Thymosin ñ<sub>1</sub> is a peptide fragment derived from prothymosin alpha, a protein that in humans is encoded by the PTMA gene.
It was the first of the peptides from Thymosin Fraction 5 to be completely sequenced and synthesized. Unlike ò thymosins, to which it is genetically and chemically unrelated, thymosin ñ<sub>1</sub> is produced as a 28-amino acid fragment having the sequence Ac-SDAAVDTSSEITTKDLKEKKEVEEEAEN, which is made from cleavage of a longer, 113-amino acid precursor, prothymosin ñ.
Thymosin ñ<sub>1</sub> is an agonist for toll-like receptor 2 and toll-like receptor 9 on both myeloid and dendritic antigen-presenting cells, thereby stimulating the adaptive immune response.
Thymosin ñ<sub>1</sub> is believed to be a major component of Thymosin Fraction 5 responsible for the activity of that preparation in restoring immune function in animals lacking thymus glands. It has been found to enhance cell-mediated immunity in humans as well as experimental animals.
Thymosin ñ<sub>1</sub> is approved in some countries for the treatment of Hepatitis B and C, and it is also used to boost the immune response in the treatment of other diseases. The synthetic version of Thymosin ñ<sub>1</sub> is known as Thymalfasin and is sold under the brand name Zadaxin.
Thymosin ñ<sub>1</sub> is usually administered by subcutaneous injection.
Clinical trials suggest thymosin ñ<sub>1</sub> may be useful in cystic fibrosis, septic shock, acute respiratory distress syndrome, peritonitis, pancreatitis, acute cytomegalovirus infection, TB, severe acute respiratory syndrome, and lung infections in critically ill patients., and for chronic hepatitis B.
For hospitalized COVID-19 patients, a 2023 review concluded thymosin ñ<sub>1</sub> was not effective in reducing mortality or length of hospitalization. A subsequent 2023 review contradicted this, showing a reduction in mortality but not length of stay.
It has been studied for possible use in treating cancer (e.g. with chemotherapy).