Aminoglycosides have played a major role in antimicrobial therapy since their discovery in the 1940s (1). Their bactericidal efficacy in gram-negative infections, synergism with β-lactam antibiotics, limited bacterial resistance, and low cost have given these agents a firm place in antimicrobial treatment. However, the successful use of streptomycin (1944), gentamicin (1963), tobramycin (1967), amikacin (1972), and netilmicin (1976) has been complicated by nephrotoxicity and ototoxicity in a significant number of treated patients.

Aminoglycosides comprise two or more amino sugars attached via glycosidic bonds to an aminocyclitol nucleus and have a molecular weight of 445 to 600 daltons. Aminoglycosides can be divided into chemical families with related structures. The relation between the structure of aminoglycosides and their activity is incompletely understood. Aminoglycosides are water-soluble, cationic at normal pH, and are distributed in plasma water. Protein binding is minimal. They have a stable structure over a wide range of temperature and pH (2,3).

Aminoglycosides are inactivated in vitro by concomitant use of β-lactam antibiotics (4,5,6). Tobramycin seems to be more easily inactivated than netilmicin or amikacin. Aminoglycosides act by altering the integrity of the bacterial cell membrane in growing bacteria by way of disturbing protein synthesis through binding to prokaryotic ribosomes (7,8). These cationic antibiotics bind rapidly and passively to the negatively charged parts of phospholipids and other proteins in the bacterial cell membrane and enter the bacterial cell by way of a self-promoted uptake process (9). The uptake process is related to the concentration of aminoglycoside and can be inhibited by low pH, anaerobic conditions, and hyperosmolarity (10,11).

Although inhibition of protein synthesis plays a major part in bacterial cell death, it is not the sole explanation for the bactericidal effect of aminoglycosides. Other antibiotics that inhibit protein synthesis, like chloramphenicol, are only bacteriostatic. Binding of aminoglycosides to the bacterial cell membrane itself may play a role in rapid bacterial cell death (12).

Aminoglycosides have a concentration-dependent bactericidal spectrum encompassing aerobic and gram-negative bacteria like Enterobacteriacae, Escherichia coli, Pseudomonas species, and Haemophilus species. The susceptibility of most gram-negative bacteria to gentamicin, tobramycin, netilmicin, and amikacin is relatively similar (13). Although susceptibility to amikacin is three- to fourfold less than that to the other aminoglycosides, this is compensated by its lower toxicity and therefore higher allowable dose. Gentamicin and tobramycin are comparable in activity, although tobramycin is slightly more active against Pseudomonas aeruginosa. They are susceptible to the same modifying enzymes and resistance rates are therefore very similar. In contrast, amikacin is resistant to many of these enzymes, and therefore is often an alternative if strains are resistant to tobramycin or gentamicin. Netilmicin susceptibility is comparable to that of gentamicin and tobramycin, although netilmicin is resistant to some of the gentamicin-inactivating enzymes, and thus in some cases is a good alternative.

The antimicrobial activity of aminoglycosides has four distinct and clinically important aspects: (a) concentration-dependent killing, (b) a postantibiotic effect (PAE), (c) adaptive resistance, and (d) synergism with other antibiotics. In vivo and in vitro studies have shown that the aminoglycoside-induced rate of bacterial killing as well as induction of resistance is peak concentration dependent (14,15,16,17,18). Other in vitro investigations, mimicking in vivo fluctuations of drug concentrations, have shown a single bolus of aminoglycoside to be superior in rate and total amount of bacterial killing to the same dose in a multiple daily-dosing regimen in nonneutropenic animals (19,20).

Aminoglycosides are often reported to have a PAE, meaning that there is suppression of bacterial growth for several hours after antibiotic serum concentrations have dropped below the minimal inhibitory concentration (MIC) (21,22,23). There are discrepancies between in vivo and in vitro studies on this effect, and studies have indicated that PAE is partly determined by diminished regrowth of
bacteria at sub-MICs (21). The clinical relevance of the PAE is unclear, and the emphasis on this effect in discussions on extending dose intervals of aminoglycosides is questionable. Synergy of aminoglycosides with other cell-wall-active antibiotics, like penicillins and cephalosporins, has been established (24,25). This synergy is the basis of the clinical choice for combination therapy of aminoglycosides with penicillins or cephalosporins. Issues of toxicity, which will be discussed later, concentration-dependent killing, a PAE (although doubtful), as well as adaptive resistance constitute the rationale for the change to extended-interval dosing of aminoglycosides (26,27,28).

