One of the first modern chemotherapeutic agents was which acts as a competitive inhibitor to an enzyme involved in folate synthesis.

One of the first modern chemotherapeutic agents was sulfonamides which acts as a competitive inhibitor to an enzyme involved in folate synthesis. 
Sulfonamides are sulfa drugs that were very first modern antimicrobial drugs that are synthetic. Human beings can not make folic acid themselves and need enzymes to synthesize it. Microbes can, they have the enzymes for it. We target those enzymes with sulfonamides and trimethoprim-competitive inhibition.
It is supplied to cells in high concentrations to make sure enzyme is constantly occupied rather than the true substrate of the enzyme.