Mechanism of Action and Selectivity
The action of sulfonamides is based on their structural similarity to para-aminobenzoic acid (PABA). Normally, bacteria utilize PABA and dihydropteridine to synthesize dihydropteroic acid via the enzyme dihydropteroate synthase.
Sulfonamides act as competitive antagonists to PABA, occupying its binding site on the enzyme. As a result, the formation of dihydropteroic acid and subsequently dihydrofolic acid is blocked. The bacterium loses its ability to synthesize purines, pyrimidines, and certain amino acids (e.g., methionine), halting DNA and RNA assembly. This produces a bacteriostatic effect — microorganisms stop growing and multiplying, but are not immediately killed.
These drugs are highly selective and safe for the human host. Human cells lack dihydropteroate synthase because we do not synthesize folic acid de novo, instead uptake preformed vitamins from the blood via specific membrane transporters.
Pharmacokinetic Classification
The primary criterion for classifying these agents is their absorption and elimination profile.
- Systemic (absorbable) agents (well absorbed from the GI tract):
- Short-acting (half-life up to 10 h): sulfanilamide, sulfathiazole, sulfaethidole. Require frequent administration (4–6 times daily).
- Intermediate-acting (10–24 h): sulfadiazine, sulfamethoxazole.
- Long-acting (24–48 h): sulfadimethoxine, sulfamonomethoxine. Administered 1–2 times daily.
- Ultra-long-acting (more than 48 h): sulfalene.
- GI lumen-restricted agents (poorly absorbed, achieve high local concentrations in the gut):
- Phthalylsulfathiazole.
- Sulfaguanidine.
- Topical agents (ointments, creams, eye drops):
- Sulfacetamide.
- Silver sulfadiazine.
Spectrum of Activity and Resistance
Originally, this group possessed a broad spectrum of activity, but due to widespread prolonged use, most staphylococci, pneumococci, and gonococci have developed resistance.
How bacteria adapt to the drugs:
- Overproduction of PABA (substrate hyperproduction).
- Mutations in the target enzyme structure, reducing its affinity for the drug.
- Decreased cell wall permeability.
Today, sulfonamides remain first-line agents for nocardiosis, toxoplasmosis, and tropical malaria (in combination therapies). They also retain activity against chlamydia, pneumocystis, and actinomycetes.
Pharmacokinetics and Drug Interactions
Systemic agents exhibit high bioavailability (70–100%) and penetrate all tissues, including the blood-brain barrier, serous cavities, and placenta.
- Plasma Protein Binding: Long-acting drugs bind avidly to plasma albumins. If a patient takes other highly protein-bound medications (NSAIDs, warfarin, phenytoin), these drugs can displace sulfonamides, sharply increasing their free fraction and toxicity risk.
- Metabolism: Occurs in the liver via acetylation. Acetylated metabolites lose antimicrobial activity and have very poor solubility in acidic environments.
- Excretion and Complications: Eliminated by the kidneys. If the urine becomes acidic (e.g., due to ascorbic acid supplementation), metabolites can precipitate, causing crystalluria. This is particularly common with short-acting agents.
- Antagonism: In purulent wounds and necrotic tissue debris, preformed purine and pyrimidine bases are abundant — bacteria utilize these to bypass the blocked pathway, dropping drug efficacy. Local anesthetics (procaine, benzocaine) also neutralize their effect because their hydrolysis releases PABA.