A 32-year-old woman with systemic lupus erythematosus is started on immunosuppressive therapy. Her physician reviews the mechanisms of action of commonly used immunosuppressants. All of the following immunosuppressants act by inhibiting calcineurin and blocking IL-2 transcription EXCEPT:

Explanation

## Calcineurin Inhibitors vs. Other Immunosuppressants **Key Point:** Calcineurin inhibitors (CNIs) form a specific drug class that blocks the phosphatase calcineurin, preventing dephosphorylation of NFAT and subsequent IL-2 transcription. Mycophenolate mofetil operates through a completely different mechanism. ### Mechanism Comparison | Drug | Class | Mechanism | Target | |------|-------|-----------|--------| | Cyclosporine | Calcineurin inhibitor | Binds cyclophilin → inhibits calcineurin | NFAT dephosphorylation | | Tacrolimus | Calcineurin inhibitor | Binds FKBP12 → inhibits calcineurin | NFAT dephosphorylation | | Pimecrolimus | Calcineurin inhibitor | Binds FKBP12 → inhibits calcineurin | NFAT dephosphorylation | | Mycophenolate mofetil | Inosine monophosphate dehydrogenase (IMPDH) inhibitor | Selective inhibition of IMPDH type II | Guanosine nucleotide synthesis in T & B cells | **High-Yield:** Mycophenolate mofetil is a prodrug that is hydrolyzed to mycophenolic acid (MPA), which selectively inhibits IMPDH type II — the enzyme responsible for de novo guanosine nucleotide synthesis in lymphocytes. This is fundamentally different from calcineurin inhibition. ### Why Calcineurin Inhibitors Work 1. Cyclosporine and tacrolimus both bind to intracellular proteins (cyclophilin and FKBP12, respectively) 2. The drug–protein complex inhibits calcineurin phosphatase activity 3. NFAT remains phosphorylated and cannot translocate to the nucleus 4. IL-2 and other cytokine genes are not transcribed 5. T-cell activation is blocked **Clinical Pearl:** Pimecrolimus is a macrolide structurally similar to tacrolimus and works identically at the molecular level, though it is used primarily in topical formulations for atopic dermatitis because it has reduced systemic absorption. **Warning:** Do not confuse mycophenolate mofetil with the calcineurin inhibitors — while all are immunosuppressants, MMF's mechanism is entirely distinct and does not involve calcineurin or IL-2 transcription inhibition. [cite:KD Tripathi 8e Ch 71]