Mechanism of Action

The mechanisms of bactericidal activity of aminoglycosides are not completely understood. In gram-negative bacteria, binding to and subsequent alteration of the cell envelope in addition to interaction with ribosomes, that causes inhibition of protein synthesis, may contribute to their bactericidal activity. Aminoglycosides are cations that bind passively to negatively charged portions of the outer membranes of gram-negative bacilli and competitively displace cell wall Mg²⁺ and Ca²⁺ that link lipopolysaccharide molecules.^1,2 The result is a rearrangement of the cell envelope and subsequent formation of transient holes in the cell wall, which interrupts normal permeability function of the bacteria.^3,4 In gram-positive bacteria, aminoglycoside uptake is decreased because of thicker outer cell wall membranes, and thus higher minimum inhibitory concentrations (MIC) are reported with these organisms.

Aminoglycosides are transported slowly across the cytoplasmic membrane via an energy-dependent process; this is the rate-limiting step in the drug action.^5,6 The transmembrane electrical potential correlates to the uptake and antibacterial effect. This energy-dependent transport mechanism is impaired in an anaerobic environment, conditions of low pH, and high osmolality. Thus in certain clinical settings such as in infections involving abscesses, aminoglycoside transport is reduced and may not be as effective.

Once across the cell membrane, aminoglycosides are trapped inside bacteria, leading to high intracellular concentration of the drug. Subsequently, aminoglycosides bind to the 16S rRNA of 30S subunits of ribosomes.⁷ This aminoglycoside-ribosome interaction causes termination and miscoding of protein synthesis, with subsequent bacterial cell death.

Spectrum of Activity

Aminoglycosides have a broad spectrum of activity against microorganisms, including gram-negative and gram-positive bacteria, mycobacteria, and protozoa. Among aerobic and facultative gram-negative bacilli, most aminoglycosides are active against Enterobacteriaceae (Escherichia coli, Proteus mirabilis, Klebsiella spp., Morganella spp., Citrobacter spp., Serratia spp., and Enterobacter spp.), Pseudomonas spp., and Acinetobacter spp. Resistance of clinical isolates to aminoglycosides varies with organism, patient population and their comorbidities, and local or regional usage patterns. According to the analyses of the Surveillance Network Database from 1998 to 2001, the susceptibility rates to aminoglycosides was higher against Enterobacteriaceae than nonfermentative organisms such as Pseudomonas and Acinetobacter spp.⁸ In addition, susceptibility to amikacin was higher than to gentamicin against species of Enterobacteriaceae, Pseudomonas, and Acinetobacter.⁹ Among ICU patients, the susceptibility of Enterobacteriaceae to gentamicin was 91.8% and amikacin was 98.5%.⁹ The susceptibility of Pseudomonas aeruginosa was 78.5% and 93.9% for gentamicin and amikacin, respectively. Against Acinetobacter baumannii, the susceptibility was 58.2% and 82.7%, respectively.⁸ For Enterobacteriaceae and P. aeruginosa, both gentamicin and amikacin susceptibility rates were similar among ICU and non-ICU patients or slightly better in non-ICU patients. However, among A. baumannii isolates, the susceptibility rates among non-ICU patients were lower than those reported for ICU patients (43.6% versus 58.2% for gentamicin and 77.2% versus 82.8% for amikacin).⁸

Since 2001, analyses of other databases indicate consistent but marginal decreases in susceptibility to aminoglycosides in comparison to other antibacterials in the United States. In 2008, a U.S. surveillance study encompassing 15 medical centers reported tobramycin susceptibility rates of 88.4% among Enterobacteriaceae, 89.1% among P. aeruginosa, and 59.1% in Acinetobacter spp.¹⁰ In 2007, a surveillance study of isolates from mostly North American sites but also including information from Europe, Asia, Latin America, Africa, and the Middle East reported susceptibility to amikacin of better than 95% among Enterobacteriaceae isolates, 92.3% among P. aeruginosa, and 69.6% among A. baumannii isolates.¹¹ In comparison to the United States, the susceptibility to aminoglycosides against P. aeruginosa in Europe and Latin was lower. A surveillance study conducted in 2007 showed tobramycin susceptibility rates among P. aeruginosa isolates of 92% in North America, 77% in the European Union, and 63.8% in Latin America.¹² A European surveillance study reported tobramycin susceptibility of 74.2% among P. aeruginosa isolates in 2007,¹³ and the International Nosocomial Infection Control Consortium (INICC) surveillance study from 2003 to 2008 reported an amikacin resistance rate of 31% among P. aeruginosa isolates collected from ICU patients from 25 countries in Latin America, Asia, Africa, and Europe.¹⁴

