Polyanhydrides are an important class of biodegradable polymers characterized by anhydride groups in the repeat unit of the polymer backbone.The anhydrides are highly susceptible to hydrolysis and degrade into non-toxic diacid monomers that can be metabolized and eliminated from the body, that is, many polyanhydrides are considered to be biocompatible. This property makes this class of polymers suitable as biodegradable controlled-release coatings for drug delivery systems.
Although, anhydride linkages are highly susceptible to hydrolysis, the crystalline polymers are very stable in an aqueous environment because the water moelcules are unable to penetrate the crystalline regions. To make the polymer more susceptible to hydrolysis, aliphatic anhydrides are copolymerized with aromatic diacids which reduce the crystallinity. The erosion rate of such polymers can vary from days to years depending on the composition. Purely aromatic polyanhydrides become brittle over time and eventually fragment after exposure to water, causing water-soluble drugs to be released more rapidly than by polymer errosion, whereas copolymers of aromatic anhydrides and anhydrides prepared from fatty acids, such as naturally occuring oleic and sebacic acid have controlled degradation rates from days to years.
Among the most widely used polyanhydrides are copolymers of sebacic acid (SA) and 1,3-bis(p-carboxyphenoxy propane (CPP)1. By varying the ratio of the hydrophobic block (CPP) and the hydrophilic block (SA), controlled degradation rates can be achieved.
Polyanhydrides find uses in the medical device and in the pharmaceutical industry. They are used as controlled-release coatings for drug delivery systems to control the rate of release of an active ingredient They can also be used as temporary medical implants that decompose safely over time, and therefore, eliminate the need for a second surgical invention for removal.
- R. Cristescu et al., Applied Surface Science 254, 116901173 (2007)
- Other important bio- and environmentally degradable polymers that are currently used or studied for controlled drug delivery include: poly(2-hydroxy ethyl methacrylate), poly(N-vinyl pyrrolidone), poly(methacrylate), poly(acryl amide), poly(acrylic acid), poly(vinyl alcohol), poly(L-lactide), poly(DL-lactide-co-glycolide), poly(ethylene-co-vinyl acetate), poly(ethylene glycol), poly(DL-lactide-co-ε-caprolactone), poly(hydroxy alkanoate)s