Download Beta-lactam antibiotics

Beta-Lactam Antibiotics
1
Introduction
β-Lactam antibiotics are the most widely produced and used antibacterial
drugs in the world, and have been ever since their initial clinical trials in
1941.
• β-Lactams are divided into several classes based on their structure and
function; and are often named by their origin, but all classes have a
common β-Lactam ring structure.
• Clinically useful families of beta-lactam compounds include the
• penicillins,
• cephalosporins,
• monobactams
• carbapenems
2
History
1928- Alexander Fleming discovers a mold which inhibits the growth of
staphylococcus bacteria
1940- penicillin is isolated and tested on mice by researchers at Oxford
1941- penicillin mass produced by fermentation for use by US soldiers in
WWII
1950’s- 6-APA is discovered and semi-synthetic penicillins are developed.
1960’s to today- novel β-lactams/ β-lactamase inhibitors are discovered and
modified from the natural products of bacteria
3
Target- Cell WallMOA
Synthesis
The bacterial cell wall is a cross linked polymer called peptidoglycan
which allows a bacteria to maintain its shape despite the internal
turgor pressure caused by osmotic pressure differences.
If the peptidoglycan fails to crosslink the cell wall will lose its strength
which results in cell lysis.
All β-lactams disrupt the synthesis of the bacterial cell wall by
interfering with the transpeptidases which catalyzes the cross linking
process.
4
Peptidoglycan
Peptidoglycan is a carbohydrate composed of alternating units of
NAMA (N-acetyl muramic acid) and NAGA (N-acetyl Gluc0samine).
The NAMA units have a peptide side chain which can be cross linked
from the L-Lys residue to the terminal D-Ala-D-Ala link on a
neighboring NAMA unit.
This is done directly in Gram (-) bacteria and via a pentaglycine bridge
on the L-lysine residue in Gram (+) bacteria.
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Mechanism
6
Mechanism of action - bacterial cell wall synthesis
NAM
NAG
NAM
L-Ala
L-Ala
D-Glu
D-Glu
L-Lys
Gly Gly Gly Gly Gly
NAG
L-Lys
D-Ala
D-Ala
D-Ala
D-Ala
SUGAR
BACKBONE
Gly Gly Gly Gly Gly
PENICILLIN
D-Alanine
NAM
NAG
TRANSPEPTIDASE
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
7
Transpeptidase- PBP
The cross linking reaction is catalyzed by a class of transpeptidases
known as penicillin binding proteins(PBPs)
A critical part of the process is the recognition of the D-Ala-D-Ala
sequence of the NAMA peptide side chain by the PBP. Interfering
with this recognition disrupts the cell wall synthesis.
β-lactams mimic the structure of the D-Ala-D-Ala link and bind to the
active site of PBPs, disrupting the cross-linking process.
8
Mechanism of action - bacterial cell wall synthesis
Normal Mechanism
Pe ptide
Chain
D-Ala
OH
Pe ptide
Chain
Pe ptide
Chain
D-Ala
CO 2 H
Pe ptide
Chain
D-Ala
Pe ptide
Chain
Gly
Gly
D-Ala
O
H
OH
9
Mechanism of β-Lactam Drugs
The amide of the β-lactam ring is unusually reactive due to ring strain
and a conformational arrangement which does not allow the lone
pair of the nitrogen to interact with the double bond of the carbonyl.
β-Lactams acylate the hydroxyl group on the serine residue of PBP
active site in an irreversible manner.
This reaction is further aided by the oxyanion hole, which stabilizes the
tetrahedral intermediate and thereby reduces the transition state
energy.
10
Mechanism of action - bacterial cell wall synthesis
Alternative theory- Pencillin mimics D-Ala-D-Ala.
Mechanism inhibited by penicillin
Blocked
Pe ptide
Chain
Blocked
H2 O
O
R
C
O
H
NH
S
N
O
H
OH
Me
Me
R
O
Gly
C
NH
H
O
R
S
NH
H
O
HN
HN
Me
CO2H
O
C
Me
CO2H
Blocked
O
S
Me
Me
CO2H
Irreversibly blocked
11
Mechanism of β-Lactam Drugs
The hydroxyl attacks the amide and forms a tetrahedral intermediate.
12
Mechanism of β-Lactam Drugs
The tetrahedral intermediate collapses, the amide bond is broken, and
the nitrogen is reduced.
13
Mechanism of β-Lactam Drugs
The PBP is now covalently bound by the drug and cannot perform the
cross linking action.
