Purine analogue

Purine analogues are antimetabolites that mimic the structure of metabolic purines.

Azathioprine is the main immunosuppressive cytotoxic substance. It is widely used in transplantations to control rejection reactions. It is nonenzymatically cleaved to 6-mercaptopurine that acts as a purine analogue and an inhibitor of DNA synthesis. By preventing the clonal expansion of lymphocytes in the induction phase of the immune response, it affects both the cell and the humoral immunity. It also successfully suppresses autoimmunity.
Mercaptopurine
Thioguanine is used to treat acute leukemias and remissions in acute granulocytic leukemias
Fludarabine inhibits function of multiple DNA polymerases, DNA primase, and DNA ligase I, and is S phase-specific (since these enzymes are highly active during DNA replication).
Pentostatin and cladribine are adenosine analogs that are used primarily to treat hairy cell leukemia.