1. Lecture 7 & 8 Medicinal Chemistry 1 PC 509
Prof. Dr/ Ghaneya Sayed Hassan

2. Antibacterial Sulfonamides
Discovery:
In 1932: Gerhard Domagk studied the antimicrobial effect of Prontosil Dye "brilliant red Dye" it was found to be active ≠ Streptococcal infection in mice [in vivo] but inactive on bacterial culture [in vitro].
In 1935: Jacques Tréfouël discovered the conversion of inactive prontosil dye -in vivo- into active Sulfanilamide "Lead compound or Prototype". This finding was confirmed by isolating free sulfonamide from blood & urine of patients treated with Prontosil.

3. Antibacterial Sulfonamides denote 3 different cases:
N.B: Side effects & resistance to sulfonamides limit its use today; Penicillins were excellent alternatives for sulfonamides.
Antibacterial Sulfonamides denote 3 different cases:
[3] Non-aniline sulfonamides
[2] Prodrugs giving active sulfonamide
[1] Aniline-substituted sulfonamide
Mafenide acetate
Sulfasalazine
Sulfanilamides
All sulfanilamides are sulfonamides,
but not all sulfonamides are sulfanilamides

4. Mechanism of action: Sulfanilamides are active BACTERIOSTATIC.
Many bacteria are impermeable to folic acid, so they rely on their ability to synthesize folate from PABA “p-Amino Benzoic Acid”, Petridine & Glutamate against MAMMALS who can't synthesize folic acid, so obtained from diet & so not affected by sulfanilamides [selective chemotherapy].
Because of their structural similarity to PABA, sulfonamides act as competitive inhibitor with this substrate for the enzyme dihydropteroate synthetase. Thus  synthesis of folic acid  thymidine, purine synthesis   synthesis of DNA   multiplication & growth of m.o.

5. Biosynthesis of folate co-enzymes:
By using sulfonamides

6. This mechanism is supported by:
1- PABA added to culture media antagonizes effect of sulfonamides.
2- Man can't form folic acid  so his cells are immune to sulfonamides.
3- M.O. which can utilize performed folic acid is less sulfonamides susceptible.
Resistance of m.o. to sulfonamide drugs is by:
The bacterial cell wall becomes more permeable to folic acid.
The m.o. learns to utilize preformed folic acid.
The organism develop alternate pathway for synthesis of folic acid.
 PAPA synthesis by m.o. to overcome inhibition of dihydropteroate synthetase.

7. Adverse effects of sulfonamides therapy:
(1) Gastrointestinal distress.
(2) Hemolytic anemia.
(3) Hepatitis.
(4) Stevens-Johnson Syndrome [sever skin eruption].
(5) Crystalluria.
General synthesis of Sulfa drugs:

8. Structure Activity Relationship [SAR]:
N.B: As structure become more close to PABA  more active.

9. Main characters of Sulfonamides:
[1] They are weak organic acids  due to SO2NH2 group [by loss of proton & stabilization of –ve charge by resonance], & this determine pka of the drug.
N.B: acidity  by attachment of e-withdrawing group to N1
[2] Metabolism:
By N4-acetylation  sulfanilamides excreted as it’s N4-acetate and glucuronide [both two metabolites are inactive]
N4-acetate is less water soluble than parent drug   tendency of crystalluria

10. Crystalluria: (1) If pH of urine = pka of drug  ionized/unionized = 1
Those sulfanilamides and their acetyl derivatives are sparingly soluble in water & excreted almost in urine precipitation in kidney  crystalluria.
According to the following equation:
(1) If pH of urine = pka of drug  ionized/unionized = 1
(2) If pka of drug > pH of urine  unionized/ionized >1 
 solubility
(3) If pka of drug < pH of urine  unionized/ionized <1 
 solubility
(4) pH of urine is about 6 & pka of sulfanilamide “prototype’ is 10.4  drug present in urine in unionized form   solubility  crystalluria.

11. So, it’s present mainly in ionized form  more soluble
Question: if pka of sulfisoxazole is 5, determine its risk of crystalluria.
So, it’s present mainly in ionized form  more soluble
  risk of crystaluria.
N.B. Sulfanilamides drugs are AMPHOTERIC in nature [with acidic & basic characters]
So, in when we make urine alkaline  we  solubility [ ionized form]

12. To solve problem of crystalluria:
Drinking large amount of water 
 urine flow by  rate of glumerular filtration.
(2) Combination therapy [triple therapy]: using mixed sulphonamides [3 sulpha drugs: Sulfadiazine + Sulfamerazine + Sulfamethazine]  only 1/3 of the amount of each drug is used giving the same bacterial action but each one is present in amount less that its solubility product  no precipitation.
(3)  pH of urine by alkalinization [using NaHCO3]
(4)  pka of drug by N1-substitution with electron-withdrawing group [as heterocycle or acyl group]

