Pharmacology: The mechanism of action of Zadaxin in treating chronic hepatitis B is not completely understood. In various in vitro assays, the peptide shows promotion of T cell maturation by mitogen-stimulated peripheral blood lymphocytes; increase in production of various lymphokines, eg interferon α, interferon γ, interleukin 2 (IL-2) and interleukin 3 (IL-3) by T cells following antigen or mitogen activation; and increase in the levels of lymphokine receptors on T cells. It also enhances both allogeneic and autologous human mixed lymphocyte reactions by activation of T4 (helper/inducer) cells. Thymosin α1 may influence recruitment of prenatural killer (NK) cells, which then become cytotoxic after exposure to interferon. In vivo, thymosin α1 enhances IL-2 receptors and production of IL-2 in concanavalin A-stimulated mouse lymphocytes.
Pharmacokinetics: At 900 mcg/m2, thymosin α1 given SC results in peak plasma concentrations of 25-30 ng/mL approximately 1 hr after injection. Peak levels are maintained for 6 hrs and return to baseline over the following 18 hrs. Repeated injections twice a week for 15 weeks result in a very slight increase in baseline plasma thymosin α1 concentration.