14
Bacterial Resistance
Bacteria have many methods with which to combat the effects of βlactam type drugs.
 Intrinsic defenses such as efflux pumps can remove the β-lactams
from the cell.
 β-Lactamases are enzymes which hydrolyze the amide bond of the
β-lactam ring, rendering the drug useless. Thus Inactivation of the
antibiotic by beta lactamase
 Imparied penetration of drug to target PBPs
 Bacteria may acquire resistance through mutation at the genes which control
production of PBPs, altering the active site and binding affinity for the β-lactam
.
15
Range of Activity
β-Lactams can easily penetrate Gram (+) bacteria, but the outer cell
membrane of Gram (-) bacteria prevents diffusion of the drug. βLactams can be modified to make use of import porins in the cell
membrane.
β-Lactams also have difficulty penetrating human cell membranes,
making them ineffective against atypical bacteria which inhabit
human cells.
Any bacteria which lack peptidoglycan in their cell wall will not be
affected by β-lactams.
16
Toxicity
β-Lactams target PBPs exclusively, and because human cell membranes
do not have this type of protein β-lactams are relatively non toxic
compared to other drugs which target common structures such as
ribosomes.
About 10% of the population is allergic (sometimes severely) to some
penicillin type β-lactams.
17
Classification PENICILLIN
Naturally
1. Ocurring
• Penicillin G
2. Semisynthetic
3. Synthetic
 Acid Resistant Penicillins
 Phenethicillin
Eg. Peniciliin V
 Penicillinase resistant P.
Eg. Methicillin, Nafcillin, Oxacillin,
Cloxacillin
 Extended spectrum Penicillins
1. Amino penicillins - Eg.
Ampicillin, Amoxicillin,
2. Carboxy PenicillinsCarbenicillin
18
Nomenclature
• Numbering of fused bicyclic ring initiates the at N atom and assigns
the ring S the 4-position.
• Thus, penicillins are named as 4-thia-l-azabicyclo[3.2.0]heptanes,
according to this system
19
Three simplified forms of penicillin nomenclature
have been adopted for general use.
• The first uses the name “penam” for the
unsubstituted bicyclic system, including
the amide carbonyl group, with
numbering systems as just described
• Thus, penicillins generally are designated
according to this system as 6- acylamino3,3-dimethylpenam-2-carboxylic acids.
20
Three simplified forms of penicillin nomenclature
have been adopted for general use.
• The second, seen more frequently in the medical literature, uses the
name “penicillanic acid” to describe the ring system with substituents
that are generally present (i.e., 3,3-dimethyl and 2-carboxyl acid)
21
Three simplified forms of penicillin nomenclature
have been adopted for general use.
• A third form, uses trivial nomenclature to name the entire
• 6-carbonylaminopenicillanic acid portion of the molecule as “penicillin” and
then distinguishes compounds on the basis of the R group of the acyl portion of
the molecule
22
• Thus, penicillin G is named benzylpenicillin and penicillin V is
phenoxymethylpenicillin
23
Degradation of Penicillin
24
25
26
Penicillin
-SAR
Penicillin
27
1.Penicillin can be modified at various positions to improve their ability to:
-be administered orally (survive acidic conditions)
-be tolerated by the patient (allergies)
-penetrate the outer membrane of Gram (-) bacteria
-prevent hydrolysis by β-lactamases
-acylate the PBPs of resistant species (there are many different PBPs)
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Penicillins- Natural
2. Natural penicillins are those which can be obtained directly from the
penicillium mold and do not require further modification. Many
species of bacteria are now resistant to these penicillins.
Penicillin G
It is not orally active
29
Penicillin G in Acidic Conditions
Penicillin G could not be administered orally due to the its inactivation
in acidic conditions of the stomach.
30
Penicillin V
3. Penicillin V is produced when phenoxyacetic acid rather than
phenylacetic acid is introduced to the penicillium culture. Adding
the oxygen decreases the nucleophilicity of the carbonyl group,
making penicillin V acid stable and orally viable.
31
PhO
H
N
CH2
H
S
C
electronegative
oxygen
N
O
O
Penicillin V
(orally active)
•Better acid stability and orally active
•But sensitive to b-lactamases
•Slightly less active than penicillin G
•Allergy problems with some patients
So Next focus was on resistance to β-lactamase
32
Semi-Synthetic Penicillins
4.The acyl side chain of the penicillin molecule can be cleaved using
enzyme or chemical methods to produce 6-APA, which can further
be used to produce semi-synthetic penicillins or cephalosporins
75% of the penicillin produced is modified in this manner.
