CHEMOTHERAPY
D\ Moustafa K Soltan
• Chemotherapy = chemical therapy
= treatment with chemicals
• Definition:
Defined by Paul Ehrlich, long time ago, as:
“the use of chemical agents to harm an invading
organism or cell without harming the host”.
• Chemotherapeutic agents:
Chemical agents that harm an invading
organism or cell without harming the host.
Invading organism or cell
Microbes
Bacteria
Fungi
Cancer cells
Parasites
Viruses
Protozoa
(Unicellular)
Helminths
(Worms)
(Multicellular)
Entamoeba
Leishmania
Giardia
Nematodes
Trematodes
T richomonas
(Flukes)
T rypanosoma (round worms)
Malaria
Cestodes
(T ape worms)
Selective toxicity
≡Affecting an invading organism or cell without
affecting the host
• Measured by the chemotherapeutic index.
Chemotherapeutic
agent
M
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Immune defence
t
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y
Invading organism
or cell
Host
Infection or invasion
Chemotherapeutic index =
Concentration harmful to the host
Concentration harmful to the microbe, parasite or cancer cell
• High chemotherapeutic index = high selective
toxicity.
• Optimally should be above 100.
In basic pharmacology:
therapeutic index is deduced from doseMedian toxic dose (T 50
D)
response
curves.
T herapeutic
index =
Median effective dose (ED
50)
• High therapeutic index = wide safety margin.
Antibacterial
agents
Antibacterial Agents
Natu ral anti bacte ri al age n ts
(An ti bi oti cs)
S ynth e ti c an tibacte rial age n ts
Topi cal age nts
S yste m i c age n ts
Disinfect ant s
Ant isept ics
P reservatives
Sulphonamides
Urinary
ant isept ics
Ant imycobact erial agent s
Ant it ubercular
agent s
Ant ilepral
agent s
Topical synthetic antibacterial agents
Disinfectant:
• A chemical agent applied to inanimate objects to destroy
pathogenic microorganisms. Required to be bactericidal.
• Inanimate objects = surgical instruments, syringes and needles,
house-hold objects.
Antiseptic:
• A chemical agent applied to living tissues to kill or prevent the
growth of pathogenic microorganisms. May be bactericidal or
baceriostatic.
• Living tissues = skin and mucous membranes.
Preservative:
• A chemical agent added to foods or pharmaceutical
formulations to prevent the growth of pathogenic
microorganisms.
• A chemical agent may act as disinfectant, antiseptic and
preservative at the same time.
The point is: how it is applied.
• Example: Alcohol (ethanol):
– when applied to syringe needles  disinfectant.
– when applied to site of injection  antiseptic.
– when added to pharmaceuticals  preservative.
Alcohols
Alcohols
When R = more than 8 carbon atoms
When R = 1-8 carbon at oms
* Increasing number of carbon at oms
* Increasing number of carbon at oms
increase in activit y
(Inverse relat ionship)
increase in activit y
(direct relat ionship)
Why ?? as nucleophilicity of oxygen and
acidit y of alcohols increases)
* Branching
Why?? as wat er solubilit y decreases, large
amounts are needed t o get t he MIC.
act ivit y.
CH3OH
CH3CH2OH
CH3CH2CH2OH
CH3CHOH
CH3
Mode of action:
Denaturation of proteins  inhibition of enzymes.
Ethyl Alcohol (Ethanol; alcohol):
Effective concentration = 60 – 95 %
Uses: disinfectant, antiseptic, preservative and solvent.
As a solvent [spirit (100% alcohol), tincture (alcohol & another
solvent)].
Problem: alcohol abuse (chronic alcoholism) due to its pleasant
odour and CNS stimulant (cortical effect causing excitation) then
depressant activity.
Treatment of chronic alcoholism: using the aldehyde dehydrogenase
inhibitor, disulfiram.
Disulfiram
CH3CH2OH
CH3CHO
Aldehyde
dehydrogenase
CH3COOH
Disulfiram inhibits aldehyde dehydrogenase leading to
accumulation of acetaldehyde which gives unpleasant feeling.
S
N
S
S
N
S
Denatured alcohol:
Alcohol to which methanol is added for use for special purposes
such as mouth washes, after-shave lotions, and preparing plant
extracts.
Methanol in this case is deliberately added to prevent its abuse.
Benzyl alcohol:
OH
Assay:
O
OH
Ac2O / pyridine
O
CH3
(Known excess)
Benzyl acetate
T it rat e wit h NaOH / ph.ph indicat or
Uses:
As a preservative (1-4 %) in vials of injectable drugs.
