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Aspirated gastric juice from eight patients was measured for surface tension, bile salt concentration and pH. Surface tension ranged between 35 and 45 mM/m, while pH was usually in the range 1-2 and bile salt concentrations were usually between 0 and 1 mM. No correlations were found between the three parameters. These findings suggest that the low surface tension of gastric juice cannot be attributed solely to refluxed bile salts. Once the source of the reduced surface tension is identified dissolution test media should be adjusted to represent these conditions.

The extent of dissolution of theophylline from sodium alginate matrices has been studied in water and HCl. Surprisingly, it was observed that the dissolution rate was very fast for the low viscosity (LV) material in water and for the high viscosity (HV) in acid, whereas slow release was seen for LV in acid with very slow release for the HV in water. The extent of erosion and swelling of the matrices (with and without drug and with and without stirring) was determined from simple weighing experiments. Based on the extent of erosion and swelling, along with visual observations, it was concluded that the rapid release from LV in water was due to rapid and complete erosion of a weak gel layer. The rapid release of HV in HCl was due to an unexpected lamination of the tablet during swelling, which resulted in exposure of more of the tablet core, and caused increased erosion of the matrix. The slow release seen with the LV in HCl was due to the acid gel being tough and resistant to erosion. The very slow release observed with the HV in water was due to very substantial swelling, with the presence of near zero-order kinetics for this system being due to a balance of swelling and erosion, keeping the diffusion process at a constant rate. Furthermore, an anisotropic swelling behavior is indicated by the preferential expansion in the axial dimension relative to the radial dimension for both LV and HV alginates.

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This study compared the release behavior of single-unit (tablets, capsules) and multipleunit (minitablets in capsules) controlled-release systems of furosemide. The swelling and erosion behaviors of these systems, which contained the swellable hydrophilic polymers sodium alginate (high viscosity) and Carbopol 974P, were compared. Swelling and erosion experiments showed a high degree of swelling and limited erosion for the Carbopol preparations, whereas less swelling but greater erosion was observed for the sodium alginate preparations. The sodium alginate preparations were eroded in 6 hours, while Carbopol preparations exhibited limited erosion within this period of time. These results appear to be attributed to the physicochemical characteristics of the polymers used in this study. Polymer characteristics greatly influenced the release of furosemide (model drug) from the formulations prepared and tested. Sodium alginate had a less pronounced sustained release effect compared with Carbopol (ie, in 8 hours all 3 sodium alginate dosage forms displayed complete release of furosemide, while only 30% of the drug was released from Carbopol dosage forms). Finally, all 3 Carbopol dosage forms (single-and multiple-unit) displayed similar release behavior while sodium alginate dosage forms displayed a different and more distinctive behavior. Minitablets and tablets showed a greater sustained release effect compared with capsules. Evaluation of the release data indicates that the release mechanism for sodium alginate formulations may be attributed to erosion/dissolution, while for Carbopol it may be attributed mainly to polymer relaxation and diffusion of the drug from the polymer surface.