Resistance to aminoglycosides can occur by three mechanisms: ribosomal resistance, decreased uptake and/or accumulation, and enzymatic modification. Ribosomal resistance will not be discussed because it is only pertinent to streptomycin.

Aminoglycosides are mainly used for treating serious gram-negative infections caused by enteric bacilli. Aminoglycosides are synergistic with cephalosporins and penicillins in the setting of gram-negative infections and are thus often combined with these cell-wall-active antibiotics (25). They are also used in combination treatment with vancomycin for Staphylococcus aureus (both methicillin-sensitive and methicillin-resistant strains), S. epidermidis, and enterococcal infections (34,35). Oral administration of paromomycin has been successfully used for treating cryptosporidiosis in patients with acquired immune deficiency syndrome (36). In case of empiric treatment with aminoglycosides, local susceptibility patterns have to be taken into account. There are many indications for use of aminoglycosides that are beyond the scope of this chapter. Three conditions will be addressed in more detail: neonatal sepsis, cystic fibrosis (CF), and urinary tract infections.

Aminoglycosides have a pharmacokinetic profile consisting of a rapid distribution phase (t_1/2α), an elimination phase (t_1/2β), and a second elimination phase (t_1/2γ). The gamma phase can only be determined after discontinuation of the drug. Distribution half-life is 5 to 10 minutes in adults but has never been measured in newborns. The gamma phase in infants is long. Netilmicin was detectable in blood and urine 11 and 14 days after discontinuation, with a t_1/2γ of 62.4 hours (58). The tissue half-life in renal cortex is 4 to 5 days (59). In general, the serum concentrations and pharmacokinetic data determined in most studies are derived from the elimination phase, which is adequately described by a one-compartment model. There are, however, some studies that have shown a two-compartment model to be superior in predicting serum t_1/2β and serum concentrations (60,61,62,63).

Because aminoglycosides are distributed over extracellular water, there are age-related changes in volume of distribution (V_d) dependent on the decrease of total body water with age. The largest V_d is encountered in premature neonates. Tables 30.1 and 30.2 display age-related pharmacokinetic parameters for aminoglycosides in neonates, infants, and children.

Many studies in neonates have been performed, and therefore only larger studies in this age group are included. In larger studies V_d of gentamicin ranges from 0.70 L per kg in neonates with a gestational age (GA) less than 32 weeks to 0.32 L per kg in children aged 11 to 18 years (64,65,66).

The V_d of most drugs is larger in neonates, especially in prematures, primarily due to a higher percentage of extracellular water (67,68). As can be seen in Table 30.1, this also holds true for aminoglycosides. There is a consistently higher V_d for prematures, especially in the group with very low birth weight (VLBW)/GA less than 30 weeks. Most authors have found birth weight (BW) to be the best predictor of V_d (62,69,70,71,72), although some have found V_d to be independent of GA (66,73). In practice, this means that prematures will end up having lower peak serum concentrations. The interpatient variability of V_d in these groups is greater, and therefore serum concentrations are difficult to predict in the individual premature infant. Total body clearance (CL), associated with GA (62,69,74) and BW (62,69,70,72), is lower, and t_1/2β is longer, in preterm infants, leading to higher serum trough concentrations in this group. This can be explained by the significant increase of glomerular filtration rate (GFR) with GA and BW described in recent studies (75,76). CL and t_1/2 are also highly variable in VLBW infants. Substantial effort has been put into developing equations, mostly based on population pharmacokinetic studies, which potentially will lead to better prediction of serum concentrations in individual patients (70,72,73). In practice, these equations are not able to adequately predict variability and subsequent serum concentrations in the same patient (64).

