Penicillin

For other uses, see Penicillin (disambiguation).

Penicillin

Penicillin core structure, where "R" is the variable group
Clinical data
AHFS/Drugs.com	Micromedex Detailed Consumer Information
Routes of administration	Intravenous, intramuscular, by mouth
Legal status
Legal status	In general: ℞ (Prescription only)
Pharmacokinetic data
Metabolism	Liver
Elimination half-life	Between 0.5 and 56 hours
Excretion	Kidneys

Penicillin (P, PCN or PEN) is a mixture of natural antibiotics derived from Penicillium moulds (principally, P. chrysogenum, P. notatum and P. rubens). The two purified natural penicillins in clinical use are penicillin G (intravenous use) and penicillin V (given by mouth). Penicillin antibiotics were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today, though many types of bacteria have developed resistance following extensive use.

About 10% of people report that they are allergic to penicillin; however, up to 90% of this group may not actually be allergic. Serious allergies only occur in about 0.03%. Those who are allergic to penicillin are most often given cephalosporin C because there is only 10% crossover in allergy between the penicillins and cephalosporins. All penicillins are β-lactam antibiotics, which are some of the most powerful and successful achievements in modern science.

Penicillin was discovered in 1928 by Scottish scientist Alexander Fleming. Penicillin was first used to treat infections in 1942. Fleming shared the 1945 Nobel Prize in Physiology or Medicine for his discovery, along with Oxford University scientists Howard Florey and Ernst Boris Chain (who developed improved ways to produce and concentrate the drug and prove its antibacterial effects).

There are several semi-synthetic penicillins which are effective against a broader spectrum of bacteria: these include the antistaphylococcal penicillins, aminopenicillins and the antipseudomonal penicillins.

Nomenclature

The term "penicillin" is defined as the natural product of Penicillium mould with antimicrobial activity. In modern usage, the term penicillin is used more broadly to refer to any beta-lactam antimicrobial that contains a thiazolidine ring fused to the beta-lactam core, and may or may not be a natural product.

Like most natural products, penicillin is a mixture of active constituents (gentamicin is another example of a natural product that is an ill-defined mixture of active components). The principal active components of penicillin are listed in the following table.

Chemical name	UK nomenclature	US nomenclature	Relative potency
2-pentenylpenicillin	penicillin I	penicillin F	70–82%
benzylpenicillin	penicillin II	penicillin G	100% (reference)
p-hydroxybenzylpenicillin	penicillin III	penicillin X	130–140%
N-heptylpenicillin	penicillin IV	penicillin K	110–120%

Other minor active components of penicillin include penicillin O, penicillin U1 and penicillin U6. Other named constituents of natural penicillin, such as penicillin A, were subsequently found not to have antibiotic activity and are not chemically related to penicillin.

The precise constitution of the penicillin extracted depends on the species of Penicillium mould used and on the nutrient media used to culture the mould. Fleming's original strain of Penicillium rubens produces principally Penicillin F, but Penicillin F is unstable and difficult to isolate. Penicillin F was named 'F' for Alexander Fleming.

The principal commercial strain of Penicillium chrysogenum (the Peoria strain) produces penicillin G as the principle component when corn steep liquor is used as the culture medium. When phenoxyethanol is added to the culture medium, the mould produces penicillin V as the main penicillin instead.

6-aminopenicillanic acid (6-APA) is a compound derived from penicillin G. 6-APA contains the beta-lactam core of penicillin G, but with the side chains stripped off; 6-APA is a useful precursor for manufacturing other penicillins. There are a number of semi-synthetic penicillins derived from 6-APA and these are in three groups: antistaphylococcal penicillins, broad-spectrum penicillins and antipseudomonal penicillins. The semi-synthetic penicillins are all referred to as penicillins, because they are all derived ultimately from penicillin G.

