Which drugs can complex with calcium to reduce absorption? How does the concentration-time curve shift?

Which drugs can complex with calcium to reduce absorption? How does the concentration-time curve shift?

• A. Macrolides; shift up
• B. Fluoroquinolones; shift horizontal
• C. Tetracyclines; shift down
• D. Penicillins; shift up

Answer: (C)

Calcium in the gut can bind certain antibiotics to form chelates that do not get absorbed. Tetracyclines are classic examples: when taken with calcium, Ca2+ binds the drug in the intestinal lumen, creating an insoluble complex that markedly reduces its absorption. Because less drug enters the bloodstream, plasma concentrations at all times after dosing are lower, so the concentration-time curve shifts downward (lower Cmax and reduced overall exposure, AUC). The peak time (Tmax) may be similar or slightly delayed, but the key effect is a decrease in extent of absorption, not just a change in rate. Fluoroquinolones can also chelate calcium, causing reduced absorption, but the scenario described centers on the downward shift due to diminished absorption.

Calcium in the gut can bind certain antibiotics to form chelates that do not get absorbed. Tetracyclines are classic examples: when taken with calcium, Ca2+ binds the drug in the intestinal lumen, creating an insoluble complex that markedly reduces its absorption. Because less drug enters the bloodstream, plasma concentrations at all times after dosing are lower, so the concentration-time curve shifts downward (lower Cmax and reduced overall exposure, AUC). The peak time (Tmax) may be similar or slightly delayed, but the key effect is a decrease in extent of absorption, not just a change in rate. Fluoroquinolones can also chelate calcium, causing reduced absorption, but the scenario described centers on the downward shift due to diminished absorption.