Antiseptic.
+ excess Ac 2O
H2O
CH3COOH
Cl CH3
Chlorbutanol:
Cl
OH
Cl CH3
2,2,2-Trichloro-1,1-dimethyl ethanol
Assay:
Cl CH3
Cl
OH
Cl CH3
+ 4 NaOH
heat
CH3
NaOOC
OH
+ 3 NaCl
CH3
determined by Volhard's met hod
Add known excess of AgNO3 in presence of HNO3.
NaCl + AgNO3  AgCl  + NaNO3
Excess AgNO3 is titrated with NH4SCN using ferric alum as
indicator.
Uses:
As a preservative in injections, ophthalmic and nasal preparations.
Phenols
OH
OH
OH
OH
OH
CH3
OH
Phenol
Resorcinol
CH3
o-Cresol
m-Cresol
CH3
p-Cresol
OH
OCH3
Cl
OH
Cl
C
H2
OH
Cl
Thymol
HO
Eugenol
Cl
Cl
Hexachlorophene
Cl
SAR of phenols:
.
Hydroxy group should be free
Halogen
Alkyl
Alkoxyl
Nitro
increase activity
* Increasing the number of halogen atoms increases bactericidal
activity (up to a limit). Polyhalogenated phenols have limited
water solubility.
* The best position for halogen is the para position.
* Increasing length of the alkyl group increases bactericidal
activity (up to a limit). Large alkyl derivatives have limited
water solubility.
* The best position for alkyl is the ortho position.
Mode of action:
** At low conc.  precipitation of proteins  inhibition of
enzymes.
(due to its acidic properties).
** At high conc.  lysis of bacterial cell membranes.
(powerful cellular poisons).
Phenol (Carbolic acid):
SO3Na
OH
ONa
+
2 NaOH
heat
Sod. benzenesulphonate
Cl
H
+ Na2SO 3
+ H2O
OH
ONa
NaOH
heat / pressure
H
Assay: Bromometry
OH
OH
Br
+
Br
3 Br2
+
3 HBr
Br
Add known excess of KBr / KBrO3 mixture to phenol.
Add acid and allow to react for a suitable time.
Excess Br2 is determined iodometrically as follows:
Add KI and titrate the liberated I2 with Na2S2O3 using starch as
indicator.
Uses:
Obsolete in the US and Europe.
Here may be used as disinfectant and antipruritic.
Resorcinol: 1,3-dihydroxybenzene
Assay: Bromometry.
Uses:
Antiseptic (less potent than phenol) and keratolytic.
These two properties are important in seborrhea and acne.
Problem: Chemically unstable in solution (easily oxidized). This led to the
development of resorcinol monoacetate.
OH
Resorcinol monoacetate:
O
O
CH3
It is a prodrug for resorcinol. It slowly llberates resorcinol by the action of
esterases.
Advantages over resorcinol:
more chemically stable in solution.
Has more prolonged action.
Picric acid:
2,4,6-trinitrophenol
Explosive, therefore it should be stored while covered with an equal
amount of water.
Preparation:
OH
OH
O2N
NO2
Nitrating mixture
H2SO 4 / HNO3
NO2
Assay:
- Direct titration with NaOH using ph.ph as indicator (due to
high acidity).
Uses:
Antiseptic in case of burns.
Hexachlorophene:
Bis(3,5,6-trichloro-2-hydroxyphenyl)methane
Preparation:
Cl
Cl
Cl
1) KOH / 1 mole
2) Acid
Cl
Cl
OH
HCHO
H2SO 4
2
Cl
Hexachlorophene
Cl
Aldehydes:
Formaldehyde gas: HCHO; CH2O
Formalin: aq. Solution containing not less than 37% formaldehyde and 10-15%
methanol to prevent polymerization.
Stability: unstable in solution.
Easy oxidation  formic acid.
Easy polymerization  paraformaldehyde.
Mode of action:
Direct alkylation of nucleophilic groups in proteins (NH2, SH, OH).
Adverse reactions:
Highly allergenic.
Cancer suspect agent.
Chlorine-containing compounds (Chlorophors
Chlorine gas
Cl 2
Sodium hypochlorite NaOCl
Calcium hypochlorite Ca(OCl)
2
Cl
Chloramine-T
H3C
SO2N
Na
Cl
Dichloramine-T
H3C
SO2N
Cl
Halazone
HOOC
To disinfect water, swimming pools
and utensils.
