<prompt> Which of the following antiumor agents works by impairing de novo purine synthesis?Acyloguanosine5-FluorouracilMethotrexate antifolateAllopurinol  <response>: Ans. is c i.e., Methotrexate antifolate o Methotrexate and permetrexed are dihydrofolate reductase DHF Rase inhibitors. o These drug also inhibit thymidylate synthase and de novo purine synthesis, which contribute to methotrexate toxicity. 