Tetracyclines
- CAS No.
- Chemical Name:
- Tetracyclines
- Synonyms
- Tetracyclines
- CBNumber:
- CB81382657
- Molecular Formula:
- Molecular Weight:
- 0
- MDL Number:
- MOL File:
- Mol file
Proposition 65 List | Tetracyclines (internal use) |
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Tetracyclines Chemical Properties,Uses,Production
Pharmacology and mechanism of action
Tetracyclines are bacteriostatic antibiotics with broad-spectrum activity. They are primarily used for the treatment of chlamydia, rickettsia, mycoplasma, spirochete as well as infections due to Gram-positive and Gram-negative bacteria. For the treatment of malaria, tetracyclines have a potent but slow blood schizontocidal effect, thus they are used together with quinine. They are also effective against the primary tissue stages of Plasmodium falciparum but lack gametocytocidal effect [1].
The mechanism of action of tetracyclines in bacteria is inhibition of protein synthesis by binding to the S-30 ribosome, inhibiting the access of tRNA to the mRNA-ribosome complex [2]. The mechanism of action against malaria is not known.
Indications
Tetracyclines are primarily used as a supplement to quinine in the treatment of P. falciparum malaria in areas with decreased susceptibility for quinine (i.e. in Thailand or adjacent countries).
Doxycycline is also used for prophylaxis against P. falciparum in areas where mefloquine resistance is frequent.
Side effects
Mild gastrointestinal symptoms such as epigastric pain, nausea, vomiting, diarrhoea, and pruritus ani are frequent. Irritative diarrhoea due to the substance should be distinguished from pseudomembraneous colitis due to the overgrowth of Clostridium difficile[2]. Pronounced photosensitivity reactions of the skin occur and seem to be more common with more intense sun exposure [3]. Renal function, in patients with renal damage, is worsened by tetracycline (not by doxycycline). Children under 8 years may develop permanent brown discoloration of the teeth. Liver damage can also be seen especially in pregnant women.
Rare hypersensitivity skin reactions such as urticaria, angio-edema and serum sickness may occur [2].
Contraindications and precautions
Tetracycline should not be given to subjects with pre-existing kidney or liver damage. Doxycycline, however, can be given in unchanged dose to patients with severe kidney failure. Due to tooth staining, tetracyclines should not be given to children under 8 years unless absolutely necessary. Tetracyclines also deposit in the human skeleton and may cause growth retardation, but this effect is reversible in short-term treatment.
Interactions
Iron, milk, and antacids reduce the bioavailability of tetracyclines. The concurrent use of tetracycline with methoxyflurane is nephrotoxic and should be avoided [4]. There are no negative reports concerning the combination of tetracyclines with other antimalarials.
Preparations
Many tetracycline and doxycycline preparations are available apart from those mentioned below.
• Achromycin® (Lederle). Tablets and capsules 250 mg, 500 mg.
• Vibramycin® (Pfizer). Tablets and capsules 100 mg, 200 mg. Oral suspension 10 mg/ml.
References
1. Practical Chemotherapy of Malaria. Report of a WHO Scientific group. Technical Report Series no. 805 (1990). (Geneva: World Health Organization), pp. 35–37.
2. Sande MA, Mandell GL (1990). Antimicrobial agents. In: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 8th edn, edited by A.G.Gilman, T.W.Rall, A.S.Nies and P. Taylor, (New York: Pergamon Press), pp. 1117–1124.
3. Meyler’s Side Effects of Drugs, 12th edn (1993), edited by M.N.Dukes (Amsterdam: Elsevier), pp. 160–161, 212–216.
4. Kuzneu EY (1970). Methoxyflurane, tetracycline and renal failure. J Am Med Ass, 221, 62–64.
Antimicrobial activity
Tetracyclines are broad-spectrum, essentially bacteristatic agents.
They share a similar spectrum of activity, though that of tigecycline
is somewhat different from that of earlier tetracyclines.
In general, tetracyclines are active against many Gram-positive
and Gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae,
coxiellae, spirochetes and some mycobacteria. Most streptococci
are sensitive, except Streptococcus agalactiae, and enterococci.
Susceptible Gram-positive bacilli include Actinomyces israelii,
Arachnia propionica, Listeria monocytogenes, most clostridia and
Bacillus anthracis. Nocardia spp. are much less susceptible, minocycline
demonstrating the greatest activity against them.