Diminishing extracellular fluid in the neonatal period (77) leads to a corresponding decrease in V_d with increasing postnatal age (PNA), again especially in the VLBW group. Recent data have shown a significant postnatal increase in GFR (75,76). A concomitant change in CL and t_1/2 has been shown for amikacin (78), gentamicin (73,74,79,80,81), netilmicin (58,62), and tobramycin (82), but this result has been refuted by others (71,83).

Patent ductus arteriosus (PDA) as well as postnatal exposure to indomethacin or other nonsteroidal anti-inflammatory drugs alters the rapid postnatal increase in GFR (84,85), possibly through decreased renal blood flow, leading to an increase in V_d and a reduction of CL. Increase in V_d of gentamicin is found in infants with PDA (66,86). In this patient group fluid overload is common. Closure of PDA leads to significant decrease in V_d of more than 30% (86,87). Dosage adjustments, based on therapeutic drug monitoring (TDM), should be made in patients with PDA. The effect of indomethacin, used for closure of PDA, as well as surgical closure itself necessitates TDM. Information on gentamicin disposition in infants on extracorporeal membrane oxygenation (ECMO) is scarce. Two small studies (63,88) showed V_d to be increased. Serum half-life in patients on ECMO was 9.55 and 9.24 hours, respectively, and decreased to 3.87 hours in the same patients off ECMO in the study by Dodge et al. (88). On the basis of these data, dosage adjustments should be made in this group of infants. Prenatal exposure to corticosteroids, which is seen increasingly in the VLBW group, leads to increased intrauterine maturation of kidney function, possibly through direct vasodilation mediated by glucocorticoid receptors (76,89). Although this point has not been investigated, it might have a significant effect on pharmacokinetic parameters in this group of infants. These studies indicate that extra TDM is warranted in patients who are on ECMO, have an open ductus Botalli, or are exposed to indomethacin. Aminoglycosides are eliminated from the body by way of glomerular filtration; it is therefore predictable that a relation between GFR and serum concentrations exists. The link between GFR and aminoglycoside pharmacokinetics is often (58,71,73,74,80,90,91,92), but not consistently (62,80,93,94), described in neonates. Furthermore, in adults it has been shown that aminoglycoside concentrations can change without concomitant change of serum creatinine (95). Keyes et al. (93) showed that serum trough concentrations could not be reliably predicted in newborns with serum creatinine. In conclusion, these studies suggest that, though there is a relation, serum creatinine in the first week of life cannot be accurately used to predict aminoglycoside CL.

There are also age-related differences in aminoglycoside pharmacokinetics in infants and older children. Figure 30.1 shows that V_d (for amikacin) decreases with increasing

PNA and CL decreases with increasing PNA (96). Changes in both V_d and CL are most pronounced in the first 2 weeks of life. V_d decreases by one-third when comparing patients younger and older than 3 months (97). A concomitant change in peak serum concentrations with age is seen (98). It is questionable whether these changes are clinically relevant for dose or dose interval in pediatric patients beyond the neonatal age.

V_d and total plasma CL for aminoglycosides are increased in patients with CF, necessitating larger doses in this patient group (46,49,50).

A relation between GFR and aminoglycoside CL in pediatric patients has been established, and aminoglycoside dosing has to be adjusted according to creatinine CL (99,100).

Efficacy of aminoglycosides is related to both the peak serum concentration-to-MIC ratio (peak/MIC) and the area under the time-versus-concentration curve (AUC/MIC) in clinical and experimental studies (14,17,101). Peak/MIC ratios of 5 to 10 are desirable for clinical efficacy. In the neonatal
period aminoglycosides are administered in combination treatment of suspected neonatal sepsis. Monitoring the efficacy of antibiotic treatment in neonates is difficult. Culture-proven early-onset sepsis occurs in approximately 2% of VLBW infants, but there are limitations to blood cultures in neonates, and single blood cultures can be false negative (102,103). Furthermore, increasing prenatal treatment of mothers with antibiotics obscures culture results in newborns. Early detection of neonatal sepsis remains difficult. Laboratory tests are unspecific and clinical signs can be ambiguous. Neonatal sepsis is suspected in many VLBW infants on clinical grounds, and antibiotic treatment is frequently started and discontinued after 48 to 72 hours when blood cultures and results of other tests remain negative (104). Given the infrequent occurrence of positive blood cultures, these cannot be used as a marker for antibiotic efficacy. Laboratory data, including C-reactive protein, complete blood cell count, and ratio of immature to total neutrophils, lack sufficient sensitivity to detect neonatal sepsis and can therefore not be used to evaluate efficacy (105). The same counts for clinical features (e.g., respiratory rate, skin color), which either cannot be quantified or lack a clearly defined relation between predictor and outcome.

