Explain about sulphonamides

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Sulphonamides (Sulfonamides)

Sulphonamides were among the first synthetic antibacterial agents ever developed - their introduction in the 1930s marked a turning point in the treatment of bacterial infections. They remain clinically relevant today, especially in fixed combinations with dihydrofolate reductase inhibitors.

1. Chemistry & Structure

Sulphonamides are structural analogues of para-aminobenzoic acid (PABA). They share the core sulfonamide group (-SO₂NH₂) attached to an aromatic amine. Different substituents on the nitrogen of the sulfonamide moiety give rise to different compounds with varying pharmacokinetic profiles.

Chemical structures: Sulphonamides and their PABA analogue

Structures of sulfanilamide, PABA, sulfadiazine, and sulfamethoxazole

Structures of sulfonamides compared to PABA - Katzung's Basic & Clinical Pharmacology, 16th Edition

2. Mechanism of Action

Key principle: Susceptible organisms (unlike mammals) cannot use exogenous folate - they must synthesize it from PABA. Sulfonamides exploit this difference.

Sulfonamides act as competitive inhibitors of dihydropteroate synthase, the enzyme that converts PABA into dihydrofolic acid. Blocking this step prevents the downstream synthesis of tetrahydrofolate, which is required for purine synthesis and ultimately DNA replication.

The folate synthesis pathway and sites of drug action:

Sulfonamides and trimethoprim block sequential steps in the folate synthesis pathway

Actions of sulfonamides and trimethoprim on folate synthesis - Katzung's Basic & Clinical Pharmacology, 16th Edition

The action is bacteriostatic (not bactericidal) when used alone. When combined with trimethoprim (a dihydrofolate reductase inhibitor), the combination achieves sequential blockade of two steps in the same pathway, producing synergistic bactericidal activity.

3. Antimicrobial Spectrum

Sulphonamides inhibit a broad range of organisms:

Category	Examples
Gram-positive bacteria	Staphylococcus, Streptococcus
Gram-negative enteric bacteria	E. coli, Klebsiella pneumoniae, Salmonella, Shigella, Enterobacter
Other bacteria	Nocardia spp., Chlamydia trachomatis
Protozoa	Toxoplasma gondii, some malarial parasites

Organisms NOT covered:

Anaerobes (poor activity)
Pseudomonas aeruginosa (intrinsically resistant)
Rickettsiae (growth may actually be stimulated by sulphonamides)

4. Pharmacokinetics

Sulphonamides are broadly divided into three groups based on route and absorption:

A. Oral, Absorbable

Absorbed from stomach and small intestine
Distributed widely to tissues, body fluids, CNS/CSF, placenta, and fetus
Protein binding: 20% to over 90% (varies by agent)
Therapeutic blood levels: 40-100 mcg/mL
Peak levels: 2-6 hours after oral administration
Metabolism: acetylation or glucuronidation in the liver
Excretion: primarily renal (glomerular filtration); dose reduction required in renal failure

Key agents: Sulfamethoxazole (used as TMP-SMX), sulfadiazine, sulfadoxine (long-acting)

B. Oral, Nonabsorbable

Sulfasalazine - acts locally in the gut; used in inflammatory bowel disease. The sulphonamide component (sulfapyridine) is split off by colonic bacteria from the active moiety (5-aminosalicylic acid/mesalamine).

C. Topical

Sodium sulfacetamide - ophthalmic drops/ointment for bacterial conjunctivitis
Mafenide acetate - topical; used for burn wounds; inhibits carbonic anhydrase as a side effect, can cause metabolic acidosis
Silver sulfadiazine - widely used for burn wound prophylaxis; may slow wound healing

5. Resistance Mechanisms

Sulphonamide resistance can arise through several mechanisms:

Overproduction of PABA - overwhelms the competitive inhibition
Altered dihydropteroate synthase - low-affinity enzyme for sulfonamides (commonly plasmid-encoded - transferable resistance)
Impaired drug permeability - reduced uptake into bacteria
Use of exogenous folate - some bacteria lack the folate synthesis pathway entirely and import folate from the environment, making them intrinsically resistant

Many formerly susceptible species (meningococci, pneumococci, streptococci, staphylococci, gonococci) are now widely resistant.