6-APA
33
Penicillins- Antistaphylococcal
5. In general, it was observed that, increasing the steric
hindrance at the α- carbon of the acyl group increased
resistance to staphylococcal β-lactamase .
• Thus Penicillins which have bulky side groups can block the
β-Lactamases (of bacteria)which hydrolyze the lactam ring.
This leads to development of penicillinase (β-Lactamases)
resistant penicillins.
34
Penicillins- Antistaphylococcal
• These lactamases are prevalent in S. aureus and S. epidermidis, and render
them resistant to Penicillin G and V. This necessitated the development of
semi-synthetic penicillins through rational drug design.
7. Methicillin was the first penicillin developed with this type of
modification, and since then all bacteria which are resistant to any type of
penicillin are designated as methicillin resistant. (MRSA- methicillinresistant S. aureus)
35
Examples – Methicillin
ortho groups
important
O
MeO
C
H
N
H
H
S
N
OMe
Me
Me
O
CO2H
•Methoxy groups block access to b-lactamases but not to transpeptidases
•Binds less readily to transpeptidases compared to penicillin G
•Lower activity compared to Pen G against Pen G sensitive bacteria
•Poor activity vs. some streptococci
•Inactive vs. Gram -ve bacteria
•Poor range of activity
•Active against some penicillin G resistant strains (e.g. Staphylococcus)
•Acid sensitive since there is no electron-withdrawing group
•Orally inactive and must be injected
36
Penicillins- Antistaphylococcal
• the -acyl carbon could be part of an aromatic (e.g.,phenyl or
naphthyl) or heteroaromatic (e.g., 4-isoxazoyl)system,which increases
antibacterial potency by making them acid stable.
• Substitutions at the ortho positions of a phenyl ring (e.g., 2,6dimethoxy [methicillin]) or the 2-position of a 1-naphthyl system
(e.g., 2-ethoxyl [nafcillin]) increase the steric hindrance of the acyl
group and confer more β-lactamase resistance than shown by the
unsubstituted compounds
37
Penicillins- Antistaphylococcal
8. Methicillin is acid sensitive hence orally inactive and has been
improved upon by adding electron withdrawing groups, as was done
in penicillin V, resulting in drugs such as oxacillin and nafcillin.
38
• Among five-membered–ring heterocyclic derivatives bulkier
substituents are required to confer effective β-lactamase
resistance. Thus, members of the 4-isoxazoyl penicillin family
(e.g., oxacillin, cloxacillin, and dicloxacillin) require both the 3aryl and 5-methyl (3-methyl and 5-aryl) substituents for
effectiveness against β - lactamase–producing S. aureus.
R'
O
C
R
H
N
N
O
H
S
N
Me
Bulky and
e- withdrawing
H
Me
Me
O
CO2H
39
Examples - Oxacillin
R'
O
C
R
H
N
N
O
H
S
N
Me
Bulky and
e- withdrawing
H
Me
Me
O
Oxacillin
R = R' = H
Cloxacillin R = Cl, R' = H
Dicloxacillin R = Cl, R' = Cl
Flucloxacillin R = Cl, R' = F
CO2H
•Orally active and acid resistant
•Resistant to b-lactamases
•Active vs. Staphylococcus aureus
•Less active than other penicillins
•Inactive vs. Gram -ve bacteria
•Nature of R & R’ influences absorption and plasma protein binding
•Cloxacillin better absorbed than oxacillin
•Flucloxacillin less bound to plasma protein, leading to higher levels of free drug
40
9. Due to the bulky side group, all of the
antistaphylococcal drugs have difficulty penetrating
the cell membrane and are less effective than other
penicillins.
• All of the clinically available penicillinase-resistant
penicillins are significantly less active than either
penicillin G or penicillin V against most non–βlactamase-producing bacteria normally sensitive to
the penicillins.
41
Penicillins- Aminopenicillins
10.In order to increase the range of activity(efficacy), the
penicillin has been modified to have more hydrophilic
groups, allowing the drug to penetrate into Gram (-)
bacteria via the porin channels.
 Introduction of an ionized or polar group into the α-position
of the side chain benzyl carbon atom of penicillin G confers
activity against Gram-negative bacilli
Ampicillin R=Ph
Amoxicillin R= Ph-(p)OH
42
Penicillins- Aminopenicillins
11. derivatives with an ionized -amino group, such as ampicillin and
amoxicillin, are generally effective against such Gram-negative
genera as Escherichia, Klebsiella, Haemophilus, Salmonella, Shigella,
and non–indole-producing Proteus.
