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Penicillin & Cephalosporin
PHRM 304
There are two major classes of antibacterial agents which act by inhibiting cell wall synthesis:
- Penicillins &- Cephalosporins
Gram (+) ve and gram (-) ve bacteria has been naming according to their staining capacity. As the gram (+) ve bacteria has higher amount of peptidoglycan in their cell wall they show (+) ve staining property.
As penicillin and cephalosporin inhibit peptidoglycan synthesis the drugs are mainly active against gram (+) ve baccteria: Staphylococci, Streptococci.
Certain moulds could produce toxic substances which killed bacteria (1877, Pasteur and Joubert)Antibacterial agents which inhibit bacterial cell wall synthesisDiscovered by Fleming from a fungal colony (1928) Shown to be non toxic and antibacterialIsolated and purified by Florey and Chain (1938)First successful clinical trial (1941) Produced by large scale fermentation (1944)Structure established by X-ray crystallography (1945) Full synthesis developed by Sheehan (1957)Isolation of 6-APA by Beechams (1958-60)
- development of semi-synthetic penicillinsDiscovery of clavulanic acid and -lactamase inhibitors (1976)
Penicillins:
History of penicillins
In 1877, 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. There was an area surrounding the fungal colony where the bacterial colonies were dying.
Penicillins:
He 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 notatum which produces antibacterial agent penicillin.
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 unstable & 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.
The synthesis of such a highly strained molecule presented a huge challenge- a challenge which was met successfully by Sheehan who completed a full synthesis of penicillin by 1957.
In 1959, Beecham Laboratories obtained 6-aminopenicillanic acid (6-APA) from Penicillium chrysogenum moulds. The availability of 6-APA made possible the introduction of semisynthetic penicillins, with new and better properties than the natural ones.
Penicillins were used widely and often carelessly, so that the evolution of penicillin resistant bacteria became more and more of a problem (Bact: Rapid division: No. double in every 20 min).
The fight against these penicillin-resistant bacteria was promoted greatly when, in 1976, Beecham Laboratories discovered a natural product called clavulanic acid which has proved highly effective in protecting penicillins from the bacterial enzymes (-lactamases) which attack penicillin.
Structure of penicillin
Fig. The structure of penicillin.
1
2
3*4
5*
6*
Penicillin contains a highly unstable-looking bicyclic system consisting of a four membered -lactam ring fused to a five-membered thiazolidine ring.
The skeleton of the molecule suggests that it is derived from the amino acids cysteine and valine. According to biogenesis, antibiotics can be derived from various natural substances like -lactam antibiotics from cysteine and valine amino acids.
Fig. Penicillin appears to be derived from cysteine and valine.
Side chain varies depending on carboxylic acid present in fermentation Side chain varies depending on carboxylic acid present in fermentation mediummedium
Penicillin GCH2 CO2H
Penicillin V(first orally active penicillin)
OCH2 CO2H
Phenylacetic acid
Phenoxyacetic-acid
All penicillins have the same -lactam-thiazolidine general structure that contains three chiral centers. Therefore, theoretically this structure could present eight optically active forms. However, the natural isomers, presumably the only one with biological activity, has the stereochemistry of 3S:5R:6R. (According to BP & USP 2S:5R:6R)
The three-dimensional shape of penicillin The three-dimensional shape of penicillin Folded ‘envelope’ shape Folded ‘envelope’ shape
CH2
Penicillin analogues
Benzyl Penicillin PEN G
- First commercially availablepenicillin.- Acid sensitive, so need to administer IV or IM route. Unable to provide through oral route.
CH2O
Phenoxymethylpenicillin PEN V
- Acid stable- so able to administer though oral route.
R= R=
Structure-activity relationships of penicillinsA large number of penicillin analogues have been synthesized and studied. The results of these studies led to the following conclusions.
• The strained -lactam ring is essential.• The bicyclic system is important.• The acidic functional group (free carboxylic acid) is essential.• The acylamino (amide) side-chain is essential.• Sulfur is usual but not essential.• The stereochemistry of the bicyclic ring with respect to the acylamino side-chain is important.• The acyl side-chain (R) can varies.
