1

Sulfonamides

• In1930s, Domagk took Ehlrich's ideas (Bayer’s plan) 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.

NH2N

N S

O

O

NH2

NH2

Prontosil

2

Sulfonamides

• It showed a remarkable activity against streptococcal

infections.

• But when he tried it on isolated bacterial cells, it showed

no activity?

• Later, 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.

• Sulfanilamide was synthesized in 1908 but never tested

for antibacterial activity.

3

Sulfonamides

NH2N

N S

O

O

NH2

NH2

Liver Reductase

H2N S

O

O

NH2

Sulfanilamide

Prontosil

4

MOA• They are synthetic compounds that get their specificity

from depletion of folic acid stores in bacterial cells.

• 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 para-Aminobenzoic acid (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 by dihydrofolate reductase (DHFR)

5

MOAN

N

N

HN

CH2O

OH

H2N

P

O

O-

O P

O

O-

O-

CH2N

O

OH

PABA

DHPS

N

N

N

HN

OH

H2N

HN COOH

N

N

N

HN

OH

H2N

HN CO Glu

DHFR NADPH

N

N

NH

HN

OH

H2N

HN CO Glu

Pteridine

-PPi

Dihydropteroic acid

Tetrahydrofolic acid

Dihydrofolic acid

6

MOA

• In most bacterial species Sulfonamides are competitive

inhibitors of DHPS, an enzyme present exclusively in

bacterial cells.

• Sulfonamides 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.

7

MOA

• 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.

8

General Characteristics

• The activity of sulfonamides depends on their

lipophilicity, which determines:

– ability to get inside the bacterial cell.

– their ionization at physiological pH.

• They have to be ionized to resemble PABA, to be better

incorporated into the active site of the enzyme and to

cause its inhibition.

• Their main use has been in the treatment of acute,

uncomplicated UTI, particularly those caused by

Escherichia coli.

9

Combination Therapy

• It has been suggested that a combination of a

sulfonamide with trimethoprim (dihydrofolate reductase

inhibitors) 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.

10

Combination Therapy

• A fixed ratio (1:5) of these agents is used in UTI, otitis,

MRSA, bronchitis, and sometimes employed in typhoid.

• It has also found application in Pneumocystis carinii

Pneumonia (PCP) in AIDS patient, a condition that can

be fatal in such immuno-compromised patients.

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SAR

• The amine and sulphonamide 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.

H2N S

O

NH2

O

12

SAR

• The amide is less active than the sulphonamide.

• Changes are allowed on the sulfonamide side:

– Monosubstitution on the sulfonamide may increase

activity (the compound still has one hydrogen to

donate and is capable for ionization) and modify

pharmacokinetic properties

– Disubstitution will abolish activity.

13

Pharmacokinetics

• They are well absorbed orally

• They are excreted in urine as the inactive acetylated

form.

• They diffuse freely throughout the body tissues and may

be detected in urine, saliva, sweat, bile, cerebrospinal,

peritoneal, ocular, synovial fluids, and in pleural.

• It crosses 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.

14

Toxicity

• Crystalluria, which can potentially lead to kidney

damage.

• The reason for this problem is that unsubstituted

sulfonamides will have low solubility around pH 6–7.

• Solutions:

– Increase urine flow (give plenty of fluids)

– Increase the pH of urine to around 10 using sodium

bicarbonate

– Use a combination of lower doses of sulfonamides

where each has its own solubility.

15

Sulfisoxazole

• Sulfisoxazole is well absorbed.

H2N S

O

NH

OO

N

H3C

CH3

Sulfisoxazole

16

Sulfadiazine

• Sulfadiazine is readily absorbed.

• It is effective against susceptible organisms in bladder

infections, ear infections, or bronchitis.

H2N S

O

NH

O

Sulfadiazine

N

N

17

Sulfacetamide

• Sulfacetamide is used, in combinations, in preparations

for vaginal, eyes, and skin infections.

