The sulfonamides are a group of compounds with the basic formula shown earlier in this chapter. By substituting various R-radicals, a series of compounds is obtained with somewhat varying physical, pharmacologic, and antibacterial proper ties. The basic mechanism of action of all these compounds is the competitive inhibition of PABA utilization. The simultaneous use of sulfonamides with trimethoprim results in the inhibition of sequential metabolic steps and possible antibacterial synergism.

The sulfonamides are bacteriostatic for some Gram negative and Gram-positive bacteria, chlamydiae, nocardiae, and protozoa.

The “soluble” sulfonamides (eg, trisulfapyrimidines and sulfisoxazole) are readily absorbed from the intestinal tract after oral administration and are distributed in all tissues and body fluids. Most sulfonamides are excreted rapidly in the urine. Some (eg, sulfamethoxypyridazine) are excreted very slowly and thus tend to be toxic. At present, sulfonamides are particularly useful in the treatment of nocardiosis and first attacks of urinary tract infections caused by coliform bacteria. By contrast, many meningococci, shigellae, group A streptococci, and organisms causing recurrent urinary tract infections are now resistant. A mixture of five parts sulfa methoxazole plus one part trimethoprim is widely used in urinary tract infections, shigellosis, and salmonellosis and infections with other Gram-negative bacterial infections and in pneumocystis pneumonia.

Trimethoprim alone can be effective treatment for uncomplicated urinary tract infections.

Resistance

Microorganisms that do not use extracellular PABA, but similar to mammalian cells, can use preformed folic acid, are resistant to sulfonamides. In some sulfonamide-resistant mutants, the tetrahydropteroic acid synthetase has a much higher affinity for PABA than for sulfonamides. The opposite is true for sulfonamide-susceptible organisms.

Side Effects

The soluble sulfonamides may produce side effects that fall into two categories: allergy and toxicity. Many individuals develop hypersensitivity to sulfonamides after initial contact with these drugs and, on reexposure, may develop fever, hives, skin rashes, and chronic vascular diseases such as polyarteritis nodosa. Toxic effects are manifested by fever, skin rashes, gastrointestinal disturbances, depression of the bone marrow leading to anemia or agranulocytosis, hemolytic anemia, and liver and kidney function abnormalities. Toxicity is especially frequent in patients with AIDS.

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Macrolides

Chloramphenicol

Tetracyclines

Other β-Lactam Drugs

Cephalosporins

Penicillins

Many Antibiotics work by Selectively Inhibiting Protein Synthesis in Bacteria

Antimicrobial Drugs: Inhibition of nucleic acid synthesis

Antimicrobial Drugs: Inhibition of Protein Synthesis