A patient taking ciprofloxacin for urinary tract infection drinks milk and takes an antacid containing aluminum hydroxide with the tablet. Serum levels of the antibiotic fall markedly below therapeutic range. The mechanism responsible is:

• A Induction of intestinal CYP3A4 increasing first-pass metabolism
• B Chelation with polyvalent cations forming insoluble complexes in the gut lumen ✓
• C Alkalinization of urine accelerating renal clearance
• D Competitive inhibition of absorption transporters at the enterocyte brush border

Explanation

Fluoroquinolones chelate di- and trivalent cations such as calcium, magnesium, aluminum, iron, and zinc, forming nonabsorbable complexes in the gastrointestinal tract. Bioavailability can fall dramatically, so dosing should be separated from antacids, dairy products, and iron or zinc supplements by at least 2 hours. Fluoroquinolones are absorbed passively without transporters, ruling out option D.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.