In vitro release behavior and stability of insulin in complexation hydrogels as oral drug delivery carriers

Abstract

Novel pH-responsive complexation hydrogels containing pendent glucose (P(MAA-co-MEG)) or grafted PEG chains (P(MAA-g-EG)) were synthesized by photopolymerization. The feasibility of these hydrogels as oral protein delivery carriers was evaluated. The pH-responsive release behavior of insulin was analyzed from both P(MAA-co-MEG) and P(MAA-g-EG) hydrogels. In acidic media (pH 2.2), insulin release from the hydrogels was very slow. However, as the pH of the medium was changed to 6.5, a rapid release of insulin occurred. In both cases, the biological activity of insulin was retained. For P(MAA-co-MEG) hydrogels, the biological activity of insulin decreased when the pendent glucose content increased. In P(MAA-g-EG) hydrogels, when the grafted PEG molecular weight increased, the insulin biological activity decreased. Finally, hydrogels of P(MAA-co-MEG) prepared with an initial ratio of 1:4 MEG:MAA and P(MAA-g-EG) hydrogels containing PEG chains of molecular weights of 200 showed the greatest change in insulin release rate from acidic to basic pH solutions and the greatest protective effect for insulin in simulated GI tract conditions.

Introduction

Oral delivery of drugs, especially therapeutic proteins, is the preferred route of administration because it offers advantages over injection, which is the presently accepted route of therapeutic protein administration. The oral delivery route is more natural and less invasive. The protein drug can be self-administered and the method is less expensive. However, there exist several problems for the development of oral protein delivery systems. One major problem is the degradation of proteins by proteolytic enzymes and the acidic environment of the stomach. Another problem is the low penetration of proteins across the lining of the intestine into the blood stream (Lee and Yamamoto, 1990, Woodley, 1994).

Among the various methods that have been developed to assist to these problems (Shichiri et al., 1974, Arrieta-Molero et al., 1982, Patel et al., 1982, Couvreur and Puisieux, 1993, Rubinstein et al., 1997), use of environmentally sensitive hydrogels, especially methacrylic acid (MAA)-based complexation and pH-sensitive hydrogels, is the most promising method. In general, MAA-based hydrogels can form polymer complexes in response to the environmental pH. In the acidic environment of the stomach, these hydrogels are in a collapsed state due to hydrogen bonding, which can protect proteins by not allowing them to diffuse out from the hydrogel. In the intestine, as the environmental pH increases, the complexes dissociate and the pore size of the hydrogels increases leading to protein (Bell and Peppas, 1996, Lowman and Peppas, 1999, Peppas et al., 1999, Peppas et al., 2000; Kim and Peppas, 2002a, Kim and Peppas, 2002b; Robinson and Peppas, 2002, Blanchette et al., 2003). Additionally, the ionized carboxylic acid groups of PMAA have the ability to bind calcium ions in the extracellular medium. Therefore, they can help to minimize the proteolytic activity of calcium-dependent enzymes like trypsin (Lueßen et al., 1995, Madsen and Peppas, 1999) and increase the paracellular permeability of epithelial cell monolayers by opening of tight junctions between two epithelial cells (Bochard et al., 1996, Kriwet and Kissel, 1996).

In our studies, insulin was used as a model protein because it is one of the well known therapeutic proteins and it has become the standard treatment for diabetes. Diabetes mellitus is a disorder caused by decreased production of insulin or by decreased ability to use insulin, leading to increase glucose levels in the blood. Diabetes affects 20 million people in the US, approximately 10% being treated with insulin (Lowman et al., 1999). Usually, insulin is injected subcutaneously two to four times a day. Therefore, there has been significant interest in the development of oral delivery systems for insulin (Saffran et al., 1997, Lowman et al., 1999, Torres-Lugo and Peppas, 1999, Torres-Lugo and Peppas, 2002, Vauthier et al., 1999).

In this study, the feasibility of MAA-based hydrogels containing various functional groups as oral delivery carriers for proteins was evaluated by investigating the pH-responsive release behavior of insulin in the physiological pH range and protective ability of hydrogels for insulin in simulated gastric solutions.