Inulin hydrogels. II. In vitro degradation study

Abstract

Inulin hydrogels have been developed as potential new carriers for colonic drug targeting. Since site-specific drug release of this delivery system is based on its bacterial degradation in the colon, the enzymatic digestibility of the prepared inulin hydrogels was assessed by performing an in vitro study using an inulinase preparation derived from Aspergillus niger. The amount of fructose liberated from the inulin hydrogels by the action of inulinase was quantified using the anthrone method. The equilibrium swelling ratio as well as the mechanical strength of the hydrogels were studied before and after incubation in inulinase solutions. The data obtained by these different methods indicate that enzymatic digestion of the inulin hydrogels appeared to be enhanced by a prolonged degradation time, a higher inulinase concentration and a lower degree of substitution and feed concentration of the hydrogel polymer. The inulin hydrogels exhibited an increase in equilibrium swelling after degradation compared to the swelling before degradation, suggesting that inulinase enzymes are able to diffuse into the inulin hydrogel networks causing bulk degradation.

Introduction

Targeting of drugs to the colon is considered to have several therapeutic advantages (Rubinstein, 1990, Ashford and Fell, 1994, Van den Mooter and Kinget, 1995): (1) By treating colonic disorders such as ulcerative colitis, Crohn's disease, colon carcinoma's and infections with a site-specific delivery system, therapy will be more effective, the required dose can be lowered and undesirable side effects can be reduced. (2) For drugs such as peptides and proteins, which are destroyed by the stomach acid and/or metabolised by enzymes of the small intestine, colon targeting can be a valuable alternative for parenteral administration, due to negligible activity of brush-border peptidase activity and much less activity of pancreatic enzymes in the colon in comparison to the small intestine. (3) colonic release of drugs can also be valuable when a delay in absorption is desired from a therapeutic point of view in the treatment of diseases that have peak symptoms in the early morning, such as nocturnal asthma, angina or arthritis.

Inulin hydrogels have been developed as potential new carriers for colonic drug targeting. Inulin is a naturally occurring polysaccharide (Van Loo et al., 1995) which consists of β 2–1 linked d-fructose molecules. Most of the fructose chains have a glucosyl unit at the reducing end (Roberfroid, 1993). Inulin was chosen as candidate polymer for the development of colon-specific hydrogels because the β 2–1 osidic bonds are not significantly hydrolysed by enzymes from the endogenous secretion of the human digestive tract (Dysseler and Hoffem, 1995); colonic bacteria however, and more specifically Bifidobacteria which constitute up to 25% of the normal gut flora in man (McKellar and Modler, 1989), are able to ferment inulin (Wang and Gibson, 1993, Gibson and Roberfroid, 1995).

In vitro degradation of inulin suspended in Eudragit^® RS films was already demonstrated in previous work (Vervoort and Kinget, 1996); in order to prepare inulin hydrogels, the fructose polymer had to be derivatised which can affect its degradation properties. Development of inulin hydrogels has been previously described (Vervoort et al., 1997). Methacryloyl groups were incorporated in the inulin chains by reaction of the fructose polymer with glycidyl methacrylate and aqueous solutions of the resulting methacrylated inulin were subsequently converted into cross-linked hydrogels by free radical polymerisation.

The aim of this study was to investigate the enzymatic degradability of the prepared inulin hydrogels, a prerequisite for their use as colon-specific drug delivery systems. Degradation of the hydrogels in the colon will result in site-specific release of drug entrapped in the polymer network. The in vitro degradation study of the inulin hydrogels was performed by incubating the hydrogels in solutions of inulinase, able to hydrolyse the β 2–1 osidic linkages of the inulin chains. With respect to inulin hydrogel degradation, information was acquired on the effect of enzyme concentration, incubation time, degree of substitution and feed concentration of methacrylated inulin.