Pharmacology and Mode of Action of the Tetracycline Antibiotics

Tetracyclines are readily absorbed in the gastrointestinal tract and are subsequently widely distributed in most tissues. These antibiotics also cross the placenta and are present in breast milk and therefore are not recommended for pregnant or lactating women. Most tetracycline antibiotics are eliminated through the kidney and are therefore not recommended for patients with renal problems

because it may lead to toxicity. The exception is doxycycline (6), which is excreted through the bile. Serum half-lives are long (6 to >20 h, depending on the agents) and thus once or twice daily dosing is recommended on the order of 100 mg per dose (Table 15.1). Metabolism of these antibiotics is minimal.

The tetracycline antibiotics in current clinical use are generally bacteriostatic, inhibiting cell growth but not actually killing the cell. Thus, if the antibiotic is removed from the medium, the cells recover and resume growth. Tetracycline freely passes through lipid bilay-ers and enters the cell in a diffusion-controlled manner and no transport proteins appear to be necessary (3, 4). The mechanism of antibiotic action involves binding to the bacterial ribosome and arresting translation. Various methods have determined (1) that the 30S subunit is the preferred site of binding and (2) that there is one high affinity binding site, located in the A-site of the ribosome, and several lower affinity sites. The crystal structure of tetracycline bound to the 30S subunit of the Thermus thermophilus 30S subunit has been determined to 3.4 A (5). In this structure, two binding sites for the antibiotic were determined, one in the A-site as predicted and another in the body of the ribosome. The first site is likely the more clinically relevant site, in that it agrees well with several decades of binding and biochemical data describing the tetracycline-ribosome interaction. The key features of the binding of tetracycline to the A-site are shown in Fig. 15.4. All of the interactions between the antibiotic and the ribosome occur with the 16S rRNA, and no contacts with proteins are observed. Tetracycline binds in a 20 x 7-A pocket above the A-site binding pocket for the aminoacyl tRNA. As predicted by structure-activity studies, all four rings of tetracycline participate in binding to the ribosome and, in particular, Ring D stacks with the pyrimidine ring of C1054. The entire pharmocophore region of the molecule (see Fig. 15.2) is involved in specific interactions with the 16S rRNA. Not surprisingly, a ion is chelated by the antibiotic and the rRNA phosphate backbone as predicted by the affinity of tetracycline for divalent ions and the requirement for Mg2+ ion in tetracycline binding. This Mg2+ is also present in the structure of the ribosome in the absence cf'the antibiotic and may represent a key conserved binding element. The tetracycline-binding region is poorly conserved between eukaryotes and bacteria and helps to explain the low toxicity and specificity of these antibiotics.

Modeling of a tRNA molecule in the structure of the tetracycline-30s complex reveals a steric clash between the tRNA and the antibiotic. Furthermore, because tetracycline binds on the opposite side of the codon-anticodon binding pocket, it is possible that tetracycline does permit presentation of the aminoacyl-tRNA by EF-Tu, which would trigger GTP hydrolysis (see Fig. 15.1). Therefore, tetracyclines may act in two ways, first by preventing occupancy of the A-site by the aminoacyl-tRNA and thus arresting translation, and second in a catalytic fashion, depleting GTP stores in the cell.

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