aminoglycoside

aminoglycoside, any of several natural and semisynthetic compounds that are used to treat bacterial diseases. The term aminoglycoside is derived from the chemical structure of these compounds, which are made up of amino groups (―NH₂) attached to glycosides (derivatives of sugar). The first aminoglycoside, the antibiotic streptomycin, was discovered in 1943 by American biochemists Selman Waksman, Albert Schatz, and Elizabeth Bugie, who isolated the compound from Streptomyces griseus, a strain of soil bacteria. Streptomycin was found to inhibit the growth of a variety of bacterial organisms, including the organism that causes tuberculosis (Mycobacterium tuberculosis). Waksman later isolated a second aminoglycoside, neomycin, from another species of soil bacteria, Streptomyces fradiae. In the following decades, natural aminoglycosides, such as gentamicin and tobramycin, and semisynthetic aminoglycosides, such as netilmicin and amikacin, were identified and developed.

Mechanism of action

Aminoglycosides are selectively active against oxygen-dependent (aerobic), gram-negative bacterial cells, since these cells possess the chemical characteristics that attract aminoglycosides and the specific transport mechanisms that facilitate the uptake of the drugs into the cells. Once inside bacterial cells, aminoglycosides exert their effects by binding to ribosomes, organelles that are fundamental to protein synthesis. As a result, protein synthesis is inhibited, and the bacterial cell dies.

Although most aminoglycosides are only effective against gram-negative bacteria, such as Escherichia coli and Pseudomonas aeruginosa, the agents streptomycin and gentamicin possess some activity against gram-positive organisms, including Streptococcus and Staphylococcus. Though these agents generally are of little clinical significance when administered alone in gram-positive bacterial infections, they can produce synergistic effects (effects greater than those expected by the addition of agents) when used in combination with other types of antibiotics. The bacterial cell-killing effect of aminoglycosides is concentration-dependent, with high concentrations of drug having high rates of bacterial killing. The aminoglycosides also have a post-antibiotic effect, in which bacterial cell killing continues for a brief period of time after the blood plasma concentration of the antibiotic has fallen below the so-called minimal inhibitory concentration (the smallest amount of drug necessary to induce cell-killing effects).

Clinical uses and toxicities

Aminoglycosides are used for a variety of infections. For example, gentamicin and tobramycin may be used in the treatment of meningitis, sepsis, and pneumonia; amikacin is commonly used in the treatment of infections that are resistant to other aminoglycosides; and neomycin is used in the treatment of skin infections associated with wounds and burns. Streptomycin is used primarily in combination with other agents, such as with tetracyclines in the treatment of plague, tularemia, and brucellosis. Gentamicin has largely replaced streptomycin in the treatment of bacterial endocarditis, which typically involves combination therapy with penicillin. The emergence of aminoglycoside-resistant organisms has limited the use of these drugs in the treatment of certain infections. For example, streptomycin was widely used in the treatment of tuberculosis, but the emergence of streptomycin-resistant tubercle bacilli led to the replacement of streptomycin with different types of antibiotics.

The aminoglycosides are poorly absorbed from the gastrointestinal tract and therefore are given parenterally, via intramuscular or intravenous injection, or topically, via application to the skin. The toxicities associated with aminoglycosides are severe and are sometimes irreversible, and the margin of safety between a toxic and a therapeutic dose is narrow. Nephrotoxicity (impairment of kidney function) and ototoxicity (impairment of the organs of hearing and balance) are the most common side effects of aminoglycosides. The risk of these reactions increases with age and with preexisting renal system diseases or hearing loss. Once-a-day dosing allows the plasma level of the drug to fall below toxic levels and does not reduce the antibacterial effect.