13. Assay:
[1] Methods depends on primary aromatic amino group [ N4 ]: Diazotization and diazocoupling.
[2] Methods depend on acidity of sulfonamide group [ -SO2NH-R]
[a] Non-aqueous titration
[b] Argentometric method [Back titration]
[3] Bromometric method
- Add known excess of standard Br2 solution in HCl  bromination of sulphonamides.
- Excess Br2 determined by adding KI  I2  titrated ≠ standard Na2S2O3
e.g. Sulfadiazine

14. [i] Systemic Sulfonamides
Classification of antibacterial sulfonamides
[i] Systemic Sulfonamides
- Used in treatment of systemic infections.
- Classified according to rate of excretion [t1/2] into:
LONG ACTING
MEDIUM ACTING
SHORT ACTING
Taken every 24 hrs
Taken every 8-12 hrs
Taken every 6 hrs
With slow excretion rate
t1/2 = hrs
t1/2 < 10 hrs

15. Systemic Sulfonamides classified into:
[1] N1-Acyl Derivatives
Sulfacetamide
Synthesis:

16. Sulfacetamide Sodium Water soluble, its solution with pka 5.4
Its Na salt is less alkaline than Na salts of other sulfonamides  non-irritant to mucous membrane  used as eye drops till 10 % concentration.
Can be used for urinary tract infection [why?]  it's highly soluble with t1/2 = 7 hrs [rapid excretion]
Sulfacetamide
Sodium
Sodium salt of N-[(4-aminophenyl) sulfonyl] acetamide OR
[N1-acetyl sulfanilamide]

17. [2] N1-Heteroatomic Derivatives [i] Pyrimidine Derivative
t1/2 = 7 hrs / pka = 7.2
More water soluble > sulfadiazine & sulfamerazine in acidic urine [pH=5.5], but lower activity in vitro & in vivo.
Used in combination sulfa [Tri-sulfapyrimidine therapy]
Short Acting
Sulfamethazine
N1( 4,6-dimethyl- 2- pyrimidinyl ) Sulphanilamide

18. Moderate Acting Long Acting Sulfadiazine Sulfamerazine
t1/2 = 17 hrs / pka = 6.3
Broad spectrum, the drug of choice in UTI.
NaHCO3 is co-given [why?]
Uses:
Na salt as 5 % solution: In Meningitis.
Ag salt: topically in burns.
Sulfadiazine
N1(2- pyrimidinyl ) Sulfanilamide
Long Acting
t1/2 = 27 hrs / pka = 6.99
Similar properties to sulfadiazine, but:
With more water solubility.
More absorbable.
Less excretion rate.
Higher blood level can be obtained with a similar dose.
Sulfamerazine
N1(4-methyl-2- pyrimidinyl ) Sulfanilamide

19. Long Acting Sulfameter Sulfadimethoxine
Long duration of action due to presence of OCH3   plasma protein binding   excretion rate.
As a result of  excretion  may cause hypersensitivity upon accumulation.
T1/2 of Sulfameter is 37-48
hrs & for Sulfadimethoxine is 40 hrs.
Sulfameter
N1(5-methoxy -2- pyrimidinyl ) Sulphanilamide
Sulfadimethoxine
N1(2,6-dimethoxy-4- pyrimidinyl) Sulphanilamide

20. [ii] Pyridazine Derivative Sulfamethoxypyridazine acetyl
N1-acetyl-N1(6-methoxy-3-pyridazinyl) Sulphanilamide
N1(6-methoxy-3-pyridazinyl) Sulphanilamide
PRODRUG for sulfamethoxypyridazine   bitter taste  used for pediatrics.
Inactive in vitro  activated by deacetylation in intestine.
t1/2 = 37 hrs  long acting [why?] , due to presence of methoxy group.
With bitter taste.

21. Sulfamethoxazole Sulfaisoxazole [iii] Isoxazole Derivative
N1(5-methyl-3-isoxazolyl) Sulphanilamide
● t1/2 = 11 hrs  short acting.
● Not rapidly absorbed as sulfisoxazole  its peak blood level is only about 50 %.
Sulfaisoxazole
N1(3,4-dimethyl-5-isoxazolyl) Sulphanilamide
● t1/2 = 6 hrs  short acting.
● Rapidly absorbed.
● Highly water soluble  no need for using alkalinizing agent with it.
● With bitter taste.
pka of sulfisoxazole is 5

22. [iv] Thiadiazole Derivative
Sulfaethidole
Sulfaimethizole
N1(5-ethyl-1,3,4-thiadiazol-2-yl) Sulphanilamide
N1(5-methyl-1,3,4-thiadiazol-2-yl) Sulphanilamide
IUPAC name: 4-amino-N-(5-methyl-1,3,4-thiadiazol-2-yl)- benzenesulfonamide
● Moderate duration.
● Has the lowest degree of acetylation of sulfonamides
● t1/2 = 2 hrs  short acting.
● Highly soluble used in UTI.

