Which of the following is true regarding the action of methotrexate?

Methotrexate is an antimetabolite and one of its primary mechanisms of action involves the inhibition of folate metabolism. Specifically, methotrexate competitively inhibits dihydrofolate reductase (DHFR), an enzyme critical for converting dihydrofolate to tetrahydrofolate. Tetrahydrofolate is essential for the synthesis of nucleic acids, as it provides the necessary one-carbon units required for the production of purines and thymidine, which are vital for DNA synthesis. By inhibiting DHFR, methotrexate effectively reduces the levels of tetrahydrofolate, thereby disrupting the synthesis of DNA, RNA, and proteins.

This inhibition of folate metabolism is particularly significant in rapidly dividing cancer cells, which require a continuous supply of nucleotides for proliferation. Thus, methotrexate's role in inhibiting folate metabolism is central to its effectiveness as a chemotherapeutic agent in various cancers.

The other options do not accurately describe the action of methotrexate. For example, methotrexate does not promote DNA synthesis or stimulate ribonucleotide reductase. While topoisomerase inhibitors are a distinct class of cancer drugs, methotrexate does not fall