A centrifuge technique for the evaluation of the extent of water movement in wet powder masses

Water retention and drainage in different brands of microcrystalline cellulose: Effect of measuring conditions

Interaction between water and microcrystalline cellulose (MCC) measured as retention and cumulative drainage of water (WR% and CDW%) is investigated for unmilled and micronized standard (Avicel and Emcocel) and silicified (Prosolv) MCC brands. A centrifuge method was applied with increasing duration and different porosity and thickness of cylindrical powder beds (specimens), in order to establish optimal determination conditions and quantify alterations in interaction between water and different MCC brands. Also, changes of specimen thickness due to presence of water (swelling) were followed. It was found that the effect of specimen porosity and thickness on water drainage (CDW%) appears to be opposite to that on water retention (WR%), while two patterns of WR% and CDW% change with specimen porosity and thickness can be distinguished depending on the centrifugation time. Also, WR% and CDW% are affected by the MCC brand and the micronization. Unmilled silicified MCC brand (Prosolv) shows significantly lower retention and higher drainage of water compared to standard unmilled brands (Avicel and Emcocel), while differences between the unmilled standard Avicel and Emcocel brands are not easily distinguished. Micronization, in general, increases greatly the WR% and decreases CDW% for all the tested MCC brands, and enhances their differences even between Avicel and Emcocel. Swelling of specimen due to presence of water was observed, which was significantly reduced with the micronization, the specimen porosity, and centrifugation as well, but showed slight variation between the different MCC brands. Values of specimen porosity between 60% and 70%, thickness/diameter ratio between 0.75 and 1.0, and centrifugation time between 5 and 20 min provide optimal measuring settings for comparison of MCC brands.

Starch–dextrin mixtures as base excipients for extrusion–spheronization pellets

Extrusion-spheronization pellets are generally produced with microcrystalline cellulose (MCC) as the principal excipient, giving rise to particles of very high quality. A number of alternative excipients have been proposed and evaluated, mostly other cellulose derivatives (e.g. different grades of Avicel), or mixtures of MCCs and other excipients. In the present study, we evaluated the possible use of starch+agglutinant mixtures as principal excipients for extrusion-spheronization pellets, with the aim of producing pellets with more suitable properties for certain types of release. We first characterized the different excipients in terms of morphometry and basic physical properties. Subsequently, torque-rheometry was used to characterize the rheology of wetted masses of the different excipients and excipient mixtures, with the aim of determining optimal amount of wetting agent (water). We also evaluated the water absorption and water retention capacities of each excipient. In view of the results obtained, we produced pellets with the different starch+agglutinant mixtures (but without drug), and used image analysis to characterize pellet morphology. Our results show that some of the mixtures-notably starch (corn starch or wheat starch)+20% white dextrin-gave high-quality pellets with good size and shape distributions. In addition, the properties of the different materials tested suggest that it may be possible to obtain pellets with very different properties.

The evaluation of the rheological properties of lactose/microcrystalline cellulose and water mixtures by controlled stress rheometry and the relationship to the production of spherical pellets by extrusion/spheronization

The consistency of wet powder masses produced from two ratios (7:3 and 8:2) of alpha-lactose monohydrate (L) and microcrystalline cellulose (MCC) mixed with a range of water contents has been assessed with a parallel plate controlled stress rheometer. The range of water contents, which could be studied, was restricted to those, which could be extruded uniformly by a ram extruder. In the creep mode, the instantaneous compliance increased as the water content increased for both L:MCC ratios illustrating the increasing deformability of the mixtures with increasing water content. The derived apparent viscosity of the mixtures as a function of shear rate, increased as the water content decreased and the values for all the systems fell on a common line. This indicates that the measurements are providing a reliable assessment of the mixtures and that the change in water content and L:MCC ratio provides systems, whose change of viscosity with rate of shear is consistent at low rates of shear. The values of the storage and loss moduli obtained from oscillatory measurements, increased with a decrease in water content but this time the two ratios of L:MCC were not on a common line when related to the water content of the mixtures. There was a range of water levels over which both the values of the storage and loss moduli were approximately constant. This corresponded to the level of water, which produced the pellets of the smallest diameter and range of diameters and were of the most spherical shape when produced by a ram extruder and spheronization. For 8:2 L:MCC ratio, there appeared to be a value for both the storage and the loss moduli above which the wet mass could not produce good pellets. For the 7:3 L:MCC these limiting levels were not achieved before extrusion with steady state conditions could be maintained without the mass being too wet or too dry. Instead, there appeared to be minimum levels of the moduli required to ensure that the mixtures were able to produce good pellets.

