Last updated on October 4, 2019 at 11:00
Tetracyclines are a group of antibiotics. The most important ones are tetracycline, doxycycline, demeclocycline, minocycline, lymecycline and oxytetracycline. Tigecycline is technically not a tetracycline but is similar enough that it is mentioned here.
Mechanism of action:
Tetracyclines bind to the 30S ribosomal subunit in bacteria and prevent elongation of the peptide chain. This elicits a bacteriostatic effect.
Mechanisms of resistance:
Tetracyclines were previously widely used, which caused extensive resistance to develop. Resistance has receded recently and so their use has become more popular.
There are three main mechanisms of resistance. Expression of an efflux pump, enzymatic inactivation and expression of a “protection protein”, which protects the ribosomes from the antibiotic.
The efflux pump and protection protein don’t work against tigecycline, meaning that tigecycline is effective against many tetracycline-resistant infections.
Tetracycline and oxytetracycline are only 60 – 70% absorbed, while doxycycline and minocycline are 100% absorbed. This makes gastrointestinal symptoms more common in tetracycline and oxytetracycline. Tigecycline is not orally absorbed and must be given parenterally.
Tetracycline and oxytetracycline form chelates with metal ions, which decreases their absorption. For this reason, these drugs shouldn’t be taken with milk, antacids or iron preparations.
Tetracyclines accumulate intracellularly and extracellularly. They penetrate the placenta and are excreted in breast milk, but don’t enter the CNS. Because they accumulate intracellularly they’re especially effective against intracellular pathogens like Chlamydia and Rickettsia.
We should only know the excretion of doxycycline, which is the most used drug. It is mostly excreted by bile and therefore good for treating gallbladder infections.
Oxytetracycline and tetracycline have short half-lives and must be taken 2 – 4 times daily. Doxycycline, minocycline and tigecycline have long half-lives and can be taken only once daily.
GI symptoms are most common in tetracycline and oxytetracycline, but all tetracyclines can directly irritate GI mucosa and even cause ulcers. These drugs should be taken with plenty of water.
Photosensitivity can occur. Both the drug itself and the person taking it should avoid sunlight to avoid dermatitis.
Tetracycline forms complexes with calcium in bones and teeth, causing deformities in growing bones and discoloration of teeth. For this reason, tetracyclines are contraindicated during pregnancy and under 8 years of age.
In high doses hepatotoxicity can occur.
Tetracyclines are mostly used for atypical bacteria like Borrelia, Mycoplasma, Rickettsia, Chlamydia, Vibrio cholerae etc. They are especially effective against intracellular bacteria. They’re also used to treat acne, as they decrease the colonization of Propionibacterium acnes. They may be used to treat H. pylori as well but are currently not part of the first-line regimen.
Tigecycline is used for complicated soft tissue infections and abdominal infections, especially when there is resistance toward the first-choice antibiotics. It’s even effective against MRSA, VRE and penicillin-resistant Strep. pneumoniae (PRSP).
Mechanism of action:
Chloramphenicol binds to the 50S ribosomal subunit and inhibits elongation of the peptide chain. This effect is bacteriostatic against most bacteria it is effective against, but is bactericidal against H. influenzae, Neisseria and Bacteroides fragilis.
Mechanism of resistance:
Resistance against chloramphenicol is common among Gram-positives and Gram-negatives. It is mediated by enzymatic inactivation or by decreased cell envelope penetration.
Chloramphenicol is orally absorbed, and it is broadly distributed, including in the CNS.
It is metabolized in the liver, mostly by conjugation with glucuronic acid by the enzyme UGT. It is excreted by the kidney and has a short half-life (3 – 5 hours).
This drug is toxic, with three significant adverse effects. It can inhibit mitochondrial protein synthesis in cells in the bone marrow, causing bone-marrow suppression. This effect is reversible in that it will be normalized after cessation of the drug. It is also dose dependent.
However, chloramphenicol can cause aplastic anaemia as well. Unlike the aforementioned bone-marrow depression this aplastic anaemia is irreversible and not dose dependent. It can occur even after the smallest dose, and it usually occurs weeks or months after the last dose. It is fatal if not treated with bone marrow transplantation. The mechanism of this adverse effect is unknown.