Among Gram-negative bacteria most enterobacteria and
most strains of Moraxella catarrhalis, Neisseria meningitidis
and Haemophilus influenzae are sensitive. Legionellae, brucellae,
Francisella tularensis, Vibrio cholerae, Campylobacter
spp., Helicobacter pylori, Plesiomonas shigelloides and Aeromonas
hydrophila are all susceptible. Many anaerobic bacteria are
susceptible, doxycycline and minocycline being the most
active. Rickettsiae are generally sensitive, especially to doxycycline,
minocycline and tetracycline. None is active against
Pseudomonas aeruginosa, Proteus spp. or Providencia spp., but
Burkholderia pseudomallei and Stenotrophomonas maltophilia are
usually susceptible.
Acquired resistance
Resistance to tetracyclines has emerged in an ever-increasing number of bacterial species and is geographically widespread. Resistance arises primarily from acquisition of genes that either encode transporters of the major facilitator superfamily (MFS), which remove the antibiotics from the cell (e.g. TetB, TetK), or encode proteins that protect the ribosome from inhibition (e.g. TetM, TetO). Tigecycline is not affected by these resistance mechanisms and consequently is active against many species resistant to earlier tetracyclines. In some Gram-negative bacteria resistance can also be due to the activity of innate (endogenous) bacterial efflux proteins such as the chromosomally encoded resistance–nodulation– cell division efflux pumps that confer resistance to several structurally unrelated biocides and antibiotics, as well as all classes of tetracyclines. The presence of these pumps explains the inherent resistance of most Ps. aeruginosa and Proteus spp. to all tetracyclines.
Pharmaceutical Applications
A group of natural products derived from Streptomyces spp. and their
semisynthetic derivatives. The minimum pharmacophore is a linear
fused tetracyclic molecule, 6-deoxy-6-demethyltetracycline.
The various members of the class contain a variety of functional groups
attached to the rings designated A, B, C and D. Natural products
include chlortetracycline, oxytetracycline, tetracycline and demeclocycline
(demethychlortetracycline). Semisynthetic derivatives include
doxycycline, minocycline, methacycline, lymecycline, rolitetracycline
and tigecycline, a glycylcycline that has been specifically developed to
overcome problems of bacterial resistance to earlier tetracyclines.
Pharmacokinetics
Absorption
Tetracyclines are usually administered by mouth. However,
tigecycline is available only for intravenous infusion.
Absorption of oral tetracyclines occurs largely in the
proximal small bowel, but may be diminished by the simultaneous
presence of food, milk or cations, which form nonabsorbable
tetracycline chelates. Cimetidine and presumably
other H2-receptor antagonists also impair absorption of
tetracyclines
by interfering with their dissolution, which is
pH dependent.
The absorption problems of earlier compounds have been
essentially overcome in the later tetracyclines. Improved
absorption is claimed for lymecycline, demeclocycline and
methacycline, but is best established for doxycycline and
minocycline, which may be administered with food and for
which the proportion of administered dose absorbed is more
than 90%.
Distribution
For orally administered tetracyclines, peak serum concentrations
follow 1–4 h after ingestion. Serum levels achieved
after normal dosage of orally bioavailable tetracyclines are of
the order of 1.5–4.0 mg/L. Most tetracyclines must be given
four times daily to maintain therapeutic concentrations in the
blood, but demeclocycline and minocycline can be administered
twice daily and doxycycline once daily.
Tetracyclines penetrate moderately well into body fluids
and tissues, reflected in relatively large volumes of distribution.
Concentrations of most tetracyclines in the cerebrospinal
fluid (CSF) are usually about 10–25% of those in the
blood, although penetration of tigecycline into the CSF is
poor. A unique feature of tetracyclines is deposition and persistence
in areas where bone is being laid down. Radioactive
tracer studies in animals suggest that tigecycline also concentrates
in the bone but concentration at this site in humans
remains unproven owing to technical difficulties with assay
methods.
Older tetracyclines are known to penetrate into the
sebum and are excreted in perspiration, properties which
contribute to their usefulness in the management of acne.
The older tetracyclines
are also known to be concentrated
in the eye.
Excretion
Excretory routes are the kidney and feces. Fecal excretion
occurs even after parenteral administration as a result
of passage of the drug into the bile. The concentrations
obtained in bile are 5–25 times those in the blood, doxycycline
attaining especially high levels. These concentrations
are lowered in the presence of biliary obstruction. The proportion
of administered dose found in the urine is, for most
tetracyclines, in the range 20–60%, but is less for chlortetracycline
and doxycycline and least for minocycline and
tigecycline.
