Download Antibacterial Agents which Act Against Cell Metabolism

Antibacterial Agents which
Act Against Cell Metabolism
(Antimetabolites)
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Mechanisms of antibacterial agents
There are Five main mechanisms by which antibacterial agents act.
1. Inhibition of bacterial cell wall synthesis: e.g. Penicillins, Cephalosporins and
Vancomycin
2. Inhibition of cell metabolism: e. g. Sulfonamides
3. Interactions with the plasma membrane: e.g. Polymyxins
4. Disruption of protein synthesis: e.g. Rifamycins, Aminoglycosides, Tetracyclines, and
Chloramphenicol
5. Inhibition of nucleic acid transcription and replication: e.g. Nalidixic acid and Proflavin
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(Antimetabolites)
Sulfonamides
4-Amino-N-substituted-benzene sulfonamide
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The history of sulfonamides

The sulfonamide story began in 1935 when it was discovered that a red dye called
prontosil had antibacterial properties in vivo (i.e. when given to laboratory
animals).

Strangely enough, no antibacterial effect was observed in vitro. In other words,
prontosil could not kill bacteria grown in the test tube. This remained a mystery
until it was discovered that prontosil was not in fact the antibacterial agent.

Instead, it was found that the dye was metabolized by bacteria present in the
small intestine of the test animal, and broken down to give a product called
sulfanilamide
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Mechanism of action
OH
N
CH2OPP
N
HO
N
N
H
Dihydropetredine
Dihydropteroate
Synthetase
O
O
HN
H2N
N
N
N
H
N
H
Act as competitive enzyme
inhibitors and block the
biosynthesis of the vitamin folic
acid in bacterial cells
CO2H
O
OH
O
CO2H
Folic Acid
OH
O
N
H
N
HN
H2N
Tetrahydrofolate
is
an
enzyme
cofactor
that
provides one carbon units
for the synthesis of the
pyrimidine
nucleic
acid
bases required for DNA
synthesis .
If
pyrimidine and DNA
synthesis is blocked, then
the cell can no longer grow
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and divide.
O
N
H
N
Dihydrofolate
Reductase
N
H
O
They prevent the cells dividing
and spreading
OH
O
OH
O
N
H
H
N
HN
H2N
O
N
H
N
N
H
Tetrahydrofolic acid
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Sulfonamides do not actively
kill bacterial cells
Antibacterial agents which inhibit
cell growth are classed as
bacteriostatic, whereas agents
which can actively kill bacterial
cells (e.g. penicillin) are classed as
bactericidal
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Mechanism of action (cont.)
The sulfonamide molecule is similar enough in structure to PABA that the
enzyme is fooled into accepting it into its active site.
Once it is bound, the sulfonamide prevents PABA from binding.
As a result, folic acid is no longer synthesized. Since folic acid is essential to
cell growth, the cell will stop dividing.
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Mechanism of action (cont.)
Why the enzyme does not join the sulfonamide to the other two components of
folic acid to give a folic acid analogue containing the sulfonamide skeleton?
Answer: This can in fact occur, but it isn’t accepted by the next enzyme
(dihydrofolate reductase) in the biosynthetic pathway
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Structure-Activity Relationships (SAR)
The synthesis of a large number of sulfonamide analogues led to the
following conclusions.
 The p-amino group (N4)is essential for activity and must be
unsubstituted .
 The only exception is when N-substituted with acyl (i.e. amides).
 The amides themselves are inactive but can be metabolized in the
body to regenerate the active compound.
 Thus amides can be used as sulfonamide prodrugs.
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Structure-Activity Relationships (SAR)
 The aromatic ring and the sulfonamide functional group are both
required.
 The – SO2NH2 group must be directly linked to the benzene ring
 The aromatic ring must be para-substituted only.
 The sulfonamide nitrogen must be primary or secondary.
 R" is the only possible site that can be varied in sulfonamides.
 Substitution with heterocyclic rings as at-N1-have variable effects on
the antibacterial activity
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Acidity of Sulfonamides
The strongly electron withdrawing character of the aromatic SO2 group makes the
neighboring nitrogen atom to which it is directly attached partially electropositive, in turn,
increases the acidity of the hydrogen atoms attached to the nitrogen and facilitates its
expulsion as a proton so that this functional group is slightly acidic (pKa 10.4).
Replacement of one of the NH2 hydrogens by an electron withdrawing heteroaromatic ring
was not only consistent with antimicrobial activity but also greatly acidified the remaining
hydrogen (pKa 5-6) and dramatically increased potency.
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Acidity of Sulfonamides (Cont.)
With suitable groups in place, the pKa came down to the same range as that of PABA itself.
The pKa of the carboxyl group of PABA is approximately 6.5
Acidity Increases
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Electronegativity of heteroatoms:
S (2.5), N (3), O (3.5)
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Metabolism of Sulfonamides
The resulting amides have reduced
solubility which can lead to toxic effects.
The metabolite formed from sulfathiazole (an early sulfonamide) is poorly
soluble and can prove fatal if it blocks the kidney tubules.
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The solubility problem could be overcome by replacing the Thiazole ring in
sulfathiazole with a Pyrimidine ring to give Sulfadiazine.
The reason for the improved solubility lies in the acidity of the sulfonamide NH proton
In sulfathiazole, this proton is not very acidic (high pKa). Therefore, sulfathiazole and its
metabolite are mostly un-ionized at blood pH
Replacing the Thiazole ring with a more electron withdrawing Pyrimidine ring
increases the acidity of the NH proton by stabilizing the anion which results.
Therefore, sulfadiazine and its metabolite are significantly ionized at blood pH. As a
consequence, they are more soluble and less toxic.
Sulfadiazine was also found to be more active than sulfathiazole and soon replaced it in
therapy.
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Crystalluria

