Living Textbook MC610

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Before antibiotics became widely used, Bayer was interested in the idea of developing antibacterials, around the turn of the century. These plans were however placed on the back burner when two drugs were touted to be the next huge marketing agents, the less popular Aspirin and the blockbuster, Heroin. So it wasn't until the thirties that Domagk took Ehlrich's ideas of using dyes as antimicrobial agents and expanded it to azodyes and came up with Prontosil. His idea was that these dyes would work like Gram stains, taken up by bacteria but not mammalian cells.

It showed a remarkable activity against streptococcal infections. But when he tried it on isolated bacterial cells, it showed no activity? That was very puzzling to him, so he set out to investigate the problem. What he found out was not only important in the field of Sulfonamides, but affected the whole medical community. He found out that Prontosil is not active by itself, but that it is metabolized by reductases to give the active form of the sulfonamide. Thus came up with the principle of prodrugs in medicine.

The irony is that Sulfanilamide was synthesized in 1908 but never tested for antibacterial activity. This discovery prompted a huge interest of this group of drugs, and researchers proceeded to synthesize over 4500 sulfonamides by 1948. They are still an attractive group of drugs since they are very cheap.

Mechanism of action:

When they were first discovered, their mechanism of action was unknown. Observations that they structurally resemble Para amino benzoic acid (PABA) prompted some scientists to suggest a connection. But it wasn't till the role of PABA in living cells was fully elucidated that the mechanism of action of Sulfonamides was fully understood.

They are synthetic compounds that get their specificity from depletion of folic acid stores in bacterial cells, while mammalian cells are unaffected since they obtain their folic acid from diet. Folic acid is important as a one carbon source in many essential biochemical pathways. Its biosynthesis involves the synthesis of Dihydropteroic acid from Pteridine and PABA, a step catalyzed by Dihydropteroate Synthase (DHPS). Several steps ensue resulting in the formation of Dihydrofolic acid, that is finally reduced to Tetrahydrofolic acid via the enzyme Dihydrofolarte Reductase (DHFR)

In most bacterial species Sulfonamides are competitive inhibitors of Dihydropteroate Synthase (DHPS) , an enzyme present exclusively in bacterial cells. In few species they are incorporated in place of PABA in the enzyme active site leading to faulty products. In both cases, the resulting low Folic acid pool will have many damaging effects including failure in biosynthesis of Purine and Thymylidate nucleotides (as Folic acid supplies carbons during their biosynthesis) and eventually inhibiting DNA synthesis.
They are Bacteriostatic in nature and have a broad spectrum, but show poor activity against pseudomonas, enterococci and anaerobes. They are slow to act, since several generations are needed before appreciable depletion of folate pool and inhibition of growth. They are considered antimetabolites.

Ionization and Lipohilicity:

The activity of Sulfonamides depends on their lipophilicity, which determines ability to get inside the bacterial cell and their ionization at physiological pH, since they have to be ionized to resemble PABA, to be better incorporated into the active site of the enzyme and to cause its inhibition.
The pKa of the anilide amine is around 2.3 in both Sulfonamides and PABA.
The pKa of the carboxylic acid in PABA is around 4.9, while the acidic Sulfonamides has a pKa around 10.4, thus modification had to be made to lower this value. Addition of electron withdrawing groups and more successfully heterocyclic rings that cause extended resonance achieved this goal. While theoretically a pKa of 6.4 - 7.4 would yield effective agents, agents with lower pKas show higher effectiveness.

Structural Activity Relationship:

The amine and sulfonamide groups have to be para to each other.

The amine has to be unsubstituted.

If the benzene ring is changed to any other ring, the compound loses activity.

The amide is less active than the sulfonamide.

Changes are allowed on the sulfonamide side of the drug. For example, Monosubstitution on the sulfonamide may increase activity and modify pharmacokinetic properties, while Disubstitution will abolish activity.


They are well absorbed orally and are excreted in urine unchanged or as the inactive acetylated form. They show high degree of binding to plasma proteins. They diffuse freely throughout the body tissues and may be detected in the urine, saliva, sweat, and bile, in the cerebrospinal, peritoneal, ocular, and synovial fluids, and in pleural and other effusions. They cross the placenta into the fetal circulation and low concentrations have been detected in breast milk. Some sulfonamides are not absorbed orally and are used in GIT infections.

Resistance usually occurs by simply increasing the concentration of PABA or a mutation in the enzyme structure to better distinguish between PABA and the drug. Decreased permeability to the drugs have been observed in some bacteria. These mechanisms will lead to cross resistance among most all sulfonamides.

Some bacteria are intrinsically resistant since they can uptake Folic acid from their surroundings.

Metabolism is mainly through glucurodination and the N-acetylation of the 4-amine. A minor route involves oxidation of the 4-amine to the hydroxyl amine. If this metabolite is not neutralized by glutathione, it can further form a nitroso compoud that can lead to both hepatotoxicity and immunological hypersenstivity reactions. This is particularly problematic in slow acetylators and G6PD patients.

Crystalluria, which can potentially lead to kidney damage, can be a serious side effect. The reason for this problem is that unsubstituted sulfonamides will have low solubility around pH = 6 - 7. Earlier solutions were to increase urine flow (give plenty of fluids), increase the pH of urine to around 10 using sodium bicarbonate, or use a combination of lower doses of sulfonamides where each has its own solubility. Newer agents are less susciptible to causing such side effect, but patients are still advised to drink plenty of fluids.

