Advances in understanding the molecular mechanisms of disease together with the advent of recombinant DNA and other technologies have opened opportunities for a vast array of novel therapeutic biopharmaceuticals and diagnostic agents. However, even natural biomolecules present a myriad of problems that limit their potential as pharmaceutical agents. Rapid degra- tion and elimination, immunological reactions, and toxicity are often asso- ated with new biopharmaceuticals, much as with conventional agents. Targeted delivery systems have the potential to increase the efficacy of existing...