Very little variation is possible in penicillin nucleus.
Fig. Structure activity relationships of penicillins.
Acyl side-chain (R) can varies
Problem one: The acid sensitivity of penicillinsThere are three reasons for the acid sensitivity of penicillin1. Ring strain
The bicyclic system in penicillin consists of a four-membered ring and a five membered ring. As a result, penicillin suffers large angle and torsional strains. Acid-catalyzed ring opening relieves these strains by breaking open the more highly strained four-membered -lactam ring.
Fig. Ring opening.
2. A highly reactive -lactam carbonyl groupThe carbonyl group in the -lactam ring is highly
susceptible to nucleophiles and it does not behave like a normal tertiary amide which is usually quite resistant to nucleophilic attack.
A normal tertiary amide is far less susceptible to nucleophiles since the resonance structures reduce the electrophilic character of the carbonyl group.
The -lactam nitrogen is unable to show such effect. To show the similar effect like tertiary amide, penicillin had to obtain a strained flat structure, which is highly unstable. As a result, the lone pair is localized on the nitrogen atom and the carbonyl group is far more electrophilic than a tertiary amide.
3. Influence of the acyl side-chain Figure demonstrates how the neighboring acyl group can actively participate in a mechanism to open up the lactam ring. Thus, penicillin G has a self-destruct mechanism built into its structure.
-H+
Fig. Influence of the acyl side chain on acid sensitivity.
Tackling the problem of acid sensitivityIt can be seen that countering acid sensitivity is a difficult task. Nothing can be done about the first two factors since the -lactam ring is vital for antibacterial activity. Without it, the molecule has no useful biological activity at all.
Therefore, only the third factor can be tackled. If a good electron withdrawing group is attached to the carbonyl group, then the inductive pulling effect should draw electrons away from the carbonyl oxygen and reduce its tendency to act as a nucleophile.
Penicillin V (Pen V) has an electronegative oxygen on the acyl side-chain with the electron withdrawing effect required. The molecule has better acid stability than penicillin G (Pen G) and is stable enough to survive the acid in the stomach. Thus, it can be given orally.
Fig. Reduction of neighboring group participation with electron withdrawing group.
Fig. Penicillin V. Fig. R= H: AmpicillinR= OH: Amoxicillin
R
A range of penicillin analogues which have been very successful are penicillins which are disubstituted on the alpha-carbon next to the carbonyl group. As long as one of the groups is electron withdrawing, these compounds are more resistant to acid hydrolysis and can be given orally (e.g. ampicillin and oxacillin).
To conclude, the problem of acid sensitivity is fairly easily solved by having an electron withdrawing group on the acyl side-chain.
X = -NH2, -Cl, PhOCONH-,Heterocycles
Problem two: Penicillin sensitivity to -lactamases (Penicillinases)
-lactamases (Penicillinases) are enzymes produced by penicillin-resistant bacteria (e.g. Staphylococcus aureus) which can catalyze the reaction shown in figure the same ring opening and deactivation of penicillin which occurred with acid hydrolysis.
-lactamase
Fig. -lactamase deactivation of penicillin.
Tackling the problem of -lactamase sensitivityThe strategy is to block the penicillin from reaching the penicillinase active site. One way of doing that is to place a bulky group on the acyl side-chain. This bulky group can then act as a 'shield' to ward off the penicillinase and therefore prevent binding.
Fig. Blocking penicillin from reaching the penicillinase active site.
Several analogues were made and the strategy was found to work. However, there was a problem. If the side-chain was made too bulky, then the steric shield also prevented the penicillin from attacking the enzyme responsible for bacterial cell wall synthesis (transpeptidase).
Methicillin is not an ideal drug. Since there is no electron withdrawing group on the side-chain, it is acid sensitive, and so has to be injected.
Oxacillin R = R' = HCloxacillin R = Cl, R‘ = HFlucloxacillin R = Cl, R‘ = F
These compounds are acid-resistant and penicillinase-resistant.
For example, the outer surface may have an overall negative or positive charge depending on its constituents triglycerides. Penicillin has a free carboxylic acid, which, if ionized, would be repelled by the cell wall.
