Aminopterin (4-aminopteroic acid), a 4-amino analog of folic acid, is an antineoplastic drug with immunosuppressive properties used in chemotherapy. Aminopterin is a synthetic derivative of pterin.

Aminopterin works as an enzyme inhibitor by competing for the folate binding site of the enzyme dihydrofolate reductase.

Its binding affinity for dihydrofolate reductase effectively blocks tetrahydrofolate synthesis.

This results in the depletion of nucleotide precursors and inhibition of DNA, RNA and protein synthesis.

http://en.wikipedia.org/wiki/Aminopterin