Regarding the mechanism of action and pharmacokinetics of NSAIDs, all of the following statements are correct EXCEPT:

Explanation

## Mechanism of NSAIDs and Pharmacokinetic Principles ### COX Inhibition and Selectivity **Key Point:** Most conventional NSAIDs (ibuprofen, naproxen, indomethacin) are non-selective COX inhibitors, blocking both COX-1 and COX-2. COX-1 is constitutively expressed in platelets and gastric mucosa; COX-2 is inducible and involved in inflammation and pain. **High-Yield:** Selective COX-2 inhibitors (celecoxib, etoricoxib) reduce GI toxicity by sparing COX-1–mediated gastric cytoprotection, but increase cardiovascular risk (thrombotic events) due to unopposed thromboxane inhibition. ### Aspirin's Unique Mechanism **Clinical Pearl:** Aspirin irreversibly acetylates a serine residue (Ser529) on COX-1, permanently inactivating the enzyme in platelets. Since platelets lack a nucleus and cannot synthesize new COX-1, the antiplatelet effect persists for 7–10 days (platelet lifespan). ### Protein Binding and Elimination — THE TRAP **Warning:** The statement "NSAIDs are primarily eliminated unchanged in urine" is **FALSE**. Most NSAIDs undergo hepatic metabolism (Phase I and Phase II) before urinary excretion of metabolites. Only a small fraction is excreted unchanged. | NSAID | Hepatic Metabolism | Renal Excretion (unchanged) | Renal Excretion (metabolites) | |-------|-------------------|---------------------------|-------------------------------| | Ibuprofen | ~90% | <1% | ~90% | | Naproxen | ~95% | <1% | ~95% | | Indomethacin | ~90% | <5% | ~90% | | Aspirin | Rapid hydrolysis | ~15% | ~85% | **Key Point:** NSAIDs are indeed highly protein-bound (>90%), but they are **metabolized hepatically** and excreted as metabolites, not unchanged. This is critical in renal impairment — accumulation of metabolites can cause toxicity. ### Why the Other Statements Are Correct - **Option 0 (COX-2 selectivity and GI safety):** Correct. Celecoxib and etoricoxib reduce GI ulceration risk by ~50% vs. non-selective NSAIDs [cite:KD Tripathi 8e Ch 12]. - **Option 1 (COX inhibition pattern):** Correct. Non-selective NSAIDs block both isoforms; most show slight COX-1 preference or equipotency. - **Option 2 (Aspirin irreversibility):** Correct. Aspirin's acetylation is covalent and irreversible, explaining its prolonged antiplatelet action.