Cl
SO2N
Cl
Mode of action:
In water, they release hypoclorous acid which can chlorinate amide nitrogen
atoms or oxidize thiol (SH) groups of proteins.
Chloramine-T: Sodium N-chloro-p-toluenesulphonamide.
Dichloramine-T: N,N-dichloro- p-toluenesulphonamide.
Halazone: N,N-dichloro- p-carboxybenzenesulphonamide.
Synthesis of Chloramine-T and Dichloramine-T:
H3C
ClSO3H
H3C
SO2Cl
NH3
H3C
SO2NH2
NaOCl
Dichloramine-T
Excess NaOCl
Chloramine-T
Cationic surface active agents (Cationic
surfactants):
Quaternary ammonium or pyridinium salts
R2
R1
X
N
R4
X
N
R
R3
Mode of action:
• They have surface active properties (ability to reduce interfacial
tension) and therefore, they can be used as detergents, emulsifiers and
germicidal as well.
N
Hydrocarbon tail
(Lipophilic)
Cationic head
(Hydrophilic)
• Adsorption on surface of bacteria and then lysis of bacterial cell wall
and cell membrane.
Advantages:
• High water solubility.
• Non-staining.
• Non-toxic.
• Non-corrosive.
• Effective at low conc.
Cl
N
CH3
R
CH3
Benzalkonium chloride
(R = alkyl C8-C16)
CH3
H3C
(CH2)15
N
Br
CH3
CH3
Cetyltrimethylammonium bromide
Disadvantages:
• Incompatible with anionic surfactants (soaps).
Therefore, any traces of soap should be
removed before the application of these agents.
• Low activity in presence of pus or serum.
Dyes
• The obvious disadvantage is that they are
staining.
• They are mainly bacteriostatic.
Gentian violet (Crystal violet): (a triphenylmethane
dye)
• Antibacterial (Gram +ve) and antifungal (tinea &
yeast).
OH
N
C
N
N
C
HCl
NaOH
N
Leucobase
Hexamethyl-prosaniline
N
Dye
N
Synthesis:
CH3
H3C
N
CH3
N
+
CH3
H3C
N,N-Dimethyl-p-toluidine
H3C
N
NaCl
Gentian violet
Nit robenzene
(mild oxidation)
CH3
Assay:
Direct titration with titanous chloride while boiling in presence of sod. pot.
Tartarate.
Activity:
Antibacterial (Gram +ve) and antifungal.
Its main use, however, is treatment of fungal infections (tinea and yeast).
NH2
Aminacrine:
N
9-Aminoacridine
Preparation:
COOH
COOH
CuO / K2CO3
+
Cl
H2 N
Ullman reaction
N
H
heat
O
Aminacrine
POCl3
heat
N
H
Mode of action
At physiological pH, aminacrine is protonated and the acquired
positive charge is delocalized between the ring nitrogen and the
side chain nitrogen.
Being a flat molecule, aminacrine acts through intercalation
between base pairs of DNA of bacteria.
The positive charge assists intercalation through electrostatic
attraction to the negatively charged phosphate of DNA.
The result of intercalation is failure of double-stranded DNA into
single strands so that cell division is arrested.
Methylene blue: (a phenothiazine dye)
Cl
N
3,7-Bis(dimethylamino)phenothiazinium chloride
S
N
N
Uses:
• Antiseptic.
• Small dose  treatment of methemoglobinemia.
Methylene blue is reduced by a dormant reductase to the leucobase
(reducing agent).
The leucobase reduces iron in methemoglobin (in the ferric state) to ferrous
thus rendering it to normal hemoglobin.
• Large dose  treatment of cyanide poisoning.
It induces a state of methemoglobinemia by converting ferrous iron in
hemoglobin to the ferric state.
Cyanide has higher affinity to methemoglobin compared to cytochrome.
Assay:
Direct titration with titanous chloride (self indicator).
Mercury-containing compounds:
HgOH
Merbromine (Mercurochrome):
Disodium 2,7-dibromo-4-hydroxyMercury fluorescin.
NaO
Br
O
O
Br
COONa
Assay:
Bromine content:
The organic bromine is converted to NaBr by the double crucible method.
The resulting NaBr is determined by Volhard’s method.
Mercury content:
Heat with NaOH / Zinc powder.
Dissolve the resulting amalgam in HNO3 to give Hg(NO3)2.
Titrate with NH4SCN using ferric alum as indicator.
Uses: antiseptic.