6. Clinical Uses

Sulphonamides are rarely used as single agents today due to widespread resistance. Their main role is in fixed-dose combinations.

Clinical Use	Drug Combination
Pneumocystis jirovecii pneumonia (PCP)	TMP-SMX (drug of choice)
Toxoplasma gondii (toxoplasmosis)	Sulfadiazine + pyrimethamine (first-line)
Nocardiosis	TMP-SMX
UTIs (uncomplicated)	TMP-SMX
Acute otitis media, sinusitis	TMP-SMX
Malaria (second-line, some regions)	Sulfadoxine + pyrimethamine (Fansidar)
Inflammatory bowel disease	Sulfasalazine
Burn wound prophylaxis	Silver sulfadiazine, mafenide acetate
Bacterial conjunctivitis	Sodium sulfacetamide (ophthalmic)

7. Adverse Effects

Sulphonamides have a significant adverse effect profile:

Hypersensitivity Reactions

Fever and rashes (most common)
Morbilliform, urticarial, scarlatinal, purpuric rashes
Stevens-Johnson syndrome (SJS) / Toxic Epidermal Necrolysis (TEN) - severe, potentially life-threatening
Erythema multiforme, erythema nodosum
Serum sickness-like syndrome
Drug reaction with eosinophilia and systemic symptoms (DRESS)

Renal Toxicity

Crystalluria and obstructive nephropathy (particularly with older, less soluble sulphonamides)
Adequate hydration and alkalinization of urine help prevent this

Hematological Effects

Hemolytic anemia (especially in patients with G6PD deficiency)
Agranulocytosis
Aplastic anemia
Thrombocytopenia

Other Effects

Kernicterus in neonates - sulphonamides displace bilirubin from albumin; contraindicated in neonates and near-term pregnancy
Hepatotoxicity (rare)
Photosensitivity
GI intolerance (nausea, vomiting)
In AIDS patients on TMP-SMX: unusually high incidence of toxic effects

8. Drug Interactions

Warfarin: sulphonamides potentiate anticoagulant effect (displace from protein binding, inhibit CYP2C9)
Phenytoin: increased phenytoin levels (protein displacement + metabolism inhibition)
Sulfonylureas: cross-reactivity concerns (structurally related); risk of hypoglycemia
Methotrexate: additive bone marrow suppression
Cyclosporine: increased nephrotoxicity

9. Contraindications

Neonates and near-term pregnancy (risk of kernicterus)
Known hypersensitivity to sulphonamides
Severe renal/hepatic impairment (use with caution/dose adjustment)
G6PD deficiency (relative contraindication - risk of hemolysis)

10. Cross-Reactivity Note

Many drugs contain a sulfonamide moiety (thiazide diuretics, loop diuretics except ethacrynic acid, sulfonylureas, COX-2 inhibitors). Historically these were considered cross-allergenic with antimicrobial sulphonamides, but more recent evidence suggests cross-reactivity between antibiotic sulphonamides and non-antibiotic sulfonamide-containing drugs is uncommon, due to structural differences. Ethacrynic acid is the only loop diuretic that is NOT a sulphonamide derivative, making it a preferred choice in patients with sulphonamide allergy.

Summary Table: Key Sulphonamides

Drug	Route	Key Use
Sulfamethoxazole	Oral	Combined with TMP - broad indications
Sulfadiazine	Oral	Toxoplasmosis (with pyrimethamine)
Sulfadoxine	Oral	Long-acting; malaria (with pyrimethamine)
Sulfasalazine	Oral	IBD, rheumatoid arthritis
Silver sulfadiazine	Topical	Burn wound prophylaxis
Mafenide acetate	Topical	Burn wounds
Sodium sulfacetamide	Ophthalmic	Bacterial conjunctivitis

Sources:

Katzung's Basic and Clinical Pharmacology, 16th Edition, Chapter 46
Goodman & Gilman's The Pharmacological Basis of Therapeutics
Sherris & Ryan's Medical Microbiology, 8th Edition
Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Edition

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