TETRACYCLINE

Tetracycline is a broad-spectrum polyketide antibiotic produced by the Streptomyces genus of Actinobacteria, indicated for use against many bacterial infections. It is a protein synthesis inhibitor. It is commonly used to treat acne today, and, more recently, rosacea, and is historically important in reducing the number of deaths from cholera. Tetracycline is marketed under the brand names Sumycin, Tetracyn, and Panmycin, among others. Actisite is a thread-like fiber formulation used in dental applications. It is also used to produce several semisynthetic derivatives, which together are known as the tetracycline antibiotics. The term "tetracycline" is also used to denote the four-ring system of this compound; "tetracyclines" are related substances that contain the same four-ring system.

MECHANISM OF ACTION

Tetracyclines bind to the 30S subunit of microbial ribosomes. They inhibit protein synthesis by blocking the attachment of charged aminoacyl-tRNA to the A site on the ribosome. Thus, they prevent introduction of new amino acids to the nascent peptide chain. The action is usually inhibitory and reversible upon withdrawal of the drug. Resistance to the tetracyclines results from changes in permeability of the microbial cell envelope. In susceptible cells, the drug is concentrated from the environment and does not readily leave the cells. In resistant cells, the drug is not actively transported into the cells or leaves it so rapidly, inhibitory concentrations are not maintained. This is often plasmid-controlled. Mammalian cells are not vulnerable to the effect of tetracyclines, as these contain no 30S ribosomal subunits, so do not accumulate the drug.

SPECTRUM OF BACTERIAL SUSCEPTIBILITY AND RESISTANCE

Tetracyclines have a broad spectrum of antibiotic action. Originally, they possessed some level of bacteriostatic activity against almost all medically relevant aerobic and anaerobic bacterial genera, both Gram-positive and Gram-negative, with a few exceptions, such as Pseudomonas aeruginosa and Proteus spp., which display intrinsic resistance. However, acquired (as opposed to inherent) resistance has proliferated in many pathogenic organisms and greatly eroded the formerly vast versatility of this group of antibiotics. Resistance amongst Staphylococcus spp., Streptococcus spp., Neisseria gonorrhoeae, anaerobes, members of the Enterobacteriaceae and several other previously sensitive organisms is now quite common. Tetracyclines remain especially useful in the management of infections by certain obligately intracellular bacterial pathogens such as Chlamydia, Mycoplasma and Rickettsia. They are also of value in spirochaetal infections, such as syphilis, leptospirosis and Lyme disease. Certain rare or exotic infections, including anthrax, plague and brucellosis, are also susceptible to tetracyclines. These agents also have activity against certain eukaryotic parasites, including those responsible for diseases such as malaria and balantidiasis.

HISTORY

The tetracyclines, a large family of antibiotics, were discovered as natural products by Benjamin Minge Duggar in 1945 and first described in 1948. Under Yellapragada Subbarao, Benjamin Duggar made his discovery of the first tetracycline antibiotic, chlortetracycline (Aureomycin), at Lederle Laboratories in 1945. In 1950, Harvard University professor Robert Burns Woodward determined the chemical structure of the related substance, oxytetracycline (Terramycin); the patent protection for its fermentation and production was also first issued in 1950. A research team of seven scientists (K.J. Brunings, Francis A. Hochstein, C.R. Stephens, Lloyd Hillyard Conover, Abraham Bavley, Richard Pasternack, and Peter P. Regna) at Pfizer, in collaboration with Woodward, participated in the two-year research leading to the discovery. Pfizer was of the view that it deserved the right to a patent on tetracycline and filed its Conover application in October 1952. Cyanamid filed its Boothe-Morton application for similar rights in March 1953, while Heyden Chemicals filed its Minieri application in September 1953, named after scientist P. Paul Minieri, to obtain a patent on tetracycline and its fermentation process. This resulted in tetracycline litigation in which the winner would have to prove beyond reasonable doubt of priority invention and tetracycline’s natural state. Nubian mummies studied in the 1990s were found to contain significant levels of tetracycline; the beer brewed at the time could have been the source. Tetracycline sparked the development of many chemically altered antibiotics, so has proved to be one of the most important discoveries made in the field of antibiotics. It is used to treat many Gram-positive and Gram-negative bacteria. Like some other antibiotics, it is also used in the treatment of acne.

INDICATIONS

It is first-line therapy for Rocky Mountain spotted fever (Rickettsia), Lyme disease (B. burgdorferi), Q fever (Coxiella), psittacosis and lymphogranuloma venereum (Chlamydia), and to eradicate nasal carriage of meningococci. Tetracycline tablets were used in the plague outbreak in India in 1992.

OTHER USES

Since tetracycline is absorbed into bone, it is used as a marker of bone growth for biopsies in humans. Tetracycline labeling is used to determine the amount of bone growth within a certain period of time, usually a period of approximately 21 days. Tetracycline is incorporated into mineralizing bone and can be detected by its fluorescence. In "double tetracycline labeling", a second dose is given 11–14 days after the first dose, and the amount of bone formed during that interval can be calculated by measuring the distance between the two fluorescent labels. Tetracycline is also used as a biomarker in wildlife to detect consumption of medicine- or vaccine-containing baits. In genetic engineering, tetracycline is used in transcriptional activation. It is also one of the antibiotics used to treat ulcers caused by bacterial infections. In cancer research at Harvard Medical School, tetracycline has been used to switch off leukemia in genetically altered mice, and to do so reliably, when added to their drinking water. A technique being developed for the control of the mosquito species Aedes aegypti uses a strain that is genetically modified to require tetracycline to develop beyond the larval stage. Modified males raised in a laboratory will develop normally as they are supplied with this chemical and can be released into the wild. Their subsequent offspring will inherit this trait, but will find no tetracycline in their environment and so will never develop into adults.

CELL CULTURE

Tetracycline is used in cell biology as a selective agent in cell culture systems. It is toxic to prokaryotic and eukaryotic cells and selects for cells harboring the bacterial tetr gene, which encodes a 399-amino-acid, membrane-associated protein. This protein actively exports tetracycline from the cell, rendering cells harboring this gene more resistant to the drug. The yellow crystalline powder can be dissolved in water (20 mg/ml) or ethanol (5 mg/ml), and is routinely used at 10 mg/l in cell culture. In cell culture at 37°C (99°F), it is stable for days, with a half-life of approximately 24 hours.

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