H2N S

O

NH

O

Sulfacetamide

CH3

O

18

Sulfamethizole

• Sulfamethizole is readily absorbed from the

gastrointestinal tract.

• It is a short-acting sulfonamide that is given by mouth in

the treatment of infections of the urinary tract, sometimes

in combination with other antibiotics.

H2N S

O

NH

O

Sulfamethizole

S

NN

CH3

19

Sulfamethoxazole

• Sulfametoxazole is readily absorbed from the

gastrointestinal tract.

• About 70% is bound to plasma proteins.

• Combination with trimethoprim due to similarity in t1l2

• (Chemotrim- Septazol- Entrim- Cortil- Co trimthoxazole- Sutrim)

H2N S

O

NH

O

Sulfamethoxazole

NO

CH3

20

Sulfadoxine

• Sulfadoxine has a half life of 4 - 9 days, and thus is a

long acting agent.

• It is used as a second line in the prophylaxis of

Pneumocystis carinii pneumonia (PCP) in AIDS patients,

and as an antimalarial agent.

H2N S

O

NH

O

Sulfadoxine

N

N

H3CO OCH3

21

Anti-Mycobacterials

• Mycobacterium are characterized by a complex cell

wall that is highly hydrophobic due to a high lipid content,

with a backbone made of mycolic acid, D-arabinose and

peptidoglycan

R1

R2

HO

COOH

Mycolic Acid

R1, R2: large fatty acid chains

CH3

H

OH

H OH

OH H

O

HO

D Aarabinose

22

Anti-Mycobacterials

• The cell wall shows low permeability, preventing many

antibiotics from being effective.

• It is identified as an acid fast Bacilli, where the cell wall

can be stained but not destained by acids.

23

TB

• Mycobacterium causes two major diseases, leprosy and

tuberculosis.

• It is estimated that one third to one half of the world

population is infected by Mycobacterium tuberculosis

and that 6% of all deaths worldwide is due to this

disease, making it the most deadly infectious bacterial

disease.

24

Isoniazid (INH) (Isocid forte)

• Synthetic agent used since 1950s.

• It is bactericidal against replicating bacteria, and

bacteriostatic against non-replicating bacteria.

• Its mechanism of action is still debated, but it is obvious

that it acts by inhibiting cell wall biosynthesis, since

bacteria treated with this agent lose their acid fast ability.

N

O NHNH2

Isoniazide

25

Isoniazid

• It is converted to isonicotinic acid and isonicotinamide in

bacterial cells via the action of a bacterial enzyme, katG.

N

O NHNH 2

Isoniazide (Nydrazid)

KatG

Catalase Peroxidase

N

O OH

N

O NH2

Isonicotinic Acid Isonicotinamide

26

Isoniazid

• It is proposed that the intermediates produced in this

reaction are reactive acylating species, which are

responsible for the antimicrobial activity.

N

O NHNH 2

Isoniazide (Nydrazid)

KatG

Catalase Peroxidase

N

O OH

N

O NH2

Isonicotinic Acid Isonicotinamide

N

O N NH

N

CO .

N

C

O O

O.

N

C

O O

OH

27

Isoniazid

• The acylating species are thought to interfere with

mycolic acid biosynthesis, possibly through acylating

NADH, a cofactor essential in the double bond reduction

during the elongation of the fatty acid side chain

N

H H

R

H3C

O

Acylating Species form

INH Oxidation

NADH

N

H

R

H3C

O

Acylated NADH

O

N

28

Isoniazid• This agent is orally absorbed, but food and antacids,

especially those containing aluminum, interfere with

absorption.

• Metabolism takes place via n-acetyl transferase that

yields inactive acetylated metabolites.

• Further metabolism will yield acetyl-hydrazide, which has

been implicated as a hepatotoxic compound.

N

O NHNH 2

N-Acyteylation

N

O NH NH C

O

CH3

NH2 NH C

O

CH3

Acetyl Hydrazide

(hepatotoxic)

29

Rifamycins

• Isolated from Streptomyces.

