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 hydroxypropylmethyl cellulose (HPMC) show different drug release behavior. In order to clarify these findings, the rheological properties of both polymers have been determined by oscillatory as well as by steady shear measurements. Aqueous solutions of 4 and 7% (w/w) polymer have been used to 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 solution. These findings are quite consistent with the reported higher ability of XG matrices to retard drug release than HPMC matrices for controlled-release formulation. The effects of differences in drug solubility and acidity, as well as the addition of lactose, and of the ionic strength of the medium on the rheological properties of XG and HPMC solutions have been studied in detail. Among these parameters, only the salt concentration exerts an enhancing effect on both moduli of XG, while no detectable influence on HPMC solution could be observed.