Which of the following is an example of a proliferation signal inhibitor?

• A. Mycophenolate
• B. Tacrolimus
• C. Cyclosporine
• D. Leflunomide

Answer: (A)

Proliferation signal inhibitors stop T-cell clonal expansion after activation by cutting off the resources or pathways needed for cell division. Mycophenolate mofetil does this by inhibiting IMP dehydrogenase, which depletes guanine nucleotide pools. With fewer nucleotides available, activated lymphocytes cannot synthesize DNA efficiently, so their proliferation is directly suppressed. This makes it a clear example of a drug that hinders the proliferation signal at the level of the cellular machinery needed for replication. In contrast, calcineurin inhibitors like tacrolimus and cyclosporine prevent the production of IL-2 to begin the proliferation process, and Leflunomide blocks pyrimidine synthesis by a different metabolic route; while both can reduce proliferation, the mechanism by which mycophenolate acts—limiting nucleotide synthesis needed for DNA replication—fits the idea of a proliferation-focused effect more directly.