Thymalfasin, commercially sold as ZADAXIN ^® , is a substance which circulates in the blood naturally and elicits a variety of immune system responses targeted against cancer cells and viruses.

SciClone believes that in both immune-mediated cancers (such as melanoma) and viral diseases, thymalfasin works through its demonstrated activation of various arms of the immune system, specifically increasing white blood cell production, including natural killer (NK) cells, cytotoxic lymphocytes (CTLs) and the T-helper 1 (Th1) subset of CD-4 helper cells. At the same time, thymalfasin increases the expression of MHC Class I proteins on the surface of cells which enable the body's immune system to recognize and target the cancer and virally-infected cells more effectively.