Ad4-PA-1

Ad4-PA-1 is an oral, replication-competent adenovirus serotype 4 (Ad4) vectored vaccine candidate developed for the prevention of anthrax infection caused by *Bacillus anthracis*. Originally developed by PaxVax and later acquired by Emergent BioSolutions, the vaccine utilizes a modified Ad4 vector to deliver the gene encoding the Protective Antigen (PA) of *B. anthracis*. Unlike traditional injectable anthrax vaccines such as Anthrax Vaccine Adsorbed (AVA), Ad4-PA-1 is designed for oral administration via a capsule, allowing the vector to replicate in the intestinal mucosa and induce both systemic and mucosal immune responses. In clinical development, Ad4-PA-1 (which expresses a secreted form of PA) has been evaluated alongside Ad4-PA-GPI, a variant that includes a glycosylphosphatidylinositol (GPI) anchor to tether the PA protein to the cell surface for potentially enhanced immunogenicity.