Development of acid-sensitive copolymer micelles for drug delivery

In recent years, supramolecular micellar assemblies formed from amphiphilic block copolymers have been receiving attention as potential drug carriers. The size of the carriers is ideal for avoiding rapid renal exclusion and reticuloendothelial uptake, and enables them to be targeted to certain tissues such as tumors. One important issue determining the effectiveness of a micellar drug carrier is the ability to control the time over which drug release takes place, or to possibly trigger drug release at a specific location or time. The mildly acidic pH encountered in tumor and inflammatory tissues as well as in the endosomal and lysosomal compartments of cells has inspired the development of micellar carriers capable of releasing their drug load in response to small changes in pH. One approach to the development of these systems has been to incorporate titratable groups such as amines and carboxylic acids into the copolymer backbone, thus altering the solubility of the polymer upon protonation and disrupting micelle formation. Another approach has been to incorporate acid-degradable linkages into the copolymer, either for direct attachment of the drug, or to cause a structural change of such magnitude that micellar integrity is lost and the drug is released.