Aminoglycosides

Discovery

The first aminoglycoside, streptomycin, was isolated from Streptomyces griseus by Selman Waksman and his group in 1943 (Comroe 1978). It was used to treat tuberculosis. Several other members of this class of antibiotics were introduced over the next few years, including neomycin (1949, S. fradiae), kanamycin (1957, S. kanamyceticus), gentamicin (1963, Micromonospora purpurea), netilmicin (1967, derived from sisomicin), tobramycin (1967, S. tenebrarius), and amikacin (1972, derived from kanamycin) (Krause et al., 2016).

Aminoglycosides are antibacterial drugs that inhibit protein synthesis in both gram-positive and gram-negative bacteria. Because active electron transport is necessary for aminoglycoside uptake, drugs of this class are ineffective against anaerobic bacteria. Aminoglycosides are used as single agents and in combination with other antibiotics in both empirical and definitive therapy for a broad range of indications (Krause et al., 2016).

Clinically, aminoglycosides are used to treat severe infections of the abdomen and urinary tract, as well as bacteremia, prophylaxis, and endocarditis. Aminoglycosides are often combined with β-lactams for the empirical treatment of severe sepsis (Dellinger et al., 2013). In the 1980s, the use of streptomycin began to decline as third-generation cephalosporins, carbapenems, and fluoroquinolones became more popular, which were believed to be less toxic and have broader coverage than the aminoglycosides.

Overview of Chemistry

The basic chemical structure of aminoglycosides that contributes to its potency and antibacterial activity is at least one aminated sugar joined by glycosidic linkages to a dibasic cyclitol. The most clinically used aminoglycoside contains the dibasic cyclitol, 2-deoxystreptamine, shown here in the example of paromomycin (Figure 1).

Figure 1. 2D structure of paromomycin showing key chemical components of an aminoglycoside. Figure drawn using ChemAxon.

Types

Aminoglycosides are classified into four groups based on the type of aminocyclitol moiety included in the molecule and their substitutions (see Figure 2). Key features of molecules in these classes (Krause et al., 2016) include:
1. has no deoxystreptamine (e.g., streptomycin, which has a streptidine ring)
2. has a mono-substituted deoxystreptamine ring (e.g., apramycin)
3. has a 4,5-di-substituted deoxystreptamine ring (e.g., neomycin, ribostamycin)
4. it has a 4,6-di-substituted deoxystreptamine ring (e.g., gentamicin, amikacin, tobramycin, and plazomicin).

These varying structures within the aminoglycoside class result in the drugs having different specificities for different regions in the target, the ribosome. The core structure of the dibasic cyclitol, along with the decorated variety of amino and hydroxyl substitutions, have a direct influence on the mechanism of action of aminoglycosides and susceptibility to different aminoglycoside modifying enzymes (AMEs) that inactivate the drug.

Figure 1. Examples of different types of aminoglycosides (AG). The streptidine group in streptomycin is highlighted in orange while the 2-deoxystreptamine group in the disubstituted aminoglycosides is highlighted in blue. Adapted from Krause et al., 2016.

Aminoglycosides inhibit protein synthesis by binding to the A-site on the 16S ribosomal RNA of the 30S ribosomal subunit (Kotra et al. 2000). Although different aminoglycosides have a different specificity for different regions on the A-site and may affect different steps in protein synthesis, all of the drugs alter the conformation of the A-site. The ribosome is seen in Figure 3 with mRNA and 2 tRNAs. Note that of the A-, P-, and E-site tRNAs the aminoglycoside (plazomicin shown here), binds to the A- site (Golkar et al., 2021)

Learn more about aminoglycoside binding for streptomycin, gentamicin, or plazomicin.

Figure 3. A ribbon representation of the subunits of the bacterial ribosome. The 23S and 5S rRNA in the large subunit are colored in shades of blue and the 16S rRNA in the small subunit are shown in pink. All the ribosomal proteins are colored grey (PDB ID: 7lh5, Golkar et al., 2021). The mRNA in the structure is colored yellow, while the P- and E-site tRNAs are colored green and red respectively, and plazomicin is colored dark blue.

Aminoglycosides are highly polar molecules and need to be actively transported into bacterial cells. They can cross the outer membrane of gram-negative bacteria by disrupting Mg²⁺ bridges between adjacent lipopolysaccharide molecules (Mingeot-Leclercq et al., 1999). Then, aminoglycosides are transported across the cytoplasmic (inner) membrane by active transport ( i.e., the step requires energy). In the cytosol, aminoglycosides bind to the 30S subunit of ribosomes after the initiation complex of peptide synthesis is formed (i.e., binding of mRNA, fMetRNA, and association of the 50S subunit).While the formation of a translational complex is not affected, aminoglycoside binding prevents the elongation of the nascent chain of amino acids by impairing the proofreading process. This leads to a misreading of the mRNA, premature termination, or both. The resulting aberrant protein products are inserted into the cell membrane, leading to altered permeability and further stimulation of aminoglycoside transport. The rapid uptake of additional aminoglycoside molecules into the cytoplasm increases the inhibition of protein synthesis, mistranslation, and accelerated cell death (Krause et al., 2016).

