Degradable polymers for site‐specific drug delivery

The present article was aimed to review the properties and applications of the actually available biodegradable polymers that have been used as targetable drug carriers and site‐specific delivery systems. We have mainly focused on microparticulate or carrier‐based drug delivery systems with little emphasis on implants as site‐specific delivery systems. The review was divided into two major sections. The first section includes a brief description of the properties and applications of the existing biodegradable polymers for site‐specific drug delivery classified in two main categories: synthetic biopolymers (as polyesters, polyanhydrides, polyorthoesters, polyphosphazenes, polycaprolactones), and natural biomaterials (as collagen, albumin and gelatin), which still remains an active area of research in spite of the extensive development of new biodegradable polymers. The second section discusses the mechanistic approaches for the selection of polymeric carriers. General guidelines for the ideal drug carrier complex were summarized. The importance of knowing and understanding the physicochemical properties of the polymer for the optimization of the final formulation and its eventual application were emphasized. The factors governing the effective drug concentration at the specific target site were reviewed. Some of the equations describing the release kinetics of a drug from a polymeric matrix were also examined. The last section of the review describes the preparation and use of polymeric macromolecular prodrugs. A diagrammatic representation of possible polymeric prodrug systems for use in site‐specific delivery was described. The basic requirements from a polymeric prodrug system were also discussed. Finally, clinical applications of polymeric prodrugs as for cancer therapy and colonic delivery were considered.