Clinical Use
The use of tetracyclines has significantly declined in
most countries as the incidence of bacterial resistance has
increased and more active and better tolerated antimicrobial
agents have been introduced. However, some new applications
have emerged, such as their use as part of multidrug
regimens for the management of gastritis and peptic ulcer
disease associated with H. pylori. Their activity against
malaria has become important for prophylaxis following
the rapid increase of chloroquine- and mefloquine-resistant
Plasmodium falciparum.
Tigecycline is currently approved
only for use in complicated skin and skin structure infections
(including those caused by methicillin-resistant Staph.
aureus), complicated intra-abdominal infections and community-
acquired bacterial pneumonia, but use for other indications, may emerge if there
is suitable clinical evidence.
Side effects
Known adverse effects are primarily based on studies with
older tetracyclines and it is not yet clear whether they also
apply to tigecycline. However, it might be expected that tigecycline
will share at least some of the unwanted side effects
associated with all members of the class.
The most important adverse effect is gastrointestinal intolerance,
reported for all tetracyclines including tigecycline.
Photosensitivity is a class phenomenon; it is most marked
for demeclocycline but is not yet reported for tigecycline.
Deposition in developing bones and teeth precludes the use
of older tetracyclines in young children and during late pregnancy;
tigecycline may also exhibit these properties and is contraindicated
in these situations. Most compounds accumulate
in renal failure, with the exception of doxycycline and tigecycline.
Nausea and vomiting are presumed to be due to a direct
irritant effect of the drug on the gastric mucosa, but diarrhea
is probably the result of disturbance of the normal flora. The
frequency and nature of superinfection with resistant organisms
depends much on local ecology. Pseudomembranous
colitis has been associated with the use of older tetracyclines,
but they do not appear to be a common precursor of that complication.
Other organisms that often become dominant in the
fecal flora after administration of tetracyclines are Candida,
Proteus or Pseudomonas spp. Staph. aureus enterocolitis, which
was described in hospital patients when tetracyclines were
widely prescribed, now seems to be rare.
Glossitis and pruritus ani, vulvitis and vaginitis are wellrecognized
side effects associated with the use of older tetracyclines;
less common side effects include esophageal
ulceration and acute pancreatitis. Changes occur in the surface
lipids of the skin, notably a decrease in fatty acids and
reciprocal increase in triglycerides that probably results from
inhibition of extracellular bacterial lipase production by
Propionibacterium acnes.
Deaths have been reported in pregnant women given large
intravenous doses (>1 g per day), usually for the treatment
of pyelonephritis. The main lesion found at autopsy was diffuse
fatty degeneration of the liver, which may also involve
the pancreas, kidneys and brain. Mild derangements of liver
enzyme function are not uncommon.
A number of infants have developed bulging of the anterior
fontanelle; benign intracranial hypertension has also been
described in older children and even in adults, with headache,
photophobia and papilledema. Symptoms disappear quickly
after the drug is withdrawn, but papilledema may persist in
some patients for many months or reappear when tetracyclines
are given again. The mechanism is unknown.
Hypersensitivity rashes, including exfoliation, occasionally
occur, but skin reactions are more often manifestations
of photosensitivity. A reaction may occur after administration
of any tetracycline, but is especially associated with demeclocycline
and may be less common with doxycycline and
minocycline. Fixed drug eruptions, onycholysis and nail and thyroid pigmentation have also been reported. Angiodema
and anaphylaxis
are rare. Hypersensitivity reactions to one
tetracycline generally infer cross-hypersensitivity to the other
agents. Reported inhibitory effects on several human polymorphonuclear
leukocyte and lymphocyte functions in vitro
have yet to be shown to have any therapeutic significance.
Drug interactions include complexes with divalent and
trivalent cations together with chelation by iron-containing
preparations. The anticonvulsants carbamazepine, phenytoin
and barbiturates decrease the half-life of doxycycline through
enzyme induction. The anesthetic methoxyflurane has been
reported to cause nephrotoxicity when co-administered
with
older tetracyclines. Tigecycline is reported to affect the pharmacokinetic
profile of warfarin such that anticoagulation tests
should be performed if it is administered with warfarin. The
efficiency of oral contraceptives is reduced by tetracyclines,
as with many other broad-spectrum antibiotics.