Synthesis:
HO
OH
HO
HO
OH
+
O
H2SO4
O
O
heat
O
OH
O
O
Fluorescein
Br2 / HOAc
Compound
Hg(OAc)2
HO
O
OH
NaOH
Br
Br
COOH
Urinary antiseptics
Hexamine (Methenamine):
N
N
O
N
Acidic pH
N
H
H
Formaldehyde
(CH2)6N4
Hexamethylenetetramine
Mode of action:
• The free base has no antibacterial action.
• Hexamine liberates HCHO in the acidic pH of urine (4.5-5.5).
Formaldehyde then exerts antiseptic effect in the urinary system.
• Acidifying the urine by the concurrent administration of NH4Cl is
advantageous.
– Do NOT consume urinary alkalinizers such as milk and antacids
• An alternative is to use methenamine mandelate which is
hydrolyzed in the body to release methenamine and mandelic
acid. Mandelic acid, in addition to having antibacterial effect,
it helps to maintain acidic urine.
N
HO
N
N
COOH
.
N
• Bacteria which elaborate urease are resistant to hexamine
because urease hydrolyze urea to ammonia which make urine
alkaline. In this case, a urease inhibitor such as
acetohydroxamic acid should be concurrently administered.
O
OH
H3C
N
H
Nitrofurans
Broad-spectrum germicidal agents (Gram +ve and Gram –ve bacteria,
fungi and some protozoa such as trichomonas vaginalis).
O
O
O2N
O2N
NH
O
C
H
N
N
O
Nitrofurantoin (Macrofuran)
(Oral)
O
C
H
N
N
H
NH2
Nitrofurazone (Furacin)
(Topical)
Nitrofurans are withdrawn in many countries due to their serious adverse
reactions such as:
– Hypersensitivity reactions.
– Suspect mutagen and carcinogen.
Fluoroquinolones
–
–
–
–
Active against a variety of Gram(+ve) & (-ve)
Relatively nontoxic, well-tolerated
Developed because of good Gram(-ve) activity
Modest Gram(+ve) activity  improving!
Required pharmacophore
O
COOH
N
N
Nalidixic acid
(NegGram)
First clinically useful agent
O
F
6
5
4
7
N
8
N1
O
COOH
3
F
2
N
HN
COOH
N
N
HN
Enoxacin
Norfloxacin
(Noroxin)
Bioisostere of norfloxacin
O
F
N
HN
O
COOH
F
N
N
N
Ciprofloxacin
(Cipro)
COOH
N
O
Ofloxacin
(Tarivid)
Conformationally restricted
SAR:
The minimum pharmacophore required for antibacterial
activity is:
* 4-pyridone ring.
* 3-carboxylic acid group.
Reduction of the 2,3-double bond  loss of activity.
The 6-fluoro group increases lipophilicity  facilitates
penetration into tissues.
The piperazine moiety broaden spectrum to include
Pseudomonas aeroginosa. On the other hand, it increases
affinity for GABA receptors resulting in CNS side effects.
Required pharmacophore
O
F
N
HN
COOH
N
R
Mode of action:
Inhibition of DNA supercoiling by inhibiting bacterial DNA
gyrase (an enzyme equivalent to topoisomerase II in human
involved in DNA synthesis).
Fluoroquinolones have selectivity for gyrase because it is
sufficiently different from topoisomerase II.
Clinical uses:
Urinary tract infections caused by Gram –ve bacteria
– E. coli, Klebsiella, Pseudomonas
Treatment of resistant gonorrhea (an alternative to G3
cephalosporins).
Treatment of atypical pneumonias.
Other indications: gastroenteritis, meningitis, bone and joint
infections, biliary tract infections, and lower respiratory tract
infections.
Incompatibilities:
Incompatible with metals due to the high chelating ability of
the 4-carbonyl group with the 3-carboxylic acid group.
– Antacids (Mg, Al),
– Hematinics (iron),
– Mineral supplements (Zn)
Incompatible with dairy products (Ca).
M
O
2+
O
F
OH
N
HN
N
R
Advise patients:
May cause dizziness or lightheadedness
DO NOT use (4h before, 2 h after taking medication)
Mg or Al containing antacids
Fe or Zn containing products
Multi-vitamins
Do not take with milk or dairy products - use a glass of
water
Take 1 hour before or 2 hours after food
Moderate to severe photosensitivity can occur – advise
patients to use a sun screen
NOT recommended for children/pregnant women
Sulfonamides
First effective antibacterial agents that could be used systemically.
Historical background:
In the search for new antibacterial agents for streptococcal and
staphylococcal infections (major killers at that time), azodyes were
screened for possible antibacterial activity.
