Penicillin

Penicillin core structure

Penicillin (sometimes abbreviated PCN or pen) is a group of Beta-lactam antibiotics used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms.

“Penicillin” is also the informal name of a specific member of the penicillin group Penam Skeleton, which has the molecular formula R-C₉H₁₁N₂O₄S, where R is a variable side chain.

History

The discovery of penicillin is attributed to Scottish scientist and nobel laureate Alexander Fleming in 1928. The development of penicillin for use as a medicine is attributed to the Australian Nobel Laureate Howard Walter Florey.

However, several others reported the bacteriostatic effects of Penicillium earlier than Fleming. The first published reference appears to have been in 1875, in the Royal Society in London by John Tyndall.^[1] Ernest Duchesne documented it in his 1897 paper which was not accepted by the Institut Pasteur because of his young age. In March 2000, doctors at the San Juan de Dios Hospital in San José, Costa Rica published the manuscripts of the Costa Rican scientist and medical doctor Clodomiro (Clorito) Picado Twight (1887–1944). They reported Picado's observations on the inhibitory actions of fungi of the genus Penic between 1915 and 1927. Picado reported his discovery to the Paris Academy of Sciences, yet did not patent it, even though his investigations started years before Fleming's.

Fleming recounted that the date of his breakthrough was on the morning of Friday, September 28, 1928.^[2] At his laboratory in the basement of St. Mary's Hospital in London (now part of Imperial College), Fleming noticed a halo of inhibition of bacterial growth around a contaminant blue-green mold in a Staphylococcus plate culture. Fleming concluded that the mold was releasing a substance that was inhibiting bacterial growth and lysing the bacteria. He grew a pure culture and discovered that it was a Penicillium mold, now known to be Penicillium notatum. Charles Thom, an American specialist working at the U.S. Department of Agriculture was the acknowledged expert, and Fleming referred the matter to him. Fleming coined the term "penicillin" to describe the filtrate of a broth culture of the Penicillium mold. Even in these early stages, penicillin was found to be most effective against Gram-positive bacteria, and ineffective against Gram-negative organisms and fungi. He expressed initial optimism that penicillin would be a useful disinfectant, being highly potent with minimal toxicity compared to antiseptics of the day, and noted its laboratory value in the isolation of "Bacillus influenzae" (now Haemophilus influenzae).^[3] After further experiments, Fleming was convinced that penicillin could not last long enough in the human body to kill pathogenic bacteria, and stopped studying it after 1931. He restarted clinical trials in 1934, and continued to try to get someone to purify it until 1940.^[4]

In 1930 Cecil George Paine, a pathologist at the Royal Infirmary in Sheffield, attempted to treat sycosis barbae – eruptions in beard follicles – but was unsuccessful, probably because the drug did not get deep enough. Moving on to ophthalmia neonatorum – a gonococcal infection in babies – he achieved the first cure on 25 November 1930. He cured four patients (one adult, three babies) of eye infections, although a fifth patient was not so lucky.^[5]

In 1939, Australian scientist Howard Florey (later Baron Florey) and a team of researchers (Ernst Boris Chain, A. D. Gardner, Norman Heatley, M. Jennings, J. Orr-Ewing and G. Sanders) at the Sir William Dunn School of Pathology, University of Oxford made significant progress in showing the in vivo bactericidal action of penicillin. Their attempts to treat humans failed due to insufficient volumes of penicillin (the first patient treated was Reserve Constable Albert Alexander), but they proved it harmless and effective on mice.^[6]

Some of the pioneering trials of penicillin took place at the Radcliffe Infirmary in Oxford, England. On March 141942, John Bumstead and Orvan Hess became the first in the world to successfully treat a patient using penicillin.^[7][8]

The challenge of mass-producing the drug was daunting. On March 14, 1942 the first patient was treated for streptococcal septicemia with U.S.-made penicillin produced by Merck & Co.^[9] Half of the total supply produced at the time was used on that one patient. By June 1942 there was just enough U.S. penicillin available to treat ten patients.^[10] A moldy cantaloupe in a Peoria, Illinois market in 1943 was found to contain the best and highest-quality penicillin after a world-wide search.^[11] The discovery of the cantaloupe, and the results of fermentation research on corn-steep liquid at the Northern Regional Research Laboratory at Peoria, Illinois, allowed the USA to produce 2.3 million doses in time for the invasion of Normandy in the spring of 1944.

