Which statement best describes the action of 6-mercaptopurine?

• A. Blocks lymphocyte proliferation.
• B. Direct smooth muscle effect.
• C. Monoclonal antibody binding TNF.
• D. 5-HT antagonist.

Answer: (A)

6-Mercaptopurine acts as an immunosuppressant antimetabolite that interferes with purine nucleotide synthesis inside cells. After activation by the enzyme HGPRT, it becomes thioinosinic acid, which blocks de novo purine synthesis and disrupts DNA and RNA production. Because lymphocytes rely on rapid DNA synthesis to proliferate in response to immune signals, this leads to reduced lymphocyte proliferation. That’s why the statement describing its action as blocking lymphocyte proliferation is the best fit. It does not work by directly affecting smooth muscle, nor is it a monoclonal antibody against TNF, and it is not a 5-HT antagonist.