Gerhard Domagk (a physician) in 1932, screened prontosil and found
that it is active against streptococcal infections in mice (in vivo) but
inactive on bacterial cultures (in vitro).
In 1935, it was concluded that sulfanilamide is the active antibacterial
agent and that prontosil is a prodrug (the first ever prodrug)
metabolized in vivo to release the active species.
H2N
N
NH2
N
Prontosil
(inactive in vitro)
(active in vivo)
SO2NH2
in vivo
H 2N
NH2
NH2
+
H2N
SO2NH2
Sulfanilamide
• Domagk won the Nobel prize in medicine for this
discovery in 1939 but the Gestapo prevented him from
receiving the prize at that time.
• Sulfonamides played a significant part in world’s history
by saving Winston Churchill’s life during World war II.
• After sulfanilamide, hundreds of sulfonamides were
developed through molecular modification.
• The use of penicillin in the 1940s largely replaced
sulfonamides due to:
– Toxicity in some patients.
– Bacterial resistance due to early worldwide
indiscriminant use.
• Today, the use of sulfonamides is limited to treatment of
• The discovery of sulfonamides triggered the modern
progress of chemotherapy and the concept of prodrugs.
• Many pharmacologically active compounds were
developed through molecular modification of
sulfanilamide.
O
O
H2N
NH2
S
R
SO2NH
HN
O
Dapson e
(anti le protic)
HOOC
O
Cl
H2NO2S
H
N
COOH
Fu rose m i de
(diu re ti c)
SO2 N
Probe n e ci d
(anti gout)
S ul fonyl u re as
(hypogl yce mi c)
Cl
N
H2NO2S
S
NH
O
O
C h loroth i az i de
(diu re ti c)
R`
SAR
Sulfanilamide is the prototype of sulfonamides
H2N
NH
SO2NH2
O
O
O
Anilino
amino group
S
Sulfonamido
amino group
It ionizes at physiological pH as follows:
NH2
H2N
SO2NH2
H2N
SO2NH
+H
The active form of sulfanilamide is the ionized
form (water-soluble).
However, adequate lipophilicity (lipid solubility)
is required for the sulfonamide to pass through
bacterial cell walls.
High degree of ionization is detrimental for
NH2
O
Sulfanilamide has weak activity How can we
explain this?
• As an acidic drug:
% ionized =
% ionized =
100
1 + 10 (pKa-pH)
100
1 + 10
(10.4 - 7.4)
= ~ 0.1
• Activity could be increased by lowering the pKa
of sulfanilamide.
How can we achieve this?
– By replacing one of the sulfonamido
hydrogens by electron-withdrawing moieties
in order to increase acidity of the remaining
hydrogen and hence lower the pKa.
Question:
• Sulfadimidine has greater activity compared to
sulfanilamide… How can you explain this?
N
H2N
SO2NH2
H2N
SO2NH
N
Sulfadimidine
pKa = 7.4
Sulfanilamide
pKa = 10.4
% ionized =
100
1 + 10 (7.4 - 7.4)
= 50
• Studies revealed that maximum activity is
exhibited by sulfonamides of pKa values 6.6 to
7.4 (% ionization ~ 50 – 80%).
• The nonionized form is insoluble at the acidic pH of urine
and tend to form crystals in the renal tubules
“crystalluria” causing renal damage.
Question:
• Sulfanilamide (pKa = 10.4) causes much more crystalluria
compared to sulfamethoxazole (pKa = 6)… why?
H2N
SO2NH2
H2N
SO2NH
N
Sulfanilamide
pKa = 10.4
O
Sulfamethoxazole
pKa = 6.0
100
= ~ 0.01 (sulfanilamide)
(10.4 - 6)
1 + 10
100
= 50 (sulfamethoxazole)
% ionized =
(6 - 6)
1 + 10
% ionized =
• In conclusion, pKa of the sulfonamide
determines both activity and toxicity.
SAR Summary
• The anilino amino group:
– Important for activity.
– Should be at para position.
– Should be unsubstituted (except acylation to make
prodrugs).
• The aromatic ring:
– Essential for activity.
– Substitution at ortho or meta position destroys
activity.
• The sulfonamido group:
– Essential for activity.
– It should be secondary and substituted with electronwithdrawing heterocycles.
Classification:
•
Classified according to therapeutic application
into:
A. Sulfonamides for systemic infections
B. Sulfonamides for urinary tract infections
C. Sulfonamides for intestinal infections
D. Sulfonamides for ophthalmic infections
E. Sulfonamides for burn therapy
Sulfonamides for systemic infections
• Well absorbed from GIT and effective orally.