Penicillin was being mass-produced in 1944

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. Penicillin are actively secreted, and about 80% of a penicillin dose is cleared within three to four hours of administration. During those times, it became common procedure to collect the urine from patients being treated so that the penicillin could be isolated and reused.^[12]

This was not a satisfactory solution, so researchers looked for a way to slow penicillin secretion. They hoped to find a molecule that could compete with penicillin for the organic acid transporter responsible for secretion such that the transporter would preferentially secrete the competitive inhibitor. The uricosuric agent probenecid proved to be suitable. When probenecid and penicillin are concomitantly administered, probenecid competitively inhibits the secretion of penicillin, increasing penicillin's concentration and prolonging its activity. The advent of mass-production techniques and semi-synthetic penicillins solved supply issues, and this use of probenecid declined.^[12] Probenecid is still useful, however, for certain infections requiring particularly high concentrations of penicillins.^[13]

The chemical structure of penicillin was determined by Dorothy Crowfoot Hodgkin in the early 1940s. A team of Oxford research scientists led by Australian Howard Florey, Baron Florey and including Ernst Boris Chain and Norman Heatley discovered a method of mass-producing the drug. Chemist Robert Burns Woodward at Harvard University completed the first total synthesis of penicillin and some of its analogs in the early 1950s, but his methods were not efficient for mass production. Florey and Chain shared the 1945 Nobel prize in medicine with Fleming for this work, and, after WWII, Australia was the first country to make the drug available for civilian use. Penicillin has since become the most widely used antibiotic to date, and is still used for many Gram-positive bacterial infections.

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 first major development was ampicillin, which offered a broader spectrum of activity than either of the original penicillins. Further development yielded beta-lactamase-resistant penicillins including flucloxacillin, dicloxacillin and methicillin. These were significant for their activity against beta-lactamase-producing bacteria species, but are ineffective against the methicillin-resistant Staphylococcus aureus strains that subsequently emerged.

The line of true penicillins was the antipseudomonal penicillins, such as ticarcillin and piperacillin, useful for their activity against Gram-negative bacteria. However, the usefulness of the beta-lactam ring was such that related antibiotics, including the mecillinams, the carbapenems and, most important, the cephalosporins, have this at the center of their structures.^[14]

Mechanism of action

Main article: beta-lactam antibiotic

β-lactam antibiotics work by inhibiting the formation of peptidoglycan cross-links in the bacterial cell wall. The β-lactam moiety (functional group) of penicillin binds to the enzyme (DD-transpeptidase) that links the peptidoglycan molecules in bacteria, which weakens the cell wall of the bacterium (in other words, the antibiotic causes cytolysis or death due to osmotic pressure). In addition, the build-up of peptidoglycan precursors triggers the activation of bacterial cell wall hydrolases and autolysins, which further digest the bacteria's existing peptidoglycan.

Gram-positive bacteria are called protoplasts when they lose their cell wall. Gram-negative bacteria do not lose their cell wall 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 its disruption of bacterial protein synthesis within the cell. This results in a lowered MBC for susceptible organisms.

Variants in clinical use

Benzylpenicillin/penicillin G

The term “penicillin” is often used in the generic sense to refer to one of the narrow-spectrum penicillins, in particular, benzylpenicillin.

Penicillin G
Systematic (IUPAC) name
4-Thia-1-azabicyclo(3.2.0)heptane-2-carboxylic acid, 3,3-dimethyl-7-oxo-6-((phenylacetyl)amino)- (2S-(2α,5α,6β))-
Identifiers
CAS number	61-33-6
ATC code	?
PubChem	?
Chemical data
Formula	C16H18N2O4S
Mol. mass	334.4 g/mol
Pharmacokinetic data
Bioavailability	?
Metabolism	?
Half life	?
Excretion	?
Therapeutic considerations
Pregnancy cat.	?
Legal status
Routes	parenteral

Benzylpenicillin, commonly known as penicillin G, is the gold standard penicillin. Penicillin G is typically given by a parenteral route of administration (not orally) because it is unstable in the hydrochloric acid of the stomach. Because the drug is given parenterally, higher tissue concentrations of penicillin G can be achieved than is possible with phenoxymethylpenicillin. These higher concentrations translate to increased antibacterial activity.

Specific indications for benzylpenicillin include:^[13]

• Cellulitis
• Bacterial endocarditis
• Gonorrhea
• Meningitis
• aspiration pneumonia, lung abscess
• Community-acquired pneumonia
• Syphilis
• Septicemia in children

Phenoxymethylpenicillin/penicillin V

Phenoxymethylpenicillin, commonly known as penicillin V, is the orally active form of penicillin. It is less active than benzylpenicillin, however, and is appropriate only in conditions where high tissue concentrations are not required.

