Which statement best describes the class of 5-fluorouracil?

• A. Antibody
• B. Antimetabolite (pyrimidine analog)
• C. Microtubule inhibitor
• D. Topoisomerase inhibitor

Answer: (B)

5-fluorouracil is an antimetabolite, specifically a pyrimidine analog. Antimetabolites resemble normal cellular building blocks and disrupt nucleotide synthesis or utilization. In cancer cells, 5-FU is converted to active metabolites that inhibit thymidylate synthase, reducing production of thymidine triphosphate needed for DNA synthesis. With less thymidine available, DNA replication slows or stalls, which is particularly harmful to rapidly dividing tumor cells. Some metabolites of 5-FU are also incorporated into RNA, disturbing RNA processing and function. This dual interference with DNA and RNA synthesis is the hallmark of its class. It’s not an antibody, nor a microtubule inhibitor, nor a topoisomerase inhibitor.