• Examples: sulfanilamide, sulfapyridine, sulfadiazine,
sulfamerazine, sulfadimidine and sulfathiazole.
N
H2N
H2N
SO2NH
Sulfadiazine
N
SO2NH
N
Sulfathiazole
S
Synthesis of sulfadiazine:
O
O
N
HN
SO2Cl
+
H2N
N
HN
N
SO2NH
N
1) NaOH
2) HOAc
N
H2N
SO2NH
Sulfadiazine
N
Sulfonamides for intestinal infections
• Sulfonamides that are so hydrophilic that they are poorly
absorbed from the GIT and exert a local action in the
intestine.
• Examples: sulfaguanidine, succinylsulfathiazole and
phthalyl-sulfathiazole.
• The latter two are designed as hydrophilic nonO
absorbable prodrugs of sulfathiazole
that are hydrolysed
NH
slowly in the large intestine
to release the active N
H2N
SO2NH
HN
SO2NH
sulfonamide.
NH2
S
COOH
Sulfaguanidine
Succinylsulfathiazole
O
N
HN
SO2NH
S
COOH
Phthalylsulfathiazole
Sulfonamides for ophthalmic infections
• Highly soluble at physiological pH.
• Example: sulfacetamide sodium. H2N
Na
O
SO2N
CH3
Sulfonamides for burn therapy
• Particularly effective against species of bacteria that are
usually associated with failures in burn therapy
Pseudomonas aeroginosa and Clostridium welchii .
• Examples: silver sulfadiazine and mafenide.
Ag
H2N
N
SO2NH
S
Silver sulfadiazine
SO2NH2
H2N
Mafenide
• Silver sulfadiazine is very slightly soluble and not
absorbed and releases sulfadiazine slowly.
• Mafenide is not a true sulfonamide and acts by a different
mechanism of action.
Sulfonamides for urinary tract infections
• Rapidly absorbed and slowly excreted so reach high
concentration in the urinary system.
• Examples: sulfacetamide and sulfamethoxazole.
O
H2N
H2N
SO2NH
Sulfacetamide
CH3
SO2NH
N
O
Sulfamethoxazole
Chemical name for sulfacetamide:
N-(p-aminobenzenesulfonyl)acetamide
* Sulfamethoxazole is usually used in combination with trimethoprim
marketed under many trade names such as Sutrim, Bacterim, Septazole,
Enterim, …
Crystalluria
• The main side effect of sulfonamides
• High pKa sulfonamides and their N4-acetylated
metabolites tend to exist at the acidic pH of urine
exclusively as the non-ionized form which is insoluble
and tend to crystallize in the renal tubules causing severe
renal damage.
• Measures to reduce crystalluria:
– The use of sulfonamides with lower pKa values such
as sulfadiazine, sulfacetamide and sulfamethoxazole.
– The use of sulfonamide combinations. This allows
reduction of the dose of each sulfonamide.
Other
side effects
– High
fluidare:
intake.
anemia in of
patients
deficient
inNaHCO
glucose-6-phosphate
–Hemolytic
Alkalinization
urine
using
3.
dehydrogenase
Compete with warfarin for plasma protein binding sites => toxicity
Mode of action:
• Sulfonamides inhibit bacterial DNA synthesis through
competitive antagonism to para-aminobenzoic acid
(PABA)… how?
O
O
H2N
H2N
N
S
O
R
O
PABA
Sulfonamides
• Being structurally similar, sulfonamides compete with
PABA forOHdihydropteroate synthetase thereby producing
O
O
COOH
..
N
non-functional
(fake) DNA.
H N
N
O
P
O
H2N
N
O
P
OH
O
N
H
Dihydropteridine
diphosphate
2
O
..
H2N
OH
PABA
Glutamic acid COOH
OH
O
O
..
N
N
O
P
O
O
H2N
N
P
COOH
H2N
OH
O
N
H
O
..
Dihydropteridine
diphosphate
Glutamic acid COOH
H2N
OH
PABA
Dihydropteroate
synthetase
Sulfonamides
Sulfones
PAS
-
OH
O
N
N
H2N
Trimethoprim
Pyrimethamine
Methotrexate
N
-
N
H
HN
Dihydrofolic acid
Dihydrofolate
reductase
Tetrahydrofolate
NH2
OCH3
N
H2N
COOH
N
H
Thymidylate
N
Trimethoprim
OCH3
OCH3
DNA
COOH
• Sulfonamides are bacteriostatic.
