7-Hydroxymethotrexate-d 3 is the deuterium labeled 7-Hydroxymethotrexate. 7-Hydroxymethotrexate is a major metabolite of Methotrexate (MTX;). Methotrexate, an antimetabolite and antifolate agent, inhibits the enzyme dihydrofolate reductase, thereby preventing the conversion of folic acid into tetrahydrofolate, and inhibiting DNA synthesis.