Sulfonamide antimicrobial agents

Question 1

sulfonamide history

Answer 1

prontosil was active in vivo but inactive in vitro - never would’ve been discovered today
this is because it’s a prodrug

Question 2

solfonamide MOA

Answer 2

In order to understand the mechanism of action of the sulfonamides, the biosynthesis of methylenetetrahydrofolate must be reviewed see notes
The incorporation of p-aminobenzoic acid (PABA) into the folic acid nucleus is inhibited competitively by the sulfonamides, which are bioisosteres of PABA. The sulfonamides inhibit dihydropteroate synthase. Since mammalian cells utilize preformed folates in the diet and some bacterial cells are required to make their own folic acid, the sulfonamides have selective toxicity for bacterial cells as opposed to mammalian cells.
A few strains of bacteria use sulfonamides as a substrate, but the product is not capable of undergoing the next reaction.
The antibiotic activity of sulfonamides can be reversed by adding large quantities of PABA to the diet.

Question 3

structure activity relationships of sulfonamides

Answer 3

The structure of sulfanilamide resembles that of PABA, so it is not surprising that sulfanilamide is able to inhibit the incorporation of PABA into the folic acid nucleus. However, there is a significant difference in the acidity of PABA (pKa 6.5) vs. sulfanilamide (pKa 10.4). PABA is mainly anionic at physiological pH, whereas sulfanilamide is a weak acid (the ratio of anion:acid is 1:1000 at physiological 7.4).
Early work on the synthesis of sulfanilamide derivatives established that the attachment of electron-withdrawing heteroaromatic rings acidified the sulfonamide nitrogen and enhanced the potency. For example, the pKa of sulfisoxazole is 5. The increase in acidity is due to the electronegativity of the aromatic substituent as well as resonance stabilization of the anion.
The increase in acidity also decreases the incidence of crystalluria** (crystallization of the sulfonamide in the urine, resulting in kidney damage), although it is still recommended to drink large quantities of water with some sulfonamides in order to avoid crystalluria.

Question 4

sulfonamide therapeutic uses

Answer 4

The following sulfonamides are used clinically: Sulfisoxazole, Sulfacetamide, Sulfabenzamide, Sulfadiazine, Acetyl Sulfisoxazole
(Deacetylated in GI tract), Sulfasalizine
Sulfisoxazole has broad spectrum** antibiotic activity and is currently the most popular sulfonamide. Sulfisoxazole and sulfamethoxazole are mainly used to treat simple urinary tract infections.
Sulfonamides in general inhibit both Gram-(+) and Gram-(–)** bacteria, nocardia, Chlamydia trachomatis, and some protozoa* and fungi*. Enteric bacteria such as E. coli, klebsiella, salmonella, shigella, and enterobacter, are inhibited.
Sulfonamides are usually used in combination. The resistance factors are too widespread** for these drugs to be used in a single drug therapy. The antifungal activity of trimethoprim-sulfamethoxazole
has become important for treatment of AIDS-infected patients that have developed infections caused by Pneumocystis carinii (changed to jiroveci). Trimethoprim inhibits dihydrofolate reductase, so the combination inhibits sequential steps in the biosynthesis of tetrahydrofolic acid.
Sulfisoxazole has broad spectrum antibiotic activity and is currently the most popular sulfonamide. Sulfisoxazole and sulfamethoxazole are mainly used to treat simple urinary tract infections.
Sulfasalazine is a prodrug that is not absorbed well from the GI tract. Bacteria in the GI tract metabolize it to sulfapyridine and 5-aminosalicylic acid, which has antiinflammatory activity. Sulfasalizine is used to treat ulcerative colitis and Crohn’s disease. Direct administration of salicylates, including 5-aminosalicylic acid, is irritating to the gastric mucosa. Note: Ulcerative colitis causes inflammation and ulcers in the top layer of the lining of the large intestine. In Crohn’s disease, all layers of the intestine may be involved.
Sulfadiazine in combination with pyrimethamine is used for first-line chemotherapy to treat acute toxoplasmosis. Toxoplasma gondii is similar to the pathogen that causes malaria, and infections are common and usually, but not always, subclinical.

