Transcript ANTIBIOTICSx

ANTIBACTERIAL ANTIBIOTICS
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ANTIBIOTICS

The word "antibiotics" comes from the Greek
anti ("against") and bios ("life"). Antibiotics are
drugs that either destroy bacteria or prevent their
reproduction. Antibiotics that kill bacteria are called
"bactericidal" and the ones that stop the growth of
bacteria are called "bacteriostatic".
 Some antibiotics are 'bactericidal', meaning that
they work by killing bacteria. Other antibiotics are
'bacteriostatic', meaning that they work by stopping
bacteria multiplying.
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 Each different type of antibiotic affects different
bacteria in different ways. For example, an
antibiotic might inhibit a bacterium's ability to turn
glucose into energy, or its ability to construct its
cell wall. When this happens, the bacterium dies
instead of reproducing.
 Some antibiotics can be used to treat a wide
range of infections and are known as 'broadspectrum' antibiotics. Others are only effective
against a few types of bacteria and are called
'narrow-spectrum' antibiotics.
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Targets for Antibiotics
Fig 24.9
pg. 741
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Antibiotics That Inhibit Cell Wall
Synthesis
Penicillin and Cephalosporins
1. One major class of antibiotics inhibit the
synthesis of peptidoglycan.
2. Once cell wall synthesis (involving penicillin
binding proteins) is inhibited, enzymatic autolysis
of the cell wall can occur.
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Antibiotic resistance
 Antibiotics are extremely important in medicine, but
unfortunately bacteria are capable of developing resistance to
them. Antibiotic-resistant bacteria are germs that are not killed
by commonly used antibiotics. When bacteria are exposed to the
same antibiotics over and over, the bacteria can change and are
no longer affected by the drug.
 Bacteria have number of ways how they become antibioticresistant. For example, they possess an internal mechanism of
changing their structure so the antibiotic no longer works, they
develop ways to inactivate or neutralize the antibiotic. Also
bacteria can transfer the genes coding for antibiotic resistance
between them, making it possible for bacteria never exposed to
an antibiotic to acquire resistance from those which have.
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ANTIBIOTICS CLASSIFICATION
Based on chemical
structure
•β-lactum antibiotics
•Aminoglycoside
•Tetracyclines
•Macrolide antibiotics
•Lincomycins
•Polypeptide antibiotics
•miscellaneous
Based on
pharmacological
activity
• Antifungal antibiotics
• Anticancer antibiotics
• Anti typhoid antibiotics
• Anti diarrheal antibiotics
• Antituberculor antibiotics
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Β-LACTAMS
β-lactam ring
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HOW DO THEY WORK?
1.
2.
3.
4.
The β-lactam binds to Penicillin Binding
Protein (PBP)
PBP is unable to crosslink peptidoglycan
chains
The bacteria is unable to synthesize a stable
cell wall
The bacteria is lysed
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Mechanism of action of β-lactam antibiotics.
Normally, a new subunit of N-acetylmuramic acid (NAMA) and Nacetylglucosamine (NAGA) disaccharide with an attached peptide
side chain is linked to an existing peptidoglycan polymer. This may
occur by covalent attachment of a glycine () bridge from one peptide
side chain to another through the enzymatic action of a penicillinbinding protein (PBP).
In the presence of a β-lactam antibiotic, this process is disrupted. The
β-lactam antibiotic binds the PBP and prevents it from cross-linking
the glycine bridge to the peptide side chain, thus blocking
incorporation of the disaccharide subunit into the existing
peptidoglycan polymer.
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Mechanism of penicillin-binding protein
(PBP)inhibition by β-lactam antibiotics.
PBPs recognize and catalyze the peptide bond between
two alanine subunits of the peptidoglycan peptide side
chain. The β-lactam ring mimics this peptide bond.
Thus, the PBPs attempt to catalyze the β-lactam ring,
resulting in inactivation of the PBPs.
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Peptidoglycan Layer
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The beta lactam antibiotics include penicillins
(e.g. ampicillin), cephalosporins and monobactams.
