Tetracyclines are a group of broad-spectrum antibiotics whose general usefulness has been reduced with the onset of bacterial resistance. Despite this, they remain the treatment of choice for some specific indications.
They are so named for their four (“tetra-”) hydrocarbon rings (“-cycl-”) derivation (“-ine”). To be specific, they are defined as "a subclass of polyketides having an octahydrotetracene-2-carboxamide skeleton".[1] They are collectively known as "derivatives of polycyclic naphthacene carboxamide".
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Tetracyclines are generally used in the treatment of infections of the respiratory tract, sinuses, middle ear, urinary tract, and intestines, and is used in the treatment of gonorrhoea, especially in patients allergic to β-lactams and macrolides; however, their use for these indications is less popular than it once was due to widespread resistance development in the causative organisms.
Their most common current use is in the treatment of moderately severe acne and rosacea (tetracycline, oxytetracycline, doxycycline, or minocycline).
Doxycycline is also used as a prophylactic treatment for infection by Bacillus anthracis (anthrax) and is effective against Yersinia pestis, the infectious agent of bubonic plague. It is also used for malaria treatment and prophylaxis, as well as treating elephantiasis.
Tetracyclines remain the treatment of choice for infections caused by chlamydia (trachoma, psittacosis, salpingitis, urethritis, and L. venereum infection), Rickettsia (typhus, Rocky Mountain spotted fever), brucellosis, and spirochetal infections (borreliosis, syphilis, and Lyme disease). In addition, they may be used to treat anthrax, plague, tularemia, and Legionnaires' disease. It is also used in veterinary medicine on pigs and alike.
They may have a role in reducing the duration and severity of cholera, although drug-resistance is occurring,[2] and their effects on overall mortality is questioned.[3]
Demeclocycline has an additional use in the treatment of SIADH.
Tetracycline derivatives are currently being investigated for the treatment of certain inflammatory disorders.
Side-effects from tetracyclines are not common, but of particular note is phototoxicity. It increases the risk of sunburn under exposure to light from the sun or other sources. This may be of particular importance for those intending to take on vacations long-term doxycycline as a malaria prophylaxis.
They may cause stomach or bowel upsets, and, on rare occasions, allergic reactions. Very rarely, severe headache and vision problems may be signs of dangerous secondary intracranial hypertension, also known as pseudotumor cerebri.
Tetracyclines are teratogens due to the likelihood of causing teeth discolouration in the fetus as they develop in infancy. For this same reason, tetracyclines are contraindicated for use in children under 8 years of age. They are, however, safe to use in the first 18 weeks of pregnancy.
Some patients taking tetracyclines require medical supervision because they can cause steatosis and hepatotoxicity.[4][5][6]
Tetracyclines should be used with caution in those with liver impairment and those that are soluble in water and urine worsen renal failure (this is not true of the lipid soluble agents doxycycline and minocycline). They may increase muscle weakness in myasthenia gravis and exacerbate systemic lupus erythematosus. Antacids reduce the absorption of all tetracyclines, and dairy products reduce absorption greatly for all but minocycline.
The breakdown products of tetracyclines are toxic and can cause Fanconi Syndrome, a potentially fatal disease affecting proximal tubular function in the nephrons of the kidney. Prescriptions of these drugs should be discarded once expired because they can cause hepatotoxicity.
It was once believed that tetracycline antibiotics impair the effectiveness of many types of hormonal contraception. Recent research has shown no significant loss of effectiveness in oral contraceptives while using most tetracyclines. Despite these studies, many physicians still recommend the use of barrier contraception for people taking any tetracyclines to prevent unwanted pregnancy.[7][8][9]
Tetracycline use should be avoided in pregnant or lactating women, and in children with developing teeth because they may result in permanent staining (dark yellow-gray teeth with a darker horizontal band that goes across the top and bottom rows of teeth), and possibly affect the growth of teeth and bones.
