Azathioprine is a prodrug of 6-mercaptopurine (6-MP). 6-MP is inactive, but it acts as a purine antagonist and requires cellular uptake and intracellular anabolism to thioguanine nucleotides (TNTs) for immunosuppression. TNTs and other metabolites (e.g., 6-methyl-mercaptopurine ribonucleotides) inhibit de novo purine synthesis and purine nucleotide interconversion. TNTs are also incorporated into nucleic acids, contributing to the drug's immunosuppressive effects.