1. Antibacterial Agents which Act Against Cell Metabolism
(Antimetabolites)
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2. Mechanisms of antibacterial agents
There are Five main mechanisms by which antibacterial agents act.
Inhibition of bacterial cell wall synthesis: e.g. Penicillins, Cephalosporins and Vancomycin
Inhibition of cell metabolism: e. g. Sulfonamides
Interactions with the plasma membrane: e.g. Polymyxins
Disruption of protein synthesis: e.g. Rifamycins, Aminoglycosides, Tetracyclines, and Chloramphenicol
Inhibition of nucleic acid transcription and replication: e.g. Nalidixic acid and Proflavin
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3. Sulfonamides (Antimetabolites)
4-Amino-N-substituted-benzene sulfonamide
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4. 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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5. Mechanism of action
Act as competitive enzyme inhibitors and block the biosynthesis of the vitamin folic acid in bacterial cells
Sulfonamides do not actively kill bacterial cells
Dihydrofolate
Reductase
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 and divide.
They prevent the cells dividing and spreading
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
Tetrahydrofolic acid
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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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
Electronegativity of heteroatoms: S (2.5), N (3), O (3.5)
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12. 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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13. 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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14. 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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15. Classes of Sulfonamides
Systemic Sulfonamides
Short-acting Sulfonamides
Intermediate- acting sulfonamides
Long-acting sulfonamides
Intestinal sulfonamides
Ophthalmic sulfonamides
Sulfonamides for burn therapy
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16. 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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17. 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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18. 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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19. C) Long-acting sulfonamides
They are rapidly absorbed but slowly excreted their half lives are 35 to 40 hours.
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20. 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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21. 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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22. Ophthalmic sulfonamides
They are used in treatment of conjunctivitis and other superficial ocular infections.
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23. 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.
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24. 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.
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25. Mixtures of sulfonamides
Sulfonamides alone
The main purpose is to reduce the risk of crystalluria
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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26. 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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27. 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 :
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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28. This approach has been described as
Trimethoprim is often given in conjunction with the sulfonamide sulfamethoxazole.
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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29. 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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30. 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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31. 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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32. Homework & Probable Quiz, Midterm and/or Final Exam Questions
Contrary to bacterial cells, sulfonamides has no lethal effect on human cell growth. Explain this fact verbally and chemically.
Illustrate by chemical equations how you could prepare (sulfacetamide sodium and/or silver sulfacetamide) from 4-aminobenzene sulfonamide.
Explain how an ‘azo-dye’ breaks down in vivo to yield sulphanilamide?
N1-substitution in sulphanilamide is more effective and useful than N4-. Explain.
Classify sulphonamide on the basis of their site of action. Give the structure, chemical name, uses and side effects of ONE drug each class.
Comment on the probable mechanisms of bacterial resistance to sulphonamides.
Write a short note on ‘chemotherapeutic consideration of sulphonamides’.
Describe the synthesis of a sulphonamide drug that could be used mostly in: a. Gas-gangrene. b. Second-and third degree burns.
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33. The end
Thanks for Attention
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