ON THE EMPLOYMENT OF LAMBDA-CARRAGEENAN IN A MATRIX SYSTEM .1. SENSITIVITY TO DISSOLUTION MEDIUM AND COMPARISON WITH NA CARBOXYMETHYLCELLULOSE AND XANTHAN GUM

Three anionic polysaccharidic materials, characterized by their differing sensitivity to pH, especially at low values, were compared in order to assess their behaviour on the performance in matrix systems. Lambda-carrageenan, a sulphated polymer, was compared with two carboxylic polymers, Na carboxymethylcelluse (Na CMC) and xanthan gum. In the case of Na CMC both the gelation rate and the ionic charge are affected by gastric pH. Gelation properties of xanthan gum are described as being more or less insensitive to pH. The more acidic lambda-carrageenan, however, was expected to maintain both gelation capability and ionization even at low pH values. Erosion in distilled water and buffers at pH 1.2 and 6.8 was evaluated. Matrices containing as model drug either salbutamol sulphate or diprophylline were prepared and tested for release rate in the same three media. Lambda-carrageenan shows characteristic behaviour. Its control of the initial drug release, ascribed to ionic drug-polymer interactions, appears to be remarkably effective and independent of the characteristics of the dissolution medium.