As was already stated, erythromycin and other antibiotics discussed in this chapter inhibit the synthesis of bacterial proteins.

Erythromycin inhibits bacterial protein synthesis by reversibly binding with their 50 S ribosomal subunit, thus blocking the formation of new peptide bonds. Erythromycin is classified as a bacteriostatic antibiotic.

However, it can also exhibit a bactericidal effect against a few types of microbes at certain concentrations.

Bacterial resistance to erythromycin can originate by two possible mechanisms: the inability of reaching the cell membrane, which is particularly relevant in the case of the microorganisms Enterobacteriaceae, or in the case of the presence of a methylated alanine in the 23 S ribosomal RNA of the 50 S subunit, which lowers the affinity of erythro-mycin to it.

Erythromycin acts on Gram-positive (staphylococci both produced and not produced by penicillinase, streptococci, pneumococci, clostridia) and a few Gram-negative microorganisms (gonococci, brucelli, hemophile and whooping cough bacilli, legionelli), mycoplasma, chlamydia, spirochaeta, and Rickettsia. Colon and blue-pus bacilli, as well as the bacilli shigella, salmonella, and others are resistant to erythromycin.

Erythromycin is used for bacterial infections such as diphtheria, whooping cough, trachoma, tonsillitis, scarlet fever, otitis, sinusitis, cholecystitis, pneumonia, gonorrhea, and so on. Erythromycin is an alternative to penicillin for treating infections caused by sensitive organisms. It is the drug of choice for pneumonia, diphtheria, enteritis, and so on. Synonyms of this drug are ilozon, meromycin, erythroped, and many others.

Clarithromycin: Clarithromycin, (2R,3S,4S, 5R,6R,8R,10R,11R,12S,13R)-3-(2,6-dideoxy-3-C-3-O-dimethyl-a-L-ribo-hexopyranosyloxy)-6-methoxy-9-oxo-11,12-dihydroxy-2,4,6,8,10,12-hexamethyl-5-(3,4,6-trideoxy-3-dimethylamino-^-D-xy/o-hexopyranosyloxy) cyclopentadecan-13-olide (32.2.2), is a semisynthetic analog of erythromycin A, in which the hydroxyl group at C6 is replaced with a methoxyl group [192,193].

Clarithromycin is better absorbed and irritates the gastrointestinal tract less than erythromycin. It is presumed that its activity exceeds that of erythromycin by 2-4 times with respect to a number of streptococci and staphylococci, and to a few other microorganisms. It is used for treating bacterial bronchitis, pneumonia, skin and sexual infections. It is believed that clarithromycin is the most active macrolide for treating atypical mycobacte-ria. Synonyms of this drug are biaxin and others.

Azithromycin: Azithromycin, [2R-(2R,3S,4R, 5R, 8R,10R,11R,12S,13S,14R)-3-(2,6-dideoxy-3-C-methyl-3- 0-methyl-a-L-nfto-hexopyranosyloxy)-2-ethyl-3,4,10-trihydroxy-3,5,6,8,10,12,14-heptamethyl-11-[(3,4,6-trideoxy-3-dimethylamino)-/to-.xyfo-hexopyrano syl]-oxy]-1-oxa-6-azacyclopentadecan-15-one (32.2.3), is also a macrolide antibiotic that is synthesized semisynthetically [194,195].

It is 2-4 times less active than erythromycin with respect to a number of streptococci and staphylococci, and to a number of other organisms; however, it is more active than other macrolides with respect to certain anaerobic organisms. Like other macrolides, it is active with respect to pathogens of the respiratory tract and pathogens transmitted sexually. It is used for treating bacterial bronchitis, pneumonia, skin, and sexual infections. Synonyms of this drug are zithromax and others.

Tetracyclines are a class of chemically similar drugs that are unified by the presence of four rings joined in a hydronaphthacene system, and the differences in activity are determined by various substituents on the base structure.

The first antibiotic of the tetracycline series, chlorotetracycline, which was isolated from a culture liquid of Streptomyces aureofaciens, was introduced into medical practice in 1948. Consequently, there were another six drugs of the tetracycline series that were introduced into medical practice between 1950 and 1972. Oxytetracycline is isolated from Streptomyces rimosus; tetracycline (semisynthetic), demeclocycline is isolated from the mutant type of S. aureofaciens; methacycline (semisynthetic), doxycycline (semisynthetic), and minocycline (semisynthetic). Methods of synthesis of the tetracycline series antibiotics have been suggested; however, they are purely of an academic interest and do not have any practical value.

Despite the few differences in these drugs in terms of pharmacokinetic features, their broad spectrum of antimicrobial action is similar in many regards.

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