The influence of model drugs on the preparation of pellets by extrusion/spheronization: II. Spheronization parameters

Five drug-models, 4-parahydroxybenzoic acid (4HBA), methyl (MBA), propyl (PBA) and butyl (BBA) paraben and propyl gallate (PG), all of similar chemical nature, were mixed in different proportions (50-73.7%) with microcrystalline cellulose (MCC) (26.3-50%) plus various levels of water (26.9-50.0%). The wet powder mass was extruded and spheronized under standard conditions. The pellets produced were evaluated in terms of their median diameter, their modal size range, the % within a given size range (0.7-1.7 mm) and their shape factor. For the majority of formulations, all drug models, except 4HBA, produced pellets. This material only had two combinations of excipients that produced acceptable pellets. For all the model drugs, two combinations of formulations could be identified; (1) a combination, which produced pellets from all the model drugs and (2) a combination, which was too wet to produce pellets with any of the model drugs. Between these two extremes, whether pellets could be made and their quality varied with the model drug. Cluster analysis was able to divide the formulations into 4 clusters. In cluster 1 all the model drugs produced pellets except 4HBA; in cluster 2 all drugs produced pellets except MBA; in cluster3, pellets were produced with PBA, BBA and PG while MBA produced agglomerates and 4HBA was too dry; in cluster 4, MBA and BBA produced pellets, PBA produced agglomerates while 4HBA was too dry to pelletise and PG too dry to extrude. The five drug models showed different relationships between the median pellet size and drug-load and initial water content in the formulation. Cluster analysis indicated that, the level of water and type of model drug were the most significant factors in determining the pellet size. Three clusters could be identified, but the response to water content was drug dependent. It was not possible to identify a relationship between the force required to extrude the wet mass and the ability to produce good pellets nor their median size. All the products, which could be classified as good pellets, when produced, had a shape factor that can be considered to be indicative of a spherical shape. The most consistent material, in terms of spheronization, as represented by median diameter, size range and roundness, was propyl gallate (PG), which throughout all the formulations produced an almost constant value for shape factor and median pellet size, which in the majority of cases fell within a limited pellet size.

The influence of type and quantity of model drug on the extrusion/spheronization of mixtures with microcrystalline cellulose. I. Extrusion parameters

Five model drugs, (methyl, propyl and butyl paraben, 4-hydroxybenzoic acid and propyl gallate), similar in their chemical structure were mixed with microcrystalline cellulose and water in different proportions and forced through a ram extruder. The overall water movement was measured by the difference between the initial water in the formulation and the water content in the plug remaining after extrusion was completed. The differences in theoretical and practical volume occupancy of the materials inside the barrel were calculated to look for trapped air inside the barrel. The steady-state extrusion force for each formulation was recorded. All five materials demonstrated differences in behaviour during extrusion. The relationship between each of the three properties measured and both the drug-load and initial water content was examined, to establish the potential relationship that existed between the differences due to the drug models. The five drug models were divided into two sub-groups, when examining the way that they underwent extrusion. Methyl, propyl and butyl paraben formed one group while 4-hydroxybenzoic acid and propyl gallate formed the other group. Within the former group the relationship between steady-state extrusion force and the percentage of drug and water present tended to be lower than those in the latter group. For the former group these relationships were non-linear.

Measuring the water retention capacities (MRC) of different microcrystalline cellulose grades

The ability of various types of microcrystalline cellulose (MCC) to hold water when subjected to an applied force has been assessed by a centrifuge technique. By considering the final percent of water retained after a standard centrifugation, as a function of the initial water content, a moisture retention capacity (MRC) can be determined. Statistical analysis of the results identified that it was possible to divide the nine samples of MCC into six sub-sets in terms of final moisture content retained. Those types which contained added polymer had by far the highest water level remaining. In terms of the MRC value, statistical analysis again sub-divided the celluloses into six sub-sets, although different from those for the final water content. Again those types containing added polymer gave much higher MRC values. As experiments were carried out with initial water/MCC ratios of different levels, statistical analysis of the influence of initial water content on the MRC values was undertaken with each type of MCC. The results showed a varying dependence on initial water level with the different types of cellulose. To provide a value of MRC which characterised the cellulose, the maximum value of MRC at the lowest initial water level was identified. Samples of MCC with low values of MRC have been shown previously to require less water for processing by extrusion/spheronization, while celluloses with a high MRC value appear better for the limitation of water movement during the process of extrusion/spheronization. The water retention must, however, also be considered in association with the rheological properties of the wet powder mass. Thus, while the Avicel RC591 had the highest MRC value, its rheological properties are so that the production of pellets with such a type can be less effective than with other types of MCC.