• Human DNA synthesis is not inhibited since human cells
use ready folic acid from diet (do not have to make folic
acid).
• Bacterial cells cannot make use of host’s folic acid since
bacterial cell walls are impermeable to folic acid.
• Sulfonamides are inactive in the presence of pus or
necrotic tissues since they are sources for nucleotides.
• Sulfonamides/trimethoprim combination has synergistic
antibacterial action due to “sequential blockade” of
bacterial DNA synthesis, i.e., they act on two successive
steps in DNA synthesis.
Spectrum of activity:
Gram-positive bacteria (especially pneumococci and
meningococci), Gram-negative bacteria, Chlamydia and
some protozoa.
Mechanism of resistance:
•
Antimycobacterial
Agents
Mycobacteria: Mycobacterium tuberculosis =>
Tuberculosis
Mycobacterium leprae => Leprosy (Hansen’s
disease)
• Slow-growing and difficult to stain (acid-fast bacilli).
• Abnormally high lipid content (mycolic acid) of the cell
envelope.
• Treatment of mycobacterial infection complicated by:
– Intracellular location of mycobacteria - phagocytes
– Drug resistance
– Chronic nature of these diseases
• In tuberculosis, the bacilli reach the alveoli, ingested by
pulmonary macrophages, fibroblasts then enclose the
infection site leading to formation of granulomas or
tubercles, hence the name Tubercle Bacillus (T.B).
Diagram of cell wall
• Leprosy primarily affects the skin as chronic
granulomatous infection.
• Tuberculosis primarily affects the lung (pulmonary
tuberculosis) but infection may extend to brain, bones,
eyes and skin (extrapulmonary tuberculosis) especially in
HIV-infected patients.
Tuberculosis
Clinical picture of leprosy:
Antitubercular agents
• Based on history of discovery, sulfanilamide
then dapsone then streptomycin (a turning point)
then PAS then isoniazid then ethambutol then
rifampin.
• Due to multiple drug resistance, a combination
of two or three of these agents is usually used.
• Classification:
A) p-Aminosalicylic acid derivatives.
B) Pyridine carboxylic acid derivatives.
C) Miscellaneous agents.
COOH
p-Aminosalicylic acid (PAS):
OH
Mode of action
Two possible mechanisms:
NH
• Similar to the mode of action of sulfonamides: 2
Being structurally similar to PABA, it acts as a
competitive antagonist to PABA by inhibiting
COO the
dihydropteroate synthetase eventually inhibits
OH
biosynthesis of mycobacterial DNA.
• It chelates trace elements.
Ca
HN
Disadvantages:
O
– Gastrointestinal irritation.
– Short half life (short duration).
• To overcome these disadvantages:
– It should be formulated in enteric-coated dosage
form or an antacid (aluminum hydroxide) is
prescribed concurrently.
– Developing less GI irritaing and longer acting
2
++
Pyridinecarboxylic acid derivatives:
Isoniazid (INH):
O
NHNH2
• Isonicotinic acid hydrazide.
Synthesis:
O
OH
N
O
OEt
O
NHNH2
OH
EtOH / H2SO4
OH
NH2NH2
OH
N
N
N
N
Pyridoxine
Mode of action:
• Inhibition of the biosynthesis of mycolic acids (branched
long chain fatty acids) which are important components
of the cell wall of mycobacteria.
• Active only against dividing mycobacteria.
NOTES:
1) INH is the most active antitubercular agent rather than
any other synthetic or antibiotic.
2) the equation of the assay is as follows:
CONHNH2
N
+ 2Br2 + H2O
COOH
N2 + HBr +
N
3)Ftivazide is hydrazone results from reaction of INH with
benzaldehyde derivatives,
It is as active as INH but non toxic.
OCH3
CONHN C
H
Ftivazide
N
OH
Side effects of INH:
long-term therapy may result in fatal drug induced
hepatitis.
It causes peripheral neuritis as it results in pyridoxine
deficiency by acting as a pyridoxine antagonist therefore,
the concurrent administration of pyridoxine (vitamin B6)
prevents the occurrence of peripheral neuritis.
INH is metabolized by acetylation. Nearly half of the
population are fast acylators of INH and the other half are
slow acylators of INH. Slow acylators (including middle
S
NH2
easterns) are genetically deficient of N-acetyltransferase
and are more prone to the side effects of INH.
Ethionamide:
Developed as a less toxic (5 times) analogue of INH.
Mode of action like INH.
