Abstract

Thymosin alpha 1 (trade name Zadaxin) is a 28 amino acid immune-stimulating peptide, which was originally isolated from thymic tissue, and is now produced by solid-phase peptide synthesis. Thymosin has been shown to activate toll-like receptor 9 (TLR9), a cell-surface receptor that plays a key role in triggering the body's innate and adaptive immune responses, allowing significant increases in antibody production and clearance of viral infections. In preclinical studies, treatment with thymosin led to increased influenza-specific cytotoxic T lymphocyte (CTL) responses following vaccination. In a Phase 2 clinical study, treatment with biweekly doses of thymosin after vaccination was shown to decrease the incidence of influenza in elderly subjects from 19% in patients receiving influenza vaccine alone to 6% in patients receiving vaccine in combination with thymosin alpha 1 (p = 0.002). Thymosin was also shown to increase the antibody response to influenza vaccines in a number of clinical studies. These data support the use of thymosin alpha 1 to increase response to vaccination.

SciClone Pharmaceuticals, Inc.

Over decades of research, thymosin alpha 1 has been shown to have a number of immunomodulating activities, centered primarily on augmentation of T-cell function. It has been shown to promote T-cell differentiation and maturation;^2–7 increase natural killer (NK)-cell activity,^8–10 production of interferon (IFN)-alpha, interleukin (IL)-2, and IL-3, expression of IL-2 receptor following activation by mitogens or antigens,^8,11–17 production of migration inhibitory factor (MIF),¹⁸ and antibody response to T-cell–dependent antigens;^19–21 and activate dendritic cell tryptophan catabolism.^22,23 Thymosin alpha 1 also has been shown to antagonize dexamethasone-induced apoptosis in thymocytes in vitro.^24,25 In vivo administration of thymosin alpha 1 to animals immunosuppressed by chemotherapy, tumor burden, or irradiation showed that thymosin alpha 1 protects against cytotoxic damage to bone marrow, tumor progression, and opportunistic infections, thereby increasing survival time and number of survivors.^{26–28 (see also 5,6,29,30)} Many of the in vitro and in vivo effects of thymosin alpha 1 have been interpreted as influences on either differentiation of pluripotent stem cells to thymocytes or activation of thymocytes into activated T-cells. The pattern of enhanced cytokine production, i.e., IFN-alpha and IL-2, suggests that thymosin alpha 1 may influence progression to a T-helper 1 (Th1) type of immune response.³¹

On a subcellular level, thymosin alpha 1 has been shown to activate toll-like receptor 9 (TLR9), a receptor that plays a key role in triggering the body's innate and adaptive immune responses.^22,32–34 TLR9 activation affects various intracellular cell-signaling pathways that have been implicated in thymosin alpha 1's effects. These include stimulation of the p38 MAPK and NFkappaB signaling pathways, which are important for the maturation of, and antigen presentation by, dendritic cells.^35–38 Together with the macroscopic effects of thymosin alpha 1 described above, these effects on signaling pathways provide a rationale for why treatment with thymosin alpha 1 could lead to significant responses to vaccination.

Thymosin alpha 1 has an excellent safety profile. Adverse experiences have been infrequent and mild in over 70 studies conducted under US corporate investigational new drug applications (INDs), physician INDs, trials in foreign countries, and market experience to date in over 2,500 individuals who have received thymosin alpha 1.^39,40 These studies have shown effectiveness in a variety of indications, including response to vaccination (described below) as well as various types of cancer (melanoma, non-small-cell lung cancer, glioma, head and neck, renal, breast, gastric, and myelodysplasia) and infectious disease (chronic hepatitis B and C, HIV, primary immune deficiency).