The poor water solubility of the earliest sulfonamides led to occasional
crystallization in the urine (crystalluria) and resulted in kidney damage because
the molecules were un-ionized at urine pH values.

Water- insoluble acetylsulfathiazole ( metabolic product) causes crystalluria and
renal toxicity

It is still recommended to drink increased quantities of water to avoid crystalluria
when taking certain sulfonamides but this form of toxicity is now comparatively
uncommon with the more important agents used today because they are at least
partly ionized and hence reasonably water soluble at urinary pH values.

Increasing the pH of the urine by oral ingestion of sodium bicarbonate used
occasionally and still used.
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Classes of Sulfonamides
Sulfonamides
Systemic
Sulfonamides
Short-acting
Sulfonamides
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Intermediateacting
sulfonamides
Intestinal
sulfonamides
Ophthalmic
sulfonamides
Sulfonamides
for burn therapy
Long-acting
sulfonamides
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I. Systemic Sulfonamides
According to their duration of action they are
divided to:
• Short-acting Sulfonamides
• Intermediate-acting sulfonamides
• Long-acting sulfonamides
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A) Short-acting Sulfonamides
• They are rapidly absorbed and rapidly excreted. Their half lives from 4-7 hours
and they are administered every 4 to 8 hours.
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B) Intermediate-acting
sulfonamides
• They are absorbed and excreted more slowly than short-acting.
•
Their half lives range from 10 to 12 hours so they are given twice daily.
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C) Long-acting sulfonamides
• They are rapidly absorbed but slowly excreted their
half lives are 35 to 40 hours.
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II. Intestinal sulfonamides
 Water soluble latent forms which are poorly absorbed from the GIT (5%) and
thus reach a high concentration in the colon lumen,
 By means of either bacterial or enzymatic hydrolysis releases the parent
sulfonamide.
 Examples: sulfasalazine, phthalylsulfathiazole and succinylsulfathiazole
(Prodrugs).
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Intestinal sulfonamides (cont.)
 Sulfasalazine is mainly used for treatment of inflammatory bowel disease,
including ulcerative colitis and Crohn's disease. It is also effective in several types of
arthritis, particularly rheumatoid arthritis
 Reductive metabolism by means of azoreductase enzyme converts the drug to
sulfapyridine and 5-aminosalicylic acid (anti-inflammatory) both components are active
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Ophthalmic sulfonamides
• They are used in treatment of conjunctivitis and
other superficial ocular infections.
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IV. Sulfonamides for
Burn Therapy
• Mafenide is not a true sulfonilamide, it is not effective
systemically, but is particularly effective topically in the
treatment of burns or for healing infected wounds.
O
NH2
S
O
H2N
Mafenide
4-(Aminomethyl)benzenesulfonamide
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Sulfonamides for burn therapy
(cont.)
• Silver sulfadiazine is used as effective topical
antimicrobial agents, especially against Pseudomonas s.
in burn therapy, where treatment failure with other
drugs may occur.
+
Ag
N
O
N
S
O
H2N
24
N
Silver sulfadiazine
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Mixtures of sulfonamides
1.Sulfonamides alone
The main purpose is to reduce the risk of crystalluria
a)Trisulfapyrimidine
Each drug is administered in one third of the total dose, they behave
independently concerning solubility but their therapeutic effects are additive.
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b. Multiple (or Triple) sulfas
• They are a 1:1:1 combination of sulfabenzamide,
