmaceutical Powders

In-die compression analysis is an effective method for the characterization of powder compressibility. However, physically unreasonable apparent solid fractions above one or apparent in-die porosities below zero are often calculated for higher compression stresses. One important reason for this is the neglect of solid compressibility and hence the assumption of a constant solid density. In this work, the solid compressibility of four pharmaceutical powders with different deformation behaviour is characterized using mercury porosimetry. The derived bulk moduli are applied for the calculation of in-die porosities. The change of in-die porosity due to the consideration of solid compressibility is for instance up to 4% for microcrystalline cellulose at a compression stress of 400 MPa and thus cannot be neglected for the calculation of in-die porosities. However, solid compressibility and further uncertainties from, for example the measured solid density and from the displacement sensors, are difficult or only partially accessible. Therefore, a mathematic term for the calculation of physically reasonable in-die porosities is introduced. This term can be used for the extension of common mathematical models, such as the models of Heckel and of Cooper & Eaton. Additionally, an extended in-die compression function is introduced to precisely describe the entire range of in-die porosity curves and to enable the successful differentiation and quantification of the compression behaviour of the investigated pharmaceutical powders.

Influence of spray nozzle aperture during high shear wet granulation on granule properties and its compression attributes

The distribution of granulating liquid is known to affect the high shear wet granulation process but the impact of the spray nozzle attributes is still unclear. While homogenous liquid distribution can be achieved by using a spray nozzle, the effect of different nozzle aperture sizes on granule properties is not well understood. In this study, nozzles of different aperture sizes were used to introduce the granulating liquid in high shear wet granulation using different process parameters. Design of experiment approach was utilised to assess effect of process parameters on granule properties. Granules produced with different spray nozzles were evaluated for binder distribution inhomogeneity, size, shape, flowability and compression attributes such as tabletability and yield pressure. Coarser granules with better flow properties were produced using the smaller aperture size nozzle. On the other hand, granules had better tabletability and lower yield pressure when larger aperture size nozzle was used. Furthermore, size of granules produced by using larger aperture size nozzle was more affected by changes in the process variables which could be influenced by the differences in granulating liquid feed rate and spray droplet size. Although the granules aspect ratios were comparable across the nozzle aperture sizes, granules produced from smaller aperture size nozzle appeared to be rounder. Regardless of the nozzle aperture sizes, homogenous binder distribution was achieved. The findings from this study could be a useful guide to the selection of the appropriate nozzle aperture size in wet granulation.

Role of Water Sorption in Tablet Crushing Strength, Disintegration, and Dissolution

Drugs formulated as tablets are subjected to accelerated stability conditions with the goal of identifying a stable formulation that will exhibit a sufficiently long shelf life. Water sorption at a condition such as 40°C/75% RH can result in significant changes in tablet properties such as a decrease in dissolution rate, the cause of which may be difficult to interpret, given the complex nature of ingredients and their interactions in a tablet. In this research, three drugs, displaying a wide range of physicochemical properties, were formulated with commonly used diluents, disintegrants, and binders, using a design of experiments approach. The tablets were stored at accelerated conditions and assessed for content, dissolution, disintegration, and crushing strength, as well as other properties. The research demonstrated many water-induced effects in tablet properties. Due to the experimental design approach that revealed many interactions, it was possible to interpret all of the changes observed in tablet crushing strength, disintegration, and dissolution for the drugs using a common set of physical principles. Specifically, the relevant factors considered were (1) mechanical properties of materials, (2) water sorption surface effects in surface diffusion and capillary condensation, (3) water sorption bulk effects for amorphous materials such as viscous flow/spreading, and (4) water-induced stress on interparticle bonding arising from volume expansion. These physical principles enable a comprehensive interpretation of the complex changes observed in tablet properties, which should be valuable in the design of tablet formulations that will be stable to accelerated storage conditions.

On Identification of Critical Material Attributes for Compression Behaviour of Pharmaceutical Diluent Powders

As one of the commonly-used solid dosage forms, pharmaceutical tablets have been widely used to deliver active drugs into the human body, satisfying patient's therapeutic requirements. To manufacture tablets of good quality, diluent powders are generally used in formulation development to increase the bulk of formulations and to bind other inactive ingredients with the active pharmaceutical ingredients (APIs). For formulations of a low API dose, the drug products generally consist of a large fraction of diluent powders. Hence, the attributes of diluents become extremely important and can significantly influence the final product property. Therefore, it is essential to accurately characterise the mechanical properties of the diluents and to thoroughly understand how their mechanical properties affect the manufacturing performance and properties of the final products, which will build a sound scientific basis for formulation design and product development. In this study, a comprehensive evaluation of the mechanical properties of the widely-used pharmaceutical diluent powders, including microcrystalline cellulose (MCC) powders with different grades (i.e., Avicel PH 101, Avicel PH 102, and DG), mannitol SD 100, lactose monohydrate, and dibasic calcium phosphate, were performed. The powder compressibility was assessed with Heckel and Kawakita analyses. The material elastic recovery during decompression and in storage was investigated through monitoring the change in the dimensions of the compressed tablets over time. The powder hygroscopicity was also evaluated to examine the water absorption ability of powders from the surroundings. It was shown that the MCC tablets exhibited continuous volume expansion after ejection, which is believed to be induced by (1) water absorption from the surrounding, and (2) elastic recovery. However, mannitol tablets showed volume expansion immediately after ejection, followed by the material shrinkage in storage. It is anticipated that the expansion was induced by elastic recovery to a limited extent, while the shrinkage was primarily due to the solidification during storage. It was also found that, for all powders considered, the powder compressibility and the elastic recovery depended significantly on the particle breakage tendency: a decrease in the particle breakage tendency led to a slight decrease in the powder compressibility and a significant drop in immediate elastic recovery. This implies that the particle breakage tendency is a critical material attribute in controlling the compression behaviour of pharmaceutical powders.

Flow and compaction behaviour of ultrafine coated ibuprofen

Good flow and compaction properties are prerequisites for successful compaction process. Apart from initial profile, mechanical properties of pharmaceutical powders can get modified during unit processes like milling. Milled powders can exhibit a wide range of particle size distribution. Further downstream processing steps like compaction can be affected by this differential particle size distribution. This has greatest implications for formulations like high dose drugs wherein the active pharmaceutical ingredient (API) contributes the maximum bulk in the final formulation. The present study assesses the impact of dry coating with ultrafine particles of same material, on the flow and compaction properties of the core material. Ibuprofen was selected as model drug as it has been reported to have poor mechanical properties. Ultrafine ibuprofen (average size 1.75 μm) was generated by Dyno(®) milling and was dry coated onto the core ibuprofen particles (average size 180 μm). Compaction studies were performed using a fully instrumented rotary tablet press. Compaction data was analyzed for compressibility, tabletability, compactibility profiles and Heckel plot. Dry coating of the ibuprofen exhibited greater compressibility and tabletability, at lower compaction pressure. However, at compaction pressure above 220 MPa, compressibility and tabletability of coated as well as uncoated materials were found to be similar. Heckel analysis also supported the above findings, as P(y) value of uncoated ibuprofen was found to be 229.49 MPa and for 2.0% ultrafine coated ibuprofen was found to be 158.53 MPa. Lower P(y) value of ultrafine coated ibuprofen indicated ease of plastic deformation. Superior compressibility and deformation behaviour of ultrafine coated ibuprofen attributed to increased interparticulate bonding area. This strategy can also be explored for improving tabletability of high dose poorly compressible drugs.