Most penicillins show a poor activity against gram-negative bacteria. There are several reasons for this resistance:
It is difficult for penicillins to invade a gram-negative bacterial cell due to the make up of the cell wall. Gram-negative bacteria have a coating on the outside of their cell wall which consists of a mixture of fats, sugars, and proteins. This coating can act as a barrier in various ways.
Problem 3 - Range of Activity
1. Permeability barrier
The transpeptidase enzyme is the enzyme attacked by penicillin. In some gram negative bacteria, a lot of transpeptidase enzyme is produced, and the penicillin is incapable of inactivating all the enzyme molecules present.
2. High levels of transpeptidase enzyme produced
A mutation may occur which allows the bacterium to produce a transpeptidase enzyme which is not antagonized by penicillin.
3. Modification of the transpeptidase enzyme
The fatty portion of the coating may act as a barrier to the polar hydrophilic penicillin molecule.
The only way in which penicillin can enter into the cell is the protein channels (porins) in the outer coating. Most of the channels remain closed.
Bacteria can transfer small portions of DNA from one cell to another through structures called plasmids. These are small pieces of circular bacterial DNA.
Through these transferred DNA the recipient cell may acquires immunity.
5. Transfer of the -lactamase enzyme DNA
-lactamases are enzymes which degrade penicillin. They are situated between the cell wall and its outer coating.
4. Presence of -lactamase (penicillinase)
Fig. Permeability barrier of a Gram-negative bacterial cell.
Overall outer negative charge inhibit the penetration of ionized (-ve) penicillin into the cell.
Factors1) Cell wall may have a coat preventing access to the cell2) Excess transpeptidase enzyme may be present3) Resistant transpeptidase enzyme (modified structure)4) Presence of β-lactamases5) Transfer of β-lactamases between strains6) Efflux mechanisms
Problem 3 - Range of Activity
Results of varying R in Pen G1) Hydrophobic side chains result in high activity vs. Gram +ve bacteria and poor activity vs. Gram -ve bacteria2) Increasing hydrophobicity has little effect on Gram +ve activity but lowers Gram -ve activity3) Increasing hydrophilic character has little effect on Gram +ve activity but increases Gram -ve activity
4) Hydrophilic groups at the α-position (e.g. -NH2, -OH, -CO2H) increase activity vs Gram -ve bacteria
Problem 3 - Range of Activity
Tackling the problem of narrow activity spectrum1. Hydrophobic groups on the acyl side-chain (e.g. penicillin G) favor activity against gram-positive bacteria, but result in poor activity against gram-negative bacteria.
2. If hydrophobic character is increased, there is little effect on the gram-positive activity, but activity against gram-negative bacteria drops even more.
3. Hydrophilic groups on the side-chain have either little effect on gram-positive activity (e.g. penicillin T) or cause a reduction of activity (e.g. penicillin N). However, they lead to an increase in activity against gram-negative bacteria.
4. Enhancement of gram-negative activity is found to be greatest if the hydrophilic group (e.g. -NH2, -OH, -COOH) is attached to the carbon, alpha to the carbonyl group on the acyl side chain.
Penicillin N
Penicillin T
Those penicillins having useful activity against both gram-positive and gram-negative bacteria are known as broad-spectrum antibiotics. There are two classes of broad-spectrum penicillin antibiotics. Both have an alpha-hydrophilic group. However, in one class the hydrophilic group is an amino function as in ampicillin or amoxycillin, while in the other the hydrophilic group is an acid group as in carbenicillin.
