Which of the following medications does NOT inhibit dihydrofolate reductase as its main MOA?

• A. Trimethoprim
• B. Methotrexate
• C. Sulfonamides
• D. Pyrimethamine

Answer: (C)

Understanding the targets in the folate synthesis pathway helps explain the answer. Dihydrofolate reductase (DHFR) converts dihydrofolate to tetrahydrofolate, a crucial cofactor for making thymidine and purines. Drugs that inhibit DHFR—such as trimethoprim, methotrexate, and pyrimethamine—block this step, hindering DNA synthesis in their respective targets (bacteria, cancer cells, or certain parasites).

Sulfonamides, on the other hand, act earlier in the pathway by inhibiting dihydropteroate synthase, the enzyme that uses PABA to form dihydropteroate. This blocks production of dihydrofolate upstream of DHFR, so their main MOA is not DHFR inhibition. They’re often used in combination with trimethoprim to shut down two consecutive steps in folate synthesis for a stronger antibacterial effect, but by themselves they don’t primarily inhibit DHFR.