Active only against dividing mycobacteria.
N
a
H
SAR:
O
N
H
b
N
H
c
N
Aromatic ring:
• If replaced by benzene, piperidine or thiazole 
loss of activity.
Hydrazide moiety:
• The -position is the best position for activity.
• Replacement of Ha with alkyl groups 
decreases activity.
• Replacement of Hb and/or Hc with small alkyl
groups  increases activity. large alkyl groups
decreases activity.
• Replacement of Hb and Hc with alkylidene 
Diazine derivatives ( pyrazinamide ).
NH2
N
O
N
Pyrazinamide
N
KMnO4
N
/ -CO2
N
N
quioxaline
benzopyrazine
Assay:
COOH
Pyrazinedioic acid
NH2
N
COOH
N
N
Kjeldahl method
COOH
N
CH3OH/HCl
COOEt
N
NH3
N
CONH2
N
O
N COONa
+ NaOH Boiling in AMMONIA
+ NH3 which is recieved in
N
distillation apparatus
N
boric acid to form ammborate which is titratrated by st HCl and methyl red indicator
Miscellaneous agents
Ethambutol (Etibi):
Mode of action:
•
Being structurally similar to cellular polyamines
(spermidine and spermine), it interferes with their
function (essential for integrity of nucleic acids).
H2N
N
H
NH2
Spermidine
•
It chelates divalent metals thus inhibiting essential
enzymes.
O
N
H
M
O
N
H
Synthesis of ethambutol:
HO
2
NH2
+ Cl
Cl
Base
- 2HCl
Antitubercular antibiotics:
Streptomycin
Cycloserine
Rifampin
Ethambutol
Antileprotic agents
A) Sulfones:
O
H2N
S
NH2
O
Dapsone:Di-(4-aminophenyl)sulphone.
• The drug of choice for the treatment of leprosy.
• In addition to the antileprotic effect, dapsone has also
antimalarial and antileshmanial activities.
Mode of action:
• Acts by a mechanism similar to sulfonamides.
Evidences:
• PABA partially antagonizes the action of sulfones.
• Cross resistance between sulfonamides and sulfones.
Disadvantage:
• Poor solubility
poor formulation
Dapsone prodrugs (soluble dapsone
analogs):
Solapsone:Tetrasodium salt of Bis[4-(3phenyl-1,3-SO Na
O
NaO S
disulphopropylamino)phenyl]sulphone
HN
S
NH
3
3
O
SO3Na
NaO3S
Solapsone undergoes acidic hydrolysis in the
stomach to release dapsone which is the active
antileprotic drug.
Synthesis of Dapsone
2 Cl
NO2
Na2S
O2N
S
NO2
OXID
K2Cr2O7
Dapsone
RED
Zn / HCl
O2N
O
S
O
NO2
Synthesis of Solapsone
O
H2N
S
O
NH2
condensation with 2 cinnamaldehyde
H
C
H
C
O
S
O
N
CHO
N
4NaHSO3
SO3Na
NaO3S
HN
O
S
O
NaO3S
Solapsone
NH
SO3Na
SAR of sulphones ( dapsone):
1) of the six isomers of diaminodiphenylsulphones only 4,4- is the active
one.
2) additional substitution of phenyl ring cause loss of activity.
3)N-alkyl and N-acyl derivatives retain activity due to deacylation and
dealkylation in vivo.
4)replacement of aminophenyl group with alkyl or aryl or heterocyclic
group will cause loss in activity.
5)activity is exhibited by several mono and bis
Schiffs base of aromatic aldehydes and aldehyde-bisulphite complexes as
in glucosulphone, and glucosulphone has weak activity in vitro as no
hydrolysis which regenerate ACTIVE DAPSONE.
6) sulphone group is essential for activity.
B) Miscellaneous agents:
Clofazimine:N,10-bis-(4-chlorophenyl)-2,10-dihydro-2[1-methylethylamino]-3-phenazinamine
Cl
• A phenazine dye
• Binds preferntially to
mycobacterial DNA
• Teratogenic
N
N
N
N
H
Cl
Thiambutosine:
• A thiourea derivative
• N-(4-butoxyphenyl)-N- -(4BuO
dimethylaminophenyl)thiourea
Other agents used for leprosy:
• Isoniazid, ethionamide, rifampin
N
S
N
H
N
H
The American Medical Association
“AMA” states:
• Treatment of leprosy is difficult and
complex.
• A combination of dapsone / clofazimine
/ rifampin should be used.
• Therapy duration = 5 years – life time.