Thymosin Alpha-1

Thymosin alpha 1 is a peptide naturally occurring in the thymus, originally isolated as the compound responsible for restoring immune function to thymectomized mice. ^{[1] [2]} As a drug it is a synthetic polypeptide, and thymalfasin is its international nonproprietary name. ^{[3] [4]} Thymalfasin, marketed as Zadaxin, is an FDA-approved thymosin-based drug used to treat chronic hepatitis B and C and as a chemotherapy inducer. ^[5] It has been utilized in the treatment of immunocompromised states and malignancies, as an enhancer of vaccine response, and to curb morbidity and mortality in sepsis and numerous infections. ^[1]

Thymosin alpha 1 is thought to modulate the immune system by augmenting T-cell function, stimulating differentiation of thymocytes or converting them to active T cells. ^[3] It has a pleiotropic mechanism of action, acting through Toll-like receptors in both myeloid and plasmacytoid dendritic cells to trigger signaling pathways and production of immune-related cytokines. ^[2] It is an immunomodulating agent able to enhance the Th1 immune response and to promote reconstitution of immune defects. ^{[6] [7]} Pharmacokinetically the drug is rapidly absorbed, achieving peak serum concentrations within two hours, with blood levels returning to baseline within 24 hours and a serum half-life of approximately 2 hours. ^[3]

A randomized controlled trial found hepatitis B virus DNA clearance in 40.6% and 25.6% of patients treated with thymosin alpha 1 for 6 and 12 months respectively, compared with 9.4% of untreated controls. ^[3] A large phase III randomized study in Europe found that in hepatitis C nonresponders thymalfasin did not improve the rate of sustained virologic responses, though in patients who completed therapy it significantly diminished the relapse rate. ^[4] A comprehensive narrative review of over 11 000 human subjects in more than 30 trials, covering COVID-19, autoimmune conditions and cancer, concluded that thymosin alpha 1 is a well-tolerated and effective immune modulator. ^[8] In sepsis, single or combined treatment with thymosin alpha 1 reduced the mortality rate, improved HLA-DR expression on monocytes and diminished the incidence of secondary infection, making it a promising alternative adjuvant therapy. ^[9] In the context of aging, preclinical and clinical studies show that thymosin alpha 1 can improve vaccine response in the elderly and mitigate immunosenescence, though further research is needed to validate long-term efficacy and safety. ^[10]

Thymosin alpha 1 is well tolerated, with most studies observing only local irritation at the injection site. ^[3] Its safety profile is described as excellent and it is virtually devoid of toxicity, having been used in thousands of patients. ^[11] Open questions remain about the optimal dose, schedule, combination treatments, and end-points to be evaluated in clinical trials. ^[11] In 2023 the FDA restricted thymosin alpha 1 along with additional peptides, though a systematic review argued that evidence of its safety and efficacy makes that restriction appear unfounded. ^[8]