Effect of drying rate on porosity and tabletting behaviour of cellulose pellets

An approach for predicting the true density of powders based on in-die compression data

Helium pycnometry, a commonly used technique for measuring the true density of powders, is sensitive to the release of volatiles during measurement. This can lead to over-estimated true density, and as such, an accurate method for determining the true density of powders containing volatile components is needed. Here, a method based on in-die compression data obtained with a compaction simulator was assessed. Specifically, the stress transmission coefficient (STC), measured using an instrumented die, was used to predict the in-die Heckel mean yield pressure (Py). A true density was derived by repeatedly performing a Heckel analysis using iteratively estimated true density values until the predicted Py value from the measured STC value is obtained from in-die density - pressure data. This novel method was validated using a set of water-free powders. Using crystalline hydrates, we further showed that the calculated true densities were closer to values calculated from crystal structure than those from helium pycnometry. Hence, this method may be used for determining the true density of powders from their STC values.

The Potential Migrated Mechanism of Water-Soluble Components in Pellets Prepared by Wet Extrusion/Spheronization: Effect of Drying Rate

INTRODUCTION

At present, there are numerous researches on the migration of components in tablets and granules, however, the investigation in the pharmaceutical literatures concerning the effect of drying rate on the migration of water-soluble components of pellets is limited. Temperature and Relative Humidity (RH) are crucial parameters during the drying process, which is an essential step in the preparation of pellets via wet extrusion/spheronization. To quantify these variables, the water loss percentage of pellets per minute is defined as the drying rate.

AIM

The study aimed to investigate the influence of drying rate on the migration of water-soluble components in wet pellets and the potential migrated mechanism.

METHODS

The pellets containing tartrazine as a water-soluble model drug and microcrystalline cellulose as a matrix former were prepared by extrusion/spheronization and dried at four different drying temperatures and relative humidity. Afterwards, the extent of migrated tartrazine was assessed regarding appearance, in-vitro dissolution test, Differential Scanning Calorimetry, X-Ray Powder Diffraction, Attenuated total reflectance Fourier transform infrared spectroscopy and Confocal Raman Mapping.

RESULTS

Results demonstrated that red spots of tartrazine appeared on the surface of pellets and more than 40% tartrazine were burst released within 5 minutes when pellets were dried at 60°C/RH 10%. When pellets were dried at 40°C/RH 80%, none of these aforementioned phenomena were observed.

CONCLUSION

In conclusion, the faster the drying rate was, the more tartrazine migrated to the exterior of pellets. Adjusting drying temperature and relative humidity appropriately could inhibit the migration of water-soluble components within wet extrusion/spheronization pellets.

Cushioning pellets based on microcrystalline cellulose - Crospovidone blends for MUPS tableting

Compaction of multiple-unit pellet system (MUPS) tablets has been extensively reported to be potentially challenging. Thus, there is a need for non-segregating cushioning agents to mitigate the deleterious effect of the compaction forces. This study was designed to investigate the use of porous pellets as cushioning agents using different drying techniques to prepare pellets of various porosities and of different formulations. The pellets fabricated were characterized for their porosity and crushing strength. Subsequently, MUPS tablets were prepared using blends of polymer-coated pellets and custom-designed cushioning pellets by compacting at different pressures. The effects of pellet volume fraction and dwell time on the pellet coat damage, as well as the tensile strength of the resultant MUPS tablets were also investigated. Compacts with coated pellet volume fraction of 0.21 exhibited the best cushioning effect when tableted at different compression speeds with both gravity and force feeders. The findings from this study showed that cushioning pellet porosity was highest when drying was carried out by freeze drying, followed by fluid bed drying and oven drying. There was an inverse relationship between cushioning pellet porosity and strength. The tensile strength of tablets prepared from freeze dried pellets was highest. The protective effect of the cushioning pellets was principally dependent on their porosity. Also, pellet volume fraction in the compacts and compaction pressure used had remarkable effect on pellet coat damage. When unprocessed powders were compacted by automatic die filling, capping and lamination problems were observed. However, tablets of reasonable quality were made with the cushioning pellets. Freeze dried pellets containing crospovidone were found to be promising as cushioning agents and had enabled the production of MUPS tablets even at higher compaction pressures, beyond the intrinsic crushing strength of the coated pellets.