They bind to and inhibit enzymes (penicillin binding
proteins) involved in the transpeptidation (cross-linking) of
peptidoglycan. These antibiotics have in common the four
membered lactam ring.
Attached to the lactam, penicillins have an additional five
membered ring and cephalosporins a six membered ring.
Monobactams consist of the lactam ring alone and display
antibiotic activity.
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penicillins
Penicillin is the oldest known antibiotic in the whole world and
what is really surprising that it was discovered by total
coincidence.
It was discovered by Alexander Fleming in the late nineteenth
century when he was trying to multiply the penicillum fungi in the
lab.
Penicillin is still used till now for the treatment of many bacterial
and fungal infections and of course it was developed immensely
since that time
.
There are now different categories of penicillin but the core of
these categories is the natural penicillin or what is known the
penicillin G. natural penicillin is effective against most of the gram
positive microorganisms like streptococcus and staphylococcus. It
is also effective against some gram negative bacteria and it is
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mainly used for treatment of oral cavity infections.
The second category of penicillin antibiotic is penicillinase resistant
antibiotics. Oxacillin and cloxacillin are prominent examples for this
group. This group does not target a wide range of microorganisms like
the natural penicillin but it targets a specific group of bacteria which
produces the beta lactamase enzyme. The lactamase producing
bacteria are resistant to natural penicillin.
The third category of penicillin antibiotics is the aminopenicillins.
Amoxicillin is the best example for this group and it is widely used
today. This is group is very potent against gram negative bacteria like Ecoli and Hemophelus influenza. The best feature of this group of
penicillin antibiotics is that they can tolerate the gastric acid. This
means that they could be administered orally without being degraded
through the acid medium of the stomach.
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The fourth group is called the extended penicillin antibiotics,
although this group is effective against many gram - negative
microorganisms but they are still inactive against beta- lacatam
producing bacteria like the aminopinicillins
.
The four categories of penicillin are still used till now to combat
different types of microorganisms and every category is used to
perform its specific role.
All penicillin drugs act in the same way by affecting the bacterial
cell wall. Suppression the formation of the bacterial cell wall will
cause the bacteria to die and omit the effect of the microorganism
on the body, the best feature of penicillin is that it develops the
least side effects on the patient.
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Penicillin
• Prevents crosslinking of peptidoglycan, gram +/some • Beta-lactam nucleus
– Resistance through beta-lactamases
• In low concentrations,kills gram + cells, gram (-) in higher
doses
• Allergic reactions
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History of penicillins
In 1875 Pasteur and Joubert discovered that certain moulds could produce
toxic substances which killed bacteria. Unfortunately, these substances
were also toxic to humans and of no clinical value. However, they did
demonstrate that moulds could be a potential source of antibacterial
agents.
In 1928, Fleming noted that a bacterial culture which had been left several
weeks open to the air had become infected by a fungal colony. Of more
interest was the fact that there was an area surrounding the fungal colony
where the bacterial colonies were dying. He correctly concluded that the
fungal colony was producing an antibacterial agent which was spreading
into the surrounding area. Recognizing the significance of this, he set out to
culture and identify the fungus and showed it to be a relatively rare species
of Penicillium. It has since been suggested that the Penicillium spore
responsible for the fungal colony originated from another laboratory in the
building and that the spore was carried by air currents and eventually
blown through the window of Fleming's laboratory.
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Fleming spent several years investigating the novel antibacterial
substance and showed it to have significant antibacterial properties
and to be remarkably non-toxic to humans. Unfortunately, the
substance was also unstable and Fleming was unable to isolate and
purify the compound. He therefore came to the conclusion that
penicillin was too unstable to be used clinically.
The problem of isolating penicillin was eventually solved in 1938 by
Florey and Chain by using a process known as freeze-drying which
allowed isolation of the antibiotic under much milder conditions than
had previously been available. By 1941, Florey and Chain were able to
carry out the first clinical trials on crude extracts of penicillin and
achieved spectacular success. Further developments aimed at
producing the new agent in large quantities were developed in the
United States such that by 1944, there was enough penicillin for
casualties arising from the D-Day landings.
