Mercaptopurine is classified as which type of chemotherapeutic agent?

• A. Alkylating agent
• B. Topoisomerase inhibitor
• C. Platinum compound
• D. Antimetabolite (cell-cycle specific)

Answer: (D)

Antimetabolites mimic normal nucleotides and disrupt DNA synthesis, especially during the S phase when cells are actively replicating DNA. Mercaptopurine is a purine analog that, after entering the cell, is activated to nucleotide-like forms that inhibit de novo purine synthesis and can be incorporated into DNA and RNA. This interference with nucleotide pools and incorporation into nucleic acids blocks replication, which is why it acts during the S phase as an antimetabolite. Other classes—alkylating agents, topoisomerase inhibitors, and platinum compounds—alter DNA in different ways (adding alkyl groups, preventing proper DNA unwinding, or crosslinking DNA), not by mimicking nucleotides. So mercaptopurine is classified as an antimetabolite (cell-cycle specific).