Influence of the porosity of cushioning excipients on the compaction of coated multi-particulates

The compaction of multiple unit-pellet system (MUPS) tablets poses considerable challenges due to potential compaction-induced damage to the functional polymer coat and segregation of pellets from other excipients during the tableting process. This study was designed to investigate the impact of porous pellets as cushioning agent without issues related to segregation while tableting. Different drying techniques were applied to produce microcrystalline cellulose (MCC) pellets with various porosities. Sodium chloride was also added to the pellet formulation as a pore forming agent to generate a porous skeleton after production and aqueous extraction. The pellets fabricated were characterized for their porosity, crushing strength and yield pressure. Tablets were prepared using unlubricated pellets and their tensile strengths determined. Blends containing polymer-coated pellets and cushioning pellets of various porosities were compacted at different compaction pressures. The porous pellets exhibiting the best cushioning effect were used for MUPS tableting at different compression speeds with both gravity and force feeders. The findings from this study showed that pellet porosity was highest when drying was carried out in a freeze dryer, followed by fluid bed and least porous from the oven. There was an inverse relationship between pellet porosity and strength. The protective effect of cushioning pellets was mainly dependent on their porosity. The porosity of pellets manufactured by leaching NaCl from MCC-NaCl (1:1) pellets were 2.14-, 2.57- and 4.88-fold higher than that of MCC PH101 only pellets for oven, fluid bed and freeze dried pellets, respectively. Although the porosity of MCC PH101-NaCl (1:3) pellets was highest, they exhibited less cushioning effect than MCC PH101-NaCl (1:1). It was inferred that a good balance between porosity and bulk density of cushioning pellets was essential to be effective at protecting the coated pellets from damage during compaction. Compared with MUPS tablets prepared using unprocessed MCC PH105, the tablets prepared with the porous freeze dried MCC PH101 (NaCl fraction leached) pellets had improved drug content uniformity and were mechanically stronger.

Mechanics of Pharmaceutical Pellets-Constitutive Properties, Deformation, and Breakage Behavior

To ensure robust manufacturing of unit-based oral solid dosage forms with minimal structural imperfections and high mechanical reliability across subsequent processing unit operations (e.g., withstanding mechanical stresses during coating, optional axial compression, handling, packaging, storage, and transport conditions), process design should include consideration of precise limits of accurate micro, macro, and bulk properties of the constituent pellets. This communication presents a comprehensive intricate database of micromechanical properties' and breakage probability distribution functions of pellets, illustrating the stiffening and strengthening effects of coatings and the softening and weakening effects of structural moisture. Further insights such as the (contact) history-dependent softening during decompression, strain hardening on repeated stressing, strength recovery by drying, and the fragmentation pattern by cracking are also presented. The contents herein are based on conveniently performable lab-scale diametrical compression measurements on model microcrystalline cellulose pellets-demonstrating feasibility of the approach and validity of the contribution.

Impact of drying on solid state modifications and drug distribution in ibuprofen-loaded calcium stearate pellets

Drying is a common pharmaceutical process, whose potential to alter the final drug properties-even at relatively low temperatures-is often neglected. The present study addresses the impact of drying at 20 and 50 °C on wet-extruded calcium stearate (CaSt) pellets. Drying at 20 °C caused the majority of ibuprofen to accumulate at the pellet surface due to a strong convective flow from the pellet's center to the surface. In contrast, pellets dried at 50 °C still contained ibuprofen in the pellet's interior due to the higher drying rate and the associated film breakage during drying. Moreover, the higher drying temperature caused CaSt to form a second lamellar phase and ibuprofen to convert (partly) into its amorphous state. Overall, the drying process affected the solid state and the spatial ibuprofen distribution within the pellet. Knowledge of these effects can aid in tailoring advanced multipellet formulations.

Ibuprofen-loaded calcium stearate pellets: drying-induced variations in dosage form properties

Pellets intended for oral dosing are frequently produced via extrusion/spheronization followed by drying. Typically, the last active process step, i.e., drying, is assumed to have little effect on the final dosage form properties (e.g., dissolution characteristics). Thus, there exist only a few studies of this subject. In the present study, calcium stearate/ibuprofen pellets were used as model system to investigate the impact of the drying conditions. Lipophilic calcium stearate matrix pellets containing 20% ibuprofen were prepared via wet extrusion/spheronization. Subsequently, desiccation, fluid-bed drying, and lyophilization were applied for granulation liquid removal. The impact of these drying techniques on the final pellet properties was evaluated. The in vitro dissolution behavior was dramatically altered by the drying techniques that were considered. The investigated pellets showed drug release rates that varied as much as 100%. As no polymorphic transitions occurred during drying, we focused on two possible explanations: (a) a change in the drug distribution within the pellets and (b) a change in pellet micro-structure (porosity, pore size). The ibuprofen distribution proved to be homogeneous regardless of the drying conditions. Pellet porosity and pore sizes, however, were modified by the drying process. Our results clearly demonstrate that a single process step, such as drying, can play a crucial role in achieving desired pellet properties and release profiles.

