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Peptide synthetase

Chloramphenicol

Chloramphenicol

Peptide synthetase

Chloramphenicol

Erythromycin

Streptomyces species

A. Protein synthesis and modes of action of antibacterial drugs mRNA

Tetracyclines are absorbed from the gastrointestinal tract to differing degrees, depending on the substance, absorption being nearly complete for doxycycline and minocycline. Intravenous injection is rarely needed (rolite-tracycline is available only for i.v. administration). The most common unwanted effect is gastrointestinal upset (nausea, vomiting, diarrhea, etc.) due to (1) a direct mucosal irritant action of these substances and (2) damage to the natural bacterial gut flora (broad-spectrum antibiotics) allowing colonization by pathogenic organisms, including Candida fungi. Concurrent ingestion of antacids or milk would, however, be inappropriate because tetracyclines form insoluble complexes with plurivalent cations (e.g., Ca2+, Mg2+, Al3+, Fe2+/3+) resulting in their inactivation; that is, absorbability, antibacterial activity, and local irritant action are abolished. The ability to chelate Ca2+ accounts for the propensity of tetracyclines to accumulate in growing teeth and bones. As a result, there occurs an irreversible yellow-brown discoloration of teeth and a reversible inhibition of bone growth. Because of these adverse effects, tetracycline should not be given after the second month of pregnancy and not prescribed to children aged 8 y and under. Other adverse effects are increased photosen-sitivity of the skin and hepatic damage, mainly after i.v. administration.

The broad-spectrum antibiotic chloramphenicol is completely absorbed after oral ingestion. It undergoes even distribution in the body and readily crosses diffusion barriers such as the blood-brain barrier. Despite these advantageous properties, use of chloram-phenicol is rarely indicated (e.g., in CNS infections) because of the danger of bone marrow damage. Two types of bone marrow depression can occur: (1) a dose-dependent, toxic, reversible form manifested during therapy and, (2) a frequently fatal form that may occur after a latency of weeks and is not dose dependent. Due to high tissue penetrability, the danger of bone marrow de pression must also be taken into account after local use (e.g., eye drops).

Aminoglycoside antibiotics consist of glycoside-linked amino-sugars (cf. gentamicin 0ยป a constituent of the gentamicin mixture). They contain numerous hydroxyl groups and amino groups that can bind protons. Hence, these compounds are highly polar, poorly membrane permeable, and not absorbed enterally. Neomycin and paro-momycin are given orally to eradicate intestinal bacteria (prior to bowel surgery or for reducing NH3 formation by gut bacteria in hepatic coma). Amino-glycosides for the treatment of serious infections must be injected (e.g., gen-tamicin, tobramycin, amikacin, netilmi-cin, sisomycin). In addition, local inlays of a gentamicin-releasing carrier can be used in infections of bone or soft tissues. Aminoglycosides gain access to the bacterial interior by the use of bacterial transport systems. In the kidney, they enter the cells of the proximal tubules via an uptake system for oligopeptides. Tubular cells are susceptible to damage (nephrotoxicity, mostly reversible). In the inner ear, sensory cells of the vestibular apparatus and Corti's organ may be injured (ototoxicity, in part irreversible).

Metabolism Chloramphenicol
A. Aspects of the therapeutic use of tetracyclines, chloramphenicol, and aminoglycosides
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