Penicillin units

• One unit of penicillin G is defined as 0.600 micrograms. Therefore, 2 million units (2 megaunits) of penicillin G is 1.2 g.
• One unit of penicillin V is defined as 0.625 micrograms. Therefore 400,000 units of penicillin V is 250 mg.

The use of units to prescribe penicillin is an historical accident and is largely obsolete outside of the US. Since the original penicillin was an ill-defined mixture of active compounds (an amorphous yellow powder), the potency of each batch of penicillin varied from batch to batch. It was therefore impossible to prescribe 1 g of penicillin, because the activity of 1 g of penicillin from one batch would be different from the activity from another batch. After manufacture, each batch of penicillin had to be standardised against a known unit of penicillin: each glass vial was then filled with the number of units required. In the 1940's, a vial of 5,000 Oxford units was standard, but the depending on the batch, could contain anything from 15 mg to 20 mg of penicillin. Later, a vial of 1,000,000 international units became standard, and this could contain 2.5 g to 3 g of natural penicillin (a mixture of penicillin I, II, III and IV and natural impurities). With the advent of pure penicillin G preparations (a white crystalline powder), there is little reason to prescribe penicillin in units.

The "unit" of penicillin has had three previous definitions, and each definition was chosen as being roughly equivalent to the previous one.

• Oxford or Florey unit (1941). This was originally defined as the minimum amount of penicillin dissolved in 50 ml of meat extract that would inhibit the growth of a standard strain of Staphylococcus aureus (the Oxford Staphylococcus). The reference standard was a large batch of impure penicillin kept in Oxford. The assay was later modified by Florey's group to a more reproducible "cup assay": in this assay, a penicillin solution was defined to contain one unit/ml of penicillin when 339 microlitres of the solution placed in a "cup" on a plate of solid agar produced a 24 millimetre zone of inhibition of growth of Oxford Staphylococcus.
• First International Standard (1944). A single large standard batch of pure crystalline penicillin was stored at The National Institute for Medical Research at Mill Hill in London (the International Standard). One penicillin unit was defined at 0.6 micrograms of the International Standard. An impure "working standard" was also defined and was available in much larger quantities distributed around the world: one unit of the working standard was 2.7 micrograms (the amount per unit was much larger because of the impurities). At the same time, the cup assay was refined, where instead of specifying a zone diameter of 24 mm, the zone size were instead plotted against a reference curve to provide a readout on potency.
• Second International Standard (1953). A large standard batch of pure crystalline penicillin G was obtained: this was also stored at Mill Hill. One penicillin unit was defined as 0.5988 micrograms of the Second International Standard.

There is an older unit for penicillin V that is not equivalent to the current penicillin V unit. The reason is because the US FDA incorrectly assumed that the potency of penicillin V is the same mole-for-mole as penicillin G. In fact, penicillin V is less potent than penicillin G, and the current penicillin V unit reflects that fact.

• First international unit of penicillin V (1959). One unit of penicillin V was defined as 0.590 micrograms of a reference standard held at Mill Hill in London. This unit is now obsolete.

A similar standard was also established for penicillin K.

Medical usage

There are only two natural penicillins currently in clinical use: penicillin G and penicillin V.

Names	Method of administration	Notes
Penicillin G, benzylpenicillin	IV or IM	Not stable to stomach acid and cannot be taken by mouth. Doses as high as 2.4 g can be achieved.
Penicillin V, phenoxymethylpenicillin	By mouth	Stable to stomach acid and can be given by mouth. Doses higher than 500 mg cannot be achieved because of poor absorption.

Penicillin G cannot be given by mouth, because it is destroyed by stomach acid. Penicillin G can however be formulated to create insoluble salts. These are then given by intramuscular injection, which creates a depot that slowly releases penicillin G into the body over some weeks. These preparations do not result in chemical modification of penicillin G: these are simply slow-release formulations of penicillin G and are not different sorts of penicillin. There are only two slow-release formulations of penicillin G still in current use: Procaine penicillin and Benzathine benzylpenicillin. These are now used only in the treatment of syphilis.