Question 5

sulfonamide adverse reactions

Answer 5

All sulfonamides and their derivatives are cross-allergenic**. Sulfonamide derivatives in use include carbonic anhydrase inhibitors (e.g. acetazolamide), thiazides (e.g. hydrochlorothiazide), furosemide, sulfonylurea hypoglycemic agents (e.g. tolbutamide), and others.
Allergic reactions are the most common and present as rash, photosensitivity, and drug fever. Stevens-Johnson syndrome is a rare skin and mucous membrane rash that is potentially fatal. Other adverse reactions include crystalluria and hematopoietic disturbances, including hemolytic or aplastic anemia, granulocytopenia, and thrombocytopenia.
Anorexia, nausea, vomiting occurs in 1%-2% of patients.

Question 6

sulfonamide resistance

Answer 6

Resistance to sulfonamides occurs through the following mechanisms:
1) Mutations that cause overproduction of PABA
2) Mutations in the target enzyme (dihydropteroate synthase) that decrease its affinity for the sulfonamides
3) Mutations that result in a decrease in cell permeability to the sulfonamides
Note: resistance to sulfonamides is very common and this has largely eliminated their effectiveness as single-use agents. Resistance to combination therapy targeting two enzymes is highly reduced relative to mono-therapy, since to survive the microorganism would have to carry mutations in two enzymes.
Resistance to trimethoprim** is on the rise as indicated by a plasmid-borne copy of the dihydrofolate reductase (DHFR) gene. The DHFR gene often bears a mutation that enables the expressed enzyme to be active in the presence of drug through a decreased binding affinity of trimethoprim. BACTRIM is composed of 400 mg sulfamethoxazole (SMX) and 80 mg trimethoprim (TMP).

Question 7

sulfonamide PKs

Answer 7

TMP is absorbed (85-90%) and distributed more rapidly than sulfonamides and dosages reflect this fact. Peak plasma levels are 2 µg/mL after 3 hours and the T1/2 is 10-12 hours. This drug and its inactive oxidized metabolites are cleared in the urine.
SMX is widely distributed in the body including the CSF and is also rapidly eliminated. It reaches peak serum levels of 30-60 µg/mL 3 hours after an oral dose of the combination therapy. The T1/2 is similar to TMP.
SMX is not as widely distributed as TMP because of the differences in lipophilicity. In tissue, the ratio of SMX/TMP is 1:2 to 1:5 indicating the preferential distribution of TMP to tissues relative to serum.

Question 8

sulfoniamide metabolism

Answer 8

The sulfonamides are generally metabolized by N-4 N-acetylation and in some cases N-1 glucuronidation. The metabolites have no antibiotic activity. Hydroxylamine and nitroso metabolites are toxic.
Note: the human population can be divided into rapid and slow acetylators. This affects the rate of metabolism of sulfonamides in humans.

Question 9

colistin

Answer 9

(polymyxin E) is a polymyxin antibiotic metabolite of Bacillus polymyxa. It is a nephrotoxic drug that is used as a last resort for treatment of multidrug-resistant Pseudomonas aeruginosa, Acinetobacter, and Klebsiella pneumoniae. Mechanism: colistin is a polycation that has lipophilic regions, so the overall structure resembles that of a cationic detergent that is able to solubilize bacterial membranes. It contains 5 unusual 2,4-diaminobutanoic acid residues that are protonated at physiological pH. The ammonium cations are able to displace cations in the bacterial cell membrane (Mg++ and Ca++) and facilitate binding of the antibiotic to anionic lipopolysaccharides in the cell membrane.

Question 10

metronidazole

Answer 10

Metronidazole is useful for treatment of anaerobic bacteria and protozoa. It is the drug of choice for treatment of first episodes of mild to moderate C. difficile infections. Mechanism: partial reduction of the nitro group in anaerobic bacteria leads to a radical anion that degrades bacterial DNA. Since this happens only in anaerobic cells and not human cells, metronidazole has selective cytotoxicity for anaerobes.

Question 11

disinfectant

Answer 11

a compound that kills the vegetative form of microorganisms, but not spores, on inanimate surfaces

Question 12

sterilant

Answer 12

a compound that kills or removes all types of living microorganisms, including spores and viruses.

Question 13

antiseptic

Answer 13

a compound that is applied to living tissue for the purpose of preventing infection.

Question 14

alcohols MOA

Answer 14

Alcohols denature proteins.