Aminoglycosides are active against methicillin-susceptible Staphylococcus aureus. For other gram-positive pathogens such as methicillin-resistant S. aureus (MRSA), Streptococcus spp., and Enterococcus spp., aminoglycosides are used in a limited fashion to provide synergistic activity with β-lactam antibiotics. In Enterococcus spp., the synergism is only observed in organisms that display low-level gentamicin resistance (4-250 µg/mL).¹⁵ A poor active transport of drug due to anaerobic metabolism and the thick cell wall is thought to be responsible for this low-level resistance. Synergism is achieved in these organisms because gentamicin uptake is enhanced when combined with β-lactam antibiotics. Enterococci may acquire one or more of the following resistance mechanisms to demonstrate high-level resistance: alteration of the target site, interference with drug permeability, or enzyme inactivation of drug.¹⁶ In organisms with high-level resistance, synergistic activity of gentamicin is not observed. In high-level gentamicin resistance, it may be worthwhile to test for high-level streptomycin resistance. High-level gentamicin and streptomycin resistance is considered with MIC ≥ 500 µg/mL and MIC ≥ 2000 µg/mL, respectively.

Rates of high-level gentamicin resistance in Enterococcus spp. varies markedly among institutions, but the nationwide prevalence is estimated at 30% to 60%.¹⁵ High-level resistance is low in Enterococcus faecalis, which is responsible for approximately 60% of nosocomial enterococcal bloodstream infections. However, this type of resistance is observed in greater than 50% of Enterococcus faecium, which causes approximately 20% of nosocomial enterococcal bloodstream infections.

Aminoglycosides have activity against less common ICU pathogens. Streptomycin has greatest activity against Mycobacterium tuberculosis and Yersinia pestis.^17,18 Both streptomycin and gentamicin have been reported to be effective in Francisella tularensis infection.¹⁹ Amikacin has the best activity among aminoglycosides against Mycobacterium avium-intracellulare. Spectinomycin is useful in treating Neisseria gonorrhoeae infection.²⁰ Paromomycin has been used against intestinal parasites.²¹

Mechanisms of Resistance

Bacterial resistance to aminoglycosides is achieved through multiple mechanisms. These include modification of the ribosomal target, enzymatic modification, decreased antibiotic uptake, and efflux of antibiotics. Mutations at the ribosomal (16S rRNA) binding sites results in resistance to aminoglycosides. This mechanism has not been detected in most clinical isolates, except for Mycobacterium tuberculosis against streptomycin.²²

The most common mechanism of resistance for aminoglycosides is inactivation by aminoglycoside-modifying enzymes. The exposed hydroxyl and amino groups of aminoglycosides are subject to structural modification and loss of antimicrobial activity by enzymes from both gram-positive and gram-negative bacteria.²³ There are three types of enzymes which transfer a functional group to the aminoglycoside structure: (1) aminoglycoside nucleotidyltransferases (ANT) that transfer nucleotide triphosphates; (2) aminoglycoside acetyltransferases (AAC) that transfer the acetyl group from acetyl-CoA; and (3) aminoglycoside phosphotransferases (APH) that transfer the phosphoryl group from ATP.^23,24 Once the structure of aminoglycosides has been modified, they bind poorly to ribosomes, and this then results in high-level resistance. Genes encoding aminoglycoside-modifying enzymes are usually found on extrachromosomal bacterial plasmids and transposons within the periplasmic space. Thus, they can be easily transferred from bacteria to bacteria.²⁵ Amikacin is the aminoglycoside most stable to these enzymatic effects because it has fewer sites for enzymatic attack.