For clinical purposes, penicillin V is used as the oral equivalent of penicillin G. The bioavailability of penicillin V is 60–73% and so for diseases where high blood levels of penicillin are required (for example, endocarditis), penicillin G injections are required and penicillin V cannot be used. Rapid clearance of penicillin by the kidney mean that both Penicillin G and V must be given four or more times a day in order to maintain a constant blood level of penicillin.

Penicillin G is licensed for use to treat septicaemia, empyema, pneumonia, pericarditis, endocarditis and meningitis caused by susceptible strains of staphylococci and streptococci. It is also licensed for the treatment of anthrax, actinomycosis, cervicofacial disease, thoracic and abdominal disease, clostridial infections, botulism, gas gangrene (with accompanying debridement and/or surgery as indicated), tetanus (as an adjunctive therapy to human tetanus immune globulin), diphtheria (as an adjunctive therapy to antitoxin and for the prevention of the carrier state), erysipelothrix endocarditis, fusospirochetosis (severe infections of the oropharynx, lower respiratory tract and genital area), Listeria infections, meningitis, endocarditis, Pasteurella infections including bacteraemia and meningitis, Haverhill fever; Rat-bite fever and disseminated gonococcal infections, meningococcal meningitis and/or septicaemia caused by penicillin-susceptible organisms and syphilis.

Penicillin G is given to treat infections caused by susceptible bacteria. Because penicillin resistance is now so common, testing for susceptibility of penicillin is mandatory to check that penicillin will actually be effective. Penicillin used to be first line treatment for Neisseria gonorrhoeae and Neisseria meningitidis, but wide-spread resistance means that it is longer recommended for treatment of these infections. Susceptibility testing must be performed by an accredited laboratory to look for penicillin resistance, where possible.

Bacterium	Susceptible (S)	Intermediate (I)	Resistant (R)
Staphylococcus aureus	≤0.12 mcg/ml	-	≥0.25 mcg/ml
Streptococcus pneumoniae meningitis	≤0.06 mcg/ml	-	≥0.12 mcg/ml
Streptococcus pneumoniae (not meningitis)	≤2 mcg/ml		≥8 mcg/ml
Streptococcus Viridans group	0.12 mcg/ml	0.25–2 mcg/ml	4 mcg/ml
Listeria monocytogenes	≤2 mcg/ml	-	-
Bacillus anthracis	≤0.12 mcg/ml	-	≥0.25 mcg/ml

Side effects

Main article: Side effects of penicillin

Common (≥ 1% of people) adverse drug reactions associated with use of the penicillins include diarrhoea, hypersensitivity, nausea, rash, neurotoxicity, urticaria, and superinfection (including candidiasis). Infrequent adverse effects (0.1–1% of people) include fever, vomiting, erythema, dermatitis, angioedema, seizures (especially in people with epilepsy), and pseudomembranous colitis. Penicillin can also induce serum sickness or a serum sickness-like reaction in some individuals. Serum sickness is a type III hypersensitivity reaction that occurs one to three weeks after exposure to drugs including penicillin. It is not a true drug allergy, because allergies are type I hypersensitivity reactions, but repeated exposure to the offending agent can result in an anaphylactic reaction. Allergy will occur in 1-10% of people, presenting as a skin rash after exposure. IgE-mediated anaphylaxis will occur in approximately 0.01% of patients.

Pain and inflammation at the injection site are also common for parenterally administered benzathine benzylpenicillin, benzylpenicillin, and, to a lesser extent, procaine benzylpenicillin. The condition is known as livedoid dermatitis or Nicolau syndrome.

Members

The penicillins can be divided in to two broad classes: the natural penicillins and the semi-synthetic penicillins.

