Penicillins

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History

There were multiple attempts to discovery of answer to a gargantuan human malady in the form of infection. Bread molds with fungus and heavy metals were used.

As we all know it was Alexander Fleming who after multiple attempts had the breakthrough. He while working as a bacteriologist At St Mary’s Hospital in London discovered the inhibitory effect of fungus Penicillium notatum on the staphylococcus colonies.

He published his work in The British Journal of Experimental Pathology in 1929. This sparked the interest in antibiotic therapy and in 1939 scientist in Oxford started the research on the new found antimicrobials.

This antimicrobial eventually named penicillin underwent cycle of refinement and purification. After many human trials this antimicrobial was finally available for prescription in 1946 A.D. For this discovery Fleming, Florey and Chain were awarded the Nobel Peace Prize in 1945. And this event laid the foundation of the discovery of many antimicrobials that are used these days.

What are the penicillins?

These are the antibiotics with beta-lactam ring sharing common feature with cephalosporin, monobactams, carbapenems, and beta-lactamase inhibitors.

What is the structure of penicillin?

All penicillins have a beta-lactam ring and a thiazolidine ring attached to this beta lactam ring.

Beta lactam ring contains the secondary amine group R-

The type of R -group determines the type of penicillin.

Mechanism of action of penicillin

Penicillin like every other beta lactam antibiotic interfere with the transpeptidation reaction of bacterial cell wall synthesis.

Bacterial cell wall is composed of a complex, cross-linked polymer of polysaccharides and polypeptides – peptidoglycan also known as murein or mucopeptide.

It is essential for maintaining cellular integrity and shape and prevent macromolecules from entering the bacterial cell.

This peptidoglycan is composed of N-acetylglucosamine and N-acetylmuramic acid sugars. 

The characteristic strength of peptidoglycan comes from the peptide cross linkages that is responsible for net like conformation of Peptidoglycan.

The peptide cross linkage occurs between the N-acetylmuramic acid with help of an enzyme called penicillin binding protein. This process of cross linkage is called transpeptidation reaction.

What happens in transpeptidation reaction?

A five amino-acid peptide is linked to N-acetylmuramic acid which terminates in D-alanyl-D-alanine. Penicillin-Binding protein is an enzyme that removes the terminal alanine in the process of forming the cross link between the terminal peptides of N-acetylmuramic acid.

The penicillin like other beta lactam antibiotics are structural analogs of D-Ala-D-Ala and binds to the PBPs thus inhibiting the transpeptidation reaction. This halts the peptidoglycan formation and the cell death occurs.

Why penicillins do not affect the eukaryotic cells?

As eukaryotic cells lack the peptidoglycan and the PBP enzyme they do not affect the eukaryotic cell.

 

Penicillins can also act on growing and multiplying bacteria as peptidoglycan formation occurs in multiplying organisms.

 

 

 

Classification of penicillins:

Penicillins are classified into 3 categories

1. Penicillin G:

Penicillin G (benzylpenicillin) was the original molecule extracted from Penicillium notatum.

Highly active against

·         Gm + ve cocci except beta lactamase staphylococci, Resistant pneumococci, some enterococci and MRSA.

·         Gm + ve rods like Listeria

·         Gm negative cocci such as Neisseria

·         Most anaerobes – Clostridium (with certain imp exceptions like Bacteroides)

·         Spirochetes like Treponema Pallidum

Penicillin G is only bacteriostatic against enterococci and there is rising resistance.

Not active against Gm -ve bacilli because of poor penetration of porin channels.

Benzylpenicillin - Penicillin G is IV formulation

IM forms are - Procaine and benzathine Penicillin G

Benzathine penicillin is used as prophylaxis against Streptococci pharyngitis for prevention of RHD. Also used in treatment of Syphilis

What is the oral preparation of Penicillin?

Penicillin V – only in minor infections due to

Poor bioavailability

Frequent dosing

Narrow spectrum of activity

Following the wide use of natural Penicillin G beta lactamase producing strains emerged among the Gram-positive species. This led to the development of semi-synthetic beta lactamase resistant penicillins.

 

2. Anti-staphylococcal penicillins –

These penicillins are staphylococcal beta lactamase resistant. They are active against staphylococci and streptococci but not against enterococci, anaerobic bacteria and gm-negative cocci and rods and Methicillin resistant staphylococcus aureus

Anti-staphylococcal penicillins are less active than Penicillin G for bacteria susceptible to both.

Drugs in this category are –

Methicillin

Nafcillin

Cloxacillin and Dicloxacillin

Methicillin was the first antibiotic in this group but no longer used because of significant adverse effects.

The 3^rd category of penicillin was developed because of the narrow spectrum of activity of category 2 drugs and for broader coverage of Gram-negative organisms.

 

3. Broad spectrum penicillins –

These drugs have greater activity against gram-negative bacteria as they have better ability to penetrate the gm – ve outer membrane.

But like penicillin G these drugs are inactivated by many lactamases.

This category includes

Aminopenicillins (2^nd generation)

Carboxypenicillins

Ureidopenicillins

 

What are the aminopenicillins?

Amoxicillin

Ampicillin

Both these drugs have very similar spectrum of activity but amoxicillin is better absorbed orally.

Both these drugs are most active of all beta lactams against pneumococci.

Ampicillin but not amoxicillin can be used against Shigella.

These antibiotics are active against majority of gm negative organisms like E coli, Salmonella, Shigella, Proteus and non-beta lactamase H influenza.