Resistance

Various mechanisms of aminoglycoside resistance have been reported. These include enzymatic modification of the antibiotic; modification of the target site via an enzyme action or chromosomal mutations, and efflux of antibiotics (Krause et al., 2016). Each of these mechanisms has varying effects on different members of the class. Often multiple resistance mechanisms are involved in any given resistant isolate.

Resistance to aminoglycosides via target site mutations has not been observed because nearly all prokaryotes, with the exception of Mycobacterium spp. (Bercovier et al. 1986) and Borrelia spp. (Schwartz et al. 1992), encode multiple copies of rRNA.

Learn more about enzymatic modification of aminoglycosides.

Safety Information

Recently, physicians have been re-evaluating the use of aminoglycosides due to two major issues: the spectrum of antimicrobial susceptibility and toxicity.

Adverse effects of aminoglycosides include kidney damage and hearing loss, limiting its use in everyday practice. Depending on the susceptibility of the patient and the duration of treatment, aminoglycosides can result in hearing loss of various degrees. It can also impair balance (Black et al., 2004). Current evidence also shows that aminoglycoside use can pose a human fetal risk (Yu et al., 2020), though potential benefits may outweigh the cons and still call for the use of members of this drug class in pregnant women.

Furthermore, advanced patterns of antimicrobial resistance have emerged. While aminoglycosides do retain activity against the majority of gram-negative bacterial strains, aminoglycoside resistance through enzymatic modification of the drug is a major mechanism of resistance. Learn more about aminoglycoside modifying enzymes (AMEs).

References

Black, F. O., Pesznecker, S., Stallings, V. (2004). Permanent gentamicin vestibulotoxicity, Otology and Neurotology. 25 (4): 559–69. https://doi.org/10.1097/00129492-200407000-00025

Comroe, J. H. Jr. (1978) Pay dirt: the story of streptomycin. Part I. From Waksman to Waksman. Am Rev Respir Dis. 117(4):773-81. https://doi.org/10.1164/arrd.1978.117.4.773

Dellinger, R. P., Levy, M. M., Rhodes, A., Annane, D., Gerlach, H., Opal, S. M., Sevransky, J. E., Sprung, C. L., Douglas, I. S., Jaeschke, R., Osborn, T. M, Nunnally, M. E., Townsend, S. R., Reinhart, K., Kleinpell, R. M., Angus, D. C., Deutschman, C. S., Machado, F. R., Rubenfeld, G. D., Webb, S. A., Beale, R. J., Vincent, J. L., Moreno, R., (2013) Surviving Sepsis Campaign Guidelines Committee including the Pediatric Subgroup., Crit Care Med. 41(2):580-637. https://doi.org/10.1007/s00134-012-2769-8

Golkar, T., Bassenden, A. V., Maiti, K., Arya, D. P., Schmeing, T. M., Berghuis, A. M. (2021) Structural basis for plazomicin antibiotic action and resistance. Commun Biol. 4(1):729. https://doi.org/10.1038/s42003-021-02261-4

Kotra, L. P., Haddad, J., Mobashery, S. (2000) Aminoglycosides: Perspectives on mechanisms of action and resistance and strategies to counter resistance. Antimicrob Agents Chemother 44: 3249–3256. https://doi.org/10.1128/aac.44.12.3249-3256.2000

Krause, K. M., Serio, A. W., Kane, T. R., & Connolly, L. E. (2016). Aminoglycosides: An Overview. Cold Spring Harbor perspectives in medicine, 6(6), a027029. https://doi.org/10.1101/cshperspect.a027029

Mingeot-Leclercq, M. P., Glupczynski, Y., Tulkens, P. M. (1994) Aminoglycosides: activity and resistance. Antimicrob Agents Chemother., 43(4):727-737. https://doi.org/10.1128/aac.43.4.727

Yu, P. A., Tran, E. L., Parker, C. M., Kim, H. J., Yee, E. L., Smith, P. W., Russell, Z., Nelson, C. A., Broussard, C. S., Yu, Y. C., Meaney-Delman, D. (2020) Safety of Antimicrobials During Pregnancy: A Systematic Review of Antimicrobials Considered for Treatment and Postexposure Prophylaxis of Plague. Clin Infect Dis., 70(70 Suppl 1):S37-S50. https://doi.org/10.1093/cid/ciz1231

March 2025, Helen Gao, Shuchismita Dutta; Reviewed by Drs. Albert Berghuis and Tolou Golkar
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/antibiotics/prot-syn/ribo/AMG