What is the primary mechanism of action of methotrexate in pharmacology?

Methotrexate primarily acts by inhibiting dihydrofolate reductase, an enzyme crucial for the conversion of dihydrofolate to tetrahydrofolate. This conversion is essential for nucleic acid synthesis because tetrahydrofolate serves as a cofactor in the synthesis of purines and thymidylate, which are vital components of DNA. By inhibiting this enzyme, methotrexate effectively reduces the availability of these nucleotides, leading to decreased DNA and RNA synthesis, which is why methotrexate is utilized not only in cancer therapies but also in autoimmune conditions such as rheumatoid arthritis.

In a clinical context, this mechanism ultimately helps limit the proliferation of rapidly dividing cells, such as cancerous cells and immune cells involved in inflammatory processes. Thus, the action of methotrexate is highly significant in managing various diseases, making the inhibition of dihydrofolate reductase its primary mechanism of action.