A 35-year-old woman with SLE presents with Class IV lupus nephritis (proteinuria 3.5 g/day, serum creatinine 1.8 mg/dL, active urinary sediment). She has failed to respond adequately to 3 months of corticosteroids and mycophenolate mofetil. What is the drug of choice for induction therapy?

Explanation

## Induction Therapy for Severe Lupus Nephritis (Class IV) **Key Point:** Cyclophosphamide is the gold-standard induction agent for severe, refractory lupus nephritis (Class III–IV) and is the preferred choice when conventional therapy (corticosteroids + MMF) has failed or disease is rapidly progressive. ### Pathophysiology of Class IV Lupus Nephritis - Diffuse proliferative glomerulonephritis with immune complex deposition - Requires aggressive immunosuppression to prevent progression to ESRD - Standard induction: high-dose corticosteroids + cyclophosphamide or MMF ### Cyclophosphamide Induction Regimens | Regimen | Dose | Interval | Rationale | |---------|------|----------|----------| | **NIH protocol** | 0.5–1 g/m² IV | Monthly × 6 months | Standard; effective but higher toxicity | | **Euro-Lupus protocol** | Fixed 500 mg IV | Weekly × 6 weeks | Lower cumulative dose; reduced toxicity; non-inferior efficacy | | **Maintenance** | Azathioprine 1–2 mg/kg/day | After induction | Sustains remission, reduces relapse | **High-Yield:** Cyclophosphamide achieves complete or partial renal remission in 60–70% of Class IV nephritis cases. The Euro-Lupus protocol is increasingly preferred due to lower cumulative toxicity while maintaining efficacy. ### Mechanism of Cyclophosphamide 1. Alkylating agent; cross-links DNA 2. Depletes T and B lymphocytes 3. Reduces autoantibody production and immune complex formation 4. Particularly effective for rapidly progressive, proliferative disease ### Toxicity Management - **Mesna:** Co-administered to prevent hemorrhagic cystitis - **Hydration:** Aggressive IV hydration to dilute urine - **Monitoring:** Baseline CBC, renal function, urinalysis; repeat before each dose - **Contraception:** Absolute requirement (teratogenic); counsel all women of childbearing age - **Infection prophylaxis:** PCP prophylaxis if CD4 <200 cells/μL **Clinical Pearl:** Cyclophosphamide is the only agent with proven superiority over MMF in head-to-head trials for Class IV nephritis, particularly in rapidly progressive disease or when renal function is declining. ### Why Other Agents Are Not First-Choice for Refractory Class IV Nephritis | Agent | Role | Limitation | |-------|------|----------| | Rituximab | B-cell depletion; emerging role | Not FDA-approved for lupus nephritis; limited evidence for monotherapy in Class IV | | Belimumab | BLyS inhibitor; approved for SLE | Modest efficacy; not recommended for active nephritis; slower onset | | Azathioprine | Maintenance after induction | Insufficient as monotherapy for severe nephritis; used after cyclophosphamide | **Mnemonic:** **CYCLO** = **C**yclophosphamide for **C**lass IV lupus nephritis (severe, refractory). [cite:Harrison 21e Ch 319; Robbins 10e Ch 6]