Sulfonamides are synthetic bacteriostatic antibiotics Overview of Antibacterial Drugs Antibacterial drugs are derived from bacteria or molds or are synthesized de novo. Technically, “antibiotic” refers only to antimicrobials derived from bacteria or molds but is often (including... read more that competitively inhibit conversion of p-aminobenzoic acid to dihydropteroate, which bacteria need for folate synthesis and ultimately purine and DNA synthesis. Humans do not synthesize folate but acquire it in their diet, so their DNA synthesis is less affected.
Sulfonamides include the following:
Mafenide
Sulfacetamide
Sulfadiazine
Sulfadoxine
Sulfamethizole
Sulfamethoxazole
Sulfanilamide
Sulfasalazine
Sulfisoxazole
Three sulfonamides, sulfisoxazole, sulfamethizole, and sulfasalazine, are available as single drugs for oral use. Sulfamethoxazole is coformulated with trimethoprim (as TMP/SMX Trimethoprim and Sulfamethoxazole Trimethoprim is available as a single drug or in combination with sulfamethoxazole (a sulfonamide antibiotic). The drugs act synergistically to block sequential steps in bacterial folate metabolism... read more ). Sulfadoxine combined with pyrimethamine is available for oral use.
Sulfonamides available for topical use include silver sulfadiazine and mafenide burn cream, sulfanilamide vaginal cream and suppositories, and sulfacetamide ophthalmic.
Resistance
Sulfonamide resistance is widespread, and resistance to one sulfonamide indicates resistance to all.
Pharmacokinetics
Most sulfonamides are readily absorbed orally and, when applied to burns, topically. Sulfonamides are distributed throughout the body. They are metabolized mainly by the liver and excreted by the kidneys. Sulfonamides compete for bilirubin-binding sites on albumin.
Indications for Sulfonamides
Sulfonamides are active against
A broad spectrum of gram-positive and many gram-negative bacteria
Plasmodium Malaria Malaria is infection with Plasmodium species. Symptoms and signs include fever (which may be periodic), chills, rigors, sweating, diarrhea, abdominal pain, respiratory distress, confusion... read more and Toxoplasma Toxoplasmosis Toxoplasmosis is infection with Toxoplasma gondii. Symptoms range from none to benign lymphadenopathy, a mononucleosis-like illness, to life-threatening central nervous system (CNS) disease... read more
species
Sulfasalazine can be used orally for inflammatory bowel disease Irritable Bowel Syndrome (IBS) Irritable bowel syndrome is characterized by recurrent abdominal discomfort or pain with at least two of the following characteristics: relation to defecation, association with a change in frequency... read more .
Sulfonamides are most commonly used with other drugs (eg, for nocardiosis Nocardiosis Nocardiosis is an acute or chronic, often disseminated, suppurative or granulomatous infection caused by various aerobic soil saprophytes of the gram-positive bacilli genus Nocardia.... read more 
, urinary tract infection Bacterial Urinary Tract Infections Bacterial urinary tract infections (UTIs) can involve the urethra, prostate, bladder, or kidneys. Symptoms may be absent or include urinary frequency, urgency, dysuria, lower abdominal pain... read more , and chloroquine-resistant falciparum malaria Treatment of Malaria in the United States 
).
Topical sulfonamides can be used to treat the following:
Superficial ocular infections: Ophthalmic sulfacetamide
Contraindications to Sulfonamides
Sulfonamides are contraindicated in patients who have had an allergic reaction to them or who have porphyria.
Sulfonamides do not eradicate group A streptococci in patients with pharyngitis and should not be used to treat group A streptococcal pharyngitis.
Use During Pregnancy and Breastfeeding
Evidence regarding an association between sulfonamides and birth defects is mixed. Animal studies with sulfonamides show some risk, and adequate studies have not been done in pregnant women.
Use near term and in breastfeeding mothers is contraindicated, as is use in patients < 2 months of age (except as adjunctive therapy with pyrimethamine to treat congenital toxoplasmosis). If used near term during pregnancy or in neonates, these drugs increase blood levels of unconjugated bilirubin and increase risk of kernicterus Kernicterus Kernicterus is brain damage caused by unconjugated bilirubin deposition in basal ganglia and brain stem nuclei. Normally, bilirubin bound to serum albumin stays in the intravascular space. However... read more in the fetus or neonate.
Sulfonamides enter breast milk.
Adverse Effects of Sulfonamides
Adverse effects of sulfonamides can result from oral and sometimes topical sulfonamides; effects include
Hypersensitivity reactions, such as rashes, Stevens-Johnson syndrome Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) Stevens-Johnson syndrome and toxic epidermal necrolysis are severe cutaneous hypersensitivity reactions. Drugs, especially sulfa drugs, antiseizure drugs, and antibiotics, are the most common... read more
, vasculitis, serum sickness, drug fever, anaphylaxis, and angioedemaCrystalluria, oliguria, and anuria
Hematologic reactions, such as agranulocytosis, thrombocytopenia, and, in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked enzymatic defect common in people with African ancestry that can result in hemolysis after acute illnesses or intake of oxidant... read more
, hemolytic anemiaKernicterus in neonates
Photosensitivity
Neurologic effects, such as insomnia, and headache
Hypothyroidism, hepatitis, and activation of quiescent systemic lupus erythematosus may occur in patients taking sulfonamides. These drugs can exacerbate porphyrias.
Incidence of adverse effects is different for the various sulfonamides, but cross-sensitivity is common.
Sulfasalazine can reduce intestinal absorption of folate (folic acid). Thus, use of this drug may trigger folate deficiency in patients with inflammatory bowel disease, which also reduces absorption, especially if dietary intake is also inadequate.
Mafenide may cause metabolic acidosis by inhibiting carbonic anhydrase.
Dosing Considerations for Sulfonamides
To avoid crystalluria, clinicians should hydrate patients well (eg, to produce a urinary output of 1200 to 1500 mL/day). Sulfonamides can be used in patients with renal insufficiency, but peak plasma levels should be measured and sulfamethoxazole levels should not exceed 120 mcg/mL.
Sulfonamides can potentiate sulfonylureas (with consequent hypoglycemia), phenytoin (with increased adverse effects), and coumarin anticoagulants.
