Sulphonamides are one of the earliest synthetic antibacterial drugs. They act by inhibiting folic acid synthesis in bacteria, preventing growth and multiplication. Sulphones are chemically related compounds with similar mechanisms but are mainly used in chronic infections like leprosy.
Introduction
Bacteria need folic acid for DNA, RNA, and protein synthesis. Humans obtain folic acid from the diet, but bacteria must synthesize it. Sulphonamides block this bacterial folic acid pathway, making them selectively toxic to microbes.
Mechanism of Action
- Sulphonamides compete with para-aminobenzoic acid (PABA), a key precursor needed to produce folic acid.
- They inhibit the enzyme dihydropteroate synthase.
- This blocks the formation of folic acid, stopping bacterial growth (bacteriostatic action).
Classification of Sulphonamides
- Short-acting sulphonamides – e.g., Sulfisoxazole
- Intermediate-acting – e.g., Sulfamethoxazole
- Long-acting – e.g., Sulfadoxine
- Topical sulphonamides – e.g., Silver sulfadiazine
- Combined preparations – e.g., Co-trimoxazole (Trimethoprim + Sulfamethoxazole)
Important Drug Profiles
1) Sulfisoxazole
Use: Urinary tract infections (UTIs) and pediatric infections.
Note: High solubility prevents crystalluria.
2) Sulfamethoxazole
Commonly used with trimethoprim.
Use: Respiratory infections, UTIs, and gastrointestinal infections.
3) Silver Sulfadiazine
A topical antibacterial agent.
Uses: Preventing and treating burns and wound infections.
Mechanism: Silver ions enhance antimicrobial activity.
4) Sulfadoxine
Long-acting sulphonamide.
Use: Combined with pyrimethamine for malaria treatment (Fansidar).
5) Co-trimoxazole (Trimethoprim + Sulfamethoxazole)
This fixed-dose combination provides a synergistic effect.
Mechanism: Sequential blockade of folic acid synthesis.
Uses: UTIs, pneumonia, bronchitis, and certain protozoal infections.
Sulphones
Sulphones are structurally related to sulphonamides but have unique therapeutic uses. The most important sulphone is Dapsone.
Mechanism of Action
Dapsone also inhibits folic acid synthesis, similar to sulphonamides, making it bacteriostatic.
Drug Profile: Dapsone
- Uses: Primary drug for leprosy therapy.
- Mechanism: Inhibits folate synthesis in Mycobacterium leprae.
- Other uses: Dermatitis herpetiformis, Pneumocystis pneumonia (as part of combination therapy).
Combination Therapy in Leprosy
Dapsone is given along with rifampicin and clofazimine to prevent resistance.
Therapeutic Uses of Sulphonamides
- Urinary tract infections
- Respiratory tract infections
- Burn wound prophylaxis
- Toxoplasmosis (with pyrimethamine)
- Malaria (with pyrimethamine or sulfadoxine)
Adverse Effects of Sulphonamides
- Allergic reactions (skin rash, fever)
- Crystalluria (precipitation in urine)
- Hemolysis in G6PD-deficient patients
- Nausea, vomiting
- Kernicterus in newborns – avoided in pregnancy
Adverse Effects of Sulphones
- Hemolytic anemia (especially in G6PD deficiency)
- Skin rashes
- Peripheral neuropathy (rare)
Detailed Notes:
For PDF style full-color notes, open the complete study material below: