Aminoglycosides are antibiotics with a broad spectrum of action used in the treatment of numerous infections in all age groups of patients ^[1].

They are used in the treatment of severe infections of early childhood but are burdened by various side effects, including ototoxicity with hearing loss. In childhood, normal hearing function is of fundamental importance to allow normal cognitive development and language development. Consequently, knowledge of the mechanisms underlying the hearing damage from aminoglycosides, and of the possible preventive strategies, can help to prevent the appearance of this irreversible side effect.

2. Pharmacokinetics and Pharmacodynamics of Aminoglycosides

2.1. Aminoglycosides Structure

Aminoglycoside antimicrobials were discovered in the 1940s. The first one, called streptomycin, was isolated from the Gram-positive bacteria Streptomyces griseus and used for the treatment of tuberculosis. Since then, other aminoglycosides have been derived from Streptomyces species (neomycin, tobramycin, kanamycin, paromomycin, and spectinomycin). Others have been derived from Micromonospora species (gentamicin and sisomicin) or from preexisting molecules of aminoglycoside through chemical modifications (netilmicin, amikacin, plazomicin, arbekacin) ^[2]. Aminoglycoside structures are formed by a hexose (aminocyclitol) bound to one or more aminosugars through glycoside bonds. Streptomycin is made up of a guanidinylated streptamine linked in 4-position to a disaccharide. In the other three classes of aminoglycosides, the aminocyclitol is 2-deoxystreptamine; it can be 4,5-disubstitued (neomycin, neamine, paromomycin, ribostamycin), 4,6-disubstitued (amikacin, arbekacin, kanamycin, tobramycin, gentamicin, sisomicin, netilmicin, plazomicin), or mono-substituted (apramycin, neamine) ^[2]. Based on the spectrum of action, four generations of aminoglycosides have been identified, according to their ability to evade bacterial resistance mechanisms: generations I (streptomycin, neomycin, kanamycin, monomycin), II (gentamicin), III (tobramycin, amikacin, netilmicin, sisomicin), and IV (isepamicin), and a new generation (plazomicin) ^[3].

2.2. Aminoglycoside Absorption and Distribution

Aminoglycosides are hydrophobic, polycationic molecules, their cationic nature resulting from a predominance of basic, ionizable amino groups in the chemical structure. They have bactericidal activity trough interference with bacterial protein synthesis ^[2]. Because of their cationic nature, aminoglycosides are not lipophilic and hence have a very poor adsorption via the gastrointestinal tract; consequently, oral administration of aminoglycosides is not recommended. Thus, they need to be administered intramuscularly or, more commonly, by the intravenous route. Aminoglycosides demonstrate remarkably similar kinetics. They exhibit low plasma protein binding (<10%) ^[1]. After parenteral administration, the volume of distribution of aminoglycosides approaches approximately total body volume, with good distribution in all tissues, mainly into the extracellular water due to their hydrophilic nature. Their apparent volume of distribution (Vd) decreases according to the increasing of age: gentamicin Vd varies from 0.5–0.7 L/kg in preterm infants to 0.25 L/kg in young adults, due to a higher proportion of total body water in neonates ^[4]. They have good penetration to several bodily fluids, including synovial fluid, peritoneal, ascitic, and pleural fluids, but penetrate poorly into the central nervous system and the vitreous. Aminoglycosides distribute quite slowly into bile, feces, the prostate, and amniotic fluid. Vd increases in conditions such as sepsis, severe burns and febrile neutropenia, congestive cardiac failure, peritonitis, in the immediate postpartum period, and on parenteral nutrition ^[1]. They enter the cell via electrostatic binding of their positive molecules to the negative components of the bacterial cell surface (lipopolysaccharide and phospholipids of the Gram-negative outer membrane and phospholipids and teichoic acid of Gram-positive bacteria). This binding allows access to the periplasmatic space. Anaerobic bacteria are generally immune to aminoglycosides due to the lack of a membrane potential and the electron transport mechanisms required for the drug uptake. From the periplasmatic space, a small number of aminoglycosides cross the inner membrane and enter the cytoplasm ^[5]. In the cytoplasm, they impact protein synthesis by inhibiting initiation of translation, blocking elongation of translation or promoting codon misreading; this process leads to the generation of mistranslated proteins, which cause damage to the inner membrane ^[6]. This event facilitates the uptake of other aminoglycosides, which accumulate within the cell and accelerate the process of mistranslation, resulting in concentration-dependent bacterial killing by aminoglycosides ^[7].

2.3. Aminoglycoside Mechanism of Action

Aminoglycoside bind the 16S rRNA (A-site) of the 30S ribosomal subunit, inducing conformational change of rRNA in the decoding region that results in the misreading of information from mRNA and errors in protein synthesis. They also inhibit ribosome translocation by immobilization of peptidyl-tRNA at the A-site ^[8]. Some aminoglycosides (kanamycin, neomycin B, and gentamicin) showed bonds to the allosteric site of 23S rRNA of the 50S ribosomal subunit ^[9]. Unlike others, the binding site of streptomycin seems to be in the immediate vicinity of the ribosomal decoding center, so it interferes with initial tRNA selection ^[7]. Furthermore, it has been shown to disrupt the formation of the initiating 70S complex, inhibiting the protein-synthesis termination step ^[10]. The bacterial cell death following aminoglycoside uptake seems to be due to insertion of misread proteins into the inner membrane, which leads to destabilization of the cell; in addition, the massive uptake of aminoglycosides leads to inhibition of ribosomal activity and blocking of the protein synthesis [11,12]^[11][12].

2.4. Aminoglycoside Excretion

Aminoglycosides are excreted renally as intact compounds, by glomerular filtration; their clearance is similar to creatinine clearance and is reduced in the setting of poor renal function. Hence, clearance is also reduced in the elderly and in the neonate ^[13]. Their urinary concentrations are 70% of the dose administered or more; thus, they are ideal for treatment of urinary tract infections ^[14]. Elimination half-lives are approximately 2–3 h in adults but are prolonged in young children, especially neonates, due to their immature renal function, and in end-stage renal disease ^[1].

2.5. Aminoglycoside Pharmacokinetics in Pediatric Patients

In pediatric patients, pharmacokinetics (PK) and pharmacodynamics of aminoglycosides is different than in adults and changes over the course of age as extracellular fluid representation and renal function change. Neonates and infants have higher extracellular fluids per kilogram than children and adults, and this affect the volume of distribution of water-soluble medications, such as aminoglycosides, resulting in a higher volume of distribution, which decreases with age ^[15]. Renal elimination is also affected by age; premature neonates have compromised renal function and glomerular filtration rate increases with age and exceeds adult values during childhood, but it gradually decreases to approximate adult values during adolescence ^[15]. In addition to these two, several other factors may influence the PK of aminoglycosides in pediatric patients, such as obesity, inflammation, organ failure, critical illness, and co-medication; these factors affect drug exposure and consequently conventional age or weight-based dosing regimens do not seem to be optimal. An inadequate dosage can lead to treatment failure or, on the contrary, an exacerbation of the side effects connected to it. In order to prevent the damage from inadequate dosage, as for other drugs, also for aminoglycosides, in recent years, population PK (PopPK) modeling has been developed, which, combined with therapeutic drug monitoring, allows one to adapt the dose to the patient ^[16]. PopPKs are currently available for pediatric patients for gentamicin, amikacin, and netilmicin, and they highlight that the main variables influencing the PK of aminoglycosides in pediatric patients are the volume of distribution and renal function [17,18,19]^[17][18][19]. Furthermore, to date, there are models available that analyze the interactions between pharmacokinetic and pharmacodynamic (PK/PD) parameters. In a recent study carried out by Zazo et al., for example, it was observed, through a PK/PD model, that the most frequently used dose of gentamicin may not be adequate in newborns, who may need administration at longer time intervals with a consequent reduction of toxic effects ^[20]. In addition, Dong et al. exploited a PK model to evaluate the variables that most influence the onset of hearing damage and observed that, in patients with cystic fibrosis undergoing therapy with Tobramycin, the main determinants are represented by repeated cycles and older age (due to the repetition of treatments over time) ^[21]. To confirm the results of this study, as the authors specified, there is a need for validation in a larger prospective sample. Prospective studies are needed to develop improved physiological-based PK (PBPK) models to predict the concentration of the drug in the inner ear ^[21]. PBPK modeling is a compartment and flow-based type of pharmacokinetic modeling in which each compartment represents a physiologically discrete entity, such as an organ or tissue, and this is combined with the blood flow into and out of those entities. The distribution of the drug into and out of that organ will be related to the blood flow into and out of that organ, the concentration in the blood, and a partition coefficient. In theory, if a PBPK model contained a compartment for each organ in the body, it could facilitate the simultaneous description of drug concentration changes over time in each organ. The compartments are not limited to entire organs, and often PBPK models contain nested compartments that represent different cell types within an organ, and even different organelles within a cell. These levels of hierarchical complexity permit the modeling of molecularly-driven events, such as specific metabolic pathway damage mechanisms. With a specific inner ear compartment model, the concentration in the inner ear would be more accurately estimated and would provide a more definitive answer as to whether dosage correlates with the amount of hearing loss.