Natural penicillins

The natural penicillins are produced by by Penicillium species and are used in the clinic without chemical modification. Different natural penicillins are produced by changing the culture media used to grow the Penicillium mould. There are only two natural penicillins currently in widespread medical use, and these are:

• Penicillin G
• Penicillin V

None of other natural penicillins (F, K, N, X, O, U1 or U6) are currently in clinical use.

Semi-synthetic penicillins

There are three major groups of semi-synthetic penicillin: these are all synthesised from 6-APA as an intermediate. These are the antistaphylococcal penicillins, the broad-spectrum penicillins and the antipseudomonal penicillins.

Antistaphylococcal penicillins

• Cloxacillin (by mouth or by injection)
• Dicloxacillin (by mouth or by injection)
• Flucloxacillin (by mouth or by injection)
• Methicillin (injection only)
• Nafcillin (injection only)
• Oxacillin (by mouth or by injection)

The antistaphylococcal penicillins are so called, because they are resistant to being broken down by Staphylococcal penicillinase. They are also therefore referred to as penicillinase-resistant penicillins or β-lactamase-resistant penicillins.

Broad-spectrum penicillins

The broad-spectrum penicillins are so called, because they are active against Gram-negative organisms such as Escherichia coli and Salmonella typhi, which are not usually susceptible to penicillin. However, resistance in these organisms is now common.

• Ampicillin
• Amoxycillin

There are a number of ampicillin precursors in existence. These are inactive compounds that are broken down in the gut to release ampicillin. None of these pro-drugs of ampicillin are in current use:

• Pivampicillin (pivaloyloxymethyl ester of ampicillin)
• Bacampicillin
• Metampicillin (formaldehyde ester of ampicillin)
• Talampicillin
• Hetacillin (ampicillin conjugated to acetone)

Epicillin is an aminopenicillin that has never seen widespread clinical use.

Antipseudomonal penicillins

The Gram-negative organism, Pseudomonas aeruginosa, is naturally resistant to many antibiotic classes. There were many efforts in the 1960's and 1970's to develop penicillins that are active against Pseudomonas species. There are two chemical classes within the group: carboxypenicillins and ureidopenicillins. All are given by injection: none can be given by mouth.

Carboxypenicillins

• Carbenicillin
• Ticarcillin
• Temocillin

Ureidopenicillins

• Mezlocillin
• Piperacillin
• Azlocillin

β-lactamase inhibitors

• Clavulanic acid
• Sulbactam
• Tazobactam

Pharmacology

Penicillin inhibits activity of enzymes that are needed for the cross linking of peptidoglycans in bacterial cell walls, which is the final step in cell wall biosynthesis. It does this by binding to penicillin binding proteins with the beta-lactam ring, a structure found on penicillin molecules. This causes the cell wall to weaken due to fewer cross links and means water uncontrollably flows into the cell because it cannot maintain the correct osmotic gradient. This results in cell lysis and death.

Some bacteria produce enzymes that break down the beta-lactam ring, called beta-lactamases, which make the bacteria resistant to penicillin. Therefore, some penicillins are modified or given with other drugs for use against antibiotic-resistant bacteria or in immunocompromised patients. Use of clavulanic acid or tazobactam, beta-lactamase inhibitors, alongside penicillin gives penicillin activity against beta-lactamase-producing bacteria. Beta-lactamase inhibitors irreversibly bind to beta-lactamase preventing it from breaking down the beta-lactam rings on the antibiotic molecule. Alternatively, flucloxacillin is a modified penicillin that has activity against beta-lactamase-producing bacteria due to an acyl side chain that protects the beta-lactam ring from beta-lactamase.

Mechanism of action

Main article: Beta-lactam antibiotic

Bacteria that attempt to grow and divide in the presence of penicillin fail to do so, and instead end up shedding their cell walls.

Penicillin and other β-lactam antibiotics act by inhibiting penicillin-binding proteins, which normally catalyze cross-linking of bacterial cell walls.

