Comparative study on xanthan gum and hydroxypropylmethyl cellulose as matrices for controlled-release drug delivery. II. Drug diffusion in hydrated matrices

Abstract

In order to look up the reason for previously observed difference in the retarding ability of drug-release from xanthan gum (XG) and hydroxypropylmethyl cellulose (HPMC) matrix tablets, diffusion of three model drugs, e.g. indomethacin, indomethacin sodium and caffeine in hydrated gels of the two polymers, were measured. For measuring the diffusivity, the drug desorption from the polymeric gels into the stirred bulk medium was continuously monitored by their UV absorbance. Under identical experimental conditions, the drug diffusivity in HPMC gel is higher than in XG gel. This difference in hindered transport of the drug molecules within the two polymeric systems brings out the real cause for the reported higher retarding ability of drug-release from a XG matrix tablet than from a HPMC matrix tablet. In view of salt effects, the diffusion through the hydrated XG matrices back up the release characteristics of water soluble drugs from XG matrix tablets, suggesting that the diffusion of drug molecules in the hydrated gel of a XG matrix tablet is the main mechanism of overall release for soluble drugs like caffeine and the sodium salt of indomethacin. On the contrary, no reflection of indomethacin diffusivity through XG gels was found in its release profiles from XG matrix tablets, suggesting that diffusion through the hydrated polymer mass is not the dominant factor for the release of an insoluble drug like indomethacin from a XG matrix tablet.