sulfacetamide, and sulfathiazole. The combination is
primarily used as topical cream for Gardnerella
vaginalis in vaginal infections
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2) Mixtures of sulfonamides with other drugs
Sulfamethoxazole and Trimethoprim
 There is a synergistic effect obtained from such combination.
 It is usually given orally and I.V. administration.
 When taken orally the tablet has a standard ratio of
5
:
1
40 mg of the sulfa and 8 mg trimethoprim.
 Both drugs are excreted in urine their half lives are about 8-10 hours
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 Trimethoprim is
sulfamethoxazole.
often
given
in
conjunction
with
the
sulfonamide
 The latter inhibits the incorporation of PABA into folic acid, while the former
inhibits dihydrofolate reductase.
 Therefore, two enzymes in the one biosynthetic route are inhibited.
 This is a very effective method of inhibiting a biosynthetic route and has the
advantage that the doses of both drugs can be kept down to safe levels. To get
the same level of inhibition using a single drug, the dose level of that drug would
have to be much higher, leading to possible side-effects.
This approach has been described as
'sequential blocking'.
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Pharmacokinetic factors of the combination
• Pairing these two particular antibacterial agents was
based upon pharmacokinetic factors and convenient
availability.
• For such a combination to be useful in vivo the two
agents must arrive at the necessary infected tissues
at the correct time and in the right ratio.
• It is used for oral treatment of urinary tract
infections, shigellosis, otitis media, traveler's diarrhea
and bronchitis.
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Selectivity of Trimethoprim
• There is a significant differences between the
bacterial and the mammalian dihydrofolate
reductases away from the active site.
• The bacterial enzyme is sensitive to inhibition by
trimethoprim by up to 100,000 times lower
concentrations than is the mouse enzyme. This
difference explains the useful selective toxicity of
trimethoprim.
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Advantages of this combination
•
The combination of sulfamethoxazole-trimethoprim is not only synergistic in vitro
but is less likely to induce bacterial resistance than either agent alone.
•
Thus, these agents block sequentially at two different steps in the same essential
pathway, and this combination is extremely difficult for a naive microorganism to
survive.
•
It is also comparatively uncommon that a microorganism will successfully mutate to
resistance at both enzymes during the course of therapy.
•
It is useful and comparatively nontoxic for AIDS patients who are infected with the
pneumonia causing opportunistic pathogen Pneumotystis carinii.
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Homework & Probable Quiz, Midterm
and/or Final Exam Questions
1.Contrary to bacterial cells, sulfonamides has no lethal effect on human cell
growth. Explain this fact verbally and chemically.
2.Illustrate by chemical equations how you could prepare (sulfacetamide sodium
and/or silver sulfacetamide) from 4-aminobenzene sulfonamide.
3.Explain how an ‘azo-dye’ breaks down in vivo to yield sulphanilamide?
4.N1-substitution in sulphanilamide is more effective and useful than N4-. Explain.
5.Classify sulphonamide on the basis of their site of action. Give the structure,
chemical name, uses and side effects of ONE drug each class.
6.Comment on the probable mechanisms of bacterial resistance to sulphonamides.
7.Write a short note on ‘chemotherapeutic consideration of sulphonamides’.
8.Describe the synthesis of a sulphonamide drug that could be used mostly in: 1. 1.
a. Gas-gangrene. b. Second-and third degree burns.
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Thanks for Attention
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