Bioadhesive Delivery Systems
INTRODUCTION Bioadhesive drug delivery formulations were pioneered in 1947 when gum tragacanth was mixed with dental adhesive powder to exhibit penicillin to the oral mucosa. Eventually, Orabase1, an adhesive paste, was developed (1). In the early 1980s, the idea to exploit bioadhesion for pharmaceutical interest became more important (2). Over the years, bioadhesive polymers were shown to be able to adhere to various mucosal membranes. Normal contact time for mucosal drug delivery ranges from a few minutes for the front of the eye to about three hours for the small intestine (3). However, the residence time can be extended with bioadhesive polymers. There are several perceived advantages by using bioadhesive drug delivery systems (BDDS). (i) As a result of the adhesion, the formulation stays longer at the delivery site, which allows a sustained drug release and improves the bioavailability of the drug. For example, bioadhesive microparticles have been investigated for the ocular delivery of acyclovir using chitosan as the bioadhesive polymer where microspheres showed increased bioavailability of the drug (4). (ii) Furthermore, an intimate contact with the absorbing mucosa resulting in a steep concentration gradient abetting drug absorption can be provided. Because of this increased contact interactions of the polymer with the epithelium such as a permeation enhancing effect (5,6), shielding of membrane bound enzymes and an increased residence time can appear (7). (iii) Bioadhesive polymers can be localized in specified regions to enhance the bioavailability, for example, specific targeting to the colon (8), specific targeting to M cells by the use of lectins for vaccine formulation (9). Because of all these benefits, many polymers are already commonly used as pharmaceutical excipients for different purposes. Several classes of polymers were found to display pronounced adhesive properties in contact with the mucosal surface. This chapter will give a general survey of BDDS and will focus on the current progress and research in bioadhesion for drug delivery applications.