Sulphonamide Mechanism Action | Classification 

Sulphonamide Mechanism Action | Classification 

The term sulphonamides are employed as a generic name for the derivatives of para amino benzene sulphonamide (sulphanilamide). The sulphonamide drugs were the first effective chemotherapeutic agents to be employed systemically for the prevention and treatment of bacterial infections in humans.

The sulphonamides are bacteriostatic antibiotics with a wide spectrum action against most gram-positive bacteria and many gram-negative organisms. Actually it was found to be the metabolic product of Prontosil, which is responsible for antibacterial activity, and this has given the initiation to develop sulphonamides as antibacterial agents.

Sulphonamides are total synthetic substances that are produced by relatively simple chemical synthesis. The advent of penicillin and, subsequently of other antibiotics has diminished the usefulness of sulphonamides. Antimicrobial compounds contain sulphonamide (SO2NH2) group. This group (SO2NH) is also present in other compounds, such as antidiabetic agents (e.g. Tolubutamide), diuretics (e.g. chlorthiazide and its congeners, furosemide, and acetazolamide), and anticonvulsants such as sulthiame.

The sulphonamides exists as white powder, mildly acidic in character, and they form water-soluble salts with bases. The pH of sodium salts with some exception, for example, sodium sulphacetamide, is very high when given intramuscular (IM), the marked alkalinity causes damage to the tissues.

Microorganisms that may be susceptible in vitro to sulphonamides include Streptococcus pyogens, Streptococcus pneumoniae, Haemophilus infl uenzae, H. ducreyi, Nocardia, Actinomyces, Calymmatobacterium granulomatis, and Chlamydia trachomatis. The minimal inhibitory concentration ranges from 0.1 μg/ml for C. trachomatis to 4–64 μg/ml for E. coli. Sulphonamides are selective drugs used to treat urinary tract infections, bacterial respiratory infections, and gastrointestinal (GI) infections.

Mechansim of Action of Sulphonamide

Sulphonamides are structure analogues and competitive antagonists of para-amino benzoic acid (PABA). They inhibit dihydropteroate synthetase, the bacterial enzyme responsible for the incorpora- tion of PABA into dihydropteric acid, and it is the intermediate precursor of folic acid. Synergistic effect is obtained by a combination of trimethoprim. The compound trimethoprim is a potent and selective inhibitor of microbial dihydrofolate reductase, the enzyme that reduces dihydrofolate to tetrahydrofolate. The simultaneous administration of sulphonamide and trimethoprim blocks the pathway of cell-wall synthesis sequentially.

Classification of Sulphonamide 

Sulphonamides can be classifi ed in various ways:

On the basis of the site of action

• Sulphonamides for general infection: Sulphanilamide, Sulphapyridine, Sulphadiazine, Sulphamethoxacine, Sulphamethoxazole.
• Sulphonamides for urinary tract infections: Sulphaisoxazole, Sulphathiazole.
• Sulphonamides for intestinal infections: Phthalylsulphathiazole, Succinyl sulphathiazole, Sulphasalazine.
• Sulphonamides for local infections: Sulpahacetamide, Mafenamide, Silver sulphadiazine.
• Sulphonamides for dermatitis: Dapsone, Solapsone.
• Sulphonamides in combination: Trimethoprim with Sulphamethoxazole.

On the basis of the pharmacokinetic properties

• Poorly absorbed sulphonamides (locally acting sulphonamides)—Sulphasalazine, Phthalylsulphathiazole, Sulphaguanidine, Salicylazo sulphapyridine, Succinyl sulpha thiazole.
• Rapidly absorbed and rapidly excreted (systemic sulphanamides): Sulphamethoxazole, Sulphaisoxazole, Sulphadiazine, Sulphadimidine, Sulphafurazole, Sulphasomidine, Sulphamethiazole, Sulphacetamide Sulphachlorpyridazine.
• Topically used sulphonamides: Sulphacetamide, Mafenide, Sulphathiazole, Silver sulphadiazine.

On the basis of the pharamacological activity

• Antibacterial agents:Sulphadiazine, Sulfi soxazole.
• Drugs used in dermatitis: Dapsone.

On the basis of the duration of action

• Extra long-acting sulphonamides (half-life greater than 50 h): Sulphasalazine, Sulphaclomide, Sulphalene.
• Long-acting sulphonamides (half-life greater than 24 h):Sulphadoxine, Sulphadimethoxine, Sulphamethoxy pyridazine, Sulphamethoxydiazine, Sulphaphenazole, Sulphamethoxine.
• Intermediate-acting sulphonamides (half-life between 10–24 h): Sulphasomizole, Sulphamethoxazole.
• Short-acting sulphonamides (half-life less than 20 h): Sulphamethiazole, sulphaisoxazole.
• Injectables (soluble sulpha drugs): Sulphafurazole, Sulphadiazine, Sulphamethoxine.

On the basis of the chemical structure

• N-substituted sulphonamide:Sulphadiazine, Sulphacetamide, Sulphadimidine.
• N-4 substituted sulphonamides (prodrugs): Prontosil.
• Both N-1 and N-4 substituted sulphonamides: Succinyl sulphathiazole, Phthalylsulphathiazole.
• Miscellaneous: Mefenide sodium.

• Microsomal Enzyme Induction 
• SAR of Morphine (Structure Activity Relationship of Morphine)

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