RHEOLOGICAL CHARACTERIZATION OF XANTHAN GUM AND HYDROXYPROPYLMETHYL CELLULOSE WITH RESPECT TO CONTROLLED-RELEASE DRUG-DELIVERY

It has been observed previously that xanthan gum (XG) and hydroxypropy lmethyl cellulose (HPMC) show different drug release behavior. In orde r to clarify these findings, the rheological properties of both polyme rs have been determined by oscillatory as well as by steady shear meas urements. Aqueous solutions of 4 and 7% (w/w) polymer have been used t o simulate the outer surface of a hydrated tablet. The dynamic moduli, i.e., storage modulus (G') and loss modulus (G'') of the two polymers have been determined in pure water and USP phosphate buffer pH 7.4 at different dilutions. In this concentration range XG solution exhibits ''gel-like'' behavior, while HPMC behaves as a typical polymer soluti on. These findings are quite consistent with the reported higher abili ty of XG matrices to retard drug release than HPMC matrices for contro lled-release formulation. The effects of differences in drug solubilit y and acidity, as well as the addition of lactose, and of the ionic st rength of the medium on the rheological properties of XG and HPMC solu tions have been studied in detail. Among these parameters, only the sa lt concentration exerts an enhancing effect on both moduli of XG, whil e no detectable influence on HPMC solution could be observed.