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AMINOGLYCOSIDE RESISTANCE BY TARGET MODIFICATION
AMINOGLYCOSIDE RESISTANCE BY ALTERED TRANSPORT
AMINOGLYCOSIDE RESISTANCE BY ENZYMATIC MODIFICATON

Aminoglycoside antibiotic resistance

Resistance to aminoglycosides results largely from interference with the drug transport mechanism following modification of the antibiotic by one or more of a series of enzymes produced by the resistant bacteria. Such aminoglycoside-modifying enzymes are often plasmid-encoded, but have been associated increasingly with the presence of transposons and integrons. The first reports of enzyme-mediated aminoglycoside resistance were published by Umezawa’s group in the 1960s,[1-2] and were preluding to an international emergence of high-level aminoglycoside resistance that has been largely spread via R-plasmids, transposons, and integrons. This emergence of transferable aminoglycoside antibiotic resistance spurred the search and development of new natural product and semisynthetic aminoglycoside drugs that retained antimicrobial activity in the face of resistance. Thus the natural product tobramycin and the semisynthetic derivative of kanamycin B, amikacin, were introduced in the early 1970s. Since the clinical introduction of these compounds, only a limited number of aminoglycosides were successfully introduced into the clinic, mostly in Japan, in response to the emergence of resistance. The most successful of these were isepamicin and arbekacin.

 

The molecular details of resistance to aminoglycosides have been explored intensively. These are understood at atomic resolution thanks to the determination of co-complex structures of aminoglycosides with the ribosome and model RNAs [3]. Aminoglycoside resistance occurs by three methods:

(1) modification of the rRNA and ribosomalprotein targets,

(2) modification of aminoglycoside transport (import and efflux), and

(3) via the synthesis of aminoglycoside-modifying enzymes.

The latter have been the most prevalent mechanism in most clinical isolates of resistant bacteria, but the other mechanisms are now emerging as more important especially in niche settings and organisms. Table 1 reports the most common aminoglycoside-modifying enzymes and their characteristic substrates.

TABLE 1. Aminoglycoside-Modifying Enzymes and Their Resistance Profiles

Enzyme Aminoglycosidesa Donor
N-acyltransferases    
AAC(6')- I(a-d,e, f-z) T, A, N, D, S, K, I AcCoA
  II T, G, N, D, S, K AcCoA
AAC(3)- I(a-b) G, S, F AcCoA
  II(a-c) T, G, N, D, S AcCoA
  III(a-c) T, G, D, S, K, N, P, L AcCoA
  IV T, S, N, D, A AcCoA
  VII G AcCoA
AAC(1)-   P, L, R, AP AcCoA
AAC(2)- Ia T, S, N, D, Ne AcCoA
O-Nucleotidyltransferase      
ANT(2")- I T, G, D, S, K ATP
ANT(3') I St, Sp ATP
ANT(4') Ia T, A, D, K, I ATP
  IIa T, A, K, I ATP
ANT(6')- I St ATP
ANT(9')- I Sp ATP
O-Phosphotransferases      
APH(3')- I K, Ne, L, P, R ATP
  II K, Ne, B, P, R ATP
  III K, Ne, L, P, R, B, A, I ATP
  IV K, Ne, B, P, R ATP
  V Ne, P, R ATP
  VI K, Ne, P, R, B, A, I ATP
  VII K, Ne ATP
APH(2") Iab K, G, T, S, D ATP
  I(b,d) K, G, T, N, D ATP
  Ic K, G, T ATP
APH(3")- I(a-b) St ATP
APH(7")- Ia H ATP
APH(4)- I(a-b) H ATP
APH(6)- I(a-d) St ATP
APH(9)- I(a-b) Sp ATP

aA, amikacin; Ap, apramycin; B, butirosin; D, dibekacin; G, gentamicin; H, hygromycin; I, isepamicin; K, kanamycin; L, lividomycin; N, netilmicin; Ne, neomycin; P, paromomycin; R, ribostamycin; S, sisomicin; Sp, spectinomycin; St, streptomycin; T, tobramycin.
bFrom the bifunctional enzyme AAC(6 )–APH(2 ).

OVERCOMING AMINOGLYCOSIDE RESISTANCE

The impact of aminoglycoside resistance on the use of these bactericidal antibiotics has been significant. They have been eclipsed by other classes less prone to preexisting resistance and with improved pharmacological properties. The emergence of resistance to other antibiotic classes, the value of bactericidal agents in the treatment of immune compromised patients, the availability of detailed information on the
molecular mode of aminoglycoside binding at the ribosomes, and the deep understanding of the molecular mechanism of resistance have converged in a renewed effort to revisit this class designing new chemotherapeutic agents. Two such approaches are inhibition of resistance and intrinsically resistant aminoglycosides.

