Which immunosuppressant agent acts primarily by inhibiting calcineurin and preventing dephosphorylation of NFAT, thereby blocking IL-2 transcription?

Explanation

## Mechanism of Calcineurin Inhibitors **Key Point:** Tacrolimus (and cyclosporine) are calcineurin inhibitors that bind to immunophilins and form a complex that blocks calcineurin phosphatase activity. ### Molecular Mechanism 1. **Binding**: Tacrolimus binds to FKBP12 (FK-binding protein) 2. **Complex formation**: Tacrolimus–FKBP12 complex inhibits calcineurin 3. **Downstream effect**: Calcineurin cannot dephosphorylate NFAT (nuclear factor of activated T cells) 4. **Transcription block**: Unphosphorylated NFAT remains in cytoplasm, preventing IL-2 and other cytokine gene transcription ### Comparison of Immunosuppressants | Agent | Mechanism | Target | Clinical Use | |-------|-----------|--------|---------------| | **Tacrolimus** | Calcineurin inhibition | NFAT dephosphorylation | Organ transplant, autoimmune | | **Cyclosporine** | Calcineurin inhibition | NFAT dephosphorylation | Organ transplant, psoriasis | | **Mycophenolate mofetil** | IMPDH inhibition | GTP synthesis in T/B cells | Transplant, lupus nephritis | | **Azathioprine** | Purine antagonist | DNA/RNA synthesis | Autoimmune diseases | | **Sirolimus** | mTOR inhibition | Cell cycle (G1→S) | Transplant, restenosis | **High-Yield:** Tacrolimus is 10–100 times more potent than cyclosporine but shares the same mechanism. Both are first-line calcineurin inhibitors in transplantation. **Clinical Pearl:** Tacrolimus has a narrow therapeutic window (10–20 ng/mL) and requires therapeutic drug monitoring to avoid nephrotoxicity and neurotoxicity. **Mnemonic:** **FKBP** = FK-binding protein (FK = forskoliin-like; tacrolimus is a macrolide from *Streptomyces tsukubaensis*)