Question 15

Use of alcohols

Answer 15

Alcohols are used as antiseptics and disinfectants. The optimum bactericidal concentration is 60-90% by volume in water. They are not sterilants because they are not sporicidal. HIV is inactivated within 1 min by exposure to 70% ethanol or isopropanol. Note: absolute ethanol is less bactericidal than 60-90% ethanol because proteins are denatured more rapidly in the presence of water.

Question 16

chlorhexidine MOA

Answer 16

Chlorhexidine strongly adsorbs to bacterial membranes, causing leakage of small molecules, and it also causes precipitation of cytoplasmic proteins.

Question 17

Chlorhexidine use

Answer 17

Chlorhexidine digluconate is used in water-based formulations as a topical antiseptic. It is active at pH 5.5-7.0 against vegetative bacteria and has moderate activity vs. fungi and viruses. It inhibits germination of spores. It is resistant to inactivation by blood and organic materials, and its capacity to sensitize or irritate skin is low. It is not absorbed through the skin or mucous membranes.

Question 18

Iodine MOA

Answer 18

Iodine iodinates phenylalanyl and tyrosyl groups in proteins and oxidizes sulfhydryl groups in proteins.

Question 19

Iodine use

Answer 19

Iodine in 1:20,000 dilution is bactericidal in 1 min and kills spores in 15 min. Tincture of iodine USP contains 2% iodine and 2.4% sodium iodide in alcohol, and is the most active antiseptic for intact skin. However, it is not used often because hypersensitivity reactions may occur, and iodine stains clothing

Question 20

povidone-iodine

Answer 20

betadine
Povidone-iodine is a water-soluble, non-covalent complex of iodine with polyvinyl pyrrolidone. The amount of iodine is low, but is released when the complex is diluted. Povidone-iodine is a member of a class of compounds known as "iodophores", which are complexes of iodine with surface active agents.

Question 21

povidone-iodine MOA

Answer 21

The action is due to the free iodine that is liberated from the complex.

Question 22

povidine-iodine use

Answer 22

Povidone-iodine is an antiseptic that is available in aerosols, ointments, surgical scrubs, antiseptic gauze pads, sponges, and mouthwashes. It kills bacteria, fungi, and lipid-containing viruses, and prolonged exposure can kill spores. The main advantage of povidone-iodine vs. iodine is that it is less irritating and less likely to produce skin hypersensitivity reactions. It is also a nontoxic, nonvolatile, and nonstaining form of iodine

Question 23

bleach

Answer 23

Hypochlorous acid is the active germicidal species that forms when chlorine is dissolved in water
Rapid evolution of irritating and toxic chlorine gas occurs when sodium hypochlorite is mixed with acid or urine (which is acidic). Ammonia in urine can react with bleach to form chloramine (ClNH2), which is toxic.
Dilutions of 5.25% sodium hypochlorite are stable for months in tap water at pH 7.5-8.0.

Question 24

bleach MOA

Answer 24

A variety of functional groups present in proteins and nucleic acids are oxidized by hypochlorous acid.

Question 25

bleach use

Answer 25

Chlorine and its derivatives have been used to disinfect water for more than a century. A 10:1 dilution of 5.25% sodium hypochlorite (household bleach), which contains 5000 ppm of available chlorine, is recommended by the Centers for Disease Control and Prevention for disinfection of blood spills.
Various organic compounds slowly liberate hypochlorous acid in water. Examples: halazone and chloroazodin
The sodium salt of halazone is used to disinfect small quantities of drinking water.
However, halazone has been largely replaced by sodium dichloroisocyanurate (NaDCC) because halazone is unstable after the bottle has been opened. In opened bottles, halazone decomposes within 3 days.
Dilute solutions of chloroazodin are used for dressing wounds, lavage and irrigation.
Oxychlorosene is a non-covalent complex of a sodium sulfonate with hypochlorous acid which slowly liberates HOCl in solution. It has rapid cidal action against most microorganisms, including both gram-positive and gram-negative bacteria, molds, yeasts, viruses, and spores. It combines the germicidal properties of HOCl with the emulsifying, wetting, and keratolytic properties of an anionic detergent. It is used as an antiseptic to treat local infections, to remove necrotic tissue in massive infections, and to treats wounds.
Oxychlorosene is supplied as a powder for dilution in water. The usual applications employ 0.1% to 0.5% concentrations in water.

Question 26

phenols MOA

Answer 26

Phenols disrupt the cell wall and membranes, precipitate proteins, and inactivate enzymes.

