Polyanhydrides

Polyanhydrides are a class of biodegradable polymers characterized by anhydride bonds that connect repeat units of the polymer backbone chain. Their main application is in the medical device and pharmaceutical industry. In vivo, polyanhydrides degrade into non-toxic diacid monomers that can be metabolized and eliminated from the body. Owing to their safe degradation products, polyanhydrides are considered to be biocompatible. Polyanhydrides are a class of biodegradable polymers characterized by anhydride bonds that connect repeat units of the polymer backbone chain. Their main application is in the medical device and pharmaceutical industry. In vivo, polyanhydrides degrade into non-toxic diacid monomers that can be metabolized and eliminated from the body. Owing to their safe degradation products, polyanhydrides are considered to be biocompatible. The characteristic anhydride bonds in polyanhydrides are water-labile (the polymer chain breaks apart at the anhydride bond). This results in two carboxylic acid groups which are easily metabolized and biocompatible. Biodegradable polymers, such as polyanhydrides, are capable of releasing physically entrapped or encapsulated drugs by well-defined kinetics and are a growing area of medical research. Polyanhydrides have been investigated as an important material for the short-term release of drugs or bioactive agents. The rapid degradation and limited mechanical properties of polyanhydrides render them ideal as controlled drug delivery devices. One example, Gliadel, is a device in clinical use for the treatment of brain cancer. This product is made of a polyanhydride wafer containing a chemotherapeutic agent. After removal of a cancerous brain tumor, the wafer is inserted into the brain releasing a chemotherapy agent at a controlled rate proportional to the degradation rate of the polymer. The localized treatment of chemotherapy protects the immune system from high levels of radiation. Other applications of polyanhydrides include the use of unsaturated polyanhydrides in bone replacement, as well as polyanhydride copolymers as vehicles for vaccine delivery. There are three main classes of polyanhydrides: aliphatic, unsaturated, and aromatic. These classes are determined by examining their R groups (the chemistry of the molecule between the anhydride bonds). Aliphatic polyanhydrides consist of R groups containing carbon atoms bonded in straight or branched chains. This class of polymers is characterized by a crystalline structure, melting temperature range of 50–90 °C, and solubility in chlorinated hydrocarbons. They degrade and are eliminated from the body within weeks of being introduced to the bodily environment. Unsaturated polyanhydrides consist of organic R groups with one or more double bonds (or degrees of unsaturation). This class of polymers has a highly crystalline structure and is insoluble in common organic solvents. Aromatic polyanhydrides consist of R groups containing a benzene (aromatic) ring. Properties of this class include a crystalline structure, insolubility in common organic solvents, and melting points greater than 100 °C. They are very hydrophobic and therefore degrade slowly when in the bodily environment. This slow degradation rate makes aromatic polyanhydrides less suitable for drug delivery when used as homopolymers, but they can be copolymerized with the aliphatic class to achieve the desired degradation rate.