Bacteria constantly remodel their peptidoglycan cell walls, simultaneously building and breaking down portions of the cell wall as they grow and divide. β-Lactam antibiotics inhibit the formation of peptidoglycan cross-links in the bacterial cell wall; this is achieved through binding of the four-membered β-lactam ring of penicillin to the enzyme DD-transpeptidase. As a consequence, DD-transpeptidase cannot catalyze formation of these cross-links, and an imbalance between cell wall production and degradation develops, causing the cell to rapidly die.

The enzymes that hydrolyze the peptidoglycan cross-links continue to function, even while those that form such cross-links do not. This weakens the cell wall of the bacterium, and osmotic pressure becomes increasingly uncompensated—eventually causing cell death (cytolysis). In addition, the build-up of peptidoglycan precursors triggers the activation of bacterial cell wall hydrolases and autolysins, which further digest the cell wall's peptidoglycans. The small size of the penicillins increases their potency, by allowing them to penetrate the entire depth of the cell wall. This is in contrast to the glycopeptide antibiotics vancomycin and teicoplanin, which are both much larger than the penicillins.

Gram-positive bacteria are called protoplasts when they lose their cell walls. Gram-negative bacteria do not lose their cell walls completely and are called spheroplasts after treatment with penicillin.

Penicillin shows a synergistic effect with aminoglycosides, since the inhibition of peptidoglycan synthesis allows aminoglycosides to penetrate the bacterial cell wall more easily, allowing their disruption of bacterial protein synthesis within the cell. This results in a lowered MBC for susceptible organisms.

Penicillins, like other β-lactam antibiotics, block not only the division of bacteria, including cyanobacteria, but also the division of cyanelles, the photosynthetic organelles of the glaucophytes, and the division of chloroplasts of bryophytes. In contrast, they have no effect on the plastids of the highly developed vascular plants. This supports the endosymbiotic theory of the evolution of plastid division in land plants.

The chemical structure of penicillin is triggered with a very precise, pH-dependent directed mechanism, effected by a unique spatial assembly of molecular components, which can activate by protonation. It can travel through bodily fluids, targeting and inactivating enzymes responsible for cell-wall synthesis in gram-positive bacteria, meanwhile avoiding the surrounding non-targets. Penicillin can protect itself from spontaneous hydrolysis in the body in its anionic form while storing its potential as a strong acylating agent, activated only upon approach to the target transpeptidase enzyme and protonated in the active centre. This targeted protonation neutralizes the carboxylic acid moiety, which is weakening of the β-lactam ring N–C(=O) bond, resulting in a self-activation. Specific structural requirements are equated to constructing the perfect mouse trap for catching targeted prey.

Pharmacokinetics

Penicillin has low protein binding in plasma. The bioavailability of penicillin depends on the type: penicillin G has low bioavailability, below 30%, whereas penicillin V has higher bioavailability, between 60 and 70%. Penicillin has a short half life and is excreted via the kidneys.

Structure

Chemical structure of Penicillin G. The sulfur and nitrogen of the five-membered thiazolidine ring are shown in yellow and blue respectively. The image shows that the thiazolidine ring and fused four-membered β-lactam are not in the same plane.

The term "penam" is used to describe the common core skeleton of a member of the penicillins. This core has the molecular formula R-C₉H₁₁N₂O₄S, where R is the variable side chain that differentiates the penicillins from one another. The penam core has a molar mass of 243 g/mol, with larger penicillins having molar mass near 450—for example, cloxacillin has a molar mass of 436 g/mol. The key structural feature of the penicillins is the four-membered β-lactam ring; this structural moiety is essential for penicillin's antibacterial activity. The β-lactam ring is itself fused to a five-membered thiazolidine ring. The fusion of these two rings causes the β-lactam ring to be more reactive than monocyclic β-lactams because the two fused rings distort the β-lactam amide bond and therefore remove the resonance stabilisation normally found in these chemical bonds.

History

Main article: History of penicillin

Discovery

File:Alexander Fleming.jpg

Alexander Fleming, who is credited with discovering penicillin in 1928.

