Febuxostat is a non-purine xanthine oxidase inhibitor approved for hyperuricemia. A patient with gout and CKD stage 3 is switched from allopurinol to febuxostat after a hypersensitivity reaction. Which statement correctly differentiates febuxostat from allopurinol pharmacologically?

• A Febuxostat is a non-purine xanthine oxidase inhibitor that binds a hydrophobic channel near the active site, inhibiting both oxidized and reduced forms of xanthine oxidase (mixed-type inhibition); allopurinol requires oxidation to oxypurinol to exert its effect and is renally cleared ✓
• B Febuxostat inhibits xanthine oxidase competitively (same mechanism) but is non-renally eliminated, making it preferable in CKD
• C Febuxostat inhibits urate transporter 1 (URAT1) in the proximal tubule, increasing uric acid excretion, whereas allopurinol reduces its synthesis
• D Febuxostat and allopurinol have identical mechanisms but febuxostat avoids the HLA-B*58:01-associated severe cutaneous reactions by binding a different protein

Explanation

Allopurinol is a purine analog that is oxidized by xanthine oxidase to oxypurinol (alloxanthine), which then tightly binds and inhibits the reduced (molybdenum cofactor-containing) form of xanthine oxidase. Oxypurinol is renally cleared and accumulates in CKD. Febuxostat is a non-purine selective xanthine oxidase inhibitor that fits into the molybdopterin binding site via a hydrophobic channel, forming contacts with residues outside the substrate binding site. It inhibits both oxidized and reduced forms of the enzyme (mechanism-based non-competitive inhibition). Febuxostat is metabolized hepatically (glucuronidation/oxidation) and does NOT require dose adjustment in mild-to-moderate CKD, which is its main advantage. It still carries an HLA-B*58:01 cardiovascular warning and may not be preferred over allopurinol in patients with established CVD.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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