Which of the following immunosuppressants is a selective inhibitor of inosine monophosphate dehydrogenase (IMPDH) and is preferred over azathioprine in lupus nephritis due to better renal outcomes?

Explanation

## Mycophenolate Mofetil (MMF): Mechanism and Clinical Role ### Mechanism of Action **Key Point:** Mycophenolate mofetil is a prodrug that is hydrolyzed to mycophenolic acid (MPA), a selective inhibitor of type II inosine monophosphate dehydrogenase (IMPDH). 1. **Selective inhibition**: Type II IMPDH is preferentially expressed in activated T and B lymphocytes 2. **GTP depletion**: Blocking IMPDH prevents GTP synthesis, essential for lymphocyte proliferation 3. **Lymphocyte-selective**: Fibroblasts and other cells use salvage pathways and are spared 4. **Result**: Selective suppression of T and B cell proliferation without broad cytotoxicity ### Comparison: MMF vs. Azathioprine | Feature | MMF | Azathioprine | |---------|-----|---------------| | **Mechanism** | IMPDH inhibition (GTP synthesis) | Purine antagonist (DNA/RNA synthesis) | | **Selectivity** | Lymphocyte-selective (Type II IMPDH) | Non-selective | | **Lupus nephritis efficacy** | Superior renal outcomes, standard of care | Older agent, less effective | | **GI side effects** | Common (diarrhea, nausea) | Less common | | **Bone marrow suppression** | Mild | More pronounced | | **Teratogenicity** | Category C (avoid in pregnancy) | Category D | **High-Yield:** MMF is now the preferred agent for lupus nephritis (Class III/IV) in combination with corticosteroids and is part of standard induction therapy protocols (Euro-Lupus, NIH guidelines). **Clinical Pearl:** MMF dosing in lupus nephritis is typically 1–3 g/day divided doses; therapeutic drug monitoring (AUC target 30–60 μg·h/mL) may optimize outcomes in some patients. **Mnemonic:** **IMPDH** = **I**nosine **M**onophosphate **D**ehydrogenase; MMF blocks the **GTP** synthesis pathway → **G**uanosine **T**ri**P**hosphate depletion