The tetracyclines

Tetracycline 11.36) and its derivatives are octahydro-naphthacenes. The tetracyclines are among the most used of all drugs in treating systemic bacterial infections. They cannot act like oxine because their action on bacteria is slow and is not promoted by iron (Albert and Rees, 1956). They have very little action on fungi. [See Section 4.1 (p. 144) for background material on tetracyclines.] 

That the tetracyclines were effective chelating agents was first demonstrated by Albert and Rees, 1956. Their stability constants were found to be similar to those for glycine (see Table 11.2). 

Because of their chelating action, tetracycline drugs are inactivated in the patient s bowel by any dietary calcium or magnesium ions, whether from milk or from antacid medication. Through such mishaps, many patients have lost the potential benefit of these antibiotics. Tetracyclines are usually given orally. Tetracycline, itself, is still much prescribed, but there are also lower-dose forms available demeclocycline and methacycline, and a sub-class of these which require less frequent dosing doxycycline and minocycline. 

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Figure 7-15 shows the time evolution of the temperature, total energy, and potential energy for a 300 ps simulation of the tetracycline repressor dimer in its induced (i.e., hgand-bound) form. Starting from the X-ray structure of the monomer in a complex with one molecule of tetracycline and a magnesium ion (protein database... 

Figure 7-16. Superimpasition of the X-ray structure of the tetracycline repressor class D dimer (dark, protein database entry 2TRT) with the calculated geometrical average of a 3 ns MD simulation (light trace). Only the protein backbone C trace Is shown, The secondary structure elements and the tertiary structure are almost perfectly reproduced and maintained throughout the whole production phase of the calculation,...

Dmg distribution into tissue reservoirs depends on the physicochemical properties of the dmg. Tissue reservoirs include fat, bone, and the principal body organs. Access of dmgs to these reservoirs depends on partition coefficient, charge or degree of ionization at physiological pH, and extent of protein binding. Thus, lipophilic molecules accumulate in fat reservoirs and this accumulation can alter considerably both the duration and the concentration—response curves of dmg action. Some dmgs may accumulate selectively in defined tissues, for example, the tetracycline antibiotics in bone (see Antibiotics,tetracyclines). 

Tetracyclines. The tetracycliaes are a small group of antibiotics characterized as containing a polyhydronaphthacene nucleus. Commercially the tetracyclines are very important. They have been used clinically against gram-positive and gram-negative bacteria, spirochete, mycoplasmas, and rickettsiae... 

The tetracyclines are a group of antibiotics having an identical 4-ring carbocycHc structure as a basic skeleton and differing from each other chemically only by substituent variation. Figure 1 shows the principal tetracycline derivatives now used commercially. 

In general, the tetracyclines are yellow crystalline compounds that have amphoteric properties (Fig. 2) (15). They are soluble in both aqueous acid and aqueous base. The acid salts tend to be soluble in organic solvents such as 1-butanol, dioxane, and 2-ethoxyethanol In fact, 1-butanol is used to extract the salts from aqueous solution. 

The tetracyclines are strong chelating agents. Both the A-ring and 11,12 P-diketone systems are active sites for chelation (16). This abiUty to chelate to metals, such as calcium, results in tooth discoloration when tetracycline is administered to children (17). 

The tetracycline molecule (1) presents a special challenge with regard to the study of stmcture—activity relationships. The difficulty has been to devise chemical pathways that preserve the BCD ring chromophore and its antibacterial properties. The labiUty of the 6-hydroxy group to acid and base degradation (12,13), plus the ease of epimerization (23) at position 4, contribute to chemical instabiUty under many reaction conditions. 

Reactions at the C-5 position of the tetracycline molecule have been limited to the iatroduction of an alkoxy group (42) and the acetylation of the hydroxyl group (43) ia 5-hydroxytetracycline. Neither of these modifications improved the biological activity of the molecule. 

X-ray crystallographic studies (59) have defined the conformations and hydrogen bonding of the tetracyclines under nonpolar and polar conditions. These are shown ia Figure 3. It is beheved that the equiUbrium between the 2witterionic and nonioni2ed forms is of importance for the broad-spectmm antibacterial activity, membrane permeation, and pharmacokinetic properties. 

Efforts have been made to correlate electronic stmcture and biological activity in the tetracycline series (60,61). In both cases, the predicted activities are of the same order as observed in vitro with some exceptions. The most serious drawback to these calculations is the lack of carryover to in vivo antibacterial activity. Attempts have also been made (62) to correlate partition coefficients and antibacterial activity. The stereochemical requirements are somewhat better defined. Thus 4-epitetracycline and 5a-epitetracycline [65517-29-5] C22H24N20g, are inactive (63). The 6-epi compound [19369-52-9] is about one-half as active as the 6a (or natural) configuration. 

Most of the fermentation and isolation processes for manufacture of the tetracyclines are described in patents (71,72). Manufacture begins with the cultivated growth of selected strains of Streptomjces in a medium chosen to produce optimum growth and maximum antibiotic production. Some clinically useful tetracyclines (2—4) are produced directly in these fermentations others (5—7) are produced by subjecting the fermentation products to one or more chemical alterations. The purified antibiotic produced by fermentation is used as the starting material for a series of chemical transformations (59). 

