A 35-year-old woman with uncomplicated urinary tract infection is prescribed ciprofloxacin 500 mg twice daily. She is also taking methotrexate 15 mg weekly for rheumatoid arthritis. On day 2 of fluoroquinolone therapy, she develops nausea, vomiting, and severe abdominal pain. Laboratory investigations show serum creatinine 2.8 mg/dL (baseline 0.9 mg/dL) and elevated liver enzymes. What is the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis This patient presents with acute renal dysfunction and hepatotoxicity in the setting of concurrent fluoroquinolone and methotrexate use. Ciprofloxacin reduces renal clearance of methotrexate, leading to drug accumulation and toxicity. ## Pharmacokinetic Drug Interaction ### Mechanism of Interaction **High-Yield:** Fluoroquinolones (particularly ciprofloxacin) inhibit renal tubular secretion of methotrexate via competition for organic anion transporters (OAT). This reduces methotrexate clearance by 20–30%, causing plasma accumulation and toxicity. ### Clinical Manifestations **Key Point:** Methotrexate toxicity presents with: - Gastrointestinal: nausea, vomiting, abdominal pain, mucositis - Renal: acute kidney injury (from methotrexate precipitation in renal tubules) - Hepatic: transaminitis, cholestasis - Hematologic: bone marrow suppression (may develop later) ## Management Algorithm ```mermaid flowchart TD A[FQ + MTX interaction identified]:::outcome --> B{MTX toxicity present?}:::decision B -->|Yes| C[Discontinue FQ immediately]:::action C --> D[Check MTX level & renal function]:::action D --> E[Aggressive IV hydration + urinary alkalinization]:::action E --> F[Consider folinic acid rescue if MTX level high]:::action B -->|No| G[Switch FQ to alternative class]:::action F --> H[Monitor Cr, LFTs, CBC closely]:::action G --> H H --> I[Renal recovery expected in 3-7 days]:::outcome ``` ### Step-by-Step Management 1. **Discontinue ciprofloxacin immediately** — Remove the causative agent of reduced methotrexate clearance. 2. **Check methotrexate level** — Quantify drug accumulation; levels >1 µmol/L at 48 hours post-dose indicate severe toxicity. 3. **Monitor renal function** — Serial creatinine, BUN, and urinalysis to assess degree of kidney injury. 4. **Supportive care:** - Aggressive IV hydration (0.9% saline) to promote glomerular filtration and prevent tubular precipitation - Urinary alkalinization (sodium bicarbonate) to enhance methotrexate ionization and renal excretion 5. **Folinic acid rescue** — If methotrexate level is markedly elevated (>1 µmol/L at 48 h), administer folinic acid 10–15 mg IV/IM every 6 hours to bypass the methotrexate block on dihydrofolate reductase. 6. **Close monitoring** — Repeat renal function, liver enzymes, and CBC every 24 hours until stabilization. ## Why Ciprofloxacin Must Be Stopped **Clinical Pearl:** Continuing ciprofloxacin perpetuates the pharmacokinetic interaction and prevents methotrexate clearance, risking fatal toxicity. The fluoroquinolone is not essential for uncomplicated UTI management and must be replaced with a safer alternative (e.g., nitrofurantoin, cephalexin, or amoxicillin-clavulanate). **Warning:** Do not increase the dose of ciprofloxacin—this worsens the interaction. Do not rely on supportive care alone without stopping the offending drug. ## Prognosis With prompt discontinuation and aggressive supportive care, renal function typically recovers within 3–7 days. Mortality is rare if toxicity is recognized early.