• Effective against a wide variety of bacteria, including

Mycobacterium, but cannot penetrate gram negative

cells.

• They inhibit bacterial RNA polymerase by binding to the

β-subunit of the enzyme, and block RNA synthesis.

• They are highly effective against rapidly dividing

Mycobacterium.

30

Rifamycins• They bind reversibly to RNA polymerase via three binding

points:

– Hydrophobic interaction between the naphthalene ring

and proteins in the enzyme via a π-π interaction.

– Hydrogen bonding between hydroxyl groups on C21

and C23 with RNA polymerase.

– Chelation with Zinc (which is present within the

enzyme) through the hydroxyl groups at C1 and C8.

31

Rifamycins

O

H3COCO

CH3

H3CO

NH

O

O

OH

N N N CH3

H3C

O

H3C

OH OH

OHOH

CH3 CH3 CH3

CH3

Naphthalene Ring (Hydrophobic interaction

with the enzyme

Hydroxyl Groups

(Hydrogen bonding

with the enzyme)

Hydroxyl Groups

(Chelation with zinc)

32

Rifampin

• Rifampicin, also known as rifampin

• Rifampin is semisynthetic agents that are used in combination with

INH.

• Common side effects include: often turns urine, sweat, and tears a

red or orange color.

• Part of the recommended treatment of active tuberculosis during

pregnancy.

• It works by stopping the making of RNA by bacteria.

O

H3COCO

CH3

H3CO

NH

O

O

OH

N N N CH3

H3C

O

H3C

OH OH

OHOH

CH3 CH3 CH3

CH3

Rifampin

33

Rifapentine

• Considered more active than rifampin, and has a longer

half-life resulting in a less frequent dose regimen.

• It differs in the 3-substitution, which is thought to be

responsible for cell entry, and thus this agent shows

better oral bioavailability.

O

H3COCO

CH3

H3CO

NH

O

O

OH

N N N

H3C

O

H3C

OH OH

OHOH

CH3 CH3 CH3

CH3

Rifapentine

34

Pyrazinamide

• A popular drug used in many combinations, but

unfortunately resistance develops very quickly.

• Its mechanism of action appears to involve its hydrolysis

to pyrazinoic acid via the bacterial enzyme pmcA.

N

N

NH2

O

Pyrazinamide

35

Pyrazinamide• The acid is believed to act as an anti-metabolite of

nicotinamide and interferes with NAD biosynthesis.

• It may also lower the pH inside the organism to a deadly

level.

• It is especially effective against semi-dormant

mycobacterium, and is used in combinations with INH and

rifampin.

• The major side effect is a dose-related hepatotoxicity

• Ricure- Rimstar (Isoniazid- Rifampicin- Pyrazinamide)

• Isorifa- Rifam plus- Rimactazid- Riozid (Isoniazid- Rifampicin)

36

Ethambutol

• Ethambutol is effective against other Mycobacterium

such as M. avium, seen in many HIV patients.

• It inhibits arabinosyl transferase, an enzyme important in

formation of the arabinogalactan portion of the

mycobacterium cell wall.

• It shows synergism with rifampin, possibly due to the cell

wall damage it causes.

C

C2H5

H

CH2OH

HN

NH

C

C2H5

CH2OH

H

Ethambutol

37

Dapsone

• Dapsone has the same mechanism of action and

properties of sulfonamides.

• It is specific to Mycobacterium.

• It is well absorbed but is not very soluble and causes GI

irritation.

• Use mainly to treat leprosy.

S

O

O

H2N NH2

4,4'-Diaminodiphenyl sulfone (Dapsone, DDS)

38

Clofazimine

• Clofazimine is especially useful in MDR regimens.

• It is dark red in color and will stain the skin, urine, tears

and faeces.

• Use mainly to treat leprosy.

N

NHN

N

CL

CL

CH3

CH3

Clofazimine