The total U.S. antibiotic market for 1990 was about 4.73 biUion, 233 million of that was tetracyclines. The development of the semisynthetic P-lactam antibiotics (see Antibiotics, P-LACTAMs) and emergence of resistance to the tetracyclines has steadily diminished the clinical usefulness of tetracyclines. 

The overall biosynthetic pathway to the tetracychnes has been reviewed (74). Studies (75—78) utilising labeled acetate and malonate and nmr analysis of the isolated oxytetracycline (2), have demonstrated the exclusive malonate origin of the tetracycline carbon skeleton, the carboxamide substituent, and the folding mode of the polyketide chain. Feeding experiments using [1- 02] acetate and analysis of the nmr isotope shift effects, led to the location of... 

It has been known for some time that tetracyclines are accumulated by bacteria and prevent bacterial protein synthesis (Fig. 4). Furthermore, inhibition of protein synthesis is responsible for the bacteriostatic effect (85). Inhibition of protein synthesis results primarily from dismption of codon-anticodon interaction between tRNA and mRNA so that binding of aminoacyl-tRNA to the ribosomal acceptor (A) site is prevented (85). The precise mechanism is not understood. However, inhibition is likely to result from interaction of the tetracyclines with the 30S ribosomal subunit because these antibiotics are known to bind strongly to a single site on the 30S subunit (85). 

Fig. 4. Comparison of the three types of tetracycline resistance where T represents the tetracycline molecule O, a tetracycline transporter and aaa/, the ribosome A shows the effect of tetracycline exposure on a sensitive cell B, the efflux of resistance where a cytoplasmic membrane protein ( D) pumps tetracycline out of the cell as fast as the tetracycline transporter takes it up C, the ribosomal protection type of resistance where the ribosome is modified by ( ) to block productive binding and D, the tetracycline modification type of resistance where t is an inactive form of tetracycline. Reproduced with...

Resistance to Tetracyclines. The tetracyclines stiU provide inexpensive and effective treatment for several microbial infections, but the emergence of acquired resistance to this class of antibiotic has limited their clinical usehilness. Studies to define the molecular basis of resistance are underway so that derivatives having improved antibacterial spectra and less susceptibiUty to bacterial resistance may be developed. Tetracyclines are antibiotics of choice for relatively few human infections encountered in daily clinical practice (104), largely as a result of the emergence of acquired tetracycline-resistance among clinically important bacteria (88,105,106). Acquired resistance occurs when resistant strains emerge from previously sensitive bacterial populations by acquisition of resistance genes which usually reside in plasmids and/or transposons (88,106,107). Furthermore, resistance deterrninants contained in transposons spread to, and become estabUshed in, diverse bacterial species (106). 

Some antibiotics, such as the tetracyclines, tetracycline (7), doxycycline (78), and minocycline (17), chloramphenicol (79), and clindamycin (23) have modest antimalarial properties, but are slow-acting. 

The tetracyclines are valuable orally active broad-spectrum antibiotics prepared by isolation from the fermentation liquors of various strains of Streptomyces or by chemical transformation of fermentation-derived substances. The basic ring system and numbering pattern is as follows ... 

Similar transformations have not as yet been successfully applied to the tetracyclines bearing a hydroxy group at Cs, and no mutant culture has been reported that biosynthesizes a 6-deoxy-5-oxytetracycline. However, other means have been found to avoid 5a,6-dehydration in this subfamily. Treatment of 3 with N-... 

An additional 50 ml of methanol was added to the flask and then 22.2 grams (0.05 mol) of tetracycline, neutral form, was added portionwise intermittently with another 50 ml of methanol. A clear solution was maintained throughout the addition of the tetracycline. After addition of all of the tetracycline, the solution was a deep orange color and the temperature in the reaction flask was 35 C. 

One hour after addition of the tetracycline, the clear reaction solution was poured into 1,500 ml of chloroform. A yellow product separated and was collected on a coarse sintered glass filter and air dried. The tetracycline-metaphosphoric acid complex weighed about 10 grams, contained 7.34% of phosphorus and had a bioassay of 634 gammas per milligram. Solubility in water is 750 mg/ml. 

Discuss the uses, general drug action, adverse reactions, contraindications, precautions, and interactions of the tetracyclines, macrolides, and... 

The tetracyclines are a group of anti-infectives composed of natural and semisynthetic compounds. They are useful in select infections when die organism shows sensitivity (see Chap. 7) to the tetracyclines, such as in cholera, Rocky Mountain spotted fever, and typhus. 

The tetracyclines exert their effect by inhibiting bacterial protein syndiesis, which is a process necessary for reproduction of die microorganism. The ultimate effect of diis action is tiiat the bacteria are either destroyed or dieir multiplication rate is slowed. The tetracyclines are bacteriostatic (capable of slowing or retarding die multiplication of bacteria), whereas die macrolides and lincosamides may be bacteriostatic or bactericidal (capable of destroying bacteria). 

These antibiotics are effective in die treatment of infections caused by a wide range of gram-negative and gram-positive microorganisms. The tetracyclines are used in infections caused by Rickettsiae (Rocky Mountain spotted fever, typhus fever, and tick fevers). Tetracyclines are also used in situations in which penicillin is contraindicated, in the treatment of intestinal amebiasis, and in some skin and soft tissue infections. Oral... 

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