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Penicillins were used widely and often carelessly, so that the
evolution of penicillinresistant bacteria became more and more of
a problem. The fight against these penicillin-resistant bacteria was
promoted greatly when, in 1976, Beechams discovered a natural
product called clavulanic acid which has proved highly effective in
protecting penicillins from the bacterial enzymes which attack
penicillin.
STRUCTURE OF PENICILLIN
Penicillin contains a highly unstable-looking bicyclic system
consisting of a fourmembered lactam ring fused to a fivemembered thiazolidine ring. The skeleton of the molecule suggests
that it is derived from the amino acids cysteine and valine (Fig.),
and this has been established.
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STRUCTURE OF PENICILLIN
O
S
C
NH
CH
CH
CH3
C
CH3
O
C
N
CH
COOH
Site of penicillinase action.
Breakage of the β lactam ring.
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STRUCTURE
O
R=
C
CH2
H
S
Me
R
Benzyl penicillin (Pen G)
R=
O
H H
N
CH2
Phenoxymethyl penicillin (Pen V)
N
Acyl side
chain
Me
6-Aminopenicillanic acid
(6-APA)
O
CO2H
Thiazolidine
ring
b-Lactam
ring
Side chain varies depending on carboxylic acid present in fermentation
medium .For example, corn steep liquor was used as the medium when
penicillin was first mass-produced in the United States and this gave penicillin
G (R=benzyl). This was due to high levels of phenylacetic acid (PhCH2CO2H)
present in the medium.
CH2
CO2H
Penicillin G
present in corn steep liquor
OCH2
CO2H
Penicillin V
(first orally active penicillin)
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Mechanism of action - bacterial cell wall synthesis
NAM
NAG
NAM
NAG
L-Ala
L-Ala
D-Glu
D-Glu
L-Lys
Gly Gly Gly Gly Gly
L-Lys
D-Ala
D-Ala
D-Ala
D-Ala
SUGAR
BACKBONE
Gly Gly Gly Gly Gly
PENICILLIN
TRANSPEPTIDASE
D-Alanine
NAM
NAG
NAM
L-Ala
L-Ala
D-Glu
D-Glu
L-Lys
D-Ala
Gly Gly Gly Gly Gly
Cross linking
L-Lys
NAG
SUGAR
BACKBONE
Gly Gly Gly Gly Gly
D-Ala
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CONSTITUTION OF PENICILLIN
Molecular formula of penicillin is C9H11N2O4SR

presence of carboxylic group:
They form mono salts indicating the presence of a carboxylic group
Penicillins are not found to posses a free amino group , thiol group

Hydrolysis:
penicillins are hydrolysed by hot dilute inorganic acids , they lose one carbon atom
in the Form of carbondioxide and they are also degraded to the equimolar
amounts of an amine, PENICILLAMINE, and an aldehyde PENILLOALDEHYDES. All
the penicillins are degraded to the Same amine but different aldehydes because
the fragement comes in the aldehyde portion.
C9H11N2O4SR + 2H2O
CO2 + C5H11NO2S + C3H4NO2R
PENICILLAMINE
PENILLOALDEHYDE
Now since the fragment R comes in the aldehyde portion, all the penicillins
give the same amine but different aldehydes.
 Structure of D-Penicillamine :
C5H11NO2S.this ha been elucidated on the basisof following facts:
Molecular formula of Penicillamine is C5H11NO2S.
As Penicillamine respond to the indigo colour reaction with ferric chloride solution
and Also gives colour reaction with sodium nitroprusside , this shows that DPenicillamine is probably a substituted cysteine.
From electrometric titrations ,it is revealed that there are three pka values 1.8, 7.9,and
10.5 corresponding to the carboxylic , α-amino and thiol groups respectively. The
presence Of these groups also reveals that D-penicillamine is probably substituted
cysteine.
