Sulfonamides or Sulfa Drugs

The term sulfonamides or sulfa drugs are commonly used for a group of antimicrobial drugs that are derived from sulphanilamide. Sulfonamides are synthetic structural analogs of para-aminobenzoic acid. Sulfonamides have considerable historical value as they remained the only antibiotic drugs until the discovery of penicillins. Chemical modification of several sulfonamides has given rise to several important groups of drugs such as diuretics (thiazides and acetazolamide), antileprotic drugs (isoniazides and depsone) and oral hypoglycemic agents (sulfonyl ureas). Presently sulfonamides do not occupy a large space in therapeutics because of the availability of more effective and less toxic drugs. But they are still widely used in the treatment of urinary tract infections, acne and infections that are resistant to other antibiotics.

Unlike mammalian cells, bacterial cells synthesize their own folic acid. Folic acid is essential for the synthesis of precursors of DNA and RNA. Sulfonamides compete with PABA (para aminobenzoic acid) for the enzyme dihydropteroate synthetase and inhibit the synthesis of folic acid, thus the synthesis of bacterial DNA and RNA is also inhibited. Thus the organisms that synthesize their own folic acid are sensitive to sulfonamides. When given alone sulfonamides are bacteriostatic in action. But in combination with other folate antagonists the effect is synergistic and often bactericidal (sulfamethoxazole + trimethoprin = septran and sulfadiazine + pyrimethamine = fansidar). The antimicrobial spectrum of sulfonamides includes pneumocystitis, Chlamydia, Nocardia and selected Enterobacteria. Fansidar is the only effective therapy for toxoplasmosis. Resistance to sulfonamides may develop by increased production of PABA, by decreased affinity of sulfonamides for enzyme dihydropteroate synthetase, or by bacterial metabolism of sulfonamides. Resistance to sulfonamides is irreversible and does not involve cross resistance to other antimicrobials.

Sulfonamides are classified as oral absorption (sulfisoxazole, sulfamethizole, sulfadiazine, sulfamethoxazole, sulfadoxine etc.), oral non-absorbable (sulfasalazine etc.) and topical (mafenide, sulfacetamide). With the exception of oral non-absorbable sulfonamides, all other drugs of this group are well absorbed orally. They are widely distributed to all tissues of the body. Sulfonamides penetrate well into the CSF and also readily cross the placental barrier. They are metabolized in the liver and excreted via glomerular filtration in urine.

Now a days, sulfonamides are more commonly used in veterinary for gastrointestinal, respiratory, urinary tract, systemic and soft tissue infections. Being very soluble and rapidly excreted in urine, sulfonamides are particularly used to treat infections of the urinary tract. In humans, they are still effective in the treatment of bacterial conjunctivitis (sulfacetamide 30% solution) and burns (silver sulfadiazine and mafenide).

The commonly observed untoward reactions of sulfonamides include anorexia, nausea, vomiting, crystal urea, hematuria, acute hemolytic anemia, agranulocytosis, aplastic anemia, hypersensitivity reactions (skin rashes, dermatitis, photosensitivity and Steven-Johnsons syndrome) and kerniterus in newborns.

The dose of sulfonamides must be adjusted when given with anticoagulants, sulfonylurea and hydentoin anticonvulsants because sulfonamides can potentiate the effect of these groups of drugs.

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