23. [2] Topical Sulfonamides
Sulfadiazine Silver
[Silvadene®, Dermazine]
Applied in water-miscible cream [Dermazine]  active topically ≠ Pseudomonas species  used in burn therapy [that Pseudomonas is responsible for failure of therapy].
Slightly soluble, Not penetrate cell wall but act on external cell structures.
Prepared by mixing equimolar amount of AgNO3 & Na sulfadiazine [both dissolved in water].

24. Sulfacetamide (Klaron®, Ovace®)
Sulfacetamide 10% topical lotion, is approved for the treatment of acne and seborrheic dermatitis.
(e.g., seborrheic dermatitis, seborrhea sicca [dandruff]); also indicated for the treatment of secondary bacterial infections of the skin due to organisms susceptible to sulfonamides

25. Mafenide acetate (Sulfamylon)
4-(Aminomethyl)benzenesulfonamide
Not true sulfanilamide compound
Not inhibited by PABA [its M.O.A. involves different mechanism than true sulfonamides].
Effective ≠ Clostridium welchii in topical use for infected wounds.
Not effective orally.
Used alone or with antibiotics in treatment of slow healing infected wounds.
If used in large quantities  metabolic acidosis. So, a series of new organic salts was prepared.
The acetate derivative in ointment base is the most efficient.

26. [3] Non-absorbable Sulfonamides
[i] Topical Sulfonamides: Sulfadiazine Ag & Mafenide acetate.
[ii] Intestinal Sulfonamides
They are Prodrugs designed to be poorly absorbed “from small intestine”
 In large intestine  cleavages  free sulfonamide "active".
Used in treatment of intestinal infections, ulcerative colitis & reduction of bowel flora.
Sulfa guanidine
N1-amidino sulfanilamide
Poorly absorbed [with additional basic group]  not absorbed from GIT  high local concentration
Synthesis:

27. N4-substituted Sulfonamides Phthalyl sulphacetamide
Phthalyl sulphathiazole
Succinic sulphathiazole
N4-acetyl-N4-phthalyl sulfanilamide
2-(N4-phthalyl sulfanilamide) thiazole
2-(N4-succinyl sulfanilamide) thiazole
They have additional acidic group  poorly absorbed from GIT.
They are prodrugs  activated in vivo by slow hydrolysis giving  local concentration.

28. 5-[4-(2-pyridyl sulfamoyl) phenyl azo] salicylic acid
Sulfasalazine
Water insoluble, broken in body giving: 5-amino saicyclic acid [anti-inflammatory] + Sulfapyridine [carrier]  so, USED IN ULCERATIVE COLITIS.
5-[4-(2-pyridyl sulfamoyl) phenyl azo] salicylic acid
Synthesis:

29. Trimethoprim Combination of sulfonamides with Dihydrofolate
Reductase Inhibitor
Remember  :  THFA synthesis   purine & pyrimidine bases synthesis   DNA synthesis  stop growth of bacteria.
Trimethoprim

30. Advantages of this combination:
It's potent anti-bacterial by inhibition of DHFR enzyme   stop bacterial growth.
Selective in action  times more active ≠ bacterial DHFR relative to mammalian DHFR. [used for UTI]
Combined with Sulfamethoxazole as both have the same pharmacokinetic properties [t1/2 = hrs like that of sulfamethoxazole]  excreted at about the same time. important condition for combination between two drugs]
Trimethoprim combined with Sulfamethoxazole [Sutrim] [Septazole] [Septrin]
The combination of a sulfonamide with trimethoprim causes
a sequential blockade of folic acid synthesis.
Advantages of this combination:
(1) Synergism due to sequential blockage.
(2) Avoid development of resistance.
(3) Broader spectrum of activity.

31. (Sulfadoxine/pyrimethamine)
Fansidar
(Sulfadoxine/pyrimethamine)
Combination of
Antibacterial Sulfadoxine [500 mg]
+ antimalarial Pyrimethamine [25 mg]
Inhibit folic acid synthesis by two different ways.
With schizonticidal effect.
Used for prophylaxis & treatment of chloroquine-resistant P.falciparum