Moreover, the chemical reactions are parrell to those of cysteine . For example
like cysteine D-penicillamine also reacts with acetone to yield an isopropylidene
derivative. The latter Compound does not contain a free amino or thiol group
and is reconverted into pencillamine On hydrolysis. These reactions reveal that
these groups,i.e, amino and thiol group , are Attached to adjacent carbon
atoms.further oxidation of penicillamine with brominewater Yields a sulphonic
acid(this reaction is characterstic of a thiol)
When kuhn-roth method for the determination of methy side chain is applied to
D-PENICILLAMINE It gives a very low value (~ 0.2molecules): this low value
reveals that D-PENICILLAMINE contains An isopropyl end-group and not a
methyl end-group.
so D-PENICILLAMINE is β,β-dimethylcysteine which has been confirmed by its
synthesis.
NaOH
ClCH2COCL
(CH3)2CHCHCO2H
(CH3)2CHCHCO2H
-HCL
NH2
NHCOCH2Cl
DL-valine
(CH3CO)2O
C
(H3C)2C
CHCO2H
(CH3)2C
SH
H2S
BOIL
NHCOCH3
CHCO2H
1.H2S
S
N
C
C
(H3C)2C
+
2.-H
3.+H+
CH3
C
N
O
H
C
SH
CH3
Azalactone
2,5-trimethyl-2thiazoline-4-carboxylic acid
HCL(boil)
pyridine
(CH3)2C
SH
CHCO2H
NH2
DL-penicillamine
From the above synthesis, the recemic miture DL is obtained.for
resolution of this racemic mixture , this is treated with formic acid to
yield the formyl derivative which is then resolved by means of brucine .
The formyl group is removed by hydrolysis.
HCO2H
(CH3)2C
SH
CHCO2H
NH2
CH3)2C
CHCHO2H
SH
NHCHO
DL-form
DL-formyl derivative
1.resolution by brucine
2.HCL
3.pyridine
(CH3)2C
SH
CHCO2H
NH2
D-penicillamine
The D- penicillamine odtained from the resoltuion of the recemic mixture has
been found to be identical with the natural penicillamine.
When the penicillamine is treated with diazomethane , it is converted into its
methyl ester.
 The lattes compound when treated with aqueous solution of mercuric chloride
yields the methyl ester of penicillamine .these reactions reveal that the
carboxylic group in penicillin itself .
Structure of penilloaldehyde:
This has been elucidated on the basis of the following facts.:
The general formula of penilloaldehyde has been found to be C3H4NO2R .
When hydrolysed vigorously , all penilloaldehydes yield a substitued acetic acid
and Amino acetaldehyde . This reaction reveals that penilloaldehyde are
RCOOH + NH2CH2CHO
RCONHCH2CHO
(or C3H4NO2R) + H2O
acyl derivatives
of aminoacetaldehyde.
substitued aminoacetaldehyde
penilloaldehydes
acetic acid
The above structure of penilloaldehyde has been confirmes by its synthesis from the
Corresponding acid chloride and amino acetal.
RCOCl + NH2CH2CH(OC2H5)2
RCONHCH2CH(OC2H5)2
HCL
RCONHCH2CHO
penilloaldehydes
In this point it was stated that acid hydrolysis of penicillinyields
penicillamine,penilloaldehyde And corbondioxide. The formation of co2 molecule is
indicative of the fact that some unstable acid is formed as an intermediate which
undergoes ready decarboxylation to yield carbon dioxide .such an acid forme as an
intermediate in the hydrolysis of penicillin.
RCONHCHCHO
COOH
penaldic acid
CO2 + RCONHCH2CHO
penilloaldehyde
Mode of lincking of penicillamine and penilloaldehyde.: now the next question arises
How are the two fragments ,penicillamine and penilloaldehyde, linked in penicillin …
So that it may explain tha formation of panaldic acid that is to explain the formation
of co2 ?
Tha mode of linkage of these fragments has been established on the basis of the
following facts;
When penicillin is hydrolysed with dil alkali or with the enzyme penicillinase it It
yields penicilloic acid which is a dicarboxylic acid and readily eliminates a molecule of
co2 to yield a monocarboxylic acid, penilloic acid . this suggest that in penicilloic acid
one of the carboxylic groups is in the β-position with respect to anelectron-attractin g
group.