R = H Carbenicillin
Ampicillin Amoxycillin
Examples of Broad Spectrum Penicillins
Class 1 - NH2 at the α-position
Ampicillin and amoxicillin (Beechams, 1964)
AmpicillinAmpicillin AmoxicillinAmoxicillin
H
C
H
O
HNC
NH2
HO
O
C
NH2
CHN
O
H
H
O
Propertieso Active vs Gram +ve bacteria and Gram -ve bacteria which
do not produce β-lactamases (Sensitive to β-lactamases)
o Acid resistant and orally active
o Non toxic
o Increased polarity due to extra amino group
o Poor absorption through the gut wall
o Disruption of gut flora leading to diarrhoea
o Inactive vs. Pseudomonas aeruginosa
Examples of Broad Spectrum Penicillins
Prodrugs of Ampicillin (Leo Pharmaceuticals – 1969)
PIVAMPICILLINR = CH2OCO
CMe3
TALAMPICILLINR = O
O
BACAMPICILLIN
R = CH
Me
OC
O
O CH2Me
ON
HNC
C
NH2
S Me
Me
CO2R
H H
H
O
PropertiesoIncreased cell membrane permeability
oPolar carboxylic acid group is masked by the ester
oEster is metabolised in the body by esterases to give the
free drug (once the prodrug has been absorbed )
Examples of Broad Spectrum Penicillins
Class 2 - CO2H at the α-position (carboxypenicillins)
R = H CarbenicillinR = H CarbenicillinR = Ph CarfecillinR = Ph Carfecillin
H H
ON
HNC
CH
CO2R
S Me
Me
CO2H
O
Propertieso Carfecillin=prodrug for carbenicillin: show an improved absorption through the gut wall. o Active over a wider range of Gram -ve bacteria than ampicillino Resistant to most β-lactamaseso Less active vs Gram +ve bacteria (note the hydrophilic group)o Acid sensitive and must be injectedo Stereochemistry at the α-position is importanto -CO2H at the α-position is ionized at blood pH
Examples of Broad Spectrum Penicillins
Class 3 - Urea group at the α-position (ureidopenicillins)
Properties•Newest class of broad-spectrum penicillins
•Have a urea functional group at the α-position.
•They generally have better properties than the
carboxypenicillins and have largely replaced them in the clinic.
There are several examples in medicinal chemistry where the presence of one drug enhances the activity of another. In many cases this can be dangerous, leading to an overdose of the enhanced drug. In some cases it can be useful.
There are two interesting examples whereby the activity of penicillin has been enhanced by the presence of another drug. One of these is the effect of clavulanic acid. The other is the administration of penicillins with a compound called probenecid.
Synergism of penicillins with other drugs
Probenecid
Clavulanic acid
Clavulanic acid has similarity with β-lactam molecules but have very weak antibacterial action. But it is a powerful and irreversible inhibitor of most β-lactamases.
It is a "suicide" inhibitor that irreversibly binds β-lactamases produced by a wide range of gram-positive and gram-negative microorganisms.
Clavulanic acid is a -lactamase (penicillinase) inhibitor sometimes combined with penicillin group antibiotics (combination of amoxicillin with clavulanic acid known as Co-amoxyclav) to overcome certain types of antibiotic resistance. Specifically, it is used to overcome resistance in bacteria that secrete beta-lactamase enzymes, which otherwise inactivate most penicillins.
The drug fits the active site of β-lactamase, and the β-lactam ring is opened by a serine residue in the same manner as penicillin.Can be given orally and intravenously.Reduces the required dose of β-lactam penicillin (This allows 1) the dose levels of amoxicillin to be decreased and 2) also increases the spectrum of activity).
Inactivation of Beta lactamase enzyme.
Probenecid
Probenecid is a moderately lipophilic carboxylic acid and, as such, is similar to penicillin. It is found that probenecid can block facilitated transport of penicillin through the kidney tubules. In other words, probenecid slows down the rate at which penicillin is excreted by competing with it in the excretion mechanism.
As a result, penicillin levels in the bloodstream are enhanced and the antibacterial activity increases.
Clavulanic acid
Cephalosporins
The second major group of -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.
Antibacterial agents which inhibit bacterial cell wall synthesis.Discovery and structure of Cephalosporin C
Cephalosporin C
Molecular modification of Cephalosporin C gave origin to cephalosporins; most of them are semisynthetic substances, obtained by reaction of 7-aminocephalosporanic acid (7-ACA) with appropriate compounds.
The structure of Cephalosporin C has similarities to that of penicillin in that it has a bicyclic system containing a four-membered -lactam ring. However, this time the -lactam ring is fused with a six-membered dihydrothiazine ring.
This larger ring relieves the strain in the bicyclic system to some extent, but it is still a reactive system.
