Which of the following is a cell-cycle specific chemotherapy agent?

• A. Asparaginase
• B. Procarbazine
• C. Thiotepa
• D. Mercaptopurine

Answer: (D)

Cell-cycle specific drugs act mainly during a particular phase of cell division. 6-Mercaptopurine is an antimetabolite that targets DNA synthesis in the S phase. It’s a purine analog that, once activated in the cell, disrupts purine nucleotide production and becomes incorporated into DNA and RNA, impairing replication and leading to cell death. This phase-restricted action is what makes it cell-cycle specific.

The other drugs don’t restrict their activity to a single phase. Asparaginase depletes asparagine, hitting protein synthesis across cycles; procarbazine and thiotepa are alkylating-type agents that damage DNA in multiple phases of the cell cycle. Because of its S-phase–targeted mechanism, Mercaptopurine is the best choice.