Which drug specifically inhibits calcineurin in activated T lymphocytes?

Tacrolimus is a powerful immunosuppressant that specifically inhibits calcineurin, an important enzyme in the signaling pathway of activated T lymphocytes. Calcineurin plays a critical role in the activation of T cells by dephosphorylating nuclear factor of activated T cells (NFAT), allowing NFAT to translocate to the nucleus and activate genes necessary for T cell activation and proliferation. By inhibiting calcineurin, tacrolimus effectively reduces the transcription of cytokines such as interleukin-2, which is essential for T cell growth and function.

This mechanism is instrumental in preventing organ rejection in transplant patients and in treating various autoimmune diseases. The specificity of tacrolimus for calcineurin makes it a unique drug among immunosuppressants, differentiating its action from other agents that may act through different pathways or mechanisms.

While other options mentioned have roles in immunosuppression, they do not specifically inhibit calcineurin—basiliximab is an IL-2 receptor antagonist, sirolimus inhibits mTOR (mammalian target of rapamycin), and mycophenolate mofetil inhibits lymphocyte proliferation by targeting the de novo purine synthesis pathway.