Grey baby syndrome is the last significant adverse effect of chloramphenicol. It occurs when a pregnant mother receives chloramphenicol during the last trimester. Neonates, especially prematures, have decreased or no UGT activity. This causes chloramphenicol to accumulate, causing muscle weakness, cyanosis, vomiting, hypothermia and circulatory shock. The cyanosis gives the baby a grey appearance, hence the name.
Lastly, chloramphenicol causes some less severe side effects as well, like GI symptoms and oral and vaginal candidiasis.
The toxicity of chloramphenicol limits its clinical use. Its systemic use is often limited to severe cases or when other safer antibiotics are unavailable. It’s more commonly used in developing countries, as it is relatively cheap.
Topical application is not associated with adverse effects. Chloramphenicol eyedrops are therefore often used in conjunctivitis.
It is sometimes used to treat bacterial meningitis, Rickettsia, Coxiella, Brucella and Salmonella typhi infections.
Clindamycin is a semi-synthetic lincosamide.
Mechanism of action:
It binds to the 50S ribosomal subunit, inhibiting peptide translocation. This effect is bacteriostatic.
The drug is completely orally absorbed. It is broadly distributed but does not enter the CNS. It accumulates in macrophages and in bones.
It is metabolized in the liver and excreted by the bile. The elimination is rapid, with a half-life of 3 hours.
Diarrhoea and C. difficile colitis are much more common with clindamycin than with other antibiotics. Rashes are relatively common, and metallic taste can also occur.
Clindamycin is effective against most Gram-positive cocci, but not Enterococci. It is also effective against anaerobes, but not against aerobe Gram-negatives.
Clindamycin is used topically to treat acne. It is used systemically to treat soft tissue infections, bone infections, joint infections and abscesses.
The important oxazolidinones are linezolid and tedizolid.
Mechanism of action:
These drugs bind to the 50S subunit of the bacterial ribosome and inhibit protein synthesis. The exact mechanism is unique for this class of drugs, and it inhibits protein synthesis at an earlier stage than other antibiotics.
Linezolid is a weak, reversible monoamine oxidase inhibitor.
These drugs are completely absorbed (bioavailability 100%) from the GI tract and are broadly distributed. They penetrate the CNS.
They are 70% biotransformed and excreted by the kidney, while the remaining 30% are excreted unchanged.
Peripheral neuropathy and bone marrow suppression are the most significant side effects, but gastrointestinal side effects can also occur. The more severe side effects usually only occur in people who have been treated with the drugs for weeks.
Unique for linezolid is that it can cause serotonin syndrome, especially if taken with monoamine oxidase inhibitors or SSRIs. It can also cause the cheese reaction.
Oxazolidinones are effective against Gram-positives and Mycobacteria. Both of these drugs are effective against multi-resistant bacteria like VRE, MRSA, VRSE and multi-resistant M. tuberculosis. These drugs are reserved for multi-resistant infections.
The most important streptogramins are quinupristin and dalfopristin. These antibiotics are usually given in combination to yield a synergic action. When used alone they’re bacteriostatic but the combination is bactericidal.
Mechanism of action:
Streptogramins bind to the 50S subunit and inhibit elongation of the peptide chain, similarly to clindamycin and macrolides. Dalfopristin enhances the binding of quinupristin.
These drugs are not absorbed and must be given IV. They accumulate in macrophages and don’t enter the CNS.
They’re metabolized in the liver by CYP450 and excreted with bile. They inhibit CYP3A4 and can therefore interact with many drugs.
The drug commonly has side effects (10 – 15%), including arthralgia, myalgia and pain at the infusion site.
Quinupristin/dalfopristin is effective against multi-resistant Gram-positives like VRE, MRSA and VRSE. They’re reserved for multi-resistant infections, often as a last resort. Resistance toward quinupristin/dalfopristin is rare.
72. Aminoglycosides. Macrolide antibiotics
74. Antituberculotic drugs. Anti-leprosy drugs