Sample of penicillium mould presented by Alexander Fleming to Douglas Macleod, 1935

Starting in the late 19th century there had been reports of the antibacterial properties of Penicillium mould, but scientists were unable to discern what process was causing the effect. Scottish physician Alexander Fleming at St Mary's Hospital in London (now part of Imperial College) was the first to show that Penicillium rubens had antibacterial properties in 1928. On 3 September 1928 he observed that fungal contamination of a bacterial culture (Staphylococcus aureus) appeared to kill the bacteria. He confirmed this observation with a new experiment on 28 September 1928. He published his experiment in 1929, and called the antibacterial substance (the fungal extract) penicillin.

C. J. La Touche identified the fungus as Penicillium rubrum (later reclassified by Charles Thom as P. notatum and P. chrysogenum, but later corrected as P. rubens). Fleming expressed initial optimism that penicillin would be a useful antiseptic, because of its high potency and minimal toxicity in comparison to other antiseptics of the day, and noted its laboratory value in the isolation of Bacillus influenzae (now called Haemophilus influenzae).

Fleming did not convince anyone that his discovery was important. This was largely because penicillin was so difficult to isolate that its development as a drug seemed impossible. It is speculated that had Fleming been more successful at making other scientists interested in his work, penicillin would possibly have been developed years earlier.

The importance of his work has been recognized by the placement of an International Historic Chemical Landmark at the Alexander Fleming Laboratory Museum in London on November 19, 1999.

Medical application

Howard Florey (pictured), Alexander Fleming and Ernst Chain shared a Nobel Prize in Physiology or Medicine in 1945 for their work on penicillin.

In 1930, Cecil George Paine, a pathologist at the Royal Infirmary in Sheffield, successfully treated ophthalmia neonatorum, a gonococcal infection in infants, with penicillin (fungal extract) on November 25, 1930.

In 1940, Australian scientist Howard Florey (later Baron Florey) and a team of researchers (Ernst Boris Chain, Edward Abraham, Arthur Duncan Gardner, Norman Heatley, Margaret Jennings, J. Orr-Ewing and G. Sanders) at the Sir William Dunn School of Pathology, University of Oxford made progress in making concentrated penicillin from fungal culture broth that showed both in vitro and in vivo bactericidal action. In 1941, they treated a policeman, Albert Alexander, with a severe face infection; his condition improved, but then supplies of penicillin ran out and he died. Subsequently, several other patients were treated successfully. In December 1942, survivors of the Cocoanut Grove fire in Boston were the first burn patients to be successfully treated with penicillin.

Mass production

A technician preparing penicillin in 1943

As the medical application was established, the Oxford team found that it was impossible to produce usable amounts in their laboratory. In 1941, Florey and Heatley travelled to the US in order to interest pharmaceutical companies in producing the drug and inform them about their process.

On March 14, 1942, the first patient was treated for streptococcal sepsis with US-made penicillin produced by Merck & Co. Half of the total supply produced at the time was used on that one patient, Anne Miller. By June 1942, just enough US penicillin was available to treat ten patients. In July 1943, the War Production Board drew up a plan for the mass distribution of penicillin stocks to Allied troops fighting in Europe. The results of fermentation research on corn steep liquor at the Northern Regional Research Laboratory at Peoria, Illinois, allowed the United States to produce 2.3 million doses in time for the invasion of Normandy in the spring of 1944. After a worldwide search in 1943, a mouldy cantaloupe in a Peoria, Illinois market was found to contain the best strain of mould for production using the corn steep liquor process. Pfizer scientist Jasper H. Kane suggested using a deep-tank fermentation method for producing large quantities of pharmaceutical-grade penicillin. Large-scale production resulted from the development of a deep-tank fermentation plant by chemical engineer Margaret Hutchinson Rousseau. As a direct result of the war and the War Production Board, by June 1945, over 646 billion units per year were being produced.