Inhibition of Aminoglycoside Resistance Enzymes

A small-molecule inhibitor of an aminoglycoside-inactivating enzyme could reverse resistance by blocking the activity of the resistance enzyme and thereby rescue antibiotic activity in the face of resistance. The inhibitory activity of 7-hydroxytropolone on ANT(2 ) was reported and shown to potentiate aminoglycosides in resistant strains [4], however this was neither sufficiently selective nor biologically active to be therapeutically interesting.

A challenge in the design of aminoglycoside potentiating agents is the large number of resistance mechanisms that exist in the clinic. The
ideal small-molecule inhibitor of aminoglycoside resistance would have activity against multiple enzymes and classes (AAC, APH, ANT). One common feature in all known aminoglycoside resistance proteins is a highly negatively charged binding site to guest the positively charged antibiotic. A series of cationic peptides were therefore screened against a several aminoglycoside-inactivating enzymes, and peptides with inhibitor activity against multiple enzymes from different classes were identified, demonstrating that it is possible to inhibit multiple enzymes with one molecule [5]. These results are encouraging and suggest that a pan-enzyme inhibition strategy is achievable by focusing on the aminoglycoside-binding pocket.

Intrinsically resistant aminoglycosides

The number of aminoglycoside antibiotic resistance strategies and associated genes means that truly intrinsically resistant aminoglycosides are likely to remain a dream. recent semisynthetic approaches have significantly expanded the chemical diversity of the class with opportunities to explore new activities. For example, the natural products tobramycin and gentamicin lack 3 -hydroxyl groups and are thus not susceptible to APH(3 ) enzymes. The semisynthetic compound amikacin (4-amino-2-hydroxybutrylamide derivative of kanamycin), which was inspired by the natural product butirosin, has improved activity against aminoglycoside resistant-strains. Efforts to expand chemical diversity have been rather successful. For example, neamine dimers maintained good antibiotic activity and inhibited APH(2 ) activity [6]. A new class of semisynthetic derivatives of neamine, termed the pyranmycins, with excellent antibiotic activity including against resistant strains has been reported [7-8]. In a similar work, a library of kanamycin derivatives has been prepared, providing several compounds showing excellent bioactivity and demonstrating the ability to greatly expand the chemical space of these compounds [9].

The structure-based drug design has allowed to rationally designed semisynthetic antibiotic using the crystal structure of the 30S ribosomal subunit as a guide [10]. Similarly, by comparing the binding of aminoglycosides to resistance enzymes and 16S rRNA in silico lead to the design of conformationally restricted aminoglycosides that would favor rRNA binding over resistance enzymes resulting in retained antibiotic activity even in the presence of resistance genes [11].

 

 


1. Umezawa, H.; Okanishi, M.; Kondo, S.; Hamana, K.; Utahara, R.; Maeda, K.; Mitsuhashi, S. Science 1967, 157, 1559.

2. Doi, O.; Miyamoto, M.; Tanaka, N.; Umezawa, H. Appl. Microbiol. 1968, 16, 1282.

3. Francois, B.; Russell, R. J.; Murray, J. B.; Aboul-ela, F.; Masquida, B.; Vicens, Q.; Westhof, E. Nucleic Acids Res. 2005, 33, 5677.

4. Allen, N. E., Jr.; Alborn, W. E. A., Jr.; J. N. H.; Kirst, H. A. Antimicrob. Agents Chemother. 1982, 22, 824.

5. Boehr, D. D.; Draker, K.; Koteva, K.; Bains, M.; Hancock, R. E.; Wright, G. D. Chem. Biol. 2003, 10, 189.

6. Sucheck, S. J.; Wong, A. L.; Koeller, K. M.; Boehr, D. D.; Draker, K.-A.; Sears, P.; Wright, G. D.; Wong, C.-H. J. Am. Chem. Soc. 2000, 122, 5230.

7. Chang, C. W.; Hui, Y.; Elchert, B.; Wang, J.; Li, J.; Rai, R. Org Lett 2002, 4, 4603.

8. Rai, R.; Chen, H. N.; Czyryca, P. G.; Li, J.; Chang, C. W. Org. Lett. 2006, 8, 887.

9. Wang, J.; Li, J.; Chen, H. N.; Chang, H.; Tanifum, C. T.; Liu, H. H.; Czyryca, P. G.; Chang, C. W. J Med Chem 2005, 48, 6271.

10. Russell, R. J.; Murray, J. B.; Lentzen, G.; Haddad, J.; Mobashery, S. J. Am. Chem. Soc. 2003, 125, 3410.

11. Bastida, A.; Hidalgo, A.; Chiara, J. L.; Torrado, M.; Corzana, F.; Perez-Canadillas, J. M.; Groves, P.; Garcia-Junceda, E.; Gonzalez, C.; Jimenez-Barbero, J.; Asensio, J. L. J. Am. Chem. Soc. 2006, 128, 100.

 

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