 Furthermore, activity against penicillin G–sensitive, Gram-positive
species is largely retained.
12. These penicillins have a wider range of activity than natural or
antistaphylococcal drugs, but without the bulky side groups are once
again susceptible to attack by β-lactamases
12.The additional hydrophilic groups make penetration of the gut wall
difficult, and can lead to infections of the intestinal tract by H. pylori
43
Penicillins- Aminopenicillins
13.Due to the effectiveness of the aminopenicillins, a second
modification is made to the drug at the carboxyl group to produce
prodrug
eg. BACAMPICILLIN prodrug of ampicillin.
14.Changing the carboxyl group to an ester allows the drug to
penetrate the gut wall where it is later hydrolyzed into the more
polar active form by esterase enzymes.
This has greatly expanded the oral availability of the aminopenicillin
class.
44
Prodrugs of Ampicillin (Leo Pharmaceuticals - 1969)
O
H
R=
NH2
C
CH2O
CMe3
PIVAMPICILLIN
C
C
H
N
O
H
H
S
O
Me
R=
TALAMPICILLIN
O
Me
N
O
O
CO2R
R=
CH
O
C
O
Me
Properties
•Increased cell membrane permeability
•Polar carboxylic acid group is masked by the ester
•Ester is metabolised in the body by esterases to give the free drug
CH2Me
BACAMPICILLIN
45
Penicillins- Extended Spectrum
15.Extended spectrum penicillins are similar to the aminopenicillins in
structure but have either a carboxyl group or urea group instead of the
amine.
• Thus Incorporation of an acidic substituent at the α-benzyl carbon atom of
penicillin G also imparts clinical effectiveness against Gram-negative bacilli
and, furthermore, extends the spectrum of activity to include organisms
resistant to ampicillin
46
Penicillins- Extended Spectrum
16.Like the aminopenicillins the extended spectrum drugs have an
increased activity against Gram (-) bacteria by way of the import
porins.
These drugs also have difficulty penetrating the gut wall and must be
administered intravenously if not available as a prodrug.
These are more effective than the aminopenicillins and not as
susceptible to β-lactamases
47
Eg. Carbenicillin
• Carbenicillin has a broad
range
of
antimicrobial
activity, broader than any
other known penicillin, a
property attributed to the
unique carboxyl group.
• It has been proposed that the
carboxyl group improves
penetration of the molecule
through cell wall barriers of
Gram-negative
bacilli,
compared
with
other
penicillins.
48
Eg. Piperacillin
• Piperacillin (Pipracil) is the
most generally useful of the
extended- spectrum
acylureidopenicillins.
• Piperacillin is destroyed
rapidly by stomach acid;
therefore, it is active only by
intramuscular or intravenous
administration
49
Therapeutic Uses
• Pneumococcal Infections
• Pneumococcal Meningitis
• Pneumococcal Pneumonia
• Streptococcal Infections
• Streptococcal Pharyngitis (including Scarlet Fever)
• Streptococcal Pneumonia, Arthritis, Meningitis, and Endocarditis
• Staphylococcal Infections
• Meningococcal Infections
• Gonococcal Infections
• Syphilis
• Actinomycosis
• Diphtheria
• Anthrax
• Clostridial Infections
• Fusospirochetal Infections
• Rat-Bite Fever
• Listeria Infections
• Lyme Disease
• Erysipeloid
• Surgical Procedures in Patients with Valvular Heart Disease
50
IUPAC names of penicillins
Penicillin G
6-Benzyl penicillin
6-(phenylacetamido)penicillanic acid
51
IUPAC names of penicillins
Methicillin
2,6-dimethoxyphenylpenicillin
52
IUPAC names of penicillins
Oxacillin
(5-methyl-3-phenyl-4-isoxazolyl)penicillin
53
IUPAC names of penicillins
Cloxacillin
[3-(o-chlorophenyl)-5-methyl-4-isoxazolyl] penicillin
54
IUPAC names of penicillins
Ampicillin
D-α-aminobenzylpenicillin
6-[D-α-aminophenylacetamido]penicillanic acid
55
IUPAC names of penicillins
Amoxicillin
6-[D- α -amino-p- hydroxyphenylacetamido]penicillanic acid
56
IUPAC names of penicillins
Carbenicillin
α -carboxybenzylpenicillin
6-[D-α-carboxyphenylacetamido]penicillanic acid
57
Cephalosporins
Cephalosporins were discovered shortly after penicillin entered into
widespread product, but not developed till the 1960’s.
