Sulphonamides
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Sulphonamides are derivatives of sulphanilamide, an active metabolite of the pro-drug and dye prontosil.
Mechanism of Action
Sulphonamides act by competing with an essential precursor in folic acid synthesis in bacteria. Bacteria need to synthesise folic acid in order to grow as they are unable to obtain it from their 'diet' like mammals can. Since the bacteria is unable to synthesise RNA or DNA, due to it's lack of folic acid, it's growth is inhibited. As such sulphonamides are bacteriostatic.
Spectrum of Activity
These are generally broad spectrum antibiotics.
- They are active against aerobic gram-positive cocci and some rods.
- They are active against gram-negative rods, including enterobacteriaceae.
- They are active against the anaerobes, Actinomyces and Fusobacterium, but inactive against clostridial species and anaerobic cocci.
- Reistance is now widespread.
- Drugs which contain a PABA nuclear core, such as procaine, antagonise their actions.
Pharmacokinetic Considerations
Sulphonamides: Lipophilic weak acids of varying pKa. • Orally active. Extent of plasma protein binding varies between individual drugs and between species. • Generally distribute very widely in body. • Rate of metabolism and rate of renal excretion varies with species and with urine pH. Not actibe in pus