Other side effects include headaches, GI discomforts, photosensitivity and rashes. Less common are kidney and liver damage with fever and various blood related problems. A rare but potentially fatal side effect is known as Stevens-Johnson Syndrome that leads to ulcerations of various mucosal membranes. As with other antimicrobials, these agents may lead to pseudomembranous colitis

Side effects where specific populations are at higher risk include hemolytic anemia in G6PD deficient patients and kernicterus in neonates.

DDIs seen with these agents may be due to displacing other drugs from serum albumin or due to their abiliy to inhibit CYP2C9. Caution should be observed when using with warfarin, methotrexate, sulfonylureas and some anticonvulsants.

Individual Agents:

Sulfisoxazole is well absorbed, with a half life of around 6 hrs. The sulfonamide end has a low pKa of 5.0. It is used in ophthalmic preparations and was used in otitis media in combinations with erythromycin.

Sulfadiazine is readily absorbed and has a 17 hr half life (medium acting agent). Its sulfonamide end has pKa around 6.5. It is used orally, as IV preparations as the sodium salt, and topically in second and third degree burns as the silver salt.

Sulfacetamide has a low pKa around 5.4. It is mainly used in ophthalmic preparations, where it shows great penetration into different ocular tissues and fluids.

Sulfamethizole is readily absorbed from the gastrointestinal tract with a short half-life of 1.5 to 3 hours. The sulfonamide's pka is around 5.5. It is unsuitable for systemic use as only relatively low concentrations of drug are achieved in the blood and tissues.

Sulfametoxazole is readily absorbed from the gastrointestinal tract and peak plasma concentrations are reached after about 2 hours. About 70% is bound to plasma proteins. The plasma half-life is about 6 to 12 hours (intermediate acting agent) and the sulfonamide pKa is around 5.6.

Sulfadoxine has a half life of 4 - 9 days, and thus is a long acting agent. The sulfonamide pKa is around 6.1. It is used in combinations with Pyrimethamine.

Dapsone is a sulfone that shares the mechanism of action of sulfonamides. It also have anti-inflammatory action as it may inhibit neutrophils.

Sulfasalazine is a prodrug, where the active ingredient is thought to be 5-aminosalicylic acid and not the sulfonamide portion of the molecule. It is poorly absorbed, so it used mainly for its local GIT effects.

Dihydrofolate reductase (DHFR) inhibitors, such as Methotrexate (anticancer agent) and Pyrimethamine (antimalarial agent) have been known for a while when George Hitchings and Gretrude Elion were able to synthesize Trimethoprim, an agent that inhibits the same enzyme. However this drug is specific to the bacterial enzyme (IC50 in bacteria is 0.1 µM compared to 300 µM in mammalian cells). Resistance to these agents arise from mutations to DHFR.

It was initially used as a single agent in uncomplicated UTI. It has been suggested that a combination of a sulfonamide with trimethoprim will have a synergistic effect since they act on successive steps in the same biochemical pathway. If the bacteria would overcome one inhibited enzymatic step, it would be extremely unlikely that it can pass the second. In addition it is highly unlikely that resistance to both drugs would arise.

Bactrim is an example of a preparation with a fixed ratio one part trimethoprim to 5 parts of sulfamethoxazole and is currently the major sulfonamide contanining preparation used clinically. Sulfamethoxazole is used in this preparation as it has an approximately equal half-life to that of trimethoprim (11 hours). While this combination is bactericidal, a precaution that should be observed is that both agents may inhibit CYP2C9.

Clinical Applications:

General: Bacteriostatic (except Bactrim®) with a broad spectrum.


Coverage includes mycobacterium spp.

Indications and Uses: It is the drug of choice for dermatitis herpetiformis and leprosy.  It is used in acne vulgaris.


Coverage includes parasitic agents.

Indications and Uses: Topically as the silver salt for burns and as an adjunct for the prevention and treatment of wound sepsis in patients with second and third-degree burns.

Orally indicated in conjunctivitis, otitis media, rheumatic fever, UTI and meningitis.  It is also used as adjunctive therapy in malaria.


Indications and Uses: Topically used in eye infections including conjunctivitis and injuries, seborrheic dermatitis, acne vulgaris, and acne rosacea.


Coverage include Pneumocystis jiroveci and parasitic agents

Indications and Uses: Often used in combinations with pyrimethamine, as a second line in the prophylaxis of Pneumocystis pneumonia (PCP) in AIDS patients, and as an antimalarial agent.


Indications and Uses: Indicated for ulcerative colitis, Crohn’s disease and rheumatoid arthritis.


Coverage includes Gram positive organisms Staphylococcus aureus (including MRSA) and Streptococcalspp.* and Gram negative organisms E. coli*.  However, resistance practically eliminates its use in Streptococcal spp. such as S. pneumoniae, and in many cases in E. Coli.

Non-susceptible organisms include Gram negative Pseudomonas aeruginosa, anaerobes Bacteroides fragilis and Gram positive enterococcus spp.

Indications and Uses: Urinary tract infections (including empiric therapy), chronic bronchitis, acute otitis media, PCP in AIDS patients, traveler’s diarrhea, community-acquired MRSA skin infections, nocardia (caused by a Gram positive aerobic actinomycetes spp.) and chlamydia trachomatis (caused by Gram negative bacteria)