Sulfonamides · Pharmacology

Why are sulfonamides bacteriostatic against many bacteria but relatively non-toxic to mammalian cells?

  1. Mammalian cells synthesize folic acid by an alternative pathway
  2. Mammalian cells use preformed folic acid from the diet
  3. The drug is rapidly metabolized by the human liver into inactive forms
  4. Sulfonamides cannot cross mammalian cell membranes
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Correct answer: Mammalian cells use preformed folic acid from the diet

Bacteria must synthesize folic acid de novo, so blocking the pathway starves them of the folate they need. Mammalian cells take up preformed folic acid from the diet and therefore do not depend on that synthetic pathway, which is why sulfonamides inhibit bacterial growth without harming the host.

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