Azathioprine and MMF
Inhibitors of purine and pyrimidine synthesis
This class of drugs is used for chronic immunosuppression. The two major antimetabolites in clinical use are azathioprine and mycophenolate mofetil (MMF).
Azathioprine is metabolized to 6-mercapto purine (6-MP), which is further converted to thioinosine monophosphate (TIMP). This molecule inhibits key enzymes in the de novo pathway of purine synthesis: adenosine monophosphate (AMP) and guanosine monophosphate (GMP).
MMF is metabolized to mycophenolic acid, which is a non-competitive inhibitor of IMPDH, the enzyme that converts inosine monophosphate (IMP) to GMP. Bone marrow suppression and increased risk of malignancy are the major toxicities associated with antimetabolites. Mycophenolate mofetil is associated with more gastrointestinal side effects than azathioprine.
Cytotoxic agents such as azathioprine are effective immunosuppressants because they
Extra info: Azathioprine is a phase-specific cytotoxic agent that functions by inhibiting purine synthesis. The other answers are wrong because azathioprine is nonspecific, is not an alkylating agent and has no effect on immune complexes.