Significance and strategies in developing delivery systems for bio-macromolecular drugs

Successful development of a new drug is prohibitively expensive, and is estimated to cost approxi- mately S100-500 million US dollars for a single clinical drug. Yet, a newly developed drug can only enjoy its patent protection for 18 years, meaning that after this protected time period, any company can manufacture this product and thus the profit generated by this drug entity would reduce dramatically. Most critically, once a drug is being synthesized, its physical, chemical, and biological attri- butes such as bioavailability and in vivo pharmacokinetics are all completely fixed and cannot be changed. In principal and practice, only the application of an appro- priately designed drug delivery system （DDS） is able to overcome such limitations, and yet the cost of developing a novel drug delivery system is less than 10% of that of developing a new drug. Because of these reasons, the new trend in pharmaceutical development has already begun to shift from the single direction of developing new drugs in the past to a combined mode of developing both new drugs and innovative drug delivery systems in this century. Hence, for developing countries with relatively limited financial resources, a smart strategic move would be to focus on the development of new DDS, which has a significantly higher benefit/risk ratio when comparing to the development of a new drug. Because of the unmatched reaction efficiency and a repetitive action mode, the therapeutic activity of a single bio-macromolecular drug （e.g., protein toxins, gene products, etc.） is equivalent to about 10^6- 10^8 of that from a conventional small molecule anti-cancer agent （e.g., doxorubicin）. Hence, bio-macromolecular drugs have been recognized around the world as the future ＂drug-of-choice＂. Yet, among the 〉 10000 drugs that are currently available, only -150 of them belong to these bio- macromolecular drugs （an exceedingly low 1.2%）, reflect- ing the difficulties of utilizing these agents in clinical practice. In general, the bottleneck limitations of these bio- macromolecular drugs are two-fold： （1） the absence of a preferential action of the drug on tumor cells as opposed to normal tissues, and （2） the lack of ability to cross the tumor cell membrane. In this review, we provide strategies of how to solve these problems simultaneously and collec- tively via the development of innovative drug delivery systems. Since worldwide progress on bio-macromolecular therapeutics still remains in the infant stage