Methotrexate - It is an anticancerous and antifolate drug. It is used in treatment of cancer, autoimmune diseases, and ectopic pregnancy. It acts by inhibiting the metabolism of folic acid.
Methotrexate is commonly employed in the treatment of choriocarcinoma, which is curable cancer. It is also useful in the treatment of acute leukaemia, where high toxic doses are first administered so as to kill maximum number of leukemia cells, and the patient is retrieved by citrovorum factor (folinic acid)
The similar structure of dihydrofolic acid(top) and methotrexate (bottom) suggests that methotrexate is a competitive inhibitor.
Metho trexate inhibits dihydroflate reducatase. When methotrexate is given, the tumour cells increase four hundred times the synthesis of the enzyme. Methotrexate has structural similarity to folic acid, and hence will competitive inhibit folate reductase. So the presence of methotrexate, tetrahydrofolic acid is not produced, which is necessary for incorporation of C2 and C8 of purine and C5 methyl group in thymidine. Thus there is inhibition of DNA synthesis and consequently of cell division.