Penicillin was being mass-produced in 1944.

World War II poster extolling use of penicillin

G. Raymond Rettew made a significant contribution to the American war effort by his techniques to produce commercial quantities of penicillin, wherein he combined his knowledge of mushroom spawn with the function of the Sharples Cream Separator. By 1943, Rettew's lab was producing most of the world's penicillin. During World War II, penicillin made a major difference in the number of deaths and amputations caused by infected wounds among Allied forces, saving an estimated 12%–15% of lives. Availability was severely limited, however, by the difficulty of manufacturing large quantities of penicillin and by the rapid renal clearance of the drug, necessitating frequent dosing. Methods for mass production of penicillin were patented by Andrew Jackson Moyer in 1945. Florey had not patented penicillin, having been advised by Sir Henry Dale that doing so would be unethical.

Penicillin is actively excreted, and about 80% of a penicillin dose is cleared from the body within three to four hours of administration. Indeed, during the early penicillin era, the drug was so scarce and so highly valued that it became common to collect the urine from patients being treated, so that the penicillin in the urine could be isolated and reused. This was not a satisfactory solution, so researchers looked for a way to slow penicillin excretion. They hoped to find a molecule that could compete with penicillin for the organic acid transporter responsible for excretion, such that the transporter would preferentially excrete the competing molecule and the penicillin would be retained. The uricosuric agent probenecid proved to be suitable. When probenecid and penicillin are administered together, probenecid competitively inhibits the excretion of penicillin, increasing penicillin's concentration and prolonging its activity. Eventually, the advent of mass-production techniques and semi-synthetic penicillins resolved the supply issues, so this use of probenecid declined. Probenecid is still useful, however, for certain infections requiring particularly high concentrations of penicillins.

After World War II, Australia was the first country to make the drug available for civilian use. In the U.S., penicillin was made available to the general public on March 15, 1945.

Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine with for the development of penicillin.

Dorothy Hodgkin determined the chemical structure of penicillin.

Structure determination and total synthesis

Dorothy Hodgkin's model of penicillin's structure.

The chemical structure of penicillin was first proposed by Edward Abraham in 1942 and was later confirmed in 1945 using X-ray crystallography by Dorothy Crowfoot Hodgkin, who was also working at Oxford. She later received the Nobel prize for this and other structure determinations.

Chemist John C. Sheehan at the Massachusetts Institute of Technology (MIT) completed the first chemical synthesis of penicillin in 1957. Sheehan had started his studies into penicillin synthesis in 1948, and during these investigations developed new methods for the synthesis of peptides, as well as new protecting groups—groups that mask the reactivity of certain functional groups. Although the initial synthesis developed by Sheehan was not appropriate for mass production of penicillins, one of the intermediate compounds in Sheehan's synthesis was 6-aminopenicillanic acid (6-APA), the nucleus of penicillin. Attaching different groups to the 6-APA 'nucleus' of penicillin allowed the creation of new forms of penicillin.

Developments from penicillin

The narrow range of treatable diseases or "spectrum of activity" of the penicillins, along with the poor activity of the orally active phenoxymethylpenicillin, led to the search for derivatives of penicillin that could treat a wider range of infections. The isolation of 6-APA, the nucleus of penicillin, allowed for the preparation of semisynthetic penicillins, with various improvements over benzylpenicillin (bioavailability, spectrum, stability, tolerance).

The first major development was ampicillin in 1961. It offered a broader spectrum of activity than either of the original penicillins. Further development yielded β-lactamase-resistant penicillins, including flucloxacillin, dicloxacillin, and methicillin. These were significant for their activity against β-lactamase-producing bacterial species, but were ineffective against the methicillin-resistant Staphylococcus aureus (MRSA) strains that subsequently emerged.

