Chapter 54

### INTRODUCTION

The aminoglycoside group includes gentamicin, tobramycin, amikacin, netilmicin (not available in the U.S.), kanamycin, streptomycin, paromomycin, and neomycin. These drugs are used primarily to treat infections caused by aerobic gram-negative bacteria; streptomycin is an important agent for the treatment of tuberculosis, and paromomycin is used orally for intestinal amebiasis and in the management of hepatic coma. In contrast to most inhibitors of microbial protein synthesis, which are bacteriostatic, the aminoglycosides are bactericidal inhibitors of protein synthesis. Mutations affecting proteins in the bacterial ribosome, the target for these drugs, can confer marked resistance to their action. However, most commonly resistance is due to acquisition of plasmids or transposon-encoding genes for aminoglycoside-metabolizing enzymes or from impaired transport of drug into the cell. Thus, there can be cross-resistance between members of the class.

These agents contain amino sugars linked to an aminocyclitol ring by glycosidic bonds (Figure 54–1). They are polycations, and their polarity is responsible in part for pharmacokinetic properties shared by all members of the group. For example, none is absorbed adequately after oral administration, inadequate concentrations are found in cerebrospinal fluid (CSF), and all are excreted relatively rapidly by the normal kidney. Although aminoglycosides are widely used and important agents, serious toxicity limits their utility. All members of the group share the same spectrum of toxicity, most notably nephrotoxicity and ototoxicity, which can involve the auditory and vestibular functions of the eighth cranial nerve.

###### Figure 54–1.

Sites of activity of various plasmid-mediated enzymes capable of inactivating aminoglycosides. The red X indicates regions of the molecules that are protected from the designated enzyme. In gentamicin C1, R1=R2=CH3; in gentamicin C2, R1=CH3, R2=H; in gentamicin C1a, R1=R2=H. (Reproduced with permission from Moellering RC Jr. Microbiological considerations in the use of tobramycin and related aninoglycosidic aminocyclitol antibiotics. MJA 1977;2S:4–8. Copyright 1977. The Medical Journal of Australia.)

History and Source. Aminoglycosides are natural products or semisynthetic derivatives of compounds produced by a variety of soil actinomycetes. Streptomycin was first isolated from a strain of Streptomyces griseus. Gentamicin and netilmicin are derived from species of the actinomycete Micromonospora. The difference in spelling (-micin) compared with the other aminoglycoside antibiotics (-mycin) reflects this difference in origin. Tobramycin is one of several components of an aminoglycoside complex (nebramycin) that is produced by S. tenebrarius. It is most similar in antimicrobial activity and toxicity to gentamicin. In contrast to the other aminoglycosides, amikacin, a derivative of kanamycin, and netilmicin, a derivative of sisomicin, are semisynthetic products. Other aminoglycoside antibiotics have been developed (e.g., arbekacin, isepamicin, and sisomicin), but they have not been introduced into clinical practice in the U.S. because numerous potent, less toxic alternatives (e.g., broad-spectrum β-lactam ...

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