Key formulation variables in tableting of coated pellets

Multiple unit controlled release dosage forms offer various advantages over their single unit counterparts. Most of these advantages are associated with the uniform distribution of multiparticulates throughout the gastrointestinal tract. Though coated pellets can be filled into hard gelatin capsules, tablet formulation is the preferred one because of various advantages associated with it. However, compression of coated pellets is a challenging task necessitating the optimization of various formulation and process variables. The key formulation variables include composition, porosity, size, shape and density of the pellets; type and amount of polymer coating; nature, size and amount of tableting excipients. The pellet core should be strong with some degree of plasticity. It should be highly porous, small, with an irregular shape. The critical density to achieve prolonged release was reported to lie between 2.4 and 2.8 g/cm³. Acrylic polymer films are more flexible and more suitable for the coating of pellets to be compressed into tablets. Thicker coatings offer better resistance to frictional forces. Solvent based coatings are more flexible and have a higher degree of mechanical stability than the aqueous based ones. The tableting excipients should have cushioning property. They should not be significantly different in size and density from those of the pellet cores in order to avoid segregation. Addition of 30%-60% of tableting excipients is necessary to avoid any damage to the polymer coat and to retain its functional property.

Incidence of drying on microstructure and drug release profiles from tablets of MCC-lactose-Carbopol and MCC-dicalcium phosphate-Carbopol pellets

The influence of intragranular excipients (lactose or dicalcium phosphate) and the drying procedure and conditions (oven-drying and freeze-drying after freezing at -30 or -196 degrees C) on the properties of tablets of MCC-Carbopol pellets was evaluated. The drying procedure caused remarkable differences in pellet size and porosity (freeze-dried pellets were 3-fold more porous than those oven dried). Theophylline release from pellets was completed in less than 30 min and followed first-order kinetics, with a rate closely related to the intragranular porosity. The total porosity of the tablets (5-10%) was conditioned by the compression force (10-20 N), the drying procedure applied to the pellets and the coexcipient nature. Their intergranular porosity ranged inversely to the initial porosity of pellets due to the greater deformability of the most porous ones. A wide range of theophylline release rates were achieved depending on the drying procedure; tablets prepared from freeze-dried pellets sustained the release for 3h. Most profiles showed a bimodal kinetics with an initial zero-order release (while the tablets did not completely disintegrate) that changed, after a certain time, to a first-order kinetics. The intergranular porosity determined drug release rate up to disintegration. Then, the release kinetics became first-order and the rate constant, which was conditioned by the intragranular porosity, showed a complex dependence on the drying procedure, the compression force, and the nature of coexcipient. In sum, the modulation of drug release profiles from tablets of MCC-Carbopol pellets through an adequate control of the effects of the coexcipient nature, the drying procedure of pellets, and the compression force on the inter- and intragranular porosity opens interesting possibilities to control the release of hydrosoluble drugs.

Stress relaxation test for the characterization of the viscoelasticity of pellets

The characterization of the mechanical properties of single uncoated pellets was performed in order to verify if these parameters could be used to predict the pellets aptitude to be compressed or utilized differently. Different ratios of microcrystalline cellulose and lactose monohydrate were used for the preparation of four batches of pellets by an extrusion/spheronization process. The 0.6-0.71 mm pellet fraction was used for the tests. Crushing strength and stress relaxation tests were carried out on the single pellets. The first test provided information of both the mechanical strength and the fragmentation aptitude. The second test provided information about their deformation ability (viscous flow) and residual elasticity (stress relaxation modulus). The results obtained from these tests were then compared with those obtained from the Heckel analysis. An excellent consistency was discovered between the parameters obtained from both the stress relaxation and crushing strength tests on one side and the Heckel parameters on the other side. Tests performed on single pellets are very useful tools to predict their deformation and fragmentation aptitude under compression and can be used for early insight of the pellet aptitude to be compressed.

Characterization of Tableting using the OSDRC System

PURPOSE

We compared the compression properties and characteristics of tablets obtained using the OSDRC system (method (OSDRC)) and conventional compression methods including multi-layer compression with a pharmaceutical powder.

MATERIALS AND METHODS

We prepared tablets using four methods of compression. The force profiles, ejection force, stress relaxation, pressure transmission ratio, and internal intensity of the tablets were measured as compression properties.

RESULTS

Method (OSDRC) gave the highest value for the crushing strength of the tablets. Although the compression properties were similar regardless of the method of compression, the internal intensity of tablets compressed by method (OSDRC) was significantly larger than that of the tablets produced by the other methods.

CONCLUSIONS

In terms of crushing strength, the tablets compressed by method (OSDRC) were superior of those obtained by the conventional compression method. Therefore, it is possible to increase the crushing strength of tablets without changing the pharmaceutical formulation.