Cephalosporins are similar to penicillins but have a 6 member
dihydrothiazine ring instead of a 5 member thiazolidine ring.
7-aminocephalosporanic acid (7-ACA) can be obtained from bacteria,
but it is easier to expand the ring system of 6-APA because it is so
widely produced.
58
Cephalosporins
Unlike penicillin, cephalosporins have two side chains which can be
easily modified. Cephalosporins are also more difficult for βlactamases to hydrolyze.
59
Cephalosporins
penicillins
cephalorsporins
b-lactam ring
60
Cephalosporins
• Mechanism of Action: Cephalosporins are composed of a
dihydrothiazine ring and a b-lactam ring. The mechanism of action
is identical to penicillins.
• Mechanism of Resistance: Same as penicillins.
• Cephalosporins are less susceptible to Staphylococcus betalactamase; therefore have a broader spectrum of activity; however
they are not the drug of choice. Other bacteria are resistant,
because they produce distinct beta-lactamases. Methicillin-resistant
Staphylococcus is resistant to most cephalosporins.
61
Mechanism of Cephalosporins
The acetoxy group (or other R group) will leave when the drug acylates
the PBP.
62
Inhibition of Beta lactamase by Cephalosporines
63
Cephalosporins- Classification
Cephalosporins are classified into four generations based on their
activity.
Later generations generally become more effective against Gram (-)
bacteria due to an increasing number of polar groups (also become
zwitterions.)
Ceftazidime (3rd gen) in particular can cross blood brain barrier and is
used to treat meningitis.
Later generations are often the broadest spectrum and are reserved
against penicillin resistant infections to prevent the spread of
cephalosporin resistant bacteria.
64
65
First generation cephalosporins:
•
•
•
cephalothin, cefazolin, cefalexin. These drugs have good activity
against most Gram positive cocci (Streptococcus, pneumococcus
but not or methicillin-resistant Staphylococcus). They are more
active against Gram negative organisms (Escherichia co1i
Kiebsiella pneumoniae, and the indole negative Proteus
mirabilis) than are the natural penicillins. They are effective
against some anaerobic cocci (Peptococcus and
Peptosteptococcus, but NOT Bacteroides fragilis).
They are ineffective against Pseudomonas aeruginosa,
Enterobacter, and indole-positive Proteus species.
These drugs do not cross the blood-brain barrier.
66
Second generation cephalosporins:
•
•
cefuroxime, cefamandole, cefoxitin, cefaclor.
The spectrum is extended to more Gram
negative bacteria Enterobacter species, Klebsiella
species, and indole-positive Proteus species.
Also, Haemophilus influenza is covered by
cefuroxime, cefamandole, cefaclor; Bacteroides
fragilis by cefoxitin.
These drugs do not achieve adequate levels in
the CSF.
67
Third generation cephalosporins:
• moxalactam, cefaperazone, ceftazidirne, ceftriaxone.
These drugs demonstrate extended Gram negative
coverage, are more resistant to non-Staphylococcus blactamase, and readily cross the blood-brain barrier.
The spectrum is extended to include: Enterobacter,
Pseudomonas (ceftazidime and cefaperazone only),
Serratia, b-lactamase producing Haemophillus
influenza and Neisseria species.
• Only cetizoxime and moxalactam retain good activity
against Bacteroides fragilis.
68
Fourth generation
• forth generation of cephalosporins (e.g. cefepime) are available,
these are comparable to third-generation but more resistant to some
betalactamases.
69
DEGRADATION OF CEPHALOSPORIN
Pharmacokinetics
• Some cephalosporins may be given orally but most
are given parenterally (IM or IV).
• They are widely distributed in the body like
penicillins.
• Some such as cefoperazone, cefotaxime,
cefuroxime, ceftriaxone, and ceftazidime (third
generation) also cross the blood-brain barrier and
are drugs of choice for meningitis due.
70
Uses:
• A cephalosporin with or without an aminoglycoside is
first-line treatment of Klebsiella.
• First generation cephalosporins are used for surgical
prophylaxis of wound infection.
• Third generation cephalosporins are used to treat
meningitis due to pneumococci, meningococci, and
Haemophillus influenza.
• Ceftriaxone is the drug of choice for treating betalactamase producing Neisseria gonorrhea.
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THANK YOU
-PHARMA STREET
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