The structure of penillonic acid has been proved on the basis of the fact that it,when
hydrolysed with aqueous mercuric chloride,yields pwnicillamine and penilloaldehyde.
This type of hydrolysis is characteristic of compounds having a thiazolidine ring. The
presence of thiazolidine type of nucleus in penicillin has been proved by the fact that
it possesses neither free amino group nor a free thiol group .thus penilloic acid could
be represented as
•Because this structure would yield the desired products
SH
S
ROCHNH2CHC
C(CH3)2
HN
CH(CH3)2
H2O
HgCl2
CHCO2H
Penilloic acid (A )
H2N
+
RCONHCH2CHO
CHCO2H
penicillamine
penilloaldehyde
If A is the structure of penilloic acid , then the structure of penicilloic acid would
be represented as
S
R.CO.NH.CH
HOOC
CH
HN
C(CH3)2
CHCOOH
PENICILLOIC ACID
CO2 + (A)
The structure B of penicilloic acid has been confirmed by the fact that
penicillin when treated with methanol yields methyl penicilloate which on
hydrolysis with aq. Mercuric chloride yields methyl penaldate
S
R.CO.NH.CH
CH
C(CH3)2
CH3OH
PENICILLIN
CO2H3C
HN
CHCO2H
Methl penicilloate
H2O
CH(CH3)2
HgCl2
HO2CHCNH2
penicillamine
+
RCONHCHCHO
COOCH3
methl penaldate
Structure of penicillin:
From the above evidences , two structures C , D are possible for penicillin . But
penicillin on treatment with dil acid undergoes molecular rearrangement readly
to yield penillic acid . Therefore , the chemical evidence could not decide about
the correct structure C (or) D of penicillin. However , the physical methods could
decide about the correct structure of penicillin.
S
N
H3CR
N
CH
O
HC
ROCHNHC
HN
oxazolone structure
C(CH3)
CH(CH3)
CO
CO
HC
N
CHCO2H
CHCO2H
(C)
beta lactum structure
(D)
Definite evidence of the existing of the β-lactum ring in penicillin was
obtained by desulphurisation of the benzylpenicillin with raney nickel to
yield desthiobenzylpenicillin (E) Which on hydrolysis by acid yields
desthiopenicilloic acid (F) or on boiling with benzylamine in Dioxane
solution yields benzylaminde of desthiobenzyopenicilloic acid(G).
S
C6H5H2COC.HN
HC
B
OC
CH3
A
N
C.(CH3)
H2C
RANEY Ni
COOH
BENZYLPENICILLIN
C6H5CH2CO.NH.CH
OC
CH2
CH(CH3)2
N
CH.COOH
desthiobenzylpenicilln(E)
C6H5CH2NH2
C6H5CH2CO.NH.CH
CH2
COOH HN
CH(CH3)2
CH.COOH
desthiobenzylpenicilloic acid (G)
C6H5CH2CO.NH.CH
CH2
(H3C)2HC
NH
CO.NH.CH2.C6H3
benzylamide of desthiobenzylpenicilloic acid (H)
CH.COOH
The compounds G , H can be obtained by the desulphation of benzyl penicilloic acid and
Its benzyl amide respectively.
However it was not possible to decide between th two possible structure of penicillami
On chemical evidenca alone , because penicillin readly undergoes moleclar rearrangem
Eg;
on treatment with dil acid , penicillin rearranges to yield penillic acid
penicillin
HCL
CHOO
S
H
C
C
N
C(CH3)2
N
C
R
PENILLIC ACID
CH.COOH
CLASSIFICATION OF PENICILLINS
NATURAL PENICILLINS
BENZYL PENICILLIN(PENICILLIN-G)
PENTYL PENICILLIN(PENICILLIN-F)
3-PENTYL PENICILLIN(PENICILLIN-K)
PHENOXY METHYL PENICILLIN(PENICILLIN-V)
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Semi-synthetic Penicillin
• Ampicillin
• Amoxicillin
• Carbenicillin
• Methicillin
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Penicillins
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Figure 20.6
Cephalosporins
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Cephalosporins
• Semisynthetic B-lactams derived
from chemical side chains added to
7-aminocephalosporanic acid.