Another development of the line of true penicillins was the antipseudomonal penicillins, such as carbenicillin, ticarcillin, and piperacillin, useful for their activity against Gram-negative bacteria. However, the usefulness of the β-lactam ring was such that related antibiotics, including the mecillinams, the carbapenems and, most important, the cephalosporins, still retain it at the center of their structures.

Production

A 1957 fermentor (bioreactor) used to grow Penicillium mould.

Penicillin is a secondary metabolite of certain species of Penicillium and is produced when growth of the fungus is inhibited by stress. It is not produced during active growth. Production is also limited by feedback in the synthesis pathway of penicillin.

α-ketoglutarate + AcCoA → homocitrate → L-α-aminoadipic acid → L-lysine + β-lactam

The by-product, l-lysine, inhibits the production of homocitrate, so the presence of exogenous lysine should be avoided in penicillin production.

The Penicillium cells are grown using a technique called fed-batch culture, in which the cells are constantly subject to stress, which is required for induction of penicillin production. The available carbon sources are also important: glucose inhibits penicillin production, whereas lactose does not. The pH and the levels of nitrogen, lysine, phosphate, and oxygen of the batches must also be carefully controlled.

The biotechnological method of directed evolution has been applied to produce by mutation a large number of Penicillium strains. These techniques include error-prone PCR, DNA shuffling, ITCHY, and strand-overlap PCR.

Semisynthetic penicillins are prepared starting from the penicillin nucleus 6-APA.

Biosynthesis

Penicillin G biosynthesis

Overall, there are three main and important steps to the biosynthesis of penicillin G (benzylpenicillin).

• The first step is the condensation of three amino acids—L-α-aminoadipic acid, L-cysteine, L-valine into a tripeptide. Before condensing into the tripeptide, the amino acid L-valine must undergo epimerization to become D-valine. The condensed tripeptide is named δ-(L-α-aminoadipyl)-L-cysteine-D-valine (ACV). The condensation reaction and epimerization are both catalyzed by the enzyme δ-(L-α-aminoadipyl)-L-cysteine-D-valine synthetase (ACVS), a nonribosomal peptide synthetase or NRPS.
• The second step in the biosynthesis of penicillin G is the oxidative conversion of linear ACV into the bicyclic intermediate isopenicillin N by isopenicillin N synthase (IPNS), which is encoded by the gene pcbC. Isopenicillin N is a very weak intermediate, because it does not show strong antibiotic activity.
• The final step is a transamidation by isopenicillin N N-acyltransferase, in which the α-aminoadipyl side-chain of isopenicillin N is removed and exchanged for a phenylacetyl side-chain. This reaction is encoded by the gene penDE, which is unique in the process of obtaining penicillins.

See also

• Medicinal fungi
• Penicillinase

Further reading

• Nicolaou KC, Corey EJ (1996). Classics in Total Synthesis : Targets, Strategies, Methods (5. repr. ed.). Weinheim: VCH. ISBN 978-3-527-29284-4.
• Dürckheimer W, Blumbach J, Lattrell R, Scheunemann KH (March 1, 1985). "Recent Developments in the Field of β-Lactam Antibiotics". Angewandte Chemie International Edition in English. 24 (3): 180–202. doi:10.1002/anie.198501801.
• Hamed RB, Gomez-Castellanos JR, Henry L, Ducho C, McDonough MA, Schofield CJ (January 2013). "The enzymes of β-lactam biosynthesis". Natural Product Reports. 30 (1): 21–107. doi:10.1039/c2np20065a. PMID 23135477.

External links

• Model of Structure of Penicillin, by Dorothy Hodgkin et al., Museum of the History of Science, Oxford
• The Discovery of Penicillin, A government produced film about the discovery of Penicillin by Sir Alexander Fleming, and the continuing development of its use as an antibiotic by Howard Florey and Ernst Boris Chain on YouTube.
• Penicillin at The Periodic Table of Videos (University of Nottingham)
• Penicillin Released to Civilians Will Cost $35 Per Patient Popular Science, August 1944, article at bottom of page