Many therapeutically active molecules do not possess the properties required to achieve efficacy in clinical medical use.  For example, therapeutically active proteins and poly-peptides are now being discovered and produced by the biopharmaceutical industry and by genetic engineering.  Although there are currently at least 80 protein based medicines marketed in the United States with at least 350 more protein based medicines undergoing clinical trails (Harris J, Chess R: Effect of Pegylation on pharmaceuticals.  Nature Review Drug Discovery, 2003, 2, 214-221), most native proteins do not make good medicines because upon administration to patients there are several inherent drawbacks that include: (1) proteins are digested by many endo- and exopeptidases present in blood or tissue, (2) many proteins are immunogenic to some extent and (3) proteins can be rapidly excreted by kidney ultrafiltration.  Other molecules used as active therapeutic agents in medicines that are systemically toxic or lack optimal bioavailability and pharmacokinetics include low molecular weight molecules where an effective dose is limited by toxicity.  Such molecules are routinely used to treat inflammation and conditions due to malignancies, infection and autoimmune disease.