Drying of poloxamer hydrogel films

The drying of hydrogel films formed by Poloxamer 407 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer was investigated at various air relative humidity (RH) conditions ranging from 11 to 97%. Initially, the amount of water lost increased linearly with the drying time. After this linear region (stage I), a nonlinear behavior was observed (stage II). The drying rate increased with decreasing RH, thus greatly shortening the drying time. A decrease of the film thickness also shortened the drying time; however, the drying mechanism did not change. Three models for one-dimensional water diffusion were used to fit the experimental results at different RH conditions and film thicknesses. Model 1 assumes semi-infinite medium and constant diffusion coefficient, and fits very well the data in stage I of the drying process. The fitted water diffusion coefficient (D) is 5 x 10(-10) m(2)/s, whereas the effects of the RH are captured by a proportionality constant (alpha) that appears in the boundary condition. Model 2 considers a finite (constant) film thickness and captures the experimental observations over the whole drying period for the same D and alpha as in Model 1. The analytical solutions available for Models 1 and 2, used together with the experimentally derived model parameters D and alpha, allow for easy estimation of drying time and water loss from Poloxamer hydrogel films of various compositions and thicknesses and at different relative humidities. Numerical solutions for water diffusion under conditions of decreasing film thickness and diffusion coefficient being a function of concentration are also presented (Model 3). It becomes apparent from the fit of the data to the different models that the drying rate is more sensitive to the boundary condition at the film-air interface (represented by alpha) than to the diffusion in the film. It is notable that the alpha values obtained from the fits of the Poloxamer hydrogel drying rate are comparable to those obtained from drying of water films under the same experimental conditions.

A study on the effect of drying techniques on the mechanical properties of pellets and compacted pellets

Microcrystalline cellulose (MCC) pellets produced by a standard extrusions/spheronisation process with a 40% ethanol/water mixture as the fluid component, were dried by four different techniques, namely: freeze-drying, fluid-bed drying, hot air oven drying and desiccation with silica-gel to less than 5% (w/w) water content. A 1.0-1.18mm size fraction of the dried pellets were characterised structurally and mechanically in terms of, shape, density/porosity (open and closed), pore volume/pore volume distribution, surface area, surface tensile strength, shear strength, deformability, linear strain and elastic modulus. An amount of 600, 700 and 750mg of the same size fraction of each pellet batch were compacted to the same tablet thickness and the tensile strength and volumetric elastic recovery of the resulted compacts were determined. Analysis of variance was used to assess the significance of the drying process on the property of the pellets and their compacts. The drying process did not influence the shape of the pellets, but all the other properties were affected to some extent. Pellets dried by freeze-drying were more porous, with most of the pores open to the atmosphere and had a higher surface area than pellets dried by the other methods. Pellets dried by desiccation contained the highest proportion of closed pores. The decrease in tensile strength of the pellets, which occurred with the increase in porosity could presumably be due to ease of crack initiation and propagation between the MCC fibres. The weaker pellets broke instantly before they were subjected to appreciable strain. The porous pellets needed a higher compressing pressure and work of compaction to produce tablets of the same mass and dimensions. This reflected their compressibility, i.e. relative decrease in volume of the pellet bed during compression. The strength and volumetric elastic recovery of the compacts increased with the increase of their porosity. The drying techniques, which produced porous, deformable and weak pellets, produced stronger tablets. The value of the volumetric elastic recovery of the compacts was also observed to increase with the value of compaction pressure.

Analysis of formulation effects in the dissolution of ibuprofen pellets

In this work the effects of citric acid and of two common fillers, lactose (soluble) and tricalcium phosphate (insoluble) are examined on the release profiles from pellets, using ibuprofen as a model drug with pH-dependent solubility. Also studied is the dependence of these profiles on the specific surface area, bulk density, apparent density, porosity and porosity parameters (pore size distribution, total pore surface area, mean pore diameter and pore shape), as determined by mercury intrusion porosimetry. Pellets with high porosity and total pore surface area but small median pore diameter (tricalcium phosphate pellets-IPM) are found to produce similar dissolution results to those of low porosity and low total pore surface area, but having a high median pore diameter (lactose pellets-ILM), irrespective of the solubility of excipients. Addition of citric acid causes a delay in the initial dissolution for both formulations. During dissolution, however, citric acid reduces the median pore diameter of lactose-based pellets. In contrast, in tricalcium phosphate/citric acid pellets (CIPM), this parameter increases considerably during dissolution, when compared to the IPM formulation. These findings may justify the contrasting dissolution behaviors of CIPM and CILM (lactose/citric acid) pellets, after their common behavior in the initial stages, and show that porosity and its related parameters, along with physical properties of excipients such as solubility, density and specific surface area, are helpful to predict pellet behavior in drug release profiles.