But many resistant cases have emerged and rarely used in cases of UTI and Typhoid

These antibiotics are not active against Pseudomonas, Klebsiella, Enterobacter, Serratia, Citrobacter, indole positive proteus and other hospital acquired gm negative organisms.

What are the carboxypenicillins?

Carbenicillin and Ticarcillin which are 3^rd generation antibiotics.

Carbenicillin are the first antipseudomonal antibiotic which is no longer used. These have activity against Pseudomonas, Proteus, Enterobacter.

Ticarcillin have same spectrum of activity but 2-4 times more active on a weight basis against Pseudomonas aeruginosa.

Ticarcillin is often used in combination with a beta lactamase inhibitor like Ticarcillin-Clavulanate.

Problems with Ticarcillin:

Disodium salt so cannot be used in volume overload state.

Inhibits platelet function thus prolongs bleeding time and causes bleeding diathesis.

 

What are ureidopenicillins?

Piperacillin

Azlocilin

Mezlocilin

These are often considered as fourth generation penicillins.

These drugs are better than previous drugs against Pseudomonas.

Better activity against Klebsiella but cephalosporins are always preferred.

Piperacillin, however are found to be ineffective as single agent against the growing resistant groups of organisms. Used in combination with beta lactamase inhibitors like Piperacillin Tazobactam which increases the spectrum of activity both against beta lactamase producing staphylococcus and beta lactamase producing gram negative organisms.

How bacteria develop resistance against Penicillin?

Resistance to penicillin develops by 1 one of the following mechanisms

1. Inactivation of antibiotic by beta-lactamase enzyme:

This is the most common mechanism of resistance.

This enzyme breaks down the basic backbone of the antibiotic – beta lactam ring producing penicilloic acid which lacks the antibacterial activity.

Various types of beta lactamases are present specific for specific beta lactams like

AmpC beta lactamases produced by Pseudomonas and Enterobacter and Extended spectrum beta lactamases (ESBL) hydrolyse both penicillins and cephalosporins

Some beta lactamases hydrolyze only penicillin

Carbapenems are resistant to hydrolysis by penicillinases and cephalosporinases but are hydrolyzed by metallo-beta lactamase and carbapenemases

2. Modification of target PBPs

3. Impaired penetration of drug to target PBPs

This mechanism occurs only in gram negative species because of the impermeable outer membrane of cell wall which is absent in gram positive organisms.

4. Antibiotic efflux

Gram negative organisms also contain efflux channels that pumps out the antibiotics from the periplasmic space to the outside of the cell

 

Units and formulations of penicillin:

Crystalline Pen G contains approx. 1600 units per mg i.e. 1 unit = 0.6 mcg and 1 million unit = 0.6 g.

Most penicillin are formulated has sodium and potassium salts of free acid.

Pharmacokinetics:

Oral administration is possible except for nafcillin

Penicillins usually have shorter half life except that of IM preparations.

Piperacillin has dose dependent pharmacokinetics and a longer half-life when higher doses are administered. 

Widely distributed throughout the body but not into the brain.

However, in CNS infection when meninges are inflamed, they can enter the CNS at higher doses.

Excretion:

Penicillin are excreted rapidly through kidney. Tubular secretion accounts for 90% and remainder by glomerular filtration. Almost all of the drugs require dose modification in cases of renal insufficiency.

Nafcillin, oxacillin, cloxacillin and dicloxacillin have major non-renal routes of clearance and need no dose adjustment.

Side effects of penicillin:

Generally, well tolerated

Most common adverse effect is d/t hypersensitivity.

Allergic reactions like anaphylactic shock, serum sickness and skin rashes.

Few rare side effects

In cases of renal failure = high doses can cause seizures

Nafcillin- neutropenia

Oxacillin – Hepatitis

Methicillin – Interstitial nephritis

Ampicillin – Pseudomembranous colitis

 

What are the MRSAs?

Methicillin Resistant Staphylococcus aureus.

Methicillin is a semi-synthetic antibiotic that was introduced in 1959 against beta lactamases producing staph but in 1960 there was an outbreak of MRSA in Europe.

MRSA have emerged as major nosocomial and community acquired pathogens.

How can we label Staph aureus as MRSA?

Methicillin resistance requires the presence of the mec gene. PCR method can be used to identify the mec gene.

As per the Clinical laboratory Standards Institute (CLSI) MRSA is defined as oxacillin Minimum inhibitory concentration (MIC) > 4 mcg/mL.

Oxacillin is used for defining MRSA as methicillin is no longer used.

 

mec gene

The structural component of mec gene is mecA which encodes the penicillin binding protein 2a which is different from the normal PBP where penicillins attach thus escaping the penicillin inhibitory action.

mecA gene confers resistance to entire beta lactam antibiotics except ceftaroline and ceftobiprole.

Apart form mecA other homologous genes like mecB, mecC and mecD have been identified.

Staphylococcal chromosomal cassette mec (SCC mec)

This is a mobile genetic element that carries the mec gene and is found in Staphylococcal species. There are about 13 SSC mec identified till date.

Type I, II and III SSC mec are associated with nosocomial MRSA and are multi-drug resistant.

Type IV and V SSC mec are a/w community acquired MRSA and are less notorious compared to HA-MRSA. CA-MRSA produce PVL toxin.

There is horizontal transfer of SSCmec from one staph to another thus leading to an outbreak.