The cephalosporin structure contains two chiral centers (6C & 7C). Thus four optically active forms are possible. The natural isomer (6R & 7R) has the following stereochemistry.
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. Cephalosporin C
7-Aminocephalosporinic acid (7-ACA)
NO
HHH2 N S
CO2 H
OC
Me
O
Many analogues of Cephalosporin C have been made and the structure-activity relationship (SAR) conclusions are as follows.
• The -lactam ring is essential.• A free carboxyl group is needed at position 4.• The bicyclic system is essential.• The stereochemistry of the side-groups and the
rings is important.
Structure-activity relationships of Cephalosporin C
These are very close to penicillins and there are only a limited number of places where modifications can be made. Those places are:
o variations of the 7-acylamino side chain o variations of the 3-acetoxymethyl side chain o extra substitution at carbon 7
Positions which can be varied
73
Cephalosporin analogues
Different cephalosporins are developed by changing the moieties attached at the 3 and/or 7 positions of the 7-ACA.
Usually, substituents at C-3 (R2) modify the overall pharmacokinetic properties, whereas those at C-7 (R1) alter the antibacterial spectrum.
N
S
O
N
R1
O
H
OHO
HH
R237
R1 – the substituents at this position effects β-lactamase resistance and its activity against Gram –ve and/or Gram +ve bacteria (its spectrum).
R2 – this substituents primarily effects the pharmacokinetics: the oral activity, the extent of metabolism, and the duration of action. Electron withdrawing group at this position provide resonance structure and thus increase stability of the structure.
Carboxylic acid group – necessary for activity. This functional group mimics the carboxylic acid group of alanine when binding the enzyme active site.
First generation cephalosporin
Cefadroxil
Second generation cephalosporin
Cefaclor
Third generation cephalosporin
Cefdinir
Fourth generation cephalosporin
Cefepime
Transpeptidase enzyme is one of the penicillin-binding proteins that normally reside in the bacterial inner membrane and perform construction, repair and housekeeping functions maintaining cell wall function and playing a vital role in cell growth and division.
Penicillin and Cephalosporin antibiotics bind to penicillin binding proteins (PBPs) of bacteria and inhibit the formation of cell wall.
Bacteria have to survive a large range of environmental conditions such as varying pH, temperature, and osmotic pressure. Therefore, they require a robust cell wall. Since this cell wall is not present in animal cells, it is the perfect target for antibacterial agents such as penicillins and cephalosporins.
The mechanism of action
N-acetylmuramic acid (NAM) N-acetylglucosamine (NAG)
The bacterial cell wall is a peptidoglycan structure. In other words, it is made up of peptide units and sugar units. The structure of the cell wall consists of a parallel series of sugar backbones containing two types of sugar [N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG)].
Peptide chains are bound to the NAM sugars, and in the final step of cell wall biosynthesis, these peptide chains are linked together by the displacement of D-alanine from one chain by glycine in another.
Peptide chain
Sugar backbone
It is this final cross-linking reaction which is inhibited by penicillins and cephalosporins, so the cell wall framework is not meshed together. As a result, the wall becomes 'leaky'. Since the salt concentrations inside the cell are greater than those outside the cell, water enters the cell, the cell swells, and eventually lyses (bursts).
The enzyme responsible for the cross-linking reaction is known as the transpeptidase enzyme.It has been proposed that penicillin and cephalosporin has a conformation which is similar to the transition-state conformation taken up by D-Ala-D-Ala— the portion of the amino acid chain involved in the cross-linking reaction. Since this is the reaction centre for the transpeptidase enzyme, the enzyme mistakes the penicillin molecule for the D-Ala-D-Ala— moiety and accepts the penicillin into its active site.
Once penicillin or cephalosporin is in the active site, the normal enzymatic reaction would be carried out on the penicillin or cephalosporin.
Penicillin can be seen to mimic D-Ala-D-Ala
Penicillin
D-Ala-D-Ala
HH
CO2H
HN
O Me
Me
N
SC
R
O
Me
CO2H
HN
O CH3
HN
HH
C
R
O
Mechanism of action - bacterial cell wall synthesis
S