Specific indications for phenoxymethylpenicillin include:^[13]

• Infections caused by Streptococcus pyogenes
  • Tonsillitis
  • Pharyngitis
  • Skin infections
• Prophylaxis of rheumatic fever
• Moderate-to-severe gingivitis (with metronidazole)

Penicillin V is the first choice in the treatment of odontogenic infections.

Procaine

Procaine benzylpenicillin (rINN), also known as procaine penicillin, is a combination of benzylpenicillin with the local anaesthetic agent procaine. Following deep intramuscular injection, it is slowly absorbed into the circulation and hydrolysed to benzylpenicillin — thus it is used where prolonged low concentrations of benzylpenicillin are required.

This combination is aimed at reducing the pain and discomfort associated with a large intramuscular injection of penicillin. It is widely used in veterinary settings.

Specific indications for procaine penicillin include:^[13]

• Syphilis
  • It should be noted that in the United States, Bicillin C-R (a injectable suspension which 1.2 million units of benzathine penicillin & 1.2 million units of procaine penicillin per 4 mL) is not recommended for treating syphilis, since it contains only half the recommended dose of benzathine penicillin. Medication errors have been made due to the confusion between Bicillin L-A & Bicillin C-R.^[15] As a result, changes in product packaging have been made; specifically, the statement "Not for the Treatment of Syphilis" has been added in red text to both the Bicillin CR and Billin CR 900/300 syringe labels.^[16]
• Respiratory tract infections where compliance with oral treatment is unlikely
• Cellulitis, erysipelas

Procaine penicillin is also used as an adjunct in the treatment of anthrax.

Benzathine

Benzathine benzylpenicillin (rINN), also known as benzathine penicillin, is slowly absorbed into the circulation, after intramuscular injection, and hydrolysed to benzylpenicillin in vivo. It is the drug-of-choice when prolonged low concentrations of benzylpenicillin are required and appropriate, allowing prolonged antibiotic action over 2–4 weeks after a single IM dose. It is marketed by Wyeth under the trade name Bicillin L-A. Specific indications for benzathine penicillin include:^[13]

• Prophylaxis of rheumatic fever
• Early or latent syphilis

Adverse effects

Common adverse drug reactions (≥1% of patients) associated with use of the penicillins include diarrhea, hypersensitivity, nausea, rash, neurotoxicity urticaria, and/or superinfection (including candidiasis). Infrequent adverse effects (0.1–1% of patients) include fever, vomiting, erythema, dermatitis, angioedema, seizures (especially in epileptics), and/or pseudomembranous colitis.^[13]

Pain and inflammation at the injection site is also common for parenterally administered benzathine benzylpenicillin, benzylpenicillin, and, to a lesser extent, procaine benzylpenicillin.

Although penicillin is still the most commonly reported allergy, less than 20% of all patients who believe that they have a penicillin allergy are truly allergic to penicillin;^[17] nevertheless, penicillin is still the most common cause of severe allergic drug reactions.

Allergic reactions to any β-lactam antibiotic may occur in up to 10% of patients receiving that agent.^[18] Anaphylaxis will occur in approximately 0.01% of patients.^[13] It has previously been accepted that there was up to a 10% cross-sensitivity between penicillin-derivatives, cephalosporins, and carbapenems, due to the sharing of the β-lactam ring.^[19][20] However recent assessments have shown no increased risk for cross-allergy for 2nd generation or later cephalosporins.^[21][22] Recent papers have shown that a major feature in determining immunological reactions is the similarity of the side chain of first generation cephalosporins to penicillins, rather than the β-lactam structure that they share.^[23]

Production

Penicillin is a secondary metabolite of fungus Penicillium, that 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 and will produce plenty of penicillin. The carbon sources that are available are also important: glucose inhibits penicillin, whereas lactose does not. The pH level, nitrogen level, Lysine level, Phosphate level, and oxygen availability of the batches must be controlled automatically.

Other areas of biotechnology such as directed evolution can also be applied to produce by mutation a much larger number of penicillin strains. These directed-evolution techniques include error-prone PCR, DNA shuffling, ITCHY, and strand over-lap PCR.

Penicillin production emerged as an industry as a direct result of World War II. During the war, there was an abundance of jobs available on the homefront. A War Production Board was founded to monitor job distribution and production.^[24] Penicillin production was a huge surplus during the war especially with all the available jobs and the industry prospered. In July 1943, the War Production Board had set up a plan to distribute mass stock of penicillin to troops fighting in Europe. At the time of this plan, 425 million units were being produced. As a direct result of the war and the War Production Board, by June 1945 over 646 billion units were being produced.^[25]

See also

• β-Lactam antibiotic

References

External links

• Model of Structure of Penicillin, by Dorothy Hodgkin et al., Museum of the History of Science, Oxford