In recent years aminoglycoside dosing in children has changed to ODD. Although theoretically this should lead to an increase of efficacy and a decrease of toxicity, this has not been clearly demonstrated in adults and has not been extensively studied in infants and children. Most studies did not address dose–response effect; some studies compared ODD to multiple daily doses (MDD) (106,107). In these studies both efficacy and toxicity were comparable. Urinary tract infections were effectively treated with ODD or thrice-daily dosing (TDD) of gentamicin in 179 pediatric patients (106). Severe bacterial infections in 52 pediatric patients were equally effectively treated with ODD and TDD of gentamicin in combination with a β-lactam antibiotic. Fever as well as C-reactive protein decreased comparably in both groups (107). ODD treatment of febrile children undergoing stem cell transplantation showed a tendency for less nephrotoxicity and more efficacy (108). The combination of amikacin with a β-lactam antibiotic showed satisfactory clinical results in 98% of severe bacterial infections in 56 infants and children (109).

The major specific side effects of aminoglycosides are nephrotoxicity and ototoxicity. Neurotoxicity by way of blockade of neuromuscular synapses with prolonged muscle weakness after the use of muscle relaxants has not been described in infants. The delayed-type hypersensitivity reaction of the skin is mostly seen after use of topical application of neomycin or framycetin and has not been described in neonates. Nephro- and ototoxicity will be described in further detail.

Dose and dosing interval are determined by the desired therapeutic range and pharmacokinetic as well as pharmacodynamic properties of a drug. It is difficult to define the desired therapeutic range for aminoglycosides. Peak concentrations of greater than 4 to 5 mg per L are generally accepted as necessary for antibacterial efficacy when dosing thrice daily (17,95,150,151). Efficacy of aminoglycosides is related to the ratio of peak serum concentration to the MIC of the infecting microorganism and the AUC. In vitro ratios of 10:1 prevent emergence of aminoglycoside-resistant pathogens (14,17). Commonly accepted trough concentration goals in adults are less than 2 mg per L, but when dosing once a day, most authors keep less than 1 mg per L as a safe limit (28,152,153). Based on the aminoglycoside susceptibility of gram-negative pathogens involved in neonatal septicemia, a reasonable target range for neonates would therefore be peak serum concentrations of 5 to 10 mg per L for gentamicin, netilmicin, and tobramycin and 15 to 30 mg per L for amikacin. Trough concentration goals are less than 2 mg per L when dosing thrice daily and less than 0.5 to 1.0 mg per L for ODD in gentamicin, netilmicin, and tobramycin and 1.5 to 3 mg per L for amikacin.

Glycopeptide antibiotics are a group of antibiotics with related structure and similar antimicrobial activity. The two major clinically important compounds in this group are vancomycin and teicoplanin. Vancomycin is an antibiotic first isolated from an Indonesian jungle soil sample in 1956, and was the first of the class of glycopeptide antibiotics. It was largely supplanted by methicillin sodium, introduced in 1960, because of the frequent occurrence of side effects associated with vancomycin use, including generalized skin eruptions, phlebitis, fever, and, more importantly, deafness and renal failure (172,173). Reduction of impurities and a concomitant decrease in incidence of side effects as well as the emergence of methicillin-resistant staphylococci have led to a resurgence of vancomycin use (174,175,176). Teicoplanin was introduced in 1982.

Despite the emergence of vancomycin-resistant enterococci, vancomycin still is the most widely used glycopeptide antibiotic and is a cornerstone in antibiotic treatment of gram-positive infections in adults, children, and neonates.