A patient on methotrexate for rheumatoid arthritis is prescribed NSAIDs for joint pain. Which mechanism best explains the increased risk of methotrexate toxicity in this combination?

Explanation

## Methotrexate–NSAID Interaction ### Mechanism of Toxicity **Key Point:** Methotrexate is a weak acid that undergoes active renal tubular secretion and glomerular filtration. NSAIDs reduce renal blood flow and inhibit prostaglandin-mediated renal function, decreasing methotrexate clearance and increasing its plasma concentration. ### Why Renal Clearance Reduction Is the Primary Mechanism Methotrexate elimination depends critically on renal function: 1. **Glomerular filtration** — accounts for ~90% of renal clearance 2. **Active tubular secretion** — via organic acid transporters (OAT) 3. NSAIDs decrease renal perfusion by inhibiting renal prostaglandins (PGE~2~, PGI~2~) 4. Result: ↓ GFR and ↓ tubular secretion → ↑ methotrexate levels → toxicity (bone marrow suppression, mucositis, nephrotoxicity) **High-Yield:** This interaction is particularly dangerous because methotrexate has a narrow therapeutic window. Even modest reductions in renal clearance can push plasma levels into the toxic range. **Clinical Pearl:** NSAIDs should be avoided in patients on methotrexate. If analgesia is needed, acetaminophen or selective COX-2 inhibitors (with caution) are preferred. Renal function (creatinine clearance) must be monitored regularly in patients on methotrexate. ### Why Other Mechanisms Are Incorrect | Mechanism | Why Wrong | |-----------|----------| | **CYP3A4 inhibition** | Methotrexate is NOT significantly metabolized by CYP3A4; it undergoes renal elimination and minimal hepatic metabolism. | | **Plasma protein displacement** | Methotrexate is only ~50% protein-bound; displacement would have minor effect on total clearance. | | **Increased GI absorption** | NSAIDs do not enhance methotrexate absorption; absorption is not the rate-limiting step in methotrexate clearance. | **Mnemonic:** **RENAL CLEARANCE** — Remember that methotrexate is primarily cleared by the kidneys, and NSAIDs impair renal function by reducing prostaglandin-mediated renal perfusion.