Effect of formulation on the rheology of theophylline compound suspensions in Gelucires

Rheological Study of Binary Gels with Carbopol UltrezTM 10 and Hyaluronic Acid

The objective of this study is to provide a rheological characterization of binary hydroalcoholic gels made with Carbopol Ultrez 10 (U10) and Hyaluronic Acid (HA) as a function of polymer concentration: U10 (0.0-2.0% w/w) and HA (0.00-0.20% w/w), and to determine the influence of this combination on the thixotropic properties of the resulting binary systems. Interaction of the two polymers was measured using the Viscose Synergy Index (I(S)) and thixotropic analysis, which indicate the structural changes that take place in binary gels attributable to molecular interactions between the gelling agents. The maximum values for viscose synergy (I(S)=1.22-1.44) are obtained for the U10 : HA mixtures with a polymer proportion of 10 : 1. The behavior of the binary gels studied is the result of the formation of a more structured three-dimensional network between the U10 and HA molecules. Shearing of this polymer network requires application of a greater force than is needed to shear the structure of the separate gels. Inclusion of HA in a proportion of 1 : 10 has a fixing effect on the polymer network, resulting in greater resistance to shearing in the compound gel. The relative thixotropic area -A(R)- shows maximum values (A(R)=17.215%) for the same polymer composition. The evolution of the two parameters indicates that restructuring of the molecular interactions for this polymer proportion (10 : 1) takes place; the result is a reinforced three-dimensional structure in the gelled system, which increases the thixotropic properties. The same composition leads to a maximum of thixotropic properties as well as viscose synergy because both characteristics are closely related to structural changes observed in the binary systems of this composition. Thixotropic systems have a very wide area of application in the pharmaceutical industry. For this reason, the results obtained here considerably increase the use of the gels studied. In fact, incorporation of HA significantly improves a property of acrylic gels which has direct repercussions on the ease and efficiency of their application to the skin.

Assessment of the extrudability of three different mixtures of saturated polyglycolysed glycerides by determination of the "specific work of extrusion" and by capillary rheometry

PURPOSE

Assessment of the extrudability of mixtures of saturated polyglycolysed glycerides used as carriers for poorly soluble drugs.

METHODS

Three different mixtures of saturated polyglycolysed glycerides were extruded in a ram extruder at different rates and using different dies. The force applied to the ram and its displacement were recorded and used to determine the "specific work of extrusion" and pursuit rheometric determinations, according to the Bagley's approach.

RESULTS

As the melting range of the mixtures, or the length of the dies increased, the higher the "specific work of extrusion" observed. From the rheometric analysis, viscoelastic properties were identified and decomposed into plastic and elastic components. The elastic component contributed more to the total pressure loss, and for the majority of the cases, was higher than the plastic component. From the Bagley plot it was possible to calculate the wall shear stress (sigma w), that increased with the melting range of the materials and the shear rate applied (gamma). The viscosity (eta) decreased as the shear rate increased, reflecting a non-Newtonian behavior of the materials. Finally, materials showing lower viscosities required less work to produce the extrudates.

CONCLUSIONS

This study has (a) shown the possibility of preparing extrudates from different mixtures of polyglycolysed glycerides alone in a non-aqueous environment, (b) demonstrated the usefulness of the "specific work of extrusion," and (c) revealed through the rheometric studies the viscoelastic nature of the materials quantifying the contribution of the elastic and the plastic components for the total pressure loss.

Rheological behavior of drug suspensions in Gelucire mixtures and proxyphylline release from matrix hard gelatin capsules

Mixtures of Gelucires 50/02 and 50/13 showing different hydrophilic-lipophilic balances (HLB) and of proxyphylline were used to prepare suspensions at a concentration of 25% and to manufacture extended release hard gelatin capsules by cooling. The rheological behaviors of Gelucire mixtures with and without drug were determined by adjustment of the rheograms to the Ostwald power-law and by statistical assessment of the flow index. Pure Gelucire mixtures were very slightly shear thickening whereas proxyphylline suspensions had a thixotropic shear thinning behavior. These rheological behaviors can be explained by the chemical composition and by the ratio of the two Gelucires used. Extended release of proxyphylline was obtained with all these mixtures. Drug release increased with Gelucire mixture HLB owing to higher erosion. A viscosity-release relationship was found and allowed, with these two Gelucires of extreme HLB and viscosities, to define the formulations which will give an optimal drug release, by the determination of their suspension viscosity. Modeling of dissolution kinetics has generally shown the predominance of surface erosion of the plugs relative to drug diffusion inside the matrix. This was confirmed by the better linearization of percentage released, according to Hixson-Crowell as compared with Higuchi.

Rheology of polyol behenates and drug release from matrix monolithic capsules

Three polyol behenates with similar melting points (MP) and different hydrophilic-lipophilic balances (HLB) were studied (MP/HLB: 70/02, 63/05 and 57/13). After melting at MP+30 degrees C, the rheological behaviour of behenates was determined by adjustment of the rheograms to the Ostwald power-law and by statistical assessment of the flow index. Behenates showed slight shear thickening. This shear thickening increased when HLB of behenates decreased. This behaviour accounted for a reorganization of the particles under the shear, which became easier when the proportion of the polyethylene glycol chains in the wax decreased. Proxyphylline was used to prepare suspensions at a concentration of 25% in the melted behenates, and to manufacture monolithic capsules by cooling. The suspensions had a shear-thinning behaviour with or without thixotropy. Colloidal particles and aggregates formed in these suspensions directly influenced the rheological properties, as observation of solidified suspensions by scanning electron microscopy confirmed. Extended release of proxyphylline was obtained with the three waxes. Behenates 63/05 and 70/02 gave inert matrices and released drug very slowly. Hydrodispersible behenate 57/13 swelled and made up a kind of hydrophilic matrix that released proxyphylline more quickly, due to slight erosion. In the three cases, the release mechanism was basically diffusional in nature.