Among the alkylating agents, which is the most common cause of hemorrhagic cystitis when used in high-dose chemotherapy regimens?

Explanation

## Alkylating Agents and Hemorrhagic Cystitis ### Cyclophosphamide and Urotoxicity **Key Point:** Cyclophosphamide is metabolized to acrolein, a toxic metabolite that concentrates in the bladder and causes direct mucosal injury, leading to hemorrhagic cystitis. **High-Yield:** Hemorrhagic cystitis is the most common dose-limiting urological toxicity of cyclophosphamide, particularly in high-dose regimens (>1.5 g/m²). It occurs in 5–40% of patients without prophylaxis. ### Mechanism of Toxicity 1. Cyclophosphamide undergoes hepatic activation to aldophosphamide 2. Aldophosphamide is converted to acrolein (the toxic metabolite) 3. Acrolein is excreted unchanged in urine 4. Direct contact with bladder urothelium causes inflammation and hemorrhage 5. Can progress to fibrosis and contracted bladder if untreated ### Prevention and Management | Strategy | Mechanism | Efficacy | |----------|-----------|----------| | Aggressive hydration | Dilutes urine, reduces acrolein concentration | Moderate | | Frequent voiding | Minimizes bladder dwell time | Moderate | | Mesna (2-mercaptoethanesulfonate) | Inactivates acrolein in bladder | High (>90%) | | Bladder catheterization | Continuous drainage in high-dose settings | High | **Clinical Pearl:** Mesna is a uroprotective agent that must be given concurrently with cyclophosphamide and for 24–48 hours after treatment. It does NOT prevent systemic toxicity—only bladder toxicity. ### Comparison with Other Alkylating Agents | Agent | Bladder Toxicity | Other Major Toxicity | |-------|------------------|----------------------| | Cyclophosphamide | **Yes (acrolein)** | Cardiac (high-dose), SIADH | | Melphalan | No | Myelosuppression, secondary malignancy | | Busulfan | No | Pulmonary fibrosis, veno-occlusive disease | | Nitrosoureas (BCNU, CCNU) | No | CNS toxicity, pulmonary fibrosis | **Mnemonic:** **CYSTO** — **CY**clophosphamide causes **CYS**titis (hemorrhagic) due to acrolein in urine.