Usage during the first 12 weeks of pregnancy does not appear to increase the risk of any major birth defects.[10] There may be a small increased risk for minor birth defects such as an inguinal hernia, but the number of reports is too small to be sure if there actually is any risk.[10]
In tetracycline preparation, stability must be considered in order to avoid formation of toxic epi-anhydrotetracyclines.
Tetracycline antibiotics are protein synthesis inhibitors, inhibiting the binding of aminoacyl-tRNA to the mRNA-ribosome complex. They do so mainly by binding to the 30S ribosomal subunit in the mRNA translation complex.[11]
Tetracyclines also have been found to inhibit matrix metalloproteinases. This mechanism does not add to their antibiotic effects, but has led to extensive research on chemically modified tetracyclines or CMTs (like incyclinide) for the treatmet of rosacea, acne, diabetes and various types of neoplasms.[12][13][14] Since incyclinide was announced to be ineffective for rosacea in September 2007, no drugs of this group will be marketed in the near-future.[15]
Several trials have examined modified and unmodified tetracyclines for the treatment of human cancers, of those very promising results were achieved with CMT-3 for patients with Kaposi Sarcoma.[16]
Tetracycline inhibits cell growth by inhibiting translation. It binds to the 16S part of the 30S ribosomal subunit and prevents the amino-acyl tRNA from binding to the A site of the ribosome. The binding is reversible in nature.
Cells become resistant to tetracycline by at least three mechanisms: enzymatic inactivation of tetracycline, efflux, and ribosomal protection. Inactivation is the rarest type of resistance, where an acetyl group is added to the molecule, causing inactivation of the drug. In efflux, a resistance gene encodes a membrane protein that actively pumps tetracycline out of the cell. This is the mechanism of action of the tetracycline resistance gene on the artificial plasmid pBR322. In ribosomal protection, a resistance gene encodes a protein that can have several effects, depending on what gene is transferred. Six classes of ribosomal protection genes/proteins have been found, all with high sequence homology, suggesting a common evolutionary ancestor.
Possible mechanisms of action of these protective proteins include:
All of these changes to ribosomes are reversible (non-covalent) because ribosomes isolated from both tetracycline-resistant and susceptible organisms bind tetracycline equally well in vitro.
When ingested, it is usually recommended that the more water-soluble, short-acting tetracyclines (plain tetracycline, chlortetracycline, Oxytetracycline, demeclocycline and methacycline) be taken with a full glass of water, either two hours after eating, or two hours before eating. This is partly because most tetracyclines bind with food and also easily with magnesium, aluminium, iron, and calcium, which reduces their ability to be completely absorbed by the body. Dairy products, antacids, or preparations containing iron are particularly recommended to be avoided near the time of taking the drug. Partial exceptions to these rules occur for doxycycline and minocycline, which may be taken with food (though not iron, antacids, or calcium supplements). Minocycline, can be taken with dairy products because it does not chelate calcium as readily, although dairy products do decrease absorption of minocycline slightly.[17]
The first member of the group to be discovered is Chlortetracycline (Aureomycin) in the late 1940s by Dr. Benjamin Duggar, a scientist employed by American Cyanamid - Lederle Laboratories, who derived the substance from a golden-colored, fungus-like, soil-dwelling bacterium named Streptomyces aureofaciens.[18] Oxytetracycline (Terramycin) was discovered shortly afterwards by AC Finlay et al.; it came from a similar soil bacterium named Streptomyces rimosus.[19] Robert Burns Woodward determined the structure of Oxytetracycline enabling Lloyd H. Conover to successfully produce tetracycline itself as a synthetic product.[20] The development of many chemically altered antibiotics formed this group. In June 2005, tigecycline, the first member of a new subgroup of tetracyclines named glycylcyclines, was introduced to treat infections that are resistant to other antimicrobics including conventional tetracyclines.[21] While tigecycline is the first tetracycline approved in over 20 years, other, newer versions of tetracyclines are currently in human clinical trials.
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Tigecycline may also be considered a tetracycline antibiotic, though it is usually classified as a glycylcycline antibiotic.
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