Resistance can also be developed by preventing penetration of the drug through the outer bacterial cell membrane or by preventing active transport through the cytoplasmic membrane.²⁶ Chromosomal mutations that alter transmembrane electrical potential may down-regulate aminoglycoside uptake into the bacterial cell after the first aminoglycoside exposure. This temporary disruption of the energy-dependent phase of aminoglycoside uptake is called adaptive resistance and lasts for several hours. Extended-interval aminoglycoside dosing may allow this effect to reverse, owing to the higher peak serum concentration (C_max)/MIC ratios achieved. Aminoglycoside exposure may also select for subpopulations of bacteria with active efflux pumps resulting in low-level resistance. The efflux pump, MexXY, in P. aeruginosa is involved in resistance to many antibacterials including aminoglycosides.²⁷

Pharmacokinetics

All the aminoglycosides have similar pharmacokinetic properties. The distribution from the vascular to the extravascular space occurs rapidly within 15 to 30 minutes post infusion.²⁸ Aminoglycosides are primarily excreted by glomerular filtration.²⁹ Thus, dosage adjustments are based on creatinine clearance (CrCl). In patients with normal renal function, the half-lives of all aminoglycosides range from 1.5 to 3.5 hours. The half-life is shortened in febrile illnesses and prolonged in any condition that decreases renal function. More than 90% of a parenterally administered dose is recovered in urine unchanged during the first 24 hours. The remainder is slowly recycled into the tubular lumen, where accumulation of the drug causes nephrotoxicity.³⁰

Aminoglycoside concentrations are generally low in infected secretions and tissues such as respiratory secretions, pleural fluid, cerebrospinal fluid, and aqueous humor. High drug concentrations are found in the proximal tubular cells of the renal cortex, which is thought to correlate with the nephrotoxic potential of aminoglycosides.³⁰

Pharmacodynamics

Pharmacodynamic principles associated with aminoglycosides include concentration-dependent bactericidal activity, post-antibiotic effect (PAE), and synergism with other cell wall–active agents.³¹ Aminoglycosides are rapidly bactericidal, and their rate and extent of bacterial killing increases as the antibiotic concentration is increased. Exposure of bacteria to a single 24-hour aminoglycoside dose with the associated high peak drug concentration results in faster and a greater extent of bactericidal activity than that noted for the same total dose administered in divided doses.³² In 236 patients with gram-negative infections, attainment of a C_max/MIC ratio of 10 and 12 exhibited a response rate of 80% or higher.³³ Another study of 78 patients with gram-negative nosocomial pneumonia suggested that achieving C_max/MIC ratio ≥ 10 within the first 48 hours of therapy had a 90% probability of temperature and leukocyte count resolution by day 7.³⁴

PAE is a persistent suppression of bacterial growth after short antimicrobial exposure.³⁵ The higher the peak aminoglycoside concentration, the longer the PAE. In vitro, the aminoglycosides consistently demonstrate a PAE that varies from 1 to 3 hours for P. aeruginosa and 0.9 to 2.0 hours for Enterobacteriaceae. A PAE is also demonstrated for S. aureus.³⁶

Synergy is frequently reported in vitro with a combination of an aminoglycoside and a cell wall–active antimicrobial (e.g., penicillin, cephalosporin, carbapenem, monobactam, glycopeptide).³⁷ Synergy is noted when significantly greater effect with two drugs is observed compared to that anticipated based on the effect of each individual drug. Enhanced aminoglycoside uptake in the presence of a cell wall–active drug has been demonstrated with Streptococcus spp., Enterococcus spp., S. aureus, and P. aeruginosa. Two meta-analyses have evaluated synergism in vivo by comparing the efficacy of monotherapy with a β-lactam antibiotics and combination therapy with a β-lactam antibiotic plus an aminoglycoside.^38,39 All-cause mortality was comparable in both groups in sepsis and in suspected ventilator-associated pneumonia. This lack of synergism observed in patients may be due to the poor quality of pooled studies, such as lack of blinding, analysis not based on intention to treat, and unspecified follow-up period. The studies included in these meta-analyses also reported high susceptibility to both β-lactam antibiotics and aminoglycosides (≥90% susceptibility) among gram-negative pathogens and included very few patients with multidrug resistant pathogens such as P. aeruginosa and A. baumannii. Therefore, it is yet to be determined if the synergism observed in vitro between a β-lactam antibiotic and an aminoglycoside translates into survival advantage in patients. However, resistance among gram-negative pathogens is increasing, especially among P. aeruginosa, and combination therapy of β-lactam antibiotics and aminoglycosides offers broader coverage in ICU patients than combination with fluoroquinolones.⁴⁰ In the future, a high percentage of adequate empirical therapy with aminoglycosides may translate into improved survival in patients infected with multidrug-resistant pathogens.