Question 27

phenols use

Answer 27

Phenols are bactericidal, fungicidal, and inactivate lipophilic viruses. They are not sporicidal. They are used to disinfect hard surfaces (eg. floors, bench tops, beds).

Question 28

quaternary ammonium compounds

Answer 28

These are cationic surfactants characterized by 1)
a cationic head, and 2) a long hydrocarbon tail. Examples:
Lauryl Triethylammonium Chloride, Benzalkonium Chloride, USP (R = C-8 and higher homologues), Cetylpyridinium Chloride, USP

Question 29

quaternary ammonium compounds MOA

Answer 29

The bactericidal action of quaternary ammonium compounds has been attributed to inactivation of energy-producing enzymes, denaturation of proteins, and disruption of cell membranes.

Question 30

quaternary ammonium compounds use

Answer 30

The quaternary ammonium compounds are fungistatic, sporistatic, inhibit algal growth, and are bactericidal for gram-positive** bacteria and moderately active against gram-negative bacteria**. Lipophilic viruses are inactivated. They are not tuberculocidal or sporicidal, and they do not inactivate hydrophilic viruses. They are inactivated by soaps, and they bind to fibers in mops and paper towels used to apply them. They are used to reduce microbial load on inanimate surfaces (e.g. floors and bench tops) to levels considered safe for public health purposes (i.e. they are sanitizers).
Note: quaternary ammonium compounds should NOT be used as antiseptics because they may contain infectious gram-negative bacteria (e.g. Pseudomonas) that in fact have occasionally caused outbreaks of infection.

Question 31

aldehydes MOA

Answer 31

Aldehydes react with amino groups in proteins and nucleic acids.

Question 32

aldehydes use

Answer 32

These agents have a broad spectrum of activity against bacteria, fungi, and viruses. They are used to sterilize instruments such as fiberoptic endoscopes that cannot be sterilized by steam in an autoclave.
Note: OSHA has declared formaldehyde to be a carcinogen and limited occupational exposure. Protection from exposure to glutaraldehyde is advisable.

Question 33

peroxygen compounds

Answer 33

The peroxygen compounds are potent and have a broad spectrum of activity against bacteria, spores, viruses, and fungi. They are powerful oxidizers that are used mainly as disinfectants and sterilants.
Hydrogen peroxide has been used to sterilize respirators, acrylic resin implants, milk and juice cartons, plastic eating utensils, and contact lenses. Concentrations of 10-25% hydrogen peroxide are sporicidal.
Peracetic acid is more potent than hydrogen peroxide as a bactericidal and sporicidal agent. An automated machine is available that sterilizes instruments with peracetic acid at 0.1- 0.5% concentration. It is used to sterilize equipment in the food and beverage industry because its breakdown products (water and acetic acid) in high dilution are safe and do not smell or taste bad.

Question 34

ethylene oxide MOA

Answer 34

Ethylene oxide reacts covalently with a variety of nucleophilic groups present in biological systems

Question 35

ethylene oxide use

Answer 35

Ethylene oxide is used to sterilize equipment that cannot be sterilized by heat in an autoclave (e.g. instruments with lenses, plastic, or rubber). It is used as a gas at 45-60 o
C and 30-60% relative humidity, diluted with carbon dioxide or fluorocarbons to decrease the explosion hazard. It readily diffuses through porous material and rapidly destroys all forms of microorganisms.
Since ethylene oxide has been classified as a mutagen and a carcinogen, alternative sterilants are being used more frequently. Personnel using ethylene oxide should use a gas mask and avoid exposure to skin and mucous membranes.

Question 36

heavy metals

Answer 36

Mercury and silver are now rarely used as disinfectants. Mercury is toxic and is an environmental hazard, and the use of silver nitrate to prevent gonococcal ophthalmitis in newborns has been replaced by antibiotic ointments. However, thimerosal 0.00012-0.010% is still used as a preservative in immune sera, vaccines, and antitoxins.

Question 37

preservatives

Answer 37

Various compounds are added to liquid dosage forms and cosmetic preparations to prevent microbial contamination.
Methylparaben is more active against molds, while propylparaben is more active vs. yeasts. Esters of p-hydroxybenzoic acid are generally not toxic because they are rapidly hydrolyzed to the corresponding benzoic acid derivatives, which are rapidly conjugated and excreted.