• Generally more resistant to Blactamases.
43
Other Inhibitors of Cell Wall Synthesis
• Cephalosporins
– 2nd, 3rd, and 4th
generations more
effective against
gram-negatives
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Figure 20.9
Other Inhibitors of Cell Wall Synthesis
• Polypeptide antibiotics
– Bacitracin
• Topical application
• Against gram-positives
– Vancomycin
• Glycopeptide
• Important "last line" against antibiotic resistant S. aureus
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The Cephalosporins (generalized)
*Not effective vs. Enterococcus or Listeria
1st Generation
Gram (+)
2nd Generation
Decreasing Gram (+) and
Increasing Gram (-)
3rd Generation
Gram (-), but also some GPC
4th Generation
Gram (+) and Gram (-)
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Discovery and structure of cephalosporin C
The second major group of β-lactam antibiotics to be discovered were
the cephalosporins. The first cephalosporin was cephalosporin C—
isolated in 1948 from a fungus obtained from sewer waters on the
island of Sardinia. Although its antibacterial properties were
recognized at the time, it was not until 1961 that the structure was
established.
It is perhaps hard for modern chemists to appreciate how difficult and
painstaking structure determination could be, even in the post-war
period. The advent of NMR spectroscopy in the sixties and seventies
has revolutionized the field so that if a new fungal metabolite is
discovered today, its structure can be worked out in a matter of
days rather than a matter of years.
.
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The structure of cephalosporin C (Fig. 10.41) has similarities to that of
penicillin in that it has a bicyclic system containing a four-membered (βlactam ring. However, this time the (3-lactam ring is fused with a sixmembered dihydrothiazine ring. This larger ring relieves the strain in the
bicyclic system to some extent, but it is still a reactive system
A study of the cephalosporin skeleton reveals that cephalosporins can be derived
from the same biosynthetic precursors as penicillin, i.e. cysteine and valine (Fig.
10.42).
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Properties of cephalosporin C
The properties of cephalosporin C can be summarized as follows.
• Difficult to isolate and purify due to a highly polar side-chain.
• Low potency (one-thousandth of penicillin G).
• Not absorbed orally.
• Non-toxic.
• Low risk of allergenic reactions.
• Relatively stable to acid hydrolysis compared to penicillin G.
• More stable than penicillin G to penicillinase (equivalent to
oxacillin).
• Good ratio of activity against Gram-negative bacteria and Grampositive bacteria.
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Cephalosporin C has few clinical uses, is not particularly potent and at
first sight seems rather uninteresting. However, its importance lies in
its potential as a lead compound to something better. This potential
resides in the last property mentioned above. Cephalosporin C may
have low activity, but the antibacterial activity which it does have is
more evenly directed against Gram-negative and Gram-positive
bacteria than is the case with penicillins.
By modifying Cephalosporin C we might be able to increase the
potency whilst retaining the breadth of activity against both
Grampositive and Gram-negative bacteria. Another in-built advantage
of Cephalosporin C over penicillin is that it is already resistant to acid
hydrolysis and to penicillinase enzymes. Cephalosporin C has been
used in the treatment of urinary tract infections since it is found to
concentrate in the urine and survive the body's hydrolytic enzymes.
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Monobactams
• Monobactams: Aztreonam
• Spectrum: ONLY  Gram negative aerobic bacteria
• Lack of Coverage:
– Some resistant P. aeruginosa, E. cloacae, and C. freundii
– Acinetobacter sp., Stenotrophomonas sp.
• Pharmacokinetics:
– Well distributed into tissues, esp. inflamed tissues
– Excretion: renal clearance
• Adverse reactions:
– Skin rash
– Very low cross-reactivity with Beta-Lactam class – highest risk
in patient allergic to ceftazidime.
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