Calcineurin inhibitors used in transplant patients include which drug?

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

Answer: (C)

Calcineurin inhibitors work by shutting down a key signal that activates T cells. They block the calcineurin phosphatase, which normally removes a phosphate from NFAT, allowing NFAT to enter the nucleus and drive IL-2 transcription. Without IL-2 production, T-cell activation and proliferation are greatly reduced, helping prevent transplant rejection.

Two drugs accomplish this by binding different immunophilins: one binds cyclophilin, the other binds FKBP-12. The resulting complexes inhibit calcineurin, so the T-cell activation pathway is dampened. That’s why these agents are classic choices in transplant immunosuppression.

Sirolimus, in contrast, targets the mTOR pathway downstream of IL-2 signaling, not calcineurin. Mycophenolate mofetil reduces lymphocyte proliferation by inhibiting purine nucleotide synthesis. So, among the options, cyclosporine is a calcineurin inhibitor, and tacrolimus is another well-known example of the same class.